Applied Geodynamics LaboratoryAnnual Progress Report

to Industrial Associates for 1994

by

D. D. Schultz-Ela, B. C. Vendeville, G. Guglielmo, Jr.,

M. P. A. Jackson, and H. Ge

Prepared for

Agip S.p.A.

Amoco Production Company

Anadarko Petroleum Corporation

Arco Exploration and Production Technology

BP Exploration Inc.

Chevron Petroleum Technology Company

Conoco Inc.

Exxon Production Research Company

The Louisiana Land and Exploration Company

Marathon Oil Company

Mobil Research and Development Corporation

Petroleo Brasileiro S.A.

Phillips Petroleum Company

Société Nationale Elf Aquitaine Production

Statoil

Texaco Inc.

Total Minatome Corporation

N. Tyler, DirectorBureau of Economic Geology

The University of Texas at AustinAustin, Texas 78713

December 1994

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Part 1: Captions for Physical and Mathematical Models

in Slide Set Number 11

Preliminary Notes:

(a) This eleventh set comprising 146 slides illustrates research carried out in 1994; theprevious slide set was distributed in March, 1994.

(b) Prekinematic layers were added before deformation began, synkinematic layersduring deformation, and postkinematic layers after deformation ended.

(c) In all vertical sections, an uppermost sand layer was added after deformation tofacilitate sectioning and to preserve the structural topography; this postkinematic sandlayer is structurally irrelevant.

(d) Model endwalls were perpendicular to the direction of tectonic transport; sidewallswere perpendicular to endwalls. Cross-sectional views were parallel to the sidewalls,unless otherwise specified.

(e) Geographic directions (N, S, E, and W) on overhead views or maps of models referto a system in which the top of each view or map is arbitrarily designated north.

(f) Prekinematic layers were deposited so that their upper and lower contacts wereplanar (section on lower left) or wedge-shaped, where specified. Any thickness ratios

quoted are the ratio of prekinematic brittle overburden thickness to viscous (“salt”)source layer thickness before deformation.

(g) Synkinematic layers all initially had planar upper surfaces (lower right of Text

Figure 11.1) but irregular lower contacts. Any primary dips are specified in the captions.

h

Text Figure 11.1

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The layer thickness, h, is defined by the difference in elevation between the topof the previous layer when it was deposited (left section of Text Figure 11.1) and thetop of the new layer when it was deposited (right section). Measuring against anexternal reference frame avoids complications due to irregular structural relief, whichcauses local increase or decrease of thickness in syndepositional layers above subsidingor rising structures. The actual thickness of the layer varied locally across fault planesand was commonly greater overall than h (right section) because of subsidence causedby regional extension and local “salt” flow.

(h) The silicone used in the physical models of this report had these properties.S6 silicone: transparent Rhône-Poulenc polymer, variety RG-20, Newtonianviscous. At strain rate of 10-4 s-1, viscosity = 2 ∞ 104 Pa s.

(i) Slides are presented in the following order:

Rejuvenation of pre-existing diapirs by regional shortening (Slides 11.1–11.8)

Rejuvenation of passive diapirs by regional shortening (Slides 11.9–11.37)

Multiple breakout of salt sheets during progradation:linear progradation front over a curved salt pinchout (Slides 11.38–11.72)

Formation of polygonal minibasins during progradation (Slides 11.73–11.92)

Progress on 3-D visualization:3-D salt layer from gravity-spreading experiment (Slides 11.92–11.97)

Structural constraints imposed by salt conservation (Slides 11.98–11.101)

Overburden response to active basement faults (Slides 11.102–11.117)

Coeval extension above and below salt sheets (Slides 11.118–11.146)

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Rejuvenation of Pre-Existing Diapirs by Regional Shortening

The experiments in the next two sections investigate the effect of superposed regionalcontraction on pre-existing buried diapirs. Regional forces squeeze the stems of thediapirs thereby pressurizing the salt. This causes an otherwise static diapir to riseactively and pierce its roof by lifting it upward and sideways. These experimentsinvestigate the differences in deformation style between diapirs with varying planformsand orientations with respect to the direction of regional shortening.

Slides 11.1 to 11.8 are top views and sections of Experiment 307. Two circular stocksand two linear diapiric walls had initially rectangular vertical sections and were laterrejuvenated during regional shortening. The model was initially 77 cm long (E-W), 40cm wide (N-S) and comprised a thin, 0.5-cm-thick layer of viscous S6 silicone overlainby a thick sand overburden. Only a narrow strip at the center of the model (darkyellow strip in Slide 11.2) was not underlain by the silicone layer. Along this strip, thefriction between the sand overburden and the base was high enough to prevent anydeformation. The silicone-free strip acted as a barrier separating the two model halves,allowing each to evolve independently.

In each model half, we built two diapirs. The left-hand half was initially 37 cmlong and contained two circular stocks (Diapirs 1 and 2 in Slide 11.2) having flat,horizontal crests and vertical flanks. Diapir 1 was initially 3.0 cm tall and had a meandiameter of 4.2 cm. Diapir 2 was initially 2.8 cm tall and had a mean diameter of 4.5 cm.The right-hand half of the model was initially 36 cm long and included two linear walls(Diapirs 3 and 4 in Slide 11.2) having flat, horizontal crests and vertical flanks. Diapir 3was initially 2.5 cm tall, 5 cm wide, and 22 cm long. Diapir 4 was initially 2.5 cm tall, 10cm wide, and 22 cm long. All diapirs were encased in a white sand layer whose top waslevel with the crest of the diapirs. This layer was overlain by a roof comprising seven0.3-cm-thick, prekinematic sand layers of various colors (from bottom to top: red,yellow, white, blue, green, white, and purple). Each half of the model was thenshortened at a rate of 0.48 cm/h for 6.5 h. The finite amount of shortening for each half

was 3.1 cm.

Slide 11.1. Overhead view after deposition of the thick white sand layer (here with ablue coating) encasing the diapirs, but before deposition of the 7 thin roof layers.The diapirs appear purple.

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Slide 11.2. Drawing of Slide 11.1 showing more precisely the location and initialplanform of the diapirs.

Slides 11.3 and 11.4. Photograph and drawing, respectively, of the model top sur-face after regional contraction. The grid lines in Slide 11.3 were initially orientedE-W and N-S with an initial spacing of 1.5 cm. These strain markers recorded surfacedeformation. The drawing in Slide 11.4 also indicates the approximate location andshape of the underlying diapirs after deformation. Top views clearly indicate thatmost of the regional shortening was preferentially accommodated in the weakestzones—the diapirs and their thin roofs—rather than distributed across the entiremodel length. All the uplifted areas are diapiric roofs, demonstrating that the diapirswere rejuvenated by a phase of active piercement driven by shortening. All reversefaults are located above the diapir flanks. Unlike active diapirs that rise withoutregional shortening, these diapirs lack any crestal grabens or normal faults in topviews. We infer therefore that the amount of horizontal shortening of the diapirsand their roofs exceeded the component of stretching caused by arching of the roofduring vertical rise. This conclusion is confirmed by the diapir geometry in verticalsections.

Slides 11.5 to 11.8 are W-E vertical serial sections cutting across or near Diapir Stock2 (location on Slide 11.4) at the end of the experiment; west is to the left. Theuppermost white sand layer is postkinematic and therefore structurally irrelevant.

Slides 11.5 and 11.8 show sections immediately S and N, respectively, ofDiapir 2. Both sections show a large W-dipping reverse fault cutting the entire sandsection. A smaller reverse fault with opposite vergence appears on Slide 11.8.Structures in both sections would be unmistakably interpreted as the result ofhorizontal shortening not vertical diapiric rise.

The section on Slide 11.6 cuts through the center of Diapir 2. The section onSlide 11.7 cuts the stock near its N edge. Both sections show an uplifted diapiric roofbounded by inward-dipping reverse faults above the diapir flanks. Both sections inSlides 11.6 and 11.7 are, at first glance, ambiguous and could be interpreted ascaused by active diapiric rise only. However, closer inspection reveals two clues thatindicate an origin by shortening rather than active diapirism. (1) The reverse faultshere are straight; those bounding active diapirs in physical and numerical modelsare curved and steepen downward (compare with Slides 6.15–6.21, 8.16, 9.83, 9.85 inprevious slide sets). (2) The reverse faults here are shallow-dipping (45° or less);

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those bounding active diapirs dip more steeply, their chord dipping at greater than45°. The dip and curvature of the reverse faults appear to be useful criteria fordistinguishing purely regional contraction from purely active diapirism. However,the essence of the tectonic setting being discussed is that these modes ofdeformation are commonly mixed because contraction pressurizes the salt, whichpromotes active diapirism. This some structures may contain reverse faults ofintermediate dip and curvature.

The elevated roof of the stock in Slide 11.6 was only slightly arched. Thusvertical uplift remained low (about 1 cm) compared with the horizontal shortening(about 3 cm). This implies that horizontal contraction was large enough tocounteract any local stretching of the crest of the slightly arched roof, and to preventformation of normal faults above the diapir. Experiment 250, illustrated in the nextsection, shows that sufficiently narrow diapirs can rise faster than they shrinklaterally, and therefore undergo more arching than lateral shortening.Consequently, narrow diapirs tend to form crestal grabens above their arched roofs(see Slides 11.9 to 11.30 in next section).

Rejuvenation of Passive Diapirs by Regional Shortening

Slides 11.9 to 11.30 illustrate Experiment 250, in which circular and linear diapirs grewpassively during sedimentation as they depleted their source layer, were buried under athick roof and then rose anew during late regional shortening. The 48-cm-long and 27-cm-wide model comprised an initially thick (2.0 cm) source layer of viscous S6 siliconeoverlain by two thin prekinematic layers of white sand, each 0.2 cm thick. To triggerpassive growth by differential loading, we locally vacuumed the prekinematic sandlayer, creating holes of varying shape in the overburden. These holes subsequentlyevolved into 4 circular stocks (Diapirs 1, 4, 5, and 6 on Slide 11.13), one E-W linear wall (Diapir 7 on Slide 11.13), and one initially N-S diapiric wall that later split into two stocks(Diapirs 2 and 3 on Slide 11.13). Local removal of the prekinematic layer by vacuumingallowed the viscous source layer to flow freely upward through the circular and linearvents and emerge diapirically above the regional datum. Diapirs in the model thengrew strictly passively as their emergent crests remained slightly above the surfaceduring sedimentation. We added new, synkinematic sand layers having their upperboundary horizontal and level with the diapiric crests. Synkinematic layer numbers,colors, thicknesses and deposition times are listed in the table below.

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Layer number Color Thickness Time(cm) (h)

7 purple 1.3 17.02

6 white 0.5 7.10

5 blue 1.1 6.35

4 white 1.0 3.75

3 red 1.1 2.57

2 white 0.6 1.01

1 purple 0.5 0.35

Slides 11.9 to 11.12. Overhead views during passive diapiric growth.

Slide 11.9. Initial stage after local removal of the prekinematic overburden.

Slide 11.10, shot immediately before deposition of synkinematic layer 5 (blue layeron sections), shows that diapirs were vigorously rising through the six vents in theoverburden. All diapirs rose above the regional datum and started to extrude,forming diapiric overhangs.

On Slide 11.11, shot immediately before deposition of synkinematic layer 7 (purplelayer on sections), the viscous silicone was still flowing fast enough in most diapirsto produce large extrusions. The diapir stems appear as dark shapes surrounded byextrusive glaciers (gray, transparent rings around each diapir) partly covered bydismembered veneers of sand (white). In the east the extrusions from three diapirstocks (Diapirs 4, 5, and 6) coalesced to form a continuous canopy. The N-S diapiricwall (NW part of the model) did not extrude much further. We assume that the flowof viscous silicone was already restricted at the base of this diapir, slowing its riseand allowing new sand layers to encroach on its crest. This onlap partly buried thediapir wall and split it into two circular stocks (Diapirs 2 and 3 on Slide 11.13).

The stage of regional extrusion illustrated on Slide 11.11 appears to mark theend of vigorous diapirism. The photograph and drawing in Slides 11.12 and 11.13

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show that diapirs did not rise much after deposition of layer 7 and did not extrude.We infer that this transition marks extreme thinning of the source layer and thatmost of the overburden blocks encasing the diapirs had grounded onto thebasement.

We then buried all the diapirs under a 3-cm-thick, post-passive-growth overburdencomprising 8 thin sand layers (from bottom to top: white, white, red, white, white,purple, white, and white). Regional shortening was applied by moving the easternendwall westward at a rate of 0.5 cm/h for 5.35 h. The total displacement was 2.6 cm.We then added a 1-cm-thick, syncontraction sand layer (uppermost blue layer withvarying thickness in sections on Slides 11.19 to 11.30) after 1.67 h (displacement 0.8 cm)of shortening.

Slide 11.14. Top view immediately after deposition of the blue syncontraction layer.Each square on the passive grid represents 1.5 cm x 1.5 cm.

Slides 11.15 and 11.16. Top views at the end of the experiment. Slide 11.16 was drawnfrom the photograph in Slide 11.15. All the passive diapirs, static after source-layerdepletion, were rejuvenated during the late contraction and lifted their thick roofs. Fourdiapirs (Diapirs 1, 2, 6 and 7) rose but did not emerge. Diapir 5 pierced its roof actively.Diapirs 3 and 4 emerged and spread as glaciers of S6 Silicone. The deformed modelcontained both normal and reverse fault scarps. Fault orientations did not appear to berelated to the direction of regional contraction (E-W) but rather follow the diapir shapesand the outline of their overhangs. An E-W crestal graben formed above the lineardiapiric wall 7. A NE-SW crestal graben oblique to the direction of regional contractionformed above the canopy between Diapirs 5 and 6 and bridges the stems of the twodiapirs. Likewise, the traces of the reverse faults appear to follow the periphery of thecanopy fed by Diapirs 4, 5 and 6, while remaining only approximately perpendicular tothe direction of regional contraction. Cross sections in Slides 11.17 to 11.30 confirm thatthese reverse faults all terminate at depth against the tip of the diapir overhangs. Onlyone reverse fault, located southwest of Diapir 7 cuts across the entire section and cantherefore be confidently attributed to regional contraction. Top views of the deformedmodel suggest that regional contraction was accommodated mostly by preferentiallydeforming the diapirs and their roofs rather than the thicker, stronger encasingoverburden.

After deformation ended, the model was buried, hardened, and cut in E-W serialsections, illustrated by Slides 11.17 to 11.30.

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Slide 11.17. Regional photograph of section X-X' (see location in Slide 11.16) cuttingthrough Diapirs 6 (right) and 7 (left). Slide 11.18 is a drawing of Slide 11.17. Slides

11.19 and 11.20 show details of Diapirs 6 and 7 in the same section. The section featuresa lack of buckling folds, throughgoing reverse faults, or any other structure commonlyassociated with regional contraction. The section shows two large diapirs with archedand faulted roofs between undeformed overburden blocks. Both diapirs havenumerous overhangs formed as extrusive flanges during passive diapir growth. Thediapirs are overlain by a thick roof comprising 8 layers deposited after passive diapirgrowth but before contraction. Because these layers were initially tabular, strain causedall thickness variations in their deformed state. The uppermost blue layer was depositedduring contraction. Thickness changes of this layer reflect the combined effect of strainand deposition. The roof of the left-hand wall (Diapir 7) was arched, but faults are notresolvable. This is partly because this strike section did not intersect the crestal graben(Slide 11.16) and partly because the faults were subparallel to the section. Thinning ofthe roof is barely noticeable. The roof of the right-hand diapir (Diapir 6) wasasymmetrically arched and broken by both normal and reverse faults. The asymmetryis due to the off-center location of the crestal graben north of the diapir stem, but is alsogreatly exaggerated by the obliquity between the section direction (E-W) and thegraben strike (NE-SW).

Likewise, reverse faulting was also asymmetric. The diapir was bounded bydivergent reverse faults rooted into the upper edge of the diapiric overhangs. Theoverall fault geometry of the diapiric roof resembles that of the arched roof of anactively rising diapir. Indeed, the steepness and curvature of the left-hand reverse faultis typical of active diapirism. However, the straightness and moderate dip of the right-hand reverse fault indicates that it was largely controlled by regional contractioninstead of active diapirism. This diapiric roof is therefore an excellent example of astructural hybrid, whose formation was heterogeneously controlled by both regionalcontraction and by the active diapirism induced by contraction.

The lack of any major contractional structure makes it easy to overlook the lateepisode of regional shortening, without knowing the history of the experiment.Regional compression preferentially shortened the viscous diapirs rather than faultedor folded the much stronger adjacent overburden. Contraction decreasedthe diapir width, squeezing the silicone upward and forcing the diapirs to rise anew.A more detailed description of this process is provided in the next section (Slides 11.31

to 11.33).

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Slides 11.21 and 11.22 show the only clear evidence for regional shortening in themodel. Slide 11.21 is a photograph of section Y-Y' south of Diapirs 6 and 7 (see locationin Slide 11.16). Slide 11.22 is a drawing of section Y-Y'. Because this section did notintersect any diapir, E-W contraction in this plane could not be accommodated bysqueezing the diapirs laterally. Instead, a large reverse fault cut the entire overburden.

Slide 11.23. The section, in the northern half of the model, cuts the entire stem of Diapir1 but misses the base of the stems of Diapirs 3 and 5. Diapir 5 (right-hand side) has thegeometry of an allochthonous sheet that later inflated and deformed its thick roofasymmetrically. However, the thrust faults over the crest of the diapir are a tell-taleanomaly. Diapir 3, in the center, rose vigorously during late contraction and emerged atthe surface (thin dark overhang at the top of the section). The prekinematic (white)sand layers at the base of the Diapir 3 were folded as the diapir stem, located off section,was shortened by regional contraction.

Although contraction occurs preferentially within salt diapirs because of theirrelative weakness, not all parts of the diapirs are necessarily squeezed. For example, theupper part of Diapir 3 opened because of contraction as the pressurized silicone burstthrough its roof. Despite regional compression, this vent was kept open by the dynamicpressure of salt escaping from the wider, but shrinking diapiric reservoir below,somewhat like a deflating balloon.

Slide 11.24. This detail from a section cut between sections X-X' (Slide 11.17) and Y-Y'(Slide 11.21) shows overhangs of Diapir 6 apparently isolated from their off-sectionfeeder stem. Arching of the roof above the overhang appears to be caused by activerise of the diapir, induced by contraction. A reverse fault (top right) cuts the upperlayers, deposited after passive growth, and roots against the uppermost diapiricoverhang. The linearity and moderate dip of this fault suggest that it was causedprimarily by regional contraction, not by arching. However, its location and vergencewere controlled by the thinness of the diapiric roof. Another reverse fault (bottom left)cuts through the lower part of the sand overburden below the diapir overhangs. Thisfault connects laterally with the large reverse fault shown on Slide 11.21 and alsoresults directly from regional contraction.

The section on Slides 11.25 and 11.26 cuts across Diapir 6 (right) and the north-ern edge of Diapir 7 (left). Diapir 7 is here disconnected from its root. Again, the faultsystem in the diapiric roof appears to be of hybrid origin: the left-hand reverse fault hasthe characteristics of active diapirism, whereas the right-hand reverse fault is controlled

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more by contraction. Another shortening structure is the subtle buckling of the whiteand purple sand layers at the base of Diapir 6 (Slide 11.26). It is unlikely that such foldswould be seismically imaged because of their small size and great depth belowoverhangs.

Slides 11.27 and 11.28 illustrate more-complex styles of roof deformation in a sectioncutting Diapirs 2 and 4 near the N edge of the model. Diapir 2 stopped rising early andwas buried under purple synkinematic layer 7, deposited while other diapirs were stillgrowing passively. Diapir 2 rose asymmetrically by lifting a roof bounded by two,high-angle, curved structures (Slide 11.28); the right-hand one was a prominent reversefault, whereas the left-hand one was merely a kink. An asymmetric graben formed inthe crest of the roof because of local stretching there.

Slide 11.29. Close-up of a section cutting the stem of Diapir 6. This image forms part ofthe report cover. During contraction the diapir arched its roof asymmetrically forminga regionally-induced reverse fault above its right flank and a diffuse array of normaland reverse faults above its left flank, locally stretched by draping above the risingdiapir. Contraction also completely pinched off the stem of the diapir, leaving a narrowscar of S6 Silicone at the base of the diapir.

