TECTONICS, VOL. 27, TC1015, doi:10.1029/2007TC002127, 2008 Click Here for Full Article Eocene exhumation and basin development in the Puna of northwestern Argentina 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 Cretaceous-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. Sandstone petrography and conglomerate 1. Introduction clast-count data document quartzose and phyllitic compositions typical of Ordovician 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 basement 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 TC1015 1of19 TC1015 CARRAPA AND DECELLES: EOCENE BASIN DEVELOPMENT IN NW ARGENTINA TC1015 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 Miocene 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 Jurassic–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 Antofalla; 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 Andean orogeny [Steinmann, 1929]. Other authors [Hartley scientific issue of whether the Puna was occupied by a 2of19 TC1015 CARRAPA AND DECELLES: EOCENE BASIN DEVELOPMENT IN NW