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RESEARCH ARTICLE Thermochronologic Evidence for Late Eocene 10.1002/2017TC004674 Andean Mountain Building at 30°S This article is a companion to Giambiagi Ana C. Lossada1 , Laura Giambiagi1 , Gregory D. Hoke2 , Paul G. Fitzgerald2, et al. (2017) https://doi.org/10.1002/ 3 3 1 3 4 2017TC004608. Christian Creixell , Ismael Murillo , Diego Mardonez , Ricardo Velásquez , and Julieta Suriano 1IANIGLA, CCT Mendoza, CONICET, Mendoza, Argentina, 2Department of Earth Sciences, Syracuse University, Syracuse, NY, Key Points: USA, 3SERNAGEOMIN, Providencia, Chile, 4IGEBA, Universidad de Buenos Aires, CONICET, Buenos Aires, Argentina • We investigate the exhumation history at the core of the Andes above the flat slab segment by performing Abstract The Andes between 28° and 30°S represent a transition between the Puna-Altiplano Plateau and AFT and (U-Th)/He thermochronology fi • The late Eocene constructional phase the Frontal/Principal Cordillera fold-and-thrust belts to the south. While signi cant early Cenozoic was recognized in the western and deformation documented in the Andean Plateau, deciphering the early episodes of deformation during eastern limits of the High Andes Andean mountain building in the transition area is largely unstudied. Apatite fission track (AFT) and (Frontal Cordillera) at 30°S • We propose a southern continuation (U-Th-Sm)/He (AHe) thermochronology from a vertical and a horizontal transect reveal the exhumation history of the Incaic relief from Puna/Altiplano of the High Andes at 30°S, an area at the heart of this major transition. Interpretation of the age-elevation through the latitude of 30°S profile, combined with inverse thermal modeling, indicates that the onset of rapid cooling was underway by ~35 Ma, followed by a significant decrease in cooling rate at ~30–25 Ma. AFT thermal models also reveal a Supporting Information: second episode of rapid cooling in the early Miocene (~18 Ma) related to rock exhumation to its present • Supporting Information S1 position. Low exhumation between the rapid cooling events allowed for the development of a partial annealing zone. We interpret the observed Eocene rapid exhumation as the product of a previously Correspondence to: A. C. Lossada, unrecognized compressive event in this part of the Andes that reflects a southern extension of Eocene [email protected] orogenesis recognized in the Puna/Altiplano. Renewed early-Miocene exhumation indicates that the late Cenozoic compressional stresses responsible for the main phase of uplift of the South Central Andes also Citation: impacted the core of the range in this transitional sector. The major episode of Eocene exhumation suggests Lossada, A. C., Giambiagi, L., Hoke, G. D., the creation of significant topographic relief in the High Andes earlier than previously thought. Fitzgerald, P. G., Creixell, C., Murillo, I., … Suriano, J. (2017). Thermochronologic evidence for late Eocene Andean mountain building at 30°S. Tectonics, 36. 1. Introduction https://doi.org/10.1002/2017TC004674 The Central Andes are the result of several episodes, or “pulses,” of deformation since the Late Cretaceous. Deciphering the discrete pulses of exhumation and crustal shortening provides crucial constraints on how Received 31 MAY 2017 Accepted 10 OCT 2017 the Andes have evolved in time and space. While Eocene deformation has been widely recognized in the Accepted article online 17 OCT 2017 Puna-Altiplano region, uncertainties remain regarding its southward extent. The Southern Central Andes (~28–36°S) exhibit remarkable along-strike variations in physiography, horizon- tal shortening, crustal thickening, and mean topography (Giambiagi et al., 2012; Isacks, 1988). The mechan- isms invoked to explain the observed trends include variations in subduction dynamics and age and geometry of the slab (Jordan et al., 1983; Ramos, 2010; Ramos et al., 2004; Yáñez & Cembrano, 2004), inherited rheological heterogeneities of the South American plate (Mpodozis & Ramos, 1989; Ramos et al., 2004; Sobolev and Babeyko, 2005), lithospheric strength variations (Oncken et al., 2006; Tassara & Yanez, 2003), and an enhanced climate erosion feedback (Lamb & Davis, 2003), yet it still remains unclear which mechan- ism has the greatest influence. While the Miocene is considered the main phase of Andean orogeny at these latitudes (e.g., Allmendinger et al., 1990; Allmendinger & Judge, 2014; Fosdick, Carrapa, & Ortíz, 2015; Jordan et al., 1983; Suriano et al., 2017), several lines of evidence point toward the importance of Paleogene com- pressive events affecting different sectors of the range. These often overlooked Paleogene events resulted in the initial thickening of the crust and development of high topography, as well as accounting for some of the observed variations in total horizontal shortening. The temporal and spatial growth of the Andes is divided into discrete pulses of shortening, traditionally designated as “tectonic phases,” separated by periods of quiescence or tectonic stability. Major, spatially extensive periods of tectonic activity are well documented during the Cretaceous (the Peruvian phase, ~110–90 Ma; Steinmann, 1929; Jaillard, 1992), the Eocene (the Incaic phase, ~45–35 Ma; Benavides-

©2017. American Geophysical Union. Cáceres, 1999; Charrier et al., 2013; Coira et al., 1982; Hong et al., 2007; Isacks, 1988; Martínez et al., 2016; All Rights Reserved. Moscoso & Mpodozis, 1988; Steinmann, 1929, among others) and the Miocene (the Quechua or

LOSSADA ET AL. EOCENE ANDEAN MOUNTAIN BUILDING AT 30°S 1 Tectonics 10.1002/2017TC004674

Pehuenche phase, ~20–10 Ma; Ramos, 1988; Steinmann, 1929; Yrigoyen, 1993). The Eocene compressive phase is widely recognized in the Puna/southern Altiplano, north of 28°S, where it represents the first stage of construction of the Andes (Coutand et al., 2001; del Papa et al., 2004; Elger, Oncken, & Glodny, 2005; Hong et al., 2007; Mpodozis et al., 2005; Oncken et al., 2006; Payrola et al., 2009). In contrast, in the Southern Central Andes, the Neogene is considered the main phase of crustal shortening and tectonic uplift (Giambiagi et al., 2012, 2015; Hoke et al., 2015; Ramos, Cegarra, & Cristallini, 1996; Ramos, Cristallini, & Pérez, 2002), and direct evidence for Eocene deformation is scarce. Evidence of Paleogene deformation in the Andes south of the Altiplano/Puna has been more difficult to constrain due to sparse, poorly dated sedimentary record, and a limited number of thermochronology studies between 28° and 36°S. This lack of evidence leads to the ten- dency to ascribe the growth of the Andes mainly to the Miocene and allows debate to persist regarding the timing of initiation of upper plate shortening in the area. Heterogeneities in crustal thickness and com- position are often proposed as the principal controls on lithospheric strength and hence the total amount of crustal shortening resulting from tectonic stresses. To explore this hypothesis, it is crucial to constrain the spatial and temporal distribution of pre-Miocene orogenic events. Apatite fission track (AFT) thermochronology and apatite (U-Th-Sm)/He (AHe) dating provide a powerful means of identifying and constraining the timing and rate of exhumation of the crust in response to moun- tain building (e.g., Farley, 2002; Fitzgerald et al., 1995; Gallagher, Brown, & Johnson, 1998; Niemi et al., 2013; Reiners & Brandon, 2006). The combination of AFT thermochronology and AHe dating provide complemen- tary but independent constraints on thermal (and hence exhumation) history of the uppermost crust and lead to more robust interpretations than that of a single thermochronometer (e.g., Fitzgerald et al., 2006; Reiners et al., 2003). The vertical profile approach is where samples are collected over significant relief, ideally from ~2 km short-wavelength topography over a short horizontal distance and parallel to structures (e.g., Braun, 2002; Fitzgerald et al., 1995; Huntington et al., 2007). Typically, the vertical profile approach provides more a robust data interpretation than a series of isolated individual samples, although different sampling approaches address different questions. Often the two approaches are paired, and this allows a good control on the spatial distribution of the exhumation. Previous thermochronology studies in the High Andes at 30°S (Cembrano et al., 2003; Rodríguez, 2013) utilized data from individual samples collected at valley level. Our study presents AFT and AHe data and thermal modeling results from a vertical transect collected in the Permian Guanta (Huanta) pluton of the High Andes of central Chile at 30°S. In addition to the Guanta vertical transect, complementary data from a horizontal transect in the vicinity of the vertical profile, and in the Colangüil Range on the eastern slope of the Frontal Cordillera, are presented (Figure 1). We interpret our new data and combine it with existing structural and thermochronology constraints to provide the first strong evidence for a major episode of Eocene exhumation related to crustal shortening in what is today the core of the Andean orogen at 30°S.

