TECTONICS, VOL. 27, TC1015, doi:10.1029/2007TC002127, 2008 Click Here for Full Article exhumation and basin development in the Puna of northwestern

B. Carrapa1 and P. G. DeCelles2 Received 6 March 2007; revised 16 July 2007; accepted 24 September 2007; published 22 February 2008. [1] The Puna is part of the larger Puna-Altiplano accumulation on the eastward propagating Plateau (also known as the Central Andean Plateau), central Andean orogenic wedge, or as a local characterized by high elevation, low relief, and aridity, intermontane basin. The similarities between wedge- located in the central Andes of Bolivia and Argentina. top deposits preserved in Bolivia and Eocene deposits Tertiary sedimentary rocks preserved within the Puna in northwestern Argentina, south of 25°S, lead us to contain a unique archive of information regarding the favor the wedge-top scenario for the Geste Formation. paleogeography, depositional environments, and If correct, this implies that the deformation front of the timing of sediment source exhumation during the Andean orogenic wedge incorporated both thin- and early stages of Andean mountain building. The Eocene thick-skinned structures as it migrated, possibly Geste Formation in the Salar de Pastos Grandes area unsteadily, from the Cordillera de Domeyko during (within the central Puna of northwestern Argentina) the -Paleocene to areas within the Puna consists of deposits that are the result of confined to and Eastern Cordillera by mid-late Eocene time. unconfined flows in a sandy to gravelly, braided Contemporaneously, a regional-scale foreland basin fluvial system and alluvial fans proximal to the source system developed over an along-strike distance of at terrane. Paleocurrent data document an overall least 650 km. Citation: Carrapa, B., and P. G. DeCelles eastward flow direction. Up-section coarsening of (2008), Eocene exhumation and basin development in the Puna of the Geste Formation suggests that topographic relief in northwestern Argentina, Tectonics, 27, TC1015, doi:10.1029/ the source area increased through time, possibly owing 2007TC002127. to enhanced tectonic activity and source terrane unroofing. petrography and conglomerate 1. Introduction clast-count data document quartzose and phyllitic compositions typical of rocks preserved [2] The Puna is the southern part of the Central Andean just west of the Salar de Pastos Grandes area. Plateau, or Puna-Altiplano Plateau (which is the second Paleocene-Eocene detrital apatite fission track age largest orogenic plateau on Earth after Tibet) and is charac- populations (P1: 35–52 Ma; P2: 52–65 Ma) of the terized by mean elevation of >3700 m, peaks over 6000 m, Geste Formation and their consistent trends up-section internal drainage, and aridity [Isacks, 1988; Strecker et al., 2007] (Figure 1). The tectonic processes responsible for the suggest moderate to rapid (0.4 mm/a to >1 mm/a) formation of the Central Andean Plateau and its marginal exhumation of western sediment sources during the areas are related to subduction of the oceanic Nazca plate early to mid-Tertiary stages of Andean mountain under the continental South American plate. Such processes building. Sedimentation rates increase up-section from mainly include distributed crustal shortening [Allmendinger 0.1 mm/a to 1 mm/a. Our data, when combined with et al., 1997], emplacement of regional thrust other structural, stratigraphic and seismic evidence sheets [Kley et al., 1997; McQuarrie, 2002], underthrusting from surrounding regions, suggest that the Geste of cratonal material beneath the plateau region and later Formation was deposited in response to crustal lithospheric thinning [Isacks, 1988] following delamination shortening and resulting erosion and sedimentation, [Kay et al., 1994; Sobolev and Babeyko, 2005] or convec- which started as early as Cretaceous in the Chilean tive removal of lithosphere [Schurr et al., 2006]. Cordillera de Domeyko and in the Salar de Pastos [3] The interconnections between such processes, the timing and mode of upper crustal shortening and the Grandes area by Eocene time. The Geste Formation resulting sedimentation patterns within the plateau interior could be interpreted either as a local wedge-top are subjects of ongoing discussion. Particularly vexing is the Paleogene history of crustal shortening, orogeny, and basin development in the region presently situated within 1Department of Geology and Geophysics, University of Wyoming, the plateau interior. Although Paleogene clastic sedimentary Laramie, Wyoming, USA. rocks are widespread [Jordan and Alonso, 1987] the depo- 2Department of Geosciences, University of Arizona, Tucson, Arizona, USA. sitional settings and mechanisms of sediment accommoda- tion for these strata are not well understood. It is also not Copyright 2008 by the American Geophysical Union. clear when the Eastern Cordillera, presently located along 0278-7407/08/2007TC002127$12.00

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et al., 1992; Flint et al., 1993; Charrier and Reutter, 1994] have interpreted Upper Cretaceous and Paleogene coarse- grained deposits in the Salar de Atacama of northern Chile as the result of back-arc extension. This interpretation is partly supported in recent papers by Pananont et al. [2004] and Jordan et al. [2007] on the basis of interpreted reflection seismic profiles correlated to strata in a borehole in the Salar de Atacama. The amounts of extension inferred from seismic data in Oligocene and lower sedi- mentary rocks are, however, very slight. In any case the extensional basin interpretation begs a tectonic explanation for regional extension in the South American plate coeval with rapid absolute westward motion during opening of the South Atlantic Ocean [Mu¨ller et al., 1997]. In contrast, Mpodozis et al. [2005] proposed that the Salar de Atacama basin formed as a consequence of tectonic inversion of a –Early Cretaceous back-arc basin within a contin- uous contractional regime in the Cordillera de Domeyko from mid-Cretaceous through Paleogene time. Similarly, other recent work that combines reflection seismic and outcrop data documents mid-Cretaceous through Tertiary shortening and synkinematic proximal foreland basin sedi- mentation in the Salar de Atacama region (Figure 1) [Arriagada et al., 2006]. The presence of a Cretaceous– early Tertiary thrust belt in northern Chile [Arriagada et al., 2006] implies that a contemporaneous foreland basin sys- tem should be preserved in the Puna and Eastern Cordillera of northwestern Argentina. However, the rocks that occupy this time interval in northwestern Argentina have been associated with regional extension in the Salta rift complex during the Early to mid-Cretaceous, followed by tectono- thermal subsidence during Late Cretaceous through Eocene time [Galliski and Viramonte, 1988; Salfity and Marquillas, 1994; Marquillas et al., 2005]. Moreover, the existing majority opinion holds that crustal shortening and related foreland basin development in northern Argentina were delayed until latest Oligocene time [e.g., Allmendinger et Figure 1. Shaded regional topography of the central al., 1997, and references therein] or perhaps even later Andes of northern Argentina and southern Bolivia, showing [Jordan et al., 2007]. On the other hand, recent studies in tectonomorphic zones (labeled), thrust faults (barbed lines), Bolivia [Elger et al., 2005; Ege et al., 2007] and north- and volcanoes (circles) (modified after Sobel et al. [2003]). western Argentina suggest local crustal shortening and rapid Grey thick lines indicate the Cordillera de Domeyko thrust exhumation during Eocene and Oligocene time [Kraemer et belt (modified after Maksaev and Zentilli [2000]). Dashed al., 1999; Coutand et al., 2001; Carrapa et al., 2005; larger box identifies the Salar de Pastos Grandes area, Hongn et al., 2007]. Thus the key question is whether the shown in more detail in Figure 2. AD, Salar de Atacama; Paleogene strata in the Puna of northwestern Argentina AR, Arizaro Basin; SA, Salar de ; P, La Poma reflect distal foreland basin deposition [e.g., Kraemer et Basin; A, Angastaco; TT, Tin Tin; CR, Chango Real. al., 1999], regional extension and thermal subsidence [Marquillas et al., 2005], or local uplift and tectonic partitioning of the Puna region [Coutand et al., 2001; Hongn et al., 2007]. the eastern margin of the Puna, began to experience regional [5] We focus here on Eocene coarse-grained clastic rocks uplift and exhumation. in the Salar de Pastos Grandes area in the central Puna [4] Jordan and Alonso [1987] and Kraemer et al. [1999] (Figure 2). We present detailed sedimentological, modal suggested that Eocene-Oligocene sedimentary rocks in the petrographic, and detrital apatite fission track (AFT) data in Puna were deposited along the eastern side of an orogenic order to constrain depositional environments, paleogeogra- highland located in the Chilean Precordillera (including the phy, sediment provenance, and exhumation ages of bedrock Cordillera de Domeyko). Eocene shortening and uplift in source terranes. The results of our study, though confined to this region is generally referred to as the Incaic phase of the a local region, nevertheless have bearing on the larger-scale [Steinmann, 1929]. Other authors [Hartley scientific issue of whether the Puna was occupied by a

