refolding of thin-skinned thrust sheets by ...refolding of thin-skinnedthrust sheets by active...

589Revista de la Asociación Geológica Argentina 61 (4): 589-603 (2006)

REFOLDING OF THIN-SKINNED THRUST SHEETS BYACTIVE BASEMENT-INVOLVED THRUST FAULTS IN THEEASTERN PRECORDILLERA OF WESTERN ARGENTINA

Andrew MEIGS1, William C KRUGH2, Celia SCHIFFMAN1, Jaume VERGÉS3 and Victor A. RAMOS4

¹ Department of Geosciences, Oregon State University, Corvallis, OR, 97331, U.S.A.2 Earth Science, Institute of Geology, ETH Zentrum, Sonneggstrasse 5, CH-8092 Zurich, Switzerland.3 Institute of Earth Sciences "Jaume Almera", CSIC, Lluís Solé i Sabarís s/n, 08028 Barcelona, Spain.4 Laboratorio de Tectónica Andina, Departamento de Ciencias Geológicas, Universidad de Buenos Aires, 1428 Buenos Aires, Argentina

ABSTRACTDevastating earthquakes like the 1944 San Juan earthquake reflect active deformation in western Argentina. Although the earthquake cau-sed considerable damage to San Juan, the source of the earthquake remains uncertain. Potential source faults occur in the thin-skinnedfold-and-thrust belt Precordillera province and in the thick-skinned Sierras Pampeanas province, to the west and east, respectively of Sierrade Villicum, a thrust sheet in the eastern Precordillera northwest of San Juan. Sierra de Villicum is a west-vergent thrust sheet bound onthe northwest by the Villicum thrust, which juxtaposes a southeast dipping panel of Cambro-Ordovician and Neogene strata in the han-ging wall with Neogene red beds in the footwall. A series of Late Pleistocene fluvial terraces developed across the Villicum thrust showno evidence of active fold or fault deformation. Terraces are deformed by active folds and faults in the middle of the southeastern flankof the Sierra de Villicum thrust sheet. A southeast-facing, southwest-plunging monocline characterizes the Neogene red beds in the regionof active folding. Co- and post-seismic surface rupture along roughly 6 km of the La Laja fault in 1944 occurred in the limb of the mono-cline. Evidence that surface deformation in the 1944 earthquake was dominated by folding includes terrace´s fold geometry, which is con-sistent with kink-band models for fold growth, and bedding-fault relationships that indicate that the La Laja fault is a flexural slip fault. Ablind basement reverse fault model for the earthquake source and for active deformation reconciles the zone of terrace deformation,coseismic surface rupture on the La Laja fault, refolding of the Villicum thrust sheet, a basement arch between the Precordillera and eas-tern Precordillera, and microseismicity that extends northwestward from a depth of ~5 km beneath Sierra de Villicum to ~35 km depth.Maximum horizontal shortening rate is estimated to be ~3.0 mmyr-1 from the terrace fold model and correlation of the terraces withdated terraces located to the southwest of the study area. Basement rocks beneath Cerro Salinas, another eastern Precordillera thrust sheetto the southwest, are also characterized by an east-facing monoclinal geometry, which suggests that blind thrust faulting on east-vergentbasement faults represents a significant, underappreciated seismic hazard in western Argentina.

Keywords: Neotectonics, San Juan, La Laja, Villicum, flexural slip fault, 1944 earthquake.

RESUMEN: Replegamiento de láminas de corrimiento epidérmicas mediante fallas inversas de basamento activas en la Precordillera Oriental del oeste de Argentina.La deformación activa en el oeste de Argentina está reflejada por terremotos devastadores como el sismo de San Juan en 1944. Aunque el terremo-to causó un daño considerable a San Juan, la fuente del terremoto permanecía incierta. Fuentes potenciales de falla ocurren en las fajas plegadas ycorridas adyacentes, la epidérmica de la Precordillera o la faja de basamento de Sierras Pampeanas, al oeste y este respectivamente de la sierra deVillicum, una lámina de corrimiento de la Precordillera Oriental, ubicada al noroeste de la provincia de San Juan. La sierra de Villicum es una lámi-na de corrimiento limitada al oeste por el corrimiento de Villicum, que yustapone un panel de estratos cambro-ordovícicos y neógenos de la paredcolgante, con estratos rojos neógenos en la pared yaciente. Una serie de terrazas fluviales desarrolladas a través del corrimiento de Villicum no tie-nen evidencia activa de plegamiento o falla. Las terrazas están deformadas por fallas y pliegues activos en el medio del flanco sudeste de la lámina decorrimiento de la sierra de Villicum. La región de replegamiento activo está caracterizada por un monoclinal buzante al sudoeste de estratos neóge-nos que inclinan al sudeste. Rupturas superficiales cosísmicas y postsísmicas de la falla de La Laja ocurrieron en 1944 a lo largo de aproximadamen-te 6 km en el limbo del monoclinal. La evidencia de deformación superficial en el terremoto de 1944 estuvo dominada por plegamiento, incluido elde terrazas con geometría de pliegues, las que son consistentes con modelos de kink-bands para pliegues de crecimiento y con relaciones de fallas deestratificación que indican que la falla de La Laja es una falla por flexo deslizamiento. Un modelo de falla inversa ciega en el basamento para la fuen-te del terremoto y para la deformación activa, reconcilia la deformación en la zona de terrazas, la ruptura superficial cosísmica en la falla de La Laja,el repliegue de la lámina de corrimiento de Villicum, el arqueamiento del basamento entre la Precordillera y la Precordillera Oriental y la sismicidadque se extiende en profundidad hacia el noroeste desde ~5 km a unos ~35 km por debajo de la sierra de Villicum. La tasa de acortamiento horizon-tal máximo es estimada en unos ~3,0 mm por año a partir del modelo de plegamiento de terraza y su correlación con las terrazas datadas ubicadasal sudoeste del área estudiada. Rocas de basamento por debajo del cerro Salinas, una lámina de corrimiento de las Sierras Pampeanas ubicada al sud-oeste de la región estudiada, también está caracterizada por una geometría monoclinal inclinada al este, que sugiere que fallas inversas ciegas en fallasde basamento con vergencia oriental representan un riesgo sísmico significativo, pero subvalorado en el oeste de Argentina.

Palabras clave: Neotectónica, San Juan, La Laja, Villicum, falla flexo-deslizante, terremoto de 1944.

0004-4822/02 $00.00 + $00.50 C 2006 Revista de la Asociación Geológica Argentina

590 A. MEIGS, W. C KRUGH, C. SCHIFFMAN, J. VERGÉS Y V. A . RAMOS

INTRODUCTION

On January 15th, 1944 San Juan, Argentinaexperienced a devastating Ms 7.4 earthqua-ke. Whereas the precise location, depth, andfocal mechanism of the earthquake arepoorly known, post-earthquake field sur-veys revealed surface deformation on theeastern flank of Sierra de Villicum. (Bastíaset al. 1985, Groeber 1944, Harrington 1944,Paredes and Uliarte 1988). Harrington(1944) documented 30 cm of vertical co-seismic and 30 cm of postseismic displace-ment along ~6km of the La Laja fault.Sierra de Villicum is the hanging wall ofone of a series of thrust sheets comprisingthe eastern Precordillera structural provin-ce, which sits between the thin-skinnedPrecordillera fold-and-thrust belt on thewest and the thick-skinned Sierras Pampea-nas structural province on the east (Gon-zález Bonorino 1950) (Fig. 1). Three faultshave been suggested as the seismogenicsource of the earthquake. Surface ruptureon the La Laja fault identifies it as a candi-date structure (Alvarado and Beck 2006,Bastias et al. 1985, Perucca and Paredes2000, 2002). A second potential candidate isthe southeast-dipping thrust, the Villicumthrust, which bounds the northwesternflank of Sierra de Villicum (Siame et al.2002), which juxtaposes Cambrian carbona-tes with Neogene red beds (Ragona et al.1995). Microseismicity beneath the rangesuggests that a third possibility is a base-ment fault zone that strikes northeast, dipsnorthwest, and extends from 5 to 35 kmdepth (Smalley et al. 1993). Thus, alternati-ve models for the source of the 1944 eventinclude surface rupturing thin-skinnedfaults (La Laja or the Villicum fault) or ablind thick-skinned thrust.In this paper, we merge new field mappingof fluvial terraces and bedrock structure,structural models of fold growth, and pu-blished seismicity in a new model for thecrustal structure of the transition betweenthe thin-skinned fold-and-thrust belt andthe thick-skinned thrust province in thefoothills between the Andes and the SierrasPampeanas in western Argentina. Field re-lationships indicate that the fault systemalong the northwestern range front ofSierra de Villicum does not cut a suite of

fluvial terraces; equivalent terraces, howe-ver, are deformed by an east-facing mono-cline and surficial thrust faults on the sou-theast flank of the range. Within this regionof active folding and faulting, flexural slipfaulting characterizes some of the activefaults, including the La Laja fault, whichindicates these are secondary faults relatedto folding. When this observation is com-bined with field data indicating the southe-astern flank of Sierra de Villicum defines a

broad, southeast-facing anticlinorium, thesedata support the inference that the seismo-genic source of the 1944 event occurred ona blind, northwest-dipping thrust in thebasement. Moreover, comparison with o-ther active structures along strike suggeststhat active deformation in the basementrefolds earlier emplaced thin-skinned thrustsheets. If correct, this interpretation haslocal implications for the tectonic develop-ment of the eastern Precordillera structural