Slide 11.30. Section through Diapir 5. Arching of the diapir roof was asymmetric.A set of small reverse faults formed on the right; their low dip and linearity suggestcontrol by regional contraction. The left side of the roof was cut by a large, singlereverse fault, whose steeper dip suggests control by active diapirism.

Slides 11.31 to 11.34 are schematic diagrams based on geometric reasoning andexperiment. They show how salt diapirs can be rejuvenated by regional or localcontraction. The diagrams all assume plane strain—that is, no salt is imported into orexported from the section. Slides 11.31 to 11.37 and their accompanying discussion areall taken from Nilsen et al. (in press).

Slide 11.31 illustrates why a diapir that has effectively exhausted its source layer canstill rise if horizontally squeezed. The top left drawing shows the simplifiedgeometry of an emergent diapir (dark red) of width w and height h, overlying adepleted source layer (light red). In vertical section, the area of salt in the diapiris Ad = h x w. The top right diagram shows that to rise to a new height h' without

changing its width, w, the diapir would have to increase its salt area to Ad'. Because

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this would require importing more salt from the source layer, a diapir above adepleted source layer would be unable to rise if its width remained unchanged. Thetwo bottom diagrams show that horizontal shortening enables diapirs to rise whileconserving their area of salt. The bottom left diagram assumes that the diapirremains continually emergent. Conversely, the diapir on the bottom right wasburied under a thick roof (plain yellow) before contraction began. A decrease indiapir width from w to w’, for example driven by thin-skinned regionalcompression, or by gravity gliding along a basin margin, forces the diapir to risefrom its initial height, h, to h' = h x w / w’, the area of salt in section remainingconstant (Ad = Ad'). Expressed as stretch, the vertical increase in diapir height,(h'/h) is the reciprocal of the horizontal decrease in diapir width, (w’/w).

Slide 11.32 shows the change in planform and vertical section of a diapir rejuve-nated by shortening. Shortening deforms the initially elliptical (elongate E-W)diapiric planform into an even tighter ellipse after N-S shortening. Sections normalto the direction of shortening (section X-X') do not show any change in diapir width.Sections parallel to the direction of regional shortening (section Y-Y') showdecreased diapir width. Nevertheless, both sections show the effects of shortening inthe form of an arched roof.

Slide 11.33 illustrates speculatively the differences in 3-D geometry between initiallytall, narrow diapirs (left-hand column) and initially wide, stocky diapirs (right-handcolumn) deformed by late shortening. Each diapir is isolated fromits neighbors. For the same amount of horizontal shortening, ∆L, the initially nar-row diapir rises higher and faster than initially wide diapirs. The increase in diapirheight, ∆hsalt, depends on the initial diapir width, Wo, and the horizontal shortening,∆L. For example, horizontal shortening by 100 m of a 5000 m wide and 3000 m highdiapir forces it to rise by only 61 m—a value less than the horizontal shortening. Bycontrast, the same 100 m of shortening applied to a diapir 1000 m wide and 3000 mhigh forces it to rise by 333 m—more than five times as much. This contrast cangreatly affect the structural style of the deformed roof because arching of thediapiric roof depends on how much the roof has been lifted rather than on the actualheight of the underlying diapir. Because the vertical displacement, ∆hsalt, duringreactivation of narrow diapirs greatly exceeds the component of horizontalshortening, ∆L, the deformation style approaches that of a vertically rising activediapir that formed in a non-contractional setting, and includes a crestal graben and

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lateral monoclinal folds or reverse faults. Normal and reverse faults above narrowdiapirs follow the diapir shape regardless of the direction of regional shortening(left-hand column). By contrast, the horizontal shortening, ∆L, during reactivation ofwide diapirs exceeds the vertical displacement, ∆hsalt. The deformation style istherefore closer to that of regional contraction and the diapir forms only a slighttopographic bulge. Reverse faults do not follow the diapir shape but formpreferentially perpendicular to the direction of regional shortening (right-handcolumn).

Slide 11.34 illustrates schematically how contraction that rejuvenates diapirs can bedriven by downslope gravity gliding of the overburden during thermal subsidence orregional extension of the basin rather than by regional compression (e.g., the inversionstructures of the southern North Sea). As the basin floor deepens (top of blue), thebasement slope steepens along the basin margins. This triggers basinward gravitygliding, normal faulting along the basin margins and horizontal shortening in the basincenter. Diapirs in the central depths of the basin would therefore be rejuvenated andrise anew.

Slide 11.35. These seismic tracings show two examples of salt diapirs rejuvenatedduring local and regional contraction in the Nordkapp Basin, Barents Sea. The verticalscale is in depth without vertical exaggeration. Diapirs formed in the Early Triassicduring basement-involved regional extension. The diapirs then rose rapidly by growingpassively and soon exhausted their source layer. Despite this depletion, the diapirscontinued to rise in the Middle and Late Triassic because regional extension and basinsubsidence triggered inward gravity gliding. The local compression induced downdip(Slide 11.34) squeezed salt out of diapiric stems and forced salt upward to form diapiricoverhangs. After burial under >1000 m of regionally uniform Jurassic (upper yellow)and Cretaceous (green) sediments, the diapirs were rejuvenated by Late Cretaceousregional extension and gravity gliding, and actively deformed their thick roofs. Afterextension, diapirs stopped rising again and were buried under 1500 m of lower Tertiarysediments (brown). Regional compression of the Barents Sea region in the MiddleTertiary caused one more episode of diapiric rise. Diapirs in the Nordkapp Basin arenow extinct. Note that because contraction squeezed the base of the diapir, nearlypinching off their stems, diapirs acquired tear-drop geometry.

Slides 11.36 and 11.37 show the geometry of other salt diapirs that were rejuvenatedduring shortening in the SW Nordkapp Basin.

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Slide 11.36. Map indicating the traces of the seismic sections illustrated on Slide

11.37. Diapirs close to the Plio-Pleistocene erosion surface are red in Slide 11.37.Surrounding yellow areas represent upturned collars in each of which the baseof the Cretaceous sediments was raised above regional.

Slide 11.37. Three drawings from seismic sections located in Slide 11.36. Verticalscale is two-way travel time. Because cross sections A-A' and B-B' pass through pre-existing diapirs, contraction is in the form of folding above the rejuvenated diapirsand squeezing of the diapiric stems, especially the stem on Section B-B'. Crosssection C-C', away from the diapir stems (Slide 11.36), shows reverse faults andfolds caused by the same episode of Late Cretaceous and middle Tertiary shorteningthat was accommodated elsewhere by diapir narrowing.

Multiple Breakout of Salt Sheets During Progradation:Linear Progradation Front Over a Curved Salt Pinchout

Sediments can prograde over a salt layer, eventually crossing its seaward limit.The salt layer can be an authochthonous evaporite unit bounded by facies changes or itcan be an allochthonous sheet. During progradation, two types of edge effects interactto control deformation: (1) the progradation front, which is the diffuse seaward limit ofthe sedimentary wedge, and (2) the salt pinchout, which is the seaward limit of the saltlayer. Experiment 146 (Slide Set 6, Slides 6.53 to 6.70) simulated the passage of a linear

progradation front over a linear salt pinchout. The experiment showed how thesedimentary differential load applied by the prograding layers squeezed salt seawardtoward its pinchout. The pinchout limited further lateral flow of the salt, which inflatedalong its pinchout, creating a box fold (Text Figure 11.38). The pressurized salt brokethrough the stretched hinge zones of the box fold,climbing stratigraphy and flowing as an allochthonous sheet out over younger strata.The site where the flow climbed to a higher stratigraphic level is termed the breakout.After salt is evacuated from the breakout, it becomes a secondary weld (Text Figure

11.38), the primary weld being in the original source layer. In nature, where sedi-mentation is typically continuous (albeit variable), allochthonous salt probably climbsstratigraphy more gradually than in this model, forming a gentle ramplike breakoutzone rather than an abrupt steplike breakout. Several allochthonous flows can joinlaterally into a salt canopy. Although the overall section did not change in length duringemplacement of the salt sheets, local deformation was extreme (Text Figure 11.38). The

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back of the allochthonous roof became highly stretched, whereas the front of the roofwas overthrust seaward, as described in the Gulf of Mexico by McGuinness andHossack (1993). A restored section of this experiment has been animated and isavailable as part of the present report.

Because the progradation front and the salt pinchout were both linear andparallel, Experiment 146 could not differentiate between the effects of these twoboundary conditions. That experiment was basically 2-D. The following Experiment 302(Slides 11.38–11.72) is one possible 3-D equivalent. It sheds light on the followingquestions.

1. What happens when a linear progradation front advances over an arcuate saltpinchout?

2. When do overthrusting or overplating predominate in forming a protective roofover extruding salt?

3. Where and how are stepped counter-regional systems likely to form?

4. How are dumbbell salt structures created? [These are introduced below]

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Slides 11.38 to 11.54 are overhead or oblique views of Experiment 302 as it deformed.The horizontal model box was 61 cm long and 59 cm wide. Its stratigraphy is shown ina vertically exaggerated dip section through the center of the model (Text Figure 11.39).This depicts the initial geometry and time of deposition of each layer. All layersconsisted of dry sand, apart from the simulated salt (hereafter referred to as “salt”),which consisted of S1 silicone. The model remained mechanically stable after the denserprekinematic layers were placed above the salt because the load was uniform.Deformation was then triggered by sedimentary differential loading as the proximalsynkinematic layers prograded; distal synkinematic layers were also added occasionallybeyond the salt pinchout to simulate slower sedimentation ahead of the progradationfront.

Slide 11.38. Overhead view of tabular salt (black) pinching out to the right along anarcuate boundary (gray) against flat strata (white).

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Slides 11.39 and 11.40. On the right is prekinematic layer 2 (white with yellow-bluecoating). On the left is the first synkinematic unit: proximal layer 3 (purple with yellowcoating). The progradation front was linear and located landward of the salt pinchout.The oblique view shows a subtle bulge in front of purple layer 3 produced by saltflowing seaward from beneath layer 3.

Slides 11.41 and 11.42. Proximal layer 5 (white with gray coating) on the left andunderlying layers squeezed salt seaward. Confined by its pinchout, the salt inflated theoverlying prekinematic layers (center) into an arcuate box fold. As the progradationfront advanced, a subtle anticline rolled ahead of it across the roof of the box fold (Text

Figure 11.41); this hinge moved seaward until it was blocked by the salt pinchout, afterwhich the box fold as a whole inflated. A proximal graben openedin the rear of the layer.

Slides 11.43 and 11.44. After proximal layer 6 (red with yellow-gray coating) and distallayer 6 (red with blue coating) were deposited, an important intersection structurebegan to form (Text Figure 11.41). Above the arcuate salt pinchout, the seaward hingezone of the box fold was locally stretched to form an arcuate foregraben (visible hereonly in the south). An analogous backgraben formed along the landward hinge zone ofthe box fold, except that this graben was linear, being controlled by the linearprogradation front of proximal red layer 6. The arcuate foregraben and the linearbackgraben intersected in the extreme north and south of the model (Text Figure

11.41). Their intersection produced maximum extension and maximum thinning, whichinduced faster reactive diapiric piercement, culminating in maximum extrusive flowover red layer 6 (Breakout 1). This enhancement and localization of deformation inintersection grabens could not occur if the progradation front paralleled the saltpinchout.

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Slide 11.45. Proximal layer 7 (white with blue coating) prograded across two-thirdsof the salt layer. The backgraben had previously stretched the prekinematic roof of thebox fold to zero thickness, exposing a diapiric wall visible here (wide black strip). Saltextruded from the intersection grabens in the extreme north and south. The more-vigorous northern flow carried the detached roof of the box fold as an allochthonous,twisted slab beyond the salt pinchout. Landward, a reactive diapir almost breached theproximal graben.

Slide 11.46. Without further sedimentation, salt extruded from the intersection grabensin the north and south, conveying the torn roof of the box fold farther seaward in thenorth. As most of this roof foundered, salt extruding from the backgraben flowed overit. This foundered roof is recognizable in cross sections cut later (Slides 11.68 and 11.70).At the left, diapiric walls actively broke through the central (most extended) parts of theproximal grabens.

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Slide 11.47. An oblique view of the same structures as in the previous slide, slightlymore evolved. Salt was expelled from beneath the prograding layers to extrude sea-ward. The front of white layer 7 thus subsided to form a rollover structure.

Slide 11.48. Proximal layer 8 (purple with yellow-gray coating) was deposited on flowsfrom the intersection graben. The progradation front reached the farthest limit of thearcuate salt pinchout.

Slide 11.49. Salt extrusion from the intersection grabens was still the most vigorous.In the center, however, salt had also begun to extrude over the top of purple layer 8(Breakout 2) and over the slumped, prekinematic forelimb of the original box fold,creating a canopy as it merged with the lateral flows. Seaward of the intersectiongrabens, the seaward margin of purple layer 8 was stretched by the vigorous under-lying divergent flows and separated into rotational slump blocks, like rafts. Thesepurple rafts are visible in sections cut later (Slides 11.56). Seaward of the rafts, purplelayer 8 was thinned by extension to about 30% of original thickness (Slide 11.57). Saltexpulsion from beneath the flexed progradation front created a subtle keystone grabenin the frontal rollover as the overburden front subsided and grounded after saltexpulsion. Near the rear of the model an elliptical dome was created by active riseof a salt wall that had been buried in Slide 11.48 after piercing a proximal graben. Thisdiapir subsequently emerged to evolve into a “dumbbell” structure (Slides 11.53,

11.54).

Slides 11.50 and 11.51. Overhead and oblique views showing deformation after addi-tion of proximal and distal layer 9 (white with blue coating). The frontal rolloverinduced by salt expulsion displays a prominent keystone graben. The central lobe of theextruding canopy increased its flow rate, extruding laterally over the distal white layer 9(Breakout 2). On either side, proximal white layer 9 was rafted obliquely seaward abovesalt extruding vigorously from the intersection grabens. On the left, the two diapiricwalls in proximal grabens were actively breaking through their domed roofs.

Slide 11.52. Layer 10 (green with yellow-gray coating) was deposited proximally anddistally. By then, the fastest extrusions were from the central lobe of the canopy,although lateral flows still carried some rafts of overburden. What caused the trend ofearly extrusion from the lateral vents to late extrusion from the central vent? Initially,the lateral parts of the salt pinchout were overrun by the progradation front; theprograding load squeezed salt seaward, and this flow conveyed lateral parts of the

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stretched roof. Later, the center of the salt pinchout was overrun; the load squeezed saltseaward, carrying the central part of the roof; by then, the lateral salt was too depletedto flow vigorously, and the overburden had grown too thick to be rafted.

Slides 11.53 and 11.54. Overhead and oblique views after deformation of the finalproximal layer 11 (red with yellow-gray coating). After burial by layer 11, the centrallobe again broke out (Breakout 3) and flowed to the seaward limit of the model. Normalfault scarps along the entire progradation front indicate that salt was being expelledalong the entire north-south width of the model. Yet only the central lobe of the canopywas actually extruding. These two observations suggest that in the extreme north andsouth, salt was being evacuated not by seaward extrusion but by flow along strike toemerge from the central extrusive vent. This has implications for the formation ofstepped counter-regional systems (see below).

On the left, a dumbbell structure formed. The underlying salt wall was buried byred layer 11, then the wall domed this roof (as in Slide 11.49) before actively piercing itthrough a crestal graben. The lowest part of the graben at each end of the crest spilledsmall lateral flows in opposite directions. This created a structure of unusual shape: twohorizontal, semicircular sheets spreading radially in opposite directions, connected by anarrow, vertical, active wall.

Slides 11.55 to 11.72 are serial vertical cross sections after deformation was terminatedby adding a white postkinematic layer. Sections are highlights from a set of serial slicescut every 2 cm from south to north. Each full-length regional section provides a frameof reference for one or more detailed views of the same section.

Slides 11.55 to 11.57 illustrate a cross section cut 2 cm north of the southern boundary.

Regional Slide 11.55. The proximal graben contains a diapiric wall that was reactiveuntil white layer 7 was deposited, when the wall actively pierced and emerged at thesurface as a passive diapir (southern proximal graben in Slide 11.46).

Detail Slide 11.56. The right-hand end of the primary salt weld is overlain byprekinematic layers 1 and 2 and by younger synkinematic proximal layers.The weld ramps up the primary pinchout into the first breakout zone of extrusiveflow (Breakout 1) (Slides 11.44, 11.45, and 11.46), now below the left-hand triangulardiapir. Below this diapir, the slumped and overturned forelimb of the box fold isonlapped by distal red layer 6 (Slide 11.44). Above the three reactive diapirs,

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produced by extension of the thin roof above the southern salt flow, are rotatedrafts (glide blocks no longer in contact) of purple layer 8 (Slide 11.49) and prerafts(glide blocks still in contact) of proximal white layer 9 (Slides 11.50 and 11.51).During extension and salt evacuation, the flanks of the small diapirs subsided, sothat the purple and white layers 8 and 9 in minibasins between them were invertedinto small extensional turtle structures. These structures characterize the extensionof the rear of the allochthonous sheet after deposition of proximal layers 8 and 9 butbefore deposition of green layer 10.

Detail Slide 11.57. Structures formed at the contractional toe of the roof to anallochthonous sheet. These are more prominently displayed in the following section(Slide 11.60), so are discussed there. The youngest salt allochthon from Breakout 3

flowed landward over the leading tip of red layer 11 (a later stage of the processvisible in Slides 11.53 and 11.54); this was an artifact caused by ponding of saltagainst the end wall of the model. The thinness of purple layer 8 is entirely due toextensional strain (compare Slides 11.48 and 11.49 before and after this layer wasstretched by divergent flow of underlying salt).

Slides 11.58 to 11.60 illustrate a cross section cut 6 cm north of the southern boundary.

Regional Slide 11.58. A more efficiently drained primary salt weld than that in Slide

11.55 floors the model.

Detail Slide 11.59. The tightly welded stem of a flow that pierced (Breakout 1) theforegraben of white layer 7 then flowed both landward (left) and seaward (right)(Slides 11.45 to 11.47). This flow was buried by purple (Slide 11.48) and white layers8 and 9, which were then extended by seaward flow of the buried sheet (Slides

11.49 to 11.51).

Detail Slide 11.60 shows the contractional toe in which proximal purple layer 8 andproximal white layer 9 on the left have been thrust seaward over distal white layer 9and distal green layer 10 on the right. During the early stage of overthrusting, thesalt flow was thick and vigorous enough to convey its roof effectively. Later, in thefinal stage shown here, the extrusive salt was welded out, largely by along-strikeflow, so the overthrust slab drapes over steps in the footwall. These steps markBreakouts 2 and 3 where allochthonous salt flowed anew over proximal purple layer

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8 and distal green layer 10 (overturned, isoclinal wisps of which are preserved belowthe purple overthrust).

Slides 11.61 to 11.63 illustrate a cross section cut 8 cm north of the southern boundary.

Detail Slide 11.62. At left center is the welded site of Breakout 1 and the edge of asmall extrusion (whose stem is off section) over proximal white layer 7 from thesame breakout. At right center, a major listric normal fault offsetting thickened redlayer 11 (Slides 11.53 and 11.54) soles out into the major salt sheet that emergedfrom Breakout 1; farther seaward, a smaller normal fault cuts the same red layer.

Detail Slide 11.63. In the center, proximal white layer 9 was thrust over the lowerstep (distal white layer 7), which marks the site of Breakout 2, but did not reach thenext step to the right (distal green layer 10), marking Breakout 3. The whiteoverthrust sheet drapes closely over the step because underlying salt was expelledafter red layer 11 was deposited. In the left center, the extensional gap between thetrailing cutoff of the allochthonous overthrust and the cutoff of its authochthonousequivalent on the left is filled by allochthonous salt overlain by proximal green layer10. This gap was created by the major listric fault described in the previous slide. Onthe right, a minibasin (green) is perched on the highest salt sheet, which extrudedduring Breakout 3.

Slides 11.64 and 11.65 illustrate a cross section in the center of the model cut 14 cmnorth of the southern boundary.

Regional Slide 11.64. On the extreme left is the main proximal graben filled by adiapiric wall that extruded repeatedly: over white layer 7 (Slide 11.47), white layer 9(Slide 11.50), green layer 10 (Slide 11.52), and red layer 11 (Slide 11.54). A keystonegraben is defined by fault offsets of the top of the purple layer 8 in the rolloveranticline above an efficiently evacuated salt weld.

