Cenozoic orogenic growth in the Central Andes: Evidence from sedimentary rock provenance and apatite fission track thermochronology in the Fiambalá Basin, southernmost Puna Plateau margin (NW Argentina)

Earth and Planetary Science Letters 247 (2006) 82 – 100 www.elsevier.com/locate/epsl

Cenozoic orogenic growth in the Central Andes: Evidence from sedimentary rock provenance and apatite fission track thermochronology in the Fiambalá Basin, southernmost Puna Plateau margin (NW Argentina) B. Carrapa ⁎, M.R. Strecker, E.R. Sobel Institut für Geowissenschaften, Universtität Potsdam, 14476 Golm, Germany Received 10 November 2005; received in revised form 2 March 2006; accepted 6 April 2006 Editor: V. Courtillot

Abstract Intramontane sedimentary basins along the margin of continental plateaus often preserve strata that contain fundamental information regarding the pattern of orogenic growth. The sedimentary record of the clastic Miocene–Pliocene sequence deposited in the Fiambalá Basin, at the southern margin of the Puna Plateau (NW Argentina), documents the late Miocene paleodrainage evolution from headwaters to the west, towards headwaters in the ranges that constitute the border of the Puna Plateau to the north. Apatite Fission track (AFT) thermochronology of sedimentary and basement rocks show that the southern Puna Plateau was the source for the youngest, middle Miocene, detrital population detected in late Miocene rocks; and that the margin of the Puna Plateau expressed a high relief, possibly similar to or higher than at present, by late Miocene time. Cooling ages obtained from basement rocks at the southern Puna margin suggest that exhumation started in the Oligocene and continued until the middle Miocene. We interpret the basin reorganization and the creation of a high relief plateau margin to be the direct response of the source–basin system to a wholesale surface uplift event that may have occurred during the late Cenozoic in the Puna–Altiplano region. At this time coeval paleodrainage reorganization is observed not only in the Fiambalá Basin, but also in different basins along the southern and eastern Puna margin, suggesting a genetic link between the last stage of plateau formation and basin response. However, this event did not cause sufficient exhumation of basin bounding ranges to be recorded by AFT thermochronology. Our new data thus document a decoupling between late Cenozoic surface uplift and exhumation in the southern Puna Plateau. High relief achieved at the Puna margin by late Miocene time is linked to Oligocene–Miocene exhumation; no significant erosion (< 3 km) has occurred since in this arid highland. © 2006 Elsevier B.V. All rights reserved. Keywords: plateau; sedimentary basin; provenance; thermochronology; exhumation; uplift; relief

1. Introduction ⁎ Corresponding author. E-mail addresses: [email protected] (B. Carrapa), [email protected] (M.R. Strecker), [email protected] (E.R. Sobel). 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.04.010

Orogenic plateaus, such as Tibet or the Andean Puna–Altiplano region, are areas of high mean-surface elevation, which exert a fundamental influence on

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atmospheric circulation, precipitation patterns, and modes of erosion and sedimentation. With 3700 m average elevation, the arid, Andean Puna–Altiplano Plateau of Argentina and Bolivia is the second largest plateau on Earth, after Tibet (e.g., [1]). Little is still known about the timing, patterns or mechanisms of plateau formation. Paleoelevation and oxygen isotope data available in the Bolivian Altiplano suggest that most of the plateau surface uplift may have occurred in

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the last 10 Ma [2–4]. However, the lack of quantitative data for most of the extensive plateau region, especially in the south, has prevented the validation of this scenario. Also, it is unclear how range exhumation and sedimentary basin architecture along the margin of the plateau would have responded to such an uplift event. Although many studies have focused on the formation of the Puna–Altiplano region, unresolved

Fig. 1. DEM of the Puna–Altiplano plateau; thick dashed black and white line denotes area exhumed between 30 and 50 Ma, based on AFT data of Maksaev and Zentilli [10]. White box shows the location of Fig. 2, the study area. A: Angastaco Basin, where the exhumation of the Eastern Cordillera between ca. 22 and 13 Ma is recorded by AFT from both detrital samples [23] and from vertical profiles [61]. C: Calalaste range, characterized by AFT ages between 24 ± 3 and 29 ± 2 Ma ages [13]. CR: Chango Real, characterized by AFT ages between 29 ± 3 and 38 ± 3 Ma [23]. SF: Sierra Famatina range, characterized by 47 ± 4 Ma AFT age; SU and SM correspond to Sierra Umango and Sierra Fertile, characterized by 147 ± 6 and 253 ± 8 AFT ages respectively [28]. Slc: Sierra de los Colorados area, where ca. 14 Ma detrital AFT population has been recorded [67]. Striped area corresponds to coeval along strike deformation and exhumation between 12 and 25 Ma, based on data from Carrapa et al. [13] and Deeken et al. [61]. White dashed line corresponds to the extension to the north of the Sierra Famatina range (SF). The inset figure shows the portion of South America with elevations over 3000 m in gray modified after Horton et al. [72].

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questions remain, including what uplift mechanism was responsible, and what spatial–temporal uplift patterns were characteristic of the evolution of this region (e.g. [1,5–7]). Recent data show that regional deformation of the present plateau region started in the Paleocene– Eocene in the Bolivian Altiplano (e.g. [6,8,9]) and in the Coastal Cordillera of Chile [10], and in the Eocene– Oligocene in the Puna Plateau and Eastern Cordillera of Argentina [11–13]. In particular, structural data suggest that most of the shortening occurred before the late Cenozoic [5,7,9,14,15]. If shortening and crustal thickening were the main driving mechanism for plateau uplift, a region with high elevation and/or high relief may have existed already at that time. Alternative mechanisms for plateau uplift include isostatic uplift following lithospheric delamination [2,16,17], magmatic addition [15], underplating of material removed from the forearc by subduction erosion [18,19], and possibly the flow of ductile lower crust from areas of excess shortening into areas that have a deficit of shortening [20,21]. Lithospheric delamination and ensuing wholesale plateau uplift have been proposed to have occurred between 8 and 3 Ma [22] in the region of the present-day southern Puna Plateau. Sedimentological and thermochronological data from the southern Puna Plateau and the Argentine Eastern Cordillera show that deformation driving exhumation started already in Eocene–Oligocene time, contributing to the development of basin compartmentalization and eventually internal drainage conditions (e.g. [11,13,23]). However, it is not clear when the plateau reached geomorphic conditions similar to present, and if and how the attainment of such a geomorphic phenomenon is related to a wholesale, late Cenozoic tectonic uplift event. In summary, different data sets from the Puna– Altiplano region and its margins suggest that widespread deformation and exhumation of individual ranges had occurred already in Oligocene time in the present-day plateau realm, which may have been followed by a surface uplift event in the Mio-Pliocene. The amount of exhumation related to these early events