2. Tectonic Setting The Andes are a noncollisional compressional orogen formed as the result of crustal shortening since at least the Jurassic, associated with the convergence of the Nazca (Farallones) and the South America plates (Charrier, Pinto, & Rodríguez, 2007; Coira et al., 1982; Mpodozis & Ramos, 1989; Oliveros et al., 2007). At ~30°S the Andes lie within the flat subduction segment (between 28° and 33°S) where the Nazca plate is sub- ducting at a near horizontal angle beneath the South American plate (Barazangi & Isacks, 1976) and arc- related activity is absent since the latest Miocene (Kay et al., 1988; Ramos et al., 2002). Within this segment, the following morphotectonic units are developed from west to east: the Coastal Range in the western slope, the Frontal Cordillera that comprises the High Andes that straddle the modern drainage divide, and the Precordillera and the Pampean Ranges on the eastern slope (Figures 1a and 1b). In contrast to adjacent regions to the north and south, there is no Central Depression between the Coastal and Frontal/Principal cor- dilleras at 30°S (Figure 1a). The eastward migration of arc-related magmatism is interpreted to indicate that the Nazca plate has shal- lowed over the last 14–10 Ma in response to the collision of the Juan Fernández aseismic ridge against the South American margin (Kay & Mpodozis, 2002; Yáñez et al., 2001). A simplified tectonic evolution of the Andes at ~30°S during the Andean cycle begins with Late Cretaceous tectonic activity in the Coastal Range, west of the Vicuña Fault (Figures 1a and 1b) through the inversion of Early Cretaceous depocenters

LOSSADA ET AL. EOCENE ANDEAN MOUNTAIN BUILDING AT 30°S 2 Tectonics 10.1002/2017TC004674

Figure 1. (a) Shaded relief map of the Andes between 29° and 35°S, showing the location of the study area in the High Andes (red rectangle), above the flat-slab segment. The dashed black lines indicate contours of the Wadatti-Benioff zone (Cahill and Isacks, 1992). Morphostructural units: CR: Coastal Range, CD: Central Depression, FC: Frontal Cordillera, PC: Principal Cordillera, PrC: Precordillera and PR: Pampean Ranges. (b) Tectonic setting of the study zone (red rectangle) in the context of the High Andes. Location of main contractional structures: VF: Vicuña Fault, RV: Rivadavia Fault, BTF: Baños del Toro Fault. The yellow segmented line is the drainage divide and border between Chile and Argentina. (c) Simplified geologic map of the study area (modified from Murillo et al., 2017; Nasi et al., 1990, Maksaev et al., 1984), showing sampling locations and location of AFT samples from previous studies (ELQ1 and LE02). (d) Schematic cross section (modified from Giambiagi et al., 2017) illustrating the structural evidence for Eocene deformation. Structures presumed to have Eocene activity are indicated with a heavy black line. Location is shown in Figure 1c.

(Cembrano et al., 2003; Pineda & Emparán, 2006; Rodríguez, 2013). In the early Eocene, deformation migrated to the east and was focused in the Vicuña-Rivadavia fault system (Cembrano et al., 2003; Pineda & Emparán, 2006; Rodríguez, 2013), contemporaneous with the expansion of the Eocene magmatic arc (Bocatoma Unit) in the High Andes. Subsequent Oligocene extension resulted in the development of the Doña Ana intra-arc basin (Maksaev et al., 1984; Martin et al., 1995) and the Valle del Cura back-arc basin (Winocur, Litvak, &

LOSSADA ET AL. EOCENE ANDEAN MOUNTAIN BUILDING AT 30°S 3 Tectonics 10.1002/2017TC004674

Ramos, 2015), which are nearly synchronous with the Abanico Basin (Charrier et al., 2002; Tapia et al., 2015) to the south. By the early to middle Miocene, the magmatic arc was established over the El Indio Belt (Figure 1b) and was accompanied by tectonic inversion of the Oligocene extensional faults and the development of new structures concentrated within this region (Giambiagi et al., 2017; Martin, Clavero, & Mpodozis, 1997; Rodríguez, 2013; Winocur et al., 2015). Finally, during middle to late Miocene magmatic activity diminished as the Nazca plate shallowed, and the deformation front migrated eastward, sequentially exhuming the Colangüil range (Fosdick et al., 2015) and the Precordillera (Suriano et al., 2017) on the eastern slope of the Andes. Since ~5 Ma active shortening has been located in the easternmost Precordillera and Pampean bro- ken foreland, as a consequence of the established flat slab setting (Ramos et al., 2002; Siame, Bellier, & Sebrier, 2006). The High Andes at 30°S are characterized by thick-skinned deformation of a Carboniferous to Triassic base- ment (Moscoso & Mpodozis, 1988) during the Cenozoic Andean orogeny through the structural inversion of fault systems bounding Late Triassic, Jurassic and Oligocene depocenters, combined with the develop- ment of new contractional structures (Mpodozis & Ramos, 1989). The High Andes are segmented by north striking reverse faults (Nasi, Moscoso, & Maksaev, 1990) and are bounded to the west by the west vergent Vicuña Fault and to the east by the east vergent Baños del Toro Fault (Figures 1b and 1c). The Baños del Toro Fault places the Permian to Triassic granitic core over the Oligocene to Miocene volcanic rocks of the El Indio-Pascua Belt (Figures 1b and 1c). The internal sector of the granitic core is affected by both west and east dipping contractional structures that include the Rivadavia, Punilla, and Los Cuartitos faults (Figures 1b and 1c). The granitic core comprises the Carboniferous to Triassic Elqui Batholith (Nasi et al., 1990; Ortiz & Merino, 2015, and references therein) that intrudes the lower Silurian metasedimentary rocks of the El Cepo Metamorphic complex (Mpodozis & Cornejo, 1988; Velásquez et al., 2015) and is associated with the Early Permian to Late Triassic intermediate to acidic volcanism (Martin et al., 1995; Martin, Clavero, & Mpodozis, 1999; Salazar & Coloma, 2016; Thiele, 1964) that crops out on the eastern side of the Elqui Batholith (Figure 1c). Within the Elqui Batholith, the Guanta unit we sampled is composed mostly of foliated coarse-grained, hornblende-biotite tonalities and granodiorites with upper Carboniferous to early Permian crystallization ages (Hervé et al., 2014; Maksaev, Munizaga, & Tassinari, 2014; Nasi et al., 1990; Ortiz & Merino, 2015).

3. Thermochronology 3.1. Sampling A major objective of this study was to constrain temporal variations in the exhumation history in the core of the High Andes, in order to unravel the discrete tectonic pulses that contributed to the creation of topogra- phy in this transitional sector, allowing a better understanding of its tectonic evolution. Our sampling strategy for low-temperature thermochronology was therefore aimed at collecting samples from suitable lithologies for AFT analyses within the core of the range in areas of short-wavelength topography with the greatest relief possible. Nine samples were collected from an ~2 km vertical profile immediately NE of Guanta, Chile, in the westernmost sector of the Frontal Cordillera at 30°S (Figures 1b and 1c). In addition, five samples were col- lected from a valley-level approximate east-west horizontal transect across the structural grain of the range in an attempt to monitor spatial variation in the exhumation. Samples for the horizontal transect were col- lected at spacing of ~10 km, around the location of the vertical transect (Figure 1c). Our sampling augments existing data from previous studies (Cembrano et al., 2003; Rodríguez, 2013). Additionally, one sample was collected at the easternmost limit of the Frontal Cordillera at 30°S, the Colangüil range (Figure 1b), with the aim to complement previous sampling from Fosdick et al. (2015). At each sampling locality, 5 kg of rock was collected. Samples were crushed and the heavy mineral fraction was concentrated by hand panning. Apatites were subsequently separated from the panned concentrate using conventional heavy liquids and magnetic separation techniques. Samples AL04 (vertical transect), H1, H2, and H3 (horizontal transect) did not yield sufficient apatite grains for FT analyses.