Detail Slide 11.65 illustrates similar features to those in Slides 11.62 and 11.63 exceptthat proximal green layer 10 overthrusts here.

Slides 11.66 to 11.68 illustrate a cross section in the center of the model cut 20 cm northof the southern boundary.

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Regional Slide 11.66 and detail Slide 11.67. At the extreme left is the northern endof the southern proximal graben; at left center of Slide 11.67 is the southern end ofthe northern proximal graben. Here, where both grabens die out along strike, theyare pierced by reactive diapirs, which initially rose then subsided.

Detail Slide 11.68. On the left, a well-defined keystone graben cuts purple layer 8and white layer 9. In the bottom center is a thin, foundered remnant of the whiteand green prekinematic roof of the box fold embedded in salt (Slides 11.46 and

11.47). The foundered roof is underlain by a primary salt weld and overlain by atertiary salt weld where Breakout 1 occurred. On the right are partially detachedfrontal pieces of proximal green layer 10 and red layer 11.

Slides 11.69 and 11.70 illustrate a cross section in the center of the model cut 32 cmnorth of the southern boundary.

Regional Slide 11.69. On the left is that part of the dumbbell structure where thevertical stem merges with the horizontal flow (compare with the more-peripheralsection in Slide 11.71). This diapiric wall passed through several cycles, eachcomprising burial, active piercement, and extrusion (over the top of proximal whitelayer 7, white layer 9, green layer 10, and finally red layer 11).

Detail Slide 11.70. At the front of the model the red layer 11 is displaced seaward bythree large listric normal faults (Slides 11.53 and 11.54). Embedded in salt are twofoundered white and green prekinematic fragments. The lower one (left center) is arelic of the roof of the box fold, as in Slide 11.68. The upper one (center) is the distalpart of the same roof transported during Breakout 1 then stranded behind Breakout 2.This higher relic is the uppermost limb of an isoclinal recumbent pair of folds thatstack the stratigraphy in three-fold repetition.

Slides 11.71 and 11.72 illustrate a cross section in the center of the model cut 44 cmnorth of the southern boundary.

Regional Slide 11.71. On the left the section cuts the edge of the dumbbell structure(compare with Slide 11.69) and two of the extrusions that emerged from this vent.

Detail Slide 11.72 illustrates three important points.Salt evacuated during green (layer 10) time caused small-scale inversion and

formation of extensional turtle structures in purple layer 8 and proximal white layer

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9. The entire salt-evacuated structure forms a double stepped counter-regionalsystem (SCRS). This consists of a major deep SCRS comprising a ramped secondaryweld (left center), and flat tertiary weld (center); and a minor shallow SCRScomprising a ramped quartic weld (right center) and flat quintic weld (right) (seeText Figure 11.72 for explanation of terminology). During evacuation of the deepSCRS, salt seemed to be flowing along strike to the vigorously flowing centralextrusive lobe (Slides 11.53 and 11.54). This raises the possibility that SCRSs form onthe margins of a major extrusive lobe and that they are drained by along-strike flowas well as by down-dip flow.

Seaward of the extensional turtle structures is an overthrust slab of proximalwhite layer 9. The age of this overthrust is significant. It is part of a structural trendfrom the south to the center to the north, in which seaward flow of allochthonoussalt has thrust hangingwalls comprising increasingly younger then older proximalroof units: purple layer 8 (Slide 11. 60), white layer 9 (Slide 11. 63), green layer 10(Slide 11. 65), then back to white layer 9 (Slide 11.72). This trend confirms theobservation from overhead views that, initially, lateral flows in the north and southwere more vigorous and efficient at conveying their roofs, so older roofs werestretched and overthrust. Later the central flow was more vigorous, so youngerroofs were stretched and overthrust.

This far north, the central flow (through Breakout 3) did not pierce red layer 11(Slide 11. 53), so the red layer was not stretched by underlying seaward flow of salt.Slide 11.72 shows a progression from early detachment and overthrusting (purplelayer 8 and white layer 9) to late overplating (red layer 11) when the roof advancedby progradation.

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The main conclusions from Experiment 302 are as follows.

1. A box fold rises in front of and in response to a differential load of sedimentsprograding toward a salt pinchout.

2. The frontal (seaward) hinge of the box fold stretches, producing a foregraben.The arcuate planform of this foregraben is directly controlled by the arcuate saltpinchout.

3. The rear (landward) hinge of the box fold also stretches, producing a backgraben.The linear planform of this backgraben is directly controlled by the linearprogradation front.

4. Because they are nonparallel, the foregraben and backgraben intersect to formtwo initial sites of maximum extension, greatest piercement by reactive diapirs, andfastest extrusion of salt.

5. Two mechanisms allow the roof of an allochthonous sheet to advance with theunderlying salt flow: (1) overthrusting, where the roof advances by slumpingupslope and by overriding younger strata below the salt sheet (McGuinness andHossack, 1993): and (2) overplating, where the roof advances by progradationaladdition of new increments of sediment without detaching upslope.

6. Overthrusting is initially more common because (1) the thickness and vigor of thesalt flow allow it to convey its overburden effectively, and (2) the thinness of theoverburden allows it to be easily detached upslope.

7. Overplating is subsequently more common because (1) the thinness and sluggishflow of the depleted salt sheet hamper its ability to convey its overburden, and(2) the increased thickness of the overburden hinders it from detaching upslope.

8. Early on, these lateral intersection zones were sites of vigorous extrusion, de-tachment and overthrusting. Later, extrusion, detachment, and overthrustingshifted to the center after flow becomes exhausted from the highly extended,lateral intersection areas. For this reason, the age of overthrusting hangingwallssystematically decreases toward the center of the central extrusive lobe.

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9. A stepped counter-regional system (SCRS) in nature is most likely to form on thelateral margins of an arcuate salt nappe rather than at its center.

10. The SCRSs in this experiment appear to form by a two-stage evacuation.First, seaward flow extends the thin roof, creating small diapiric walls separatedby welds (a fault-segmented sheet or roho). Second, along-strike salt flow into themiddle of the salt nappe evacuates the SCRS, causing the small walls to subside andthe intervening minibasins to invert into extensional turtle structures.

11. A dumbbell salt structure forms by flow out of spillpoints at each end of thecrestal graben of an actively breaching salt wall.

Formation of Polygonal Minibasins During Progradation

The experiment in Slides 11.73 to 11.92 investigates deformation by gravity spreadingof a lobate wedge of brittle sediments, modeled by sand, prograding above a thick,less dense, salt layer, modeled by S6 silicone. Gravity spreading was driven by theacceleration of gravity on the dipping frontal zone of the prograding sediments.However, because gravity spreading is permitted by the lack of a buttress at the frontof the prograding wedge, a wedge with a circular planform expands radially. Both theperimeter and the radius of the wedge increase during spreading by means of two setsof intersecting normal faults, approximately radial and concentric. Their interactionforms a network of polygonal grabens. Reactive diapirs rise below the intersectinggrabens, forming a similar network of polygonal walls.

Slide 11.73. Overhead view of the initial model, which comprised a 2-cm-thick tabularlayer of S6 silicone resting on a flat, horizontal base, overlain by a 2.5-cm-thick tabularprekinematic overburden made of five 0.5-cm-thick sand layers (from baseto top: white, white, purple, white, and white). Initially, the prekinematic layers coveredthe entire model. Deformation started immediately after we vacuumed off most of theprekinematic overburden, leaving the semicircular lobe (18-to-19-cm radius) shown inSlide 11.73. The lobe had a horizontal upper surface (patchy blue veneer), and a frontalslope dipping at 35–45˚, appearing as a arcuate, narrow (4–6 cm) white rim in Slide

11.73). We added three synkinematic layers during deformation. Synkinematic layernumbers, colors, thicknesses and deposition times are listed in the following table.

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Layer Number Color Thickness Time(cm) (h)

3 purple 0.0 3.67

2 white 0.0 2.18

1 red 0.5 0.47

The layer thicknesses in the table represent regional values and do not includelocal thickening due to subsidence by faulting or salt withdrawal. The zero depositionalthickness of synkinematic layers 2 and 3 means that local thickening was entirely causedby deformation and thinning of the source layer, rather than by regional aggradation.

Slide 11.74. Overhead view after 0.38 h, before deposition of synkinematic layer 1. Thisview and all following vertical overhead views of this experiment are lit from the S. Ifoptical illusions prevent you from visualizing the graben as a trough, try rotating theslide 90° or 180°. The model was stretched by normal faults trending roughly radiallyand concentrically. Radial faults formed in response to the extension of the perimeter ofthe wedge. They are more pronounced at the peripheryand form V-shaped structures down the frontal slope. Two concentric fault zonesformed as well: a graben bounded by conjugate normal faults in the core of the wedge,and a half graben near the periphery of the wedge. South of the overburden, theresidual sand veneer over the salt buckled to form tiny wrinkles. This contractionbalances the extension farther N and appears in all subsequent top views as well.

Slide 11.75. Overhead view after 0.50 h, a few minutes after deposition of synkinematiclayer 1 (red with blue coating). Structures formed earlier simply propagated upwardinto the new layer and accommodated the rapid radial extension. The two sets of faultsintersected on the W fringe of the overburden.

Slide 11.76. Overhead view after 1.00 h. With further spreading of the sand wedge, thetwo sets of normal faults intersected throughout the overburden. Like a chocolatetablet, high blocks of intact overburden are separated by polygonal depressions createdby grabens.

Slide 11.77. Overhead view after 2.18 h, shot just before deposition of synkinematiclayer 2. The intersecting grabens widened. New stairstep faults cut the graben floors

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and allowed the underlying silicone to rise reactively in the subsurface below thethinned grabens.

Slide 11.78. Overhead view after 2.50 h and shows the top of the deformed synkine-matic layer 2 (white with blue-white coating). Only a few of the structures visible on theprevious slides have remained active and propagated upward by this stage.

Slide 11.79. Overhead view shot after 3.67 h, shot immediately before deposition ofsynkinematic layer 3. Continued extension along the grabens forced the underlyingreactive diapirs to fall as the underlying source layer had become depleted. Theoverburden blocks between diapirs grounded onto the base of the model once theintervening silicone had been expelled. As the silicone diapirs subsided, the adjoiningoverburden also subsided to form an arch; this arching is especially clear in thenorthernmost block of overburden. This block inverted as a turtle structure, as indi-cated by the presence of a large crestal graben marked by many parallel normal faults(also visible in section in Slides 11.86 and 11.87).

Slides 11.80 to 11.83 illustrate the latest stages in the experiment after deposition ofsynkinematic layer 3 (purple with blue-white coating).

Slide 11.80. Overhead view after 4.50 h.

Slides 11.81 to 11.83. Overhead view at the end of the experiment. The white layer ispostkinematic sand deposited around the deformed wedge to act as a buttress toprevent further spreading. Slides 11.82 and 11.83 are oblique views after 4.78 h ofdeformation, shot from the SE (lit from the E) and the E (lit from the S), respectively.All three slides show an advanced stage of diapir fall. Most of the older fault blockshave been yoked together by layer 3, leaving only a few narrow linear, troughs thatrapidly subsided (e.g., large E-W trough in the center of the wedge, Slide 11.81).Bending was accommodated in part by small-scale normal faulting (see the N edgeof the large E-W trough in Slide 11.81).

Slide 11.84. Overhead view after burial by the thick, white postkinematic layer after5.42 h of deformation, showing the location and orientation of serial sections. We cutthe E half of the model along N-S serial sections, and the western half along E-W serialsections. Because the extension was multidirectional and the progradation front wassemicircular, both lines of section were variably oriented with respect to strike.

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Slides 11.85 to 11.87. N-S sections cut in the E half of the model. Numbers on the scalein the sections indicate the distance between the section and the N-S center line. Right isN, left is S. All sections show rafts of extended prekinematic overburden (thin white andpurple layers) separated by early normal faulting and wide, fallen diapirs (black).Thickness changes in the prekinematic layers were caused by strain, not by differentialdeposition.

Slide 11.85, cut 2 cm from the model center shows a large central block (below scalemarks 11 to 16) cut by an oblique radial fault (scale mark 13). The rightmost(northernmost) block clearly shows the crestal graben that formed during the earlystage of diapir fall (also visible in overhead view in Slide 11.79). Also note thedownward flexing and thickening of synkinematic layers 2 and 3 (thick white andpurple) above the fallen diapir (scale mark 18). On the left, the prograding wedgethins to a white veneer over the frontal bulge of thickened source layer (black)expelled from beneath the thicker sediments.

Slides 11.86 and 11.87 were cut at 14 cm and 18 cm, respectively, from the center.Because the section in Slide 11.86 cuts across many more radial faults than Slide

11.87, the rafts of prekinematic overburden are narrower and have been thinned byboth radial and concentric normal faults. The section in Slide 11.87 from near theedge of the wedge is mostly a strike section cutting the lowest part of the slope, andhence sporadically shows only the base of the prekinematic overburden. Most of thesection comprises synkinematic layers that prograded beyond the initial wedge,leaving a huge apparent stratigraphic gap between the prekinematic blocks (left-and right-hand sides).

Slides 11.88 to 11.92. E-W sections cut in the W half of the model. Numbers on the scalein the sections indicate the distance between the section and the N of the model. Right isW, left is E. Structures are closely similar to those in perpendicular sections, whichdocuments that extension was multidirectional.

Progress on 3-D visualization:3-D salt layer from gravity-spreading experiment

Being able to visualize (slice, peel, etc.) physical models in full three dimensions greatlyimproves the ability to elucidate the formation of structural traps for hydrocarbons. Weare currently beta testing the visualization program EarthVision®, and working with its

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creators at Dynamic Graphics Inc. so that future versions of this program will includetools better tailored to our purposes. Preliminary results described below include 3-Dvisualization of a salt surface (Slides 11.94 to 11.97) constructed from 2-D serial slices(Slide 11.93) of one of our physical models. Upcoming reports will include complete3-D analysis of this and other models, as well as 3-D images showing sedimentarylayers, fault surfaces, isopach surfaces, and quantitative volumetrics.

Slide 11.93. Serial vertical sections of the salt layer of Experiment 50 (see Slides 4.8

to 4.16, and 4.30 from Slide Set 4). This physical model simulates syndepositionalextension of a slowly accumulating synkinematic overburden during gravity spreadingand gravity gliding down a 2° slope. Stacking sections like this conveys only the barestoutlines of the shape of these diapiric walls. Particularly difficult to visualize by thismethod are the transitional regions where one wall merges into another.

Slide 11.94, 11.95, and report cover. 3-D surface generated by digitizing, interpolating,and rendering the cross sections shown in Slide 11.93. Horizontal grooves parallel tored arrows are the actual impressions of horizontal sand layers on steeply dippingflanks of the diapirs. These bedding traces illustrate the fidelity of detail that thecomputer-rendered surface can provide.

Slow aggradation rates (relative to extension rates) favor shallowly buried steepsalt walls that can develop overhangs (A, B, C, D) at the landward edge of the basin.Although deeply buried salt rollers having gently dipping flanks (E) may formthroughout the basin, they are more common at the seaward edge of the basin wherelocal aggradation rates were faster.

Salt walls change markedly along strike in 3-D (F, see also F in Slide 11.96) toproduce diapirs with triangular sections (G), stocks (H), immature salt rollers (I), andmature diapirs with overhangs (J).

Slide 11.96. Top view of surface in Slide 11.94, lit from the right. Salt ridges connect toeach other at angles (red lines show trends) to form branching patterns in 3-D(E, see also E in Slide 11.94). These branching patterns could link laterally to formpolygonal ridges. However, the model is too narrow to confirm this. These branchingpatterns are not recognizable from the serial sections in Slide 11.93.

Also difficult to recognize from the serial sections in Slide 11.93 are 3-D relay

patterns (K and P). Salt walls (L and M in Slide 11.94) gradually curve and become loweralong strike to form salt rollers (N and O in Slide 11.94). The result is an asymmetric,curved, relay pattern in map view.

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The location of diapiric walls was controlled by the overlying grabens that ini-tiated the walls. Significantly, neither the walls nor the grabens (peeled off here)individually trended perpendicular to the direction of extension and progradation,except locally. Rather they trended obliquely in such a way that their mean trend wasperpendicular to the direction of extension.

Slide 11.97. Horizontal and vertical sections of the surface in Slide 11.94 can be com-pared, respectively, with time slices and seismic sections of natural rocks.

Structural Constraints Imposed by Salt Conservation

Structures above and below a salt sheet may interact, even if flow effectively relaxesshear-stress perturbations that could couple those structures. This interaction resultsfrom a requirement for conservation of salt volume, or salt area in the two-dimensionalexamples considered here. A balanced cross section through encased salt must conservearea through time (adjusted for flow out of the section). Within a section, deformationin one area may be accommodated by salt flow from another deforming area (Hossack,1993). We build on this balancing concept by applying it to the area changes above andbelow a salt sheet required to maintain constant salt area. Rather than using these ideasto reconstruct the evolution of a section, we will examine the major influence of theseconstraints on the type of structures that can evolve.

In nature, all salt sheets are three dimensional, allowing salt flow into and out ofthe cross-section plane to compensate for unbalanced structural movement above andbelow a sheet. Moreover, in two dimensions salt area balance constraints arecamouflaged in very long or laterally continuous sheets, which can maintain areabalance by subtly subsiding over large areas that may not be readily apparent.However, the principles discussed here may still be useful for interpreting naturalexamples for the following reasons. In the models, conservation of salt area canstrongly influence formation of the first-formed structures, which then determine muchof the subsequent evolution. In nature, salt flow out of the section may be minimal atthe earliest stages of deformation, so some of the formative influences seen in themodels may still apply. In addition, given the difficulties inherent in interpreting subsaltstructures, even guidelines of uncertain reliability may be useful to test.

Slide 11.98. Constraints imposed by salt conservation: Theory. The basic rule of saltconservation is this: the area change caused by faulting above the salt sheet must equalthe area change caused by faulting below the sheet. For example, normal faulting

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below the salt sheet in Slide 11.98a creates potential excess area (pink) above the sub-salt graben block. Salt flows into this new area. In addition, the entire sheet extendslaterally by the heave across the faults, also creating new area (pink). Movement ofblocks above the salt sheet must fill a space (light blue) equivalent to the sum of thesenew areas to maintain constant salt area while at the same time accommodating theregional extension.

Terminology for area change depends on the reference frame (country rockor salt) and whether we talk about a potential change or the resulting response. Becausethe salt must conserve area, it seems natural to discuss change relative to that constantarea. In addition, we will talk about how deformation alters the space available for thesalt, rather than the actual response of the salt. For example, we will say that subsidenceof the subsalt graben in Slide 11.98a creates an area excess,or gain for the salt, and subsidence of the roof results in an area deficit, or loss for thesalt by impinging on it. Our terminology refers to these potential changes. Remember,however, that the salt flow and structural movement are concurrent, and that the saltdoes not actually change area, despite the potential gains or losses by surroundingblock movements.

The geometry needed to balance the area excess and deficit is illustrated inText Figure 11.98. The unknown, x, is the separation between the locations of the faultsabove and below the salt. Extension of the salt sheet creates an area ht, labeled A.Additional area, C, is created by dropping of the subsalt graben and is equal toav – 0.5hv = h(tanθ)(a - 0.5h). Area consumed by dropping of the roof block, labeled B,is equal to a + x – 0.5hv = h(tanθ)(a + x - 0.5h). Equating expressions for A+C with B andsimplifying yields

t/x = tanθ.

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This equation simply states that the horizontal separation between balancedfaults above and below a sheet depends only on the sheet thickness and fault dip.Furthermore, this separation is exactly the same as the fault dip projected acrossthe thickness of the sheet. Therefore, the faults must align across the salt for the area to

remain balanced; that is, slip on a single normal fault above the sheet will exactlycompensate for the sheet extension and salt redistribution if that fault projectsacross the salt to a normal fault below the sheet. Any other relative location of the faultsrequires additional deformation to maintain compatibility. This additional deformationis common in our models, and typically involves tilting of the blocks, formation of newfaults, or flow of salt out of the section.