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Figure 2. (a) Geological map of central Andes (modified after Reutter et al. [1994]). Square corresponds to area TC1015 shown in Figure 2b. 1, Eocene deposits in the Salar de Pastos Grandes area; 2, Eocene deposits in the La Poma Basin [Hongn et al., 2007]. (b) Geological map of Salar de Pastos Grandes area (modified after Alonso [1992]). (c) Cross section modified after San Antonio de los Cobres geological map [Blasco et al., 1996]. TC1015 CARRAPA AND DECELLES: EOCENE BASIN DEVELOPMENT IN NW ARGENTINA TC1015 regional-scale foreland basin system or local extensional Chile, approximately 200 km west of the Salar de Pastos basins during Eocene time. Grandes, document the Cordillera de Domeyko fold-thrust belt and associated proximal syntectonic deposits of mid- Cretaceous through Eocene age [Arriagada et al., 2006]. 2. Geological Setting Contractional growth structures in these strata imply that [6] The Puna resides in the hinterland of the Andean they were deposited in the wedge-top depozone of a thrust belt, bounded to the west by the modern Andean foreland basin system that may have extended into northern magmatic arc and to the northeast by the Eastern Cordillera. Argentina. Thermochronological AFT data from plutonic East of the Eastern Cordillera lies the Santa Barbara thrust intrusions in the Cordillera de Domeyko document Eocene belt and its along-strike equivalent Subandean thrust belt to Oligocene cooling ages (between 50 Ma and 30 Ma) (Figure 1). The Eastern Cordillera and Santa Barbara belt [Maksaev and Zentilli, 2000]. both contain basement-involved reverse faults, whereas the [11] In the Eastern Cordillera, sparse AFT data from the Subandean belt is a typical thin-skinned thrust belt [Kley et Chango Real Range document late Eocene–Oligocene al., 1999; Allmendinger and Zapata, 2000; Echavarria et (38.3 ± 3 Ma to 29.0 ± 3 Ma) cooling ages [Coutand et al., 2003]. Directly west of the magmatic arc is the al., 2006]; however, it remains unclear if the Chango Real Cordillera de Domeyko in northern Chile (Figure 1), which (CR in Figure 1) and other ranges in the Eastern Cordillera comprises and Mesozoic rocks that experienced constituted topographically uplifted areas and sediment significant uplift and erosion during at least Paleocene- sources at that time. Recent sedimentological and detrital Eocene time [Hartley et al., 1992; Flint et al., 1993; U-Pb geochronological data from the Angastaco and Tin Charrier and Reutter, 1994; Maksaev and Zentilli, 2000; Tin areas (A, TT, in Figure 1) suggest that a genetic link Mpodozis et al., 2005; Arriagada et al., 2006]. may exist between the Eocene sedimentary deposits of the [7] The Puna interior is characterized by internally plateau interior and those located within the Eastern Cor- drained topographic basins separated by generally north- dillera [Carrapa et al., 2006a]. AFT data and thermal trending mountain ranges composed of and modeling support this concept, showing that ranges within lower Paleozoic low-grade metasedimentary and plutonic the Eastern Cordillera must have been buried by a thick pile rocks, Neogene ignimbrites and volcanic rocks, and Paleo- of pre-Miocene sediments and that exhumation of those gene-Pliocene clastic and evaporitic sedimentary rocks. ranges commenced at 20 Ma [Coutand et al., 2006; Numerous, generally steeply eastward and westward dip- Deeken et al., 2006]. ping, reverse faults cutting the older rocks and locally juxtaposing them against the Tertiary sedimentary rocks, 3. Sedimentology are depicted in the cross section shown in Figure 2c derived from the 1:250,000 (San Antonio de los Cobres) geological 3.1. Facies Analysis map [Blasco et al., 1996] and integrated with the [12] The following sedimentological descriptions and 1:1,000,000 geological map [Reutter et al., 1994]. interpretations are based on detailed, bed-by-bed measured [8] The Salar de Pastos Grandes area contains more than stratigraphic sections (Figure 3). Individual beds were 3.5 km of Eocene to Quaternary strata [Alonso, 1992]. The measured at centimeter scale. We measured a composite section investigated here is part of the Geste Formation of section totaling 2019 m in thickness, and correlated between late Eocene age [Pascual, 1983; Alonso, 1992; DeCelles et offset sections by tracing marker beds and projecting along al., 2007]. In this area the Geste Formation unconformably strike. Because the lithofacies encountered are well known overlies Ordovician quartzite and phyllite of the Copalayo in the sedimentological literature, we employ the lithofacies Formation in the Copalayo Range. Other pre-Tertiary rocks codes of Miall [1978] with some minor modifications west of the Ordovician outcrops include Precambrian plu- (Table 1). Paleoflow directions were determined by mea- tonic rocks and sparse Paleozoic plutonic rocks. To the east suring limbs of trough cross strata according to method I of of the study area, the Eastern Cordillera is composed of DeCelles et al. [1983], and the dip directions of imbricated Precambrian- meta-sedimentary (Puncoviscana clasts in conglomerates. Approximately 20 trough limbs or Formation) and plutonic rocks, and Cretaceous sedimentary at least 10 imbricated clasts were measured per station. rocks. Mean paleocurrent directions are plotted in Figure 3. [9] Detrital zircon U-Pb ages show that fluvial sand- stones in the Geste Formation were derived in part from 3.2. Depositional Environments of the Geste Formation quartzite in the underlying Ordovician Copalayo Formation 3.2.1. Description [DeCelles et al., 2007]. Volcanogenic air fall derived zircons constrain the depositional age of the Geste Formation [13] Overall the Geste Formation exhibits a marked to the late Eocene (39.0 ± 0.6 Ma to 35.4 ± 0.55 Ma; upward coarsening trend in grain size over the >2 km thick [DeCelles et al., 2007]). A late Eocene depositional age also section that we measured (Figure 3). On the basis of is consistent with vertebrate paleontological ages from the lithofacies assemblages, we divide the Geste Formation into Geste Formation in the Salar de Pastos Grandes area [Pascual, informal lower, middle, and upper members. 1983] and in the La Poma Basin to the east [Hongn et al., 2007]. [14] The lower member is 370 m thick and extends from the base of measured section 1SP to the base of section [10] Recently published seismic, sedimentological, and structural data from the Salar de Atacama basin in northern 2SP (Figures 3 and 4). It mainly consists of fine- to medium-grained sandstone, bright red siltstone, and pebble

