Late Miocene Upper-Crustal Deformation Within the Interior of the Southern Puna Plateau, Central Andes

Late Miocene upper-crustal deformation within the interior of the southern Puna Plateau, central Andes

Renjie Zhou1,2,* and Lindsay M. Schoenbohm1,2 1DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF TORONTO, 22 RUSSELL STREET, TORONTO, M5S 3B1, ONTARIO, CANADA 2DEPARTMENT OF CHEMICAL AND PHYSICAL SCIENCES, UNIVERSITY OF TORONTO MISSISSAUGA, 3359 MISSISSAUGA ROAD NORTH, MISSISSAUGA, L5L 1C6, ONTARIO, CANADA

ABSTRACT

The origin and evolution of the central Andes, a noncollisional orogenic system, have been hypothesized to evolve as a result of several dynamic processes, including formation of an eastward-propagating orogenic wedge, segmentation into rhomb-shaped basins as a result of N-S gradients in crustal shortening, reactivation of inherited deep structures, and lithospheric foundering. How these proposed processes dominate the orogen spatially and temporally is uncertain; however, constraining the timing of upper-crustal deformation is critical for investigating these models. We document the formation and deformation of the Pasto Ventura basin (NW Argentina) in the southern Puna Plateau. Through field mapping, deformation analysis, secondary ion mass spectrometry U-Pb dating of zircon from interbedded volcanic ashes, and 40Ar/39Ar geochronology of volcanics, we show that major basin formation started ca. 11.7–10.5 Ma and continued until at least ca. 7.8 Ma. The basin underwent syndepositional faulting and folding from ca. 10 to 8 Ma. Contractional deformation in the Pasto Ventura basin ended between ca. 7.3 and 4 Ma, based on the onset of regional horizontal extension. Data from the Pasto Ventura region allow us to bridge existing data and complete a regional compilation of upper-crustal deformation for the Puna Plateau. Our analysis shows that late Miocene formation and deformation of the Pasto Ventura basin represent an important out-of-sequence contractional event in the southern Puna Plateau. While a number of geodynamic processes likely shape the evolution of the southern Puna, multidisciplinary data sets, including deformation in the Pasto Ventura basin studied here, highlight the role of the formation and detachment of a late Miocene lithospheric drip in causing the upper-crustal deformation on the southern Puna Plateau since the mid–late Miocene.

LITHOSPHERE; v. 7; no. 3; p. 336–352; GSA Data Repository Item 2015112 | Published online 12 March 2015 doi:10.1130/L396.1

INTRODUCTION mass of the subcontinental lithospheric mantle beneath this region, either through partial melting or lithospheric foundering, appears to control rates Studies of the central Andes have enriched our understanding of oro- of growth and the width of the central Andes during modulated periods genic plateaus, such as the Altiplano-Puna Plateau, along noncollisional of active thrusting in the orogenic foreland (Dahlen, 1990; DeCelles et plate boundaries. In the absence of plate collision, other processes must al., 2009; Garzione et al., 2006; Kay et al., 1994; Kay and Coira, 2009; account for generating the high strain, thick crust, and topography cur- Molnar and Garzione, 2007; Schoenbohm and Carrapa, 2015). Locally, rently observed in the interior part of the overriding South American the formation and detachment of foundering lithospheric mantle would plate. Understanding the evolution of parts of this broad orogenic sys- stretch the upper crust. As predicted by numerical and analog models, tem requires understanding the overall physical and chemical parameters crustal shortening takes place in the region above a lithospheric founder- controlling tectonic coupling between the subducting plates (e.g., Capi- ing event during the formation of the foundered block prior to its detach- tanio et al., 2011; Lamb and Davis, 2003; Sobolev and Babeyko, 2005). ment; crustal extension then takes over immediately after the lower litho- Horizontal forcing from subduction of the Nazca plate and formation of spheric block detaches and the asthenospheric materials upwell (Göğüş the associated orogenic wedge would result in a deformation front in the and Pysklywec, 2008; Pysklywec and Cruden, 2004). central Andes that has migrated in an in-sequence manner from W to E While a better understanding of geodynamic mechanisms operating in through the interior of the plateau to the modern foreland (e.g., Carrapa the central Andes requires a multidisciplinary approach, one key to evalu- et al., 2011; DeCelles et al., 2011; Horton, 2005; McQuarrie, 2002a). Yet, ating these models is a comprehensive deformation history of the plateau horizontal forcing may preferentially activate preexisting crustal weak and its margins. In the southern Puna Plateau, existing data are extremely zones inherited from the Cretaceous rifting event, causing distributed, out- sparse. Questions remain over the timing and causes of deformation and of-sequence crustal shortening (e.g., Hongn et al., 2007; Sobel et al., 2003; the applicability of orogen-wide geodynamic models for the southern Sobel and Strecker, 2003). Neogene compressional basins, bounded by Puna Plateau (e.g., Allmendinger et al., 1997; Carrapa and DeCelles, orogen-parallel and NE-striking fault zones, form rhomb-shaped crustal 2008; Carrera et al., 2006; DeCelles, et al., 2011; Hain et al., 2011; Hongn segments in map view (Fig. 1B; Riller et al., 2012; Riller and Oncken, et al., 2007; McQuarrie et al., 2005; Kay et al., 1994; Sobel et al., 2003; 2003). Together with an overall N-S gradient in crustal shortening, these Strecker et al., 2007). In this investigation, we present mapping and defor- structures have accommodated deformation propagating north and south mation analysis for the Pasto Ventura basin, the only exposed basin on the from the center of the Bolivian orocline since the Eocene (Riller et al., southernmost Puna Plateau (NW Argentina). High-precision secondary 2012; Riller and Oncken, 2003). In addition, significant reduction in the ion mass spectrometry (SIMS) zircon U-Pb geochronology of interbedded ash beds and 40Ar/39Ar geochronology of newly identified, deformed *[email protected] basaltic trachyandesite flows provide age constraints on sedimentation

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Figure 1. (A) Overview of the Andes. Shaded areas indicate region above 3000 m in elevation, and dark shading denotes the modern surfaces that have a surface slope of less than 4° (e.g., modern basins), both derived from 90 m SRTM (Shut- tle Radar Topography Mission) data. Black line encompasses the internally drained part of the Central Andean Plateau, from HydroSHEDS data (hydrological data and maps based on SHuttle Elevation Derivatives at multiple Scales). Reconstructed historical plate boundaries (dashed lines) are from McQuarrie (2002b). Black arrow shows the kinematics of the modern subducting Nazca plate (Marrett and Strecker, 2000). Inset: historical variations in plate velocity (compiled by Capitanio et al., 2011, from Pardo-Casas and Molnar, 1987; Sdrolias and Muller, 2006; Somoza, 1998). (B) Close-up of Central Andean Plateau indicating tectonomorphic zones (after Strecker et al., 2007). Major faults are shown for the Puna Plateau (from Riller and Oncken, 2003).

and contractional deformation. We then discuss deformation of the south- offset from the modern arc (e.g., Kay et al., 2010; Guzmán and Petrinovic, ern Puna Plateau and adjacent regions, supplemented with other geochro- 2010; Guzmán et al., 2011). Across the Puna Plateau, there are well- nological, geochemical, and geophysical studies, and we explore possible preserved monogenetic volcanic cinder cones and associated lava flows; geodynamic processes in the southern Puna Plateau. these are dominantly mafic, late Miocene to Holocene, confined to the plateau region, and compositionally distinct from arc volcanics (Figs. 2; BACKGROUND Kay et al., 1994; Risse et al., 2008). Pre–late Miocene mafic to intermediate volcanic lavas are also present on the southern Puna Plateau, but their The Andes is the major topographic feature in South America, extend- extents and ages are poorly constrained, especially for the southern Puna ing from tropical Venezuela to glaciated southern Chile. In the central (Fig. 2A; Roy et al., 2006; Schnurr et al., 2006). Andes (~15°S to ~27°S), the Central Andean Plateau (the Altiplano-Puna The deformation history of the Puna Plateau, the adjacent Eastern Cor- Plateau) is characterized by a high-elevation (>3 km), low-relief interior dillera, and the Sierra Pampeanas has been studied for several decades, plateau, surrounded by the higher, more rugged Western and Eastern Cor- where shortening is primarily accommodated by steeply dipping, biver- dilleras, the Sierra Pampeanas, and the Santa Barbara Ranges (Fig. 1; All- gent thrust faults that cut deeply into the crust (>25 km; Allmendinger et mendinger et al., 1997; Isacks, 1988; Strecker et al., 2007). The plateau al., 1997; Cristallini et al., 1997; Cristallini and Ramos, 2000; Jordan and region currently experiences semiarid to arid climate conditions, is inter- Allmendinger, 1986; Kley and Monaldi, 1998; Kley et al., 1999; Monaldi nally drained, and is divided at ~22.5°S latitude into the Altiplano Plateau et al., 2008; Pearson et al., 2013). This thick-skinned tectonic style may to the north and the Puna Plateau to the south (Fig. 1). The Nazca plate has be genetically linked to the Salta Rift, which evolved through Cretaceous- subducted nearly orthogonally beneath the South America plate boundary Paleogene time (ca. 160–60 Ma) in NW Argentina and cuts across the since the Cretaceous, with a present speed of ~8 cm/yr (Fig. 1; Marrett current Santa Barbara system, parts of Eastern Cordillera, and the Puna and Strecker, 2000). Geophysical investigations indicate that the crustal Plateau (Galliski and Viramonte, 1988; Marquillas et al., 2005; Salfity and thickness in the Puna is spatially variable, but it ranges from ~50 to 68 km Marquillas, 1994). (McGlashan et al., 2008; Whitman et al., 1992, 1996; Yuan et al., 2002). Despite several studies from the Eastern Cordillera, data on the tim- Cenozoic arc volcanic rocks are widespread along the Western Cor- ing of Cenozoic upper-crustal deformation such as basin deformation, dillera and the western Altiplano-Puna Plateau (Fig. 1). The post–late sedimentation, and bedrock exhumation in the Puna Plateau are sparse, Miocene Altiplano-Puna volcanic complex between latitudes of 21°S and particularly in the southern plateau, mainly due to the extensive volcanic 24°S, covering ~50,000 km2, represents an intense episode of felsic volca- rocks and colluvial cover (Fig. 3). This paucity of data has allowed a num- nism of the central Andes (de Silva, 1989). Ignimbritic complexes are also ber of different, sometimes opposing models for the causes of deforma- present across the Puna Plateau, often as collapsed calderas, and they are tion in the Puna Plateau. Some models emphasize the W-E propagation

