DUCTILE AND BRITTLE EXTENSION IN THE SOUTHERNLOFOTEN ARCHIPELAGO, NORTH NORWAY:

IMPLICATIONS FOR DIFFERENCES IN TECTONICSTYLE ALONG AN ANCIENT COLLISIONAL MARGIN

ANDRE C. KLEIN*‡, MARK G. STELTENPOHL*, WILLIS E. HAMES*,and ARILD ANDRESEN**

ABSTRACT. The Lofoten archipelago, north Norway, occupies the most inter-nal position of the Caledonian belt in northern Scandinavia, and rocks andstructures exposed there are crucial to understanding processes of how the Balticbasement and its cover allochthons responded to continental lithospheric subduc-tion and subsequent continental separation. Relatively little is published aboutthe structural and metamorphic development and especially the timing of theseevents; consequently, it is unknown how features exposed on these spatiallyisolated islands relate to those of the adjacent mainland. Rocks in Lofoten wereaffected by Caledonian regional metamorphism, and structures record tops-eastcontraction and later extension related to late- to post-Caledonian basementexhumation. Tops-west extension is preferentially developed in meter to km-scale ductile shear zones containing west-dipping extensional shear bands, west-verging rootless folds, and asymmetric feldspar porphyroclasts. West-plunging,sinistral-oblique elongation lineations in the mylonitic foliation are interpretedto indicate the line of transport. Mesoscopic backfolds are locally developedwithin the tops-west shear zones and further document west-directed transport ofstructurally higher rocks. The ductile extensional shear zones locally are cross cutby low-angle, tops-west cataclastic normal faults, reflecting progressive unroofingof the shear zones to shallower crustal levels during late- and post-Caledonianextensional events. Northeast-striking, high-angle, brittle normal faults truncateall other fabrics and structures and juxtapose structurally deep undeformedPrecambrian basement with the Caledonian nappe sequence. 40Ar/39Ar dataindicate that ductile extension in Lofoten likely initiated soon after the Caledo-nian metamorphic peak (at �430 Ma) and continued episodically until at leastPermian time. Rocks passed through the brittle/ductile transition rapidly at �275Ma. Recent paleomagnetic data also point to later phases of brittle faulting inLofoten, one in the Jurassic/Cretaceous and one related to Tertiary opening ofthe Norwegian Sea. The magnitude and style of late- to post-Caledonian exten-sion and basement exhumation in Lofoten differs markedly from the formation ofphenomenal Devonian basins and exposures of Caledonian ultra-high pressureassemblages in the Western Gneiss Region (WGR) of southern Norway. Differ-ences in the styles of extension between northern and southern Norway areinterpreted to reflect inherited differences in crustal architecture that may havebeen enhanced during the Caledonian orogeny.

INTRODUCTION

Extensional structures and fabrics in the Western Gneiss Region (WGR) of south-western Norway expose deep sections of continental lithosphere exhumed followingcontinent-continent collision (Norton, 1986; Seranne and Seguret, 1987; Andersen andJamtveit, 1990; Andersen and others, 1991). Comparable crustal-scale extension hasbeen reported in the Caledonides of East Greenland (Hartz and Andresen, 1995). Overthe past decades there has been little record of syn- to post-Caledonian extensionreported in the basement of northwestern Norway, although it has recently beenrecognized in the Lofoten archipelago (Holloman and others, 1995, 1996; Hames and

* Department of Geology, Auburn University, Auburn, Alabama 36849‡ Present address: Department of Geology and Geophysics, Rice University, MS-126, 6100 Main Street,

Houston, Texas 77005-1892** Department of Geology, University of Oslo, P.O. Box 1047, Blindern, 0316 Oslo, Norway

[AMERICAN JOURNAL OF SCIENCE, VOL. 299, JANUARY, 1999, P. 69–89]

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Andresen, 1996a; Holloman, ms; Klein, ms; Mooney, ms; Waltman, ms). The evolution-ary history of the northern WGR (Lofoten block) differs strikingly from its southernWGR counterparts, especially in terms of metamorphism, deformation, pressures at-tained during subduction, and residence time in the footwall of a major fault zone. Forexample, the WGR of southern Norway was penetratively-deformed and subjected toultra-high pressure (UHP) metamorphic conditions during the Caledonian orogeny(Bryhni and Sturt, 1985). Many basement exposures in the Lofoten region of northernNorway, however, appear to have experienced little or no Caledonian deformation(Hakkinen, ms; Griffin and others, 1978; Tull, 1978; Bartley, 1981, 1982). The differ-ences between the WGR and Lofoten block are enigmatic given that the two areas have asimilar structural setting. Explanations for these differences are examined herein basedon the results of new structural studies aimed at characterizing the Caledonian and latertectonic evolution of the southern Lofoten islands (Holloman and others, 1995, 1996;Holloman, ms; Klein, ms; Mooney, ms; Klein and Steltenpohl, 1999). This paper focuseson the islands of Vestvågøy, Værøy, and Røst (fig. 1), because they are the only islands insouthern Lofoten with exposures of aluminous metasedimentary rocks, making themideal for an integrated structural, geochronologic, and thermobarometric study.