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Figure 3. Stratigraphic sections of the Geste Formations based on detailed bed-by-bed field measurements. TC1015 CARRAPA AND DECELLES: EOCENE BASIN DEVELOPMENT IN NW ARGENTINA TC1015

Table 1. Facies Codes After Miall [1978] Integrated With Our Descriptions

Facies Codes Lithofacies Sedimentary Structures Interpretation

Gmm conglomerate, matrix-supported structureless, disorganized high-strength (cohesive) debris flow Gcm conglomerate, clast-supported structureless to crude horizontal clast-rich (noncohesive) debris flow stratification, imbrication and sheetflood deposits Gch conglomerate, clast-supported horizontal stratification, longitudinal gravel bars, lag deposits local imbrication Gct conglomerate, clast-supported trough and planar cross-beds minor channel fills, 3D gravelly bedforms Gci conglomerate, clast-supported imbricated clasts channel fills and longitudinal bar deposits Sm sandstone, fine- to coarse-grained massive or faint lamination hyperconcentrated flows, slurry flows St sandstone, fine- to very coarse-grained, trough cross-beds subaqueous 3D dunes, sustained locally pebbly unidirectional currents Sh sandstone, fine- to very coarse-grained, horizontal laminations/low angle shallow supercritical flows locally pebbly (<15°) cross-beds Fsm fine-grained sandstone, siltstone, mudstone massive, desiccation cracks overbank deposits, abandoned channel fills and drape deposits to cobble conglomerate. Sandy lithofacies include massive between samples 1SP32 and 1SP238, and 1.1 mm/a, between (Sm), horizontally laminated (Sh), and trough cross-stratified sample 2SP277 and 3SP431. These rates, and their increase (St) sandstone in 1- to 5-m-thick, broadly lenticular beds with up-section, are typical of sedimentary basins related to erosional bases. These beds commonly exhibit upward fining contractional orogenic systems such as wedge-top, foreland, grain size trends. Siltstone (Fsm) intervals are massive, and intermontane basins [Allen, 1983; Horton, 1998; Allen bioturbated, and locally mottled, with clay cutans and peds. and Allen, 2005]. Conglomerate beds are lenticular and typically intercalated 3.2.2. Interpretation within units of St. The most abundant gravelly lithofacies are clast-supported, imbricated (Gci), horizontally stratified [19] The Geste Formation contains abundant evidence for (Gch), and trough cross-stratified (Gct) conglomerates. deposition in fluvial and alluvial fan environments, includ- [15] The middle member is 1482 m thick (Logs SP2 and ing lenticular bodies of conglomerate and sandstone with SP3 in Figures 3), and consists of 1- to 10-m-thick lenticular erosional basal surfaces, sedimentary structures formed by bodies of upward fining conglomerate (lithofacies Gcm, traction currents, paleosols, and sediment-gravity flow Gch, and Gci) with erosional basal surfaces; the tops of deposits. The sandstone and conglomerate with sedimentary these units are capped by thin beds of medium- to coarse- structures formed by unidirectional traction currents (e.g., grained sandstone (lithofacies Sm, Sh, and St). Massive, trough cross stratification, imbrication) were deposited in burrowed siltstone (Fsm) is abundant in the lower 250 m of fluvial channels and gravelly and sandy macroforms [Smith, the middle member, but becomes sporadic from there 1970; Hein and Walker, 1977; Miall and Turner-Peterson, upward (Figures 3 and 4). 1989; Lunt and Bridge, 2004; Wooldridge and Hickin, [16] The upper member of the Geste Formation (Log SP 2005]. Upward fining in these channel deposits resulted 4 in Figure 3) consists of thick, amalgamated beds of cobble from gradual filling and abandonment. The absence of to boulder conglomerate (Figure 4). Although we measured typical features associated with meandering channels and only 167 m of the upper member before encountering anastomosing fluvial systems, such as lateral accretion poor outcrop, it is clear that at least an additional several deposits, prominent levees, oxbow lake deposits, and cre- hundred meters of this unit are present [cf. Alonso, 1992]. vasse splay deposits [Smith and Perez-Arlucea, 1994; Miall, The upper member is mainly characterized by massive to 1996; Makaske, 2001], suggests low-sinuosity bed load– horizontally stratified, clast- and matrix-supported conglom- dominated braided channels. Although fine-grained deposits erates (Gcm, Gch, Gmm), with limited massive to horizon- are abundant in the lower member, paleosols are neither tally stratified, medium- to coarse-grained sandstone (Sm, well developed nor particularly common, and in general we Sh). The conglomerates occur in beds ranging from 1 m to found little evidence for abundant vegetation, again sug- 16 m thick, and no obvious grain size trends are present. gesting that channel banks were unstable. The middle [17] Bedding in the Geste Formation dips eastward away member is dominated by coarse conglomerate and sand- from the Copalayo Range. Although the exposure is insuf- stone, with relatively little fine-grained sediment. The ficient to document unequivocally the presence of a growth abundance of imbricated and horizontally stratified con- structure, dip of bedding decreases systematically up-sec- glomerate in beds with erosional bases suggests deposition tion and eastward from 60°E in the lower member to 20°E in shallow, gravelly braided channels [Nemec and Steel, in the upper member. 1984]. The upper member contains evidence for deposition [18] Sedimentation rates are calculated using the strati- by viscous debris flows, hyperconcentrated flows, and graphic thickness presented in Figure 3 and the mean U-Pb coarse-grained fluvial channels. The massive matrix-sup- ages of Eocene zircon grains (refer to Table 3 in section 5.1) ported conglomerates and extreme coarseness are best for samples in which they were found [DeCelles et al., explained by deposition in an alluvial fan system [e.g., 2007]. Sedimentation rates are on the order of 0.08 mm/a Heward, 1978; Pierson,1980;Nemec and Steel, 1984; Schultz, 1984; Flint, 1985; DeCelles et al., 1991]. Most

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Figure 4. Typical facies of the (a) lower, (b) middle, and (c) upper members; (d) typical burrows. Refer to Figure 3 and text for more details. likely, these alluvial fans experienced a combination of sheet order to identify the sediment source area and its erosional flood and sediment-gravity flow deposition. The abundance history. Sixteen sandstone samples (Figure 5 and Table 2) of boulders larger than 50 cm in diameter indicates depo- were analyzed using a modified Gazzi-Dickinson method sition within a few kilometers of the source terrane. Clast [Gazzi, 1966; Dickinson, 1970, 1974; Ingersoll et al., 1984]. composition data, discussed below, support this concept of Half of each thin section was stained for K and Ca feldspars. proximal deposition. All constituents and at least 250 framework grains were counted, and an additional 100 lithic grains were counted in each thin section in order to provide better control on source 4. Provenance rock types. The point-counting parameters and normalized 4.1. Methods modal petrographic data are given in Table 2 and plotted on standard ternary diagrams in Figure 5. Conglomerate com- [20] Sandstone petrography and conglomerate clast positions (Figure 6) were determined by counting at least counts were conducted throughout the Geste Formation in

