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 For permission to copy, contact [email protected] 1203 © 2014 Geological Society of America 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 SILA 0 20406080100 TOFOTO Brazil GNDL km FO Chile Argentina Peru Bolivia Eastern Precordillera JUNT AT17 MORA AT24 AT30 AT23 AT12 AT20 AT PAGN High 10 POBR Argentina Sierras AT25 Cordillera Pampeanas Bermejo AT Iglesia 29 Basin Basin AT AT 05 AT03 06 COGO PA LO CNGT MGCR X-Line 15 CHI BDSD CFAG G 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 1204 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 to present Andean orogeny (Ramos et al., 1986, micity beneath the eastern Precordillera con- Niquivil Plate 2002; Ramos, 2008).