Figure 1: a) Regional tectonic map of Argentine Andes between 31° and 32° south. Four structu-ral provinces comprise the crustal structural at this latitude, including the Frontal Cordillera(beige), the Precordillera (pale green), an east vergent thin-skinned fold-and-thrust belt, the eas-tern Precordillera (pink), a zone of west vergent thrust sheets, and the thick-skinned Sierras Pam-peanas (turquoise) structural provinces in the vicinity of San Juan, Argentina (modified fromRagona et al. 1995). LL marks the La Laja fault and TVS indicates the Tulum Valley Syncline. Abox marks the location of the map in figure 2. Yellow star marks the location of terraces dated bySiame et al. (2002). Cross-section location in figures 11 and 13 are indicated. b) Shaded relief mapof the San Juan region showing focal mechanisms of historical seismicity (1977 to the present) forshallow crustal events (<50km) from the Harvard Centroid Moment Tensor Catalog. Depth is gi-ven below beach balls. White boxes are 1944 earthquake epicentral locations summarized in Alva-rado and Beck (2006).

591Refolding of thin-skinned thrust sheets by active basement-involved ...

transition region, seismic hazard for largecities along the eastern foothills of the A-ndes in Argentina, and global implicationsfor understanding the development ofthick-skin thrust structural provinces andfor recognition of blind thrust fault seismichazards.

THRUST ARCHITECTURE BETWE-EN THE PRECORDILLERA ANDSIERRAS PAMPEANAS

Outcrops in the Precordillera, eastern Pre-

cordillera, and Sierras Pampeanas constrainthe stratigraphic location and depth of theprincipal thrust systems. Ordovician andyounger rocks are exposed in the Precor-dillera (Ragona et al. 1995, von Gosen 1992)(Fig. 1), which forms the basis for the inter-pretation that the Precordillera is an east-vergent thin-skinned thrust belt with a basaldécollement in Orodovician-Lower Devo-nian sediments (Cristallini and Ramos 2000,von Gosen 1992). Precambrian and earlyPaleozoic basement rocks are exposed inPie de Palo and other ranges in the Sierras

Pampeanas province farther east (GonzálezBonorino 1950). Historic earthquakes, struc-tural geometry, and active faults indicatethat east-dipping, west-vergent basement-involved faults are the dominant structureof the Sierras Pampeanas province (Costa etal. 1999, Costa and Vita-Finzi 1996, Jordanand Allmendinger 1986, Jordan et al. 1983a,b, Kadinsky-Cade et al. 1985, Ramos et al.2002, Zapata and Allmendinger 1996b).Bedrock geology indicates that many of thebasement faults bounding the Sierras Pam-peanas ranges are reactivated Paleozoic and

Figure 2: a) Bedrock geologic map of the southwestern end of Sierra de Villicum. Brown shading of Neogene redbeds highlights repetition of Cambro-Ordovician-Neogene red bed thrust imbricates. Grey color used to show Neogene red bed distribution on southeast flank. b)Terrace map. The loca-tions of figures 3 and 5 are shown. Structural data are omitted from (b) to highlight distribution of terraces. Note that thrust faults at southwestern endof Sierra de Villicum do not extend southwestward into Neogene red beds.

592 A. MEIGS, W. C KRUGH, C. SCHIFFMAN, J . VERGÉS Y V. A. RAMOS

older sutures and terrane boundaries (Ra-mos et al. 2002). Eastern Precordillerathrust sheets define a narrow belt betweeneast-directed thrust sheets of the Precor-dillera and west-directed basement reversefaults of the Sierras Pampeanas (Fig. 1).Strata exposed at the base of east-dippingthrust sheets are restricted to Cambro-Or-dovician carbonates (Fig. 2). Thus, the eas-tern Precordillera has characteristics incommon with the Precordillera, in that co-ver sedimentary rocks are exposed in thrustsheets at the surface and characteristics incommon with Pampean range thrusts, inthat faults dip east. Whether the easternPrecordillera has a thin-skinned structuralstyle, in which upper plate faults root to adécollement at the base of Cambrian strataor into the basement is unresolved(Alvarado and Beck 2006, Ramos et al.2002, Siame et al. 2002, von Gosen 1992). Sierra de Villicum is a N-S trending rangelocated in the eastern Precordillera (Fig. 1).Bedrock structure exposed in the core ofthe range reveals a series of west-vergentimbricate thrust sheets involving an UpperCambrian through Middle Miocene strati-graphic sequence (Fielding and Jordan1988, von Gosen 1992). An angularunconformity marks the contact betweensteeply-tilted Cambro-Ordovician rocksand overlying moderately-tilted Neogenered beds (Figs. 2a and 3) (von Gosen 1992).A range front fault system, the Villicumthrust fault (Siame et al. 2002, von Gosen1992), marks the contact between foldedNeogene red beds in the footwall and theCambro-Ordovician sequence in the han -ging wall along the northwestern flank ofthe range (Figs 2a and 3). On the southeas-tern flank of Sierra de Villicum, an angularunconformity separates the Cambro-Ordovician rocks from shallowly dippingNeogene conglomerates, sandstones, andsiltstones (Krugh 2003). A second uncon-formity marks the contact between theNeogene red beds and the overlying MognaFormation, a conglomerate characterizedby abundant volcanic and plutonic clasts(Fig. 2a). Five terraces were identified around theperiphery of Sierra de Villicum (Fig. 2b).An individual terrace comprises an erosio-nal surface on bedrock, a strath, and an

overlying capping gravel, typically 1-3meters thick (Krugh 2003). Clast composi-tion of the capping gravels is dominated bylimestone, dolostone, and chert. Gravel interraces on the southwest end of Sierra deVillicum includes volcanic and plutonicclasts as well. The relative ages of the terra-ces were determined by their distribution,strath elevation above modern channels,and differences in surface morphology andcomposition. Sequentially younger terracesare inset into older terraces and occupylower elevations with respect to modernchannels. Older terraces are more dissected,have lower surface relief, and have a higherconcentration of varnished chert clasts.Based on these observations, the terraceswere classified T1 = oldest to T5 = youn-gest. Four of the terraces (T1-4) are regio-nally extensive, well preserved around the

periphery of Sierra de Villicum, and can becorrelated throughout the study area.

METHODOLOGY

Geologic and geomorphic mapping anddetailed topographic surveying of terracesurfaces was conducted along the flanks ofSierra de Villicum. Elevation and form ofmodern channels were used to constrainthe magnitude and style of terrace defor-mation. Terraces were physically correlatedwith a suite of dated surfaces to the south-west of the study area (Siame et al. 2002).Deformation rates were calculated by assig-ning ages to the terraces of this study onthe basis of the correlation with the datedsurfaces to the southwest (Siame et al.2002). A structural model at the crustalscale was developed from field relationships

Figure 3: Detailed bedrock and geomorphic map of southwestern end of Sierra de Villicum alongthe northwestern range front. 1, 2, and 3 mark the three thrust imbricates that duplicate the se-quence of Cambro-Ordovician carbonates unconformably overlain by Neogene red beds on sou-theast-dipping reverse faults. Photos in 4a and b are from imbricate 1 and figures 4c and d arefrom along the Villicum thrust (VT) along the northwestern margin of imbricate 2. Neither theVillicum thrust nor the thrust on the northwest margin of imbricate 3 continues southwest in thered beds


and published microseismicity (Smalley etal. 1993). Together, the crustal model andpatterns of Quaternary and younger defor-mation were used to propose a model forthe seismogenic source of the 1944 earth-quake, to characterize the regional structu-ral style, and to relate the rate and style ofcrustal deformation to GPS and other data(Brooks et al. 2000, Kadinsky-Cade et al.1985, Ramos et al. 2002, Smalley et al. 1993).