Synchronous contraction and extension have been documented from severalorogenic belts around the world (the Higher Himalaya, Burchfiel and Royden, 1985;southwestern Norway, Norton, 1986; the Variscan belt in Europe, Steltenpohl andothers, 1993), and these examples have added greatly to our understanding of thestresses present in an evolving orogen. Modern subduction zones, like the one along thewestern coast of North America, contain a great deal of along-strike variability. Suchvariability in ancient subduction zones is commonly masked by later deformation orunfavorable levels of erosion, but the Norwegian Caledonides, with excellent exposureof subequal amounts of basement and cover, provide a paramount opportunity to studyan ancient subducted margin. The ancient A-type subduction zone boundary (Hodgesand others, 1982) is exposed along the west coast of Norway and has been the subject ofnumerous reports. Most studies have focused on the southern WGR, where large-scaleextension driven by post-Caledonian gravitational collapse of the orogen apparently ledto the formation of the Devonian molasse basins and juxtaposed them against basementeclogitized during Caledonian subduction (Norton, 1986; Seranne and Seguret, 1987;Andersen and Jamtveit, 1990; Andersen and others, 1991).

It must be noted here that the final closure of Iapetus during the Silurian involvedboth contractional and extensional stresses at various levels of the Caledonian nappestack at various times (Andersen and others, 1991; Northrup, 1996). Therefore, thedistinction between Caledonian and post-Caledonian, as used in this paper, is not relatedto an absolute change in stress regimes. The boundary is drawn, instead, at the end ofprograde regional metamorphism of the Baltic basement and its overlying cover at �430Ma (Steltenpohl and Bartley, 1988; Hames and Andresen, 1996a; Northrup, 1996;Dunlap and Fossen, 1998).

Recent detailed geologic mapping of the southern Lofoten archipelago has revealedthat post-Caledonian extension significantly modified the earlier contractional edifice(Holloman, ms; Klein, ms; Mooney, ms; Waltman, ms; Klein and Steltenpohl, 1999). Ofsingular interest is how the continental margin at the present-day latitude of Lofoten(68°N) responded to the waning stages of continent-continent collision and subduction todepths approaching 40 km. In contrast to the spectacular examples of deep crustalexhumation reported from southern Norway, mentioned above, a master extensionaldecollement (Nordfjord-Sogn Detachment Zone of Andersen and Jamtveit, 1990) hasnot been reported. In addition, Lofoten does not expose UHP assemblages of Caledo-nian age, such as the coesite-eclogite facies basement of the WGR of southwest Norway(Griffin and Brueckner, 1980, 1985; Gebauer and others, 1985). Given that the rocks

Andre C. Klein and others—Ductile and70

exposed in these two terranes are lithologically similar and occupy a comparablestructural setting, why did they behave so differently in response to subduction andsubsequent eduction? This paper addresses this question in light of new structural,petrologic, and geochronologic data from the southern Lofoten archipelago.

Fig. 1. Location of the Lofoten archipelago and study area. Lithologies exposed in Lofoten are primarilyPrecambrian basement gneiss of the Lofoten block (light gray) and allochthonous metasedimentary rocks of theLeknes group (black). The Ofoten region of north Norway is indicated.

brittle extension in the southern Lofoten archipelago, north Norway 71

GEOLOGIC SETTING

The Lofoten archipelago (68°N) is a basement ridge in the north Norwegian shelf,associated with gravity, magnetic, and topographic highs. Exposures in Lofoten repre-sent the most distal component of the Caledonides in the north Norwegian margin(fig. 1). The Caledonides are composed of a variety of vertically stacked nappes, thrustfrom west to east on to the Baltoscandian platform during partial subduction beneathLaurentia in early to middle Paleozoic time (Gee, 1975; Roberts and Gee, 1985). Therelationship of the Lofoten region to the Caledonides on the Norwegian mainland hasbeen debated since detailed mapping of the area was begun in the early 1970’s (Griffinand Taylor, 1978; Tull, ms and 1978), although it is now generally accepted that Balticbasement exposed on the mainland is continuous with Lofoten basement (Olesen andothers, 1996). The Lofoten block is composed of primarily Precambrian basement gneiss(Foslie, 1941; Vogt, 1942; Andresen and Tull, 1986). Griffin and others (1978) reportareally extensive mangeritic and charnokitic intrusions in Lofoten dated at �1700 to1800 Ma. The so-called ‘‘mangerite suite’’ was emplaced under granulite-facies (clinopy-roxene-zone) conditions. The Lofoten basement was locally retrograded to amphibolite-facies in shear zones accompanying allochthonous Caledonian rocks, although, as awhole, the basement was little-affected by Caledonian reheating (Griffin and others,1978).

In central Lofoten, allochthonous cover rocks, informally known as the ‘‘Leknesgroup’’ (Tull, ms and 1978; Sigmond and others, 1984), lie in thrust contact withPrecambrian gneisses and granitic plutons typical of the northern WGR exposed in theLofoten islands (Griffin and others, 1978). Based on poorly-constrained Rb-Sr and K-Ardates from the Leknes group, Tull (ms and 1978) and Griffin and others (1978) suggestedthat the Leknes group was emplaced onto the basement during an amphibolite-faciesmetamorphic event that occurred between 1200 to 1100 Ma. Tull (ms) indicated that themetamorphism (his M2) of the Leknes group is unrelated to the Caledonian metamor-phism described by Bartley (ms) on Hinnøy (�100 km northeast of Vestvågøy). Newisotopic data, however, indicate development of amphibolite-facies porphyroblasts inschists and amphibolites of the Leknes group occurred during Caledonian metamor-phism (Hames and Andresen, 1996a, b). Records of this event are variably overprintedby subsequent retrogressive events (Klein and Steltenpohl, 1999).