3.2. Analytical Techniques 3.2.1. Apatite Fission Track Apatite grains were mounted on glass slides with epoxy resin, then ground and polished to expose the inter-

nal surfaces of the grains, and finally etched with 5N HNO3 for 20 s at room temperature (21°C) to reveal

LOSSADA ET AL. EOCENE ANDEAN MOUNTAIN BUILDING AT 30°S 4 Tectonics 10.1002/2017TC004674

spontaneous fission tracks. A thin sheet (external detector) of low-uranium muscovite was attached to each sample to detect induced tracks, and mineral-mica pairs were stacked along with CN5 glass neutron dosimeter-mica pairs. Samples were irradiated in the slow soaker position B-3 (thermal column number 5) at the Oregon State University Reactor Facility with a total requested fluence of 1 × 1016ncm 2. Upon return from the reactor, micas were detached from the mount and etched with 40% HF at room temperature for 20 min to reveal induced tracks. The mounts were counted at a magnification of 1,250X on a computerized stage, and ages were calculated using the external detector method (Gleadow, 1981). A minimum of 25 grains per sample were counted. Zeta calibration utilized age standards Durango, Fish Canyon Tuff and Mt. Dromedary (Hurford & Green, 1983). A zeta of 362 ± 14 was obtained by AL. Confined horizontal spontaneous track length distributions were determined, with ~100 individual lengths measured, when available, using a digitizing tablet. Dpar (diameter of etched spontaneous fission tracks measured parallel to crystallographic c axis) values were measured for each counted grain and also from each grain where confined track lengths were measured (Burtner, Nigrini, & Donelick, 1994). Single-grain age data, track lengths, and their angle to the c axis, as well as Dpar values, were used as inputs for the inverse thermal modeling (Ketcham, 2005). 3.2.2. Apatite (U-Th-Sm)/He (U-Th-Sm)/He analyses on 14 apatite grains from five of the vertical transect samples (AL02, AL03, AL07, AL08, and AL09) were performed at the University of Michigan, following standard procedures (Farley, 2002). High- quality, crack- and inclusion-free apatite grains were selected. Individual grains were photographed and mea-

sured to determine the alpha ejection correction (FT), assuming a spherical geometry of grains (Farley, Wolf, & Silver, 1996; Ketcham, Gautheron, & Tassan-Got, 2011). Single-grain aliquots were loaded into Nb tubes and sent to the University of Michigan Thermochronology Lab. Single-grain ages presented good reproducibility within samples, for samples with more than three grains. For those samples (AL02, AL03, and AL08), the mean age was calculated as a standard average, after performing a Dean and Dixon’s Q-test (Dean & Dixon, 1951) for outliers. 3.2.3. Age-Elevation Profile We construct an age-elevation profile by plotting AFT and AHe ages versus sample elevation (Figure 2). Age- elevation profiles can be used to evaluate temporal variations in the exhumation rate and potentially identify the presence of exhumed partial annealing zones (PAZ). However, there are many caveats, such as advection of isotherms during rapid exhumation, to be aware of in the interpretation of such plots (e.g., Braun, 2002; Fitzgerald et al., 1995; Huntington et al., 2007; Reiners & Brandon, 2006; Stüwe, White, & Brown, 1994). In addi- tion, topographic relief results in deflection of the near-surface isotherms, such that they are compressed beneath valleys relative to ridges. As a “vertical sampling profile” is seldom actually vertical, but is usually col- lected down a valley wall or ridge, the slope of a simple age-elevation plot does not represent a “true” exhu- mation rate, and will usually overestimate the “true” exhumation rate, with the overestimation greater for the lowest temperature thermochronometers, such as apatite (U-Th-Sm)/He data. We do not have to correct for advection as the estimated exhumation rates (see below) are low enough that this effect can be neglected (Gleadow & Brown, 2000; Reiners & Brandon, 2006). 3.2.4. Thermal Modeling AFT ages, confined track lengths, and Dpar values for individual grains are used as inputs for inverse model- ing in HeFTy (Ketcham, 2005), in order to constrain time-temperature (t-T) envelopes that best match our observed FT results. We use the annealing algorithm from Ketcham et al. (2007a). C axis correction, which takes into account the angle of the confined track with respect to the C axis of the grain, was also selected for the track length data (Ketcham et al., 2007b). Dpar values relate to the variation in the kinetics of anneal- ing of fission tracks that occur due to compositional differences (Burtner et al., 1994; Carlson, Donelick, & Ketcham, 1999; Ketcham, Donelick, & Carlson, 1999). In this study, Dpar varies from 1.6 to 2.5 μm, with a mean of 1.9 μm. This compares to the age standard Durango apatite, which has a Dpar of 2.2 μm measured under same etching conditions. An AFT closure temperature of ~110°C (similar to the Durango apatite age stan- dard) is appropriate for the samples from Guanta pluton. For those samples with available (U-Th-Sm)/He data, the [U], [Th], and mean age were used as inputs. We obtained best fit thermal histories for the samples in the vertical profile (AL01 to AL09), the horizontal transect (H4 and H5) in the western limit of the Frontal Cordillera, and for sample C4 at the Colangüil Range on the eastern slope of the Frontal Cordillera (Figure 1b). A final temperature between 10°C and 20°C was used depending on the present elevation of the sample. Models were constrained very simply, with one initial t-T window and one final lower tempera- ture constraint window (as shown in Figure 3). In these modeled paths, HeFTy generates new nodal points

LOSSADA ET AL. EOCENE ANDEAN MOUNTAIN BUILDING AT 30°S 5 Tectonics 10.1002/2017TC004674

Figure 2. Age-elevation profile collected from Guanta range and track length distributions. AFT ages error bars are 2σ, AHe age error bars are 1σ. Confined track length distributions are shown with sample numbers, mean length (e.g., 14.9 μm) and standard deviation (e.g., 1.2, in microns). Track length distribution for sample AL08 is not shown because of the low number of tracks measured.

between constraint envelopes with the mode between nodal points being episodic monotonic-variable, allowing trajectories to have the most freedom incorporating heating and cooling and also sudden changes, such as episodes of unroofing or exhumation associated with normal faulting (HeFTy user manual; e.g., Ketcham, 2005). Modeled t-T paths were run until the ending condition of 10.000 paths or 5 good fit paths was reached. The merit values for “good” and “acceptable” fits were 0.5 and 0.05, respectively. The available AHe ages were consistent with best fit t-T envelopes determined using AFT data (Figure 3).

3.3. Results and Interpretation 3.3.1. AFT and AHe Data of the Guanta Profile The nine samples (AL01 to AL09) of the Guanta profile were collected between elevations of 4,250 m to 2,250 m. Sample AL04 did not contain sufficient apatites for analysis. For the samples in the horizontal trans- ect (H1 to H5), only two (samples H4 and H5) were suitable for AFT analysis. Analytical data are presented in Table 1. Good quality grains for AHe were only recovered from samples AL02, AL03, AL07, AL08, and AL09. Analytical data are reported in Table 2. In the vertical profile, AFT ages range from 39 to 24 Ma (Figure 2) and define the usual systematic age- elevation trend, with AFT ages older at higher elevations decreasing in age with decreasing elevation. We divide the age-elevation profile into two parts based on both age and track length variation. In general, ages in the structurally highest part of the section (4,250–3,000 m) are nearly invariant (~35 Ma) with elevation,

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Figure 3. Inverse thermal models constrained from AFT data from the Guanta pluton (a) vertical and (b) horizontal profiles produced using HeFTy (Ketcham, 2005) and the AFT annealing algorithm of Ketcham et al. (2007a). The pink envelope represents a good fit (i.e., the T-t path is supported by the data), and the green envelope is an “acceptable fit” (T–t path not ruled out by the data), for a duration of 10,000 random paths or until five paths with a good fit were produced. The solid black line corresponds to the “best fit model.” The black dots and quadrangles represent the measured AFT and AHe ages, respectively, placed on the “best fit model” line.

with long mean track lengths (>14 μm; Figure 2) indicative of rapid cooling (e.g., Gleadow et al., 1986). The slope of this part of the profile is steep, but there is too much scatter in the ages to generate a reliable slope, although the rate of exhumation can be estimated using AFT-AHe age pairs (see below). In the lower part of the AFT age-elevation profile, ages decrease from ~30 to ~25 Ma, with decreasing mean track lengths and broadening of the track length distributions. The shorter mean track lengths (~13 μm) for these lower elevation samples indicate more time resident within an apatite PAZ, that is, slower cooling. In general, a decrease in slope reflects a slower rate of cooling through the PAZ, due to slower exhumation. However,