and the timing of the inferred last surface uplift are still poorly constrained. In particular, it is not clear whether exhumation and surface uplift occurred at the same time and were linked (i.e. were coupled). Sedimentary rocks preserved within and along the margin of the Puna Plateau provide important information about the timing and potentially the processes responsible for establishing the morphological features characteristic of the present-day plateau. For example, sedimentary basins located within the present-day plateau, such as in the Calalaste region (Fig. 1), record the direct response of drainage basin reorganization to exhumation of intra-basin ranges in Eocene–Oligocene time [13]. Likewise, intramontane basins in the Eastern Cordillera, and in the transition between the Argentine Sierras Pampeanas province and the Puna, show a similar response to the progressive unroofing of bounding ranges that deform and exhume as the plateau grows spatially and temporally along its margins (e.g. [11,24–26]). With this study we aim to determine (1) when exhumation occurred in the southern plateau margin and if and how it is related to previously proposed mechanisms; (2) whether exhumation was coupled with a wholesale late Cenozoic surface uplift event; and (3) how such late stage plateau uplift was reflected in the geological record. In particular, we present new sedimentologic and apatite fission track (AFT) thermochronologic data from Mio-Pliocene sedimentary rocks and from Paleozoic basement rocks from the southern margin of the Puna Plateau that constrain the exhumation history and paleogeographic evolution of this region with respect to plateau–margin growth. Our data document early to middle Miocene exhumation of the Puna Plateau margin associated with the creation of high topography that provided sediments to the Fiambalá Basin by ca. 6 Ma. This was coeval with an important paleogeographic reorganization and change in sediment source regions, inferred to be the direct result of widespread plateau uplift.

Fig. 2. (A) Geological map of the Fiambalá Basin and surrounding region (redrawn from the geological map of Catamarca, 1 : 500,000 [73]): 1) Quaternary volcanics, 2) Mio-Pliocene volcanics (andesite–dacites), 3) Quaternary evaporites, 4) Cambrian–Ordovician volcanics (dacites) and metamorphic rocks, 5) Paleozoic granites, 6) Triassic–Jurassic gabbros, 7) Permo-Triassic granites, 8) Ordovician phyllites, gneiss, and minor metavolcanites, 9) Cambrian phyllites, schists and metavolcanics, 10) Permo-Triassic red sandstones, conglomerates, marls, and related volcanics (Grupo Paganzo), 11) Ordovician granodiorites and minor gabbros, 12) Miocene (Guanchin and Tamberia formations), 13) Pliocene (Punashotter Formation) and Quaternary, 14) late Miocene–Pliocene distal facies: a) Guanchin Formation, b) Punaschotter Formation. The black line correspond to the transect along which the stratigraphic sections were measured. Distal facies have been measure at the far NE end of this transect. White inset rectangle: northern vertical profile (Cerro Negro: CN); gray inset rectangle: eastern vertical profile (Alto Grande: AG); dark gray square corresponds to sample UP78-9 (Sierra de las Planchades: Pl); light gray square corresponds to sample UP78-3 (Filo Negro: FN). The ages of these samples are listed in Table 1. (B) Simplified profile (A–A′) through the investigated section in the Fiambalá Basin; T: Tamberia, G: Guanchin, P: Punaschotter, formations; Q: Quaternary.

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2. Geological setting Our study area comprises the southernmost basement ranges of the Puna Plateau margin, ca. 4000 m high, and the adjacent Fiambalá Basin, a Cenozoic sedimentary basin immediately to the south at ca. 1650 m elevation (Fig. 1). Deformation and exhumation of reverse-fault bounded basement ranges within and on the eastern margin of the present southern Puna began during the Oligocene (e.g., [23,27]). AFT cooling ages from western areas record deformation and exhumation commencing during Eocene and early–middle Miocene time, respectively [10,28]. To the southeast the Puna is transitional with the reverse-fault bounded Sierras Pampeanas basement uplifts, a broken foreland province overlying the flat subduction segment of the Nazca Plate between 27° and 33°S latitude [29]. The arid Fiambalá Basin contains over 4 km of upper Mio-Pliocene rocks belonging to the Tamberia, Guanchin and Punaschotter formations [30] that record the exhumation and erosion of the surrounding ranges [31– 34]. These sediments are distributed throughout the entire basin and reflect ephemeral braided fluvial and alluvial fan depositional environments [35]. The lack of plant fossils and the presence of mud cracks, halite and gypsum layers [36] suggest that the depositional environment was already arid at time of the Tamberia sediments deposition (ca. 8 Ma). To the west and north– northwest the basin margin mainly comprises CambroOrdovician dacites and sedimentary rocks, Paleozoic (Ordovician–Carboniferous) granites, Carboniferous, Permian and Triassic sedimentary rocks as well as Tertiary and Quaternary volcanic rocks of predominantly andesitic composition [30]. To the northeast and east the basin is bounded mainly by Cambrian schists and phyllites and minor sectors with Ordovician–Carboniferous granites that constitute the southernmost end of the Puna Plateau (Fig. 2). 3. Methods In order to constrain the exhumation history and paleogeography of the southern Puna Plateau margin and the adjacent ranges during the late Cenozoic, we choose a multidisciplinary approach involving quantitative sedimentological investigations and thermochronological analysis using apatite fission track dating. 3.1. Sedimentary provenance Sedimentary provenance analysis was carried out on sedimentary rocks from the Fiambalá Basin in order to