The type locality of the Leknes group on Vestvågøy is composed of aluminousschist, calc-silicate gneiss, quartzite, marble, amphibolite, and voluminous quartzofelds-pathic mica schist (fig. 2). Two lithologically distinct sequences are found within theLeknes group on Vestvågøy (Klein, 1997). The ‘‘southern units’’ (fig. 3) are in thrustcontact with the mangeritic basement and contain the bulk of the quartzofeldspathicmica schist and amphibolite, as well as thin (�5 m) layers of quartzite, marble, andcalc-silicate gneiss. The ‘‘northern units’’ have been faulted over the southern units andcontain predominantly amphibole-rich schist, two-mica schist, and quartz-rich amphibo-lite.

STRUCTURAL DEVELOPMENT

The structural development of the southern Lofoten archipelago is outlined in table1 and covers mainly the Caledonian and later history of the region. Due to the localintensity of Caledonian deformation and metamorphism, evidence relating to thepre-Caledonian geologic history has been largely obscured near Caledonian shearzones. With depth below Caledonian thrusts or across post-Caledonian normal faultswith large displacements, Caledonian deformation generally is absent, and Proterozoicstructures remain intact. In general, the pre-Caledonian geologic history is dominated bydevelopment of the Baltic Shield and the northern WGR by a series of orogenic and

Andre C. Klein and others—Ductile and72

Fig. 2. Lithotectonic map of the study area on Vestvågøy and diagrammatic cross section A-A� (samehorizontal scale as map). In general, the Leknes group is a complexly interlayered and anastomosing sequenceof variably felsic and mafic schists and gneisses with minor amounts of quartzite, marble, and amphibolite. Seefigure 3 for a measured section of part of the Leknes group.

brittle extension in the southern Lofoten archipelago, north Norway 73

intrusive events that are difficult to evaluate in rocks of the study area due to the highmetamorphic grade and localized migmatization of many of the rocks.

Caledonian orogeny.—The onset of Caledonian orogeny in the Lofoten region ischaracterized by east-directed thrusting of Leknes group assemblages onto the Balticcraton. Thrusting was accompanied by local retrogression of basement rocks fromgranulite-facies to amphibolite-facies metamorphic assemblages. Metamorphic grade ofallochthonous cover rocks increased from east to west, with schists exposed in the Røstdistrict having experienced kyanite-orthoclase zone conditions and local migmatization(Holloman, ms; Mooney, ms; Waltman, ms). Thermobarometric data reflect this west-

Fig. 3. Diagrammatic illustration of the tectonostratigraphic architecture of the Leknes area (no scale).Small arrows are Caledonian thrust faults. Excellent exposure in the southern units allowed for the develop-ment of a detailed tectonostratigraphic column. Lithologies are summarized from Klein (ms).

Andre C. Klein and others74

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ward increase, with conditions on Vestvågøy averaging 600°C and 6 kb, increasing to750°C and 8.2 kb on Røst (Mooney, ms; Waltman, ms). Similarly, 40Ar/39Ar ages ofmuscovite porphyroblasts indicate that rocks of the Røst district cooled through theirrespective blocking temperatures �100 my later than rocks exposed on Vestvågøy(Hames and Andresen, 1996a). These data are not too surprising considering the locationof these rocks in the footwall of a decollement. Rocks exposed on Vestvågøy and Røstwere most likely farther apart during the onset of Caledonian orogeny than they aretoday.

The Caledonian orogeny was a major contractional event (D2 of Klein, ms) at thelatitude of Lofoten, producing numerous thrust faults, large-scale basement-cored folds(F2), and pervasive metamorphic and/or mylonitic foliations (S2) (Bartley, 1982; Stelten-pohl, 1987; Holloman, ms; Klein, ms; Mooney, ms). The Caledonian foliation disap-pears with depths of less than 0.5 km into the basement below the basal Caledonianthrust. This phenomenon was recognized by early workers throughout Lofoten-Ofoten(Tull, ms; Griffin and others, 1978) and initially was attributed to the Lofoten terranehaving occupied a high crustal level during Caledonian orogenesis (e.g. Hakkinen, ms;Griffin and others, 1978). It is now generally accepted, however, that the Lofoten blockwas tectonically buried by the composite Caledonian allochthon, and existing pressure-temperature data suggest that Lofoten was subducted to depths of 25 to 35 km (Mooney,ms; Waltman, ms).

Late- to post-Caledonian.—Following the peak of Caledonian metamorphism (at �430Ma), rocks of the Lofoten archipelago underwent two kinematically distinct foldingevents (F3 and F4 of Klein, ms; F3a and F3b of Waltman, ms) related to regionallydiachronous deformation (D3 and D4 on Røst, Holloman, ms; D3/D4 on Vestvågøy,Klein, ms; D3 of Waltman, ms). F3 folds on Vestvågøy have a dominant west-northwesttrend, parallel to the Caledonian (and post-Caledonian, see below) transport direction(fig. 4A). These folds are termed crossfolds and appear to represent shortening directedat a small angle to the trend of the orogen (Steltenpohl and Bartley, 1988). F4 fold axes,on the other hand, parallel the orogenic trend (north-northeast) but verge to the west inthe opposite sense from that of nappe transport (fig. 4B) and thus are consideredbackfolds (Steltenpohl and Bartley, 1988). Interestingly, the crossfolds are concentratedat the highest exposed structural levels on Vestvågøy, whereas the backfolds only occurproximal to the basement-allochthon contact. As a result, fold interference patternsbetween F3 and F4 were never observed.

