The Argentine Precordillera: a Foreland Thrust Belt Proximal to the Subducted Plate

The Argentine Precordillera: A foreland thrust belt proximal to the subducted plate

Richard W. Allmendinger and Phoebe A. Judge Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York 14853, USA

ABSTRACT of constructing balanced cross sections with- thickening in the lower crust, much as described out worrying about exactly where lower crustal by Bird (1988) for the Laramide Rocky Moun- The Precordillera thrust belt of west- shortening commences, how the transition from tain foreland of the western United States. ern Argentina is anomalously close, both upper crustal to whole crust shortening occurs, horizontally and vertically, to the coeval or even how the thrust plates will restore relative TECTONIC AND GEOLOGIC SETTING subduction zone of the Nazca plate. The to the trench. thin-skinned part of the belt has an unusu- The Precordillera thrust belt of western The Argentine Precordillera overlies a region ally deep décollement that is well defi ned by Argentina, in contrast, is located 350 km from of fl at subduction of the Nazca plate (Cahill industry seismic refl ection and recent broad- the Chile Trench and just 100 km above the and Isacks, 1992; Gans et al., 2011) located at band experiments. New area and line-length subducted Nazca plate. Thus the amount of the southern end of the Central Andes (Fig. 1). balanced cross sections show that the cen- crust to work with when attempting to bal- This region of fl at subduction has been linked tral Precordillera has accrued ~90 ± 21 km ance the shortening in the thrust belt is signifi - to the subduction of the Juan Fernández Ridge, of shortening since 13 Ma; much of that cantly less than elsewhere, raising questions which, because of a dogleg in its now sub- shortening occurred between 12 and 9 Ma. about how and where the shortening observed ducted trace, swept southward along the South Fault-slip data generally show shortening at the surface is accommodated at depth. The American margin from 22 to 10 Ma. However, approximately west-northwest–east-south- fi rst balanced section in the Precordillera (All- since 10 Ma the segment entering the trench east, orthogonal to the traces of the thrust mendinger et al., 1990) attempted to address is nearly parallel to the convergence direction, and folds in the Precordillera and oblique to this problem. Since that time, a great deal of resulting in a stable confi guration since then the mean vector of local global positioning new fi eld and geophysical data (e.g., Jordan (Yáñez et al., 2002). The link between ridge system (GPS) data. The GPS strain rate is et al., 1993, 2001; Zapata and Allmendinger, subduction and the fl at geometry appears to be –63 ± 9 × 10–9/yr, whereas strain rate in the 1996a; Pardo et al., 2002; Brooks et al., 2003; supported by anomalously high frequency of thrust belt, averaged over 13 m.y., is –56 ± Gans et al., 2011; Judge, 2012) for the region seismicity in the subducted plate aligned with 4 × 10–9/yr. Although the décollement of the have become available and new structural the ridge (Pardo et al., 2002; Gans et al., 2011). Precordillera cannot cut into Paleozoic Cuy- algorithms allow us to specify the uncertain- Progressive enrichment of arc magmatic rocks ania(?) terrane basement east of the crest of ties inherent in balanced sections (Judge and indicates that the main phase of shallowing of the high Andes, broadband receiver function Allmendinger, 2011; Allmendinger and Judge, the subducted plate occurred between 10 and data show that signifi cant crustal thicken- 2013). With a new generation of Precordillera 5 Ma (Kay and Abbruzzi, 1996), in broad agree- ing must occur beneath and even east of the studies underway (e.g., Fosdick and Carrapa, ment with the history of subduction of the Juan thrust belt. We suggest that top-to-the-west 2012), it is timely to reexamine the question of Fernández Ridge. shear and thickening of the lower crust due shortening in the Precordillera. Study of seafl oor magnetic lineations, global to fl at subduction explains the distribution of In this paper we present new fi eld data and plate circuits, and GPS geodesy has shown that crustal thickening. balanced cross sections of the Precordillera the convergence rate at the plate boundary has between lat 30°S and 30.5°S. Our fault-slip data decreased by a factor of 2 in the past 15 m.y. INTRODUCTION demonstrate that the Miocene to Holocene his- (Pardo-Casas and Molnar, 1987; Somoza, 1998; tory of this part of the Precordillera is character- Angermann et al., 1999; Kendrick et al., 2003). Most retroarc foreland thrust belts, such as ized by thrust faulting and shortening that devi- Currently, convergence is ~63 mm/yr in a direc- the Bolivian Subandean belt or the Mesozoic– ates by as much as 40° from the mean vector of tion 079.5° at the latitude of the Precordillera. early Cenozoic thrust belt of western North global positioning system (GPS) geodetic data. This convergence produces GPS measurable America, are 600 km or more inland from the The shortening values from balanced sections, displacements of ~10 mm/yr with respect to coeval trench and 400–600 km above the sub- similar to those previously determined, yield a stable South America in the central Pre cordi- ducted plate. This inboard position raises impor- yearly average strain rate that is indistinguish- llera, which is thought to be due to elastic defor- tant scientifi c questions, including the nature of able from the GPS strain rate. The observation mation from a combination of locking of the the driving mechanism of a belt so distant from that crustal thickening extends east of the defor- interplate subduction zone and locking of the the trench, but it has a useful practical benefi t: mation front of the Precordillera thin-skinned Precordillera décollement (Brooks et al., 2003). the broad swath of hinterland allows the luxury belt requires top-to-the-west simple shear and We discuss the relationship between long-term

Geosphere; December 2014; v. 10; no. 6; p. 1203–1218; doi:10.1130/GES01062.1; 14 ﬁ gures; 1 plate; 1 supplemental table. Received 21 April 2014 ♦ Revision received 21 August 2014 ♦ Accepted 22 August 2014 ♦ Published online 30 October 2014

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Figure 1. Regional location map. Inset shows the location within South America and contours on the Wadati-Benioff zone from the SLAB 1.0 model (Hayes et al., 2012). The box shows the location of the geological map in Figure 3. The eastern Precordillera is shaded light yellow . Locations of global positioning system (GPS) stations with velocity vectors are from Brooks et al. (2003); triangles show locations of SIEMBRA (Sierras Pampeanas Experiment Using a Multicomponent Broadband Array) broadband stations (Gans et al., 2011). Part of X-Line 15 of Gans et al. (2011) is shown in Figure 12.