constrain the spatial–temporal evolution of the contributing sediment sources. Sedimentological investigations were conducted along a NE to SW transect (Fig. 2), the only transect in the basin along which the complete Cenozoic sequence is exposed; this includes the easternmost outcrops in the basin, which record more distal sources. Clast composition analysis was performed throughout the complete stratigraphic sequence at eighteen localities (Figs. 2 and 3). Pebbles of different lithologies were counted every 5 to 10 cm (depending on granulometry) within a 50× 50 cm2 grid. The grid was shifted parallel to bedding until at least 100 clasts were counted at each locality, for a total of over 1800 clast counts. 3.2. Apatite fission track thermochronology Apatite Fission Track (AFT) thermochronology was performed on both basement rocks surrounding the Fiambalá Basin and sedimentary rocks from the Cenozoic basin sequence (detrital samples), in order to constrain the cooling and exhumation histories of the source areas as well as the relationship between exhumation, relief, uplift, and sedimentation. AFT thermochronology provides information on the timing and rates of cooling occurring at temperature (T) between ca. 60 and 110 °C, defined as the Partial Annealing Zone (PAZ). The exact T of the upper (hotter) boundary depends on the kinetic characteristics of the apatites and the cooling rate; the former can be quantified by measuring the diameter of track etch pits, known as Dpar [37–39]. In general, apatites with smaller Dpar are typical of fluorine-rich apatite and are characterized by lower temperatures of the upper boundary. Fission track-lengths provide information on the proportion of the cooling history that the sample experienced within the PAZ, and hence how quickly the apatite passed through the PAZ. Therefore, in order to interpret the AFT data in terms of a T–t path an integrated analysis of fission track age, track length distribution, and kinetic characteristics of the apatite grains is required. Samples were prepared and analyzed following the procedure described by Sobel and Strecker [40]. About 20 grains for basement samples and 100 grains for detrital samples were dated (Table A1 in the Appendix). Confined track-lengths were measured in both basement and detrital samples together with the angle between the confined track and the C-crystallographic axis (C-axis projected data). Use of the angular data mitigates track-measurement bias [41] and improves annealing model results, as confined tracks anneal anisotropically as a function of orientation [37,38]. Apatite etch pit diameter (Dpar) and grain shape were also determined (Table A1 in the Appendix).

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Fig. 3. Clast counts (see text for explanation) and simplified stratigraphic logs from the Fiambalá Basin. Paleocurrents are indicated with rose diagrams; the number of measurements is noted on the side. AFT detrital sample numbers shown in Fig. 6 are marked in italics. Depositional ages are based on magnetostratigraphy [31] and zircon fission track dating on ash layers [32]. Mrl: marls; fS: fine sandstones; mS: middle sandstones; mcg: micro conglomerate; cgl: conglomerates.

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Pooled ages are reported for the basement samples, calculated using the Trackkey program [42] (Table 1), as they all pass the chi squared test [43,44]. For each detrital sample fission track grain-age distributions were decomposed following the binomial peak-fit method [45] incorporated in the Binomfit program [46]. In automatic mode, the program provides an iterative search of peak ages and number of peaks to find the optimal (best-fit) solution. The best-fit solution is determined by directly comparing the distribution of the grain data to a predicted mixed binomial distribution. The related best-fit peaks are reported by age, uncertainty, and size (Table 2). The uncertainty for the peak age is given at 95% confidence intervals. The size of the individual peaks is reported as a fraction (in percent) on the total (Table 2). Basement samples were collected along two elevation transects from basin bounding ranges to the north and east and from different ranges to the west of the basin (Fig. 2). The vertical profiles were collected along the steepest possible routes between 2291 m to 4072 m elevation (northeast, Cerro Negro – CN), and 2165 and 3127 (east, Alto Grande – AG). Results from seven samples from the CN profile are presented in Fig. 4 and Table 1. Unfortunately, only the lowest sample (2165 m) from the AG profile yielded sufficient apatite to provide results (Table 1). Out of many samples collected to the west of the basin only two, Sierra de las Planchadas (Pl; Fig. 2) and Filo Negro (FN; Fig. 2), yielded sufficient apatite to provide results. In total, we present results from ten basement samples and one cobble from the Punaschotter Formation (Table 1). In addition, five detrital samples from the Tamberia, Guanchin and Punaschotter formations were selected for AFT analysis (Table 2), representing ca. 1 sample per Ma of depositional time. Assuming that the temperature in the sedimentary basin was never high enough to overprint the original thermochronological signal (discussed in the following), detrital thermochronology provides fundamental information on characteristic cooling ages of rocks originally present in the adjacent source (e.g. [47,48]) and the timing, rates and patterns at which these rocks were exhumed (e.g. [49–51]). Most detrital fission track thermochronology studies have utilized zircons (e.g. [52,53]) while apatite has only rarely been analyzed (e.g. [54]). Only recently, detrital AFT analysis has been recognized as an important tool in resolving young exhumation histories [11,55]. The advantage of analyzing apatite rather than zircon fission tracks is that the former mineral provides information on the thermal history of shallower crustal levels due to the lower closure temperature, thus enabling the identification of cooling events in regions where little erosion has occurred. When

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Table 1 Apatite fission track analytical data of the NE vertical profile (VP) and single basement samples and pebble Lithology

Elevation (m)

No. Xlsa

Rho-S (e5)b

NSc

Rho-I (e5)b

NIc

P(χ)2 (%)d

Rho-D (e5)e

NDf

Age (Ma)

±1σ

U (ppm)

Mean length (μm)

Dpar (μm)

S.D.

041B(UP61-10) 042B(UP61-12) 042A(UP61-13) 042B(UP60-1) 045A(UP60-2) 046A(UP60-3) 048(UP60-4) 149(UP78-9) 116-1(UP78-3) 161-1(UP79-5) 177 (79-11)

VP (CN), granite, NE VP (CN), granite, NE VP (CN), granite, NE VP (CN), granite, NE VP (CN), granite, NE VP (CN), granite, NE VP (CN), granite, NE (P1), granite, W (FN), phyllite, W Granite east (Alto Grande) Granite cobble, (Pliocene Punaschotter Fm.)