Slip on different combinations of faults intersecting a salt sheet greatly varies thearea gained or lost, and therefore the required amount of compensation by other faults.Therefore, the spacing and type of fault systems above and below a salt sheet tend tobe related in a way that maintains constant salt area. For example, slip on a singlenormal fault above a salt sheet lowers a long length of hangingwall, impinging on alarge salt area to create a large deficit. To compensate, a similarly large block mustmove to create excess space beneath the block. However, if a graben forms above the

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sheet, the system can extend by the same amount but now with only a very small arealoss at the point of the graben block (Slide 11.98b). The only significant change ofavailable space is an addition due to extension of the sheet itself.To maintain area balance, a structure must form beneath the sheet that moves a smallfault block into the sheet to compensate for area increase by sheet extension. Thesubsalt horst in Slide 11.98b could balance the salt area and accommodate the incre-mental regional extension. However, in the case where projections of the suprasaltfaults cross to align with the subsalt faults, as in Slide 11.98b and c, salt area is onlytemporarily balanced at that stage because continued extension on the faults shown willincrease area faster than area is lost. In Slide 11.98b and c, if salt area balances at aparticular moment, continued slip on the same faults requires additional deformation tomaintain area conservation. In contrast, in Slide 11.98a continuous extension and slipon the faults automatically conserves salt area.

In nature, other processes relax the strict constraints indicated by Slide 11.98 forspacing and type of fault systems above and below a salt sheet. For example, the blocksadjoining faults can rotate or flex and new faults can form. Slides 11.99–11.101 showsome simple illustrations of the constraint of salt conservation and how additionaldeformation maintains salt area when faults are not aligned.

Slides 11.99–11.101. Effects of salt conservation constraint in numerical models. Thisset of slides shows seven simple models with different deformation imposed on thesame initial geometry. A salt sheet 0.75 km wide by 10 km long underlies overburden2.5 km-thick regionally (see Slide 11.99a for the basic configuration). A free-slip verticalboundary on the left end of the models corresponds to a symmetry condition (i.e., amirror image of the model could be connected to the left end). The base of the salt andoverburden is bonded to the bottom boundary. Deformation was initiated bydisplacing a portion of the salt base in a specified direction to simulate movement onsubsalt faults having various orientations, locations, and senses of slip. The top rightcorner of the salt sheet consistently initiated overburden faulting due to the stressconcentration there. Therefore, the location of the subsalt fault could be moved relativeto the projection of the corner fault to study how the alignment of faults across the saltgenerates additional deformation to maintain constant salt area.

All numerical modeling was done with the commercial software GEOSIM-2D,described in Slide Sets 7–10. Specified material properties corresponded to mid-rangevalues for natural sedimentary rock and salt. In particular,

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ρsalt = 2200 kg m-3, ρrock = 2400 kg m-3, K = 16.7 GPa, G = 10 GPa, ϕ = 31°/26°,

where ρsalt and ρrock were the densities of rock and salt, and K, G, and ϕ were respec-tively the bulk modulus, shear modulus, and friction angles of the sedimentary rock.The friction angle weakened from its intact value to its residual value from 0.01 to 0.02equivalent plastic strain (see Slide Set 7 for details). Salt viscosity was uniformly 1×1018

Pa s. Slip rates on various faults in different models all resultedin bulk horizontal displacement rates of 10 mm/a.

Slide 11.99. Simple extension and one normal fault. The model in Slide 11.99a to cdeformed due to simple horizontal displacement of the overburden. The first faultgrew upward from the stress concentrator at the corner (Slide 11.99a). Part b showsthe surface profile at that stage with vertical exaggeration. Extensional thinning ofthe salt accommodated tilting of the hangingwall of the half graben. Slight kinksmark the nascent boundaries of the adjoining graben, fully developed in part c,along with another fault that forms a graben across the left symmetry line. Ofinterest here is the overall subsidence of the roof of the salt sheet due to extension ofthe salt. Structures are dominantly symmetric, resulting in only minor tilting of thefault blocks.

Part d) and e) show displacement of the subsalt rocks along a single normalfault positioned to align with the projection of the overburden fault that propagatesfrom the corner of the sheet. Theoretically, this alignment should have produceduniform subsidence of the roof because the salt area created exactly balances thearea consumed (as in Slide 11.98a). In practice, however, drag on the fault and shearresistance from the salt slightly tilted the roof away from the fault (part d). To keepthe salt area constant, the middle of the roof sagged slightly. These effects magnifiedwith further displacement (part e).

Slide 11.100. One normal fault, imposed salt excess or deficit. Slip on faults that donot align to balance salt area causes additional compensating deformation,illustrated here. The roof deforms in the vicinity of its active fault, in a zoneseparating footwall roof that maintains regional elevation and hangingwall roof thatsubsides uniformly with the moving subsalt rocks. Moving the subsalt fault ofSlide 11.99a–c to the right created a salt area deficit when the roof subsided alongthe fault formed at the upper corner of the sheet (Slide 11.100a–b). To conserve saltarea, the roof sagged toward the fault which was the active site of salt extension. Incontrast, moving the fault to the left created a potential area excess, with com-

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pensating tilt away from the fault (Slide 11.100c–d). These latter offset locations ofsubsalt and suprasalt faults are common for drape folding above salt covering anactive basement fault. Thus the folding of the bed, rather than uniform subsidence,can be seen as 1) a consequence of flexure of the bed above locally flowing salt, anexplanation invoked in previous slide sets, and 2) the constraint that salt area mustbe conserved given the relative location of subsalt and suprasalt faults in this two-dimensional case. Also note the difference in overburden fault patterns between thetop and bottom models of the slide. An area deficit (Slide 11.100a–b) induces strongasymmetry in the overburden deformation; a half graben is bounded by a singlenormal fault that propagates with a steeper dip than in the balanced case (Slide

11.99d–e). An area excess (Slide 11.100c–d) preserves elevation near the suprasaltfault, resulting in a more symmetric structure. The greater the potential area excess(caused by greater offset in locations of subsalt and suprasalt faults), the less the roofsubsidence and the greater the graben symmetry.

Slide 11.101. Subsalt horsts. Movement on subsalt horsts of different widths cor-responds to the situation shown in Slide 11.98b and c, in which upward projectionsof the subsalt faults cross within the salt. Uplift of a narrow horst at the lower cornerof the salt sheet (Slide 11.101a) causes a roof fault to propagate from the uppercorner of the sheet, nearly on line with one of the basement faults.The other roof faults form substantially offset to the left from the projection of theright horst fault, causing a large salt area deficit. As a result, a graben grows nearlysymmetrically between the left roof faults. The graben absorbs some of the regionalextension that would otherwise occur on the rightmost roof fault, therebydecreasing the roof subsidence to the right of the graben. The insignificant areachange caused by uplift of the narrow horst in Slide 11.101a–b produces roofstructures resembling the case of pure extension (Slide 11.99c). Conversely, thehorst faults in Slide 11.101c initially projected upward to intersect at the base of theroof. Movement on the horst should be exactly balanced by downdroppingof a roof graben, but growth of a reactive diapir degraded the balance. Salt flow intothe diapir was compensated by gentle tilting of the top surface toward the graben.Slide 11.101d shows a horst built slightly wider. At 31 000 a fault slip above andbelow the salt exactly balanced the salt area. Continued balance would require thehorst to narrow with further uplift, that is, following the green dashed lines inSlide 11.98b–c. In reality, the horst maintains its width, creating a salt excess. Theroof footwalls respond beyond the fault zones by tilting upward slightly. These tilts

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are perceptible in the coarse resolution of Slide 11.101e to the left of the graben inthe top and bottom roof contacts.

Overburden Response to Active Basement Faults

Previous slide sets have shown examples of the effects of faulting beneath a salt sheet.The slides in this section represent a more complete and systematic study.For brevity, faults offsetting the base of salt will be termed “basement” faults, althoughthe term carries no implications of age, rock type, etc. The following finite elementmodels display variation in (1) fault dip, (2) displacement rate, (3) salt thickness,(4) overburden material properties, (5) multiple non-rotational faults, and(6) single and multiple rotational (domino) faults. The last slide of the section shows asimple case of inversion.

All models were built with a length of 60 km to minimize any boundary effects,salt thickness of 0.5 km, and overburden thickness of 1 km. The overburden thickenedby about 100 m near both ends to inhibit edge effects (half grabens at the boundaries).The mesh was coarser away from the basement fault(s) at the center. Only the centralportion of the model is displayed in subsequent sections to increase resolution of thedeformed area. For single-fault models, the same 10 km section appears in each slide;up to 16 km appears for multiple-fault models. The models were deformed after all ofthe rocks were in place. No synkinematic layers were added. Unless otherwisespecified, models used the same material properties as the salt balance models of thepreceding section. The base of the salt was constrained bya no-slip condition, concentrating all deformation at the salt base to separation acrossthe basement fault. Faulting was simulated by displacing basement nodes at the speci-fied fault dip. The fault trace shown on the slides connects the last fixed node with thefirst displaced node. As a result, that line may dip at an angle slightly smaller than thefault angle because of the initial horizontal separation between the nodes. Overburdendeformation developed spontaneously in the models; no local forces or geometryvariations were imposed in addition to the rigid displacement of the model base.

Slides 11.102–104. One fault, varied dips. These slides display the results for a singlebasement fault dipping at various angles. Slip on the basement fault resulted in ahorizontal displacement rate of 5 mm/a (extension or shortening), except in the case ofa vertical fault.

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Slide 11.102. One extensional fault dipping 60°. The top section of the slide showsthe scale of the entire model. The bottom three sections show the central 10 km,with plastic strain contoured from 0.02 to 0.35. Overburden deformation created anasymmetric graben, offset in location toward the footwall of the basement fault. Thegraben accommodated the horizontal extension and flexure at the top hinge of anoverburden monocline. Additional grabens with minor slip formed later to the leftof the major graben. The bottom hinge of the monocline showed less intense strainin the form of intersecting “hourglass” conjugate faults(Text Figure 11.102). The outer arc of the bottom hinge extended, and a horst blockrose between the faults. The inner arc shortened at the top surface, causing theV-shaped block there to rise also (rather than dropping as a graben). This faultingin response to flexure produced an odd slip pattern, with normal slipon the lower faults and reverse slip above. We will refer to the top block of thesystem as a “pop-up” for brevity.

Slide 11.103. One extensional fault dipping 30°. The same dimensions and exten-sion rate of Slides 11.102 were used with a fault dip of 30°. The top section of theslide is contoured from 0.02 to 0.35, the bottom two sections from 0.02 to 0.6.Greater horizontal extension relative to vertical subsidence resulted in morenumerous and closely spaced extensional faults. The dominant overburden faultaligned closely with the basement fault (but with a higher dip). As a result, in theearly stages the overburden subsided more uniformly and the major graben wasmore asymmetric. Less flexure above the basement fault delayed faulting at thebottom monocline hinge until late in the history.

Slide 11.104. One fault, vertical or reverse movement. The top three sections showdeformation above a vertical basement fault, the first two contoured from 0.02 to0.1 and the third from 0.02 to 0.35. Relative to extensional faults, pure vertical slipincreased the flexure of the overburden. Without regional extension, the layer

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folded like a beam with a central neutral surface. The upper monoclinal hingedeveloped a typical arching graben; the normal faults only cut downward to thelevel of no strain. The inner arc of the fold was in compression. Strain at the lowerhinge of the monocline increased relative to the models with extensional basementfaults. The neutral surface passed through the intersection of the conjugate faultsthere. Later (third section), a curved reverse fault developed as another crestalgraben formed slightly closer to the basement fault.

Changing the basement fault to reverse slip with a 60° dip (bottom threesections, strain 0.02 to 0.1) greatly diminished strain at the top hinge of the mon-ocline. Shortening deformation eventually localized on a thrust extending upwardfrom the basement corner. The early stages show a different type of deformationbefore the thrust propagated. A pop-up formed at the bottom monoclinal hinge, asin other models. In addition, a low-angle fault cut the middle of the monocline’slimb. This fault had left-lateral slip, corresponding to the sense of shear that wouldoccur on horizons of a multilayer overburden. This response demonstrates thatdeformation tends to create multilayers where there were none, because easier slipbetween multilayers facilitates flexure. In this model, compression in theoverburden suppressed the early normal faulting that hinged the overburdenflexures in the preceding models. Later, localized deformation on the thrust rapidlyreplaced the dominant folding response (note the short time change between thefirst and second reverse-fault sections).

Several trends are evident in Slides 11.102–104. The overburden invariablyfolds as a drape monocline due to local salt flow over the growing basement fault.As a basement fault steepens from low-angle normal through vertical to reverse,faulting at the top monoclinal hinge changes from an intense, very asymmetricgraben to a weak, shallow symmetric graben. The bottom hinge compensates withincreasing faulting intensity and asymmetry. The basal horst that dominates abovenormal basement faults shrinks and then disappears as the dip is changed to areverse basement fault. The pop-up capping the hourglass structure enlarges andextends deeper as the horst shrinks. Above a reverse basement fault, theseconjugate faults hinging the monocline are rapidly replaced by a single thrust fault.For moderate basement fault dips, either with normal or reverse slip, offset on adominant single fault on line with the basement fault characterizes the overburdendeformation, rather than the fault-accommodated folding that typically accompaniessteeper basement fault dips.

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Slides 11.105–108. One fault, varied thicknesses, rates, and material properties. Theseslides display the results for a single basement fault dipping at 60° with normal slip. Theoverburden structures changed with salt thickness, horizontal displacement rate, andthickness and material properties of the overburden. Unless specified otherwise, theplastic strain contours range from 0.02 to 0.5, and the horizontal component of fault slipwas 5 mm/a.

The variations in overburden structure can be understood by viewing theresponse to basement faulting as a compromise between two processes with competingtendencies. Specifically, the observed structures minimize the total work done by thebasement to deform the overlying layers. This balance can understood by consideringthe work required first to deform the overburden, then second, to deform itsunderlying salt.

First, the vertical component of basement movement drapes the overburdeninto a monoclinal fold. The early elastic folding changes the stress at the hinges of themonocline, thereby controlling the location of the dominant later faults. Therefore, tounderstand the variation in location of structures, we need to analyze the overburden’selastic response. Far to either side of the active basement fault the basement andoverburden move in tandem. Nearer the basement fault the work to overcome theoverburden’s elastic resistance to folding is minimized if the fold is as gentle and broadas possible. Changes that increase flexural rigidity of the overburden, such as a thickeror stiffer layer, increase the broadening tendency to produce a long-wavelength, gentlemonocline.

Countering this tendency is the viscous resistance to flow in the underlying salt.The longer the wavelength of the overburden monocline, the more salt must flow toaccommodate it. Conversely, very local, sharp overburden folding requires only verylocal salt redistribution, minimizing the work dissipated by viscous flow. Viscousresistance to salt flow tends to localize deformation. However, viscous stresses, andtherefore the work needed to move the salt, depend on time. Workin moving the salt decreases if the time for a deformation increment is increased,whereas work needed to deform the overburden is independent of time.

The drape structure formed above the basement fault balances the workrequired to fold the overburden and move the salt to accommodate the flexure. There-fore, changes that increase resistance to salt flow, such as increased displacement rate ordecreased salt thickness, tend to create a sharper, more localized overburden mono-cline. In extreme cases, the overburden is forced to fault on line with the impingingbasement fault. Conversely, changes that decreased viscous stresses, such as slower

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deformation, thicker salt, or lower viscosity, increase the relative influence of theoverburden to form a long, gentle monocline with only minor faulting in its openhinges.

Slide 11.105. One fault, varied salt thickness. Increasing the salt thickness from0.25 km (top section) to 1 km (bottom section) diffuses the flow resistance in the salt.As a result, the overburden deformation changes from a dominantly fault responseclosely coupled to basement fault slip to a dominantly fold response only looselylinked to the subsalt forcing. With the change, the single normal fault in theoverburden becomes a progressively more symmetric graben located farther fromthe basement fault. Viewed from a perspective of salt balance, the closely alignedbasement and overburden faults in the thin salt case result in nearly parallelsubsidence of the basement and overburden hangingwalls. With thicker salt, shiftingthe overburden faults away from the basement fault increases the salt excess anddecreases the subsidence, resulting in a gently tilted monocline limb.

Slide 11.106. One fault, varied slip rate. Most of the models in this slide vary onlythe slip rate on the fault, with tenfold decreases from 50 mm/a (top two sections,with strain in the first from 0.02 to only 0.1) to 5 mm/a to 0.5 mm/a. Because theviscous resistance to overburden movement is decreased by either displacementrate or salt thickness, the trends in this slide are nearly identical to those in thepreceding slide. Therefore, decreasing slip rate changes the overburden responsefrom a single fault to a progressively more symmetric and laterally offset graben.The bottom sections shows a doubled salt thickness at 0.5 mm/a, which exacerbatesthe effect of slowed displacement rate to produce only minimal coupling betweenbasement and overburden response. Salt redistribution over a wide area allows theoverburden monocline to become a very gentle tilt, with the extensional componentaccommodated by the graben at the upper hinge.

Slide 11.107. One fault, varied overburden material properties. These slides showvariation in overburden response as several material properties change. All sectionsare shown after approximately the same time, except for the bottom section whichdid not evolve that far. For reference, a model with standard properties (Slide

11.102) is shown as the top section. The next section decreased the overburdendensity from 2400 kg m-3 to 2000 kg m-3. Salt density remained at 2000 kg m-3.Removal of the density inversion had little effect. Increased development of a

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curved reverse fault above the basement fault was apparently due to enhanced saltflow across the basement step, thinning the decoupling salt there faster.

The model in the third figure effectively reduced the flexural rigidity of theoverburden with a hundred-fold decrease in Young’s modulus of elasticity.Increased ease of folding in this model reveals a reason for the spacing of structuresto the left of the main graben. Early folding was not confined to the monocline, butcontinued outward periodically with rapidly diminishing wavelength. As the yieldstrength was reached in extension, normal faults propagated to define grabens atthe synclines and horsts at the anticlines. These undulations are more apparent inthis model than with a stiffer overburden. The faults also had substantially greaterplastic strain.

A hundred-fold decrease in the shear modulus of elasticity (fourth section)also effectively reduces Young’s modulus, but makes the overburden nearly incom-pressible. Easy shear results in more folding and less faulting. Somewhat surpris-ingly, the changes in material properties in the top four sections negligibly affect theposition of the monocline hinges, and therefore the offset of the main graben fromthe basement fault. Therefore, this offset is only sensitive to viscous stresses in thesalt, which depend on salt thickness and deformation rate, andto overburden thickness (see next).

Halving the overburden thickness (bottom section) also halves its flexuralrigidity. In contrast to the sections above, the monocline forms with shorter wave-length, moving the hinge structures closer to the basement fault. Fewer finiteelements in the overburden decrease resolution of the structures, but they appear tobe similar to the standard model (top section) except for their spacing.

Slide 11.108. One fault, varied overburden cohesion. The magnitude of cohesionfor bulk sedimentary rocks is probably the most poorly known of the materialproperties that need to be specified in the finite element models. Unfortunately, arecent theoretical and numerical analysis of a brittle layer deforming above aviscous layer found that cohesion had a greater influence on the deformation thanfriction angle (Triantafyllidis and Leroy, 1994). Friction angle is generally consideredto be the prime determinant of overburden strength and therefore deformationbehavior. Cohesion on the scale considered here could be increased by lithification,cementation, and metamorphism. Fracturing and jointing could decrease cohesionin nature.

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We ran a series of models with varying cohesion values to study this effect.The standard model, with a cohesion of 200 kPa is shown for reference as the thirdsection in this slide. Decreasing the cohesion by six orders of magnitude (top section)had little effect on structures in the overburden. Faults appeared to form a littlemore easily and accumulate slightly more strain. Increasing the cohesion by onlyone order of magnitude had a much more profound effect (bottom two sections).Deformation accumulated on the initially formed faults without appearance of anynew ones. The initial failures were nearly vertical in the stretched top hinge of themonocline. Additional deformation created a rough graben shape, but without thenicely defined bounding faults and less-strained graben block of the other models.Thus, it appears that cohesion has a large effect but only at a threshold value ratherthan as a gradual change. All of the models in this slide set, and most of those inprevious ones, were run witha cohesion value below this threshold.