and short-term upper crustal shortening toward SIEMBRA (Sierras Pampeanas Experiment Fosdick and Carrapa, 2012). Perhaps most ger- the end of this paper. Using a Multicomponent Broadband Array) mane to the current study is the thrust timing A considerable amount of geophysical and (Gans et al., 2011). Several geological studies in the Precordillera established by Jordan et al. geological information about the Precordillera have incorporated seismic reﬂ ection data from (1993, 2001); they showed that the Precordil- and westernmost Sierras Pampeanas is now the Yacimientos Petrolíferos Fiscales (YPF; All- lera thrust belt east of the Iglesia Basin initiated available (Fig. 1). Campaign-style GPS mea- mendinger et al., 1990; Beer et al., 1990; Zapata between 21 and 19 Ma with progressive east- surements are available from the Central Andes and Allmendinger, 1996a, 1996b; Zapata, 1998). ward migration of the thrust front through time Project (Brooks et al., 2003). Early local seismol- Previous geological studies concentrated and abundant evidence of simultaneous and out- ogy networks focused on the region around San on both the foreland and intermontane basin of-sequence thrust motion. Juan city in the aftermath of the 1977 Caucete stratigraphy and structural geology of the belt earthquake (Kadinsky-Cade et al., 1985; Smal- (Furque, 1979, 1983; Ortíz and Zambrano, STRUCTURAL GEOLOGY OF THE ley and Isacks, 1987; Regnier et al., 1992, 1994; 1981; Ramos et al., 1984, 2002; Johnson et al., PRECORDILLERA BETWEEN Smalley et al., 1993). More recently, the region 1986; Allmendinger et al., 1990; von Gosen, JÁCHAL AND GUALILÁN has seen two signiﬁ cant broadband seismo- 1992, 1995; Jordan et al., 1993, 2001; Zapata graph deployments, the 2000–2002 CHARGE and Allmendinger, 1996a, 1996b; Siame et al., The Precordillera thrust belt is built on a foun- (Chile-Argentina Geophysical Experiment) 1997, 2002, 2005; Colombo et al., 2000; dation of a Paleozoic terrane, Cuyania, accreted (e.g., Alvarado et al., 2005) and the 2007–2009 Alvarez-Marrón et al., 2006; Vergés et al., 2007; to South America prior to the start of the Jurassic

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to present Andean orogeny (Ramos et al., 1986, micity beneath the eastern Precordillera con- Niquivil Plate 2002; Ramos, 2008). The central part of the Pre- fi rms its thick-skinned nature (Smalley et al., The Niquivil thrust constitutes the lead- cordillera has a layered sequence of Cambrian 1993). To the west, the Iglesia Basin (Fig. 1) ing edge of the thin-skinned belt at these to Permian strata dominated by the Cambrian– has several structures with signifi cant strike-slip latitudes and has been active for about the Ordovician San Juan Limestone found at the base components, including the well-known El Tigre past 5 m.y. (Jordan et al., 1993, 2001). The of many of the thrust plates. The remainder of the fault (Bastías and Bastías, 1987; Siame et al., thrust’s ongoing activity is demonstrated by succession is composed of siliciclastic rocks. 1997, 2002) and local features visible on seismic a 10–15-m-high fault scarp where its frontal Several low-angle unconformities within the refl ection data that resemble fl ower structures trace is crossed by the Río Jáchal at the vil- Paleozoic section occur throughout the belt, and (Alvarez-Marrón et al., 2006). As shown in the lage of Niquivil (Fig. 4). The fault plane is pre-Andean deformation becomes increasingly following, strike-slip faulting is not signifi cant not exposed anywhere, but the probable sur- important to the west. In the westernmost part of farther east. Thus, our study focuses primarily face trace of the Niquivil thrust is ~50 km the Precordillera, the Ordovician changes facies on the thrust plates of the central Precordillera, in length. To the north, the thrust dies out to slope and basinal fl ysch with pillow basalts though we also present fault slip data and dis- into the Cuesta de Huaco fault-propagation and ultramafi c rocks that signal the allochthonous cuss GPS data from the neighboring areas. fold and to the south it terminates at the Río boundary between the Cuyania and Chilenia ter- Francia tear fault–transfer zone. In Zapata and ranes (Ramos et al., 1986). Thrusts within these Major Structures of the Central Allmendinger (1996b) it was reported that the rocks are impossible to balance due to completely Precordillera Niquivil thrust plate is cut and deformed by unknown initial thickness of the deposits and sig- the thick-skinned fault coring the Niquivil nifi cant pre-Andean folding. South of ~31°S, the The main thin-skinned thrusts of the Precor- anticline that is just to the east, locally revers- Precordillera is broken up by Triassic grabens dillera between the Río Jáchal and the Cienega ing the vergence of the Cuesta de Huaco (Ramos and Kay, 1991), but those structures are de Gualilán are, from east to west (foreland to anticline. not present in our fi eld area. hinterland), Niquivil, San Roque, Blanquitos, The base of the Niquivil thrust plate con- To the east, the eastern Precordillera is com- Blanco, Caracol East, Caracol West, and Tranca tains the thickest exposures of the Ordovician posed of thick-skinned, west-verging structures (Figs. 2 and 3). We fi rst describe the surface San Juan Limestone of any plate in this seg- more reminiscent of the neighboring Sierras geology and structures of these thrust plates, ment of the Precordillera. The thickness of the Pampeanas (Ortíz and Zambrano, 1981; Zapata then address the upper crustal geometry and remainder of the Paleozoic section is highly and Allmendinger, 1996b). Deep crustal seis- shortening. variable: at Cuesta de Huaco, no Silurian

Figure 2. Digital elevation model of the study area visualized as a shaded relief block diagram with the Landsat enhanced thematic mapper image draped over it, looking north. Principal structures of the Precordillera are shown in white. Location approximately the same as the region depicted in Figure 3.

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Figure 3. Simpliﬁ ed geological map of the Precordillera (area shown in Fig. 1). The map was compiled based on ﬁ eld work (Judge, 2012), satellite imagery interpretation, and published geological maps (Furque, 1979, 1983; Ragona et al., 1995; Furque et al., 1998; Cárdo and Díaz, 2005). Barbs are on the upper plate of thrust faults. Fm.—formation. (Plate 1 contains a large-scale version of the map including topographic contours and strikes and dips.)