LOSSADA ET AL. EOCENE ANDEAN MOUNTAIN BUILDING AT 30°S 7 OSD TA.ECN NENMUTI ULIGA 0S8 30°S AT BUILDING MOUNTAIN ANDEAN EOCENE AL. ET LOSSADA Tectonics

Table 1 Fission Track Analytical Results, Guanta Range Location Elevation Standard Fossil track Induced Age fi track density density track density dispersion P(χ2) Central age ± 1σ [U] Con ned track (mean length ± Dpar (mean ± Sample Lat/long (m)No. of grains (×106 cm 2 ) (×106 cm 2 ) (×106 cm 2 ) (%) (%) (Ma) (ppm) SE, SD, (N); μm) S.D.; μm)

AL01 29°48.8650S 4244 75 1.007 0.978 4.81 20.3 <0.1 36.9 ± 1.3 60 14.9 ± 0.1 1.9 ± 0.2 70°16.3720W (3469) (3015) (14823) 1.10 (100) AL02 29°48.6810S 3994 25 1.036 0.582 3.16 51.3 <0.1 38.8 ± 4.4 38 14.9 ± 0.1 1.9 ± 0.3 70°16.6790W (3469) (647) (3511) 1.2 (73) AL03 29°48.2200S 3746 25 1.064 0.7134 4.37 22.5 <0.1 30.7 ± 1.9 51 14.3 ± 0.2 2.5 ± 0.2 70°16.7110W (3469) (847) (5189) 1.3 (66) AL05 29°47.7890S 3215 25 1.079 1.06 6.28 20.6 <0.1 34.2 ± 2.0 73 14.4 ± 0.1 1.9 ± 0.3 70°16.0830W (3469) (1605) (6300) 1.3 (100) AL06 29°47.4730S 3000 25 1.093 0.803 5.48 10.8 6.8 29.1 ± 1.4 63 14.1 ± 0.1 1.6 ± 0.2 70°16.3210W (3469) (733) (5001) 1.3 (100) AL07 29°47.4560S 2754 25 1.114 0.449 3.51 26.6 0.3 26.7 ± 2.0 37 13.8 ± 0.2 1.9 ± 0.2 70°17.6220W (3469) (406) (2926) 1.5 (87) AL08 29°47.2900S 2512 47 1.157 0.283 2.24 28.3 0.3 25.2 ± 1.6 26 12.8 ± 1.5 1.9 ± 0.3 70°18.1720W (3469) (335) (2651) 3.0 (4) AL09 29°47.2660S 2244 25 1.164 0.148 1.18 20.3 12.9 24.3 ± 2.1 14 13.2 ± 0.3 1.9 ± 0.2 70°19.2390W (3469) (208) (1661) 1.9 (40) H4 29°50.8620S 1180 25 1.136 0.182 1.36 18.2 23.9 27.4 ± 2.4 15 13.8 ± 1.8 1.5 ± 0.3 70°24.9530W (3382) (210) (1571) 1.6 (50) H5 29°54.7420S 947 24 1.149 0.144 0.892 <0.1 74.7 33.5 ± 3.6 10 13.0 ± 2.0 1.7 ± 0.3 70°31.7930W (3382) (101) (625) 1.7 (25) C4 29°55.5670S 3159 24 1.084 0.179 0.969 31.7 9.1 36.2 ± 4.6 11 10.9 ± 3.3 1.6 ± 0.5 10.1002/2017TC004674 69°28.2920W (3382) (109) (587) 1.6 (8) Note. Parentheses enclose number of tracks counted (density) or measured (track lengths). Standard and induced track densities were measured on mica external detectors (geometry factor of 0.5), and fossil track densities were measured on internal mineral surfaces. The errors were calculated using the conventional method (Green, 1981). In this paper central ages are reported rather than pooled or mean ages. The central age allows for non-Poissonian variations in the counts of fission tracks, providing a more robust measure of the central tendency of single-grain ages. The relative error or age dispersion (spread of the individual grain data) is given by the relative standard deviation of the central age. Where the dispersion is low(<~15) the data are consistent with a single population, the mean/pooled ages and the central age converge and the sample should pass a chi-square test. The chi-square test performed on single-grain data (Galbraith, 1981) determines the probability that the counted grains belong to a single age population (within Poissonian variation). If the chi-square value (P(x2)) is less than 5%, it is likely that the grains counted represent a mixed-age population with real age differences between single grains. Three Dpar measurements in 25 grains were collected for each sample. Tectonics 10.1002/2017TC004674

the form of the lower elevation samples’ (AL07 and AL09) track length distribu- tions, which tend toward bimodality, suggests residence within the PAZ for some period of time. We interpret this part of the AFT age-elevation profile as (Ma) (Ma) age the upper part of an exhumed apatite PAZ that became established after rapid exhumation (from at least ~35 to ~30 Ma) when the rate of exhumation slowed

error Mean Error considerably. In this interpretation, within the track length distributions, the Age shorter-track length peak indicates annealing and shortening of confined tracks while the samples are resident within a PAZ (i.e., following rapid cooling since ~30 Ma), and the longer-length peak indicates later, more rapid cooling once these samples have cooled subsequent to the age of the youngest sample, that

(Ma) (Ma) (Ma) ≤ “ ” fi — age is, 25 Ma. Thus, the break in slope in this pro le (at ~3,200 m elevation) which is different from a typical “convex-up” break in slope that marks the base of an exhumed PAZ (i.e., the paleo-~110°C isotherm, dependent on composition m) μ of the apatites; e.g., Gleadow & Fitzgerald, 1987) marks the upper part of an exhumed PAZ and represents a paleo-~60°C isotherm. The slope of this part fi m) ( of the AFT age-elevation pro le (AL05-09) is ~100 m/Myr, estimated using a line μ fitted by “eye.” However, this slope does not represent an apparent exhumation rate; it is simply part of an exhumed PAZ (Fitzgerald & Gleadow, 1990). The for- mation of an exhumed PAZ within uplifted high topography, as compared to a present-day PAZ observed in a drill-hole, for example, must be followed later by more rapid cooling. The slope of an exhumed PAZ provides a rough approxima- tion as to how long the samples were resident in the PAZ before later rapid cool- ing and exhumation. The longer the time over which a PAZ is allowed to develop, the more gentle the slope (see Fitzgerald, 1994; Gleadow & Brown, 2000). Assuming an ~27°C/km geothermal gradient (see below), between

. eU is effective uranium calculated as [U]~110°C + 0.235*[Th] (e.g., Flowers et al., 2009). and ~60°C, there would be ~1.5 km of crustal section; thus, simple trigo- σ nometry gives a time of ~15 Myr to generate a slope of ~100 m/Myr. In this case, if this paleo-PAZ took ~15 Myr to form, later rapid cooling and exhumation would occur at approximately ≤15 Ma, although this is a very rough estimate.