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Figure 2. (A) Geological map of southern margin of the Puna Plateau (modified after Allmendinger et al., 1989; Martinez, 1995; Schnurr et al., 2006; Schoenbohm and Strecker, 2009; Sobel and Strecker, 2003; Zhou et al., 2013). (B) Quaternary and Neogene volcanism in the Pasto Ventura region, NW Argentina. Black polygons are Quaternary volcanic cinder cones and lava flows; geochronology data are from Risse et al. (2008), Zhou et al. (2013), and this study. Dark-green polygons are Neogene mafic flows (Martinez, 1995; Schnurr et al., 2006; Roy et al., 2006; this study). Numbers correspond to age data in Table 2.

of both deformation and basin sedimentation, arguing for a propagating, bedrock cooling episodes have been documented in the Puna Plateau and thick-skinned wedge (e.g., Carrapa et al., 2011; DeCelles et al., 2011). its adjacent Eastern Cordillera (e.g., Carrapa et al., 2014; Deeken et al., Others argue that upper-crustal deformation is driven by a northward 2006; Löbens et al., 2013; Sobel and Strecker, 2003) and could be related increase in shortening in the plateau, resulting in a southward young- to exhumation associated with extension along the Cretaceous Salta Rift ing of deformation and basin sedimentation (Fig. 1B; e.g., Riller and (Carrapa et al., 2014; Sobel and Strecker, 2003). However, the pre-Ceno- Oncken, 2003). Other models draw attention to out-of-sequence defor- zoic history of the central Andes is beyond the scope of this paper and is mation observed across the plateau and its adjacent Eastern Cordillera, not included in the following summary. attributing this to irregular reactivation of faults in a broken foreland (e.g., In the northwestern Puna Plateau, modeled apatite fission-track data Hain et al., 2011), change of the mass of a critical taper (e.g., DeCelles et reveal that deformation to the west of the Salar de Arizaro (AR in Fig. 3) al., 2009), or to lithospheric foundering (e.g., Kay et al., 1994). We sum- took place ca. 42–33 Ma, followed by exhumation around 20 Ma (Schoen- marize existing shortening data in the following paragraphs, separating bohm and Carrapa, 2015). To the east of the Salar de Arizaro, isotopic ages deformation inferred from rapid cooling deduced from bedrock thermo- from deformed strata constrain deformation to after ca. 14 Ma (Alonso chronology (exhumation in Fig. 3) from deformation inferred from dating et al., 1991; Boyd, 2010; Marrett et al., 1994), consistent with proposed of sedimentary strata that predate, overlap, or postdate deformation (pre-, 15–8 Ma exhumation based on thermochronology data (Fig. 3C, section syn-, and post-deformation in Fig. 3). We note that several pre-Cenozoic A–A′; Carrapa et al., 2009). To the east of the northern Puna Plateau in the

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Figure 3. Compilation of documented deformation across the southern Altiplano Plateau and the Puna Plateau. (A) Topographic map (Shuttle Radar Topography Mission [SRTM] 90 m digital elevation model) of the southern Altiplano and the Puna, central Andes. The 3 km contour is indicated in black, and the internal drainage area is outlined with blue lines (derived from U.S. Geological Survey HydroSHEDS data). It is noted that some studies directly documented the timing of deformation based on structural relationships (structural data), while others made use of low-temperature thermochronology to date exhumation (exhumation data). The timing of exhumation may relate to structural deformation, but it could also reflect other possible causes such as climate-driven erosion (e.g., Barnes et al., 2012; Sobel and Strecker, 2003). Deformation from locations north of ~24°S is shown in circles, with references in square brackets (sources: 1—Hammerschmidt et al., 1992; 2—Reutter et al., 1996; 3—Mpodozis et al., 2005; 4—Arriagada et al., 2006; 5—Siks and Horton, 2011; 6—Cladouhos et al., 1994; 7—Echavarria et al., 2003; 8—Horton, 1998; 9—Ege et al., 2007). (B) Detailed map of documented deformation in the Puna Plateau and its adjacent Eastern Cordillera (sources: a—Alonso, et al., 1991; b—Marrett et al., 1994; c—Boyd, 2010; d—del Papa et al., 2013, Hongn et al., 2007; e—Cristallini et al., 1997; f—Carrapa et al., 2011; g—Mortimer et al., 2007; h—Carrapa et al., 2009; i—Carrapa and DeCelles, 2008; j—Schoenbohm and Carrapa, 2015; k—Deeken et al., 2006; l—Carrapa et al., 2005; m—Sobel and Strecker, 2003; n—Coutand et al., 2001; o—Carrapa et al., 2006; p—Schoenbohm et al., 2007; q—Löbens et al., 2013; r—Vezzoli et al., 2012; s—Pearson et al., 2013; t—Carrapa et al., 2014). (C) Plots of the timing of deformation along topographic gradients. The elevation swath profiles are extracted from 50-km-wide swath boxes (shown on Fig. 3B), based on 90-m-resolution SRTM data. On the plots, the constrained timing of pre-/syn-/postdeformation is marked by short, horizontal bars. On the topographic swath profiles, the plateau is the higher-elevation region, bounded by the more steeply sloping eastern/southeastern flanks of the plateau.

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Eastern Cordillera, apatite fission-track thermochronology indicates range- scale exhumation and adjacent intramontane basin development migrating eastward into the Eastern Cordillera (Cumbres de Luracatao Range) by 21 Ma (Fig. 3C, section A–A′; Deeken et al., 2006). At the southern end of the Eastern Cordillera, syndepositional deformation occurred at ca. 40 Ma (Fig. 3C, section A–A′; del Papa et al., 2013; Hongn et al., 2007). In the southern Puna Plateau, especially south of 26°S, there are few data constraining deformation, partly because of limited access and partly because of extensive surficial deposits from the Cerro Galán ignimbrite complex (GL, Fig. 3B), which formed from 6.6 to 2 Ma (Fig. 3; Kay et al., 2010), obscuring observations of the underlying geology in much of the region. The 29–24 Ma exhumation of the Calalaste Range west of the Salar de Antofalla (ANT, Fig. 3) is the only other region containing documented Cenozoic shortening on the southern Puna Plateau (Carrapa et al., 2005). Deformation of the eastern and southern flanks of the southern Puna Plateau and in the adjacent Sierras Pampeanas was diachronous. Apatite fission-track data from an E-W transect (Fig. 3C, section B–B′) across the Chango Real pluton (CR, Fig. 3) indicate that the southeastern flank of the Puna Plateau was exhumed ca. 38–29 Ma (Figs. 3B and 3C; Coutand et al., 2001). Farther east, U-Pb geochronology of intercalated ashes and detrital fission-track data from strata within the El Cajón basin (Caj, Fig. 3) indicate uplift of the eastern plateau margin (west margin of basin) from 13.6 to 10.7 Ma, and of the Sierra de Quilmes to the east of the basin from ca. 10 to 6 Ma (Mortimer et al., 2007; Schoenbohm et al., 2007). Exhumation of the next range to the east, the Sierra del Aconquija (Acon, Fig. 3), occurred around 5.5 Ma (Fig. 3C, section B–B′; Sobel and Strecker, 2003). On the southern flank of the Puna Plateau (Fig. 3C, section C–C′), apatite fission-track and (U-Th)/He thermochronology indicate rapid exhumation at ca. 21–14 Ma in the Cerro Negro (Carrapa et al., 2014). This exhumation is also recorded by detrital apatite fission-track data in sediments preserved in the Fiambalá basin (Fia, Fig. 3) with a strong 14 Ma signal, suggesting the southern margin of the Puna Plateau was exhuming during the middle Miocene (Carrapa et al., 2006), and this is in line with the estimate of pre–9 Ma local relief establishment (Mon- tero-López et al., 2014). Figure 4. Field photos of map units. (A) Unit Ns-1, eolian sandstone. (B) Unit Ns-1 eolian sandstone, hydrothermally altered. Note that the eolian METHODS AND RESULTS cross-bedding is preserved. (C) Unit Ns-2, gray fine to medium sandstone, with parallel lamination. (D) Unit Ns-3, mudstone. (E) Unit Ns-4, Geological Mapping in the Pasto Ventura Basin, NW Argentina medium to coarse, cross-bedded red sandstone and siltstone with occasional cobble layers. (F) Paleosol within unit Ns-4, showing pedogenic The landscape of the southern Puna Plateau is dominantly covered by carbonate nodules. (G) Unit Nfl, deformed basaltic trachyandesite. Note recent lava flows, ignimbrites, and colluvium. The Pasto Ventura region that the fractures are parallel to each other and are dipping to the right in the photo (dipping to the east in the field). Outcrop location: 26°46.39′S, (~26°48′S, ~67°16′W) is located on the southern Puna Plateau (Fig. 2) 67°13.97′W. Sample PV11B-01 was sampled from this outcrop. (H) Unit M, and outcrops in a narrow, elongated, roughly N-S–striking basin that the metamorphic bedrock. contains sedimentary rocks of primarily Neogene age (Schoenbohm and Carrapa, 2015; Zhou et al., 2013; Fig. 4). The Pasto Ventura basin is the only exposed, relatively continuous sedimentary record in the southern- Most of the basin-fill units are exposed along a river-cut valley run- most Puna Plateau (Fig. 2A). We mapped this basin using an aerial photo- ning approximately N-S through the Pasto Ventura basin, with the rest of graph base (Instituto Geografico Militar, Argentina) through both field and the region largely covered by Quaternary sediments and Quaternary mafic remote mapping, supplemented with satellite images and digital elevation volcanic rocks. Quaternary units overlying the deformed Neogene strata models (DEMs), including Landsat, Google Earth, ASTER (Advanced include modern eolian sand dunes (Qe), Quaternary alluvial and fluvial Space-borne Thermal Emission and Reflection Radiometer), and SRTM sediments (Qaf), and Quaternary colluvial and terrace sediments (Qct). (Shuttle Radar Topography Mission). We present two maps of the Pasto Unit Qe is usually found along the lee side of river-cut channels or hills, Ventura basin, covering the majority of its extent: the north Pasto Ventura where locally high topography favors the accumulation of large volumes map (the NPV map; Fig. 5) and the south Pasto Ventura map (the SPV of sand carried by prevailing northwesterly winds. Unit Qaf is sand-/ map; Fig. 6). Our maps update previous work in parts of the Pasto Ventura gravel-grade sediments found within river channels (fluvial origins) or basin (Allmendinger et al., 1989; Schoenbohm and Carrapa, 2015; Zhou fans (alluvial origins). Unit Qct comprises low-slope surfaces covered by et al., 2013) and, along with new structural, geochronological, and geo- various unconsolidated, thick (up to several meters) sediments. The clasts chemical data, allow us to explore the deformation history and dynamics composing unit Qaf and unit Qct are mainly derived from metamorphic of this region. bedrock (the Puncoviscana Formation) and mafic and intermediate lava