D4 was a major extensional episode throughout the entire Lofoten archipelago(Løseth and Tveten, 1996; Holloman, ms; Mooney, ms; Klein, ms). Late-phase, tops-west, ductile shear zones that affect the basement and cover units on Vestvågøy aremainly localized within 1 km of the basement-allochthon contact (fig. 5). Rare tops-westshear zones occur higher in the nappe stack, where they were observed to truncate F3folds; hence the late-phase shear zones are interpreted as D4 structures. The east-northeast-trending, northwest-dipping shear zones are defined by an S4 extensional crenulationcleavage (Platt and Vissers, 1980) that is subparallel to S2 but generally has a steeper dip(fig. 6A). These relationships lead to the development of local S-C composite fabrics(Lister and Snoke, 1984) in the basement and S-NSC (Dennis and Secor, 1987)composite fabrics in the cover units (fig. 6B). Local high-strain mylonite zones aredefined by stretched quartz and feldspar and may represent competence contrastsbetween different lithologies. A mineral elongation lineation (L4) in the S4 myloniticfoliation plunges west northwest and is interpreted to indicate the line of transport (Klein,ms; Klein and Steltenpohl, 1999). These relations suggest that the D4 slip line was largelycoaxial with D2, and that post-Caledonian transport of structurally-higher rocks was inthe opposite direction from that of Caledonian nappe emplacement. The west-northwestplunge of L4 in the north-dipping D4 shear zones indicates a component of left-slip and

Andre C. Klein and others—Ductile and76

Fig. 4(A) Field photograph of an F3 crossfold within mafic gneiss of the Leknes group, �2 km west ofLeknes. Surface being folded is S2 gneissosity developed during Caledonian thrusting. Hammer is �30 cmlong. (B) Outcrop photo of a mesoscopic F4 backfold in basement mangerite orthogneiss, looking north anddown plunge, �4 km southwest of Leknes. Note west vergence of axial surface and lack of axial planar fabric.Lens cap is �5 cm in diameter.

brittle extension in the southern Lofoten archipelago, north Norway 77

characterizes the D4 extensional event. Local S-NSC relations also indicate a strongsinistral component to D4 (Klein and Steltenpohl, 1999).

Argon isotopic analysis of metamorphic minerals recrystallized during D4 providesclues as to the absolute timing of this late-phase shearing event. Metamorphic conditionsduring D4 were in the greenschist facies, with chlorite commonly overgrowing biotitewithin D4 shear zones. The shear zones also commonly contain fine-grained, recrystal-lized muscovite that clearly is younger than the coarse mica flakes formed during D2 (thatis, the coarse-grained muscovite is often preserved in boudins within S4, whereas thefine-grained mica defines S4). Recrystallized micas in S4 fabrics on Vestvågøy yield agesof �365 Ma (Hames and Andresen, 1996a). Similar fabrics along basement-covercontacts on Værøy and Røst yield lepidoblastic muscovite with ages of �275 Ma. Theserelationships suggest a diachronous or otherwise time-transgressive evolution of D4structures from northeast to southwest Lofoten.

D5 brittle faulting.—Brittle faults truncate earlier ductile structures and fabrics and cangenerally be divided into two main types: fabric forming and non-fabric forming faults.Fabric forming faults range in orientation from low-angle to high-angle. The low-anglefaults occur only in the cover rocks, and their orientation may have been influenced by

Fig. 5. Map of D4 ductile shear zones within the Leknes group and subjacent basement mangerite(outlined in bold lines, gradient pattern). The two shear zones may be connected but are not shown to do so dueto lack of exposure. Lower-hemisphere equal-area projections are of L4 mineral elongation lineation data fromeach shear zone. Contours are 2, 6, and 8 percent.

Andre C. Klein and others—Ductile and78

Fig. 6(A) Interlayered quartzite and quartz-rich mica schist exposed on the beach, �3.25 km west ofLeknes. View is looking north and down the dip of the dominant schistosity. The schistosity in the rock is offsetin a top-down-to-the-west sense of shear. Hammer head is �10 cm across. (B) Oriented hand sample of shearedbasement mangerite from quarry �1.5 km southwest of Leknes. Sample was cut perpendicular to the myloniticfoliation and parallel to the elongation lineation. Note subhorizontal S2 fabric offset along S4 extensional shearbands dipping moderately to the right (west).

brittle extension in the southern Lofoten archipelago, north Norway 79

the strong fabric already present; they were never observed to juxtapose differinglithologies. High-angle faults were easier to recognize in the field. A subvertical,northwest-striking D5 fault on Værøy juxtaposes undeformed deep crustal basementrocks with the strongly Caledonized Leknes group. Brittle fractures associated with D5faults locally contain celadonitic muscovite with 40Ar/39Ar ages of �265 Ma (Hames andAndresen, 1996a). The fact that this fault contains a fabric defined by metamorphicminerals and strained quartz (Mooney, 1997) suggests that faulting occurred near thebrittle-ductile transition, where the rocks were hot enough to deform semi-plastically. OnVestvågøy, a subvertical northeast-striking D5 breccia zone truncates the basement-allochthon contact and is interpreted to be a late-phase, down-to-the-east normal fault.Cataclastic rocks related to this fault zone are exposed also in the northwestern part ofthe study area and on the northeastern tip of neighboring Flakstadøy. The breccia zoneslack any planar fabric and have a similar orientation to the late-stage faults described onHinnøy (Bartley, 1981), on Grytøya (Van Winkle, ms; Van Winkle and others, 1996),and in Skånland (Steltenpohl, 1987) on the adjacent mainland. The breccia zones mayalso be related to the Vestfjorden-Vanna fault complex (Andresen and Forslund, 1987;Olesen and others, 1997) that trends northeast-southwest throughout Lofoten-Vest-erålen, and to which Olesen and co-workers assigned a Permian age based on paleomag-netic studies. Other late-phase D5 brittle faults recognized in the area deform units of theoverlying allochthon.