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present, but chert was not detected (Table 2). Lithic grains (grain size < 62 mm) consist mainly of quartzite, schist, and phyllite (Table 2 and Figure 5). No volcanic lithic grains were found (Table 2). [22] Sandstone samples from the Geste Formation show a trend from quartzose to quartzolithic compositions, with average Qm/F/Lt = 66/8/25. Following the classification of Dickinson and Suczek [1979] and Dickinson [1985], these compositions overlap the fields of continental block and recycled orogenic provenance (Figure 5a) and are typical of Andean-type orogenic systems [DeCelles and Hertel, 1989; Garzanti et al., 2007]. The Qm/P/K diagram is mainly dominated by quartz with wavy extinction, Qz (grain size >62mm) in polycrystalline and monomineralic grains and Qz (grain size > 62 mm) in plutonic and metamorphic rocks. The Qm/P/K diagram shows a decrease up-sequence of both P and K (Figure 5b). The extra diagram for fine-grained lithics (with grain constituents <62mm; refer to Table 2 for more details) shows a source initially (lower member) characterized by quartzite (90% quartz) and fine polycrys- talline Qz (Quartzite/Qz pole, in Figure 5c). An increase up- section in schist and phyllite fragments is observed (Figure 5c). [23] Conglomerate clasts are composed of phyllite, quartzite, fine-grained quartz-mica schist, and milky vein quartz (Figure 6). Quartzite and phyllite are typical of Ordovician metasedimentary rocks in the Copalayo Forma- tion. Mica-schist may be representative of Cambrian and Ordovician metamorphic rocks [Coira et al., 1982]. These rock types are typical of western source terranes, in agree- ment with paleocurrent data documenting eastward flow (Figure 3).

5. Apatite Fission Track Thermochronology 5.1. Method

[24] AFT thermochronology provides information on the timing and rates of cooling between temperatures of 60°C and 120°C. This temperature range defines the AFT Partial Annealing Zone (PAZ) [Wagner, 1968; Gleadow and Fitzgerald, 1987]. The exact temperature of the base of the Figure 5. Modal compositions of Geste Formation PAZ depends on the kinetic characteristics of the apatites calculated using the Gazzi-Dickinson method and the cooling rate. Apatite kinetic characteristics can be (for more details refer to text): (a) QmFLt diagram based on quantified by measuring the diameter of fission track etch the technique described by Gazzi [1966] and Dickinson pits, which is defined as Dpar [Donelick et al., 1999; [1970, 1985]; (b) QmPK; and (c) Quartzite/Qz-Schist- Ketcham et al., 1999]. In general, apatites with smaller Dpar Phyllite. See text for explanation, and see Table 2 for data are typical of flourine-rich apatite and are characterized by and parameter definitions. cooler temperatures of the upper PAZ boundary, whereas apatites with larger Dpar are typically chlorine-rich and are characterized by hotter temperatures of the upper PAZ boundary. Fission track lengths provide information about 100 clasts per location within a 10- to 20-cm grid that was the proportion of the cooling history that the sample moved along strike until enough data were collected. experienced within the PAZ, and hence how quickly the 4.2. Sandstone and Conglomerate Provenance apatite passed through the PAZ. Therefore, in order to interpret the AFT data in terms of a temperature-time path, [21] Framework grains in the Geste Formation sandstones an integrated analysis of fission track age, track length include monocrystalline quartz (Qm), quartz (Qz) (>62 mm) distribution, and kinetic characteristics of the apatite grains in plutonic and metamorphic rocks, plagioclase (mainly is required. In the case of detrital AFT thermochronology, calcic, P), K-feldspar (K), and P or K in plutonic and multiple populations of apatites may be present in a single metamorphic rocks. Fine-grained polycrystalline quartz is

8of19 TC1015 Table 2. Sandstone Petrography Raw Data and Parametersa

Sandstone Parameters QmFLt QmPK Lithics SP11 SP23 SP77 SP99 SP123 SP201 SP237 SP239 2SP100 3SP10 3SP95 3SP149 3SP203 3SP627 3SP925 4SP111

Qz straight extinction Qm Qm 2.2 2.3 0.9 3.7 4.3 2.2 1.9 2.5 0.3 2.8 22.2 2.3 1.4 1.0 1.3 1.3 Qz extinction Qm Qm 16.7 12.2 14.4 23.4 18.4 16.5 23.2 24.9 20.2 24.1 6.9 20.5 8.9 14.0 15.2 12.2 Coarse policrystalline Qz (>62 mm) Qm Qm 25.2 22.5 9.3 16.3 17.0 22.9 11.7 4.3 3.9 4.0 0.2 8.2 3.4 2.6 5.2 3.3

Fine policrystalline Qz Lt 1.3 0.2 0.3 0.4 0.2 ARGENTINA NW IN DEVELOPMENT BASIN EOCENE DECELLES: AND CARRAPA Chert Lt Qz (>62 mm) in plutonic rock Qm Qm 2.4 0.4 1.6 0.4 1.6 2.6 2.9 1.0 4.7 2.8 5.9 0.9 1.0 0.8 0.5 Qz (>62 mm) in metamorphic rock Qm Qm 0.7 2.5 4.6 2.6 3.3 3.5 4.2 2.5 12.3 4.2 4.1 6.5 7.1 5.5 6.0 7.1 Qz (>62 mm) in volcanic rock Qm Qm monocrystalline K not altered K K 2.4 1.2 3.5 2.9 1.8 0.8 1.2 0.5 0.4 K (>62 mm) in plutonic rock not altered K K 2.0 1.6 0.2 0.2 0.6 1.6 0.2 0.2 K (>62 mm) in metamorphic rock K K 0.9 0.4 2.4 0.2 0.8 0.0 0.0 0.4 0.3 0.0 0.0 0.5 0.0 0.2 0.0 0.0 K (>62 mm) in volcanic rock K K Monocrystalline Ca-P

9of19 not altered P P 0.2 7.6 0.5 1.8 0.2 1.8 3.5 1.2 1.0 2.6 2.3 0.0 0.4 1.2 0.2 0.5 Monocrystalline Na-P not altered P P 0.2 0.4 0.4 0.2 0.0 P (>62 mm) in plutonic rock not altered P P 0.9 0.8 0.2 0.6 2.0 1.1 1.0 1.2 0.5 0.7 1.1 0.8 0.5 P (>62 mm) in metamorphic rock P P 0.7 0.4 0.4 0.2 0.2 0.7 0.4 0.2 0.3 0.9 0.7 0.7 0.4 0.2 0.2 0.4 P (>62 mm) in volcanic rock P P 0.3 Metamorphic lithic Lt 3.3 5.4 6.6 2.2 3.9 5.7 3.6 12.7 23.4 18.5 16.0 24.9 27.9 24.7 17.8 22.8 Volcanic lithic Lt Clastic lithic Lt 0.2 0.2 0.2 0.2 0.2 0.7 Single biotited/chlorite 0.2 0.2 1.0 0.3 0.7 0.7 0.2 0.2 0.6 1.0 0.2 Single muscovite 1.1 0.2 1.6 2.0 1.0 1.9 0.8 1.2 0.5 0.2 0.4 1.2 0.3 Mica in crystalline rock fragment 0.2 0.2 0.2 0.4 0.2 0.5 1.0 0.6 0.3 Single heavy mineral 0.6 1.2 1.4 0.7 1.8 0.6 0.5 0.9 Heavy mineral in rock fragment 0.2 0.2 0.2 0.5 0.2 Calc-schist Lt Sparry calcite Dolomite Impure calcareous grain Mudstone-wackstone Lt Packstone-grainstone Lt Total framework grains 57 56 47 56 55 57 53 53 69 61 60 65 51 52 46 50 TC1015 Altered grain of unknown origin 2.7 3.1 2.6 2.9 3.3 1.3 2.9 2.0 2.4 2.8 2.5 0.5 0.4 4.7 2.8 0.4 TC1015