or Devonian strata are present and the Car- San Roque Plate ~200 m of separation across the Río Francia boniferous and Permian are unconformable The San Roque thrust trace is at least 120 km tear fault, which dies out into the San Roque on the Ordovician. Due east of the town of long; it probably extends from the Guandacol plate. This suggests that at least some move- Jáchal, thin remnants of Silurian Los Espejos area in the north (outside our map area) to the ment on the Niquivil thrust postdates the San and Devonian Talacasto Formations appear Cienega de Gualilán in the south. It was active Roque thrust. beneath the late Paleozoic unconformity. At from 10 or 9 Ma to 3 or 2 Ma (Jordan et al., The San Roque plate contains a thinner sec- the south end of the Niquivil plate, just north 1993, 2001). The fault has a signifi cant lateral tion of San Juan Limestone than the Niquivil of the Río Francia , Silurian strata are present ramp just west of the village of Niquivil: to the plate. North of the Río Jáchal, the rest of the but the Devonian section is missing (Fig. 3; north, the fault is within the San Juan Limestone lower Paleozoic section is completely missing Plate 1). As described in the following, the but to the south it steps upsection to within the and the upper Paleozoic directly overlies the Niquivil thrust is the only one crossed by Silurian Los Espejos Formation. Both north limestone. To the south, a thicker Silurian and YPF seismic lines (Allmendinger et al., 1990; and south of the hanging-wall lateral ramp, the substantial Devonian section overlie the lime- Zapata and Allmendinger, 1996b). Although footwall strata are Miocene sandstones. There stone, with a thin upper Paleozoic sequence the fault plane is not exposed anywhere, inter- is a duplex at the base of the thrust plate, 10 km unconformably overlying the Devonian. On the pretation of the YPF seismic lines indicates south of the lateral ramp, and a thin sliver of western fl ank of the San Roque range proper, that the 35° west-dipping thrust places Ordo- Ordovician limestone is present at the base of a series of small thrusts and tight folds thicken vician limestone over Miocene sandstone, the plate. The San Roque fault and its hanging the Silurian and Devonian section with minor with a stratigraphic throw of 12–15 km. wall go through an 80° bend and appear to have involvement of the Carboniferous. Signifi cant

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Figure 4. Google Earth satellite image and ﬁ eld photo of the active fault scarp, indicated by the large white arrows, of the Niquivil thrust just outside the village of Niquivil. The composite scarp is ~10 m high. Field photo is looking due west, from the footwall of the thrust toward the uplifted hanging wall. Osj—Ordovician San Juan Limestone.

folding of the Devonian section is also prevalent anticline. Due to extensive Quaternary cover wavelengths, including spectacular outcrop- farther south, complicating the task of determin- north of this point, it is unclear whether the anti- scale kink folds that can be seen on the main ing original stratigraphic thickness. cline represents a north-plunging tip-line fold road between Jáchal and the village of Rodeo in or if the fault continues northward and the fold the Iglesia Basin west of the Precordillera. Blanquitos Plate represents a hanging-wall lateral ramp where The Blanquitos plate, the shortest and most the fault once again steps upsection northward Blanco Plate enigmatic in this segment of the Precordillera, into the Silurian. There are excellent exposures The highest relief in the region, including the underwent a brief period of activity between of the main thrust plane where the limestone highest peak in this segment of the Precordi- 11.5 and 9.5 Ma (Jordan et al., 1993, 2001). Its overlies the Tertiary on the backlimb of the San llera at more than 3600 m elevation, occurs in surface trace is <50 km long and along most of Roque plate. The fault in these exposures has the upper plate of the Blanco thrust. The Blanco that length Devonian strata are at the base of the a south to south-southwest strike and dip that thrust was active from 13 to 9 Ma, overlapping upper plate. At 30.37°S, there is a lateral ramp varies between 30°W and 55°W. activity on the Blanquitos thrust (Jordan et al., and the thrust cuts downsection northward to South of the lateral ramp, 1800–2700 m of 1993, 2001). Although the Blanco thrust dies out include a thin sliver of San Juan Limestone at Devonian Punta Negra Formation are overlain within our map area at ~30.3°S, it is undoubt- the base of the upper plate, with Miocene sand- directly by a thin sequence of Cenozoic foreland edly one of most important thrust faults in the stone in the lower plate. The Cerro Bayo anti- basin strata, including a distinctive eolian cross- central Precordillera and can be traced south cline (Fig. 3; Plate 1) generated by this lateral bedded sandstone, which overlies a redbed with to the Río San Juan for a total map length of ramp propagates across the entire upper plate a tuff dated as 21.6 ± 0.8 Ma, found through ~120 km. Throughout most of that trace length, and appears to plunge beneath the Blanco thrust much of the region (Jordan et al., 1993; Milana, San Juan Limestone is found at the base of the to the west. Less than 10 km north from where 1993). To the north of the lateral ramp and Cerro plate. Between the Cienega de Gualilán and the the limestone ﬁ rst appears at the base of the Bayo anticline, the Silurian and Devonian form Río Jáchal, the Blanco plate contains a tight syn- upper plate, it disappears in a north-plunging a broad expanse of strata folded at multiple cline in thick Silurian and Devonian strata. On

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Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/6/1203/3336361/1203.pdf by guest on 30 September 2021 Allmendinger and Judge Rodeo Fm. Iglesia Fm. Las Flores Fm. Tertiary intrusives Lomas del Campanario Iglesia Group Fault, dotted where buried or inferred Right-lateral Left-lateral Iglesia Basin clastic & subvolcanic igneous rocks Conjugate faults of the Niquivil anticline Plate 1 Los Azules San Juan San Roque Cuculi El Corral tear fault south of Río Francia (to accompany Allmendinger, R.W., and Judge, P.A., 2014, The Argentine Precordillera: A foreland thrustproximal belt to the subducted plate: Geosphere.) c contours, showing strikes and dips in their c contours, showing strikes and dips in their mafic igneous bodies Strike and dip of bedding, measured in the field Strike and dip of bedding, calculatedproblem from (not 3-point field checked) Anticline axial trace; arrows on tracedirection(s) show of the plunge of the folds Syncline axial trace; arrowsdirection(s) on of trace plunge of show the the folds undifferentiated Mio-Pliocene Mogna Rio Jáchal Quebrada del Cura Huachipampa Quebrada del Jarillal Rio Salado Cauquenes Cerro Morado Vallecito Cañon Colorado Ojo de Agua Panacán Volcán Punta Negra Talacasto Los Espejos La Chilca Yerba Loca Sierra de la Invernada modern alluvial fans ancient alluvial fans Alluvium 52 24 52

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Basin 0 http://www.ngdc.noaa.gov/geomag-web/ Iglesia 100 2 60 2 Base map uses a UTM 19Sdatum. projection Topographic contours and from WGS 30 84 m ASTER digital elevation model Plate 1. Geologic map of the Jáchal Argentina Precordillera thrust belt study area at a scale of 1:150,000 with 20 m topographi thrust belt study area Argentina Precordillera Plate 1. Geologic map of the Jáchal proper location. To view the full-sized version of Plate 1, please visit http:// dx .doi .org /10.1130 /GES01062 .S2 or the ful .S2 or /GES01062 /10.1130 .org .doi dx view the full-sized version of Plate 1, please visit http:// To location. proper