Slides 11.109–111. Multiple faults, no rotation. These slides show two or three base-ment faults dipping at 60° with normal slip. Motion parallel to the faults produceda downward-stepping basement surface. Fault blocks did not rotate. Salt thickness, faultspacing, and displacement rate were systematically varied. Plastic-strain contours rangefrom 0.02 to 0.4.

Slide 11.109. Two faults, varied slip rate and spacing. With 0.5 km salt and ahorizontal extension rate of 5 mm/a, overburden structures above two faults(top section) resembled those above a single fault. The only effect was to increasethe length of the monocline limb, and therefore the separation between structuresat the upper and lower hinges. In this case, the salt sufficiently decoupled the over-burden from the basement that the individual offsets on the faults were irrelevant— a single shallow fault with the same bulk offset would presumably cause similaroverburden deformation. Increasing the extension rate to 20 mm/a localized shearstresses in the salt that link the basement faults to overburden faults accompaniedby poorly developed grabens. Most of the extension occurred on the upper (left)overburden fault. Slip on the right fault was impeded by compression from thehigher blocks to the left sliding down the salt slope created by the basement faults.

Doubling the fault spacing separated the overburden fold into individualmonoclines over the two basement faults (bottom two sections). The structuresabove each basement fault resembled the structures above a single fault. The only

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interaction between the sets of structures was gravity gliding which enhancedextension on the upper faults and hindered slip on the lower faults.

Slide 11.110. Two faults, thin salt, varied basal friction, slip rate and spacing.

Reducing the salt thickness to 0.25 km increased coupling across the salt andseparated the overburden structures (compare the top two sections with the top sec-tion of Slide 11.109). In addition, the overburden impinged more on the basementsteps. The later-stage impingement tended to produce curved rotational faults in theoverburden linked to the lower (rightmost) basement fault. The curvature of therotational fault results in normal slip at their base and reverse slip near the surfacewhere the dip direction reverses.

The third and fourth sections of this slide changed the condition at the base ofthe salt from a no-slip condition to a frictional slide line. Setting the friction angle to26° (third section) and 6° (fourth section) produced a negligible change in theoverburden response; most of the differences derive from the slightly differentstages of development shown. This result justifies use of the numerically simplerno-slip condition along the base of the salt.

A fourfold increase in slip rate (fifth section) strongly linked the basementand overburden faults. Their alignment removed the between-fault tilting seen inthe monoclines of more decoupled models. Wider basement fault spacing (bottomtwo sections) simply separated the sets of structures in the overburden farther(compare with top two sections).

Slide 11.111. Three faults, varied slip rate and salt thickness. Three relativelyclosely spaced faults acted as a single low-angle fault beneath 0.5 km of salt (topsection). As with two faults, decreasing the salt thickness (second section) began toproduce separated sets of overburden structures, and increasing the displacementrate coupled the basement and overburden structures closely (third and fourthsections). Left-dipping reverse faults at the base of the overburden slope appearedwith large vertical offset across the three closely spaced faults (particularly evident inthe second and third sections).

Slides 11.112–116. Rotational “domino” faults. Rotational faults are idealized hereas producing no regional vertical offset of the basement surface. Only the basementbetween the faults rotated downward. All basement fault(s) began with a 60° dip.In all but Slide 11.112, slip on the basement faults duplicated domino motion. In thisidealization, the fault blocks do not change shape, but simply topple over like books on

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a shelf to create a series of half-grabens. Lengths of all originally horizontal basementsegments are conserved between fault cut-offs. Regional horizontal extension rateacross the fault array was constant at 5 mm/a (the more faults in the array, the slowerthe extension rate across each). The vertical component of slip decelerates with time asthe blocks rotate their bounding faults to lower dips. Unfaulted basement on either sideof the array does not change elevation. Plastic strain contours range from 0.02 to 0.3.

Slide 11.112. One fault, varied vertical relative to horizontal slip. Rotational faultscan produce various combinations of vertical and horizontal slip components. Thethree models shown here may not have good analogs in nature, but they shouldrepresent end members of overburden response.

The top two sections show the rotating hangingwall cutoff moving downa fault with a fixed dip of 60° fault. The basement on either side of the rotatingsegment is stationary. The constraints result in the displaced segment decreasing inlength initially, then increasing after it rotates past a 30° tilt (the label “trap door” is aslight misnomer). Although this history is unlikely in nature, it serves to show theoverburden response where there is no regional extension. The overburden initiallysagged symmetrically above the fault basin, with bending grabens at the monoclinalhinges and a horst capped by a pop-up at the synclinal hinge. Only in the lateststages did the overburden “feel” the asymmetry of the basement motion. A reversefault grew near the right terminations of the faults of the trough structure, and theright graben migrated closer to the basement step.

In the third section, the basement to the left of the hangingwall cutoff movedleftward at 5 mm/a relative to the fault and fixed basement to its right. The cutoffmoved downward with the leftward extension to follow a 60° fault dip. In this case,the rotating segment increased in length, negligibly at first, but accelerating withincreasing tilt (the stretch is the reciprocal of the cosine of the dip angle). At the timeshown, the length had only increased about 5%. Adding the component of regionalextension to the fault rotation propagated the graben faults through the overburdenand increased the slip on them. Additional normal faults appeared outward of themajor graben on the left. The synclinal hinge structure remained more symmetricfor the same amount of vertical movement than in the section above.

The bottom section conserved the length of the hangingwall from the pivotpoint to the cutoff. Horizontal extension to the right of the footwall cutoff was5 mm/a. In consequence, the fault dip shallowed with time. This movement historyaccords with a domino fault or bookshelf model. Less vertical motion accumulated

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for the time shown than in the models above, resulting in less strain on the synclinalstructure. The overburden grabens were more asymmetric, which created a moreuniformly subsiding block than in the other models. The intensity of the strainedzone directly above the basement fault is a numerical aberration, but it appears thatfailure really was propagating through the overburden there.

To summarize, increasing the component of horizontal extension on thesystem relative to the vertical motion on the fault enhances slip on the overburdengrabens above the basement fault and hinge point, and increases their asymmetry.The same trend occurred for a single nonrotational fault with change in dip fromvertical to low angle (Slides 11.102–104). With a single rotational fault, increasedextension shifts the stacked horst and pop-up system at the synclinal hinge awayfrom the basement fault, separating it from the late-stage fault that propagatesdirectly above the basement step as the salt thins there.

Slide 11.113. Two domino faults, varied salt thickness. With 0.5 km salt thicknessand 3 km fault spacing (top two sections), the overburden only responded to offsetsat the ends of the basement fault array. A long, almost uniformly subsiding blockwas bounded by monoclines. Fault patterns in the fold hinges resemble the typicalones above a single basement fault (Slide 11.102). There was surprisingly littledifference between overburden structures developed above the hinge point (leftend of faulted basement) and the basement fault step (right end of faultedbasement). The faults in the bottom hinges of the monoclines showed the greatestdifference. Above the basement hinge, the overburden develops a bending horstpropagating upward from the base, but never completely cutting the layer. Abovethe fault step, a similar horst appeared but with more slip on the throughgoing faultdipping the same direction as the basement fault. Normal slip changed to reversewhere the dip reversed at the top of the overburden fault.

With 0.25 km of salt (bottom two sections), the overburden response wasstrongly coupled to the basement faults, resulting in two similar and relativelyindependent sets of faults above each basement fault step, similar to a model withtwo nonrotational faults (Slide 11.110). In a trend that continues with threebasement faults, the graben and single normal fault above the rightmost basementfault were more separated and showed higher strain than those over the fault in themiddle. This difference occurred because the middle structures were in a blocksubsiding between the outermost structures. This subsidence hastened impingementon the basement and increased the horizontal compression (as with arrays of

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nonrotational basement faults). Together these effects lessenedthe deformation across the middle structure relative to the outermost structures.A simple bending graben appeared above the left hinge of the basement fault array,with most of the extension taken up by the other overburden faults.

Slide 11.114. Three domino faults, varied extension rate. Adding another dominofault basically added another structure if fast extension either coupled the basementand overburden faulting (bottom section), or increased the length of the uniformlysubsided block in a more decoupled case (standard displacement rate, top twosections). The structures over the outermost (rightmost) basement fault closelyresembled those over a single basement fault (Slide 11.106), with the samevariations with displacement rate. The inner overburden structures accommodateless bending and extension. Above the left hinge of the basement fault array, slip onthe dominant fault of the asymmetric graben decreases as the inner structures takeup more of the extension where coupling was greater and extension was faster.

Slide 11.115. Three domino faults below thin salt, varied extension rate.

A thinner salt layer affects the overburden deformation similarly to an increasein extension rate. Therefore, the models in this slide resembled those in Slide 11.114

for an extension rate several times faster. In addition, the thinner salt promotedformation of bending grabens overlapping the normal faults above the basementfault steps. The thicker salt in the preceding slide allowed the overburden betweenthe outermost faults to subside as a single block. In this slide, the thinner salt causedmore folding over each step, with bending grabens hinging the anticlines.

Slide 11.116. Three domino faults, tilted model. The four models shown all hada small tilt to the right. Adding even a small tilt to the model caused a major changein overburden deformation. The relative effect of the tilt increased as couplingdecreased by changes in other factors, such as thicker salt or slower displacement.In relatively decoupled models, faulting initiated far upslope to the left (not visiblein the slide) rather than initiating extensional faulting in the perturbed zone abovethe moving basement faults. As a result, most of the overburden layer slid relativelyrightward on the salt the same distance as the extensional heave across thebasement faults, that is, extreme decoupling between locations of basement andoverburden faulting. Because the regional extension was accommodated farupslope, the overburden faults that did form above the basement faults tended to

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be arching-type grabens and horsts that relieved local stretching on the outer arcs offlexures.

With 0.5 km of salt and a relatively slow displacement (top section), only veryweak overburden structures appeared to accommodate the single gentle sag overthe three basement faults. With thinner salt (second and third sections), closercoupling with the basement movement folded the overburden into three synclinesbounded by anticlines or monoclines. Horsts and graben hinged the foldtroughs and crests, respectively. Finally, new normal faults developed inthe overburden immediately above the impinging basement steps. Greater viscousresistance in the thin salt resisted wholesale sliding of the entire overburden, somore extension coupled directly to the zones of basement extension than to gravitygliding. An increase in slip rate (fourth section) enhanced this trend, withoverburden faulting very similar to that in the untilted model (Slide 11.115, bottomtwo sections). An increase in tilt to 2° at the standard slip rate (compare bottomsection to the second and third sections) again stimulated sliding of the entireoverburden, suppressing extension above the basement fault array. The resultsresemble the model with thicker salt and a smaller tilt (top section).

To summarize, tilting suppresses extension above the basement fault array asoverburden and basement deformation become increasingly decoupled by in-creased salt thickness, decreased component of extension rate, and/or increasedtilting. Faults in the overburden only form on the outer arcs of folds to accom-modate local stretching. Increased coupling of basement and overburden defor-mation nullifies the effects of tilting.

Slide 11.117. Reversal of slip on single normal fault (“inversion”). The model shownin this slide began identically to the model in Slide 11.102, but then reversed the slip onthe basement fault at 50 ka (top three sections) or at 67 ka (bottom two sections). Theoverburden structures formed during normal slip remained dominant throughout theinversion. The only indicator of moderate amounts of reverse slip was a subtle reversefault appearing nearly on line with the basement fault. This fault did not becomeevident until nearly half of the normal slip was recovered. Thus, without the benefit ofa null point created by synkinematic sediments, substantial inversion may occur andleave very little outward evidence. By the time of complete inversion, the newoverburden fault had grown to absorb most of the imposed shortening. The result wasa wavy top surface, with troughs over the early extensional grabens and a larger bulgeformed by uplift of the late reverse fault’s hangingwall.

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Unfolding of the drape monocline by inversion had two effects. First, the pop-upat the lower hinge of the monocline reversed motion (not visible at the scale of theslide) and grew slightly outward and downward as a graben. Similarly, the horst belowreversed motion to pop down. These reversals were less complete if inversion wasdelayed (compare bottom model to top model), and the new reverse fault took upmore of the reversed deformation. Second, a curved, near-horizontal fault appeared inthe middle of the monocline’s limb (faintly shown by the first nonzero contour color).Resembling a similar structure seen as the monocline grew in compression in thereverse fault model of Slide 11.104, this fault slipped with the opposite sense (rightlateral) as tilt of the limb was removed in compression.

Coeval Extension Above and Below Salt Sheets

The models in this section expand a preliminary series distributed in Slide Set 10

(Slides 10.66–10.88). Salt sheets that are thin relative to their wide areal extent mayhave structures beneath them that are favorable targets for exploration, but are difficultto image seismically. The following numerical models (Slides 11.118–11.146, andText Figs. 11.148–11.153) were designed to simulate extensional structures that coulddevelop above and beneath salt sheets. Keep in mind that in these models, the rocksabove and below the salt sheet are extending concurrently, causing interaction betweendeveloping subsalt and suprasalt structures. Structures would likely develop differentlyif, for example, the rocks above the sheet were extending as the salt extruded andspread over static rocks beneath (we are currently investigating this topic).

These two-dimensional, cross-sectional models have two important charac-teristics.

First, all of the models contained an isolated, elongate salt sheet without aconnection in the section plane to deeper salt bodies. An isolated, perched salt sheetcould occur where a feeder has pinched off or where adjoining salt has flowed away toform a weld. The same configuration could appear where the section plane cuts a saltsheet to one side of a feeder, although these two-dimensional models cannot simulateflow in or out of the section. All of the models were also run with an additional deepersalt layer that was continuous at the base.

Second, the horizontal base of the model is a free-slip surface, either beneath thecountry rock or beneath the continuous salt layer. This condition could occur in naturewhere a décollement forms along a thin salt layer or overpressured zone beneathcountry rocks. Most models (Slides 11.118–11.141) contain only prekinematic rocks.

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Five of the models were run again with synkinematic sediments periodically added oreroded to restore the top surface to its original elevation (Slides 11.142–11.146).

Rock properties: All rocks modeled in this section were prekinematic. For brevity,the simulated salt and surrounding strata are termed salt and country rock, respectively.The salt had the same material properties as in the models above, and the country rockhad the same density, cohesion, and friction angle. The elastic moduli of the countryrock was less, with K = 8 GPa, G = 4.8 GPa, where K and G were the bulk and shearmoduli, respectively. This two-fold decrease from the above models presumablychanged the results little, increasing slightly the amounts of folding and the strain onindividual structures (see Slide 11.107, for the effects of a hundred-fold decrease inelastic moduli).

Basic dimensions: All models had a country-rock total thickness of 5 km. The saltsheets vary in maximum thickness from about 0.75 km to 1 km, and the continuousbasal layer, where present, was 1.5 km thick. The basic model geometries are shownin Text Figure 11.118. The symmetric models (Text Fig. 11.118a and c) had country-rock lengths of 40 km and salt sheet lengths of 20 km. The asymmetric models (Text

Fig. 11.118b and d) had country-rock lengths of 30 km and salt-sheet lengths of 10 km.For the symmetric geometries (Text Fig. 11.118a and c), only the right half wasmodeled, bounded by a symmetry line on the left. The mirrored right half is included inall following slides. To save space, the complete suite of models in Text Figures

11.148–11.153 show only the part of the model actually run.

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Imposed displacements: The right boundary of the models was extended at dis-placement rates at both 1 mm/a and 10 mm/a for all models. Both lateral boundariesallowed free slip vertically. Although in most models the displacements acted on theactual rock boundaries, some models used a rigid wall moving to the right forcomparison. Opening of a gap was possible in the latter models if tension developed atthe boundary between the wall and the rocks. Results were identical in models in whichonly this boundary condition was altered, proving that the models deformed bygravity spreading (lateral expansion under their own weight) rather than by an activelateral pull. All deformation developed spontaneously during extension; no notches,weak elements, or local applied forces were needed to initiate deformation.

Model variations: Because the shape of the perched salt sheet primarily influencedthe location of structures in the country rock, we built models with sheets in five basicshapes (Text Fig. 11.119), described in detail in Slide Set 10, Slide 10.66. Additionally,each model was then flipped about a horizontal line at the center of the country rock,which reversed the relative thicknesses of country rock above and below the salt lens.Each of those 10 models run with or without a basal salt layer and at the twodisplacement rates constituted the 40 models analyzed for this study. All of the modelsare shown in Text Figures 11.148–11.153. A subset that shows distinctive or interestingfeatures is shown in Slides 11.118–11.146 and discussed below.

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Basic structures: Faults in the models form a wide variety. However, we haverecognized three recurring patterns that either individually or in combination composethese fault arrays. Describing these patterns here will aid interpretationof model geneses in the following slides. Furthermore, these patterns can indicate link-ages between structures above and below the sheet, so may have predictive value. Thethree basic types of extensional structures are: (1) a graben over a décollement, (2) agraben over salt that rises as a reactive diapir, and (3) an asymmetric graben above saltthat partially decouples it from normal movement on an active fault beneath the salt.For brevity, we will informally term these structures a simple graben, a reactive graben,and a drape graben.

Simple graben: Extension of sedimentary rock above a flat, slippery décollementcreates a simple graben (Text Fig. 11.120). The rigid boundary condition imposed by thedécollement prevents isostatic uplift of the footwalls and also impedes subsidence of anintact hangingwall block. However, the extensional thinning at the graben makes itweaker and prone to further deformation. To continue deformation without openinggaps, new normal faults dissect the initial graben block inward to create a series ofprogressively smaller grabens in the original graben floor. This dissection processoccurs symmetrically (Text Fig. 11.120a), or, more commonly asymmetrically (Text

Fig. 11.120b), where movement remains localized on one bounding fault of the originalgraben as normal faults step inward from the other fault. Continued extension can thinthe layer to nothing. The resulting structure resembles a rollover fold formed above alistric normal fault by antithetic faulting of the hangingwall.

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Reactive graben: A salt layerbeneath an extending graben removesthe constraints on vertical motion ofthe blocks and isostaticallycompensates for thickness changes.This combination produces the secondbasic structure, a graben overlying areactive diapir (Text Fig. 11.121), called a reactive graben here. Reactive grabens appearin many physical and numerical models in preceding slide sets where a rock layerextends above a uniform salt layer. A symmetric graben above rising salt distinguishesa reactive graben.

Drape graben: The third basicstructure is an asymmetric graben thatforms in response to active movementon a normal fault below an in-tervening salt layer (Text Fig. 11.122).Drape grabens appeared with most ofthe basement faults in this slide set,exemplified in Slides 11.105 and 106.The degree of coupling between thesubsalt and overburden faulting isexpressed by the relative offset of the graben from the subsalt fault, and the degree ofasymmetry of the graben, ranging from a nearly symmetric graben to a single normalfault. Drape grabens are important interpretation tools because they are coupled tosubsalt structures. A drape graben above a salt sheet generally indicates a normal faultbelow the sheet (Text Fig. 11.122). The subsalt fault is laterally offset in the dip directionof the dominant fault in the drape graben and dips in the same direction.

The slides discussed below (Slides 11.118–11.146) add to the preliminary series inthe previous slide set (Slides 10.66–10.88). For that reason, none of those models are

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duplicated here, except where they have been run substantially further or helpto explain features in newer models. Because thickness variations are a primary influ-ence on locations of failure, the models are classified according to the initial shape of thesalt sheet, specifically whether it has curved or flat contacts. Additionally, each modelwas run at moderate (10 mm/a) or slow (1 mm/a) horizontal displacement rates andwith or without an additional basal salt layer.

Format of slides: The following slides show strain at an intermediate stage (top), anend stage (middle), and a plot of material lines (bottom) representing layers. The straincontours record the equivalent plastic strain, defined as the non-reversible octahedralshear strain. For our purposes, relative values of strain are more significant than theirabsolute values (Slide Set 7 includes a more complete discussion of plastic strain andstrain weakening). Plastic strain in the top sections of all slides is contoured from 0.02 to1.0; the middle section (end stage) ranges to a higher maximum of 1.6. An orangedashed line outlines the original salt and country-rock boundaries in the contouredsections. The material lines were initially horizontal and evenly spaced every 0.33 kmvertically, so any later variation in layer thickness records deformation or syntectonicsedimentation on a deformed top surface. Two arbitrary layers are also subdividedwith vertical lines at the same spacing to record extension and shear. The basal saltlayer, where present, contains vertical lines defining an initially uniform grid (except forthe lowermost layer which is slightly thicker at 0.5 km). The material lines and thelayers they define deform passively with no mechanical significance; the country rockhas isotropic material properties (until locally strain-weakened).