Table 2. (continued) ARGENTINA NW IN DEVELOPMENT BASIN EOCENE DECELLES: AND CARRAPA

Sandstone Parameters QmFLt QmPK Lithics SP11 SP23 SP77 SP99 SP123 SP201 SP237 SP239 2SP100 3SP10 3SP95 3SP149 3SP203 3SP627 3SP925 4SP111

Authigenic mineral in unknown grain 0.4 0.3 0.2 0.2 0.2 0.2 1.5 Coarse silt 0.9 0.6 4.7 0.6 0.4 0.8 0.4 1.4 0.5 1.9 1.2 0.4 1.0 1.5 Siliciclastic matrix 11.4 8.1 12.4 7.7 8.4 1.3 1.0 3.5 4.7 11.2 3.7 12.1 10.5 14.4 11.5 6.2 Carbonatic matrix 1.3 0.4 0.0 0.4 0.4 0.2 0.2 3.5 1.6 1.1 0.7 0.2 0.4 1.6 0.0 Quartzitic cement 0.2 0.2 0.2 0.2 0.4 0.2 Phyllosilicatic cement 6.0 6.0 28.1 5.9 8.8 0.2 12.5 6.5 9.4 4.2 6.4 16.1 6.5 23.1 8.6 1.3 Carbonatic cement 10.5 20.0 2.6 19.1 11.0 35.7 21.5 22.0 6.0 4.2 19.7 0.5 0.2 0.8 5.5 0.0 Other cements Inter grains porosity 7.3 2.7 0.0 2.6 9.6 0.0 0.0 4.5 6.3 11.5 0.5 2.3 28.7 0.8 20.6 38.6 Calcite overgrowth 1.3 2.5 0.4 2.4 0.6 4.0 4.0 0.8 0.0 0.2 2.1 0.3 Total Rock 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 Qm 83 71 66 83 80 83 82 66 60 62 66 59 43 46 60 49

0o 19 of 10 F 9192013136111067832613 Lt + C 8 10 14 4 7 10 7 24 34 31 27 39 55 48 39 48 Qm 81 62 69 81 80 88 85 86 91 89 90 95 96 88 99 94 P 7325712121385111024916 K 1262513803640130300

Fine Lithic Components SP11 SP23 SP77 SP99 SP123 SP201 SP237 SP239 2SP100 3SP10 3SP95 3SP149 3SP203 3SP627 3SP925 4SP111

Fine QZ Qz 18.3 7.8 1.1 7.6 4.6 4.0 0.9 0.0 0.9 1.9 0.9 Quarzites quartzite 18.3 7.8 11.6 6.3 4.6 41.0 0.0 7.9 8.0 1.9 1.9 3.5 0.9 10.3 0.9 Schist QZ + Biotite schist 1.0 1.2 2.0 1.9 1.9 0.7 0.0 0.9 13.4 Schist QZ + Biotite + Musc. schist 6.3 4.6 2.0 3.6 3.0 11.6 9.3 5.6 1.9 2.1 0.9 8.0 + K + heavy min. Phyllate phyllite 26.8 41.2 30.5 27.8 66.2 28.0 0.0 46.5 38.4 39.3 46.3 41.7 49.3 65.7 60.7 25.0 Microcrystalline mica schist 4.2 3.9 3.2 6.3 1.5 0.0 39.8 2.0 0.9 12.1 5.6 8.3 0.7 15.7 3.7 Schist QZ + Muscovite schist 29.6 33.3 45.3 39.2 16.9 19.0 4.8 28.7 39.3 29.9 28.7 41.7 28.9 13.0 17.8 39.3 Schist QZ + Musc. + heavy min. schist 2.8 5.9 2.1 6.3 1.5 5.0 28.9 8.9 0.9 5.6 8.3 5.6 14.1 1.9 6.5 9.8 Total Fine Lithics 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 Quartzite-Qz 37 16 13 20 9 45 2 8 9 2 3 0 4 3 10 2 Schist-phyllite 32 39 54 46 23 27 58 43 52 47 45 50 46 16 25 72 phyllite 31 45 34 34 68 28 40 49 39 51 52 50 50 81 64 26

aRefer to text for further explanations. TC1015 TC1015 CARRAPA AND DECELLES: EOCENE BASIN DEVELOPMENT IN NW ARGENTINA TC1015

2006b; Coutand et al., 2006; Mortimer et al., 2007; van der Beek et al., 2007] because it places constraints on cooling events at temperatures lower than those recorded by zircon fission tracks and other higher temperature thermochron- ometers [e.g., Garver et al., 1999]. Detrital AFT thermo- chronology is particularly suitable in geological settings where shallow exhumation and erosion (4–6 km; consid- ering a paleogeothermal gradient of 20°–30°C/km and a closure T of 120°C) has occurred, such as in the central Andes. [26] Samples were prepared and analyzed following the procedure described by Sobel and Strecker [2003]. An average of 100 grains for each detrital sample of the Geste Formation was dated (Data Set S1 in auxiliary material1). Confined track lengths, when present, were measured in the same grains counted for age determination. The angle between the confined track and the C-crystallographic axis (C-axis projected data) was measured in order to mitigate track-measurement bias [Barbarand et al., 2003], because confined tracks anneal anisotropically as a function of orientation [Donelick et al., 1999; Ketcham et al., 1999]. Grain shape was also determined for each analyzed grain (Figure S1 in auxiliary material). For each detrital sample, fission track grain-age distributions were decomposed fol- lowing the binomial peak-fitting method [Galbraith and Green, 1990] incorporated in the Binomfit program [Brandon, 2002]. All populations are calculated in automatic mode, in which 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 age data to a predicted mixed binomial distribution. The related best fit peaks are reported by age, uncertainty, and size (Table 3). 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) of the total (Table 3). Track length and Dpar data were compiled for each detrital population to allow comparison (Table 2). The complete AFT data set, including radial plots and probability density diagrams, is represented in Figure 7 and Table 3. 5.2. AFT Data and Interpretation

Figure 6. Conglomerate clast-composition data from the [27] Five sandstone samples from the Geste Formation Geste Formation based on 100 clast counts per location; exhibit four distinctive AFT populations (P1–P4; Table 3). arrows indicate mean flow directions based on imbrications The depositional age of each sample is determined from the (see text for explanation). For location, refer to Figure 3. youngest cluster of U-Pb zircon ages in each sample [DeCelles et al., 2007]. This approach assumes that the depositional age is equal to the zircon U-Pb age (mean or single age), which is consistent with paleontological data sample. Therefore careful measurement of Dpar values and track lengths for each population is necessary. [Pascual, 1983] and arguments for the volcanogenic air fall origin of the Eocene zircons [DeCelles et al., 2007]. Sample [25] Assuming that the apatites in the sedimentary basin never were subjected to temperatures high enough to 1SP238 only produced one Eocene grain age (39.8 ± 0.6 Ma); we used this age as a maximum stratigraphic age constraint. overprint the original thermochronological signal (80°C; discussed below), detrital AFT thermochronology provides Overall the AFT age populations can be traced up-section information about characteristic cooling ages of rocks and the youngest ones show consistent upward younging originally present in the source terrane and the timing, rates, trends (Figure 8). The AFT populations are generally older and spatial patterns of exhumation [e.g., Carrapa et al.,

2006a; Coutand et al., 2006]. Detrital AFT has received 1 increasing attention in recent years [e.g., Carrapa et al., Auxiliary material data sets are available at ftp://ftp.agu.org/apend/tc/ 2007tc002127. Other auxiliary material files are in the HTML.