Summary.—The Caledonian orogeny at the latitude of Lofoten was characterized byeast-southeast-directed, basement involved thrusting and prograde amphibolite faciesmetamorphism of the allochthonous cover rocks. On Vestvågøy, D2 was the dominantdeformational episode, creating an S2 schistosity throughout the Leknes group and todepths of generally less than 0.5 km into the basement. Following the metamorphic peak,the basement-allochthon contact was reactivated and again locally sheared as the coverallochthons moved to the west-northwest. This event, D3/D4, created two kinematicallydistinct fold sets at different levels within the nappes. Due to their lack of foldinterference and similar metamorphic conditions of formation, F3 and F4 may haveformed at roughly the same time (Steltenpohl and Bartley, 1988). The later stages of folddevelopment were accompanied by ductile shearing and retrograde metamorphism.The ductile extensional shear zones locally are cross cut by low-angle, tops-westcataclastic normal faults, reflecting progressive unroofing of the shear zones to shallowercrustal levels during late- and post-Caledonian extensional events. Northeast-trending,high-angle, brittle normal faults truncate all fabrics and structures and juxtapose structur-ally-deep, undeformed Precambrian basement with the Caledonian nappe sequence.

DISCUSSION

Kinematic significance of late-stage crossfolds and backfolds.—Crossfolds and backfoldslocally developed within the Leknes group give map-scale evidence for sinistral shearingand extension. Cross-strike structures such as crossfolds have been reported throughoutwestern Ofoten (Steltenpohl and Bartley, 1988; Van Winkle, ms; Van Winkle andothers, 1996) and can be related to the finite strain ellipse (fig. 7) of a large-scale sinistralshear zone (as in Wilcox and others, 1973). The crossfolds lie within the compressionalfield of the strain ellipse documenting orogen-parallel movements. F3 crossfolds onVestvågøy are geometrically, kinematically, and temporally related to post-metamor-phic D4 shear zones occurring in the lower part of the nappe stack implying that D3 andD4 may have been contemporaneous. This is consistent with microstructural observa-tions that indicate similar deformational conditions for D3 and D4. The fact thatcrossfolds and backfolds formed at different structural levels within the Leknes groupmay reflect extensional strain partitioning in a transtensional event.

Andre C. Klein and others—Ductile and80

The east-northeast-trending, D4 ductile shear zones and their associated backfoldscan be related to an extensional kinematic plan (Osmundsen and Andersen, 1994).Backfolds have axes that are parallel to the trend of the orogen but verge in the oppositesense from that of nappe transport (Steltenpohl and Bartley, 1988). D4 shear zones trend

Fig. 7. Map-scale features reflecting orogen-parallel and extensional motion along the basement-covercontact of west-central Vestvågøy. Gray scale map image is of thrusts and the shoreline as illustrated in figure 2.See text.

brittle extension in the southern Lofoten archipelago, north Norway 81

east-northeast, and L4 stretching lineations trend oblique to the dip direction, predomi-nantly west, imparting a component of sinistral shear to the extension. The relationshipof the crossfolds and backfolds to the late, D4 ductile shear zones suggests that the foldsformed in response to late- or post-Caledonian extension with a sinistral strike-slipcomponent. The west-vergent, late-stage backfolds have upright hinge planes in Ofoten(Steltenpohl and Bartley, 1988) but progressively become recumbent in Lofoten (fig. 8).The fact that backfolds in Lofoten are restricted to discrete, late-stage shear zonessuggests that extension may have been accommodated along a system of west-dippingnormal faults rather than along a single detachment. Alternatively, D4 extensionalshearing and backfold development may have been restricted to discrete zones of thecrust by the relative paucity of water available to promote ductile deformation (seebelow). This alternative is consistent with Bartley’s (1982) interpretation to account forthe downward disappearance of Caledonian thrust-related fabrics on the neighboringisland of Hinnøy. Thus, a model invoking the importance of a single master extensionaldecollement in unroofing the Lofoten block is equally attractive. The diachronous natureof extension across Lofoten, borne out in the mineral cooling ages reported by Hamesand Andresen (1996a), is consistent with either interpretation.

Comparison of extensional development throughout the southern Lofoten Islands.—Detailedfield mapping throughout southern Lofoten (fig. 1), that is, Vestvågøy (Klein, ms), Værøy

Fig. 8. Cartoon diagram depicting progressive fold axial plane rotation in the direction of extension if theWGR of northern Norway occupied a position in the footwall of a major west-dipping normal fault (Coker andothers, 1995). The figure represents D4 evolution of Vestvågøy at �365 Ma. Folds have been dragged in thedirection of extension and become progressively more overturned as the fault is approached. Short dashedlines represent ductile conditions. Topmost diagram is modified from Model III of Brun and Choukroune(1983).