AHe ages are consistent with the AFT ages (Figure 2), in that they are younger; the age difference is more precisely constrained for the three samples where multiple single-grain ages were obtained. There is some dispersion among single-grain AHe ages, but this is to be expected given the multitude of factors such as variable [eU], grain size, zonation, and cooling rate that may cause single grain age variation (Ehlers & Farley, 2003; Farley, 2002; Fitzgerald et al., 2006; Flowers et al., 2009; Reiners & Farley, 2001). Distributions of ages with respect to grain radius and effective uranium concentration are presented in Figure S1 in the supporting information. For samples with large age dispersion (e.g., sam- ple AL02), the mean or weighted mean age typically lacks geological meaning and the youngest AHe age is more useful for interpreting the thermal history (Fitzgerald et al., 2006). AHe ages vary from ~26 Ma (youngest AHe age for AL02) at high elevations to ~14 Ma (mean age for AL08) at low elevation, a simi- lar trend to that of the AFT ages. Note that it is difficult to constrain cooling/exhumation trends at the younger end of the data as there is a big gap in ages between AHe ages from ~25 to ~14 Ma. AFT and AHe age trends from the vertical profile overlap; therefore, we can constrain a paleogeothermal gradient. At ~25 Ma the elevation of the AHe age trend is ~3,750 m (elevation of AL03, which has a tight cluster of individual AHe ages of ~25 Ma). At ~25 Ma the elevation of the AFT age trend is between ~2,250 and 2,500 m (elevation of AL09 and AL08, respectively). The elevation difference between the AFT and AHe ap02 91.24 1.32 49.8 0.72 102.943 55.63 0.82 0.37 0.00067 272573.784 0.0013 0.69 38 116 21.7 31.5 0.3 ap02 70.35 1 98.92 1.42 93.5962 150.31 2.28 0.61 0.00082 236368.142 0.0026 0.75 47 145 20.7 27.7 0.2 ap03 36.55 0.53 74.98 1.07 54.1703 174.74 2.73 0.09 0.00034 53115.418 0.0017 0.71 39.5 138 7.9 11.2 0.1 ap03 9.76 0.15 10.24 0.18 12.1664 141.47 2.1 0.03 0.00028 20626.686 0.0015 0.7 39 120 12.9 18.4 0.2 Grain (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ncc) (ncc) (ncc/g) (mg) ( is fraction of alphas retained (Farley et al., 1996), used to correct He ages. The errors are 1 ap 04ap 05 27.39 46.85 0.4 0.67 28.54 55.29 0.41 0.81 34.0969 59.84315 36.45 66.3 0.56 1 0.21 0.00041 0.21 109324.513 0.00053 0.002 137789.969 0.71 0.0015 0.71 40 42 152 106 26.2 18.8 36.8 26.4 0.3 0.2 ap 03ap 04 87.69 66.85 1.25 0.95 123.12 112.65 1.78 1.63 116.6232 161.55 93.32275 160.2 2.34 2.32 0.5 0.6 0.00075 258907.073 0.00067 0.0019 215409.075 0.72 0.0028 41.5 0.74 45.5 138 167 18.2 18.9 25.4 25.4 0.2 0.2 ap 04 37.74 0.55 74.29 1.09 55.19815 161.52 2.36 0.14 0.00041 67702.178 0.0021 0.71 40 163 9.9 13.9 0.1

T – F trends is on the order of 1,250 1,500 m. Hypothetically, we can use the differ- . ence in closure temperature between AFT (~110°C) and AHe (~70°C) to estimate Sample, U U SD Th Th SD eU Sm Sm SD He He error He Mass FT Radius Length Uncorr. Age AL 02 ap01 44.46 0.64 38.67 0.56 53.54745 64.5 0.96 0.25 0.00056 135995.835 0.0018 0.68 35 186 20.8 30.5 0.3 31.3 2.1 AL 03 ap01 55.59 0.8 66.69 0.96 71.26215 121.33 1.75 0.29 0.00046 149542.937 0.0019 0.71 39.5 154 17.2 24.2 0.2 25.7 0.7 AL 07AL08 ap 02 ap02 46.94 20.18 0.67 0.3 102.37 1.47 50.4 70.99695 0.72 161 32.024 2.32 125.59 0.38 1.84 0.00078 140487.509 0.13 0.0027 0.00041 0.75 47136.088 47.5 0.0025 0.73 150 43.6 16.2 164 21.6 11.9 0.2 16.2 0.1 - 13.8 - 1.5 AL09 ap01 66.37Note 0.95 87.16 1.25 86.8526 143.25 2.1 0.41 0.00067 207627.073 0.002 0.73 43 134 19.6 27.0 0.2 - - Table 2 (U-Th-Sm)/He Single-Grain Laser Analytical Data, Guanta Range a paleogeothermal gradient at ~25 Ma of ~27 to ~32 °C/km. This

LOSSADA ET AL. EOCENE ANDEAN MOUNTAIN BUILDING AT 30°S 9 Tectonics 10.1002/2017TC004674

paleogeothermal gradient is typical of “continental” geotherms that are on the order of 25–30°C/km (Hamza & Muñoz, 1996; Morgan, 1984) and indirectly supports the inference that advection of isotherms due to rapid exhumation has not occurred. We can also compare AFT and AHe ages for samples at similar elevations to constrain a cooling rate for the steep upper part of the profile. For example, for samples AL02 (elevation ~4,000 m) and AL03 (elevation ~3,750 m), the difference between AFT ages and mean (or even youngest) AHe ages is ~5 Myr. Taking the difference in closure temperatures (~40°C), the cooling rate is ~8°C/Myr. Using that estimated paleogeothermal gradient of ~27°C/km yields an exhumation rate of ~250–300 m/Myr for the steep upper part of the age profile. This rate is consistent with a reasonably rapid synorogenic erosion/exhumation rate. Figure 4. Inverse thermal model constrained from AFT data from sample C4 at To summarize, the AFT and AHe data and interpretation of the age pro- the Colangüil range, eastern Frontal Cordillera, produced using HeFTy file indicate that rapid cooling due to rapid exhumation at a rate of (Ketcham, 2005) and the AFT annealing algorithm of Ketcham et al. (2007a). ~250–300 m/Myr occurred from ~35 to 30 Ma, although we cannot Same color code used in Figure 3. The solid black line corresponds to the “best fit ≤ model.” The black dot represents measured AFT age, placed on the “best fit constrain the start of exhumation. From at least ~30 to ~25 Ma sam- model” line. ples were resident in an PAZ, with the evidence for this coming from the track length distributions and the slope of the profile. The slope of ~100 m/Myr of the lower part of the AFT age profile formed in a paleo-PAZ and does not represent an exhumation rate, although the slope does suggest a subsequent period of rapid cooling/exhumation was initiated at ~15 Ma. This interpretation is discussed in more detail below when we incorporate HeFTy inverse thermal modeling and nearby samples (H4 and H5) collected along the Río Turbio valley west of the vertical transect. 3.3.2. t-T trajectories for the Guanta Profile, Río Turbio Valley, and Colangüil Range HeFTy inverse thermal modeling show that thermal histories that best fit our observed data for the high- elevation samples in the vertical profile (Figure 3a) are consistent with our qualitative interpretation. That is, rapid cooling (~10–22 °C/Myr) through the PAZ at ~35 Ma, with samples at decreasing elevation reflecting rapid cooling at progressively younger times, as expected. Samples at lower elevations (<3,000 m) present a progressively slower cooling (e.g., AL-09 ~3°C/Myr) starting at ~30–25 Ma. While we do not see HeFTy best fit model paths indicating that samples were isothermal in the PAZ before later rapid cooling, such a thermal history is revealed in sample H4, collected nearby in the Río Turbio valley west of the vertical transect. The two samples collected along the Río Turbio valley west of the ver- tical transect (H4 and H5; Figures 1c and 3b) are close to the Guanta vertical profile but were collected at elevations ~1,000 m lower. The AFT ages, track length distributions, and the t-T paths constrained with HeFTy agree well with the thermal history suggested by the vertical profile. H4, the closest sample to the Guanta vertical profile, is about the same age as the lowermost samples but has a very well defined t-T envelope indicating residence within a PAZ until rapid cooling (~16°C/Myr) starting at ~18 Ma. This ~18 Ma pulse of rapid cooling is similar to the ~15 Ma onset of rapid cooling we predicted based on the slope of the exhumed PAZ in the age-elevation plot. Sample H5 is less well constrained, but it is pos- sible that both the earlier (~35 Ma) and the ~18 Ma rapid cooling events are identifiable in the thermal model from this sample. Concordantly, sample ELQ1 from Cembrano et al. (2003) located 10 km to the NW of sample H4 (Figure 1c) yields an AFT age of ~29 Ma, and sample LE02 from Rodríguez (2013) located just a couple of kilometers east of H4 (Figure 1c) yielded an AFT age of 22 ± 3 Ma with mean track length of 14.1 μm. This reinforces the idea of an episode of accelerated exhumation beginning in the early Miocene. Finally, sample C4 from the Colangüil range (Figure 1b and Table 1) has an AFT age of 36.2 ± 4.6 Ma, and a short mean track length (<11 μm). This is indicative of a more complex thermal history with more residence time in the PAZ. The HeFTy thermal modeling best fit t-T paths from sample C4 (Figure 4) suggests an initial cooling at ~40 Ma, followed by a period of deceleration in the cooling/exhumation rate between <40 Ma and 10 Ma, and final exhumation in the late Miocene (~10 Ma). It is worth highlighting that this is a provisional result given the limited number of confined track lengths measured. More work needs to be done on the eastern flanks of the range and is currently underway.