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Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/7/3/336/3051194/336.pdf by guest on 25 September 2021 Late Miocene southern Puna Plateau deformation | RESEARCH Ar geochronology result for sample PV11B-01, which was sampled from the deformed basaltic trachyandesite unit (unit Nfl). basaltic trachyandesite the deformed sampled from was which sample PV11B-01, for result Ar geochronology 39 Ar/ 5. Geological map of the northern Pasto Ventura basin (the NPV map). Ages with (*) are from Schoenbohm and Carrapa (2015). The map base is aerial photography. (Upper right) (Upper right) photography. The map base is aerial (2015). and Carrapa Schoenbohm from with (*) are Ages basin (the NPV map). Ventura Pasto Geological map of the northern 5. Figure 40

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Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/7/3/336/3051194/336.pdf by guest on 25 September 2021 ZHOU AND SCHOENBOHM 6. (A) Geological map of the southern Pasto Ventura basin (the SPV map). The map base is aerial photography. (B) Cross sections for the geological map of the southern Pasto Ventura Ventura Pasto map of the southern the geological sections for (B) Cross photography. The map base is aerial basin (the SPV map). Ventura Pasto (A) Geological map of the southern 6. Figure 2A. in Figure shown Locations are basin (the SPV map).

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flows. The clasts are generally ~5–10 cm in size and are characterized on The bedrock (metamorphic basement, unit M) in this area belongs to the surface by wind-blown erosion surfaces. the Puncoviscana Formation and consists of clastic, weakly metamor- The Quaternary mafic volcanic units include Qbc (cinder cones), Qbf phosed, sedimentary rocks (Allmendinger et al., 1997; Ramos, 2008; (lava flows), and Qwb (wind-blown clasts). Unit Qwb is characterized by Fig. 4F). On images, the bedrock is characterized by high topography and dark colors on images and composed of irregular-shaped clasts derived a fracture-rich, dark-colored appearance. directly from nearby lava flows (Qbf) or cinder cones (Qbc). The recent volcanism is characterized by monogenetic volcanoes with associated lava U-Pb Geochronology of Volcanic Ashes flows and is spatially and temporally linked with post-Pliocene extension (e.g., Allmendinger et al., 1997). We sampled volcanic ash layers from deformed sedimentary units in We divide the Neogene sedimentary rocks into four units (units Ns-1, order to place age constraints on deposition and deformation (Figs. 5 and 6; Ns-2, Ns-3, and Ns-4; Figs. 5 and 6). The thickness of these units var- Table 1). Zircons were separated following standard methods at the Jack ies across the region, ranging from 100 to 200 m to up to 1 km, and it Satterly Geochronology Laboratory at University of Toronto (e.g., Barker may reflect varying depositional conditions throughout this relatively et al., 2011). U-Pb measurements were performed at the SIMS Center small basin. Unit Ns-4 is inferred to be the oldest among the four units at University of California Los Angeles (UCLA) following methods because it does not contain interbedded volcanic ashes, whereas this described by Schmitt et al. (2003). region has been experiencing felsic volcanism since ca. 20–16 Ma (Kay We dated 16 ashes, six of which are from unit Ns-3 and 10 of which et al., 2010). Unit Ns-4 outcrops along and beyond the northwest edge are from unit Ns-2 (Table 1). For each ash, ~10–20 grains were dated. The of the NPV map (Fig. 5), where it lies in depositional contact with the age of an ash was calculated from individual grain ages in a clustered age bedrock basement (unit M). It consists of medium to coarse cross-bedded population, excluding very young (Quaternary age) and old (up to 100s red sandstone and siltstone with occasional cobble layers (~5–10 cm in or 1000s Ma) ages that likely reflect contamination by Quaternary vol- thickness) and pedogenic carbonate nodules (Fig. 4). In the central part canic activities in the region and detrital input from the plateau bedrock, of the basin, unit Ns-3 is a mudstone with thin (up to 20 cm) interbedded respectively. We combine our new data with published data (Schoenbohm fine sandstone to siltstone layers (Fig. 4B). This unit, as well as units Ns-2 and Carrapa, 2015) to present a complete data set for all of the dated vol- and Ns-1, is interbedded with volcanic ash layers or pumice layers, with canic ashes for the Pasto Ventura basin (Table 1). Due to uncertainties in thicknesses varying from several centimeters to ~1 m (Fig. 4B). Locally, the dating technique, some ages overlap or are identical with each other. cross-bedding is observed within fine sandstone to siltstone layers. The We group them into Ns-1, Ns-2 upper, Ns-2 lower, Ns-3 upper, and Ns-3 mudstone alternates between green and red in color on a scale of several lower based on field relationships (Table 1). Ages of volcanic ashes in tens of centimeters to ~1 m. Within the red mudstone, gypsum deposits Ns-3 range from 10.4 ± 0.10 Ma to 9.8 ± 0.10 Ma. Those from Ns-2 range are found as thin (1–2 cm) layers. Conformable with unit Ns-3, unit Ns-2 from 9.59 ± 0.10 Ma to 8.32 ± 0.12 Ma. In addition, Schoenbohm and is dominantly composed of fine- to medium-grained sandstones, with Carrapa (2015) reported 7.77 ± 0.21 Ma and 7.88 ± 0.58 Ma ashes from interbedded conglomerate layers (Fig. 4C). These conglomerate layers unit Ns-1, and a 10.50 ± 0.10 Ma for unit Ns-2 (Table 1). are typically ~0.5 m thick with clasts of dominantly phyllite, basalt, and granite, ranging in size from 1 to 5 cm. The clasts are well rounded and 40Ar/39Ar Geochronology of Lava Flows and Monogenetic relatively uniform in size within each bed. Conglomerate beds make up a Cinder Cones larger portion of the unit in the southern part of the basin. Within the sandstones, cross-bedding is widely observed. Soft-sediment deformation such The 40Ar/39Ar geochronology experiments were conducted at the U.S. as convolute bedding is also observed and is only found locally within Geological Survey in Denver, Colorado. For this study, neutron irradi- isolated beds. Conformable with unit Ns-2, unit Ns-1 is a medium-grained ated basalt rock fragments, free of obvious alteration and phenocrysts, eolian sandstone featuring low-angle (usually <~30°) and large-scale (up of ~1 mm3 were analyzed. A summary of the methods and the tabulated to several meters) eolian cross-beds (Fig. 4A). In the southeast part of the results are given in the GSA Data Repository (see footnote 1). NPV map, outcrops of unit Ns-1 found along a river-cut valley appear to Unit Nfl was deformed and exhumed in a dome-like feature, as the be hydrothermally altered and are brown to black in color, but the strong result of interference between fold 1 and fold 2 structures (see later herein; eolian cross-beds are preserved (Fig. 4B). Fig. 5). Unit Nfl also constitutes the floor of the eastern Pasto Ventura Unit Nfl constitutes volcanic flows that crop out in the center of the basin (Fig. 5), thus providing important constraints on the timing of depo- NPV map (~67°14′S, 26°47′W; Fig. 5). Nfl flows contain phenocrysts sition and deformation for the Pasto Ventura basin. We sampled unit Nfl that are mainly plagioclase (up to ~2 cm) with sparse olivine; the ground- from an outcrop immediately below the base of unit Ns-3 (sample PV11B- mass contains plagioclase and olivine that have been mostly altered to 01) for 40Ar/39Ar dating. A 40Ar/39Ar plateau age (three or more contiguous iddingsite, and calcite-filled vesicles are also present (Fig. DR31). Unit heating steps comprising ≥50% of the 39Ar released with ages overlapping Nfl is overlain by unit Ns-3 red mudstone and therefore predates deposi- at the 2s level of uncertainty) of 11.69 ± 0.01 Ma was determined for this tion of units Ns-3, Ns-2, and Ns-1. However, the relationship between sample (11 heating steps and 55.2% total 39Ar released; Fig. 5). Nfl and Ns-4 is unconstrained by mapping. We performed geochemi- The study area also contains several monogenetic mafic volcanic cin- cal and 40Ar/39Ar geochronological analysis on samples from unit Nfl in der cones and associated lava flows (Fig. 2). The cinder cones are rela- order to get a better understanding of its origins and to put better con- tively small in size (typically 500 to 800 m in diameter), and the flows are straints on basin sedimentation and deformation. Details are included in usually confined to within 1–3 km from the sourcing cinder cones. They the following sections. are important indicators for geodynamic processes such as lithospheric foundering and are clearly associated with upper-crustal extension (e.g., Allmendinger et al., 1989; Marrett and Emerman, 1992; Kay et al., 1994; 1 GSA Data Repository Item 2015112, supplementary data for U-Pb geochronol- Ruch and Walter, 2010). In this study, we compiled existing geochrono- ogy, Ar/Ar geochronology, and volcanic samples, is available at www.geosociety .org/pubs/ft2015.htm, or on request from [email protected], Documents logical data for the mafic monogenetic volcanism for the Pasto Ventura Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA. region, and we add five new40 Ar/39Ar ages (Table 2). Five mafic volcanic