Slide 11.118. Salt sheet with curved top and base, 1 mm/a. The early stages of thismodel were shown previously (Slides 10.68) and discussed extensively there. Aperched salt sheet has the shape of Text Figure 11.119a. The country rock faulted first atits thinnest points, at the central symmetry line both above and below the salt sheet.Salt rising beneath the roof graben created a reactive graben. The basal décollementconstrained the subsalt structure to become a simple graben. The slow extension raterelaxed the viscous stresses throughout the sheet as it flowed toward the center,allowing the roof to tilt gently inward along hinges accommodated by weak bendinggrabens above the tips of the salt sheet. Nearly all of the extension occurred across thecentral grabens. Near the end stage, the floor of the suprasalt reactive graben thinnedsufficiently that it bulged upward by active diapirism (middle section of slide). Thestructures above and below the salt were almost entirely decoupled, as shown bycomparison with the effects of an increased extension rate in the next slide.

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Slide 11.119. Salt sheet with curved top and base, 10 mm/a. The country rock faultedfirst at its thinnest point, in the roof of the salt sheet at the center, to create a reactivegraben. Decrease of vertical stress, created by salt rising into the reactive diapir, inhib-ited underlying faulting at the thinnest subsalt country rock (for more details, the earlystages of this model were shown previously in Slides 10.69–72 and discussedextensively there). Instead, simple grabens formed to either side of the centralsymmetry line. Extensional thinning of the sheet and flow of salt into the subsaltgrabens caused the roof of the sheet to sag over the outermost faults of the subsaltgrabens, which acted as basement faults to initiate drape grabens in the roof. Slip on thecentral suprasalt faults waned because the reactive graben was “pinched” in the centerof the sag and the salt balance was better maintained by relatively uniform subsidencebetween the drape grabens. Additional extension propagated outward above the endsof the sheet for two reasons. First, the surface slope that steepenedas the central roof subsided caused the relatively high footwalls to extend by gravityspreading. Second, salt flowed from the ends of the sheet into the subsalt grabens andwas thinned by extension, causing faulting at flexures as the salt roof drooped. Extremethinning of the sheet in the late stages draped the roof over the central subsalt high,reactivating the central roof graben. This folding created gentle synclines above thesubsalt grabens. The centers of the synclines were hinged by weak conjugate faultsbounding a horst capped by a pop-up, the same system that was seen in the basementfault slides (Text Figure 11.102).

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Guidelines for predicting subsalt structure. The evolution of this model demonstratessubstantial interaction between structures above and below the salt. The structuralpatterns resulting from coupling across the salt sheet will be repeated in most of themodels shown below, so are emphasized here. The coupling was minor where ex-tension was sufficiently slow that the influence of thickness variations could overwhelmthe diminished interaction of country rock structures. But in the moderate extensionrate models, the following observations serve as guidelines for using structures abovethe salt sheet to predict structure beneath the salt sheet:

1) Asymmetric drape grabens are the most useful indicators of subsalt structure,because they develop above the salt sheet in response to faulting below the sheet.As demonstrated by the basement fault models, the form of drape grabens varieswith the amount of their coupling with the subsalt faults. The lateral offset betweenthe location of a subsalt fault and its related drape graben at the surface increasesas coupling between the structures decreases. The graben becomesmore symmetric as the offset increases. A very asymmetric graben, or only ahalf-graben, indicates that the faults line up above and below the sheet. A nearlysymmetric graben implies large offset of its location from the subsalt fault, and alack of strong structural coupling across the sheet.

2) Symmetric or nearly symmetric reactive grabens above a salt sheet generally indicatea lack of subsalt structure directly below them. The dominant fault in a slightlyasymmetric graben points downdip toward the nearest subsalt fault. Symmetricgrabens may overlie subsalt structure if geometric effects prevail where couplingacross the salt is weak (e.g., Slide 11.118). Thus, symmetric reactive grabens are notas reliable an indicator of subsalt structure, or lack thereof,as are asymmetric drape grabens.

3) The roof of a salt sheet directly above a subsalt graben tends to have little fault-ing. Instead, a pronounced sag develops there as extension grows and salt thinsat the margins of the graben. Flexure of the sag may be hinged by a stacked horstand pop-up. The relief across this sag is much greater in décollement-based modelsthan in models with a salt layer at the base. That difference arises because isostaticflow of the basal salt layer diminishes surface relief.

Slide 11.120. Inverted salt sheet with curved top and base, 10 mm/a. This model resem-bles the model in Slide 11.119, but with the salt sheet flipped about a horizontal axis

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passing through its tips. This inversion put the thinnest country rock below the saltsheet, rather than above, so extension initiated faulting beneath the sheet to create asimple graben. In the moderate displacement-rate case shown, the locations of thesesubsalt faults are evinced by their offspring, the offset drape grabens above the salt. Asin the preceding slide, extensional faults formed outward from the main drape grabensas the roof neared the subsalt contact, inhibiting uniform subsidence. Salt conservationthen requires subsidence elsewhere, appearing as dropping and spreading of the outerparts of the roof and sagging of the central roof with a horst and pop-up in the synclinalhinge zone.

Slide 11.121. Salt sheet with curved top and base, 10 mm/a, basal salt layer. Adding ofa continuous salt layer beneath the model shown in Slide 11.119 frees movement of thesubsalt country rock, so that early failure occurred at the thinnest rock both above andbelow the salt sheet. However, the oldest structure, the upper graben,still influenced subsalt structure. Flow upward into the upper reactive diapir createdexcess salt area, which spurred formation of a horst below it (as shown for an ideal casein Slide 11.98b). Initial movement on the horst system better balanced the potentialexcess space than could the space-creating grabens beneath the salt sheetin the preceding examples.

Effects of a basal salt layer: Relative to models floored by a décollement, the basalsalt layer allows a wider variety of potential structures beneath the perched sheet. Inaddition, widespread salt redistribution brings the models with a basal salt layer closerto isostatic equilibrium. At the sides of the model, small subsidence of the regionalcountry rock replaces some of the large subsidence at the center of the model.Extension of the salt sheet and subsalt rock above a décollement (Text Fig. 11.120)requires the roof to drop. Flow in a basal salt layer removes this restrictionso that the regional rock may actually drop as the rock in the center rises, both rela-tively, and occasionally, absolutely (e.g., Slides 11.122 and 11.126).

The subsalt horst system in Slide 11.121 developed precisely the same as areactive graben system (Text Fig. 11.121) turned upside down. A reactive diapir grewdownward above the tip of the relatively upthrown horst block. We define this “down-ward growing diapir” as a structurally discordant salt body that propagated oppositethe direction of stratal younging. Salt flow into the downward growing diapir soonstimulated formation of drape grabens above the salt sheet as the initial suprasaltgraben was pinched in the central sag and effectively stopped its growth. Later, themodel only extended across the drape graben above the sheet and across the horst

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system below. At the latest stage (middle section of slide), the country rock beneath thesheet had thinned to nothing across the horst system. There the basal salt layer couldconnect with the salt sheet. Note that an interpretation based only on this end stagemight erroneously conclude that the connection was the original feeder for theallochthonous sheet. Instead, the connection formed late, marking the location ofextreme concentrated extension.

Because the horst floor rose to meet the diapir growing downward from above,the shape of the contacts at the end stage are nearly symmetric across a horizontalplane. There is no sign that the central subsalt structure grew as a horst. However, thehorizon lines beneath the sheet clearly reveal that history (bottom section of slide). Fol-lowing the horizons toward the center, the upper ones converge close to the saltcontact and bend downward, whereas the lower horizons remain roughly parallel asthey bend upward across the inward-stepping faults. Contrast this pattern with hori-zons approaching a severely extended graben, for example, above the sheet inSlide 11.118, bottom section. Approaching a graben the upper horizons gradually benddownward in parallel, whereas the lower horizons converge and bend sharply upwardclose to the salt contact.

Slide 11.122. Inverted salt sheet with curved top and base, 10 mm/a, basal salt layer.This model adds a continuous basal salt layer to the model in Slide 11.120. Unlike themodel in Slide 11.120, inverting the shape of the salt sheet from that of Slide 11.121

did not shift the initial faulting to the thinnest country rock beneath the sheet.A graben formed first above the sheet at the central symmetry line. We surmisethis happened for two reasons.

First, consider that faulting initiates when the stress on a plane reaches the yieldstress. Vertical compressive stress increases downward due to weight of overlyingmaterial, and therefore so does the horizontal stress which derives from the elasticresponse to the vertical stress. The yield stress depends on normal stress, so it alsoincreases downward. Every depth requires the same proportional reduction in horizontalstress to reach the yield stress, but deeper levels require a greater absolute reduction.Roughly speaking, extending the model decreased the horizontal stress by a uniformabsolute amount, regardless of depth. Therefore, shallower levels reached the yieldstress and faulted first, all else being equal. For the country rock beneath the sheet tofault first, it must be substantially thinner than that above to overcome this effect.

Second, the central part of the model with its greater percentage of less-densesalt rose relative to the outer, regional parts. This uplift further decreased the horizontal

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compressive stress in the roof of the sheet, which extends on the outer arc of theflexure.

Once the roof graben formed, the salt flow into the reactive diapir appears tohave affected the country rock beneath the sheet like an upside-down basement faultand drape graben system. That is, if you invert the model, the subsalt faults look likedrape grabens above the “basement” faults of the reactive graben in the roof of thesheet, and it appears that they formed for the same reasons. As the reactive grabengrew, the horst block beneath the sheet rose dramatically as salt in the lower layerflowed to the center to isostatically compensate for the thinning of the roof. Regionalsubsidence of the roof of the basal salt, shown by the deformed and initial levels in themiddle section and by the deformed grid in the bottom section, record the inward flowof salt to feed the central diapir. The absolute uplift of the central horst block apparentlycontravenes the well-known rule that extension lowers rocks below their original level.Accordingly, this rule may not hold if salt beneath a block is pressured from elsewhereand flows upward to support extending fault blocks.

Why the subsalt faults form a pointed horst in Slide 11.121 but occur farther outto form a wider horst in Slide 11.122 is complex and relies on technical subtleties. Ourgeneral conclusion is that the pattern in Slide 11.122 represents the more common andreproducible response to extension of a salt sheet with curved top and bottom perchedabove another laterally continuous salt layer. The subsalt fault pattern in Slide 11.121

was unusually sensitive to various properties. For example, a decrease in numericalconvergence tolerance changed it to a reactive graben. In nearly all other models, sucha tolerance change merely widens shear zones but leaves the basic structural patternunaltered. A different change resulted from a reversal of the density contrast inSlide 11.121 so that the overburden was less dense than the salt (2000 kg m-3 vs.2200 kg m-3): the faults moved outward to a position similar to Slide 11.122.

Slide 11.123. Salt sheet with flat base and curved top, 1 mm/a. Flattening the base ofthe salt sheet removes the geometrical preference for subsalt faulting at the center(salt sheet shape of Text Fig. 11.119b). Instead, simple grabens formed beneath thesheet from faults propagating from the stress concentrations at the ends of the sheet.Unlike the moderate displacement rate in Slides 10.73–74, with the slow extension inSlide 11.123 the structures were essentially decoupled above and below the sheet. Notethe active diapir breaking through the central reactive graben in the end stage (middlesection) and the inversion of the shape of the salt sheet at its ends.

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Slide 11.124. Inverted salt sheet with flat base and curved top, 10 mm/a. Inverting thesalt sheet in Slide 11.123 about a horizontal axis places the thinnest country rock belowthe sheet and a uniform thickness above. A simple graben appeared first beneath thesheet, which initiated coupled drape grabens farther outward in the roof. As in Slide

11.120, the late stage shows a central horst and pop-up at the central sag and additionalextensional faults outward from the main drape grabens. Distributed extension acrossthe many roof faults reduces the local subsidence and better balances the small increasein salt area produced by the single subsalt graben. The predictive rules apply here:major subsalt faults occur offset in the dip direction from the dominant faults of thesuprasalt drape grabens.

Slide 11.125. Inverted salt sheet with flat base and curved top, 1 mm/a. Slowing thedisplacement in the model of Slide 11.124 removes most of the coupling between struc-tures above and below the sheet. The subsalt simple graben formed first, but then theroof extended across reactive grabens at the stress concentrators of the ends of thesheet. These grabens are somewhat asymmetric, so are really drape grabens looselylinked to the distant subsalt faults. As noted in Slide 11.106, the small degree ofasymmetry, which is caused by a slow deformation rate, indicates a large horizontalseparation between surface and subsalt structures.

Slide 11.126. Salt sheet with flat base and curved top, 1 mm/a, basal salt layer.Flooring the model of Slide 11.123 with a salt layer created only minor changes whereextension rate was slow. What were simple grabens beneath the ends of the sheetbecame asymmetric hourglass structures across which the base of the subsalt blockrose,both relatively and absolutely, above the regional base of the country rock. Regionalsubsidence balanced central uplift beneath the sheet. Isostatic compensation is morepronounced in these models where faults beneath the sheet define a large block thatcan rise without the flexure required in this model such as in Slide 11.121, and isparticularly enhanced in slow extension rate models. As in Slide 11.122, the subsaltfaults can be viewed as upside-down systems of drape grabens responding to faultingat the suprasalt reactive graben. To maintain salt area, the drape grabens could not bedominated by a single normal fault accommodating the extension. Only a small uplift ofthe wide block beneath the sheet was necessary to balance the extension of the sheetand the flow into the reactive diapir.

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What Defines a Regional Datum? The model in Slide 11.126 illustrates a dilemma thatwill recur. Defining a “regional” datum elevation in the commonly used way createserrors in interpreting the history of the section from the end stage shown. With theforward modeling, we have the luxury of comparing the final, deformed state with theknown, unique initial state.

Generally the regional datum is the elevation of a key horizon in an apparentlyundeformed area. Comparing the elevation of the same horizons in a deformed zoneelucidates the history of deformation. For example, a guideline states that “strata dropbelow regional datum during extension, reactive diapirism, salt withdrawal, or saltdissolution; strata rise above regional datum during contraction or active diapirism”(Jackson et al., 1994, p. 98). Although our numerical models do strictly follow theseguidelines, correct specification of the regional may require knowledge of the initial,as well as the final state.

The model in Slide 11.126 apparently subverts the rule of the regional datum.With respect to the elevation of the outer, undeformed “regional” blocks, the centralroof above the salt sheet has lifted. Even more notable is the uplift of the blockbeneath the salt sheet relative to contacts in the regional blocks, and also absolutelyin comparison with the original outline. Obviously, the section has not undergonecontraction or active diapirism. In fact, the uplifted roof blocks did not rise absolutely;instead, salt flow toward the center dropped the regional blocks more than the centralblocks. Isostasy acting on the greater thickness of less dense salt at the center supportsthe higher elevation there.

Many of the salt-floored models having slow displacement (1 mm/a) showedsome uplift above the salt sheet relative to the outer, regional blocks, although mostof those cases are local footwall uplift near a normal fault. The slow extension allowsbroad redistribution of the basal salt toward the center with its less-dense salt sheet.Faster displacement inhibits this flow, so extensional thinning of the salt dominates inthe vicinity of the active country rock structures, causing subsidence in the structurallyactive areas relative to a regional datum.

In summary, the concept of a regional datum must be used carefully. Elevationchanges relative to this datum must be evaluated keeping in mind the possibility ofisostatic compensation in zones where salt may easily flow long distances and whichcontain substantially more or less salt than nearby, leading to an overall lateral densitycontrast. In this case, elevation changes should not be ascribed purely to extension orcontraction.

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Slide 11.127. Salt sheet with flat base and curved top, 10 mm/a, basal salt layer.A tenfold increase in the extension rate relative to Slide 11.126 caused predictablechanges. More numerous structures formed and they were closer together. The oldeststructures resembled those in Slide 11.126, except the central graben had a widerdeformed zone at its base. Sagging of the roof tended to pinch this subsalt graben,so a horst overprinted it to hinge the flexure, and the original inward dipping faultsmigrated outward to form the paired graben system flanking the central horst. Belowthe sheet, the oldest faults were the outward dipping ones near the ends of the sheet.Salt balance favored rise of the floor of the sheet. Although the initial stressperturbations generated short spur faults at the end of the sheet, an outward-dippingnormal fault there would have required salt flow at the very thin end of the sheet anduplift of the footwall would have been limited by the unfaulted roof of the sheet.Consequently, the short faults were replaced by throughgoing subsalt faults slightlyinward from the sheet ends. These faults stimulated drape grabens in the roof early inthe history when these roof and subsalt faults were closer together. The top sectionshown in this slide is much later (note that almost 4 km of extension had occurred,which exceeds the total in most of the other models shown with moderate extensionrates).

Given the outward dip of the main roof faults, salt balance had to be maintainedeither by downward flexure of the ends of the salt sheet roof or by uplift of the subsaltblock. The subsalt block was too long to uniformly uplift and balance the salt area, sothe ends of the block bent upward. This bending spurred formation of another set ofoutward dipping faults in the subsalt block closer to the center before the earliest stageshown. Later (top section) the pairs of subsalt faults were joined by inward dippingfaults to form a system with an “N” shape on the left side. Slip on the three faults ateach end of the subsalt block resulted in interesting offsets of the top and bottomcontacts. The top contact (base of the sheet) only shows a fault stepat the innermost fault. Much greater slip on the outer two faults exactly matches toleave a flat surface across their intersection. On the base of the subsalt block (top of thebasal salt layer), the contact is mostly offset across the outermost fault. Slip on the twoinner faults nearly matches to create little relative offset of the contact on either sideof the reactive diapir beginning to grow at the fault intersection.

The end stage (middle section) shows more total extension than any other modeldue to the cumulative heave across the numerous faults. Extension rotated most of thefaults to lower dips and greatly thinned the entire roof of the sheet. Note howcompensation from flow in both salt layers subdued relief on the top surface, even

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though it was greatly extended. The deformed grid in the lower part of the basal saltlayer (bottom section) records the large regional flow toward the center that causedregional subsidence as it buoyed the central zone. The reactive diapirs piercing thecountry rock beneath the sheet had nearly broken through, and were actively archingthe overlying floors of the reactive grabens. This example shows that active diapirs mayoccur beneath salt sheets, as well as at the surface.

The predictive rules hold well for this model. The drape grabens at the surfacepoint to, and are offset from, the major subsalt faults. Symmetric reactive grabens inthe roof overlay unfaulted subsalt country rock.

Slide 11.128. Inverted salt sheet with flat base and curved top, 10 mm/a, basal salt

layer. Inverting the shape of the sheet in Slide 11.127 changes the complex structuralpatterns of the preceding slide to a particularly simple pattern. Faulting initiated in thesubstantially thinner country rock beneath the sheet as a reactive graben. Salt flow intothis graben stimulated formation of drape grabens above the sheet. The nearly perfectalignment of faults above and below the sheet resulted in uniform subsidence of theroof block, with only small flexure to create additional minor faults by the end stage.With only a single graben beneath the sheet to accommodate extension, the subsaltlayer thinned to nothing rapidly and terminated the run before the sheet thinnedsufficiently to cause local bending and additional faulting in the roof. The passive lineplot in the bottom section illustrates well the bedding pattern around a severelyextended graben. Compare this with the subsalt horst in Slide 11.121 that producedthe same shape of salt contacts, but an inverted pattern of horizons within the countryrock.

Slide 11.129. Inverted salt sheet with flat base and curved top, 1 mm/a, basal salt

layer. The uniform roof subsidence of Slide 11.128 was destroyed by two factors in thisslower displacement model, both related to greater time for flow in the salt andrelaxation of viscous stresses. First, the drape grabens propagated farther from thecenter with the reduced coupling. Second, the footwalls of the reactive graben beneaththe sheet rose. Both effects consumed salt area relative to the preceding slide.Therefore, slip on the inward dipping faults alone would have caused too muchsubsidence of the roof block. Therefore, outward-dipping faults formed asymmetricgrabens that accommodated the necessary extension while lessening the centralsubsidence of the roof to conserve salt area. The relatively thin salt sheet in this model

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caused significant coupling of structures across the sheet, even though the displacementwas slow.