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3.3.3. Thermal History Summary and Significance The AFT and AHe data and best fit t-T paths, both from the Guanta vertical profile and nearby samples, sug- gest the following thermal history for this region: Eocene rapid cooling was underway before ~35 Ma and then slowed at ~30 Ma, before later rapid cooling beginning in the lower Miocene (~18 Ma). We interpret cooling due to exhumation of rocks associated with the formation of topography as a result of compressional Eocene tectonics (see below). The rate of Eocene exhumation was on the order of ~250–300 m/Myr, then slo- wed considerably. The form of the track length histograms for the lower elevation samples in the vertical pro- file suggests that the samples were resident in a PAZ for long periods of time and that the profile below ~3,200 m is the upper part of an exhumed PAZ. That is, this part of the AFT profile represents a time of relative thermal and tectonic stability. The slope of the profile, ~100 m/Myr, is consistent with the formation of an AFT age profile over a time period of ~15 Myr within a static PAZ although the HeFTy models for samples from the vertical profile (Figure 2) suggest that cooling of samples was ongoing at slow rates. Sample H4, close to the base of the vertical profile, indicates nearly isothermal residence of this sample in the PAZ before rapid cool- ing in the early Miocene starting at ~18 Ma. The identification of an exhumed PAZ allows us to place constraints on the amount of exhumation, since that PAZ formed. We can use both the top (~60°C paleoisotherm) and the base (~110°C paleoisotherm) of the exhumed PAZ. Convex-up inflections in the AFT age-elevation profile that represents the base of an exhumed PAZ (Fitzgerald et al., 1995; Gleadow & Fitzgerald, 1987) are quite common in the literature, but a concave-up “break in slope” representing the upper part of an exhumed PAZ is actually quite rare. However, a good exam- ple is seen in an AFT age-elevation profile collected from the Maladeta massif from the Orri thrust sheet within the Axial Zone of the Pyrenean Orogen (Fitzgerald et al., 1999). There, rapid cooling due to rapid exhu- mation ceased at ~30 Ma and a PAZ established, which was later exhumed in the late Miocene. In that situa- tion, the southern flank of the Axial Zone and the southern fold-and-thrust belt was buried by syntectonic conglomerates that were re-excavated beginning at ~9 Ma. Apatite (U-Th)/He data, 1-D forward thermal modeling, inverse thermal modeling, higher-temperature thermochronologic techniques, and thermo- kinematic modeling (e.g., Fillon & van der Beek, 2012; Fitzgerald et al., 1999; Gibson et al., 2007; Metcalf et al., 2009) are all consistent with and confirm the presence of this apatite PAZ and the various thermochro- nologic age trends. The upper limit (concave-up inflection, paleo-~60°C) of the exhumed PAZ is at ~3,200 m elevation. Using an assumed paleo-mean annual temperature (paleo-MAT) of ~15°C (there was high topography and mountains formed following late Eocene rapid exhumation—see below), the calculated paleogeothermal gradient of ~27°C/km means that there was ~1.7 km of material above this inflection point or ~0.7 km above the upper- most sample (AL01, 4,244 m). This material was not removed until after ~18 Ma, as the PAZ, representing rela- tively stable tectonic and thermal conditions, formed from ~30 to 18 Ma. Moreover, using equation 1 from Brown (1991), the same paleogeothermal gradient of ~27°C/km and paleo-MAT of ~15°C as well as a present-day mean surface elevation of ~2,800 m for the Guanta Range yield exhumation of ~1.7 km since the early Miocene. Thus, it is apparent that the amount of exhumation since the early Miocene (~18 Ma) is only on the order of a few kilometer or less. The implication for this is that majority of exhumation and hence rock uplift likely occurred in the late Eocene event, with relatively minor erosion, exhumation, and modifica- tion of topography since the early Miocene.

Using the concept of the exhumed PAZ, along with a pseudo age-elevation profile serves as a summary figure (Figure 5). This composite profile is constructed combining the AFT and AHe data by changing the relative elevations of (U-Th-Sm)/He ages by the difference in closure temperatures between AFT and AHe systems (~40°C), divided by the paleogeothermal gradient (~27°C/km). Summarizing, interpretation of the composite AFT and AHe vertical profile in conjunction with inverse ther- mal modeling from all the samples suggests that the recent thermal evolution of the area was composed of two episodes of rapid cooling; one during the late Eocene and the other since the early Miocene, separated by a period of relative tectonic and thermal stability from ~30 to ~18 Ma. We interpret the observed pulses of rapid cooling as the result of erosional exhumation related to the creation of topography by reverse move- ment along basement-bounded faults, with the event in the late Eocene being much more significant than since the early Miocene. Samples from the vertical transect were collected in the hanging wall of the east ver- gent Punilla reverse fault (Figure 1c). We interpret the observed thermal history as the result of reverse fault

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Figure 5. Composite AFT and AHe vertical profile for Guanta pluton summarizing cooling information. Elevation of AHe ages was changed relative to the elevation of AFT ages, using a difference between closure temperatures between AFT and AHe systems of ~40°C and a paleogeothermal gradient of ~27°C/km.

activity over this structure. The hypothesis of a thermal reset of the samples can be easily rejected if proximity of magmatic sources—coetaneous with the obtained cooling ages—is considered. The Eocene arc at this latitude is poorly developed and represented only by its plutonic components, and no contemporaneous volcanic-arc-related rocks are observed. These plutons (the Bocatoma Intrusive Unit; Nasi et al., 1990) (Figure 1c) are widespread, but in small bodies, with the closest located ~10 km away from our study area. During the Miocene, the magmatic arc was located even further east, straddling the international border in the El Indio-Pascua Belt.

4. Discussion and Tectonic Implications According to our data, the High Andes at 30°S experienced two important periods of exhumation during the Cenozoic that we link to rock uplift and propagation of the deformation front. The first tectonic event took place during the Eocene, contemporaneous with the “Incaic orogeny” widely recognized in the Andes of Perú, Bolivia, and northwestern Argentina (e.g., Mégard, 1984; Mpodozis et al., 2005; Steinmann, 1929). The sec- ond pulse of uplift and erosion was during the early Miocene, when the Andes began to take its present form.

4.1. Late Eocene Deformation Previous work identified the existence of Eocene relief tens of kilometers to the west of our study area at the western boundary of the Frontal Cordillera, linked to the activity of the Vicuña and Rivadavia faults (Figure 1b). Pineda and Emparán (2006) proposed that the pop-up system bounded by the Vicuña Fault to the west and