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TABLE 1. 206Pb/238U GEOCHRONOLOGY RESULT OF ZIRCONS FROM VOLCANIC ASHES IN THE PASTO VENTURA BASIN, NW ARGENTINA

206 238 † Location Number of zircons dated Pb/ U age (Ma) Sample ID Unit MSWD* (°S, °W) Total Excluded Calculatedw.m. 1 s.e. (±) PVN260§ 26.764433, 67.219417 Ns-1 18 16 20.057.770.21 PVN226§ 26.764100, 67.219950 Ns-1 15 14 1n/a 7.88 0.58 PV10A-02 26.807267, 67.233150 Ns-2 upper 15 10 52.438.320.12 PV11A-22 26.839994, 67.280485 Ns-2 upper 25 10 15 1.78 9.11 0.07 PV10A-01 26.806400, 67.228083 Ns-2 upper 20 8121.039.270.09 PV11A-61 26.800374, 67.251767 Ns-2 upper 20 5150.669.170.1 PV11A-06 26.798655, 67.249178 Ns-2 upper 20 0201.119.220.08 PV11A-04 26.807071, 67.247294 Ns-2 upper 19 0191.699.010.08 PV11A-05 26.811840, 67.247901 Ns-2 upper 19 6130.659.090.09 P09A-01 26.854220, 67.306940 Ns-2 lower 20 6140.449.3 0.13 PV11A-07 26.759477, 67.238497 Ns-2 lower 25 9161.249.510.1 PV10A-04 26.869967, 67.310300 Ns-2 lower 20 11 90.449.590.19 PV10A-06 26.876950, 67.299067 Ns-3 upper 15 3121.479.990.12 PV10A-03 26.878117, 67.298500 Ns-3 upper 15 5101.519.8 0.1 P09A-03 26.864950, 67.295200 Ns-3 upper 15 5101.489.9 0.1 PV10A-05 26.867433, 67.301950 Ns-3 upper 20 2181.2910.15 0.1 PVN75§ 26.762433, 67.223467 Ns-3 lower 13 0130.6310.50.1 PV11A-72 26.881525, 67.291895 Ns-3 lower 84 40.8510.33 0.29 P-Ash-01 26.902010, 67.299200 Ns-3 lower 90 90.9210.36 0.14 *MSWD—mean square of weighted deviates. †w.m.—weighted mean; 1 s.e.—one standard error. §Cited from Schoenbohm and Carrapa (2015).

TABLE 2. 40Ar/39Ar AGES FOR MAFIC CINDER CONES AND LAVAS IN THE PASTO VENTURA REGION, NW ARGENTINA (RISSE ET AL., 2008; ZHOU ET AL., 2013; THIS STUDY) Number* Sample ID Location Age 1 s.e. References (°S, °W) (Ma) (±) 1 SAF71 26.471 67.416 0.75 0.08 Risse et al. (2008) 2 P07-PV1-02 26.7020 67.2480 0.68 0.06 This study 3 P07-PV1-01 26.7050 67.2490 0.27 0.04 Zhou et al. (2013) 4 P07PV2-01 26.7320 67.2110 0.76 0.16 Zhou et al. (2013) 5 P07PV2-02 26.7290 67.2050 0.47 0.03 Zhou et al. (2013) 6 P07PVSUR3-01 26.865 67.255 0.42 0.05 This study 7 P07SUR1-01 26.9000 67.2750 0.43 0.07 This study 8 P09B-09 26.84584 67.32280 0.57 0.04 This study 9 P09B-12 26.86422 67.31136 0.45 0.02 This study *Numbers are labeled on Figure 2.

samples (PV07-PV1-02, P07PVSUR3-1, P07SUR1-01, P09B-09, P09B- analytical Laboratory in the School of Earth and Environmental Sciences at 12) were collected for geochronology using 40Ar/ 39Ar step-heating analy- Washington State University (e.g., Johnson et al., 1999; Table 3). ses, and these yielded ages of 0.68 ± 0.06 Ma (sample PV07-PV1-02), The geochemical similarities between samples PV11B-01 and PV11B- 0.42 ± 0.05 Ma (sample P07PVSUR3-1), 0.43 ± 0.07 Ma (sample 04 confirm our field and petrographic observations for the extent of unit P07SUR1-01), 0.57 ± 0.04 Ma (sample P09B-09), and 0.45 ± 0.02 Ma Nfl. Results also show that the newly identified unit Nfl in the center of the (sample P09B-12) (Table 2). NPV map area shares similarities with a sample (PV10B-03) from the previously mapped, voluminous intermediate-mafic unit to the east (e.g., Roy et Whole-Rock Geochemistry al., 2006; Schnurr et al., 2006). Given the very similar geochemical signatures between PV10B-03 and PV11B-01/04, we describe them as one group Two samples of volcanic rocks (PV11B-01 and PV11B-04) obtained (called the Nfl samples) in the discussion. The Nfl samples are shoshonitic from the northeastern and southwestern edges of the mapped Nfl unit, basaltic trachyandesites. They have low Mg# (20.6–43.4), low MgO (1.2–

respectively (Fig. 5), were processed for whole-rock geochemistry (Table 3). 3.8 wt%), high K2O (3.3–4.1 wt%), and high Al2O3 (17–20 wt%; Table 3). We also collected one sample from a similar flow to the east of the NPV map They are depleted in Ni and Cr and high field strength elements (HFSEs) (sample PV10B-03; Fig. 2), which was assigned to a dacite/andesite unit or such as Ta, Nb, and Ce. Some of the large ion lithophile elements (LILEs), Tertiary mafic volcanic rocks by previous workers (e.g., Roy et al., 2006; especially Ba and Nb, are also depleted. The 11.69 ± 0.01 Ma 40Ar/39Ar Schnurr et al., 2006). Major and trace elements including rare earth ele- plateau age of PV11B-01 might be indicative of the more extended Tertiary ments (REEs) were determined by whole-rock X-ray fluorescence (XRF) andesite/dacite unit on the SE edge of the Puna Plateau (Roy et al., 2006; and inductively coupled plasma–mass spectrometry (ICP-MS) at the Geo- Schnurr, 2006), the age of which has not previously been documented.