Slide 11.130. Inverted salt sheet with curved base and flat top, 10 mm/a. This modelinverts the shape in Text Figure 11.119c. The slight primary central thinning of the roofinitiates faulting there as a typical drape graben. Subsequent deformation proceededsimilarly to Slide 11.119 but with greater lateral offset between structures above andbelow the sheet, reflecting the greater thickness of the sheet. Subsalt grabens appearedfarther from the center because the lack of central thinning removed a geometricpreference for failure there. The predictive rules apply well to this model.

Slide 11.131. Salt sheet with curved base and flat top, 10 mm/a, basal salt layer. Thismodel added a basal salt layer to the model of Slide 10.77–78 (salt sheet shape of Text

Fig. 11.119c). Country rock above the sheet was thinner than that below. Extensionalfaulting propagated from the stress concentrators at the ends of the flat top of the sheetto form grabens. But early centerward salt flow to isostatically compensate the less-dense salt sheet tended to drop the regional overburden, so the outward-dipping faultof the graben dominated. Rise of the suprasalt hangingwalls was compensated by smalluplifts in the subsalt country rock below. Between them the subsalt block sagged, so theinitial faulting toward the slightly thinner center formed a horst resembling the one inSlide 11.122. Gravity and salt area conservation resisted rise of the wide roof block, soit began subsiding along the inward-dipping faults of the drape grabens and anotherweaker set closer the center. Salt area conservation during dropping of this wide roofblock inhibited rise of the subsalt horst, as seen in Slide 11.122, so the additional faultsdeveloped to create two reactive grabens beneath the sheet. The grabens allowedextension with little relative elevation change of their footwalls.

This model represents an apparent exception to the predictive value of drapegrabens. The dominant faults of the roof’s drape graben point outward, whereas theassociated basement faults are located offset inward. The clue to this anomaly is that theasymmetry of elevation change across the graben is downward toward the center, inthe direction of the subsalt faults, which derives from slip on the additional inward-dipping faults. Thus, the most reliable asymmetry property of drape grabens is therelative elevation of their footwalls, rather than the dominant fault of the graben.However, in nearly all cases these two indicators point the same way, so this distinctionis generally unnecessary.

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Slide 11.132. Salt sheet with curved base and flat top, 1 mm/a, basal salt layer.Decreasing the extension rate of Slide 11.131 markedly changes the fault pattern. Thesalt had more time to flow centerward in response to the pressure gradient beforeextension initiated in the overburden. Therefore the center of the model archedsignificantly as the regional country rock sank. Faulting initiated in the sheet’s roof asreactive grabens on the stretched outer arc of the arch. Faulting also initiated as areactive graben beneath the sheet, where the subsalt country rock was thinnest. Here,the thicker salt of this sheet effectively decoupled older structures across the salt, in con-trast to the thinner sheet of Slide 11.129, for example. However, flow into the subsaltgraben stimulated later formation of drape grabens away from the center of the roof,which killed extension across the central suprasalt reactive graben. A weak asymmetryon the outer grabens signals their large distance from the related subsalt faults.

Slide 11.133. Inverted salt sheet with flat base and top, 10 mm/a. Inversion of theshape in Text Figure 11.119d about a horizontal axis makes uniform thickness countryrock above and below the salt sheet. The initial faults therefore propagated above andbelow the stress concentrations at the ends of the salt sheet. Further growth formed asimple graben beneath the sheet and a half graben above the sheet. Withdrawal intothe subsalt grabens, enhanced by tilting of the half grabens, created drape grabens nearthe center of the roof (upper section). Further extension rotated these graben faults tolower dips. After extreme extension (middle section), the ends of the sheet welded,arresting fall of the roof there and folding the roof into a syncline. Curvedsubhorizontal zones of weak strain (next-to-lowest contour) propagated inward inthe mid-level of the roof to the curved high-angle fault formed above the inward sidesof the basal simple grabens. These subhorizontal faults resemble those formed inSlides 11.104 and 11.117 where the layer is shortened as it folds. Presumably theshortening as the syncline folds in Slide 11.133 derives from gravity spreading awayfrom the surface highs at the model center and edges of the regional country rock.Therefore, surface slope can create structures indicative of shortening even though theoverall deformation is extensional.

Slide 11.134. Salt sheet with flat base and top, 10 mm/a, basal salt layer. Adding abasal salt layer to the shape in Text Figure 11.119d created this model. Deformationbegan similarly to Slide 11.131, but the lack of primary thinning in the country rock be-neath the sheet moved the faults in it toward the ends of the sheet. The footwalls flexedupward above and below the sheet. Additional structures were not required to

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conserve salt area because the faults nearly aligned across the sheet and had the sameslip direction. Withdrawal of basal salt from both the regional country rock and thecenter of the model fed the growing reactive diapirs beneath the outer subsalt half-grabens, dropping the regional elevation and strongly warping the subsalt centralhorst.

Slide 11.135. Further deformation. By the final stage in Slide 11.134, faulting hadthinned the country rock to almost nothing beneath the salt sheet. Because of thecontinuity assumption in the finite element method, stretching the minuscule thicknessrock farther was not a realistic analog of nature. As an alternative, this slide shows amodel with the element mesh rebuilt within the same outlines as the last stage in Slide

11.134, but with a small salt gap connecting the basal salt layer and the salt sheet. Thestress gradients at the thinned spot prevented convergence of the runs if the mechanicalstate was preserved. Consequently, the model was run without the mechanical history ofthe parent model. Therefore, all of the country rock was again homogeneous andisotropic, without strain weakened fault zones, and the stress in the salt initiallyapproached a lithostatic stress gradient.

Breakthrough of salt isolated the country rock block beneath the original saltsheet. Considering only relative densities and thicknesses (fluid statics), the blockshould have sunk in the sea of slightly less dense salt. However, the dynamic pressureexerted by upward flow from the basal salt layer dynamically lifted the ends as the saltfilled the widening gap. The bottom section of the slide shows cumulative displacementvectors. Note that upward flow of salt at the base the narrow neck at the upper centerof the section accompanied rise of the subsalt block. But at the top of the channel,the flow reverses direction, dragged downward by the sinking tip of the regionalcountry rock. Salt withdrawal from the center of the model further drops the centerof the subsalt block, yielding the rotation recorded by the vectors.

Slide 11.136. Salt sheet with flat base and top, 1 mm/a, basal salt layer. Slowing theextension rate of Slide 11.134 produced familiar effects. Wider redistribution of saltmitigated local bending effects. Drape grabens in the roof were loosely coupled to horstfaults propagated downward from the tips of the sheet. Of further interest here was thesubstantial uplift of the central blocks above the apparent regional elevation of thecountry rock. The surface uplift was wider here than in Slide 11.126 and less easilydismissed as local footwall uplift. The caveats discussed for Slide 11.126 should beapplied here when interpreting changes of relative elevation.

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Slide 11.137. Inverted salt sheet with flat base and top, 10 mm/a, basal salt layer.Adding a basal salt layer to the model in Slide 11.133 made little difference, an unusualresult. The order of fault formation was still controlled initially by stress perturbationsat the ends of the sheet. The simple grabens above the décollement in Slide 11.133

became more complex hourglass structures here, allowing moderate uplift of the cen-tral subsalt block and moving their linked drape grabens somewhat closer. Otherwisethe two models were remarkably similar at equivalent times.

Slides 11.138–141. Asymmetric salt sheets. The following four models contained astrongly asymmetric salt sheet shorter than in the preceding models (Text Figs.

11.118b, 11.118d, and 11.119e). The asymmetric shapes could be due to a previoushistory of sediment progradation or salt extrusion onto a surface of accumulation.Asymmetric sheets imply marked thickness changes in the surrounding rock. Thesevariations strongly controlled the pattern of subsequent faulting in the models. The thincountry rock adjoining the thickest salt always faulted first, placing a strong geometriccontrol on the fault pattern and its evolution.

Slide 11.138. Asymmetric salt sheet, 10 mm/a. This model contained a flat-bottomedasymmetric sheet, with a free-slip décollement at the base. Faulting initiated at thethinnest country rock above the sheet, evolving into a system of several faults thatproduced enormous local subsidence. Subsalt faulting propagated from the stressconcentrator at the left end of the sheet to form the typical simple graben seenbeneath the sheet in all décollement-floored models. A drape graben cut the roof assalt flowed leftward and downward into the subsalt graben. In the latest stage(middle section) the faulting above and below the thickest part of the sheet almostcompletely inverted its shape. The largest sag in the roof overlay the subsalt grabenin partial agreement with the third predictive guideline (see Slide 11.119), but theroof was heavily faulted because of the initial failure there. The asymmetric drapegrabens reliably indicate the location of subsalt faulting, with their symmetryincreasing with lateral offset from the location of the subsalt faults.

Slide 11.139. Inverted asymmetric salt sheet, 1 mm/a. Inverting the shape of thesheet caused strong initial faulting at the décollement. Location of the resultantdrape grabens in the roof was controlled by stress perturbing tips of the sheet.However, the asymmetry of their development still accurately reflected their offsetfrom the basement faults. The model with moderate extension rate (not shown)evolved virtually identically to the model shown here.

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Slide 11.140. Asymmetric salt sheet, 10 mm/a, basal salt layer. Adding a basal saltlayer to the asymmetric model markedly changed the subsalt deformation pattern.Subsalt faulting initiated at the other end of the sheet, evolving as a single dominantfault. The country rock beneath the sheet rose isostatically to balance structuralthinning of the sheet’s roof. Rather than creating large surface relief as in thedécollement-floored model (Slide 11.138), flow in the basal layer allowed largefootwall uplift of the subsalt normal fault to compensate for extensional thinning ofthe salt sheet, resulting in low relief on the top surface of the country rock. The zoneof extremely thin country rock covering the high point of the sheet would likely bebroken apart in nature to expose emergent salt.

Slide 11.141. Inverted asymmetric salt sheet, 10 mm/a, basal salt layer. This modeladded a basal salt layer to the moderate extension rate version of Slide 11.139.Early faulting at the base of the sheet again triggered a drape graben above thesheet. However, the drape graben shifted to the left because the footwall upliftof the subsalt country rock increased the effect of the subsalt fault. Flow in the saltlayers buoyed the roof of the sheet, removing the large subsidence at the surfaceseen in Slide 11.139 as well as the deformation needed to accommodate thesubsidence.

Slides 11.142–146. Synkinematic sedimentation. Because horizontal extension typicallycauses vertical subsidence, synkinematic sediments commonly accumulate over zonesof extension. These new sediments may affect subsequent deformation, which in turndetermines the likely sites for further accumulation. The response to synkinematicsedimentation may be particularly evident if a body of salt below the sediment pileflows relatively rapidly in response to pressure gradients and therefore to changes inoverburden load. To understand these effects, we ran five of the above models with thesame starting configurations, but periodically added or eroded sediments to restore thetop surface to horizontal at its original elevation.

The sedimentation intervals were constant in each model, but varied betweenmodels from 10 000 a to 25 000 a depending on the subsidence rate and total expectedextension of the model. The number of layers deposited ranged from six to twenty andare depicted in greens and yellows. For each cycle the model was rebuilt with new rockraising the top surface to the original level, or eroding existing rock if it rose above that.The preserved mechanical state was combined with the new geometry, and the modelwas run for another specified interval. Because the passive lines best display the

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sedimentation patterns, the following slides show plastic strain contours only at the endstage, with plots of lines shown for an intermediate stage and the end stage.

Slide 11.142. Salt sheet with curved base and top, 10 mm/a. Periodically adding sed-iment (every 17,400 a for 20 layers) to the model in Slide 11.119 adds strength and anonuniform load. The deformed areas were filled and loaded without increasing theregional thickness of sediment. Maximum subsidence of the top surface was alwaysin the graben at the central symmetry line. Subsidence lessened progressively witheach cycle, from 325 m initially to 169 m during the last deformation. Increasing loadintensified deformation at the boundaries of the subsiding areas, making themmajor growth faults or tilt hinges. Any tilting in the prekinematic sediments wasamplified by the differential loading of the new sediment layers, particularly wherethe salt layer beneath could flow to make room for the subsiding rock. Minorstructures covered with new sediment tended to die outor become less active because the new rock layer had to be broken after each sedi-mentation cycle. Activity also decreased on extensional structures elsewhere, forexample above the ends of the sheet in this model. Without synkinematic sedi-mentation (Slide 11.119), faulting was induced in two ways in undeformed countryrock above the ends of the salt sheet. First, the relatively high footwalls above theends of the sheet responded to the growing surface slope by gravity spreading.Second, the salt sheet in those locations thinned with extension, inducing flexuralfaulting as its roof sagged. Synkinematic sedimentation leveled the surface slope andsuppressed local gravity spreading. Increased slip on the major basin-boundingfaults accommodated all regional extension. Furthermore, each addition of sedimentdifferentially loaded the salt, driving it toward the thinner, undeformed parts of itsroof and removing the tendency to sag in those outer regions.

Synkinematic sedimentation perturbed the patterns of flow in the salt sheet.These perturbations were then sheared further to create isoclinal recumbent folds(visible in the bottom section just inward from the outer reactive diapirs). In thecenter of the salt sheet, strong upward flow near the basal contact fed the diapirthen was dragged downward by the sinking overburden near the top salt contact.The beginnings of this circulation are apparent in the middle section. By the endstage, overturn of the passive lines in the salt at the center so severely folded andstretched them that they lost resolution. Keep in mind that there was no discretefaulting at the top salt contact, as it seems to appear from the large displacements ofthe layers. Shear in the salt accommodated all of the movement.

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Slide 11.143. Salt sheet with curved base and top, 10 mm/a, basal salt layer. Thismodel adds a basal salt layer to the model in Slide 11.142. New layers of sedimentwere deposited every 10 000 a. Because of the concentrated thinning of the subsaltcountry rock, the total duration of the model was only 70 000 a, resulting in onlyabout one-fifth the extension of Slide 11.142. Synkinematic sedimentation did notgreatly affect the results of the model in this slide. Thin layers of new sedimentaccumulated on the regional country rock to balance salt flowing centerward in thebasal layer. The downward-propagating reactive diapir below the sheet pierced agreater proportion of subsalt country rock to form a steeper walled contact andterminating the run earlier than in the prekinematic case (Slide 11.121). Piercementrate increased because the greater load further pressurizedthe salt. A similar effect occurs for an upward-propagating reactive diapir if differen-tial load outside the reactive graben increases pressure in the salt, or equivalently,if surface load decreases by thinning of the floor of the reactive graben, whichincreases the pressure difference between the salt and the surface.

Slide 11.144. Inverted salt sheet with curved base and top, 10 mm/a, basal salt

layer. Layers of sediment were deposited every 15 000 a to the model in Slide

11.122 for a total duration of 120 000 a (seven new layers). As in Slide 11.143 with abasal salt layer, synkinematic sedimentation did not greatly affect the results of themodel in this slide. New layers of new sediment smoothed the downward bendingof the roof toward the center, diminishing the strain due to local flexures. Additionalload on the reactive graben floor at the center kept the underlying reactive diapirpointed, rather then broadening as it neared emergence in Slide 11.122.

Slide 11.145. Salt sheet with flat base and top, 10 mm/a. Layers of sediment weredeposited every 25 000 a to the model in Slide 10.81–82 (previous set) for a totalduration of 262 000 a (ten new layers). The largest change from the prekinematiccase was the reversed and enhanced asymmetry of the subsalt graben. Extreme fallof the sheet’s roof at its ends completely inverted the original shape of the sheet. Saltsqueezed out of the peripheral “horns” marking the high point of the salt, leavingsalt welds at the ends. Shear of perturbed zones and overturn of rising salt can beseen in the outer parts of the sheet (compare the middle and end stages).Depocenters in the oldest synkinematic layers record a shift of maximum subsidencerate from above the subsalt graben to the central drape grabens. Subsidence withinthe central reactive graben accelerated with time.

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Slide 11.146. Salt sheet with flat base and top, 10 mm/a, basal salt layer. Layers ofsediment were deposited every 20 000 a to the model in Slide 11.134 for a totalduration of 200 000 a (nine new layers). The synkinematic sediments damped flowinto reactive diapirs in the basal layer by removing surface relief, which almosttotally prevented the marked upwarping of the ends of the subsalt country rockseen in Slide 11.134. Basal salt flowed to the center to isostatically balance the saltsheet. This flow dropped the regional country rock, making it an accumulation zone.As a result of this increased differential loading and continuing isostatic adjustment,the center of the model eventually rose above the original surface elevation andunderwent erosion of up to 30 m of older layers at each stage of sedimentationelsewhere. This erosional thinning caused the normal faults in the roof to migratecenterward with time to minimize their length and frictional resistance. Therefore,the thickest zone of the synkinematic sedimentary layers also shifted centerwardthrough time.

Text Figures 11.147–11.153. Strain contours and passive lines for complete models

series. The following six foldout figures (Text Figures 11.148–11.153) show the resultsfrom the complete series of 40 models with only prekinematic sediments.

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Text Figure 11.147 shows the layout of models in each of the foldout figures, arrangedby the initial shape of the salt sheet and displacement rate. To save space, only the halfof the models actually run (to the right of the central symmetry line) is shown in thefigures. The contour ranges in Text Figures 11.148–11.151 corresponds to that in theslides discussed above, but with only 10 contour intervals instead of the 14 intervals inthe slides. The undeformed salt contacts are superimposed on each model as thin dottedlines. Because models differed in their total amount of extension, the total simulatedtimes differ for each of the models. The initial spacing of passive material lines inText Figures 11.152 and 11.153 correspond to those in the models of the slides(see description immediately preceding Slide 11.118).

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Text Figure 11.148. This figure shows contoured plastic strain in models with a basaldécollement at approximately their middle stage of development.

Text Figure 11.149. This figure shows contoured plastic strain in models with a basaldécollement at their termination.

Text Figure 11.150. This figure shows contoured plastic strain in models with a basalsalt layer at approximately their middle stage of development.

Text Figure 11.151. This figure shows contoured plastic strain in models with a basalsalt layer at their termination.

Text Figure 11.152. This figure shows passive lines representing layering in modelswith a basal décollement. Models deformed at the moderate displacement rate(10 mm/a), first and third columns, appear at their termination. Because structurallyirrelevant strain anomalies accumulated in the thousand of iterations required forthe slow displacement models, the passive line plots in the salt became progressivelymore contorted with run time. These perturbations didnot seem to affect the overall structural development, but they obscure the flowpatterns in the salt. For this reason, the slow displacement models (second andfourth columns) are shown at approximately their intermediate stage rather than attheir termination.

Text Figure 11.153. This figure shows passive lines representing layering in modelswith a basal salt layer. Moderate displacement rate models appear at theirtermination, slow displacement rate models appear at approximately their inter-mediate stage.

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Part 2: Animations

Introduction

Each animation produced at AGL concentrates data from hundreds of images into one,easily visualized, compact presentation. Accordingly, the time and effort required tocomprehensively absorb complex concepts are reduced by orders of magnitude.

Animations can be effectively used for training. Viewers learn at their own paceby replaying animations until movement patterns are absorbed. Playing animations atnormal speeds shows structures and processes in detail, whereas accelerating playbackby manually dragging the control slider emphasizes overall displacements.Geoscientists can replay in seconds the complex experiments that took days or weeks todeform.

Animations also allow viewers of differing expertise (e.g., sedimentologists,structural geologists, geophysicists, managers) to grasp quickly even structurallycomplex simulations or restorations. This promotes multidisciplinary cross-fertilizationof ideas on the timing of oil migration and development of structural traps.

The current report includes four animations: a restoration of a physical model,a restoration of a seismic section, and two finite-element models. These respectivelyillustrate the following aspects of salt tectonics: (1) evolution of a complex allochthonoussalt sheet, (2) raft tectonics, (3) reactive diapirism, and (4) subsalt structures.

Obtaining animations

Most of our Industrial Associates can obtain and play our animations by following theinstructions in the section “(a) Users who have previously imported our animations” below.Industrial Associates who joined us after March, 1994, should refer the section “Users

who have NOT previously imported our animations” instead. Please contact GiovanniGuglielmo (E-mail: giovanni@mail.utexas.edu, Tel: 512 471-6373) if you have anyproblems in obtaining or running these animations.

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(a) Users who have previously imported our animations

To obtain and play our animations, please locate the software included in the Macintoshfloppy disks provided to our Industrial Associates in our last report of March, 1994.Then, follow the steps below to “Upgrade Fetch and AGL access key”, then “Get the

animations”.