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the Rivadavia Fault to the east was active during the late Eocene, based on structural relationships and 40 Ma whole rock K-Ar dates on syntectonic granitoids (Figure 1c) (Emparán & Pineda, 1999). Similar late Eocene activity on this fault system was described just north of the study area by Creixell et al. (2013) and Salazar, Coloma, and Creixell (2013). Previous thermochronology (Cembrano et al., 2003; Rodríguez, 2013) interpreted an Eocene to early Oligocene deformation event affecting the westernmost sector of the Frontal Cordillera, linked to the activity of the Vicuña Fault. These studies proposed exhumation at ~30 Ma, slightly younger than what our data suggest. This is likely because they analyzed single valley-level samples, rather than incor- porating high-elevation samples (vertical transect), thus limiting their ability to capture a more complete thermal evolution of the area. In the eastern slope of the Frontal Cordillera, at the Colangüil range (Figure 1b), Fosdick et al. (2015) reported a late Eocene-early Oligocene AFT cooling age (31.2 ± 7.7 Ma), interpreted as the initial exhumation of this sector, followed by a main episode of exhumation across the Precordillera during the middle Miocene. Our provisional AFT age of 36.2 ± 4.6 Ma (sample C4) also points towards a late Eocene initiation of Cenozoic exhumation of the Colangüil range. Combined, these results provide substantial evidence in support of Eocene activity in the Colangüil range at the eastern limit of the High Andes (Frontal Cordillera), at 30°S. These new data indicate that contraction during the Eocene phase was not restricted to the westernmost edge of the Andes as previously thought. This allow two possible regional tectonic scenarios: (i) deformation could have been localized at the eastern and western boundaries of the orogen at that time, similar to that observed in the Eastern and Western Cordilleras during the middle Eocene (Elger et al., 2005; Mpodozis et al., 2005; del Papa et al., 2004; Hong et al., 2007, and references therein) in the margins of the Puna-southern Altiplano plateau, north of the study area; or (ii) Eocene deformation was widespread in the High Andes at 30°S but still not identified in the core of the range at the El Indio Belt. In this sense, the Baños del Toro Fault (Figure 1b)—as the main contractional structure present in the core of the range (Maksaev et al., 1984)—may have originated during Miocene compression, or if it was a prior structure, it could have remained locked dur- ing Eocene contraction. The migration of the deformation east to the Colangüil range would be consistent with both scenarios outlined before. At present, there is no evidence for pre-Miocene activity on the Baños del Toro Fault; however, it does not preclude a phase of Eocene deformation in the highest sector of the Andes at this latitude. At the Río Seco river (Figure 1c) in the southern part of El Indio-Pascua Belt, a sedimen- tary sequence of sandstone and conglomerates with granitic pebbles (Los Cuartitos Fm., Martin et al., 1995; Merino, 2013; Murillo, Velásquez, & Creixell, 2017), with similar characteristics of those from Guanta Pluton, has been recently redefined as late Eocene (35.47 ± 0.35 Ma, amphibole Ar-Ar; Merino, 2013; Rossel et al., 2013; Murillo et al., 2017) and probably represents the proximal facies associated with erosion during the rise of the Eocene relief. This suggests that the El Indio Belt area could have remained unaltered during Eocene con- traction, acting as a topographic low where synorogenic sediments derived from erosion of the Guanta Pluton were deposited, while deformation was concentrated to the west and east, in the Guanta and Colangüil ranges. Despite our data suggesting >5 km of exhumation since ~35 Ma, the Rio Seco sedimentary succession pre- served in an isolated outcrop is relatively thin (with an estimated thickness of ~200 m; Murillo et al., 2017) and begs the question of where the products of Eocene exhumation were deposited. In the Bermejo foreland basin and related intermontane basins within and adjacent to the Argentine Precordillera (Figure 1a), Neogene foredeep deposits linked with the Miocene deformation are well recognized (Jordan et al., 1983) and Eocene to early Oligocene foredeep deposits were absent. However, recent detrital zircon maximum depositional ages from Fosdick et al. (2017) and Suriano et al. (2016) reveal Eocene to early Oligocene ages for red beds and eolian deposits, previously interpreted as Permo-Triassic (Borello & Cuerda, 1968; Limarino et al., 2000) or lower Miocene (Jordan et al., 1993, 2001). These deposits are likely related to this older pulse of tectonic uplift of Frontal Cordillera at 30°S. Moreover, backstripping analysis that includes the newly identified early Cenozoic sediments in the Bermejo basin suggests that flexural subsidence begin in the Eocene, and U-Pb detrital zircon are dominated by ages predominantly sourced in the Frontal Cordillera (Fosdick et al., 2017). This indirect evidence for the onset of distal foredeep sedimentation in the Eocene, and coincident topographic growth in the hinterland, strongly supports our results documenting Eocene exhumation in the Frontal Cordillera. The widespread and poorly developed subvolcanic bodies of the Bocatoma Intrusive Unit represent the late Eocene magmatic arc in the study area (Figure 1c), with Ar- Ar and K-Ar ages ranging between 39.5 and 30 ± 1.9 Ma (Bissig, Lee, & Clark, 2001; Martin et al., 1995; Mpodozis & Cornejo, 1988; Nasi et al., 1990) and U-Pb ages of 36.1 ± 0.6 Ma (Jones et al., 2016). A

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Figure 6. (left) Shaded relief map of the Andes between 22° and 35°S showing the regional extension of the Eocene relief. After Hong et al. (2007) and Oncken et al. (2006) between 23° and 27°S, Rossel et al. (2016) between 28° and 29°S (Valeriano Fault zone), and Fosdick et al. (2015) (Colangüil Range), and this work at 30°S (Guanta and Colangüil ranges). PrC: Precordillera; PR: Pampean Range; SBS: Santa Bárbara System. (right) Magnitude of horizontal shortening estimated for the different latitudes, after McQuarrie (2002), Anderson et al. (2017), Allmendinger et al. (1990), Allmendinger and Judge (2014), Cristallini and Ramos (2000), Cegarra and Ramos (1996), Giambiagi et al. (2015), and Mescua et al. (2014).

geothermal barometry study of the phenocryst phases of the extrusive Bocatoma Unit determined 7.2 to 7.6 (± 1.8 km) emplacement depths for the late Eocene arc magmas (Jones, 2014). This is in good agreement with our estimates of >5 km of exhumation. In the light of our new data, we believe that the compressional stresses associated with the Eocene orogenesis may be likely responsible for the absence of arc-related volcanic activity, coupled with an oblique subduction (Figure 7), which would be inefficient to generate large volume of arc magmas (Jones et al., 2016). Between the Puna/Altiplano and the thick-skinned fold-and-thrust belt developed at 30°S and further south, there is transition zone between ~29° and 27°S. Eocene deformation similar to that observed in the Puna/Altiplano was described based on low temperature thermochronometry (Aguilar et al., 2014), structural evidence (Martin et al., 1997; Salazar & Coloma, 2016), and stratigraphic and geomorpohological characteri- zation of the intramontane basin deposits that record the activity of the Valeriano Fault (Rossel et al., 2016). Thus, our new evidence of a late Eocene compressive phase at the latitude of 30°S validates previous studies and extends the belt of Eocene relief creation southward, outside the limits of the Puna/southern Altiplano and the transitional segment (Figure 6). Based on these observations, we propose the existence of two con- tinuous Eocene belts: a western one that comprises, from north to south, the Western Cordillera (north of 27°S), the Valeriano Fault zone (28°–29°S), and the Vicuña-Rivadavia to Guanta area (30°S); whereas the eastern belt includes the Eastern Cordillera (in the north) and the Colangüil range (at 30°S). The

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intervening area that comprises the present-day El Indio Belt was likely an undeformed topographic low at this time, analogous to the Puna/Altiplano basins prior to its uplift. The relationship between pre-Miocene crustal thickness and deformation with the final amount of crustal shortening along the different segments of the Central Andes remains overlooked. Figure 6 shows the close relationship between the presence of Paleogene relief, with a broad cordillera in the Altiplano (19°–22°S) and Puna (22°–25°S); the initial uplift of the High Andes further south (30°S) and the absence of it southward; and the final amount of estimated crustal shortening carried out by different authors. We argue that the Eocene deformation has an important impact over the final shortening of the continental crust achieved during the Cenozoic and should be considered when addressing the mechanisms invoked to explain the latitudinal var- iations in horizontal shortening, crustal thickness, and mean topography.

4.2. Early Miocene Deformation The main Andean deformation in the High Andes at 30°S occurred in the early Miocene and is located to the east of the sampling zone, in what comprises the highest part of the range today. During this compressive period, crustal thickening led to the topographic growth of the El Indio Belt, and the Baños del Toro Fault probably developed. The observed renewal of exhumation during the Miocene in the Guanta sector coincides with other evidence of tectonic uplift related to crustal thickening and horizontal shortening documented in the El Indio Belt area (Bissig et al., 2001; Giambiagi et al., 2017; Martin et al., 1997; Winocur et al., 2015). The structural characteriza- tion carried out by Martin et al. (1997) suggests that deformation occurred between 18 and 16 Ma, as do sub- sequent interpretations by Giambiagi et al. (2017), who propose that the El Indio Belt area was under a compressive tectonic regime between 18 and 13 Ma. The timing of deformation is coincident with the tec- tonic inversion of the Oligocene intra-arc basin (~18–14 Ma; Winocur et al., 2015; Winocur & Ramos, 2015) developed in the El Indio Belt and in the Valle del Cura area further east. Our results are also consistent with previous thermochronology studies. In the granitic core, very close to our sampling area, 1-D thermal models of AFT and AHe systems from Rodríguez (2013) suggest that this sector experienced an episode of acceler- ated exhumation at ~22–18 Ma. The early Miocene pulse of exhumation at Guanta and the El Indio Belt reflects the deeper structure of the range. Following the pioneering approach of Allmendinger et al. (1990), Giambiagi et al. (2017) proposed that a single active décollement exists beneath the High Andes at the latitude of 30°S (Figure 7). The shared basal detachment favors the renewed exhumation observed in the Guanta area during the Miocene deformation of the El Indio Belt, as the crust in the innermost sector of the range passively moves through this deeply seated discontinuity resulting in its (rock) uplift.