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both limbs of the syncline, dips are steep, rang- et al., 1990). However, the rocks of the two roads and more than 50% of their measurements ing from 40° to 80°. This fold, named Mogotes upper plates are entirely different; the Caracol are in Paleozoic bedrock, we collected a more Quebraditas syncline (Fig. 3; Plate 1), is geneti- East plate is dominated by Silurian and Devo- extensive data set (Judge, 2012) that we know cally related to the Blanco and the Caracol East nian strata, whereas the Caracol West plate con- to be completely of late Cenozoic age, as all of thrust located on the west fl ank of the syncline, tains exposures of the Ordovician Yerba Loca our faults are located within Cenozoic strata or and its signifi cance is discussed in the follow- Formation, a deep-water fl ysch deposit. There- on faults that place Paleozoic over Tertiary (see ing section. fore the two faults are unlikely to be the same. the Supplemental Table1). In addition, we prefer Just north of the Cienega de Gualilán at the We consider the Caracol West and the Tranca an analysis in terms of infi nitesimal strain (Mar- Cerro Portezuelo Blanco (Fig. 3), the Blanco thrusts farther west to be a single en echelon rett and Allmendinger, 1990) rather than stress. thrust overrides and cuts downsection eastward system that carries the western deep-water The strain from multiple faults sets is cumulative across the back limb of the Blanquitos plate. facies of the Ordovician in its upper plate. The (additive in the case of infi nitesimal deformation Three-point calculations show that the fault Caracol West thrust dies out southward and and multiplicative for fi nite strain; Cladouhos plane has a dip of <15°W here, whereas the the Tranca thrust dies out northward at the Río and Allmendinger, 1993), whereas stress is bedding in the footwall dips 30°–40°W. This Jáchal with an overlap of ~22 km immediately instantaneous only, and thus one must assume geometric relationship indicates that at least the south of the river. Along the Río Jáchal, a few that all the faults in a data set formed at the same youngest motion on the Blanco thrust is out of highly deformed slivers of limestone are imbri- time or that the same homogeneous stress state sequence and postdates the tilting of the strata in cated in the Yerba Loca, but near the Cuesta del persisted for the duration of the faulting. the Blanquitos plate. Viento (Fig. 3; Plate 1) the Ordovician strata are We plot the P-axes and T-axes (pressure and At the northern end of the Blanco plate, the tightly deformed into pre-Andean west-verging tension) for individual faults associated with Ordovician limestone and overlying Silurian and overturned folds associated with mafi c and each major structure (Fig. 6) and summarize Devonian sequence are deformed into an over- ultramafi c igneous rocks. each data set with a composite fault-plane solu- turned fault-propagation fold. The forelimb of The age relations of the Caracol and Tranca tion calculated using a moment tensor sum this fold is complexly imbricated with the lime- thrusts were described by Jordan et al. (1993, where all the faults are assumed to have the stone thrust over the Silurian–Devonian, which 2001). The Tranca thrust defi nitively had activity same weight (Marrett and Allmendinger, 1990). is in turn thrust over the Cenozoic of the Blanco prior to 19 Ma, and the Caracol West possibly had P-axes and T-axes, despite their names, are sim- valley. The Blanco fault ramps into the Devonian activity in the same time frame. Both members ply the infi nitesimal strain axes for the faults. in the hanging wall and continues for another of the en echelon system also display younger In detail, individual data sets show a broad dis- 8–10 km northward before dying out completely deformation; in the case of the Tranca thrust, it tribution of individual P-axes and T-axes, but into an anticline separating two large synclines. is a subsidiary structure that placed ca. 21 Ma taken at a broader scale, two fundamental obser- The exposures of the Silurian and Devonian redbeds over the Chestnut Conglomerate (Jordan vations emerge from our fault-slip data. along the Río Jáchal belong to the combined et al., 1993, 2001), and for the Caracol West, the First, there is very little evidence for signifi - upper plate of the Blanquitos and Blanco thrust. outcrops near the Río Jáchal show thrusting over cant strike-slip faulting in the Neogene rocks of mid-Miocene eolian beds. In summary, initial the Precordillera, except for the Niquivil anti- Caracol East Thrust motion on the Tranca and Caracol West thrusts, cline of the Eastern Precordillera (Fig. 7). Imme- The next fault to the west is an east-dipping, which carry Ordovician Yerba Loca Formation diately to the west between the Iglesia Basin and west-verging structure we refer to as the Cara- in their upper plates, is ~6–7 m.y. older than the the Precordillera, the El Tigre fault (Fig. 3) has col East thrust. This structure tracks the west- other thrusts in this segment of the Precordillera. documented Quaternary right-lateral displace- ern limb of the Mogotes Quebraditas syncline. The Blanco thrust and the more eastern thrusts ment (Bastías and Bastías, 1987; Siame et al., On the eastern side of the Caracol Valley, thin, are all younger than 13 Ma. 1997), although this fault dies out just south of highly deformed slivers of San Juan Limestone the Río Jáchal. Thus, there must be a partition- are present locally along the base of the thrust Fault-Slip Data ing of displacements very much as described plate. Farther south in the Cordón del Peñon by Siame et al. (2005). Alvarez-Marrón et al. (Fig. 3) more extensive outcrops of limestone Siame et al. (2005, 2006) presented fault-slip (2006) proposed that the thrust faulting in the and overlying Silurian rocks are tightly folded data for the Jáchal segment of the Precordillera; central Precordillera was Paleozoic in age and into anticlines and synclines with westward ver- they suggested that the predominant fault popu- that Neogene deformation is characterized by gence. The upper plate of Caracol East is also lation shows approximately horizontal east- minor high-angle strike-slip fault dismember- the upper plate of the Blanco thrust, and the tight northeast–oriented maximum principal stress, ment of the ancient thrust belt. Our fault slip σ syncline between the two thrust traces suggests 1, though a secondary, and in their interpreta- data provide no evidence for this hypothesis; that Caracol East is a back thrust off the Blanco tion, older, population that records northwest- instead they are completely consistent with σ thrust. We hypothesize that Blanco–Caracol southeast–oriented horizontal 1. The criteria East formed a triangle zone early in the history used to separate their data into older and younger 1Supplemental Table. A tab-separated, column- σ oriented text fi le that contains all of our fault slip of the thrust belt before it was subsequently 1 directions are unclear. A remarkable aspect of deformed and moved again out of sequence. their data set is that the primary, younger σ is data. The Supplemental Table can be displayed in 1 a spreadsheet or opened and manipulated by Fault- not orthogonal to the strikes of the thrust faults Kin, a freely available program for Macintosh and Caracol West and Tranca Thrust Plates or the trends of the fold axes, but makes an angle Windows operating systems that can be down- A west-dipping thrust occurs on the west side of as much as 45° to the structures, even though loaded from Allmendinger’s web site: http:// www of the Caracol Valley (Fig. 5), and it is initially there is little evidence for strike slip or oblique .geo .cornell .edu /geology /faculty /RWA /programs /faultkin .html. If you are viewing the PDF of this tempting to interpret the Caracol East and Cara- slip in their data set. paper or reading it offl ine, please visit http://dx .doi col West faults as a continuous fault forming a Because the Siame et al. (2005, 2006) data .org /10 .1130 /GES01062 .S1 or the full-text article on fenster into the lower plate (e.g., Allmendinger appear to have been collected primarily along the www .gsapubs .org to view the Supplemental Table.