4072 3785 3463 3190 2889 2584 2291 2952 4635 2165 1648

25 10 20 22 20 20 20 16 15 20 19

3.200 1.040 1.193 0.418 4.172 2.880 1.679 2.279 2.119 0.621 7.767

493 22 141 45 724 547 201 82 107 69 1310

27.409 9.265 12.356 5.356 54.501 46.752 23.312 13.204 12.495 8.429 16.186

4222 196 1460 576 9458 8879 2791 4775 631 939 2730

90.15 92.46 96.05 84.08 0.09 98.72 60.8 89.6 63.5 11.33 42.62

10.771 10.688 10.647 13.232 13.144 13.057 12.97 11.037 11.943 9.7876 9.3689

4444 4444 4444 5168 51.68 5168 5168 4612 4612 3922 3922

23.1 22.0 18.9 19.0 18.4 14.8 17.1 34.6 43.9 13.1 81.3

1.2 5.0 1.7 3.0 0.8 0.7 1.3 4.2 4.4 1.7 3.4

30.33 10.69 14.02 4.83 52.00 44.29 20.73 17.31 15.32 12.18 21.28

NA NA NA NA 14 ± 1.0 NA NA NA NA NA NA

1.8 2.1 1.9 1.8 2.1 2.1 1.9 1.8 1.9 1.7 1.9

0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.3 0.1 0.2 0.1

AFT analytical data for the vertical profiles (VP), basement samples and pebble; the sample indicated in bold is the one modeled in Fig. 7. CN: vertical profile along the Cerro Negro (north-east); Pl: single sample from the Sierra de las Planchadas to the west; AG: single sample from a vertical profile along the Alto Grande to the east. Sample 177 is a cobble from the Punaschotter Formation (refer to Fig. 3). Samples analyzed with a Leica DMRM microscope with drawing tube located above a digitizing tablet and a Kinetek computer-controlled stage driven by the FTStage program [74]. Analysis performed with reflected and transmitted light at 1250× magnification. Samples were irradiated at Oregon State University. Samples where etched in 5.5 M nitric acid at 21 °C for 20 s. Following irradiation, the mica external detectors were etched with 21 °C in 40% hydrofluoric acid for 45 min. The pooled age is reported for all samples as they pass the χ2 test, suggesting that they represent a single population. Error is 1σ, calculated using the zeta calibration method [75] with zeta of 364.1 ± 4.8 for apatite [unpublished data, 2006, B. Carrapa]. a No. Xls is the number of individual crystals dated. b Rho-S and Rho-I are the spontaneous and induced track density measured, respectively (tracks/cm2). c NS and NI are the number of spontaneous and induced tracks counted, respectively. d (χ)2 (%) is the chi-square probability [45,76]. Values greater than 5% are considered to pass this test and represent a single population of ages. e Rho-D is the induced track density in external detector adjacent to CN5 dosimetry glass (tracks/cm2). f ND is the number of tracks counted in determining Rho-D. Dpar: fission track etch pit measurements, SD is the related standard deviation.

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Sample number

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Table 2 Detrital populations of samples from the Fiambalá clastic sequence Sample

Approximate depostional age

N

P1

P2

P3

P4

Punaschotter 177

t < 3.6

100

92.9 ± 18.3

3.6 < t < 5.7

129

Guanchin 054⁎

30.0 ± 2.8 23.5% 36.3 ± 3 22%

57.2 ± 5.5 10.7%

Guanchin 053

3.6 < t < 5.7

110

Tamberia 050⁎

5.7 < t < 8

117

11.1 ± 1 60.60% 14.0 < 1.5 25.10% 14.4 ± 1.6 23.20% 13.8 ± 1.7 31.60%

Tamberia 003

5.7 < t < 8

108

43.0 ± 3.7 32.10% 37.2 ± 4.6 23.70%

81.7 ± 7.9 15.50% 92.0 ± 6.5 22.40% 71.4 ± 6.1 21.60%

49.7 ± 4.6 57.30%

P5

P6

127.1 ± 10.6 34.10% 167.8 ± 9 22.20% 123.3 ± 11.6 19.10% 121.9 ± 12.2 42.70%

217.5 ± 92.1 3.30%

206.0 ± 47.3 4%

⁎ Modeled samples in Fig. 6.

applying detrital AFT thermochronology, a large number of grains and related lengths must be analyzed in order to obtain a statistically significant representation of ages present in the source region. 3.3. Track-length modeling Track-length modeling was carried out on basement and detrital samples using the multikinetic annealing model AFTSolve [56] in order to better define the exhumation history of specific sediment sources and to constrain the maximum degree of burial re-heating, respectively. In the latter case, modeling was performed on selected detrital populations from the stratigraphically oldest (and deepest) samples 050 and 054 from the Tamberia and Guanchin formations, respectively, for which enough data for modeling were available. Populations were selected for which sufficient lengths could be associated with specific age grains, in order to

check on possible annealing due to burial-related reheating. When modeling a specific age detrital population, an important issue is that the ages belonging to a single population might be derived from a broad spectrum of possible sources that experienced similar, but not necessarily identical thermal histories. Furthermore, a detrital apatite population is typically composed of a range of ages that may be derived from different elevations within the same range. Consequently, the spectrum of lengths in a specific population may reflect both multiple cooling events prior to exhumation and variations due to different elevations of the source unit, potentially overprinted by re-heating due to burial and subsequent cooling. Considering that multiple cooling events may have affected the original source prior to deposition, correctly constraining this portion of the cooling path in the model is difficult. Therefore, it is clear that modeling detrital samples is challenging and must be undertaken with caution. Therefore, we can only make reasonable hypotheses about the thermal history of the original source prior to deposition in the basin if independent geological constraints are available. In the following section, track-length modeling is applied to test hypotheses about the amount of reheating experienced due to burial of the samples during the formation of the Fiambalá Basin. The initial time constraint is set at double (at least) the pooled age of the sample or detrital population to ensure that the firstformed tracks are all completely annealed, thereby avoiding potential boundary condition artifacts [56]. 4. Results and discussions 4.1. Sedimentary evolution

Fig. 4. Plot of AFT age versus elevation in km for basement samples collected from the southern Puna margin (CN, see Fig. 2 for location and Table 1 for tabulated data); the grey sample (UP60-2) is modeled in Fig. 7.

Sedimentation in the Fiambalá Basin commenced between ca. 8 and 5.7 Ma with the deposition of the Tamberia Formation [30,32]. It comprises massive,