4.3. Comparison Between Late Eocene and Miocene Orogens In the Central Andes, discrete episodes of shortening during the Andean cycle are often linked to periods of accelerated convergence between the South American and Nazca plates (e.g., recent review by Charrier et al., 2013). Convergence rates have fluctuated between rapid (12–16 cm/yr) during 60–40 Ma and 20–10 Ma and slower (5–8 cm/yr) rates during 70–60, 40–20, and 10–0 Ma (Pardo-Casas & Molnar, 1987; Sdrolias & Muller, 2006; Somoza, 1998; Somoza & Ghidella, 2005). The two episodes of exhumation recognized here at ~40– 35 Ma and at ~20 Ma overlap with or directly postdate periods of high convergence rates between the South American and Nazca plates (Figure 7). This suggests that a causality relationship exists between the absolute velocity of the plates, the thickening of the crust and the rise of the inner sector of the range (the Guanta Range in the granitic core), and probably the westernmost (Colangüil range) at around 40–35 Ma, and the El Indio-Pascua Belt in the highest part of the range at ~20 Ma, with reactivations in the areas pre- viously deformed during Eocene phase. The lack of an exhumation younger than 18 Ma in our data suggests that the inner sector of the Frontal Cordillera analyzed here did not experienced significant exhumation dur- ing the peak of contraction associated with the flattening of the slab at this latitude (at around 10 Ma) and the migration of the subducting Juan Fernández ridge. Taken together, it is clear that the Central Andes cannot be treated as a critical Coulomb wedge (e.g., Dahlen & Suppe, 1988; Hilley & Strecker, 2004; Horton & DeCelles, 1997), constantly growing from the hinterland toward the foreland, and that the geometry of the subducting slab may not have a strong impact on the exhumation of the internal sector of the orogen.

LOSSADA ET AL. EOCENE ANDEAN MOUNTAIN BUILDING AT 30°S 15 Tectonics 10.1002/2017TC004674

Figure 7. Cross section (location in Figure 6, modified from Giambiagi et al., 2017) and location of the different tectonic events in the Andes at 30°S. Average convergence rates and orientation of convergence after Somoza and Ghidella (2005).

Fault activity in the High Andes (Frontal Cordillera) at this latitude does not seem to reflect simple “in sequence” propagation of deformation toward the foreland: that is, from west to east as commonly observed in other thick- and thin-skinned fold-and-thrust belts in the Andes. Instead, when compression occurs, differ- ent sectors of the range are active at the same time, as is the case of the observed late Eocene activity in the western and easternmost boundaries of the range (Figure 6). This irregular distribution of deformation is likely related to preexisting heterogeneities in the crust (Alvarado, Beck, & Zandt, 2007; Ammirati et al., 2016; Ramos et al., 1986; Sánchez et al., 2017), inherited from Paleozoic terrain collisions, and Triassic and Oligocene extensional basin borders. Even so, some differences can be highlighted between the Eocene and the Miocene compressional episodes. The Eocene event occurred simultaneously over a broad area of the High Andes (Frontal Cordillera) rather than concentrated over a localized deformation front, mostly through high angle reverse faults that accommodate little horizontal shortening, generating comparatively less synorogenic erosion products. During this time, the intrusion of small plutons occurred (Bocatoma Unit) as part of an eastward expansion of a diminished magmatic arc. As mentioned before, it is likely that the compression associated with the Eocene orogeny may be responsible for the absence of arc-related vol- canic activity. During this period, reactivation of preexisting weaknesses in the crust is considered to be sig- nificant, resulting in a disorganized and disrupted pattern of deformation where double vergence is common. Similar asymmetrical deformation pattern related to inversion of Mesozoic basin edges was described by Martínez et al. (2016) to the north in the transition area (27°–28°S) between Puna and 30°S. Miocene activity, in contrast, is clearly defined by a broad, eastward progression of the deformation front beginning with activity in the El Indio Belt (at around ~18 Ma). Crustal shortening migrated sequentially across the High Andes (El Indio Belt and Colangüil range; lower to middle Miocene), the Precordillera (middle Miocene to Pliocene), and the Pampean Range (Pliocene to present; Figures 1a and 7). Neogene tectonic activity during this phase resulted in the generation of major and secondary contractional structures that accommodated major horizontal shortening, at various stratigraphic levels, favoring the progress of a more spatially distributed deformation and a large volume of synorogenic materials. Even the western flank of the

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Andes was active, as revealed by the valley bottom thermochronology results presented as well as the Miocene surface uplift of pediplains between 29° and 30°S (Rodríguez et al., 2015).

5. Conclusions In this contribution we unravel the Cenozoic exhumation history of the inner sector of the Andean orogen at 30°S by applying AFT thermochronology and AHe dating to samples from vertical and horizontal profiles. The interpretation of the vertical profile coupled with inverse thermal modeling reveals two episodes of cooling: one during late Eocene (underway by at least ~35 Ma) and the other in the early Miocene (initiated ~18 Ma). We interpret the two episodes of rapid cooling to reflect exhumation associated with the two main tectonic events that occurred in this part of the Andes during the Cenozoic. Our data support a Eocene contractional phase previously hinted at by indirect structural evidence (Pineda & Emparán, 2006), reconnaissance thermo- chronology (Cembrano et al., 2003; Rodríguez, 2013), and retroarc basin provenance analyses (Fosdick et al., 2017). Exhumation rate estimates for this compressional phase is on the order of 250–300 m/Myr. Fission track length data suggest that activity slowed down at ~30–25 Ma, resulting in a significant slowdown in exhumation rates. There is also evidence for a contemporaneous cooling related to late Eocene exhumation in the Colangüil range, in the most eastern sector of the High Andes at this latitude, consistent with previous results from Fosdick et al. (2015) for the same range. Our results suggest that Eocene contraction extended further south than previously thought and localized in zones of shortening at the margins of the High Andes. Further work is needed in order to better characterize the Eocene compressional setting and its occur- rence in the highest-interior part of the range, at the El Indio-Pascua Belt. The tectonic scenario presented for the Eocene is analogous to that developed north of 27°S for the same time period, where two orogenic chains (Eastern and Western cordilleras) are separated by a topographic low, prior to the rise of the Puna/Altiplano (Elger et al., 2005; Hong et al., 2007; Oncken et al., 2006). We propose the southward continuation of this Incaic relief (Figure 6) through the transitional zone (27°–29°S, following Rossel et al., 2016 observations) to 30°S. The Miocene compressional phase witnessed the final rise and eastward broadening of the Andes at this lati- tude, tracing the landscape and topographic features observed today. The lower Miocene cooling event recognized in this study was recorded in samples from the horizontal transect that correspond to a lower structural position with respect to those in the vertical transect but are spatially close enough to consider a common thermal history. Thermal modeling indicates a new phase of rapid cooling and renewal of exhu- mation occurring at ~18 Ma, in response to the tectonic reactivation of the area after the deceleration observed starting at around 30 Ma, that lead to the final denudation of involved rocks through its present position. This Miocene event is contemporaneous with well-documented deformation in the El Indio Belt (Martin et al., 1997; Giambiagi et al., 2017 among others) resulting from the inversion of late Oligocene exten- sional depocenters (Winocur et al., 2015), and deformation in the eastern flank of the Frontal Cordillera at Colangüil (Fosdick et al., 2015). The amount of Miocene exhumation that our data suggest is restricted (less Acknowledgments Ana Lossada acknowledges the than 2 km), which means that high topography we observe today in the inner sector of the High Andes was Tectonics Group—IANIGLA for the sti- mainly developed during the Eocene compressional event. mulating discussions and the Chilean Geological and Mining Survey The two tectonic pulses recognized in this work—the late Eocene and the early Miocene—correlated well (Sernageomin) for the field work facil- with periods of high convergence rate between the Nazca and South America plates, rather than with the ities and fruitful networking. This geometry of the subduction system and the establishment of flat slab conditions. The two tectonic pulses research was supported by a grant from the Argentine Agencia de Promoción partially overlapped in space, which precludes the statement of the critical wedge model of an orogen con- Científica y Tecnológica (PICT 2011- tinuously migrating to the foreland. 1079), and an Argentine Presidential- Fulbright Fellowship in Science and Technology for Lossada’s 9 months in residence at Syracuse University during References 2015–2016. Our work benefited from Aguilar, G., Reverman, R., Salazar, E., Rodríguez, M., & Rossel, K. (2014). Thermochronometric constraints on the timing and rates of late discussion with Constantino Mpodozis. Miocene-Pliocene exhumation in flat-slab subduction zone of north-central Chile. Abstract EP21B-3540 Presented at the 2014 AGU Fall We thank Nathan Niemi and the Meeting, San Francisco, CA. University of Michigan Allmendinger, R. W., Figueroa, D., Snyder, D., Beer, C., Mpodozis, C., & Isacks, B. L. (1990). 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