BEDROCK AND TERRACE DEFORMATION, NORTHWESTRANGE FRONT, SIERRA DE VILLICUM

A prominent topographic escarpmentmarks the northwest range front of Sierra

de Villicum. Whereas the range front hasbeen considered a simple thrust contact be-tween Cambro- Ordovician carbonates andNeogene red beds in the hanging wall andfootwall of the Villicum thrust, respectively(Siame et al. 2002, von Gosen 1992), severalthrust imbricates involving the Cambro-Ordovician and Neogene red beds are ex-posed with both faulted and folded con-tacts at the range front (Fig. 3). Northwest-dipping carbonates are unconformablyoverlain by less-steeply tilted red beds in thenorthwestern-most exposures in the studyarea (imbricate #1, Figs. 3 and 4a). To thesoutheast, the carbonates and red beds dipsoutheast (Fig. 4b). An anticline with a ste-eply dipping northwest limb and a modera-tely dipping southeast limb are defined by

the contact trace and bed dips (#1, Fig. 3).Red beds in the southeast limb of the anti-cline are cut by the Villicum thrust, an obli-que slip reverse fault (imbricate #2, Figs. 3,4b, and 4c). The Villicum thrust can be tra-ced in outcrop along strike to the south-west, but disappears into the subsurfaceand the range front is characterized at thesurface by northwest dipping Cambro-Or-dovician rocks unconformably overlain bynorthwest-dipping red beds (Figs. 3 and4d). This contact traces around the north-west end of Sierra de Villicum, which defi-nes a southwest plunging anticline, the axialtrace of which continues to the southwest.Terraces T1-3 are well preserved along thenorthwest range front (Fig. 3). Both T1 andT2 are continuous across the axial trace of

Figure 4: Views of the range-front fault system on the northwest flank of Sierra de Villicum. a) Western-most outcrop is the northwest limb of afold marked by an unconformable northwest-dipping depositional contact between Cambro-Ordovician strata and Neogene red beds (ball-arrowsymbols give dip direction). Terrace gravels of T2 depositionally overlie the contact and are not deformed. b) Cambro-Ordovician and Neogene stra-ta dip southeast in the southeastern limb of the fold. Red beds are truncated by the Villicum reverse fault on the southeast (trace marked by whitearrows). c) Dip of Villicum fault varies between 45° and ~50° southeast and has right-oblique shear sense striations on the fault plane. Cambro-Ordovician rocks in the hanging wall are juxtaposed with Neogene rocks in footwall. d) Reverse fault continues at surface to yellow arrow on thesoutheast (Fig. 3). Southwestward from yellow arrow to the southwestern end of Sierra de Villicum the range-front structure is a fold characterizedby northwest-dipping Cambrian unconformably overlain by Neogene red beds. Whereas the fault is well exposed in deeply incised washes along stri-ke, neither scarps nor steps occur in terraces where they overlie the fault trace.


the northwestern folded unconformity bet-ween the Cambro-Ordovician and the Neo-gene (Figs. 3 and 4a). The terraces extendacross the fold axial trace. To the southeast,the Villicum thrust occurs midslope abovethe elevation of the terraces along the rangefront (Fig. 4d). Because the Villicum thrustlooses elevation to the southwest, however,terrace relationships with faults andbedrock contacts can be resolved (Fig. 3).On the southwest, terraces T2 and T4 con-tinue without scarps across the Villicumfault trace and the northwest-tilted uncon-formity between the Cambro-Ordovicianand Neogene strata. Whereas the Villicumthrust has been interpreted to cut surficialdeposits (Siame et al. 2002), these field ob-servations indicate that although the rangefront is characterized by both folded andfaulted Cambro-Ordovician and Neogenered beds, neither the folds nor the Villicumfault affect the terraces along the northwes-tern range front. These observations sug-gest that the Villicum thrust and rangefront folds are either inactive or growingimperceptibly on the timescale of develop-ment of the terraces. Moreover they reaf-firm Whitney's (1991) conclusion, on thebasis of detailed mapping on aerial photo-graphs and field mapping, that the faultsystem along the northwestern range frontdoes not cut terraces of any age.

BEDROCK AND TERRACE DEFOR-MATION, SOUTHEAST FLANK,SIERRA DE VILLICUM

A 10 km-wide panel of shallowly- to mode-rately-inclined southeast-dipping red bedscharacterizes the southeast flank of Sierrade Villicum (Fig. 2a). A ~20° angular un-conformity marks the contact between theCambro-Ordovician sediments and theNeogene red beds. Two structural domainscan be differentiated within the red bedsexposed on the southeastern flank (Fig. 5).Shallow southeast bed dip (<15°) and nofaults or small-scale folds characterize thewestern domain. A laterally continuoussoutheast dipping thrust fault marks theboundary between the western and easterndomains. Small displacement thrust faultsand a southeast-facing monocline are deve-loped throughout the eastern domain (Figs.

2 and 5). Bedding dip changes from 10°-15°to 35°-45° from northwest to southeast,respectively, across the eastern domain.Mogna Formation, the youngest stratigra-phically differentiable bedrock unit, is wellexposed in three outcrop belts. On the nor-theast edge of the area the Mogna For-mation dips moderately southeast. On thesouthwest end of the area, in contrast, theMogna formation dips < 15° and faces

south (Fig. 2a). Bedding orientation in theunderlying Neogene red beds also changesfrom southeast facing in the northeast tosouth facing along strike to the southwest(Fig. 2a).A low relief topographic surface comprisedof a series of fluvial terraces etched intoNeogene red beds typifies the southeastflank of the range (Fig. 5). Individual terra-ces consist of a low-relief bedrock erosio-

Figure 5: Map of terrace surfaces T1-T3 on the eastern flank of Sierra de Villicum, San Juan,Argentina. Faults and folds that deform terraces define an eastern domain, which contrasts withthe western domain where terraces are undeformed. The La Laja fault, which ruptured in the1944 earthquake (Fig. 7a), and a regionally extensive active axial surface (red dashed line) mark thesoutheastern edge of the eastern domain. Figures 7b, 7c and 8 depict structural relationships inthe wash indicated by the red box near 'Calle La Laja'.

Figure 6: Profile of terraces and modern wash across eastern domain. Faults are steep due to 5xvertical exaggeration (VE) (upper profile; lower profile has no VE). Equivalent horizontal shorte-ning across faults for the T2 surface (Ht2) is calculated from vertical separation and a 40° faultangle. Box indicates structural location of photos in figure 7 and structural data summary offigure 8. LL marks the La Laja fault.


nal surface (a strath), which is overlain by 1-3 meters of poorly sorted limestone-and-dolomite-clast gravels. Map relationshipsand terrace profiles indicate that the terra-ces are undeformed in the western domain.Both terrace surfaces and modern channelprofiles are concave up and have a nearlyconstant gradient of ~ 3° across the wes-tern domain. In contrast, folds and faults ofthe eastern domain affect terraces of nearlyall ages (Fig. 6). Displacement of terraces

across faults varies as a function of age anddoes not exceed ~10 m for T1. Roughly 17m of horizontal shortening is deducedfrom the fault dip and separation of T2straths across faults in the eastern Domain(Fig. 6).The most pronounced deformation of te-rraces is localized in a narrow region nearthe La Laja fault scarp on the southeasternedge of the eastern domain (Figs. 2, 5 and7). In this region, the shallowly dipping

Neogene strata are tilted to the southeast ~40° (Fig. 7). Photographs taken after the1944 event show that surface rupture onthe La Laja fault is approximately parallel tobedding (Fig. 7a) (Harrington 1944). Rou-ghly 60 cm of uplift was measured from aroad warped by surface rupture on thefault. Total displacement of individual te-rrace surfaces is larger than that observedfrom the 1944 earthquake (Fig. 7b), sugges-ting that the terrace surfaces provide a com-

Figure 7: Photos of deformation style on southeastern limit of the eastern domain, southeast flank Sierra de Villicum. (A) Surface rupture on the LaLaja fault in 1944 (from Harrington 1944). Resistant beds are inclined at a similar angle to the surface rupture trace. (B) Exposure of La Laja faultsouthwest of La Laja road. Bed and fault dip (ball-ended arrow) are similar. Terraces 2 and 4 occur in the hanging wall and footwall, respectively. (C)(D) Views to the southwest and northeast of deformed terraces in a wash ~100 m southwest of La Laja road (red dot, Fig. 5). Solid lines mark strathsurfaces at the base of terraces; dashed lines mark the abandoned surface. Monoclinal fold hinges within individual terraces do not fold older oryounger terraces or underlying Neogene bedrock. Fold hinges in structurally lower and geomorphically lower terraces are located systematically to thesoutheast of hinges in older, higher surfaces. (E) Synclinal hinge controlling the southeastern break in slope exposed in Neogene bedrock (red dashedline, Fig. 5).