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generally structureless sandstones, with occasional planar cross-bedded strata grading into well sorted, parallel bedded conglomerates toward the top (Fig. 3). The clast composition of the upper Tamberia Formation is characterized by dacitic volcanics and red sandstones and minor andesitic volcanics; granites are also present. These lithologies, particularly the red sandstone clasts, are typical of Cambro-Ordovician and Permo-Triassic rocks from western sources. Granites are typical of both western and north-eastern sources. Paleocurrent directions measured in an imbricated conglomerate layer (Pc10), from the upper part of this formation, suggest a first contribution from the north-east (Fig. 3). The Guanchin Formation was deposited between ca. 5.7 Ma and 3.6 Ma [32]. It comprises mainly trough and planar cross bedded sandstones with occasional CaCO3 bearing paleosols and silicified tree trunks, alternating with conglomerate lenses with silt intraclasts. Toward the top of the formation these rocks grade into coarse sandstones and conglomerates. The lower part of the Guanchin Formation is mainly composed of Ordovician dacite clasts typical of western sources. The upper part of the Guanchin Formation is still dominated by volcanic clasts. However, an increase in the abundance of granite clasts in the upper Guanchin Formation with respect to the underlying Tamberia Formation may either represent the progressive unroofing of western crystalline sources or/and an expansion of the source area involving north-eastern granitic sources. Importantly, distal facies of these sediments, exposed farther basinward, record the first unequivocal influx of schist/ phyllite sourced in the southern Puna margin. The Guanchin Formation is separated by an angular unconformity and overlain by the Punaschotter Formation [32,34], which should be younger than 3.6 Ma [32]. To date, no other radiometric ages are available for these sediments in the Fiambalá Basin. The Punaschotter Formation consists of ca. 500-m-thick, disorganized and poorly sorted conglomerates that are deeply incised and preserved in isolated outcrops in the basin. Clast counts reveal a broad compositional variation, but with a large component of the same Puna-related schist/phyllite lithologies found in the upper Guanchin Formation. Paleocurrent data measured on ca. 100 imbricated clasts

(Pc19-20) from the easternmost preserved units clearly record a provenance from the north–northeast (Fig. 3). 4.2. Exhumation history of the southern Puna revealed by apatite fission track thermochronology of basement sources The ca. 1800 m AFT vertical profile (CN; Fig. 2) documents that basement rocks at the southern Puna margin passed through the partial annealing zone (PAZ: ca. 110°– 60° [57,58]) between 14.7 ± 0.7 Ma and 23.1 ± 1.2 Ma (Fig. 4), from which an apparent mean exhumation rate of ca. 0.2 mm/yr can be inferred. This estimate neglects possible affects of advection; however, this is justified by the relatively slow exhumation rate [59]. The onset of more rapid exhumation is represented by a break in slope on an age–elevation plot [60]. Unfortunately, this feature apparently occurred at an elevation above the presently preserved ridge crest and therefore can potentially only be preserved in the detrital record. Although we cannot determine the precise onset of more rapid exhumation, it must have been prior to ca. 24 Ma, when the highest elevation sample cooled through the PAZ. Length measurements were only possible on sample UP60-2 (2889 m) and yield an average value of 14.5 ± 1 μm. A single sample from the eastern vertical profile (UP79-5), collected at 2165 m, has a pooled age of 13.1 ± 1.7 Ma (Fig. 2; Table 1). This age is remarkably similar to ages characteristic of the lower portion of the northern vertical profile, suggesting that the eastern basinbounding range is the southward structural continuation of the southern Puna margin, and that eastern sources are characterized by early to middle Miocene ages (Fig. 2). A single granite sample (UP78-9) collected at 2952 m west of the basin has a pooled age of 34.6 ± 4.2 Ma (Fig. 2; Table 1). A single sample of phyllite (UP78-3), from the west, yields a pooled age of 43.9 ± 4.4 Ma (Fig. 2; Table 1). Both ages are in agreement with other ages reported from regions to the west and southwest [10,28] and document that these source regions were subject to an important Eocene–Oligocene cooling and exhumation event.

Fig. 5. Radial plots of detrital ages recorded in the Tamberia, Guanchin and Punaschotter formations and corresponding populations calculated in automatic mode using the Binomfit program of Brandon [46]. Mean Dpar values and related standard deviations (S.D.) are provided). Ages on radial plots shown in Ma; (L) denotes mean length. Arrows in sample 177 indicate the two components forming the binomial fitting curve (discussed in the text); histograms are provided for the lengths counted for each sample. Note that lengths are in this case non-corrected for c-axis to allow comparison with previous studies; corrected values are indicated. The light gray area corresponds to acceptable fits, the dark gray area to good fits, and the dashed black line to the best fit.

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4.3. Detrital apatite fission track thermochronoloy Using the Binomfit program on the five analyzed detrital samples decomposes them into 2 to 5 components per sample. Examination of these components suggests that they belong to 6 discrete age groups (populations: P) (Table 2). Based on these age clusters we assigned to the age populations P1 through P6. This implies that a specific sample has only a subset of the 6 possible populations; these population numbers are not always consecutive. Table A1 in the Appendix contains raw data of all detrital samples including length and Dpar values for each grain; mean length and Dpar values are presented in Fig. 5. In order to interpret detrital AFT ages in terms of provenance and/or cooling and exhumation events of a specific sediment source we need to be able to exclude important annealing due to post-depositional burial re-heating. Considering the total estimated thickness of ca. 4 km of the Fiambalá Basin, postdepositional heating, annealing, and subsequent cooling are carefully addressed here. Several lines of evidence strongly argue against significant burialrelated annealing following deposition in the Fiambalá Basin. Firstly, all detrital age populations are older than the depositional ages and all samples contain at least two discrete populations, indicating that none of the analyzed samples were subjected to total or significant annealing after deposition. This is also confirmed by the mean length trend that tends to decrease up-section (Fig. 5). If significant partial annealing due to burial induced re-heating had occurred, shorter track lengths in the bottom (deepest) sample would be expected. Secondly, the central ages increase systematically down-section while the population ages are relatively consistent, whereas partially annealed samples would show the opposite pattern (Table 2; Fig. 5). Thirdly, the mean track lengths observed from the adjacent ranges, which are the likely source of our sediments as indicated by sediment provenance data, are from 10 to 13 μm [28,61] and are very similar to the ones observed in the detrital samples, indicating that no significant partial annealing has occurred after deposition. 4.4. Estimation of annealing due to post burial heating from heat flow density analysis The calculation of the maximum amount of heating related to burial is here attempted by first performing heat flow density analysis. Heat flow data are not available in the study area; however, data from Bolivia