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TABLE 3. GEOCHEMICAL RESULTS FOR Nﬂ SAMPLES

† § Sample ID Latitude Longitude SiO2 TiO2 Al2O3 FeO MnO MgOCaO Na2OK2OP2O5 LOI Total (°S) (°W) PV10B-03 26.805 67.182 53.537 1.403 19.511 7.219 0.157 2.0097.101 3.8753.483 0.9620.479 99.258 PV11B-01 26.773 67.232 50.088 1.482 18.281 8.546 0.1633.678 6.8524.038 3.2211.078 2.15597.428 PV11B-04 26.796 67.241 47.942 1.379 17.097 8.154 0.200 1.19011.1843.375 3.9380.898 4.84195.359

Sample ID Ni# Cr Sc VBaRbSrZrYNb Ga Cu Zn Pb PV10B-03 7.976 2.991 17.946 116.450 445.360 246.857 612.956 948.34663.010 89.331 24.028 200.39791.226 19.180 PV11B-01 20.738 15.154 20.538 200.596 380.954 148.354 663.902 301.69235.194 37.188 20.937 265.50184.147 9.914 PV11B-04 14.157 24.427 20.239 222.331 467.194 148.055 581.949 231.70332.801 24.227 20.837 160.716100.099 10.364

Sample ID La Ce Th Nd UPrSmEuGdTbDyHoErTm PV10B-03 105.995 213.293 61.111 95.116 12.447 25.743 17.8862.722 14.455 2.21512.663 2.4856.615 0.967 PV11B-01 49.230 100.167 13.937 48.540 3.041 12.354 9.720 2.2908.214 1.2016.960 1.3643.595 0.517 PV11B-04 41.530 84.545 9.594 42.435 5.493 10.663 8.861 2.2337.678 1.1466.526 1.2653.376 0.482

Sample ID Yb Lu Hf Ta La/Ta Ba/Ta Sm/Yb La/Yb Hf/Sm Mg#**Eu/Eu*†† PV10B-03 6.049 0.937 21.614 5.559 19.067 80.113 2.95717.522 1.20833.162 0.432 PV11B-01 3.259 0.520 6.593 2.091 23.548 182.225 2.983 15.106 0.67843.409 0.670 PV11B-04 2.953 0.459 5.586 1.430 29.040 326.686 3.00114.064 0.63020.648 0.701 Note: Sample locations are shown in Figures 2B, 5, and 8. †Concentrations for major elements are listed in weight percent. §LOI—loss on ignition, in weight percent. #Concentrations for trace elements are listed in ppm. **Mg# = [mol Mg/(mol Mg + mol Fe)] × 100, assuming all Fe2+. ††Eu* is determined from interpolation of rare earth element pattern between Sm and Tb (Kay et al., 1994).

Deformation Analysis: Faults, Folds, and Syndepositional NPV map. From the north to the south, fold 1 becomes more open, with Deformation the interlimb angle changing from ~30°–40° to ~125°. Based on cross sections reconstructed from surface data, shortening accommodated by A major NW-dipping thrust fault (fault 1), with an ~N32°E strike, cuts fold 1 is estimated to be ~0.81 km (~32%; Fig. 5, section A–A′) between much of the NPV map, dying out to the southwest by ~26°50′S, before fault 1 and fault 2 and ~0.94 km (~34%; Fig. 5, B–B′) between fault 1 reaching the SPV map. This thrust fault is a member of the El Peñón– and fault 3 farther south. We suggest that there is a WNW-ESE–directed, Pasto Ventura fault group (Allmendinger et al., 1989). Although this major open anticline (fold 2), which subsequently folded fold 1 and is respon- thrust fault has undergone recent extensional reactivation (Allmendinger sible for forming the axial culmination of fold 1. Volcanic unit Nfl is thus et al., 1989), as evidenced by normal sense displacement of two ca. 0.8 Ma exposed at the base of the eastern section in an interference dome-like and ca. 0.38 Ma mafic cinder cones to the north (out of the NPV map; structure. This second folding event postdates most of the ESE-WNW– Zhou et al., 2013), we focus on its earlier, contractional phase because it directed deformation in the region. appears to have controlled the deformation of the Pasto Ventura basin. It Fault 3 is paired with a syncline in its hanging wall to the east (fold 3), carries bedrock (unit M) in its hanging wall over sedimentary units in its which is open in shape and has only minor significance for accommodat- footwall along the western margin of the basin. Because of modern eolian ing shortening (~10%). Fault 3 dies out and is replaced by a south-plung- deposition and pervasive physical weathering and colluvial transport of ing anticline (fold 4) to its south. The axial plane of fold 4 is vertical with material, we were unable to locate any well-preserved exposures of the an interlimb angle of ~150°. The fold 4 anticline is paired with a syncline fault plane. However, the fault trace on the surface is clearly evident by a (fold 5) to its east, which is also south-plunging and gently folded. Across distinct topographic rise, and therefore it enables us to calculate an ~33° the cross section C–C′ (Fig. 5), the shortening accommodated by fold 1, to 42° dip for fault 1 based on mapped fault trace and topography. To the fold 4, and fold 5 is ~0.46 km (10%). east of fault 1, a secondary thrust (fault 2) bounds the fold 1 syncline on Unit Nfl, the volcanic flow, contains ~50-cm-spaced layers bounded the west. Fault 2 strikes ~N40°E and dips ~45° to the west based on field by parallel parting surfaces that consistently dip ~22° to the east along the observations. It brings unit Ns-3 to the surface and tilts the strata ~28° eastern edge of the unit, similar to the underlying unit Ns-3 (Fig. 4G). On to 35° to the east. Displacement along the ~2.7-km-long fault decreases satellite and air-photo images, the layers can be traced along strike, fol- to both the N and S (Fig. 5). Fault 3 is also present to the east of fault 1, lowing the base of the mapped unit Ns-3, forming “V-shaped” curvatures but in contrast to fault 2, it dips to the east (toward ~N80°E). This fault at river valleys. The orientation of the layering in unit Nfl is close to the carries Ns-3 above Ns-2. Along this fault, the maximum displacement is orientation of bedding of the base of overlying unit Ns-3, consistent with estimated to be ~0.5 km (Fig. 5, section B–B′). primary lavas flow within unit Nfl that were once flat-lying. Deformation Strata of unit Ns-3 have been folded into a syncline (fold 1) and are by approximately NNE-striking folding and thrust faulting, and possibly locally overturned within ~300 m of fault 1 (Fig. 5). Fold 1, with an subsequent anticline folding (fold 2), tilted unit Nfl. Therefore, unit Nfl 80° ~W-dipping axial plane, runs across the entire NPV map, with an (11.7 Ma, see following sections) predates deformation of units Ns-3 to ~N30°E strike in the north and an ~N10°E strike in the south. This syn- Ns-1 in the Pasto Ventura basin. cline has an axial culmination, and thus younger units make up the core In southern Pasto Ventura (the SPV map; Fig. 6), fault 4 and fault 5 of the syncline in the north and south compared to the central part of the thrust faults mark the southern continuation of fault 1 from the NPV map.