11 of 19 TC1015 CARRAPA AND DECELLES: EOCENE BASIN DEVELOPMENT IN NW ARGENTINA TC1015

Table 3. Detrital AFT Populations Calculated Using Binomfit in Automated Modea

Sample Depositional Age, Ma N

Code P1 P2 P3 P4 4SP7 34.7 ± 3.5b 100 35.4 ± 3.3 51.7 ± 4.3 ND 106.3 ± 15 (UP80-10) 46.3% 51.1%ND2.6% L(n:1): 13.9 L(n:4): 13.8/0.7 ND L(n:2):11.4/1.8 Dpar/stdev: 1.9/0.2 Dpar/stdev: 1.9/0.2 ND Dpar/stdev: 2/0.1 3SP431 35.4 ± 0.55 Ma 100 35.8 ± 1.7 54 ± 5.5 64.7 ± 13.8 ND (UP80-9) 39.8% 40.2% 20.0%ND L(n:7)/stdev: 13.3/0.9 L(n:10)/stdev: 12.7/2 L(n:2)/stdev: 13.3/1.6 ND Dpar/stdev: 2.3/0.4 Dpar/stdev: 2.4/0.5 Dpar/stdev: 2.4/0.7 ND 2SP277 35.9 ± 6.4 27 46.5 ± 3.25 ND ND 112.3 ± 26.4 (UP80-5) 25.3% ND ND 1.7% L(n:11)/stdev: 13.2/0.8 ND ND ND Dpar/stdev: 2.0/0.3 ND ND ND 2SP38 37.3 ± 1.5 100 43.7 ± 3.2 56.2 ± 2.7 88.1 ± 31.1 ND (UP 80-6) 41.0% 58.0% 1%ND L(n:3)/stdev: 12.9/0.6 L(n:18)/stdev: 12.8/0.8 ND ND Dpar/stdev: 1.9/0.2 Dpar/stdev: 1.9/0.2 Dpar: 1.6 ND 1SP32 39.8 ± 0.6c 100 51.6 ± 4.1 65.7 ± 6.9d 93.3 ± 10.9 ND (UP80-3-4) 27% 63% 10%ND L(n:6)/stdev:11.6/3 L(n:27)/stdev:11.5/1.2 L(n:1): 13.0 ND Dpar/stdev: 1.9/0.2 Dpar/stdev: 1.9/0.2 Dpar/stdev:2.0/0.2 ND

aDetrital AFT populations are calculated using Binomfit from Brandon [2002]. Refer to text for mode details. N, number of grains counted; ND, no data; stdev, standard deviation; Dpar, etch pit diameter (mm). L denotes length (mm); the values reported are not corrected for c axis. bDepositional age calculated assuming a sedimentation rate of 1.1 mm/a (calculated between sample 2SP277 and 3SP431). cDepositional age is assumed equal to the youngest zircon U-Pb age of sample 1SP238 [after DeCelles et al., 2007]. dPopulation is modeled in Figure 9.

than, or equal to (for the stratigraphically higher samples), [28] The youngest population P1 is composed of early to the associated zircon U-Pb ages from the same samples. late Eocene ages (35.4 ± 3.3 Ma to 51.6 ± 4.1 Ma) and shows Detailed inspection of individual AFT ages (Figure 7) relatively short to extremely short lag times (11–0.5 Ma). indicates that some grains have mean ages younger than P1 could have different explanations discussed below. the depositional age inferred from zircon U-Pb geochronol- [29] 1. The Eocene grains reflect plutonic or volcanic ogy. However, less than 10% of the AFT ages are younger input. A plutonic origin is unlikely because no Eocene than the associated depositional age (i.e., fewer than plutons have been reported in the region. The presence of 10 grains); moreover, these young ages have errors gener- zircons in Geste Formation sandstones with Eocene U-Pb ally greater than 20%. Therefore, statistically they cannot be ages might support this interpretation, but we prefer a considered meaningful. This supports the hypothesis that volcanogenic origin for the Eocene zircons because of their the investigated samples did not suffer significant annealing fresh and angular appearance, coupled with the absence of after deposition. Had these samples been annealed after Eocene plutons in the region [DeCelles et al., 2007]. In deposition, older, more deeply buried samples would have addition, Eocene tuffs have been documented in northern experienced greater amounts of annealing and, in turn, Chile [Ramı´rez and Gardeweg, 1982; Hammerschmidt et recorded younger ages than the stratigraphically higher al., 1992; Mpodozis et al., 2005]. However, the ashfall samples. This would most likely result in detrital popula- derived zircons account for less than 5% of the total detrital tions becoming older up section, which is the opposite trend zircon age spectrum, whereas the ages belonging to P1 from the younging up-section trend observed in Figure 8. In constitute more than 30% of the detrital apatite age spec- the same way, track lengths, even though limited, do not trum in each sample. Moreover, the measured track lengths show an apparent decreasing trend down-section as would (even though limited) are not typical of instantaneously be expected if the Geste Formation apatites experienced cooled ashfall material, for which values of 15–16 mmare significant postdepositional annealing. All available con- expected. A detailed analysis of grain shapes on all counted fined track lengths were measured on double mounts for the grains (Data Set S1 and Figure S1 in auxiliary material) investigated samples (i.e., no extra length data were avail- failed to show any distinguishing shape characteristics of P1 able). Negligible postdepositional annealing (<80°C) is also grains. We therefore view volcanic input as an unlikely supported by the modeling results described in the follow- explanation for P1. ing paragraph. Therefore we consider the AFT age popula- [30] 2. The Eocene grains could represent partial thermal tions as representative of regional cooling events owing to overprinting of the apatite-bearing source rocks owing to tectonic exhumation and erosion affecting source rocks Tertiary magmatic intrusions that may not surface today. If located to the west of the study area. such a process were responsible for the Eocene AFT signal then we would expect to find a broader spectrum of Tertiary

12 of 19 TC1015 ARP N EELS OEEBSNDVLPETI WARGENTINA NW IN DEVELOPMENT BASIN EOCENE DECELLES: AND CARRAPA 3o 19 of 13 TC1015 Figure 7. Detrital AFT data from the Geste Formation. Radial plots are calculated using Trackkey program after I. Dunkl (Trackkey, windows program for calculation and graphical presentation of EDM fission track data, version 4.2, 2002, http://www.sediment.uni-goettingen.de/staff/dunkl/software/trackkey.html). Probability density plots are calculated using Binomfit [Brandon, 2002] in automated mode; refer to text for more details. TC1015 CARRAPA AND DECELLES: EOCENE BASIN DEVELOPMENT IN NW ARGENTINA TC1015