[62], suggest that heat flow in the Eastern Cordillera is between 60 and 40 mW m− 2 (Q0). We consider a range of plausible thermal conductivity values based on literature (e.g., [63]); we apply values from 3.0 to 2.0 W m− 1 K− 1 for the 4 km thick sandstones and conglomerates of the Fiambalá Basin. The following simplified equation, assumes thermal steady state and it is used to calculate the maximum temperature (T) beneath a sedimentary layer with 4 km thickness (zsed), assuming thermal steady state: T ¼ Ts þ ðQ0 zsed =KÞ where Ts is the temperature at the surface (∼ 10 °C; Climatic Atlas of South America [64]), Q0 is the heat flow, zsed is the thickness of the sedimentary pile and K is the thermal conductivity. Following this equation we obtain values between 63 °C (Q0 = 40 mW m− 2 and K = 2) and 130 °C (Q0 = 60 mW m− 2 and K = 3) for the base of the stratigraphic sequence. It must be noted that these values are obtained on a calculation based on thermal steady state and therefore can only be used as maximum estimates of the re-heating caused by burial. 4.5. Thermal modeling of detrital populations: implications for maximum burial temperatures Combined, the presented evidence documents that annealing related to burial in the Fiambalá Basin played a minor role in the thermal history of the detrital samples. However, thermal modeling of the P4 population from sample 050 (Tamberia Formation) and sample 054 (Guanchin Formation) is presented in order to further examine cooling following heating due to burial between ca. 8 and 3.6 Ma (Fig. 6). T–t constraints have been applied based on independent data from the Sierra Pampeanas ranges presented by Coughlin et al. [28] and Jordan et al. [65]. AFT data from the Sierra Famatina range, directly to the west of the Fiambalá Basin (SF; Fig. 2), suggest that this range underwent two phases of cooling: the first one between ca. 40 and ca. 60 Ma and the second at ca. 10 Ma. Track length modeling suggests that a re-heating event occurred between these two cooling episodes [28]; this re-heating is attributed to burial by a hypothesized foreland basin sequence [28]. Isopach map reconstructions and AFT data suggest that the Sierra Famatina range, together with other ranges in the Sierra Pampeanas, were once covered by a thick pile of sediments that were eroded in the Miocene during the main deformation and exhumation phase in this region [28]. In particular, farther south, at ca. 29°S, Jordan et al. [66] show the presence of a continuous foreland

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Fig. 6. AFTSolve thermal modeling of population P4 in sample 050 and 054 from the Tamberia and Guanchin formations respectively. For T–t constraints refer to the text. Lengths reported are corrected with respect to the c-axis for modeling purposes (see text for more details).

basin at ca. 20 Ma; this may also have formerly been present in the north. In accord with this geological evidence, a T constraint of 60–120 °C has been applied at 2 Ma steps between 25 and 15 Ma to test the hypothesis of cooling following burial caused by the sediments of a foreland basin in the early Miocene (Fig. 6). The best solution was obtained at 18 Ma; acceptable and no fits were obtained when such constraint was not applied for samples 050 and 054, respectively. Specific model input parameters for sample 050 (Tamberia Formation) include a T constraint of 10– 20 °C at ca. 8 Ma because the source for the Tamberia sediments must have been at the surface at the time of sediment deposition; a T constraint of 40–70 °C at 5.7 Ma was applied to test burial heating caused by deposition of sediments belonging to the Tamberia Formation; a T constraint of 50–86 °C at ca. 3.5 Ma was applied to test burial heating caused by deposition of sediments corresponding to the Tamberia, Guanchin and Punaschotter formations. Modeling results show that the best fit for the maximum burial T experienced by the sample is ca. 60 °C (Fig. 6).

An additional modeling exercise was performed on the P4 population from sample 054 from the Guanchin Formation, stratigraphically above sample 050 (Figs. 3 and 6). The same general T–t constraints as for sample 050 were used. A T constraint of 10–20 °C was applied at ca. 5.5 Ma and one at 35–72 °C was applied at ca. 3.5 Ma to test burial heating caused by deposition of Guanchin and Punaschotter sediments. Our modeling results show that the best fit for the maximum burial T experienced by the sample is less than 60 °C (Fig. 6). These results support the evidence presented above, documenting negligible annealing due to post-depositional burial in the Fiambalá Basin. A possible explanation for this negligible amount of burial-heating annealing during the evolution of the Fiambalá Basin could be the combination of a relatively low and/or unsteady heat flow and syn-depositional deformation. This last could have involved migration of the basin depocenter during thrust propagation and formation of growth strata. Such processes might have prevented the clastic sequence from ever reaching a thickness of ca. 4 km. Lack of seismic data and 3D outcrops prevent us from holding such a scenario as the

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sole responsible mechanism; however, evidence of synsedimentary deformation in the Fiambalá Basin may support this hypothesis [35]. Interestingly, a similar example of minimally reset detrital samples analyzed from a ca. 6-km-thick stratigraphic sequence in the Angastaco Basin, northeast of the study area (Fig. 1), also documents negligible annealing associated with post-depositional re-heating. A combination of scenarios, similar to those discussed here, was invoked to explain observations there [11]. 4.6. Implications for provenance and sediment source rock exhumation The detrital thermochronologic data obtained from the Tamberia, Guanchin, and Punaschotter formations in the Fiambalá Basin provide important insights into the evolution of the paleogeography of the surrounding ranges. Below, we interpret the detrital age populations in terms of provenance proxies and as representing cooling and exhumation events of the sediment source. Sample 003 from the lower part of the Tamberia Formation has two main age populations (P): 121.9 ± 12.2 and 49.7 ± 4.6 Ma, respectively (Fig. 5). Sample 050 from the upper part of the Tamberia Formation presents a wider age spectrum with five populations: 206.0 ± 47.3, 123.3 ± 11.6, 71. 4 ± 6.1, 37.2 ± 4.6, and 13.8 ± 1.7 Ma. While older populations are similar to the first sample, the youngest middle Miocene population (P1; Table 2) suggests that a new, different source terrain began contributing sediment to the Tamberia Formation at about 5.7 Ma. Sample 054 from the lower part of the Guanchin Formation contains age populations of 167.8 ± 17.8, 92.0 ± 12.8, 43.0 ± 7.2, and 14.4 ± 3.1 Ma. Sample 053 from the upper member of the Guanchin Formation is characterized by age populations of 217.5 ± 194.2, 127.1 ± 21.0, 81.7 ± 15.5, 36.3 ± 5.8, and 14.0 ± 2.8 Ma. Sample 177 from the distal Punaschotter Formation (location Pc19 in Fig. 3) contains age populations of 92.9 ± 18.3, 57.2 ± 5.5, 30.0 ± 2.8, and 11.1 ± 0.9 Ma. A single granite cobble, from the same location as the sandstone sample 177, records a pooled age of 81.3 ± 3.4 Ma (Table 1). Paleocurrent measurements from this location unambiguously show provenance from the N–NE (Fig. 3). The Punaschotter Formation reflects a significant contribution (60%) of middle Miocene ages (P1). P1 from sample 177 is slightly younger compared to P1 of the underlying samples. However, detailed examination of the youngest grains in sample 177