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Fault 4 brings the bedrock (unit M) to the surface in its hanging wall, their geometry satisfies features for syndepositional fault-bend folding. whereas fault 5 is confined to unit Ns-1. We suspect both faults dip rela- Volcanic ashes within this unit, from below and above the observed inter- tively steeply because of their linear map trace, and they die out rapidly nal unconformity, yield ages of 9.22 ± 0.08 Ma (sample PV11A-06) and to the south. The anticline, fold 8, dominates the deformation in the SPV 9.17 ± 0.10 Ma (sample PV11A-61), respectively. Taken together, these map. The axial plane of fold 8 strikes ~N20°–30°S, although the trace is internal unconformities and age data for interbedded volcanic ashes imply somewhat sinuous. This fold brings the lower unit, Ns-3, to the surface. In that deposition and deformation must have taken place synchronously in the middle of the SPV map, strata from two limbs of fold 8 dip toward the the Pasto Ventura basin around ca. 9–10 Ma. NE and NW, indicating this fold is plunging to the north. Fold 8 is open in the south (interlimb angle ~110°) and becomes tighter to the north (inter- DISCUSSION limb angle ~40°). Fold 8 is paired with syncline fold 6 in the northwest and syncline fold 7 in the middle and south of the map. Axial planes of Timing of Formation and Deformation of the Pasto Ventura both fold 6 and fold 7 strike ~N30°E, parallel with syncline fold 8, and are Basin (NW Argentina) tight (interlimb angle ~70°–80°). The axial planes of fold 6, fold 7, and fold 8 are subvertical (~80°, ~85°, ~80°, respectively), but dip slightly to The sedimentation history for the Pasto Ventura basin began with the west. The amount of shortening estimated from the cross section to deposition of the oldest sedimentary unit in the basin, unit Ns-4. We do the north (D–D′) is ~1.45 km (~47%) and from the south section (E–E′) is not have direct age constraints for unit Ns-4, and no stratigraphic relation- ~3 km (~45%; Fig. 6). ship between units Ns-4 and Nfl has been observed in the field. However, The mapped NNE-striking, dominantly WNW-dipping thrust faults we argue that unit Ns-4 likely predates unit Nfl (11.69 ± 0.01 Ma, sam- and associated folds suggest that the Pasto Ventura basin was deformed ple PV11B-01 in this study) given the fact that the Puna Plateau started primarily under approximately WNW-ESE contraction. We are able to to be characterized by felsic volcanism since 20–16 Ma (mostly since constrain the amount of shortening based on cross sections. The amount 14–12 Ma), and unit Ns-4 does not contain any of the volcanic ashes so of approximately E-W shortening ranges from ~35%–45% to 10%–15% prevalent in younger strata (e.g., Kay et al., 2010). The presence of paleo- (Figs. 5 and 6). Some minor NNE-SSW contraction is possible as a later sol horizons within unit Ns-4 (Fig. 4F) indicates that the Pasto Ventura phase of deformation, evidenced by the formation of a WNW-ENE–strik- basin was undergoing slow accumulation and prolonged subaerial expo- ing fold 2 anticline in the NPV map. sure, with possible periods of erosion or at least sedimentary hiatuses. Several lines of evidence point to syndepositional deformation in the Following deposition of unit Ns-4, a basaltic trachyandesite flow (unit Pasto Ventura basin. In the SPV map, an angular unconformity formed Nfl) covered the basin at ca. 11.7 Ma. The onset of major accumulation within the Ns-3 mudstone-siltstone (Figs. 7A and 7B). Units below and in the basin, units Ns-3, Ns-2, and Ns-1, which consist of fluvial/allu- above the angular unconformity are indistinguishable in lithology but are vial, lacustrine, and eolian sediments, started after ca. 11.7 Ma and before oriented differently (the beds above the unconformity are oriented 230/06; ca. 10.5 Ma, the ages of the oldest volcanic ashes obtained from the Pasto the beds below the unconformity are oriented 216/28; Figs. 7A and 7B). Ventura basin (Table 1; sample P-Ash-01 in this study; sample PVN75 in SIMS-dated zircons from ashes above this unconformity yield an age of Schoenbohm and Carrapa, 2015). Basin strata continued to accumulate 9.99 ± 0.12 Ma (sample PV10A-06), and ashes below yield an age of 9.80 until at least ca. 7.8 Ma, the age of the youngest ash dated in the basin ± 0.10 Ma (sample PV10A-03), which are the same within uncertainty, (Schoenbohm and Carrapa, 2015). As no outcrop of any younger basin indicating only a short interval of section missing across the unconformity. fill is observed in this region and the paleoenvironment for Ns-1 is similar In the NPV map, an internal unconformity is found within the lower unit to modern conditions in the Pasto Ventura region, we infer that unit Ns-1 Ns-2 of the NPV map (Figs. 7C and 7D), in which older beds are folded is the last unit deposited in the Pasto Ventura basin and continued to be and truncated. Schoenbohm and Carrapa (2015) were able to retrodeform deposited after 7.77 ± 0.21 Ma (Schoenbohm and Carrapa, 2015). the beds (right of the photo view in Figs. 7C and 7D) and showed that Volcanic flow unit Nfl floors most of the exposed Pasto Ventura basin, dips consistently with immediately overlying unit Ns-3, and was tilted during basin deformation (Fig. 5). Therefore, its age of 11.7 Ma must predate the onset of the late Miocene contractional deformation in the Pasto Ven- tura basin. Angular, internal unconformities indicate ongoing syndepositional deformation of the Pasto Ventura basin from 10 to 9 Ma. The major thrust fault in the NVP map developed around the same time and truncates a 9.51 ± 0.10 Ma ash (sample PV11A-07) in its footwall. Unit Nfl, underlying unit Ns-3, was possibly folded by a NNE-SSW–striking anticline, enhancing its exposure at the surface. The influence of this anticline is minor and could be coeval with fault 2 and fault 3, but its exact timing is unclear. In the southern NPV map (Fig. 4), an ash close to the core of a syncline is dated at 8.32 ± 0.12 Ma (sample PV10A-02), and Schoenbohm and Carrapa (2015) documented tilted strata younger than 7.77 ± 0.21 Ma (in Ns-1 eolian sandstone). We therefore infer that contractional deformation, in the form of thrust faulting, folding, tilting, and formation of internal unconformities, initiated between 11.69 ± 0.01 Ma and 9.99 ± 0.12 Ma and continued until after 7.77 ± 0.21 Ma in the Pasto Ventura basin. Maxi- mum shortening occurred in the southern part of the basin, in which 47% Figure 7. Uninterpreted (A, C) and interpreted (B, D) field photos for syn- shortening took place in a WNW-ESE direction (Fig. 6). depositional structures and growth strata. Photo locations are marked in Across the Puna Plateau, contractional deformation ended and was Figure 5 (for C) and Figure 6 (for A). succeeded by extensional deformation during the late Miocene and

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early Pliocene (e.g., Allmendinger et al., 1997). We use the age of recent tional deformation in the Pasto Ventura region, likely took place between regional extension of the southern Puna Plateau to constrain the cessa- 7.8 Ma and 5–4 Ma based on regional observations (Montero-López et al., tion of contractional deformation of the Pasto Ventura basin. One impor- 2010; Schoenbohm and Carrapa, 2015; Schoenbohm and Strecker, 2009), tant indicator constraining the timing of regional extension is the age of and certainly by 0.8 and 0.5 Ma based on constraints within the basin post–mid-Miocene monogenetic mafic volcanism, because dense mafic (Zhou et al., 2013). magmas could not have traveled easily through continental crust under a compressional stress regime (Allmendinger et al., 1997; Marrett and Mafic Volcanism on the Southern Puna Plateau Emerman, 1992) and because of the association of mafic cinder cones with normal and strike-slip faults (e.g., Allmendinger et al., 1989). Sev- On the southern Puna Plateau, the most recent phase of mafic volca- eral authors have suggested that the shift from contraction to extension nism is represented by widely distributed, late Miocene monogenetic cin- likely took place diachronously across the plateau (Marrett and Emerman, der cones and lava flows (e.g., Allmendinger et al., 1997; Kay et al., 1994). 1992; Montero-López et al., 2010; Riller and Oncken, 2003; Riller et al., The volcanic rocks have experienced little erosion and are associated with 2001; Risse et al., 2008; Schoenbohm and Strecker, 2009; Eckelmann et crustal extension, as described already. The oldest post–mid-Miocene al., 2013; Zhou et al., 2013). Fault 1 cuts two mafic cinder cones with monogenetic mafic volcanism clusters around the Antofagasta-Antofalla 0.76 ± 0.16 Ma and 0.47 ± 0.03 Ma ages to the north of the NPV map and region (~26°S; Fig. 8A; Risse et al., 2008; Schoenbohm and Carrapa, offsets them by different amounts in a normal, right-lateral sense (Zhou et 2015; Zhou et al., 2013), with ages of up to 8.7 ± 0.04 Ma (Schoenbohm al., 2013), which indicates ongoing extension in the Pasto Ventura region and Carrapa, 2015). The Pasto Ventura region is located in the southern- between ca. 0.8 and 0.5 Ma. Other crosscutting relationships between most Puna Plateau, and our newly dated samples in this study agree with dated units and extensional faults indicate the extension from south of the results from previous studies in the area (Table 2; Fig. 8A; Ducea et al., 26°S on the southern Puna Plateau was established by 5 Ma (north of the 2013; Risse et al., 2008; Zhou et al., 2013; this study). The post–8.7 Ma Pasto Ventura basin; Montero-López et al., 2010) and 4 Ma (south of the mafic magmatism on the southern Puna has been linked to lithospheric Pasto Ventura basin; Schoenbohm and Strecker, 2009). Taken together, we foundering, notably because of a lack of geochemical evidence for sub- conclude that the onset of extension, and thus termination of the contrac- duction (described as intraplate volcanism; Kay et al., 1994). Recent

Figure 8. Mafic volcanism on the southern Puna Plateau. (A) Geochronology for post–mid-Miocene monogenetic mafic volcanism on the Puna Plateau (Risse et al., 2008; Ducea et al., 2013; Kay et al., 1994; Schoenbohm and Carrapa, 2015; Zhou et al., 2013; this study). Circles denote the geographic distributions for all existing geochemical analyses for this volcanic group. (B–F) Selected geochemical plots, denoting contrasts between Nfl samples and post–mid-Miocene monogenetic mafic volcanics (compiled from Ducea et al., 2013; Drew et al., 2009; Kay et al., 1994, 1999; Knox et al., 1989; Risse et al., 2013; Murray et al., 2015). REE—rare earth elements. Plots in Figs. E, F are primitive mantle normalized (Sun and McDonough, 1989).