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E W

NeogeneNeogene

Figure 5. Field photo of the Caracol West thrust fault in the Caracol Valley, looking south. The thrust places the (greenish) Ordovician Yerba Loca Formation over the ca. 21 Ma Neogene redbeds. Bushes in foreground are ~1 m high. See Figure 3 (upper left of center, outlined by white box) for location.

Neogene horizontal shortening on dip-slip map-scale structures (i.e., strikes of thrust faults, in this part of the Precordillera (Brooks et al., faults. Alvarez-Marrón et al. (2006) also ignored trends of fold axes; Fig. 8). For example, data 2003) have a consistent mean orientation of the evidence from seismic refl ection data across from near or in the Blanquitos fault zone yield 075° (Figs. 1 and 8). To compare GPS data the frontal thrust fault (Allmendinger et al., a west-dipping nodal plane that strikes 020°, the to deformation features of the Earth’s crust, 1990; Zapata and Allmendinger, 1996a, 1996b) local strike of the thrust fault. Conjugate strike- however, one must use the gradient of the GPS and seem not to have considered the possibility slip faults from the Niquivil anticline of the velocity fi eld rather than the velocity fi eld (All- that, in an imbricate stack of thrusts such as the eastern Precordillera (Fig. 7) yield a shortening mendinger et al., 2009). We do that using the Precordillera, older thrusts are rotated to a high axis that is orthogonal to the fold axis (Figs. 6 program SSPX by Cardozo and Allmendinger angle by younger more eastern thrusts. Alvarez- and 8). To the south at the Río Francia tear fault, (2009) and the most recently available GPS Marrón et al. (2006) are correct that consider- shortening axes are anomalous with respect to data for the region (Brooks et al., 2003). The able pre-Andean deformation in the Paleozoic the rest of the belt, but then so is the local strike results presented here could change when the rocks of the Precordillera is indicated by numer- of the Francia thrust. same analyses are applied to updated data, ous unconformities in the Paleozoic section and which are still being processed (B. Brooks, by tight folding of the Paleozoic rocks that are Comparison to GPS Geodesy 2013, personal commun.). There are several unconformably beneath the Neogene strata. ways to calculate strain from GPS data (All- The second major observation from our fault- The orthogonal shortening is important mendinger et al., 2009); we use two, distance slip data is that shortening everywhere within because the strikes and trends of the structures weighting and a simpler nearest neighbor the belt is mostly orthogonal to the primary vary by 70° or so, even though GPS vectors approach, which calculates a single best-fit

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D B C 3 2 E 1 HuacoHuaco 1 1 3 1 RíR í 3 o

H 3 u 2 RodeoRodeo ac 2 o 2 JáchalJáchal F 30.25°S30.25° Iglesiagle BBasina

Niquivil 3

G R

1 J á 1 330.5°S0 c 3 h a l

ndelPeñ

e 2 l P

CerroCerro PPortezueloortezuelo BlancoBlanc

3 J H 1 30.75°S I

2 CCienegaienega ddee Gualilán 69.25°W 69.0°W 20 km 68.75°W 68.5°W

Figure 6. Summary of >200 fault slip measurements, all involving Tertiary strata (Judge, 2012). The P-axes and T-axes (P—pressure, T—tension) of individual faults are shown by the blue and red dots, respectively; the overall kinematics are indicated by the underlying fault-plane solution (T-quadrant is shaded; same convention as used in earthquake focal mechanisms). Sites shown: A—Niquivil anticline, B—footwall Tertiary rocks and San Roque thrust zone (does not include normal faults due to thrust load), C—Blanquitos footwall Tertiary and thrust zone, D—Caracol Valley Tertiary and Caraco West thrust zone, E—Iglesia Valley Tertiary, F—northern Tranca Valley Tertiary and thrust zones, G—southern Tranca Valley, H—Blanco footwall Tertiary and thrust zone, I—north side, Río Francia tear fault, J—Río Francia thrust zone south of tear fault.

strain rate ellipse to the group of GPS stations shortening and the north-northeast–striking the Precordillera, assuming that strain is homo- closest to our study area. structures in many parts of the belt is only ~60° geneous, yields a principal horizontal shortening Regional smoothed two-dimensional (2D) (Fig. 1). Because our fault-slip data sets tend to strain rate of –63 ± 9 × 10–9/yr in the direction principal horizontal shortening strain-rate be diverse, we could arbitrarily select subsets 093° ± 8° (Fig. 8). The GPS-derived horizontal axes, using a distance-weighted algorithm that of faults that will give shortening parallel to maximum extension value is more than an order smooths out the strain-rate solution at long the modern-day GPS vectors or the distance- of magnitude lower than the concomitant short- wavelengths (Allmendinger et al., 2009), are weighted shortening axes, but we have no objec- ening, suggesting that modern-day deformation rotated ~5° clockwise from the mean GPS vec- tive basis for doing so. is approximately plane strain with no signiﬁ cant tor in the Jáchal Precordillera (Fig. 8). With an The best-ﬁ t horizontal strain ellipse to the active strike-slip faulting in the area covered by orientation of 080°, the angle between GPS eight stations nearest our studied segment of the GPS stations. The shortening value obtained

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Figure 7. Google Earth image of conjugate faults in Miocene rocks of the Niquvil anticline in the eastern Precordillera. (A) Uninterpreted image. (B) Interpreted image. See Fig- B ure 3 for location (right of center, outlined by black box).