Figure 8. Lag time plot, showing AFT age populations on the x axis and depositional ages on the y axis (from Table 3). Depositional ages are based on the youngest U-Pb ages reported for the Geste Formation samples by DeCelles et al. [2007]. Refer to text for more details.

ages in the original source, characterized by younger ages youngest sample 3SP431 for which the lowest error is closer to the locus of the intrusive body and older ages reported in Table 3) and 11.8 Ma (obtained using the oldest farther away from it. Sediment derived from a partially reset sample 1SP32 for which the lowest error is reported in source terrane would contain a broad spectrum of Tertiary Table 3) we obtain exhumation rates on the order of 0.4 mm/a detrital ages, which, most likely, would not result in a to 1.0 mm/a. A lower paleo-geothermal gradient and a consistent up-section trend. higher Ta would result in even higher cooling and exhuma- [31] 3. The short lag time may indicate rapid source tion rates. We consider paleo-geothermal gradient >30°C/km terrane exhumation and cooling. In this case we would for Eocene time as unlikely. The lowest estimated exhuma- expect a general up-section decrease in population mean tion rates are similar to values reported from the Oligocene age, which is evident in P1. Moreover, the clear younging and middle-upper Tertiary in neighboring areas [Carrapa et up-section trend of P1 is consistent with the trend observed al., 2005; Coutand et al., 2006; Deeken et al., 2006] for P2, suggesting real exhumation and erosional signals whereas the highest exhumation rates are much greater than affecting either a single source characterized by different values previously reported. This could be representative of a ages belonging to populations P1 and P2 (e.g., from strong tectonic exhumation signal related to the early stages different elevations) or different sources characterized by of Andean mountain building, or an overestimate related to similar exhumation patterns. Given the objections raised to the fact that we are using detrital populations as proxies for the other two explanations, either of these two scenarios a specific exhumation age. Even considering an overesti- would seem to be equally valid. We assume a conservative mation of 50% of the maximum calculated exhumation rate, paleo-geothermal gradient of 20°C/km, which is consistent we still obtain values that are much higher than previously with modeling results presented below and with AFT data reported for the Andes. We interpret this result as a signal of and thermal modeling presented by Deeken et al. [2006]. strong tectonic exhumation related to early Tertiary Andean We use the minimum total annealing temperature of 105°C shortening and crustal thickening. (in order to limit possible overestimations), indicated by [32] Limited volcanogenic contamination remains a pos- modeling of P2 presented in Figure 9; note that P1 and P2 sibility that we cannot completely rule out and its potential apatites have the same Dpar values and therefore the same influence may have been greater in the upper (younger) total annealing temperature (Ta). A crustal thickness of samples for which lag times <5 Ma are observed. In any 5.2 km is obtained with the constraints specified above. case, the fact that locally derived coarse-grained Eocene Considering a lag time of 0.4 Ma (obtained using the conglomerates were deposited in this region unequivocally

14 of 19 TC1015 CARRAPA AND DECELLES: EOCENE BASIN DEVELOPMENT IN NW ARGENTINA TC1015

Figure 9. Modeling results of P2 in sample 1SP32 obtained using HeFty model after Ketcham [2005]; T-t constraints are explained in the text. Ta is total annealing T and T-window (gray transparent area) calculated using the oldest track age.

documents active exhumation and erosion of the sediment population might be derived from a broad spectrum of source during the Eocene. possible sources, and/or elevations, which experienced [33] P2 is composed of early Paleocene to early Eocene similar but not necessarily identical thermal histories. In ages (51.7 ± 4.3 to 65.7 ± 6.9 Ma) showing a clear younging such a case, the spectrum of track lengths in a specific up-section trend. P2 in sample 1SP32 was modeled in order population may reflect both multiple cooling events prior to to gain a better understanding of the thermal history of the exhumation and variations owing to different elevations of original sediment source and of the sedimentary basin; the the source rock, potentially overprinted by reheating due to method and results are discussed in the following section. burial and subsequent cooling. Considering that multiple P3 and P4 consist of limited Paleocene and Late to mid- heating/cooling events may have affected the original Cretaceous ages (64.7 ± 13.8 to 112.3 ± 26.4 Ma). Such source prior to deposition, correctly constraining this por- ages may correspond to rocks in the Cordillera de Domeyko tion of the cooling path in the model is challenging. Inverse fold-thrust belt undergoing deformation and exhumation modeling was performed using HeFTy [Ketcham, 2005]. during middle Late Cretaceous-Paleocene and Oligocene time [Maksaev and Zentilli, 2000; Mpodozis et al., 2005; 6.2. Track-Length Modeling Constraints and Results Arriagada et al., 2006]. [35] The initial time constraint is set at approximately double the pooled age of the sample or detrital population to 6. AFT Length Modeling ensure that the first-formed tracks are all completely annealed, thereby avoiding potential boundary condition 6.1. Method artifacts [Ketcham, 2000]. Other geologically plausible con- [34] Track-length modeling was attempted on sample straints were applied in order to test for: (1) exhumation- 1SP32 for which the most track length data per population burial of the source before deposition of the Geste Formation exist (Table 3). Inverse modeling is used to test hypotheses (65–45 Ma); (2) Eocene source exhumation (50– concerning the thermal history of the sediment source prior 37 Ma); (3) peak burial heating during deposition of the to and after deposition, and to define the likely exhumation Geste Formation (38–30 Ma) and subsequent cooling; history of the specific population source and of the host and (4) the possibility that the maximum temperature of the sedimentary basin. An important issue when modeling a deepest sample in the basin was not high enough (<80°C) to detrital population is whether the ages belonging to a single reset the AFT signal. A slight overlap of temperature-time

15 of 19 TC1015 CARRAPA AND DECELLES: EOCENE BASIN DEVELOPMENT IN NW ARGENTINA TC1015

logical evidence and therefore the output can be considered realistic.