596 A. MEIGS, W. C KRUGH, C. SCHIFFMAN, J . VERGÉS Y V. A. RAMOS

posite record of earthquakes prior to andincluding the 1944 earthquake. Dip of thered bed sequence and the La Laja fault arethe same suggesting the fault result frombed-parallel slip during the earthquake (Fig.7b) (Krugh 2003). Therefore, the La Lajafault is interpreted to be a flexural slip fault(Krugh 2003), which are secondary faultsrelated to folding (Yeats 1986). Folded te-rraces in the hanging wall to the southeastof the La Laja fault also record deforma-tion related to folding (Figs. 7c-e).Geometric relationships between individualterraces and underlying bedrock are wellexposed in southeast-draining, transversestream channels (Figs. 5 and 7). Southeast-facing monoclines typified by a change indip from ~ 3°SE to ~15°-20°SE are pre-sent in T1-3 (Figs. 7c and d). Hinges preser-ved in individual terraces are unique to agiven terrace and affect neither younger norolder surfaces. Hinge location change inspace systematically as a function of age;the hinge in T3 is located to southeast ofT2 (Fig. 7d), which is to the southeast ofthe T1 hinge (Fig. 7c). Underlying Neogenebedrock is not folded about any of the hin-ges, in contrast, and has a uniform dip of~40°SE beneath each of the terraces. Aprominent break in slope marks the southe-astern limit of the tilted terraces. TerraceT5 is undeformed and inset into older te-rraces in transverse channels but onlap ol-der surfaces on interfluves. Thus the breakin slope is a depositional contact betweenolder, tilted terraces to the northwest and

the youngest undeformed terraces on thesoutheast. Along strike to the northeast,however, a folded unconformity within theNeogene red beds reveals the nature of thesame break in slope in terms of bedrockstructure. Dip of strata above the uncon-formity shallows from ~30° to ~10° andfrom ~60° to ~40° below the unconfor-mity from northwest to southeast, respecti-vely (Fig. 7e). The break in slope is therefo-re interpreted as the trace of a fold hingeseparating more steeply from shallowlysoutheast dipping strata in the Neogenebedrock. At the structural and stratigraphiclevel of the terraces, the hinge separates til-ted terraces on the northwest from equiva-lent undeformed sediments on the southe-ast. The youngest terrace deposits are a thinveneer deposited across the hinge.

TERRACE FOLD MODEL

The observed geometric relationships bet-ween bedrock structure, folded terraces,and topography constrain the style of foldgrowth and shortening rate across the sou-theastern flank of Sierra de Villicum. Anymodel for fold growth must account for theprincipal structural and geomorphic obser-vations along the southeastern edge of theeastern domain, as summarized in figure 8.At the structurally lowest level in thebedrock, bedding changes from shallow tosteeper dip across a synclinal hinge thatcoincides with the topographic break inslope at the southeastern edge of the eas-

tern domain (Figs. 5 and 7e). Bedding to thenorthwest of the hinge has invariant dip,despite the complex folding of terraces atshallower structural levels. The La Laja faultis a flexural slip fault parallel to bedding(Figs. 7a and b). Terrace folds have a mono-clinal geometry marked by a hinge uniqueto each terrace (Figs. 7c and d). Hingesdeveloped in a terrace of a given age do notaffect bedrock or older or younger terraces.Hinge position in space increases with dis-tance northwest from the synclinal hinge asa function of age. A model of fold growthvia kink-band migration best reconcilesthese features (Fig. 9). Kink-band migration is a mechanism offolding whereby beds change dip due torotation about axial surface(s) (Suppe 1983,Suppe et al. 1997). The region between twocorresponding axial surfaces is termed a"kink-band". Fold growth results from mi-gration of one or both axial surfaces defi-ning a kink band, which translates materialinto the fold limb. In growing structures,one or both of these axial surfaces maymove relative to bedrock (Suppe et al. 1992).An active axial surface refers to a hinge thatmoves with respect to rock whereas a fixedaxial surface is one that is stationary withrespect to rock. As an active axial surfacemigrates, rock is instantaneously rotatedabout the axis and translated onto the foldlimb. Fold growth rate can be determinedwhen the position of the fixed and activeaxial surface can be determined in time(Mueller and Suppe 1997, Shaw and Suppe

Figure 8: Summary sketch (to scale) ofstructural relationships in region ofwash southwest of La Laja road (redbox, Fig. 5; photos in Figs. 7b and c).Topographic profile, T1, T2, and bed-ding are from ridge on southwest sideof wash. T4-5 are from outlet ofwash on southeast. T3 projected onto profile from ridge on northeast sidewash (Fig. 7c). T2-4 in footwall of theLa Laja fault are projected from washadjacent to the La Laja road (Fig. 5).Bed form lines in Neogene bedrockschematically illustrate bedding.Relationships above ~610 m elevationare observed in the field. Structurebelow 610 m is pro-jected on the basisof surface data.


1996, Suppe et al. 1992).Strata deposited coevally during foldgrowth are referred to as syntectonic orgrowth strata (Riba 1976). Growth stratageometry depends on geometrical changesof fold limbs during folding (DeCelles et al.1991, Ford et al. 1997, Suppe et al. 1997,Vergés et al. 1996). Hinge migration relativeto rock and to other hinges dictates growthstrata geometry for folds formed due tokink band migration (Suppe et al. 1992).For the case of motion of an active axialsurface relative to a fixed axial surface, de-position across the fixed hinge results information of a new hinge, a growth axialsurface, which is fixed with respect to theaccumulating growth strata (Suppe et al.1992). At the moment after a bed is deposi-ted, but prior to its incorporation into thefold, the growth and active axial surfacesintersect at a point at the surface. Conti-nued migration of the active axial surfacerelative to the growth axial surface creates agrowth triangle, the size of which is pro-portional to the total amount of syndeposi-tional fold growth (Suppe et al. 1997).Growth axial surfaces, active axial surfaces,and growth triangles thus preserve a recordof fold growth because the displacement ofthe active axial surface relative to the

growth axis is represented by their offset inspace. Because older growth strata haveendured more folding, growth trianglewidth progressively decreases in youngergrowth strata. When sedimentation is unsteady relative tofold growth, such as during punctuatedevents like earthquake induced folding orclimatically variable deposition or erosion,discrete growth axial surfaces form in sedi-ments deposited between growth events(Mueller and Suppe 1997). To illustrate asequence of alternating erosion/depositionand uplift events and the consequent affecton terraces and bedrock, a simple model ofmigration of a single active hinge was cons-tructed (Fig. 9). A channel forms a strath asit bevels an erosional surface into titledbedrock and deposits a gravel cap in step 1(Fig. 9a). Migration of the active hinge bysome arbitrary horizontal amount (H1) ro-tates rock and the terrace as they are drawninto the fold limb, although the amount ofrotation is a function of initial dip (Fig. 9b).Incremental folding results in formation ofa growth hinge offset a distance H1 withrespect to the active hinge. Uplift of thestrath at the base of the terrace (V1) de-pends on H1 and tilt of the bedrock outsi-de of the kink band. Renewed down cutting

of the channel forms a new strath/ gravelcap pair comprising a younger terrace (Fig.9c). A second folding event de-forms thenewly formed terrace, creates a growthhinge in the new surface, and the incre-mental fold growth is given by horizontaloffset of the growth hinge with respect tothe active hinge and the uplift of the strathrelative to the modern channel, H2 andV2, respectively (Fig. 9d). Whereas thecumulative growth is given by the sum ofthe vertical and horizontal motions, allstructures on the side of the hinge opposi-te to the direction of active hinge migra-tion experience uplift, but no additional til-ting. Thus, tilted bedrock and terraces andearlier-formed growth axes are not tiltedfurther during subsequent folding events.The position of a growth axial surface istherefore dependent on the migration rateof the active axial surface relative the timerequired to form the strath/gravel cap pairof a terrace. Discrete earthquake eventsare recorded by growth strata formed by

kink band migration only under specialconditions that include depositional eventswith greater frequency than folding events,fold growth in scale that exceeds the scaleof interseismic sediment accumulation, andstructural tilting that is greater than primarydepositional dip (of bedding and bedforms).