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(Mooney, ms; Waltman, ms), and Røst (Holloman, ms; Waltman, ms), has revealedductile and brittle extensional structures and fabrics previously undocumented in thisregion. Geologic mapping on west-central Vestvågøy documents thrust-related struc-tures and fabrics (Tull, 1978) that were later overprinted by discrete extensional shearzones (Klein, ms). The shear zones contain both meso- and microscopic extensionalshear bands (fig. 6), that cross cut the Caledonian fabric and indicate tops-west move-ment. Asymmetrical feldspar and mica porphyroclasts that record a tops-west sense ofshear were observed within the lowest 1 km of structural thickness in the Leknes groupand topmost 0.5 km of underlying basement, suggesting that extensional movements onVestvågøy were concentrated along the basement-cover contact. The S4 myloniticfoliation developed during extension dips moderately to the north, and a mineralelongation lineation (L4) developed in the plane of S4 plunges to the west-northwest,indicating a sinistral strike-slip component to the extension. 40Ar/39Ar ages of �365 Mafrom recrystallized muscovite in the Leknes group mylonites on Vestvågøy are inter-preted to reflect the timing of mylonite development during extension (Hames andAndresen, 1996a).

The island of Værøy (fig. 1) exposes west-dipping Caledonian shear zones witheast-vergent fabrics and structures that have been locally overprinted by late(?)- topost-Caledonian, tops-west, mylonitic and cataclastic structures and fabrics (Mooney,ms; Waltman, ms). West-dipping zones of mylonite-ultramylonite contain asymmetricalfeldspar and mica porphyroclasts with fine-grained, recrystallized tails indicating atops-west sense of shear when viewed on a surface parallel to the mineral elongationlineation and perpendicular to the mylonitic foliation (Mooney, ms). Muscovite frommylonitic rocks on Værøy yields ages of 270 Ma (Hames and Andresen, 1996a). Alarge-scale brittle normal fault that cuts the island in half dips steeply to the southwestand offsets the Caledonian fabric in a top-down-to-the-west sense of shear. Hydrothermalmuscovite from fractures associated with this normal fault yields 40Ar/39Ar ages of about265 to 255 Ma (Hames and Andresen, 1996a), interpreted to record the timing ofextensional movements across the fault.

The islands of the Røst district, at the southernmost terminus of the Lofotenarchipelago, expose imbricated Caledonian (?) thrusts that juxtapose rocks lithologicallysimilar to the Leknes group (Tull, ms) with Precambrian basement (Holloman, ms). Therocks of the Røst district presently have the geometry of a northeast-southwest doubly-plunging antiform (Holloman, ms). Thrust-related fabrics and structures have locallybeen overprinted by tops-east and tops-west, mylonitic extensional fabrics and structures(Holloman, ms). 40Ar/39Ar ages of muscovite porphyroblasts and recrystallized musco-vite are also Permian in Røst (�275 Ma) and are interpreted to reflect an episode ofunroofing and extension (Hames and Andresen, 1996a).

In summary, the findings of this study are consistent with abundant evidencedocumenting post-orogenic extensional shearing throughout various segments of theCaledonian belt (Andersen and Jamtveit, 1990; Andresen and Forslund, 1987; Bukovicsand Ziegler, 1985; Coker and others, 1995; Norton, 1986; Olesen and others, 1997).40Ar/39Ar cooling dates indicate that brittle and ductile extensional shearing in Lofotenoccurred over a protracted span, lasting nearly 150 my and broken up into pulsesseparated by 30 to 50 my intervals of quiescence (Hames and Andresen, 1996a). Thesame evidence indicates that extensional shearing did not last the full 150 my at any oneplace but was diachronous throughout Lofoten, younging to the southwest. Olesen andothers (1997) provide temporal constraints on episodes of brittle faulting in the Lofoten-Lopphavet region using paleomagnetic methods. They conclude that two phases offaulting and brecciation affected rocks in this region, one Permian in age and the otherrecent/Tertiary, and that there may have been a westward shift of fault activity fromCarboniferous-Permian to Late Jurassic-Early Cretaceous time. The westward younging

brittle extension in the southern Lofoten archipelago, north Norway 83

pattern of metamorphic cooling ages (Hames and Andresen, 1996a) and brittle faultpropagation is consistent with the Lofoten terrane occupying a position in the footwall ofa west-dipping, crustal-scale extensional fault located somewhere west of the presentNorwegian coast (Coker and others, 1995; Hames and Andresen, 1996a).

Differences in tectonic styles between the WGR and the Lofoten block.—The structural andmetamorphic styles in Lofoten appear to differ substantially from those of the WGR ofsouthwestern Norway. Brittle and ductile extensional structures and fabrics are locallywell-developed in southern Lofoten, but they are not as regionally pervasive as theextensional fabrics found in the WGR (Andersen and Jamtveit, 1990; Andersen andothers, 1991). Large areas of the southwest Norway WGR have yet to be explored,however, and undeformed Precambrian granites have been discovered structurallybeneath the Devonian mylonites (J. Bartley, personal communication, 1998). Even so,the scale difference in the amount of Caledonian and post-Caledonian sheared rocksbetween the two regions is remarkable. Furthermore, the presence of Caledonianeclogite bodies in the WGR contrasts sharply with the lack of such ultra-high pressure(UHP) rocks of the same age reported in Lofoten. Finally, the WGR of the Nordfjord-Sognefjord region is perhaps best known for its large-scale basins filled with several (?)km of Devonian sediments deposited on the hanging wall of a crustal-scale extensionaldetachment (Norton, 1986; Seranne and Seguret, 1987; Andersen and Jamtveit, 1990).No Devonian deposits have been reported in Lofoten. It is important to note here,however, that basement exposures in southwest Norway reach as much as 75 to 100 kmfarther into the hinterland of the orogen than the exposures in Lofoten, relative to thebasal Caledonian thrust (fig. 1). If any Caledonian UHP rocks exist at the latitude ofLofoten, they may be farther west and submerged in the continental shelf or removed bylater normal faulting.