400 m

in this 2D calculation is essentially the same as est to our study area, is rotated clockwise by 2001) but are probably also due to variations in that yielded by a 1D transect using the stations nearly 20° from the mean GPS vector, a result coupling on both the subduction megathrust and between PAGN and AT30, located on the Sierra produced by north-south gradients in the GPS the Precordillera décollement. Nonetheless, the de Valle Fértil (Fig. 9). velocity vectors. These north-south gradients GPS shortening direction is still not orthogonal The orientation of the 2D nearest-neighbor are produced in part by subduction that deviates to many of the eastern structures, including the GPS shortening, using just the stations clos- by ~20° from perpendicular (e.g., Bevis et al., San Roque and Niquivil thrusts and the folds

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of the eastern Precordillera in the Sierras de AT17 Huaco, which have strikes or trends of 025°, 69.2°W 69.0° 68.8° 68.6° 68.4°W and thus one would expect shortening azimuths to be ~115° (Fig. 8). At an azimuth of 093° ± 8°, the 2D GPS shortening rate is also not parallel to 30.2°S AT24 AT12 fault-slip shortening orientations. It is possible AT23 AT20 that more dense GPS data would better resolve shortening directions. AT10 Assuming that GPS data are representative Central & Western of the present-day strain rate fi eld, one must Precordillera postulate that signifi cant vertical axis rotation 30.4° Thrust fault has occurred since the major structures and the Fold axial trace minor faults we have measured formed. Cur- rently available paleomagnetic data were col- Fault-slip shortening lected in the Sierras de Huaco for magnetic GPS vector used in reversal stratigraphy (Johnson et al., 1986; Beer 2D nearest neighbor 30.6° and Jordan, 1989; Beer, 1990) and, because strain analysis they were based on oriented cubes collected AT25 GPS vector (length = just for polarity, are not very reliable for verti- 5 mm/year) cal axis rotations. Thus, we await the paleomag- Horizontal shortening direction from 2D netic resampling of the Miocene and Pliocene regional distance rocks of the region for fi nal resolution of this 30.8°S weighted solution conundrum. Best-fit strain ellipse AT05 (highly exaggerated) Shortening Magnitudes from AT06 for nearest neighbor AT03 calculation using red Balanced Sections vectors. Dashed 0 1020304050km ellipses = uncertainty The magnitude of shortening in this segment of the Central Andes is of primary interest, given Figure 8. Summary of shortening directions from instantaneous global positioning system its proximity to the subducted plate. We estimate (GPS) velocities and geological fault-slip data, compared to the traces of thrust faults and horizontal shortening in two ways (Figs. 10 and folds in the Precordillera between the Río Jáchal and Río Huaco and the Cienega de Gualilán 11): the fi rst is via classic line-length balancing (2D—two dimensional). See text for discussion. and the second, using area balancing, allows us to estimate the uncertainties of our calculations. The depth to décollement is a key factor for 0.020 both types of balancing. On the east side of High Central Sierras Andes Precordillera Pampeanas the belt, we use the value derived from indus- Iglesia Eastern try seismic refl ection data that yields a current Basin Precordillera depth at the eastern side of the belt of ~13.7 km below sea level (described in Allmendinger et al., 1990; Zapata and Allmendinger, 1996a, 1996b). This depth puts the décollement close to 0.015 the top of the very bright positive polarity mid- PAGN crustal converter imaged in the receiver function AT24 stacks of Gans et al. (2011). Although this con- AT23 verter extends east of the thrust front, we interpret it as the top of Cuyania basement and the de facto décollement (Fig. 12). This converter continues west, uninterrupted, to at least 70°W. 0.010

Velocity (075° East), m/yr AT12

AT20 AT17 Figure 9. Strain rate from a one-dimensional AT10 transect of global positioning system (GPS) stations from the Agua Negra Pass (PAGN), 63.5±7×10–9yr–1 AT30 along the Río Jáchal to the Sierra de Valle Fértil (AT30). Original GPS data are from 0.005 Brooks et al. (2003). Dashed lines show the 0 50,000 100,000 150,000 200,000

α95 uncertainty envelope on the slope of the linear regression. Distance (075° East), m

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Cenozoic 5 0 5 10 15 20 Devonian km Silurian

Ordovician western/eastern facies

B 97 km line-length shortening

C 88 ± 22.5 km area balance shortening

uncertainty ± 22.5 km

Figure 10. Line length and area balance of section A-A′ (see Fig. 3 for location of A-A′). Stratigraphy has been simpliﬁ ed. Diagonal ruled area in the deformed section at the top corresponds to the area-balanced section in the area balance at the bottom. (A) Deformed section. Error bars show uncertainties applied to individual vertices of the deformed area polygon for use in the error analysis. Caracol E—Caracol East (W—West). (B) Line-length restoration. (C) Area-balance restoration. os l it u a o vi i c u n nq a anc

Cenozoic A iq l ra T San Roque Caracol E Caracol W Bl B N Upper Paleozoic 505101520 Devonian km Silurian

Ordovician western/eastern facies

B 99 km line-length shortening

C 84 ± 21 km area balance shortening

uncertainty ± 21 km

Figure 11. Line-length balanced section B-B′ (see Fig. 3 for location of B-B′). Stratigraphy has been simpliﬁ ed. Diagonal ruled area in the deformed section at the top corresponds to the area-balanced section in the area balance at the bottom. (A) Deformed section. Error bars show uncertainties applied to individual vertices of the deformed area polygon. Caracol E—Caracol East (W—West). (B) Line-length restoration. (C) Area-balance restoration.

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error is rectifi ed here. Second, the Blanco and Caracol East thrust are linked; we have interpreted them to have initially formed a triangle zone sometime between 13 and 9 Ma. Third, the most recent movement on the Blanco thrust was out of sequence and younger than that on the Blanquitos thrust, because the former locally cuts downsection across the latter. Taking into account point two, after the triangle zone was formed, the Blanquitos thrust moved and then the Blanco thrust broke through from the blind tip of the triangle zone and cut downsection across the Blanquitos sheet. Chronology that is independent of the geometrical arguments (Jordan et al., 1993, 2001) clearly permits this interpretation. Both balanced sections yield similar line- length shortening values: The leading edge of the Caracol West plate must restore to 95 km west of its current position on A-A′ and to 99 km Figure 12. Crustal-scale section showing how industry seismic refl ection data and the bal- west on B-B′; this gives us 72% shortening on anced section relate to fi rst-order events on the receiver function stack (from Gans et al., A-A′ and 77% on B-B′. These large values are 2011). The strong upper mid-crustal converter is interpreted as the top of Cuyania base- similar to those obtained in Allmendinger et al. ment and the décollement of the central Precordillera, based on its correlation with the top (1990) and fundamentally arise because the of industry seismic data on the eastern side of the thrust belt. The receiver function data on unusually deep décollement requires a tripling X-line 15 of Gans et al. (2011) has been foreshortened into the east-west plane of the section. of the stratigraphic section to fi ll the space. The 0 datum is sea level. Area Balance and Uncertainties