Upgrade Fetch and AGL access key

1-Double click on the AGL Access icon.

2-Click OK button.

3-Click OK button if an error message appears.

4-Double click on AGL folder name.

5-Click on file names Fetch2.1.2 and AGL_access_8_94 to select files (press shift

key for multiple selection).

6-Click Get files button.

7-Click Save button; The files will be saved by default in the same folder thatFetch is.

8- Choose Quit from the File menu.

9- Drag your old Fetch2.1.1 and AGL Access icons to the Trash, and chooseEmpty Trash from the Finder’s Special menu.

Get the animations

1-Double click on AGL access 8/94 icon, click OK.

2-Double click on movies folder name.

3-Click on file names to select files (press Shift key for multiple selection).

4-Click Get file button.

5-Click Save button.

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NOTE: Animation files will be saved by default in the same folder as Fetch.Depending on your network, it may take several minutes or longer to transferthe files. So you may prefer to let Fetch run unattended during lunch or over-night. Some companies have internal security software that interferes withFetch’s operation. Please contact Giovanni if you cannot use Fetch.

6- Quit Fetch from the File menu.

7-Please do not keep copies of the file AGL_access_8_94 in your Mac. Put copiesof it in two floppy disks and keep them in a safe place.

(b) Users who have NOT previously imported our animations

Industrial Associates who have joined us after March, 1994, (Statoil and The LouisianaLand and Exploration Company) are receiving animations for the first time. For thoseAssociates we include two Macintosh floppy disks containing updated instructions,computer requirements, and software to obtain our animations through the Internetand play them in Macintosh computers.

Playing animations in low-end Macintoshes

Quicktime 2.0 will improve playback of our animations on both high-end and low-endMacintoshes without extra hardware, according to Apple. This improved version ofQuicktime is included with the Macintosh System 7.5 released by Apple in August, 1994.

Playing animations in PCs soon

By our next report we expect that you will be able to run the animations in PCs. Wehave slightly changed the format of our animations, which now have file names withthe extension “.mov” . They still run on Macintoshes, but you will also be able to playsee “.mov “ animations in IBM PCs under Windows 3.1 when Quicktime for Windows2.0 is released soon. We will keep you informed.

Feedback request

We would like you to use these animations as effectively as possible for teaching,research, or brainstorming. If you let us know how these animations are being used

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within your company, we can select future themes, captions, features, distributionmedia, and formats to accommodate your needs as much as possible. Also, please let usknow if you have releasable sequential images from which we could create animations.Please send even minor comments on animations to:

Dr. Giovanni Guglielmo Jr.E-mail: giovanni@mail.utexas.eduFax: (512) 471-0140Tel: (512) 471-6373Bureau of Economic GeologyUniversity of Texas at AustinUniversity Station, Box XAustin, Texas 78713-7508

Animation Captions

BREAKOUT.mov

This animation of a computer-restored section from a scaled physical model illustratesthe complex evolution of an allochthonous salt sheet during progradation. Only theright part of the experiment (yellow rectangle) shown in the opening photograph wasrestored and animated. Throughout the experiment, synkinematic sediments (sand)prograde from left to right across a flat-topped horizontal salt basin or sheet (viscoussilicone animated in black) that pinches out seaward below a thin prekinematic layer(sand, animated in light and dark blue). The prekinematic layer at the beginning of theanimation comprises irregular fragments (deformed fault blocks) reassembled into thenearest semblance of its undeformed state as a horizontal, tabular layer. The resultingdeformation can be described in nine sequential stages. The section “Potential structural

traps for hydrocarbons” suggests how structural traps could be created or destroyed ateach stage. Percentages of elapsed time listed below show the approximate age andduration of each stage with respect to the entire history.

Stage 1 (0-12% time elapsed): At the onset of deformation, the weight of overlyingprograding sediments (visible in the opening photo) pressurizes the salt andsqueezes it seaward (to the right). The pressurized salt lifts and deforms the pre-kinematic layer into a seaward-verging bending (not buckling) box fold.

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Stage 2 (12-16% time elapsed): A foregraben and a backgraben form at the crests ofthe box fold. These grabens were very clear in top view of the actual experiment butare not readily distinguishable in the restoration because of later faulting in the roofof the box fold. The grabens are best recognized in the animation by their latereffects: the foregraben becomes the site of the first breakout of salt in Stage 4; thebackgraben becomes overlain by locally thick, dark-green strata in Stage 3. Thesegrabens locally cause differential loading and structural weakening of theoverburden, triggering reactive diapirs below each graben.

Stage 3 (16-22% time elapsed): The oversteepened forelimb of the box fold collapsesand sheds pale-blue sediments into a local basin at the base of the forelimb.

Stage 4 (22-40% time elapsed): The weight of prograding sediments continuouslyincreases salt pressure. The reactive diapir in the foregraben becomes active andemergent. Salt expelled from the foregraben (breakout 1), flows seaward down theresedimented forelimb. Traction exerted by the base of the extrusion folds thepartially collapsed forelimb into a small isoclinal recumbent syncline. The rear diapir(below scale mark 2-6) continues to rise reactively. Extrusive salt spreads radially aslobes in the abyssal plain that resemble piedmont glaciers.

Stage 5 (40-44% time elapsed): These salt flows coalesce laterally to form a canopy(allochthonous sheet 1) with a scalloped leading margin (not visible in vertical section).The diapir at the backgraben evolves to a passive stage after emerging. The roofand backlimb of the box fold are stretched by the underlying salt and segmentedinto two rafts that migrate seaward as overthrusts carried by the salt sheet. Theoverthrusting rafts above the isoclinal syncline create a triple stratigraphic repetitionof the prekinematic layer.

Stage 6 (47-51% time elapsed): Further sedimentation (orange and yellow layers)buries these allochthonous rafts, the diapir at the backgraben, and the salt extrusion.This confinement of allochthonous sheet 1 causes it to inflate actively because ofcontinuous salt expulsion from the left.

Stage 7 (51-62% time elapsed): Pressurized salt eventually breaks through the newlydeposited yellow sediments (Breakout 2, below scale mark 20), creating a secondallochthonous salt layer. This second breakout causes local folding and slumping

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downslope from, and next to, the breakout point (not visible at the scale of theanimation).

Stage 8 (62-72% time elapsed): The salt layer is completely covered by further sed-imentation (yellow). More extension causes differential loading and thinning of theyellow overburden triggering a large seaward-dipping listric fault (below scale mark12), and two small reactive diapirs at the seaward edge of the basin, which continueto evolve in Stage 9.

Sedimentary loading and evacuation of salt enhance subsidence of the reardiapir (below scale mark 7) as salt escapes seaward through breakouts 1 and 2. Bothflanks subside and tilt toward the diapir as the yellow layers locally thicken above itscrest. Continuous rotation of the left footwall ultimately produces a structure thatresembles a counter-regional growth fault along the left flank of the diapir at the leftend of the animation window. However, no slip occurred along this pseudo-fault.

Stage 9 (72-85% time elapsed): The weight of overlying sediments, combined withoverthrusting, join strata at the breakout conduits created in Stages 4 and 7,restricting flow of salt, and eventually producing salt welds.

Remaining time (85-100% time elapsed): The animation dissolves back into thephotograph of the final cross section.

Potential structural traps for hydrocarbons: Each stage described above could create ordestroy structural traps for hydrocarbons. The following assessment is based on thetwo-dimensional section and would depend on structural or stratigraphic closure in thethird dimension and on the existence of reservoirs.

Stage 1: Shaly facies of prograding sediments may seal the faulted culmination of thebox fold. Local traps for hydrocarbons could form in crestal culminations of this foldif the box fold consisted of reservoir rocks.

Stage 2: The reactive salt wall beneath the backgraben could produce diapir-relatedstructural traps (a) where upraised sediments are truncated against the deepestflanks of the diapir; (b) in graben footwalls around and above the diapir.

Stage 3 and 4: Newly migrating hydrocarbons could be trapped at the breakout ofthe pressurized salt. These traps could be (a) within the forelimb of the box fold, and(b) within sediments slumped from this crest. Both these traps are local structural

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highs sealed by the overlying salt flow. These salt-breakout traps also form at theedge of the basin during Stage 7.

Stage 5 and 6: The salt flow (allochthonous sheet 1) could produce traps by sealingprimary topographic highs of underlying prekinematic sands downslope. Thesebroad traps become even larger as flows coalesce into canopies. After the ends ofthe rafts subside to form extensional turtle structures (when the seaward raftreaches scale mark 20), hydrocarbons could be trapped in their culminations.Additionally, cemented fault planes and prograding muds draping rafts (e.g., belowscale mark 13) may provide the necessary seal for a trap. These rafts could formisolated reservoirs between widely spaced drill-holes or seismic sections.

Stage 7: As in stages 5 and 6, this second salt sheet could also seal underlyingstructures creating additional subsalt traps. Salt-breakout traps could form as inStages 3 and 4: Folding and slumping associated with the salt breakout creates alocal structural high downdip, which is immediately sealed by the overlying saltflow to originate a potential subsalt trap.

Stage 8: The two late diapirs at the seaward edge of the basin (below scale marks26 and 31) could produce diapir-related structural traps as described in Stage 2.

Also, gradual inward tilting of flanking strata toward the backgraben reactive diapir(below scale mark 7) could redirect hydrocarbons away from the original traps atthe overburden/diapir interface. Hydrocarbons would migrate (a) landward, intothe broad crest of the pseudo-rollover anticline shown in the opening photograph,and (b) seaward, to be possibly trapped in the footwall of the next seaward-dippinggrowth fault (below scale mark 12).

Stage 9 : Complete evacuation of impermeable salt during welding could juxtaposepermeable strata from both sides of the salt layer. This would create windowsthrough which hydrocarbons could drain upward away from the subsalt reservoirsformed at Stages 3, 4, and 7. The latest draping of sediments (light yellow) over thestructural high between the larger sagging salt bodies (the backgraben diapir andthe first salt flow formed at Stage 5) might create an anticline and associated onlaps(below scale mark 11). This anticline could be a structural trap, whereas the onlapscould form stratigraphic traps.

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The animation was digitally interpolated from 14 key restoration frames. We areindebted to Dr. J. R. Hossack (BP Exploration) for supplying his Locace restoration,which represents 8 of these key frames. An additional 6 were created by us. Physicalmodeling directed by B. Vendeville (Jackson and Vendeville, 1993; Slides 6.53 to 6.70

of Slide Set 6, see also Slides 11.38 to 11.72). The 14 s of animation represent 54 h ofthe original experiment.

KNZARAFT.mov

This animation of a computer-restored seismic section from the Kwanza Basin, Angola,illustrates aspects of raft tectonics during thin-skinned extension. Before restoration,the time-migrated section was roughly depth converted by hand.

Stage 1 (0-7% time elapsed): Early extension creates a reactive diapir, which couldproduce typical diapir-related hydrocarbon traps.

Stage 2 (7-12% time elapsed): This diapir emerges actively through the dark-blueoverburden, potentially compartmentalizing any reservoirs in its roof.

Stage 3 (12-27% time elapsed): During deposition of the green layer, the diapir growspassively in the widening gap between two isolated fault blocks, called rafts. Subtletilting of the rafts at this stage or throughout the deformation history could redirectthe flow of oil (a) away from the diapir to be trapped by sealing extensional faultswithin each raft, or (b) toward the diapir to accumulate at the diapir-raft interface.

Stage 4 (27-43% time elapsed): Eventually, widening of the diapir and constriction ofsalt flow diminishes the rate of salt supply. Consequently, during deposition of theyellow layer, the diapir starts subsiding and accommodates a local depocenter aboveit.

Stage 5 (43-68% time elapsed): This depocenter evolves into a half-graben boundedon the right by a major growth-fault. Clockwise tilting of the right-hand raft coulddirect hydrocarbons from the green raft to the footwall of the half graben. Thehalf-graben hangingwall depocenter consistently rotates clockwise, which favorsmigration of any hydrocarbons away from the growth fault, to be trapped againstthe structurally higher left boundary of the graben, where strata are bent downto the right, forming a keystone graben in the olive layer. The age of the oldestsediment in the depocenter core indicates (a) the time when the original diapir

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began to subside, and (b) the earliest time that hydrocarbons could have migratedinto the half graben (perhaps along strike). The diapir shrinks in cross-sectional area,perhaps partly by dissolution while the salt was shallow, but most likely by flow outof the plane of section when the salt was deeply buried. The final width of the half-graben is roughly proportional to the amount of extension since rafting began.

Stage 6 (68-82% time elapsed): Ultimately the diapir splits into two smaller remnantdiapirs, and the Tertiary depocenter (yellow to brown) welds directly on the pre-saltCretaceous strata (white) producing a stratigraphic jump of 60-90 Ma (Duval et al.1992). Remaining elapsed time (82-100%) dissolves back into the seismic section.

Animation based on Restore restoration by Schultz-Ela (1992) of a seismic sectionprovided by Total S. A. (see also Slides 4.63 and 4.64 of Slide Set 4). Duration: 16 s.

REACTGEO.mov

Animation of a finite-element (GEOSIM-2D) model of a reactive diapir produced byregional extension. The initially blue layer is the brittle overburden, overlain by air.The white substratum is viscous salt. Plastic strain increases from blue to red contoursand indicates zones of fracturing and faulting. A relatively low contour minimumhighlights the oldest faults. Salt viscosity is 1 ∞ 1018 Pa.s. Material properties are typicalof natural rocks (see introductory caption for Slides 8.9 to 8.23 of Slide Set 8). 5.5 kmof sedimentary rocks overlies 4 km of salt. The base of the overburden thickenedgradually toward the ends of the 48-km model along a smooth elliptical curve. Only aregion 15 km wide from the central part of the model is shown in the animation. Thismodel illustrates how changing stress fields may alter the sequence and orientation offaults, changing an initially asymmetric structure into a symmetric one. This change insymmetry, which is a common characteristic of our finite element models simulating awide range of initial geometries, affects fluid migration paths and the geometry oftraps.

Stage 1 (0-15% time elapsed): The overburden initially failed along a single left-dippingfault.

Stage 2 (15-33% time elapsed): The hangingwall accommodates further extension byflexing downward, which creates an incipient extensional graben to the left of themain fault. The footwall flexes slightly up, which creates a small horst at the base of

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the blue layer. The upward flexure produces a subtle but broad structural high thatcould direct hydrocarbon migration from the right to the center of the model. Anyhydrocarbons would be trapped in the footwall mostly against the major left-dipping fault but also against the faults of the horst. The overall structure at thisstage is a highly asymmetric salt roller overlain by a half graben.

Stage 3 (33-75% time elapsed): A new right-dipping fault forms in the hangingwall,surprisingly at an angle of about 15° from the existing outermost fault of theextensional graben. Stress concentrations at the tip of the newly formed reactivediapir probably alters the overall stress field to produce the fault in a neworientation instead of following the existing zone of weakness. The new faultquickly accumulates most of the current slip and, with the major left-dipping fault,forms a graben. The underlying asymmetric salt roller evolves into a symmetricreactive salt wall. Despite the small – now inactive – horst immediately to the rightof this graben, the overall structure is at this stage symmetric.

Stage 4 (75-100% time elapsed): All the faults and fractures contribute to weakeningand thinning the overburden, which locally reduces the load on the salt layer. Theseprocesses cause a symmetric reactive diapir to rise where the overburden is thinnestand weakest. The rate of diapiric rise is increased by high extension rates and lowsalt viscosities. At this stage, any hydrocarbons would tend to migrate laterallyinward toward the uplifted strata on the flanks of the diapir. Oil could be trappedagainst (a) the diapir, (b) the two faults bounding the major graben, and (c) the rightside of the inactive fault that bounds the small horst.

Extension rate is 10 mm/a (10 km/Ma) for a total extension of 1.95 km. The 7 s ofanimation represent 195,000 a.

SUBSALT.mov

This animation shows a finite-element (GEOSIM-2D) model of regional extension of asedimentary sequence containing an isolated salt sheet. The salt sheet (white) pinchesout laterally, and is encased in sedimentary rocks (blue) that overlie a continuous,tabular basal salt layer (white). When undeformed, the sheet has a flat basal contact anda gently elliptical upper contact. Plastic strain increases from blue to red contours andindicates zones of fracturing and faulting. Salt viscosity is 1 ∞ 1018 Pa.s, and density

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contrasts are typical of rocks in real sedimentary basins (see introductory caption forSlides 8.9 to 8.23 of Slide Set 8).

Stage 1 (0-22% time elapsed): Initially, regional extension creates a symmetric grabenin the thinnest part of the overburden above the salt sheet. Below the sheet,extension produces faults that propagate downward from stress concentrations nearthe ends of the sheet. These subsalt faults act like basement faults to initiate newasymmetric grabens offset laterally above the sheet to each side of the centralgraben. Extension triggers low, reactive salt rollers above and below the salt sheet.

Stage 2 (22-38% time elapsed): In the middle stages, two horst-and-graben pairs,comprising three faults each, appear beneath the salt sheet. Slip on the three faultsexactly balances to produce only a single step in the top and bottom contacts of thesubsalt country rock (see also Slide 11.127). In a seismic section, this step could bemistakenly interpreted as created by a single fault. This misinterpretation can beavoided if a diapir can be detected below the salt sheet in the seismic section becausethe reactive stage of this diapir would have produced additional faults.

Stage 3 (38-90% time elapsed): Stress concentrations produce incipient symmetricgrabens above the edge of the salt sheet. These grabens allow stretching, whichenhances inward tilting of the suprasalt overburden. The displacement of blocksabove and below the salt sheet changes the shape of the salt sheet itself. The sheetdeforms into a broad, stair-stepped, anticline.

Stage 4 (90-100% time elapsed): The thin residual roof, between a subsalt diapir andthe salt sheet, arches, indicating that even active diapirs may grow below salt sheets.A seismic section of the final stage of deformation could mistakenly suggest that thesubsalt diapirs were stems that fed the overlying salt sheet. The location of subsaltdiapirs is not necessarily controlled by the diapirs of the salt sheet. However,grabens above the salt sheet appear to have been controlled, in part, by theinitiation of subsalt symmetric and asymmetric grabens. The position of theseshallow grabens could indicate the position of subsalt diapirs and associated oil traps(see caption of Slide 11.127 for details).

One second of animation represents 30 000 a. Model is 20 km wide by 6.5 km deep,and extension rate is 10 mm/a. The 12 s of the animation represents 362 000 a ofdeformation.

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References

Duval, B., Cramez, C. and Jackson, M. P. A., 1992. Raft tectonics in the Kwanza Basin,Angola: Marine and Petroleum Geology, v. 9, p. 389-404.

Hossack, J. R., 1993. Geometrical rules of section balancing of salt structures in the Gulfof Mexico (abs.): In Jackson, M. P. A., Roberts, D., and Snelson, S. (eds.), Salt Tectonics:AAPG International Research Conference, Bath, England, unpaginated.

Jackson, M. P. A., and Vendeville, B. C., 1993. Extreme overthrusting and extensionabove allochthonous salt sheets emplaced during experimental progradation (abs):American Association of Petroleum Geologists 1993 Annual Convention Program, p.122-123.

Jackson, M. P. A., Vendeville, B. C., and Schultz-Ela, D. D., 1994. Structural dynamics ofsalt systems: Annual Review of Earth and Planetary Sciences, v. 22, p. 93-117.

McGuinness, D. B. and Hossack, J. R., 1993, The development of allochthonous saltsheets as controlled by the rates of extension, sedimentation, and salt supply: Society ofEconomic Paleontologists and Mineralogists, Gulf Coast Section, 14th Annual ResearchConference Program and Extended and Illustrated Abstracts.

Nilsen, K. T., Vendeville, B. C., and Johansen, J.-T., in press. Influence of regionaltectonics on halokinesis in the Nordkapp Basin, Barents Sea: In Salt Tectonics, AAPGMemoir.

Schultz-Ela, D. D., 1992. Restoration of cross sections to constrain deformation processesof extensional terranes: Marine and Petroleum Geology, v. 22, p. 372-388.

Triantafyllidis, N, and Leroy, Y. M., 1994. Stability of a frictional material layer restingon a viscous half-space: Journal of Mechanics and Physics of Solids, v. 42, p. 51-110.