suggests volcanic contamination, responsible for making P1 in sample 177 younger (see Table A1 in the Appendix) compared to other samples. Indeed, the Gaussian distribution (Fig. 5) shows that the binomial fitting curve contains two components, at ca. 10 and 15 Ma. The younger one could denote contamination from reworked ashes while the older one is closer in age to P1 detected in other samples. Jurassic and Eocene ages are characteristic of western sources (e.g., [10,28]). Such a source is also supported by the late Eocene age recorded in samples UP78-9 and UP78-3, collected immediately west of the basin. However, Eocene ages are also characteristic of eastern sources, as shown by sample 177 (sandstone matrix of conglomerate) from the Punaschotter Formation. The Punaschotter conglomerates at this location are derived from the east as indicated by paleocurrent data (Fig. 3). A Late Cretaceous age recorded by a single granite cobble derived from the east in the distal Punaschotter Formation (sample 177, location Pc19; Fig. 3) may also represent an easterly source. Cretaceous cooling ages in this region are typical of eastern sources that have been influenced by events in the vicinity of the Cretaceous Salta Rift, such as sectors including the Eastern Cordillera further north [61] and parts of the northern Sierra Pampeanas [40]. In contrast, middle Miocene ages, as recorded in the P1 population are more typical of sources located within the southern portion of the Puna Plateau (e.g. [61]) (Fig. 1). Grains recording the youngest detrital Miocene population (ranging between 11.1 and 14.4 Ma) are generally rounded and not as translucent as volcanically derived crystals, but are instead more typical of grains derived from crystalline basement rocks. This suggests that this age population represents an exhumation signal rather than a Miocene volcanic input (i.e. from ashes). Although a limited amount of ash contamination may be potentially present in the Punaschotter Formation, this would not significantly alter the main population age. Grains derived from unconsolidated volcanic sources (i.e. ash flow) are typically euhedral and translucent (see Table A1 in the Appendix). In particular, similar detrital middle Miocene ages (P1) have been detected in the Sierra de los Colorados area, immediately southwest of the study area (Slc in Fig. 1), and are considered to represent the exhumation of southern Puna sources [67], thus corroborating our interpretation. In particular the lower part of the northern vertical profile is characterized by ages typical of middle Miocene detrital population (P1). A sample from the AG range to the east of the basin, at a similar elevation as the lower samples of the CN

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Fig. 7. AFTSolve thermal modeling of basement sample UP60-2 (2889 m; lower part of the northern vertical profile), indicated in grey in Fig. 4 and in bold in Table 1. The model was initiated at 200 Ma at T > 180 °C; T constraints of 170–90 °C, 140–60 °C and 17–70 °C were applied at 24 and 15 Ma and 8 Ma, respectively, to examine cooling indicated by the age–elevation profile (Fig. 4) and the possibility that this part of the range might have been at the surface by late Miocene time. Lengths reported are corrected for c-axis for modeling purposes. Refer to text for more details. The light gray area corresponds to the acceptable fit, the dark gray area to the good fit and the dashed black line to the best fit. Note that lengths are corrected for c-axis for modeling purposes explained in the text.

profile, shows similar middle Miocene ages, suggesting that the two ranges have similar exhumation histories and that they are not separated by major structures. Thus, we hypnotize that rocks from structural positions similar to the lower part of the northern and eastern vertical profiles were the likely source for P1, first detected in the upper Tamberia Formation (ca. 6 Ma). This is supported by paleocurrent data from ca. 6 Ma strata documenting the first input from northeastern sources at this time (Fig. 3). The same P1 detrital age population prevails in samples from the Guanchin Formation. The composition of this unit in turn is similar to the sediments from the Punaschotter Formation, which is clearly sourced from the southern Puna margin. By the time the Guanchin Formation was deposited (3.6 < t < 5.7 Ma), the sediment source had thus evolved toward the north, suggesting that rocks from the vicinity of the northern profile supplied sediments. In order to test the hypothesis that the range to the northeast of the basin (CN vertical profile) was the source for P1 in the Fiambalá sediments track length modeling on a sample from the lower part of the vertical profile is performed in the following. 4.7. Track length modeling of the northern vertical profile: implications for the creation of a high relief plateau margin The AFT ages and track-length modeling from the basement profile from the southern Puna margin suggest that this area was exhumed during the early–middle Miocene. Thermal modeling was performed on sample UP60-2 (Fig. 7). The model was initiated at 200 Ma at T > 180 °C; T constraints of 170–90 °C and 140–60 °C

were applied at 24 and 15 Ma, respectively, to examine cooling indicated by the age–elevation profile (Fig. 4). In order to test the hypothesis that the lower part of the range was exposed by late Miocene time, a T constraint between ca. 17 and 70 °C was applied at different times from 10 to 6 Ma at intervals of 2 Ma. The best run was obtained with the T constraint at 8 Ma and shows that the sample could have cooled below ca. 60 °C by ca. 8 Ma. We hypothesize that rocks characterized by ca. 14 Ma ages were at the surface by ca. 6 Ma based on the model of landscape development shown in Fig. 8. The region between the vertical profile and the Fiambalá Basin does not contain any mapped structures; therefore, exhumation on this flank of the range must occur by a combination of rock uplift and headward erosion, requiring the exhumation path to approach the surface obliquely. However, isochrons representing the layer of rock that passed through the ca. 110 °C closure isotherm at a particular time extend subhorizontally; rocks with roughly the same age as the base of the profile would be exposed on the flank of the range prior to exposure of the particular rock that was sampled. As headward erosion removed the flank of the range, the area representing the vertical profile continues to cool as it approaches the surface; rock uplift due to the isostatic response to erosion will continue even after tectonism has ceased. Alluvial fans sourced from the growing topography will onlap the eroding flank of the range. 5. Conclusions Provenance and thermochronologic data from the Fiambalá Basin at the southern margin of the Puna

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Fig. 8. Schematic depiction of the exhumation pathways followed by the 1.8 km vertical profile at 3 time steps. Please note that the figure is not at scale. For simplicity, surface topography is depicted as being in a steady state; dash lines represent the topography at various t-stages. Stars represent the AFT sample locations; light grey swath represents position of the apatite partial annealing zone (PAZ). Lower temperature isotherms are more strongly warped due to topographic effects [59]. 14 and 17 Ma isochrons depicts layers of rock that passed through the lower portion of PAZ. Note that these are approximation values from the P1 population (ca. 14 Ma) and the pooled age of sample UP60-2 (Fig. 5). The 17 Ma AFT sample (black star) crosses 60 °C isotherm at 8 Ma; by 6 Ma, samples with 14 Ma fission track age are exposed at the surface and deposited in the adjacent basin. At this time, the relief must be at least 1.8 km. Curved grey arrows show cooling paths for the lowest and highest samples of the vertical profile; curvature with respect to the surface is caused by effects of headward erosion and rock uplift. Sedimentary basin progressively onlaps topography as flank of range retreats from the depocenter.