The structural and petrologic nature of the Baltic basement in the WGR and in theLofoten block also provide clues on the differing tectonic styles that resulted in the abovementioned variations. A long-standing problem connected with rocks in Lofoten is thatvast regions of Precambrian basement lack Caledonian deformational fabrics andstructures; undeformed rocks occur to within �0.5 km structurally beneath the thrustfault along the base of the main Caledonian allochthon (Tull, 1978; Griffin and others,1978; Bartley, 1982; Hodges and others, 1982; Van Winkle and others, 1996; Klein, ms;Klein and Steltenpohl, 1999). This observation indicates that deformation decreasedstructurally downwards during Caledonian orogenesis, in sharp contrast to the classicmodel of penetrative flow below the brittle-ductile transition proposed by Armstrongand Dick (1974). Rocks of the WGR, on the other hand, were penetratively tectonized atmost presently exposed levels during the Caledonian orogeny, with relatively unde-formed rocks commonly occurring as lenses or pods within a highly strained matrix.Basement rocks of the WGR are mainly polydeformed, kyanite-zone gneisses withstrong gneissic foliations or schistosities and numerous ductile shear zones that locallyencapsulate coesite-eclogite lenses (Andersen and Jamtveit, 1990). Associated with theseeclogite lenses are granulite-facies basement gneisses that have been retrograded toamphibolite-facies schists and gneisses. The protolith lithologies (for example, Protero-zoic granitic rocks) in both southern and northern Norway are similar, however,requiring an explanation to account for these along-strike differences.

One possible explanation for the differences in metamorphism and deformationalstyle is the residence time of the Baltic basement in the footwall of the Caledonian A-typesubduction zone (Hodges and others, 1982). The response of rocks to changes inpressure is nearly instantaneous whereas temperature adjustments are much slower dueto the low thermal diffusivity of silicate minerals. In addition, a depressed geotherm in asubduction zone will also cause rocks to experience temperatures that are lower thannormal for a given depth. It follows from this reasoning that the WGR of southwest

Andre C. Klein and others—Ductile and84

Norway would have resided at depth for a longer period of time than the rocks of theLofoten block, allowing the temperature to equilibrate with the pressure and causing therocks to deform in a more uniformly plastic manner. Geochronologic data indicate thatthe WGR underwent rapid exhumation prior to molasse deposition in the Devonianbasins (�390 Ma) (Norton, 1986; Fossen and Dunlap, 1998; Dunlap and Fossen, 1998).Ages reported for initial exhumation of the Lofoten block, based on 40Ar/39Ar analysis ofhornblende from the Leknes group (Hames and Andresen, 1996b), suggest that theLofoten block maintained high temperatures (�500°C) for a length of time comparableto that of southwest Norway (�30 my). Prograde dynamothermal metamorphism of thefootwall block had effectively ceased in both areas by 390 Ma.

The response of the Lofoten block to subsequent exhumation was different fromthat of the WGR. Recent thermochronologic data from the WGR of southern Norwaysuggest a period of relative tectonic and thermal stability during the interval from 380 to330 Ma (Dunlap and Fossen, 1998); most of the exhumation and cooling through�350°C in the allochthonous nappes (Dunlap and Fossen, 1998, p. 607) took place priorto �390 Ma (Fossen and Dunlap, 1998). In contrast, allochthonous rocks in Lofotenunroofed through metamorphic temperatures in several stages over �150 m.y. (Hamesand Andresen, 1996a) (fig. 9). D4 ductile extension on Vestvågøy occurred at �365 Maand was accompanied by fabric development and metamorphic mineral recrystallization(Klein and Steltenpohl, 1999). A similar D4 event on Røst occured �90 my later, at 275Ma (Hames and Andresen, 1996a; Holloman, ms; Waltman, ms). Interestingly, ductileshearing and cooling through 350°C on Røst (Hames and Andresen, 1996a) wasconcurrent with brittle faulting and cooling through �200°C in southwest Norway(Torsvik and others, 1992; Dunlap and Fossen, 1998). Not only was exhumation of the

Fig. 9. Comparison of the thermal history of the Lofoten block (Hames and Andresen, 1996a) with that ofsouthern Norway (adapted from Dunlap and Fossen, 1998). The thermal path of southern Norway was derivedusing K-feldspar 40Ar/39Ar data, whereas the Lofoten paths were derived using muscovite 40Ar/39Ar data. As aresult, overall temperatures are different. It is important to note, however, the discordance in the shapes of thepaths, which indicate relative thermal stability for southern Norway between 380 to 330 Ma (Dunlap andFossen, 1998) and thermal instability of Lofoten in the same interval. Thermal instability in Lofoten is related todiscrete stages of tectonic unroofing (Hames and Andresen, 1996a). See text.

brittle extension in the southern Lofoten archipelago, north Norway 85

Lofoten block less ‘‘instantaneous’’ than the WGR, but unroofing was also diachronousfrom northeast to southwest across Lofoten.