ACTIVE DEFORMATION ANDSHORTENING RATES CROSS THEEASTERN PRECORDILLERA

Active deformation is localized on the sou-theast flank of Sierra de Villicum (Figs. 2, 3,and 5). No evidence of folding or thrustfaulting of terraces is observed along theVillicum thrust fault or folds cropping outalong the northwestern range front (Figs. 3and 4d). In contrast, abundant evidence foractive folding is represented on the southe-ast edge of the southeast flank of Sierra deVillicum (Figs. 5, 6, and 7). The La Lajafault, for example, is a flexural slip faultthat ruptured in the 1944 earthquake(Harrington, 1944) and offsets all of theterraces, older terraces have greater verticalseparation than younger surfaces (Fig. 8).This later observation argues for the long-term flexural slip faulting accompanying

Figure 9: Terrace fold growth model. (A) Fluvial incision cuts a strath terrace into folded bed-rock. A thin (1-3 m-thick) gravel layer overlies the strath (grey bar). (B) A fold event deforms theterrace due to a migration of a hinge (dash) through the rock. Strata to the right of the hinge in(A) uplift but do not tilt, whereas strata to the left uplift and tilt, which forms a hinge (solid)fixed within the terrace. Incremental shortening (H1) and uplift (V1) are given by the offset ofthe hinges and uplift of the strath, respectively (C) Erosion into the fold results in formation of asecond terrace, represented by the strath and gravel cap (dotted bar). (D) A second fold eventforms a hinge restricted to the second terrace. Total shortening and uplift are equal to the sum ofthe incremental horizontal (Hn) and vertical (Vn) motion. Compare (D) with figures 6b, 6c, and 7.

598

fold growth in earthquakes. Terraces of allages are folded as well. A kink-band migra-tion model in which uplift, erosion, anddeposition are out of phase fits the geo-metry of deformed terraces on the southe-ast flank well (compare Figs. 6, 7 and 8).Determination of shortening rates acrossthe zone of active folding thus requiresabsolute age assignments for the varioussurfaces. To date, no age constraints, unfor-tunately, are available from the terraces inthe study region.Six kilometers southwest of Sierra de Villi-cum, however, a suite of three terraces hasbeen dated using cosmogenic radionuclides(Siame et al. 2002). Siame et al. (2002) map-ped terrace surfaces A3, A2, and A1 thatare elevated 11±1 m, 6±2 m, and ~0.6 mabove the modern channel, respectively.Radionuclide concentrations from A3, A2,and A1 indicate ages of 18.7 - 18.0 ka, 6.9 -5.3 ka and 1.9 - 1.1 ka, respectively. Theterraces are cut into Neogene red beds thatare structurally contiguous with the southe-astern flank (Fig. 1). Because the terraces ofthis study and those of Siame et al. (2002)occur within the same structural domain, aphysical correlation between the two areaswas performed to provide estimates on theages of the terraces in this study. TerracesT1-4 were physical correlated by followingterraces up and down channels and acrossadjacent drainages from the northeast cor-ner of the study area on the southeast flankacross strike to the southwestern end ofSierra de Villicum and around to the north-

west range front (Fig. 2b). Strath and gravelcap elevation with respect to modern chan-nels form the basis of the correlation.Uncertainty in the correlation was introdu-ced by correlation across regions with limi-ted preservation of T1-3, correlation acrosssmall-displacement faults, by variable eleva-tion of modern channels, and by paleoto-pography, paleocanyons in particular, pre-served between strath surfaces and cappinggravels. Uncertainty in the age determina-tions are introduced because inheritancewas not specifically considered in the sam-pling or subsequent evaluation of the nucli-de concentrations (Siame et al. 2002). Mo-reover, inspection of the Siame samplingsites suggests the possibility that one ormore of the surfaces are cut-in-fill terracesand therefore have a more complex expo-sure history than a simple abandoned fillterrace. Either of these two sources of un-certainty implies that the terrace ages maybe minimum ages, which means that struc-tural rates based on correlations are maxi-mum values.Horizontal shortening rates are invertedfrom the horizontal separation of growthaxial surfaces in T1, T2, and T3 relative tothe active axial surface to the southwest(Figs. 8-10). On the basis of our terracemapping, older and younger correlationswith the terraces dated with cosmogenicradionuclides of Siame et al. (2002) are per-missible (Fig. 10). A 'younger' correlationmakes T1 equivalent with A3, T2 correlati-ve with A2, and yields a maximum shorte-

ning rate of 5 mmyr-1 or greater (Fig. 10).An 'older' correlation ties T2 to A3, T3 orT4 to A2, and implies a minimum shorte-ning rate of ~3 mmyr-1 (Fig. 10). These rateapproximations assume constant shorte-ning since the formation of T2 or T1 forthe older and younger correlations, respec-tively. Independent age determinations inprogress for the terraces of this study willprovide independent constraint on terraceage and therefore structural rates.

CRUSTAL SCALE MODEL FORREFOLDING OF THIN SHINNEDTHRUST SHEETS BY BASEMENTDEFORMATION

A number of observations suggest that theSierra de Villicum thrust sheet is being acti-vely refolded. The thrust sheet extendsfrom the Villicum thrust on the northwestto the Tulum Valley syncline on the southe-ast (Fig. 1). Dip data and stratigraphic con-tact relationships reveal the presence of astructural high in the middle of the southe-ast flank of Sierra de Villicum. Dip in thered beds defines a southeast facing mono-cline characterized by a change from sha-llowly southeast- to moderately southeast-dipping within the western domain (Fig. 2).Active folding is localized in the moderatelydipping limb of the monocline, which iswhere the La Laja fault and folded terracesare located (Figs. 5 and 8). One key activestructure in this region is the active axialsurface that crops out in both the terracesand in the bedrock (Figs. 2 and 7). Strike ofthe contact between the red beds and theoverlying Mogna Formation changes fromnortheast to west along strike from northe-ast to southwest. In the region where thecontact is west striking, both the MognaFormation and the red beds dip south (Fig.2b). Together, these observations indicatethat the middle of the Sierra de Villicumthrust sheet is marked by a structural high,the structural high is a southeast-facingmonocline, and active folding is concentra-ted on the southeast limb of the monocline(Fig. 11).A structural high between the Precordilleraon the west and the eastern Precordillera onthe east is required by structural and strati-graphic relationships (Cristallini and Ramos

Figure 10: Horizontal shortening rate estimates based on alternative terrace correlations with Siameet al. (2002) and horizontal displacement measurements of growth hinge relative to active hinge interraces and bedrock (Fig. 8). Constant maximum (long dash) and minimum (solid) rates (in millime-ters per year (mm/yr)) depend on whether T1 ('younger' correlation, open diamonds) or T2 ('older'correlation, closed diamonds) correlate with Siame's 18.7 ka terrace. Variable shortening rate in timeis implied by the 'younger' correlation (short dash). Age error is from Siame et al. (2002) (± 2.3 and± 1.0 ka for 18.7 and 6.8 ka surfaces, respectively). Hinge position is assigned a ± 5 m error.


2000, Ramos et al. 2002, von Gosen 1992).The basal décollement of the Precordilleradips west (Fig. 11). If the hanging wall ofthe Villicum thrust fault is a hanging wallflat, which is consistent with the dip of thefault and hanging wall bedding (Figs. 3 and4), the Tulum Valley syncline would repre-sent the down dip branch line of the Villi-cum thrust system with a regional décolle-ment near the base of the Cambrian Strata(von Gosen 1992). This geometry indicatesthat the Ullum Valley to the northwest ofSierra de Villicum sits above an arch in thebasement separating opposing regional dipsof the décollement beneath the Precordi-llera and eastern Precordillera (Figs. 1 and11). Microseismicity beneath this region issuggestive of an active fault(s) between 5and 35 km depth in the basement (Smalleyet al. 1993). The principal plane revealed bythese data is N45E° striking and dips 35° tothe northwest (Fig. 11). Focal mechanismsand focal depths of current seismicity indi-cate active reverse faulting concentrated be-tween 20 and 30 km at depth (Fig. 1b)(Kadinsky-Cade et al. 1985, Ramos et al.2002, Siame et al. 2002, Smalley et al. 1993).Thus, field relationships, the loci or activedeformation, historical seismicity, and mi-croseismicity can be linked in a structural

model that relates active refolding of theSierra de Villicum thrust sheet to an activenorthwest-dipping fault that extends to themiddle crust at depth (Fig. 11).Four alternative models relate basementfaulting in the middle crust with accommo-dation of slip on upper crustal structures(Fig. 12). A simple fault-propagation foldwhere the east-facing monocline and struc-tural high between the Precordillera andeastern Precordillera are the forelimb andcrest of the fold, respectively, growing abo-ve fault tip below the cover sequence (Fig.12a). Slip from the basement may be trans-ferred to the detachment at the base of theSierra de Villicum thrust sheet and daylighton the east side of the Tulum Valley syncli-ne (Fig. 12b). Third and fourth alternativesare represented by a wedge geometry whe-reby east-directed slip on the basementfault is accommodated in the cover sequen-ce by west-directed thrusting (Figs. 12c, d).Transfer of slip to the La Laja fault was firstproposed by Smalley et al. (1993) to accountfor the fact that La Laja and basementfaults have opposing dips (Fig. 12c). Sliptransfer to the Villicum thrust system couldalso absorb slip at depth on the basementfault (Fig. 12d).Neither the transfer of slip to a thrust on

the east side of the Tulum Valley synclinenor to the La Laja fault is supported by fieldobservation (Figs. 12b, c). No active eastvergent faults are present between the westflank of Pie de Palo and the hinge of theTulum Valley syncline (Fig. 1a) (Ragona etal. 1995). Stratigraphic separation across theLa Laja fault would have to be proportiona-te to slip on the basement thrust fault, inthe case of the latter alternative, yet field re-lations suggest that separation is effectivelyzero (Figs. 7 and 8). Whereas none of theimbricates of the Villicum thrust systemapparently affects the terrace sequence, it ispossible that shortening on the fault systemis reflected at a longer wavelength. There-fore, it is reasonable to conclude that slipon the basement fault system is accommo-dated by a combination of monoclinal fol-ding above the fault tip, which is potentiallyaccompanied by displacement transfer toone of the faults within the Villicum thrustsystem (Figs. 12a, d).We propose that the 1944 earthquake occu-rred on the northwest-dipping basementfault that extends into the mid-crust bene-ath Sierra de Villicum. This model con-trasts, however, with current models for the1944 source. Those models propose thatthe earthquake occurred to the east of Sie-