Plateau show that late Miocene–Pliocene sedimentary rocks preserved in the basin are derived from the progressive exhumation and erosion of the basinbounding ranges. This was coupled (i.e. contemporaneous and genetically linked) with a northward shift in the evolution of the paleo-drainage system. Initially, the primary source of the Tamberia Formation (ca.

8 Ma) was located to the west, in the Precordillera. With the deposition of the upper Tamberia (sample 050) and Guanchin formations (3.6 < t < 5.7 Ma), a shift of the source towards the north–northeast is recorded by paleocurrent data and by the first appearance of clast lithologies typical of the present-day southern margin of the Puna Plateau.

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Our thermochronological data document that rocks that supplied middle Miocene detrital apatites to the upper Tamberia and Guanchin strata must have been exposed at surface at about ca. 6 Ma. Middle Miocene cooling ages are only characteristic of basement rocks that constitute northern and eastern source areas. In particular, 14 Ma ages are characteristic of the lowest part of the vertical profile to the north, this suggests that this structural level of the range (ca. 2300 m) was at the surface at ca. 6 Ma. Consequently, at least the entire crustal column that presently overlies this sample (crestal elevation of 4072 m) must had also been exhumed by then (Fig. 8). The crestal elevation is a minimum estimate, as there has likely been erosion during the last 6 Ma. Moreover, the fact that the same ages recorded in the 6 Ma basin strata are still recorded by the range to the north suggests that not enough tectonic and or erosional exhumation has occurred since then to be recorded by the AFT system. Assuming 60 °C as the upper limit of the PAZ, a mean surface temperature of ca. 10 °C and a conservative 20 °C/km paleo-geothermal gradient, less than 3 km of upper crust would have been exhumed by erosion since middle Miocene time. Our data show that the southern margin of the Puna Plateau expressed an high relief by late Miocene time, when sedimentary strata hosting AFT detrital ages of 14 Ma (P1) were deposited in the Fiambalá Basin. Rocks of the northern and eastern ranges that provided sediments to the basin in late Miocene time are presently at ca. 2300 m. Although our new data may indicate a relief similar or higher than present in the source region during the latest Miocene, they do not provide direct information on absolute paleo-elevations. Instead, our data may document a minimum crestal height of ca. 1800 m above the elevation of the Fiambalá Basin at that time. However, more detailed information on paleo-elevation and relief conditions are not available for this region in order to further constrain the paleo-geographic evolution. Paleo-elevation proxies from the Bolivian Altiplano have been used to suggest that present plateau elevations further north may have been acquired within the last 10 Ma [2–4], implying a rapid uplift event subsequent to the main phase of shortening and crustal thickening. Indeed, this timing for wholesale plateau uplift broadly coincides with an inferred isostatic uplift event following mantle delamination in the southern Puna Plateau sometime between 8 and 3 Ma [16,17]. Interestingly, sedimentologic, thermochronologic, paleontologic, and oxygen-isotope data in Mio-Pliocene sedimentary sequences deposited in other intramontane basins along the eastern margin of the Argentine Puna

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Plateau record a similar paleogeographic reorganization with evolution of headwaters from the west to the north and north–east [11,24,25,68,69]. In particular, in the neighboring El Cajon Basin (Fig. 1), a reorganization of the source and basin depositional environment is recorded by both seismic reflection and thermochronological data at ca. 6 Ma and is interpreted to result from plateau–margin growth [13]. Cooling ages from basement ranges in the southern Puna Plateau and detrital AFT data from adjacent intramontane basins [11,13] suggest that the main phase of exhumation was prior to a late Miocene–early Pliocene event during which significant changes occurred in paleo-drainage conditions. In particular, our data suggest that range uplifts existed at the present-day southern Puna margin by late Miocene time and were the result of Oligo-Miocene distributed shortening and exhumation [13]. This is in agreement with data from the Eastern Cordillera of Argentina to the northeast [61] and the Bolivian Altiplano [70]. If a genetic link between shortening, crustal thickening, exhumation, and uplift, exists, then a high-elevation and/or high relief region may already have been in place in the Puna region by Oligo-Miocene time. Although not recorded by AFT, there certainly has been some amount of rock uplift since then, as documented by regional paleo-drainage reorganization and Pliocene reverse faulting observed in the Fiambalá Basin and other intramontane sedimentary basins along the plateau margin [11,25,71]. Thus, while reverse-fault bounded ranges and intervening, internally drained basins are a typical feature of the present-day plateau morphology, the foundation for this setting may already have been attained in Oligo-Miocene time. The filling of these basins and consequent reduction of local relief within the present plateau region also began at that time and has continued since then. However, the pronounced changes at the immediate eastern and southern plateau margin in late Miocene–early Pliocene time suggest that wholesale plateau uplift may have affected this region, subsequent to the earlier period of distributed shortening and crustal thickening. This younger event is not reflected in the AFT data, presumably due to the arid climate and associated reduced erosion rates in the Puna region; the southern Puna Plateau margin has experienced less than ca. 3 km exhumation since the middle Miocene. Acknowledgements We thank Glen R. Murrell for his fundamental help with part of fission track analysis, A. Villanueva Garcia

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and J. Sosa Gomez for their hospitality and help in the field. We appreciate the constructive reviews of two anonymous reviewers as well as the useful comments of Teresa Jordan on an earlier version of this manuscript. The Alexander von Humboldt Foundation is kindly acknowledged for supporting B. Carrapa's research at Potsdam. We thank the German Science Foundation for financial support (Leibniz-Prize to M. S.) as well as the A. Cox Fund (M.S.), Stanford University.

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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j. epsl.2006.04.010. References

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