The décollement is interpreted on our sections from one plate to the next and even from north To carry out the area balance we use meth- to be at ~16 km below sea level beneath the to south within a single plate, as described ods that allow us to assess uncertainties (Bird, Iglesia Basin. The current, unusually deep posi- herein. Thus, the balancing is based on the 1988; Judge and Allmendinger, 2011). The bal- tion of the décollement refl ects the magnitude of Cambrian–Ordovician San Juan Limestone, and anced polygons with their error bars are shown shortening and foreland basin strata accumula- only includes from the footwall of the Niquivil in Figures 10 and 11. We use a uniform initial tion; at the start of deformation the stratigraphic thrust to the footwall of the Caracol thrust to stratigraphic wedge of just Ordovician strata level of the décollement would have been just the west. The Caracol West and Tranca thrust because the stratigraphy of the Precordillera is 5–8 km deep. The low metamorphic grade of the plates cannot be balanced because the thickness not suffi ciently well known to allow us to defi ne rocks at the base of each thrust sheet is prob- of the Yerba Loca Formation is not well known, a nonuniform taper. The greatest unknowns are ably indica tive of this initial depth rather than and these plates contain no markers of known the horizontal position of the western edge of the fi nal depth. In the western Precordillera, the orientation at the start of Andean deformation the deformed polygon on the décollement (we rocks in the thrust sheets are incipient green- that can be correlated with the thrust plates somewhat arbitrarily assign ±5 km to that point) schist facies, though that metamorphism could farther east. Because the Tranca and probably and the thickness of the Ordovician Invernada have long predated the Neogene thrusting. the Caracol West plates moved several million and Yerba Loca Formations on the west side of years before the other plates (according to the the initial wedge. We assign an uncertainty of Line-Length Balanced Sections chronology described in Jordan et al., 1993, 25% of the thickness, although this is admit- 2001), our shortening values apply to the past tedly little more than a guess. We report Gauss- The two line-length sections (Figs. 10 and 11) 13 m.y. only. ian errors, based on the square root of the sum of have several basic features in common. Based We have no illusions that the geometries shown the squares, in the following analysis; maximum on the seismic refl ection data and on the array of in sections A-A′ and B-B′ are anything more than errors are much larger (Taylor, 1997; Bevington back-limb dips, it appears that the initial cut up plausible. Nonetheless, we note a few key geo- and Robinson, 2003). angle for the belt is steep, between 30° and 40°. metric elements. First, the easternmost Niquivil Determination of shortening magnitude and This value makes it impossible to preserve bed- thrust contains the thickest section of the Cam- percent depends on the relationship between ding thickness on the forelimbs of fault-bend brian–Ordovician limestone and the décollement the deformed polygon and the initial wedge. To folds (Suppe, 1983), but that is of little concern, is at the base of the limestone. Therefore all of make our area balance as close as possible to our because most folds in the belt are better inter- the thrusts to the west should have hanging-wall line-length balance, we use the same deformed preted as fault-propagation folds and outcrops ramps at depth across the unexposed parts of the width. Thus, cross-section A-A′ has a shorten- show signifi cant thinning and thickening of fold section producing thickening in the subsurface; ing magnitude of 88 ± 22.5 km and a shorten- forelimbs. this helps to fi ll the space down to the unusually ing of 71% ± 5.4%, very close to the line-length Because of pre-Andean deformation, most deep décollement. This geometry was not rec- balance of that section. Section B-B′ yields stratigraphic units change thickness abruptly ognized in Allmendinger et al. (1990) and that a shortening magnitude of 84 ± 21 km and a

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shortening of 73% ± 5%. The line-length short- Central & Western Precordillera ening for B-B′ is within the error limits from the

area balance. oque R If we assume that the central Precordillera acol an thrusts balanced here were active for the past 0 Tranca Car Blanco Blanquitos S Niquivil 13 m.y., then the yearly average strain rate is AA′ Shortening Rate BB′ Shortening Rate 56 ± 4 × 10–9/yr; recall that the 2D GPS strain 2 calculation gives 63 ± 9 × 10–9/yr over the geo- logically instantaneous time span of a decade. 4 Thus, we can say that the rate of strain accu- 6 mulation today in the Precordillera matches the average rate of strain accumulation over the past 8 13 m.y. In detail, of course, over geological time the shortening rate (Fig. 13), and thus the strain 10 rate, is not constant. Furthermore, the GPS strain

Time (Ma) 12 is thought to be due to elastic deformation due to locking of both the subduction zone and locking 14 of the Precordillera décollement (Brooks et al., 2003). Given the 50% decrease in plate conver- 16 0 5 10 15 20 25 30 gence rate over the past 15 m.y. (Kendrick et al., Shortening Rate (km/Ma or mm/yr) 2003), the instantaneous strain rate has probably 18 not been constant. Our result, though, suggests 20 that elastic strain that accumulates in the Pre- cordillera ultimately stays in the Precordillera 22 when it is converted into permanent deformation across numerous earthquake cycles. Figure 13. Thrust duration in the central and western Precordillera (after Jordan et al., 2001) and shortening rate through time based on the balanced sections presented here. The CRUSTAL SHORTENING OF THE spike in shortening at 12–9 Ma is due to simultaneous motion on Blanco, Blanquitos, and ANDEAN FORELAND AT 30°S San Roque thrusts.

The cross-section balancing demonstrates that the décollement of the thrust belt continues horizontal particle velocity profi les depicting velocity gradient in the middle crust between westward at least ~115 ± 21 km from the thrust the coeval fl ow of rocks in different parts of the western and central velocity profi les, there must front of the central Precordillera without involv- belt (Fig. 14). The weak lower crust is probably be middle crustal shortening there as well. ing basement, a position beneath or west of the advected eastward during fl at subduction, much These schematic horizontal velocity pro- crest of the high Andes (Fig. 12). This raises a as described by Bird (1988) for the Laramide fi les are, of course, oversimplifi ed; they do not space problem because, at the west end of the deformation of the western United States. Con- address shortening in the Sierras Pampeanas, décollement, the South American crust is already ceivably, part of the lower crustal material could strike slip along the El Tigre fault, or forearc thinning toward the anomalously close plate be tectonically eroded from the leading edge of deformation. Nonetheless they explain, to a fi rst boundary and no obvious structural geometry South America (Goss et al., 2013). approximation, the patterns of crustal thicken- exists for transferring the upper crustal shorten- In all three of our particle velocity profi les, ing and surface deformation that are observed in ing into lower crustal thickening. Furthermore, the strong mantle lid is simply translated east- the Andes at lat ~30°S. broadband geophysical experiments (Gans et al., ward (Bird, 1988). The eastward fl ow of the The crustal deformation scenario proposed 2011) show that the thickest crust in this seg- lowermost crust diminishes gradually eastward for this segment of the Andes may have broader ment is not underneath the high topography, but extends well into the Sierras Pampeanas. application. Kley and Monaldi (1998) docu- but instead is beneath the western margin of the Beneath the Precordillera, the middle crust mented a defi cit of surface shortening in the Iglesia Basin. The crust is >60 km thick east of under the décollement has a velocity close to retro arc thrust belts relative to the crustal thick- the deformation front of the thin-skinned thrust zero with respect to South America. Beneath the ness throughout much of the Central Andes. belt. How does the substantial surface shortening High Andes, the crust near the western end of Translation of lower crustal material eastward, in the Argentine Precordillera produce shorten- the décollement probably behaves more like a perhaps linked to subduction erosion, could ing and thickening at deeper levels in the crust? ductile shear zone than a discontinuity, and the account for at least some of this defi cit, although Allmendinger et al. (1990) noted the same middle crust below has some fi nite eastward the mechanical model proposed by Bird (1988) problem: with much poorer data, they attempted velocity. The horizontal velocity gradient in the is only applicable to fl at subduction regimes some fanciful geometries (triangles zones and middle crust between the high Andes and the such as exist beneath the Precordillera. duplexes) to solve the problem. Here we do not Precordillera produces shortening in the middle attempt to provide specifi c structural geometries, crust but not at the surface. In the western veloc- CONCLUSIONS as even today’s data are not suffi cient to defi ne ity profi le beneath the Miocene forearc (there is the lower crustal deformation style. Instead, the no arc in this segment of the Andes today), there Horizontal shortening in the Argentine Pre- simplifi ed deformation across this segment of is no difference in horizontal velocity between cordillera over the past 13 m.y. is ~86 ± 22 km, the Andes can be visualized as three schematic, the upper and middle crust; because there is a or ~72% ± 5%. The line-length balances of two