Figure 11: Crustal scale model relating basement seismicity and upper crustal structure beneath the eastern Precordillera. Whereas a thrust sheetcarrying Cambro-Ordovician sedimentary cover strata underlies Sierra de Villicum, active deformation is restricted to folding on the southeastflank. Microseismicity between ~5 and 35 km depth defines a northwest-dipping fault plane (black dots) (Smalley et al. 1993), which is inferred tobe the source of the 1944 San Juan earthquake. Active folding of terraces and flexural slip faulting on the La Laja fault are secondary effects ofthrust sheet refolding. A basement high between the Precordillera and the eastern Precordillera is required by the opposing regional dips betweenthe two structural domains. Precordillera structure and stratigraphy are modified from von Gosen (1992). Eastern Precordillera structure is modi-fied from Krugh (2003). Sierras Pampeanas structure and seismicity (light dots) modified from Ramos et al. (2002).


rra de Villicum on an east-dipping faultsystem that extends into the basement(Alvarado and Beck 2006, Perucca and Pa-redes 2000, 2002, Siame et al. 2002). Anearthquake source on an east dipping faultsystem is based on the inference that theVillicum thrust system ruptured in the 1944event and is therefore the up-dip continua-tion of a fault that extends to ~20 kmdepth beneath the Tulum Valley (Siame etal. 2002). Others contend that because ofthe surface rupture in the earthquake, theeast-dipping La Laja fault is the up-dip con-tinuation of the seismogenic fault (Alva-rado and Beck 2006, Perucca and Paredes2000, 2002, Smalley et al. 1993). A sourceon an east-dipping fault is consistent withtwo of the three reported locations for theepicenter of the 1944 earthquake, which lieto the southeast of Sierra de Villicum (Fig.1) (Alvarado and Beck 2006). The La Lajafault can be connected with a hypocenter at~11 km depth beneath the Tulum Valley,which was determined via an inversion ofhistoric seismograms based on assumingapriori that epicenter is to the east (Alva-rado and Beck 2006). In this model the LaLaja fault is an imbricate splay off the faultsystem proposed in the Siame et al. (2002)model.A number of observations can be used toevaluate the competing structural modelsfor the eastern Precordillera and thereforethe potential seismic sources for the 1944event. The key observations include the re-lative activity and style of faulting on theVillicum and La Laja faults, the location and

geometry of active folding and faulting,alternative epicentral locations for the 1944earthquake, and the regional distribution ofseismicity with depth. Field data indicatethat the Villicum thrust system is eitherinactive or moving at a rate slower than canbe recorded by geomorphic markers, giventhat none of the individual thrust imbrica-tes of the fault systems cuts or folds terra-ces of any age (Figs. 3 and 4). Althoughthere is no field evidence for active slipalong the fault trace, it is possible that someslip on the thrust may accumulate as theconsequence of folding or slip transferfrom the basement fault (Fig. 12). Bedding-fault relationships across the La Laja faultare consistent with a flexural slip interpreta-tion of the active faulting (Figs. 7a, 7b, and8). Thus, surface rupture of the La Lajafault reflects coseismic fold growth and notsurface rupture of the primary seismogenicfault, which is consistent with the locationof the fault within the actively growingmonocline.Bedrock and terrace geometries indicatethat the east-facing monocline is refoldingthe Sierra de Villicum thrust sheet (Figs. 2and 11). A model where the active defor-mation at the surface is dominated foldingabove a northwest dipping basement thrustreconciles the fold-related flexural slip faul-ting on the La Laja fault, bedrock and terra-ce structural relationships, and the spatialdistribution of active structures. A faultthat extends westward to ~35 km depth, assuggest by microseismicity (Smalley et al.1993), is consistent with an epicentral loca-

tion to the west of Sierra de Villicum (Fig.1), which is one of three proposed loca-tions (Alvarado and Beck 2006). Moreover,the majority of earthquakes occur between20 and 30 km (Fig. 1b) (Smalley and Isacks1990). If the hypocenter of the earthquakeis shallow (~11 km) and to the east ofSierra de Villicum, as suggested by the epi-central locations in the vicinity of San Juan(Fig. 1) and the inversion of historic seis-mograms (Alvarado and Beck 2006), thewest-dipping crustal fault proposed hererepresents a second and previously unre-cognized active seismic source in the boun-dary region between the Precordillera andSierras Pampeanas.Global positioning system (GPS) data indi-cate that the rate of convergence of thePrecordillera with respect to the SierrasPampeanas across the eastern Precordillerais ~4.5 mmyr-1 (Brooks et al. 2003), whichprovides a framework for evaluating shorte-ning rates determined from the deformedterraces. Horizontal shortening rate acrossthe southeastern flank of Sierra de Villicumis estimated to be no more than 3 mmyr-1

(Fig. 10). That rate is determined from thesum of shortening of local folding of terra-ces where the folding is being forced by alocal change in dip (Fig. 8) and by the dis-placement of faults in the eastern domain(Figs. 5 and 6) (Krugh 2003), of a regionallyextensive long-wavelength monocline (Fig.11). If the fold is growing via kink bandmigration, the horizontal component offold growth is controlled by the underl-ying fault orientation and slip rate (Suppe

Figure 12: Alternative models for theaccommodation of slip in the coversedimentary rocks (grey shading) dueto fault displacement on the basementreverse fault. A) Fault slip is accom-modated by fault-propagation foldingabove the fault tip. (B) Fault slip istransfered to a fault system beneaththe Tullum Valley syncline (TVs). (C)Fault slip is transfered to the La Lajafault on the east flank of Sierra deVillicum (SdV). (D) Fault slip istransferred to the imbricate fan thrustsystem bounding the west flank ofSierra de Villicum. Depth of seismi-city is indicated by cross-hatched pat-tern (after Smalley and Isacks 1990).


et al. 1992). Shortening at ~3 mmyr-1 across the easternPrecordillera due basement thrust faultingis consistent with seismicity and short- andlong-term shortening rates measured regio-nally. Evidence of active folding and faul-ting is present across Pie de Palo to the eastof Sierra de Villicum (Fig. 1a) (Ramos et al.2002). Shortening rate across Pie de Palo isof 4 mmyr-1 since 3 Ma (Ramos et al. 2002),The sum of the rates across Pie de Palo andSierra de Villicum (~7 mmyr-1) are consis-tent with, albeit higher, than the GPS-deter-mined rate change between the Precor-dillera and the Sierras Pampeanas (4.5mmyr-1, Brooks et al. 2003). If the sum ofthe two rates represents the long termregional shortening rate, despite the factthat rates are measured over differenttimescales, it seems likely that the ~3 mmyr-

1 horizontal rate across the eastern Precor-dillera is a maximum estimate (Figs. 8 and9). Independent age determinations for theterraces in the study are will provide betterconstraints on the deformation rates withinthe eastern Precordillera. Shortening rate at31° S north of the study area is ~5 mmyr-1

since 2.7 Ma (Zapata and Allmendinger1996a). Seismicity is focused beneath theeastern Precordillera and Sierras Pampea-nas region (Alvarado and Beck 2006,Kadinsky-Cade 1985, Kadinsky-Cade et al.1985, Siame et al. 2005, Smalley and Isacks1990, Smalley et al. 1993), which supportsthe inference that active deformation isconcentrated deformation in eastern Pre-cordillera-western Sierras Pampeanas region. Fault dip is a key difference between thecrustal model proposed in this study andthe structural model used to interpret theGPS velocity field. An elastic model of the