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geochemical studies indicate that the melts for the small-volume, mono- strain field resulted in the compressional reactivation of orogen-parallel genetic mafic volcanics were derived primarily from pyroxenites located thrusts and the formation of new NE-SW–trending thrust faults that seg- in the lower parts of the lithosphere (Ducea et al., 2013; Murray et al., ment the plateau into rhomb-shaped structural basins (Figs. 1B and 9; 2015), and they seem to have become progressively hotter within a short, Riller and Oncken, 2003; Riller et al., 2012). This model predicts that ~1.7 m.y. time window (Ducea et al., 2013). This model suggests that shortening, and therefore the onset of basin formation, should propagate the foundered lower lithopheric blocks were likely small in size (~50 km; in time from N to S. The timing of onset of sedimentation in the Corque, Ducea et al., 2013; Murray et al., 2015). Lipez, Atacama, Antofalla, Hombre Muerto, Santa Maria, Pipanaco, and The 11.7 Ma unit Nfl lava in this study represents a distinct group of La Rioja basins supports this model (Riller and Oncken, 2003), as these voluminous mafic volcanics on the southern Puna Plateau that are overlain basins decrease in age from Eocene in the north to Miocene–Pliocene in by sedimentary units and that were incorporated in upper-crustal deforma- the south. The timing of sediment accumulation in the Pasto Ventura basin tion (Figs. 2A and 5). They outcrop mostly along the southern margin determined in this study is consistent with this north to south progression, of the Puna Plateau (Fig. 2A; Roy et al., 2006; Schnurr et al., 2006) and but the chronology of the onset of deformation is too tentative to allow a are older than late Miocene (this study). They are geochemically distinct rigorous evaluation of this model (Figs. 7B and 7C). Further, the forma- from the post–mid-Miocene monogenetic mafic volcanics (Fig. 8). The tion of individual basins is likely more complex than envisaged by this lower MgO and Mg# indicate that they are more evolved than post–mid- model; studies of sedimentary facies and subsidence history for basins Miocene monogenetic lavas. The less depleted HREEs in the Nfl samples in the northern Puna Plateau have argued various origins, including, but imply that the melts were unlikely to have been sourced from a garnet- not limited to, foreland-type sedimentation and rapid surface subsidence bearing source indicative of lithospheric foundering, such as eclogite (Lee related to lower lithospheric foundering (e.g., DeCelles et al., 2011, 2015; et al., 2006; van Westrenen et al., 2001), as were the younger mafic vol- Schoenbohm and Carrapa, 2015), raising challenges in simply evaluat- canics in the region. While the origin of Nfl lavas is not resolvable in this ing the success of this model based on the onset of sedimentation. Thus, study, they are not readily explained by the same models invoked for the although we cannot exclude this model based on available data, we argue post–8.7 Ma monogenetic mafic volcanism from the same region (Drew et that other models, which incorporate wedge dynamics and lithospheric al., 2009; Ducea et al., 2013; Murray et al., 2015; Kay et al., 1994). foundering, have the potential to explain more of the observations. A second set of models characterizes the central Andes as a classic Deformation and Dynamics of the Southern Puna Plateau orogenic wedge, highlighting west-to-east propagation of a deformation front and initiation of Cenozoic sedimentation, albeit at an unsteady pace Our data show that basin sedimentation and deformation occurred (e.g., Arriagada et al., 2006; Carrapa et al., 2011; DeCelles et al., 2011; between 11.7 and 10.5 and <7.8 Ma in the Pasto Ventura region of the Horton and DeCelles, 2001; McQuarrie, 2002a). In the northern Puna southern Puna Plateau. In this section, we explore our data within the (~24°S–25°S), the succession of sedimentary units across the eastern context of regional upper-crustal deformation and geodynamic models for Puna Plateau and adjacent lowlands has been interpreted to result from development of the Puna Plateau, and we focus on revealing causes for the an eastward-migrating foreland basin system, and it is thought to be the documented late Miocene upper-crustal shortening event in the southern southern extension of the foreland basin system in the Altiplano, despite Puna region. the striking difference in deformation style as compared to the Altiplano A first model notes the importance of a gradient in crustal shortening (DeCelles et al., 2011). This model emphasizes flexural compensation of from the middle of the Altiplano-Puna Plateau at ~21°S to the southern the lithosphere due to end loading and involves age reinterpretation of the tip of the Puna Plateau (Riller and Oncken, 2003; Riller et al., 2012). This Santa Barbara Subgroup, which was proposed to be a product of thermal

Puna Plateau east Cretaceous rift basin south ~24°S

Eastward propagation of the Central Andean orogenic front (e.g., Carrapa et al., 2011 DeCelles et al., 2011)

~26°S

reactivation of deep-seated Cretaceous rift structures (patched: extent of syn-rift sediments) (e.g., del Papa et al., 2013; Hongn et al., 2007) subducting Nazca plate

This study lithospheric foundering-induced Rhomb-shaped crustal segmentation late Miocene basin formation and crustal surface contraction and basin formation by orogen-parallel and NE-striking fault zones shortening in the southern Puna Plateau (e.g., DeCelles et al., 2015; Schoenbohm (e.g., Riller and Oncken, 2003; Riller et al., 2012) and Carrapa, 2015; this study)

Figure 9. Schematic illustration of multiple processes operating in the southern Puna Plateau (not to scale). Dynamic processes for the Puna Plateau are listed in the lower panel.

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subsidence following the demise of the Salta Rift (Marquillas et al., 2005). first proposed that the lithosphere beneath the Puna Plateau may have In this model, the Western Cordillera was deforming by early Paleogene thinned since the mid-Miocene through delamination, based on geochem- time (e.g., Arriagada et al., 2006; Cladouhos et al., 1994; Mpodozis et ical signatures of mafic volcanism. Mineralogical and geochemical stud- al., 2005; Kennan et al., 1995). Deformation migrated quickly across the ies also reveal that ignimbrite complexes such as Agua Calientes and the plateau but stalled in the Eastern Cordillera from ca. 25 to 19 Ma (e.g., Cerro Galan may be related to melting due to asthenospheric upwelling Deeken et al., 2006). Deformation has been propagating east through in the wake of foundering lithosphere (Kay et al., 2010, 2011). Geophysi- the Subandes and Santa Barbara Ranges since ca. 15–8 Ma (Fig. 3; e.g., cal studies have supported this hypothesis, documenting thinned crust and Echavarria et al., 2003; Marrett et al., 1994; McQuarrie, 2002a; Pear- lithosphere (Tassara et al., 2006, Yuan et al., 2002) and potential foundered son et al., 2013). However, for the southern Puna Plateau, especially for blocks in the asthenosphere (Bianchi et al., 2013; Calixto et al., 2014; Heit regions south of ~26°S, a similar pattern is difficult to establish given the et al., 2014). In the northern Puna region, structural data suggesting out-of sparse data (Carrapa et al., 2011; DeCelles et al., 2011). Around the Salar sequence deformation and basin formation in the Salar de Arizaro region de Antofalla (~26°S; ANT, Fig. 3), ~130 km NW of the Pasto Ventura have been interpreted to reflect lithospheric foundering (DeCelles et al., region, several studies have suggested a foreland-like sedimentary record 2015). In the southern Puna, the spatio-temporal pattern and geochem- containing Upper Eocene upward-coarsening units with growth strata istry of small-volume mafic volcanics, and surficial extensions are con- (Adelmann, 2001; Carrapa et al., 2005; Kraemer et al., 1999; Voss, 2002). sistent with the formation of small (~50-km-diameter) lithospheric drips Although the record is incomplete in the Pasto Ventura basin, the older, (Fig. 8A; Ducea et al., 2013; Marrett et al., 1994; Murray et al., 2015; fine-grained, paleosol-bearing depositional unit Ns-4 might reflect fore- Risse et al., 2008; Schoenbohm and Carrapa, 2015; Zhou et al., 2013). bulge deposition. It likely predates felsic volcanism in this region, which A small-scale, late Miocene lithospheric dripping event beneath the began in the early Miocene (Kay et al., 2010). After 11.7 Ma, the Pasto southern Puna Plateau may be responsible for the late Miocene upper- Ventura basin was filled with fluviolacustrine-eolian, mostly medium- to crustal deformation in the Pasto Ventura region. Subsidence and contrac- fine-grained sediments and was accompanied by syndepositional defor- tion of the basin since 11.7–10.5 Ma and until 7.8 Ma may reflect the mation. In a foreland system, wedge-top sediments are commonly associ- formation of a drip. After the drip detached, the contractional deforma- ated with syndepositional deformation due to the active thrust faulting in tion was replaced by upper-crustal extension, evidenced by transtension- the deformation front, but they are typically coarser than we observe in ally reactivated older thrust faults and the eruption of monogenetic mafic the Pasto Ventura basin, given their alluvial and fluvial origins in proxim- lavas (Allmendinger et al., 1989; Schoenbohm and Strecker, 2009; Zhou ity to high topographic relief (DeCelles and Giles, 1996). Moreover, if et al., 2013). Evidence exists to support ongoing upper-crustal Quater- the southern Puna was part of the large-scale, N-S–striking foreland basin nary extension in the Pasto Ventura region (Zhou et al., 2013; this study). system in the central Andes (e.g., DeCelles et al., 2011), the deformation Additionally, geochemical signatures from deformed basin-floor unit Nfl front during late Eocene–late Oligocene time should already have been are distinct from those of recent monogenetic mafic lavas, which presum- located in the Eastern Cordillera, east of the Pasto Ventura region, as is ably resulted from lithospheric foundering (Fig. 8; e.g., Drew et al., 2009; seen in the northern Puna and the Altiplano (e.g., Carrapa et al., 2011; Ducea et al., 2013; Kay et al., 1994; Murray et al., 2015), and may suggest Horton, 2012). The post–11.7 Ma basin formation and deformation events alternative origins for 11.7 Ma Nfl lavas, prior to the detachment of the late in the Pasto Ventura region therefore need an alternative explanation. Miocene lithospheric drip. The center of this drip may be located beneath Other models emphasize out-of-sequence deformation, describing the northern part of the Pasto Ventura region, around ~26°S, which is thick-skinned, post-Miocene deformation as nonsystematic and pure evident by the oldest post–mid-Miocene monogenetic mafic volcanism shear–like (e.g., Allmendinger and Gubbels, 1996). A study in the Eastern (Schoenbohm and Carrapa, 2015) and by an azimuthal change in recent Cordillera at ~23°S suggests the development of hinterland basins by fold- extension of the southern Puna Plateau (Zhou et al., 2013). This inferred and-thrust deformation and fault reactivation on the eastern flank of the location is also close to the Cerro Galen, which erupted 6.6–2.06 Ma and Puna Plateau during the Miocene (Siks and Horton, 2011). Another recent is thought to be a result of lithospheric foundering (Kay et al., 2010). model characterizes the Eastern Cordillera at ~25°S as a broken foreland, Lithospheric foundering (Kay et al., 1994) may be a key element in where deformation and relief development took place along steeply dip- controlling upper-crustal deformation in the central Andes, acting together ping inherited faults from the Salta Rift, which do not necessarily show with other important geodynamic processes (Fig. 9; e.g., Allmendinger et any directional gradients in timing (Hain et al., 2011). Although different al., 1997; DeCelles et al., 2011; Horton, 2005; Riller and Oncken, 2003; authors highlight various specific mechanisms, these irregular basement Sobel et al., 2003; Strecker et al., 2007), all of which have been supported uplift and deformation models agree on the important role of inversion of by geological observations from various locations, and all of which may preexisting Salta Rift structures (e.g., Carrapa et al., 2014; Cristallini et al., contribute to the development of the Puna Plateau. Particularly in the Pasto 1997; Deeken et al., 2006; Hongn et al., 2007; Insel et al., 2012; Riller et Ventura region, the nature and timing of basin sedimentation and deforma- al., 2012; Riller and Oncken, 2003). However, it is not clear if the southern tion, when combined with other geophysical and chemical data, are sup- Puna Plateau inherited any structures from the Salta rifting during the Cre- portive of models for a small-scale lithospheric drip beneath the southern taceous, which is underscored by the lack of Cretaceous sediments or exhu- Puna Plateau during the late Miocene (Schoenbohm and Carrapa, 2015). mation (Carrapa et al., 2014; Löbens et al., 2013; Marquillas et al., 2005), meaning these models may not be readily applicable to our study area. CONCLUSIONS Out-of-sequence deformation may also be a result of lithospheric foundering. A key prediction for the formation and detachment of a litho- Sedimentary basins on the Puna Plateau, although isolated and with spheric drip is that during drip formation, the upper crust would experi- only limited exposure, serve as important recorders of upper-crustal defor- ence contraction accompanied by sedimentary basin formation, and after mation and geodynamic processes in the central Andes. This work docu- drip detachment, it would experience extension, basin inversion, and ments a basin formation and deformation event in the Pasto Ventura region ignimbritic and mafic volcanism (e.g., DeCelles et al., 2009, 2015; Elkins- of the southern Puna Plateau. Most recent sediment accumulation within Tanton, 2007; Göğüş and Pysklywec, 2008; Kay et al., 1994; Krystopow- the basin started after ca. 11.7 Ma (the age of the underlying basaltic tra- icz and Currie, 2013; Schoenbohm and Carrapa, 2015). Kay et al. (1994) chyandesite flow, unit Nfl) and before ca. 10.5 Ma (the oldest volcanic