Existing geothermobarometric data reveal that pressure and temperature differ-ences between the two regions during Caledonian orogenesis may have played impor-tant roles in determining later extensional styles. Rocks of the Leknes group in Lofotenexperienced temperatures higher than those reported for allochthonous nappes insouthwest Norway (550-750°C for Lofoten, 350-450°C for SW Norway) (Hames andothers, 1992; Hames and Andresen, 1996a; Dunlap and Fossen, 1998). Basement rocksof the WGR suffered high temperatures (550-750°C, Griffin and Carswell, 1985),whereas the Lofoten basement stayed relatively cool (�300°C, Griffin and others, 1978).Rocks from both areas experienced vastly different pressures, as well. Quantitativegeobarometry involving compositions in the allochthonous kyanite-bearing migmatitesof the Røst District give peak metamorphic pressures of 9.2 kb, corresponding to crustaldepths of �30 km (Mooney, ms; Waltman, ms). Estimates of Lofoten basement pressuresbased on fluid inclusion studies are even lower (Griffin and others, 1978). Numerouspublished reports, however, indicate that the WGR basement likely reached muchhigher pressures (�16 kb, Andersen and Jamtveit 1990; �17-21 kb, Griffin and Carswell,1985; crustal depths approaching 100 km for the southern WGR, Smith and Lappin,1989).

Such vast pressure differences likely correspond to subduction depth along a singlesubduction zone, although the cause for such variability is unresolved. The subductionangle in southern Norway may have been greater than that for northern Norway,allowing the WGR basement to attain greater crustal depths and generate the UHPassemblages. Alternatively, variations in the thickness of the overriding plate may haveplayed a role in subduction depth. Contrasts in deformational style, temperatures, andpressures attained during subduction may have set the stage for subsequent responses tocrustal extension and gravitational collapse along the subducted western margin ofBaltica. In fact, differences in crustal response to Caledonian contraction may also havebeen influenced by earlier crustal formation and deformational events. In the long andvaried history of the Scandinavian Caledonides, some event produced differencesbetween the Lofoten block and the WGR such that their deformational and thermobaro-metric paths diverged. The event in question is considered responsible for dewateringthe Lofoten basement to such a degree that deformation was only possible in narrowshear zones where fluids were present (Bartley, 1982). This is in sharp contrast to theWGR, where deformation is more pervasive and the extensional shear zones are major,large-displacement features (Dunlap and Fossen, 1998), suggesting that metamorphicfluids had more wholesale effects on the evolution of the WGR of southern Norway.

SUMMARY

Recent detailed field mapping in southern Lofoten has led to the recognition ofsinistral-oblique-slip, top-west movement along moderately north-northwest dippingshear zones concentrated near basement-allochthon thrusts. There are many structuraland kinematic similarities between the Lofoten shear zones and the extensional shearzones in southern Norway. In both regions, ductile extension was accommodated byexploiting earlier fabrics. Furthermore, both areas have well-developed sets of backfolds(Steltenpohl and Bartley, 1988; Osmundsen and Andersen, 1994) and abundant shearsense indicators that document west-directed movement of structurally higher rocks. Animportant difference is that the shear zones in southern Norway record displacements onthe order of several kilometers to many tens of kilometers (Dunlap and Fossen, 1998).Ductile shears in Lofoten, on the other hand, do not juxtapose different lithologies ormetamorphic grades and thus are interpreted to have much smaller displacements.

Andre C. Klein and others—Ductile and86

We suggest that the extensional style of the Lofoten block is different from itssouthern counterpart only in terms of timing and scale. It is likely that these differencesstem from the influence of basement dewatering resulting from along-strike variations insubduction depth during Caledonian orogeny. Devonian extension in Lofoten is inter-preted to have initiated during the waning stages of Caledonian orogenesis and contin-ued episodically through middle Carboniferous time at progressively higher crustallevels, based on microstructural analysis of extensional mylonites from Vestvågøy(Klein, ms). Late Carboniferous to Permian 40Ar/39Ar muscovite dates document that theLofoten block remained at elevated temperatures much later than its southwest Norwaycounterpart (Hames and Andresen, 1996a). Furthermore, the tectonic and thermalinstability of Lofoten during the interval from 380 to 330 Ma is in sharp contrast to therelative stability of the WGR during that time (Dunlap and Fossen, 1998). The deforma-tional history of the two regions became coincident again in the Permian, when brittlefaulting in the WGR (Torsvik and others, 1992; Dunlap and Fossen, 1998) wascontemporaneous with the onset of brittle crustal extension in Lofoten (Hames andAndresen, 1996a). The mechanism that caused the geologic path of the Lofoten block todiverge from that of the WGR prior to or during the Caledonian orogeny is unresolved,but the most important result of that event was the relative dewatering of the Lofotenbasement with respect to the WGR of southern Norway. The paucity of metamorphicfluids in the basement limited Caledonian deformation (Bartley, 1982) and, as a result,post-Caledonian extension in Lofoten was mainly accommodated by reactivating Cale-donian thrusts and deforming hydrous basement enclaves. Because these features arelimited in lateral extent in Lofoten, the ductile shear zones that exploited them are notcrustal-scale detachments like their WGR counterparts.

ACKNOWLEDGMENTS

Portions of this work constitute part of the first author’s M.S. thesis at AuburnUniversity. This work was supported by the NSF through grant EAR 9506698 to W.Hames and M. Steltenpohl, a Norwegian Marshall Foundation grant to M. Steltenpohl,and a Geological Society of America Grant-in-Aid to A. Klein. Support for travel tovarious meetings to present preliminary findings was provided by the College of Scienceand Mathematics, Auburn University, Auburn, Alabama to A. Klein. Careful reviews byJ. Bartley, C.J. Northrup, and an anonymous reviewer greatly improved the content ofthe manuscript.

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