GPS data using a 10° west dipping planefits well the velocity gradient across thePrecordillera-Sierras Pampeanas transition(Brooks et al. 2003). The rationale for thisgeometry is a cross-section across the Ber-mejo basin north of 31° S (Zapata andAllmendinger 1996a). A key feature of thismodel of the GPS velocity field is that themodel fault represents the basal décolle-ment of the Precordillera (Brooks et al.2003). At 32° S in the study area, however,there is little evidence that the frontal thrustof the Precordillera is active. Historical seis-micity, recent earthquakes, and the crustalscale model presented here suggest thatactive structures involve the basement todepths of ~35 km (Figs. 1 and 10) (Costa etal. 2000, Costa et al. 1999, Kadinsky-Cade etal. 1985, Ramos et al. 2002, Smalley et al.1993). Active, strain-accumulating faultsthat continue at moderate dips to depths of~35 km are permissible from the perspecti-ve of the GPS velocity field given that thefact the data are consistent with a range ofmodel fault dips (Brooks et al. 2003).Other thrust sheets in the eastern Precor-dillera have similarities with the Villicumthrust sheet. Cerro Salinas is an isolatedoutcrop of Cambro-Ordovician carbonatesunconformably overlain by Neogene redbeds (Fig. 1) (Comínguez and Ramos 1991,Vergés et al. 2002). An east-dipping thrustfault marks the western boundary of thethrust sheet and both long-term and activefolding are concentrated on the eastern,down-dip end of the thrust sheet (Fig. 13).A monoclinal geometry marks the style offolding and the geometry of growth stratain the red beds is consistent with foldgrowth due to kink band migration. Thebasement, be-low the basal décollement is

structurally higher in the footwall on thewest than it is beneath the undeformedforeland on the east (Fig. 13). Cerro Salinasis apparently underlain and folded by anantiform that involves the basement.Moreover, active deformation is concentra-ted in the east, which is the forelimb of thefold, rather than on the west along the sur-face trace of the thin-skinned thrust sheet.Thus, the model for basement faulting pro-posed for Sierra de Villicum (Fig. 11) mayalso apply to Cerro Salinas (Fig. 13), whichsuggests that the structural style of activedeformation in the boundary region betwe-en the thin- and thick-skinned Precordilleraand Sierras Pampeanas provinces involvesactive basement-involved thrusting that isoverprinting earlier-formed thin-skinnedthrust sheets. The onset of this transition isuncertain, but has implications for seismicsources for large cities in western Argentinathat lie in the foothills of the Andes.

CONCLUSIONS

1. The principal structure in the easternPrecordillera northwest of San Juan, Ar-gentina is a southeast dipping thrust sheetbeneath Sierra de Villicum, which is boundat the base on the northwest by theVillicum reverse fault along the range frontand on the southeast by the Tulum Valleysyncline. Cambro-Ordovician carbonatesunconformably overlain by Neogene redbeds comprise the bedrock of the thrustsheet.2. A suite of 5 fluvial terraces beveled intoNeogene red bed bedrock provide markersthat allow the late Quaternary deformationin the eastern Precordillera at ~32° to becharacterized.

Figure 13: Interpretation of seismic line 31017 from Repsol-YPF (from Vergés et al. 2002). The Las Peñas thrust marks the leading edge of thePrecordillera thin-skinned thrust belt and is active (Costa et al. 2000). Note that Cerro Salinas has an east-facing monoclinal geometry in the cen-ter of the thrust sheet. The basement is shallower on the west than it is on the east. Growth strata document the long-term growth of the fold.


3. The Villicum thrust system along thenorthwestern range front of Sierra de Vi-llicum juxtaposes thrust imbricates of fol-ded Cambro-Ordovician carbonates againstNeogene red beds. All the field evidenceindicates that (A) none of the imbricatethrusts extends southwest of the Paleozoicrocks holding up Sierra de Villicum, (B) theterraces overlie faults of the thrust system,and (3) no fault scarps are preserved in te-rraces of any age.4. Active deformation in the eastern Pre-cordillera at 32° is localized on the southe-astern flank of Sierra de Villicum, in themiddle of the Villicum thrust sheet.5. A number of active small displacementactive faults cut fluvial terraces. A narrow,well-defined region on the southeast flankof Sierra de Villicum contains evidence offolding, which includes the La Laja faultthat ruptured in the 1944 San Juan earth-quake.6. Evidence for active folding includes fle-xural slip faulting on the La Laja fault,which is a bedding parallel fault in theNeogene bedrock that cuts terraces of allages. The oldest three terraces (T1-3) arefolded and the fold geometry is characteri-zed by monoclinal form. Individual terra-ces are folded about hinges that are uniqueto that terrace and affect neither older noryounger terraces or bedrock. Hinges inyounger terraces are located to the southe-ast with respect to hinges in older terraces.7. A kink band model of fold growth satis-fies all of the key field observations at thestructural level of the bedrock and theterraces. From this model the shorteningsince abandonment of the oldest terrace,T1, can be estimated.8. Correlation with dated terraces roughly15 km to the southwest (18.7-18.0 ka. 6.8-5.8 ka, and 1.9-1.1 ka) (Siame et al. 2002)allows for older and younger age assign-ments for terraces T1-3 of this study. Ma-ximum shortening rate across the easternPrecordillera is ~3 mmyr-1, as determinedfrom the integration of the fold model andterrace correlation. These rates are consis-tent with short-term rates determined fromGPS (Brooks et al. 2003) and with long-term rates determined via cross-sectionsand growth strata to the north of 31° S(Zapata and Allmendinger 1996a).

9. The loci of active folding, dip data andbedrock contacts suggest that the Sierra deVillicum thrust sheet is being actively fol-ded by a deeper structure. A crustal modelis proposed to reconcile the pattern of fol-ding of the thrust sheet, a northwest-dip-ping zone of microseismicity, and a structu-ral high in the basement between thePrecordillera on the west and easternPrecordillera on the east. In this model,active deformation of the eastern Precor-dillera is a function of folding of uppercrustal thin-skinned structures due to slipon a northwest-dipping blind thrust faultthat extends to ~35 km depth.10. Near surface structures record activefolding and coseismic faulting during the1944 earthquake, which implies that thebasement reverse fault is the source of the1944 San Juan earthquake.11. Basement rocks beneath Cerro Salinas,another eastern Precordillera thrust sheetto the southwest, are also characterized byan antiformal geometry, which suggeststhat blind thrust faulting on east-vergentbasement faults represents a significant,underappreciated seismic hazard in westernArgentina.

ACKNOWLEDGMENTS

Drs. Ernesto Cristallini and Matias Ghiglioneprovided careful and thoughtful reviewsthat improved the quality of this work.Carlos Costa, Daniel Ragona, Robert Yeats,and Emily Schultz are thanked for nume-rous discussions of this paper and the seis-motectonics of western Argentina. AJMwas supported by U.S NSF grants EAR-0232603and EAR-0409443 and a ResearchEquipment Reserve Fund grant fromOregon State University. We thank Repsol-YPF S.A. for providing us with the seismicline and for the permission to publish it(Fig. 13). J.V. was partially supported by99AR0010 CSIC-CONICET project andGrups de Recerca Consolidats (II Pla deRecerca de Catalunya) Projects 1997 SGR00020.

WORKS CITED IN THE TEXT

Alvarado, P. and Beck, S. 2006. Source characte-rization of the San Juan (Argentina) crustal

earthquakes of 15 January 1944 (M7.0) and11 June 1952 (M6.8). Earth and PlanetaryScience Letters 243(3-4): 615-631.

Bastias, H. E., Weidmann, N. E. and Perez, A. M.1985. Dos zonas de fallamiento Pliocuater-nario en la Precordillera de San Juan Trans-lated: The Plio-Quaternary fault zone in thePrecordillera of San Juan. 9°Congreso Geo-lógico Argentino, Actas 2: 329-341.

Brooks, B. A., Bevis, M., Smalley, R., Kendrick,E., Manceda, R., Lauria, E., and Araujo, M.2003. Crustal motion in the southern Andes(26°-36° S): Do the Andes behave like amicroplate? Geochemistry, Geophysics, Geo-systems 4(10): 1-14.

Comínguez, A. H. and Ramos, V. A. 1991. Laestructura profunda entre Precordillera ySierras Pampeanas de la Argentina: Eviden-cias de la sísmica de reflexión profunda. Re-vista Geológica de Chile 18: 3-14.

Costa, C. H., Gardini, C. E., Diederix, H., andCortés, J. M. 2000. The Andean orogenicfront at Sierra de Las Peñas-Las Higueras,Mendoza, Argentina. Journal of SouthAmerican Earth Sciences 13(3): 287-292.

Costa, C. H., Rockwell, T. K., Paredes, J. D., andGardini, C. E. 1999. Quaternary deforma-tions and seismic hazard at the Andean oro-genic front (31°-33°, Argentina): a paleoseis-mological perspective. 4° International Sym-posium on Andean Geodynamics (Goettin-gen), Proceeding 187-191.