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ash from unit Ns-3) and consists of fluvial/alluvial, lacustrine, and eolian Barker, I., Moser, D., Kamo, S., and Plint, G., 2011, High-precision U-Pb zircon ID-TIMS dating sedimentary rocks. The youngest volcanic ash is dated at ca. 7.8 Ma from of two regionally extensive bentonites: Cenomanian Stage, Western Canada Foreland Basin: Canadian Journal of Earth Sciences, v. 48, no. 2, p. 543–556, doi:10.1139/E10-042. unit Ns-1, and no outcrop of younger basin fill is observed in this region, Barnes, J.B., Ehlers, T., Insel, N., McQuarrie, N., and Poulsen, C., 2012, Linking orography, cli- suggesting unit Ns-1 is the last unit deposited in the Pasto Ventura basin. mate, and exhumation across the central Andes: Geology, v. 40, no. 12, p. 1135–1138, doi:10.1130/G33229.1. Several examples of syndepositional deformation were identified within Bianchi, M., Heit, B., Jakovlev, A., Yuan, X., Kay, S.M., Sandvol, E., and Comte, D., 2013, Tele- the strata of the Pasto Ventura basin. An angular unconformity formed seismic tomography of the southern Puna plateau in Argentina and adjacent regions: within the Ns-3 mudstone-siltstone and two ashes below and above this Tectonophysics, v. 586, p. 65–83, doi:10.1016/j.tecto.2012.11.016. Boyd, J., 2010, Tectonic Evolution of the Arizaro Basin of the Puna Plateau, NW Argentina: unconformity yield identical ages (9.99 ± 0.12 Ma above the unconfor- Implications for Plateau-Scale Processes [M.S. thesis]: Laramie, Wyoming, University of mity; 9.80 ± 0.10 Ma below the unconformity). The major basin-bounding Wyoming, 81 p. thrust fault (fault 1 on the NPV map) truncates a 9.51 ± 0.10 Ma ash. These Calixto, F.J., Robinson, D., Sandvol, E., Kay, S., Abt, D., Fischer, K., and Alvarado, P., 2014, Shear wave splitting and shear wave splitting tomography of the southern Puna Plateau: Geo- observations, together with growth strata within unit Ns-3, indicate that the physical Journal International, v. 199, no. 2, p. 688–699, doi:10.1093/gji/ggu296. Pasto Ventura basin was undergoing deformation around ca. 10–9 Ma. The Capitanio, F.A., Faccenna, C., Zlotnik, S., and Stegman, D.R., 2011, Subduction dynamics and the origin of Andean orogeny and the Bolivian orocline: Nature, v. 480, p. 83–86, youngest dipping strata are 7.77 ± 0.21 Ma (Schoenbohm and Carrapa, doi:10.1038/nature10596. 2015), indicating deformation continued until after this time. Contractional Carrapa, B., and DeCelles, P.G., 2008, Eocene exhumation and basin development in the Puna deformation of the Pasto Ventura region ended 7.3–4 Ma, accompanied by of northwestern Argentina: Tectonics, v. 27, TC1015, doi:10.1029/2007TC002127. Carrapa, B., Adelmann, D., Hilley, G.E., Mortimer, E., Sobel, E.R., and Strecker, M.R., 2005, up to 10%–47% upper-crustal shortening, and was thereafter replaced by Oligocene range uplift and development of plateau morphology in the southern central horizontal extension. The post–mid-Miocene Pasto Ventura basin is floored Andes: Tectonics, v. 24, no. 4, TC4011, doi:10.1029/2004TC001762. Carrapa, B., Strecker, M., and Sobel, E., 2006, Cenozoic orogenic growth in the central Andes: by part of the voluminous pre–late-Miocene mafic volcanics of the south- Evidence from sedimentary rock provenance and apatite fission track thermochronology ern Puna Plateau. They are geochemically distinct from the well-studied in the Fiambalá Basin, southernmost Puna Plateau margin (NW Argentina): Earth and post–mid-Miocene monogenetic mafic lavas from the same region, imply- Planetary Science Letters, v. 247, p. 82–100, doi:10.1016/j.epsl.2006.04.010. Carrapa, B., DeCelles, P.G., Reiners, P.W., Gehrels, G.E., and Sudo, M., 2009, Apatite triple ing origin mechanism(s) other than lithospheric foundering. dating and white mica 40Ar/39Ar thermochronology of syntectonic detritus in the cen- Regional analysis of the Puna and southern Altiplano of the central tral Andes: A multiphase tectonothermal history: Geology, v. 37, no. 5, p. 407–410, Andes, including deformation documented from sedimentary basins and doi:10.1130/G25698A.1. 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Coutand, I., Cobbold, P.R., de Urreiztieta, M., Gautier, P., Chauvin, A., Gapais, D., Rossello, E.A., southern Puna Plateau during late Miocene time. and Lopez-Gamundi, O., 2001, Style and history of Andean deformation, Puna Plateau, northwestern Argentina: Tectonics, v. 20, no. 2, p. 210–234, doi:10.1029/2000TC900031. ACKNOWLEDGMENTS Cristallini, E., and Ramos, V.A., 2000, Thick-skinned and thin-skinned thrusting in the La This work is funded by a NSERC (the Natural Sciences and Engineering Research Council Ramada fold and thrust belt: Crustal evolution of the High Andes of San Juan, Argentina of Canada) Discovery grant to L. Schoenbohm, a Geological Society of America Student (32°S): Tectonophysics, v. 317, no. 3–4, p. 205–235, doi:10.1016/S0040-1951(99)00276-0. Research Grant to R. Zhou (10256-13), and a National Science Foundation (NSF) Earth Sci- Cristallini, E., Cominguez, A., and Ramos, V., 1997, Deep structure of the Metan region: Tectonic ences (EAR) grant (0911577). We thank Michael Cosca for assistance with 40Ar/39Ar analysis and inversion in northwestern Argentina: Journal of South American Earth Sciences, v. 10, edits on an earlier draft. We thank Jack Satterly Geochronology Laboratory at University of no. 5–6, p. 403–421, doi:10.1016/S0895-9811(97)00026-6. Toronto for assistance with mineral separation, and F. Zambrano and S. Uriburo for assistance Dahlen, F.A., 1990, Critical taper model of fold-and-thrust belts and accretionary wedges: in the field. R. Zhou thanks K. Murray and M. Kirsch for discussion on geochemistry data. The Annual Review of Earth and Planetary Sciences, v. 18, p. 55–99. secondary ion mass spectrometry facility at University of California Los Angeles is supported DeCelles, P.G., and Giles, K.A., 1996, Foreland basin systems: Basin Research, v. 8, p. 105–123, by a grant from the Instrumentation and Facilities Program, Earth Sciences (EAR) of NSF. 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