7. Discussion and Conclusions

[38] The fluvial and alluvial deposits of the Geste For- mation in the central Puna record a response to source terrane exhumation and deformation within the plateau interior and its marginal areas during Eocene time. Sedi- mentological data indicate that the lower and middle mem- bers of the Geste Formation are the results of confined to unconfined flows in a sandy to gravelly, braided fluvial system characterized by shallow, unstable channels and overbank deposits. The upper member was deposited in alluvial fans proximal to the source terrane. Paleocurrent data document an overall eastward flow direction. The up- section coarsening of the Geste Formation suggests that topographic relief in the source area increased through time, possibly owing to enhanced tectonic activity and source terrane unroofing. [39] Sandstone petrographic data show compositions that overlap the field of continental block and recycled orogenic provenance but may be considered more typical of a recycled orogenic provenance [Dickinson, 1985; Garzanti et al., 2007]. Conglomerate clast-count data document that the source of the Eocene conglomeratic facies was com- posed of quartzose and phyllitic rocks typical of the Figure 10. Block diagram showing different, direct Ordovician metasedimentary basement (Copalayo Forma- (growth-structure apparent in seismic and field relation- tion) directly beneath the Geste Formation and in ranges to ships; black boxes) and indirect (based on coarse-grained the west. Detrital zircon U-Pb ages support the interpreta- sedimentation, proximal source, rapid syn-tectonic source- tion of Ordovician provenance [DeCelles et al., 2007]. terrane exhumation; grey boxes) evidence for wedge-top Sedimentation rates increased up-section from 0.1 mm/a deposition in the Chilean Precordillera, Puna-Altiplano to more than 1 mm/a, consistent with deposition in a wedge- Plateau, and Eastern Cordillera regions. Sources are as top depozone or proximal foreland basin setting [e.g., follows: (1) Arriagada et al. [2006]; (2) Voss [2002], Johnson et al., 1986; Jordan, 1995; Ojha et al., 2000]. Kraemer et al. [1999], and Carrapa et al. [2005]; (3) Jordan [40] AFT data indicate primarily Paleocene-Eocene (P1: and Mpodozis [2006]; (4) present work; (5) Hongn et al. 35–52 Ma; P2: 52–65 Ma) and limited mid-Cretaceous [2007]; and (6) Horton [1998]. cooling ages (P3 and P4: 88–112 Ma) of the rocks in the source terranes of the Geste sediments. The Cretaceous signal may be related to a distal source located in the constraints was applied in order to allow the model a higher Chilean Cordillera de Domeyko thrust belt, which is known degree of freedom. to have been actively shortening at that time [Mpodozis et [36] The best solution, with the highest number of good al., 2005; Arriagada et al., 2006]. The Paleocene and fits (50 good solutions), is the one presented in Figure 9. Eocene cooling ages (35–65 Ma) document active tectonic This model shows that the main cooling events occurred exhumation and suggest that the orogenic belt directly west between 90 Ma and 75 Ma; and between 60 Ma and 45 Ma of the Salar de Pastos Grandes area was in a constructional when the sample reached surface temperature. This is orogenic phase at that time. Lag times suggest that exhu- generally in agreement with provenance data documenting mation rates increased from 0.4 mm/a to 1mm/a Ordovician rocks as a source for the Geste sediments. between 40 Ma and 35 Ma, again consistent with proximity [37] Between 40 Ma and 10 Ma the sample was heated to a rapidly exhuming, tectonically active source terrane. owing to burial following deposition and subsequently [41] Overall our new data are consistent with existing exhumed. The model predicts that maximum temperature sedimentological, structural, seismic, and the thermochro- in the basin never exceeded 80°C, consistent with our nological evidence from the Salar de Atacama basin and inference that negligible fission track annealing took place Cordillera de Domeyko documenting that the Chilean after deposition of the investigated samples. Even if we do Precordillera thrust belt and areas to the east were actively not attempt to draw any major conclusions based on track- deforming since at least the Paleocene (35–65 Ma), length modeling (because of the limited number of length and possibly as early as the mid-Late Cretaceous (88– measurements), overall the exercise reproduces other geo- 112) [Mpodozis et al., 2005; Arriagada et al., 2006]. Also, our study corroborates existing data from the southern Puna

16 of 19 TC1015 CARRAPA AND DECELLES: EOCENE BASIN DEVELOPMENT IN NW ARGENTINA TC1015 and the Eastern Cordillera in Argentina and Bolivia doc- alent to the Quebrada de los Colorados Formation to the umenting proximal foreland basin sedimentation and local east (Figure 2). This implies that the orogenic wedge, which crustal shortening and uplift during the early Tertiary exhibits both thin-skinned and thick-skinned deformation [Horton, 1998; Carrapa and DeCelles, 2005; Hongn et (Figure 2c), was located between the Puna interior and the al., 2007]. Eastern Cordillera of northwestern Argentina during the [42] However, we emphasize that the regional tectonic middle to late Eocene. Similar coarse-grained deposits of context of the Geste Formation remains in question. In Oligocene-Miocene age are preserved in the Tupiza Basin southern Bolivia, Eocene deposits clearly fit into a long- complex of the Bolivian Eastern Cordillera [Horton, 1998, term Paleocene through Miocene stratigraphic sequence that 2000], directly along strike from the Salar de Pastos Grandes can be interpreted as a regional foreland basin succession area (Figures 1 and 10). As in the Salar de Pastos Grandes, [Horton et al., 2001; DeCelles and Horton, 2003], whereas Tertiary sedimentary rocks in the Tupiza Basin were depos- in the case of the Geste Formation, the deposits are isolated ited directly on top of Paleozoic rocks in a wedge-top and rest directly upon Paleozoic basement [Alonso, 1992]. depozone characterized by growth structures related to both The isolated nature of Geste depocenters, and their clear ties west- and east-verging thrusting [Horton, 1998]. to local source terranes suggest that the Geste Formation [44] Overall, what is clear from our data and other recent represents either intermontane basin deposition (i.e., it is not work in Chile, Bolivia, and Argentina is that by Eocene part of a single regional foreland basin system), or local time the Puna-Altiplano Plateau and Eastern Cordillera were wedge-top basins. Wedge-top basins are characterized by experiencing local tectonic shortening, rapid exhumation, growth structures related to progressive syndepositional and coarse-grained basin development (Figure 10). Un- deformation, local sources of coarse-grained sediment, and equivocal growth structures within Paleocene units (Naranja increasing up-section sedimentation rates on the order of Formation) in the Salar de Atacama basin [Arriagada et al., 0.5mm/a. They usually overlie proximal foredeep deposits 2006] to the west and Eocene growth structures in the La [DeCelles and Giles, 1996]. In northern Argentina, however, Poma Basin (Quebrada de los Colorados Formation [Hongn vertical stacking of regional foreland basin depozones could et al., 2007]) to the east of the study area, together with our be missing because of progressive deformation and rework- new multidisciplinary data set, suggest that the orogenic ing of older strata following thrust propagation, coupled front was migrating eastward and incorporating areas located with inherited topographic highs. The systematic up-section within the central Puna and Eastern Cordillera by Eocene decrease in dip of bedding in the Geste Formation may time. If correct, our interpretation would suggest that the indicate syndepositional growth related to a blind structure Andean orogenic strain front shifted eastward from middle beneath the Copalayo Range as suggested by Figure 2. Cretaceous through Eocene time (Figure 10), though not Overall, the Geste Formation has many of the characteristics necessarily smoothly, incorporating both thin-skinned and common to wedge-top deposits, including a 2000 m thick thick-skinned structures. upward coarsening sequence with sedimentation rates that increase up-section from 0.1 to 1 mm/a, deeply eroded [45] Acknowledgments. This work was supported by DFG (Deut- local sources, and transverse paleoflow directions. For these schen Forschungsgemeinschaft) and in part by National Science Foundation grant EAR 0710724 (Tectonics program) and a generous grant from reasons we favor the interpretation that the Geste Formation ExxonMobil. B. Carrapa gratefully acknowledges M. Strecker for invalu- represents wedge-top deposition rather than local intermon- able scientific support and the University of Potsdam for providing tane basin deposits. analytical facilities. We kindly thank Brian K. Horton, Ricardo Alonso, Daniel Starck, and Suzanne M. Kay for useful scientific discussions. [43] The presence of middle Eocene growth structures Salvatore Critelli, Eduardo Garzanti, and Marco Malusa` are kindly ac- within the Eastern Cordillera (La Poma Basin; Figure 2) knowledged for their constructive reviews. [Hongn et al., 2007] supports the wedge-top interpretation, and suggests that the Geste Formation is genetically equiv-

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