1216 Geosphere, December 2014

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/6/1203/3336361/1203.pdf by guest on 30 September 2021 The Argentine Precordillera

W High Andes dillera E modern trench Miocene Forearc Iglesia BasinPrecor

0 translation translation top-to-E shear &shortening South American –20 crust shortening shortening –40 top-to-W shear & shortening top-to-W shear & top-to-W shear & shortening shortening –60 ocea nic c –80 rust –100

–120 depth below sea level (km)

Figure 14. Schematic diagram showing postulated geological horizontal velocity proﬁ les in the Central Andes at lat 30°S. Crustal thickness and location of the top of the subducted oceanic slab are from Gans et al. (2011). The 0 datum is sea level.

parallel sections across the belt are also within broadband seismic data and balanced sections latitude: Tectonics, v. 9, p. 789–809, doi:10 .1029 /TC009i004p00789. this range. The average yearly strain rate for this show that the Precordillera décollement projects Allmendinger, R.W., Loveless, J.P., Pritchard, M.E., and deformation is 56 ± 4 × 10–9/yr, although the westward beneath the high Andes (Fig. 12), just Meade, B., 2009, From decades to epochs: Spanning temporal history of motion (Jordan et al., 1993, 220 km horizontally and 80 km vertically from the gap between geodesy and structural geology of active mountain belts: Journal of Structural Geology, 2001) in this segment of the Precordillera shows the plate boundary. This result, combined with v. 31, p. 1409–1422, doi: 10 .1016 /j .jsg .2009 .08 .008 . that this strain rate must have varied with time. the well-imaged Moho, shows that lower crustal Alvarado, P., Beck, S., Zandt, G., Araujo, M., and Triep, E., Nonetheless, the average value is indistinguish- thickening also occurs beneath the thin-skinned 2005, Crustal deformation in the south-central Andes backarc terranes as viewed from regional broad-band able from the 1D and 2D strain rates across the thrust belt. We suggest that lower crustal fl ow seismic waveform modelling: Geophysical Journal Precordillera calculated from available geodetic similar to that proposed by Bird (1988) for the International, v. 163, p. 580–598, doi:10 .1111 /j .1365 -246X .2005 .02759 .x . GPS data (Brooks et al., 2003), which give us Laramide Rocky Mountains is responsible for Alvarez-Marrón, J., Rodriguez-Fernández, R., Heredia, N., 63 ± 8 × 10–9/yr. Although the GPS strain rate the crustal thickening pattern beneath this seg- Busquets, P., Colombo, F., and Brown, D., 2006, Neo- is undoubtedly mostly elastic due to loading of ment of the Andes. gene structures overprinting Palaeozoic thrust systems in the Andean Precordillera at 30°S latitude: Geologi- the locked décollement and the locked subduc- ACKNOWLEDGMENTS cal Society of London Journal, v. 163, p. 949–964, doi: tion megathrust, it appears that the contempo- 10 .1144 /0016 -76492005 -142 . rary strain in the Precordillera will be converted Angermann, D., Klotz, J., and Reigber, C., 1999, Space- Judge is grateful to fi eld assistants Jordan Garro- geodetic estimation of the Nazca–South America Euler into permanent deformation in the Precordillera way, Rowan Gaffney, Bill Barnhart, and Rachel vector: Earth and Planetary Science Letters, v. 171, by future earthquakes. The nonorthogonal- Valletta for thoughtful fi eld conversations and good p. 329–334, doi: 10 .1016 /S0012-821X (99)00173 -9 . ity between GPS mean vector orientation and company while collecting fi eld data. We thank our col- Bastías, H.E., and Bastías, J.A., 1987, Fallamiento rumbo- leagues Chris Andronicos, Terry Jordan, Laura Giam- deslizante en el borde oriental de los Andes entre los geological shortening is in part related to north- biagi, Greg Hoke, Sue Kay, and Manfred Strecker for 32 y 26 grados de latitud sur: X Congreso Geológico south gradients in the vector fi eld producing a many fruitful discussions and fi eld visits, and Susan Argentino Actas, v. 1, p. 207–210. Beck for providing a high-resolution version of the Beer, J.A., 1990, Steady sedimentation and lithologic com- rotation of the infi nitesimal strain ellipse, but pleteness, Bermejo Basin, Argentina: Journal of Geol- probably also to some component of permanent receiver function data for inclusion in Figure 12. We ogy, v. 98, p. 501–517, doi: 10 .1086 /629421 . are grateful to Peter DeCelles and Raymond Russo for vertical axis rotation over geologic time. Beer, J.A., and Jordan, T.E., 1989, The effects of Neo- very helpful critical reviews of the manuscript. This gene thrusting on deposition in the Bermejo basin, The décollement of the Precordillera is well research was supported by National Science Founda- Argentina: Journal of Sedimentary Petrology, v. 59, identifi ed by industry seismic refl ection data at tion grant EAR-0510785. p. 330–345. the eastern limit of the belt, and its westward Beer, J.A., Allmendinger, R.W., Figueroa, D.E., and Jordan, REFERENCES CITED T.E., 1990, Seismic stratigraphy of a Neogene piggy- projection coincides with the high-amplitude back basin, Argentina: American Association of Petro- middle and/or upper crustal event on the receiver Allmendinger, R.W., and Judge, P.A., 2013, Stratigraphic leum Geologists Bulletin, v. 74, p. 1183–1202. function stacks given by Gans et al. (2011). 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