Costa, C. H. and Vita-Finzi, C. 1996. Late Holo-cene faulting in the Southeast Sierras Pampe-anas of Argentina. Geology 24(12): 1127-1130.

Cristallini, E. O. and Ramos, V. A. 2000. Thick-skinned and thin-skinned thrusting in the LaRamada fold and thrust belt; crustal evolu-tion of the High Andes of San Juan,Argentina (32° SL). Tectonophysics 317(3/4): 205-235.

DeCelles, P. G., Gray, M. B., Ridgway, K. D.,Cole, R. B., Srivastava, P., Pequera, N., andPivnik, D. A. 1991. Kinematic history of aforeland uplift from Paleocene synorogenicconglomerate, Beartooth Range, Wyomingand Montana. Geological Society of AmericaBulletin 103: 1458-1475.

Fielding, E. J. and Jordan, T. E. 1988. Activedeformation at the boundary between thePrecordillera and Sierras Pampeanas, Argen-tina, and comparison with ancient RockyMountain deformation. Geological Society ofAmerica, Memoir 171: 143-163.


Ford, M., Artoni, A., Williams, E. A., Vergés, J.,and Hardy, S. 1997. Progressive evolution ofa fault propagation fold pair from growthstrata geometries, Sant Llorenç de Morunys,SE Pyrenees. Journal of Structural Geology19(3-4): 413-441.

González Bonorino, F. 1950, Algunos problemasgeológicos de las Sierras Pampeanas: Revista dela Asociación Geológica Argentina 5: 81-110.

Groeber, P. 1944. Movimientos tectónicos con-temporáneos y un nuevo tipo de dislocaciones:Notas del Museo de La Plata 9(33): 363-375.

Harrington, H. J. 1944. El sismo de San Juan; del15 de Enero de 1944. Corporación para lapromoción del intercambio, S. A., 79 p.,Buenos Aires.

Jordan, T. E. and Allmendinger, R. W. 1986, TheSierras Pampeanas of Argentina; a modernanalogue of Rocky Mountain foreland defor-mation. American Journal of Science 286(10): 737-764.

Jordan, T.E., Isacks, B., Ramos V.A., andAllmendinger, R.W. 1983a. Mountain buil-ding in the Central Andes. Episodes 1983(3):20 26, Ottawa.

Jordan, T. E., Isacks, B. L., Allmendinger, R. W.,Brewer, J. A., Ramos, V. A., and Ando, C. J.1983 b. Andean tectonics related to geometryof subducted Nazca Plate: Geological So-ciety of America Bulletin 94(3): 341-361.

Kadinsky-Cade, K. 1985. Seismotectonics of theChile margin and the 1977 Caucete earthqua-ke of western Argentina. PhD Dissertation,Cornell University, 253 p.

Kadinsky-Cade, K., Reilinger, R., and Isacks, B.1985. Surface deformation associated withthe November 23, 1977, Caucete, Argentina,earthquake sequence. Journal of GeophysicalResearch B90(14): 12,691-12,700.

Krugh, W. C. 2003. Fold growth due to kink-band migration in repeated earthquakes, Sie-rra de Villicum, San Juan, Argentina. Master'sthesis, Oregon State University, 54 p.

Mueller, K. and Suppe, J. 1997. Growth ofWheeler Ridge anticline, California: geomor-phic evidence for fault-bend folding behaviorduring earthquakes. Journal of StructuralGeology 19(3-4): 383-386.

Paredes, J. D. and Uliarte, E. 1988. Análisis mor-fotectónico del sistema de fallamiento Pre-cordillera oriental, San Juan, Argentina. 2°Reunión Sudamericana del Proyecto 206 delIGCP, Santiago.

Perucca, L. P. and Paredes, J. D. 2000. Falla-miento activo y su relación con la magnitudmáxima del sismo probable en la zona de lalaja. Departamento Albardón. San Juan. Re-pública Argentina. 9° Congreso GeológicoChileno, Actas 4 p.

Perucca, L. P. and Paredes, J. D. 2002. Peligro sís-mico en el departamento Albardón y su rela-ción con el área de fallamiento La Laja, pro-vincia de San Juan. Revista de la AsociaciónGeológica Argentina 57(1): 45-54.

Ragona, D., Anselmi, G., González, P., andVujovich, G. 1995. Mapa Geológico de laProvincia San Juan, República Argentina:Ministerio de Economía y Obras y ServiciosPúblicos, scale 1:500,000.

Ramos, V. A., Cristallini, E. O., and Perez, D. J.2002. The Pampean flat-slab of the centralAndes. Journal of South American EarthSciences 15(1): 59-78.

Riba, O. 1976. Syntectonic unconformities ofthe Alto Cardener, Spanish Pyrenees: A gene-tic interpretation. Sedimentary Geology 15:213-233.

Shaw, J. H. and Suppe, J. 1996. Earthquakehazards of active blind-thrust faults underthe central Los Angeles basin, California.Journal of Geophysical Research 101(B4):8623-8642.

Siame, L. L., Bellier, O., Sebrier, M., Bourles, D.,L., Leturmy, P., Perez, M., and Araujo, M.2002. Seismic hazard reappraisal from com-bined structural geology, geomorphologyand cosmic ray exposure dating analyses; theeastern Precordillera thrust system (NWArgentina). Geophysical Journal Internatio-nal 150(1): 241-260.

Siame, L. L., Bellier, O., Sebrier, M., Bourles, D.,L., Leturmy, P., Perez, M., and Araujo, M.2005. Corrigendum - Seismic hazard reap-praisal from combined structural geology,geomorphology and cosmic ray exposuredating analyses; the eastern Precordillerathrust system (NW Argentina). GeophysicalJournal International 161(2): 416-418.

Smalley Jr., R. and Isacks, B. L. 1990. Seismotec-tonics of thin- and thick-skinned deforma-tion in the Andean foreland from local net-work data; evidence for a seismogenic lowercrust. Journal of Geophysical Research 95(B8): 12,487-12,498.

Smalley, R., Jr., Pujol, J., Regnier, M., Chiu, J.-M.,Chatelain, J.-L., Isacks, B. L., Araujo, M., and

Puebla, N. 1993. Basement seismicity bene-ath the Andean Precordillera thin-skinnedthrust belt and implications for crustal andlithospheric behavior. Tectonics 12(1): 63-76.

Suppe, J. 1983. Geometry and kinematics offault bend folding. American Journal ofScience 283: 648-721.

Suppe, J. S., Chou, G. T., and Hook, S. C. 1992,Rates of folding and faulting determinedfrom growth strata. In McClay, K. R. (ed.)Thrust Tectonics, Chapman and Hall, p. 105-122, London.

Suppe, J., Sàbat, F., Muñoz, J. A., Poblet, J., Roca,E., and Vergés, J. 1997. Bed-by-bed foldgrowth by kink-band migration: Sant Llorençde Morunys, eastern Pyrenees. Journal ofStructural Geology 19(3-4): 443-461.

Vergés, J., Burbank, D. W., and Meigs, A. J. 1996.Unfolding: An inverse approach to fold kine-matics. Geology 24: 175-178.

Vergés, J., Ramos, V.A., Bettini, F., Meigs, A.,Cristallini, E., Cortés, J. M., and Dunai, T.2002. Geometría y edad del anticlinal falladode Cerro Salinas. 15° Congreso GeológicoArgentino, Actas 3: 290-295, Calafate.

Von Gosen, W. 1992. Structural evolution of theArgentine Precordillera; the Rio San Juansection. Journal of Structural Geology 14(6):643-667.

Whitney, R. A. 1991. Faulting and Tectonics inthe Foreland Basin Fold/Thrust Belt of theSan Juan Province, Argentina, and a Com-parison of the Yakima Fold/Thrust Belt ofthe Northwestern United States. Ph.D. thesis,University of Nevada-Reno, 176 p.

Yeats, R. S. 1986. Active faults related to foldingIn Wallace, R. E. (ed.) Active Tectonics,National Academy Press: 63-79, Washington.

Zapata, T. R. and Allmendinger, R. W. 1996 a.Growth stratal records of instantaneous andprogressive limb rotation in the Precordillerathrust belt and Bermejo basin, Argentina.Tectonics 15: 1065-1083.

Zapata, T. R. and Allmendinger, R. W. 1996 b.Thrust-front zone of the Precordillera, Ar-gentina; a thick-skinned triangle zone.American Association of Petroleum Geolo-gists, Bulletin 80(3): 359-381.

Recibido: 30 de junio, 2006Aceptado: 15 de noviembre, 2006

refolding of thin-skinned thrust sheets by ...refolding of thin-skinnedthrust sheets by active...

Documents