Apatite triple dating and white mica 40Ar/ 39Ar thermochronology of syntectonic detritus in the Central Andes: A multiphase tectonothermal history B. Carrapa1, P.G. DeCelles2, P.W. Reiners2, G.E. Gehrels2, and M. Sudo3 1Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA 2Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA 3Universität Potsdam, Institut für Geowissenschaften, 14476 Golm, Germany ABSTRACT We applied apatite U-Pb, fi ssion track, and (U-Th)/He triple dating and white mica 40Ar/39Ar thermochronology to syntectonic sedimentary rocks from the central Andean Puna plateau in order to determine the source-area geochronology and source sedimentary basin thermal his- tories, and ultimately the timing of multiple tectonothermal events in the Central Andes. Apa- tite triple dating of samples from the Eocene Geste Formation in the Salar de Pastos Grandes basin shows late Precambrian–Devonian apatite U-Pb crystallization ages, Eocene apatite fi s- sion track (AFT), and Eocene–Miocene (U-Th)/He (ca. 8–47 Ma) cooling ages. Double dating of cobbles from equivalent strata in the Arizaro basin documents early Eocene (46.2 ± 3.9 Ma) and Cretaceous (107.6 ± 7.6, 109.5 ± 7.7 Ma) AFT and Eocene–Oligocene (ca. 55–30 Ma) (U-Th)/He ages. Thermal modeling suggests relatively rapid cooling between ca. 80 and 50 Ma and reheating and subsequent diachronous basin exhumation between ca. 30 Ma and 5 Ma. The 40Ar/39Ar white mica ages from the same samples in the Salar de Pastos Grandes area are mainly 400–350 Ma, younger than apatite U-Pb ages, suggesting source-terrane cooling and exhumation during the Devonian–early Carboniferous. Together these data reveal multiple phases of mountain building in the Paleozoic and Cenozoic. Basin burial temperatures within the plateau were limited to <80 °C and incision occurred diachronously during the Cenozoic. INTRODUCTION ern Argentina using the apatite U-Pb, apatite fi s- answering the following questions, which have The provenance, geochronology, and thermal sion track (AFT), and apatite (U-Th)/He meth- implications for paleogeographic reconstruc- history of syntectonic sedimentary rocks pro- ods. We also apply 40Ar/39Ar thermochronology tions and tectonic models of Andean evolution. vide valuable information about the location, on detrital white micas from the same samples in (1) What is the geochronological source-terrane age, and exhumation history of source terranes order to determine the mid-temperature cooling signature of Eocene sedimentary rocks? (2) Are and dynamics of orogenic processes (e.g., Ber- history of the detritus. The closure temperatures Eocene AFT ages widespread within the pla- net et al., 2001; Najman et al., 2001; Carrapa et of these systems are ~450–550 °C for apatite teau, and do they represent regional exhuma- al., 2003; Hodges et al., 2005). Although much U-Pb (e.g., Flowers et al., 2007), ~350 °C for tion, rather than magmatic input? (3) What is the progress has been made during the past decade white mica 40Ar/39Ar, ~120–60 °C for AFT (e.g., magnitude of basin burial (heating) and subse- toward routine detrital thermochronology, the Green et al., 1989), and ~80–60 °C for apatite quent exhumation, and is basin exhumation syn- combination of multiple thermochronological (U-Th)/He (e.g., Farley, 2000). chronous within the plateau? (4) Did the Central and geochronological methods on individual The Central Andes have been the site of arc- Andes undergo pre-Cenozoic exhumation and, detrital grains (multidating) is still in its infancy. related and foreland basin deposition since the if so, when, and what were the magnitude and The advantage of multidating is that a high-tem- Paleozoic, and therefore are an ideal place in cause? In order to answer all these questions, a perature method can reveal the crystallization which to investigate the thermal effects of multi- multidating approach covering a large tempera- age and a lower-temperature method can reveal ple orogenic phases. In the Central Andes, docu- ture window (~500–60 °C) is necessary. the cooling and exhumation age of a grain, thus mented Cenozoic exhumation rates are ~0.2 mm/ providing valuable information about source and yr (e.g., Carrapa et al., 2005, 2006; Deeken et al., GEOLOGICAL BACKGROUND basin histories. Although several studies using 2006; Coutand et al., 2006). However, recently The region defi ned as the Puna Altiplano, multiple chronometers on individual zircon published AFT detrital thermochronologic data or central Andean plateau, is characterized by grains have been published (Rahl et al., 2003; document relatively rapid exhumation rates (0.5 high mean elevation (>3500 m), internal drain- Campbell et al., 2005; Reiners et al., 2005; Ber- to >1 mm/yr) during Paleocene–Eocene time age, and aridity resulting from geodynamic net et al., 2006; van der Beek et al., 2006), this is (Carrapa and DeCelles, 2008), coeval with con- and surfi cial processes related to convergence the fi rst work using three methods on individual tractional deformation. It remains unknown if between the Nazca and South American plates detrital apatite grains. the Central Andes underwent earlier phases of since the mid-Cretaceous (e.g., Isacks, 1988; We present triple dating of detrital apatite grains rapid exhumation, because 40Ar/39Ar ages do not Allmendinger et al., 1997; Strecker et al., 2007). from Eocene syntectonic sedimentary rocks of record early Cenozoic signals. Rocks now in the central Andean plateau record the Geste Formation in the Pastos Grandes and With this study we demonstrate the unique deposition in a backarc basin during the Cam- Arizaro basins in the Puna plateau of northwest- power of the detrital multidating approach by brian–Ordovician, a foreland basin during the © 2009 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, May May 2009; 2009 v. 37; no. 5; p. 407–410; doi: 10.1130/G25698A.1; 3 fi gures; Data Repository item 2009103. 407 68°W 67° 66° 24°S ple 2SP38 because of its abundant high-quality B apatites and lowest (i.e., deepest) stratigraphic C position, which provides the best constraints on Silt Sand Pebble Boulder basin burial and exhumation history. 2000 Three cobbles from an ~500-m-thick sec- Salar Pocitos 4SP700 tion of fl uvial, eolian, and alluvial fan deposits of equivalent Geste Formation in the Arizaro Salar de basin (Fig. 1), farther to the west, were selected, Pastos Grandes Salar de Pastos Grandes Macon Range and apatites were analyzed for fi ssion track Salar de Arizaro 1500 and (U-Th)/He ages (Table DR2). We also ana- 40 39 Nevados de Palermo lyzed detrital white micas for Ar/ Ar ther- 25° mochronology from two samples, one from the lowest part (1SP32) and one from the top LEGEND A Peru 3SP431 Bolivia Mean paleocurrent (4SP700) of our measured sections in the Salar Salar de direction 16 S de Pastos Grandes basin (Fig. 3C). We picked Hombre Muerto Conglomerate 1000 > 3 km those two samples to check for possible strati- elevation Eolian sandstone 20 S 40 39 Sandstone graphic shifts in the detrital Ar/ Ar signatures. e l i 24 S h Partially covered We analyzed 19 grains from sample 1SP32 and C Study Siltstone area 25 grains from sample 4SP700 by single fusion Phyllite 28 S Argentina Pacific 3SP431 Sample site 2SP277 analysis (Fig. 2C). One grain from each sample 0 20 2SP238 Ocean 500 500 was selected for step-heating analysis (Table km 74 W 66 W 26° TG408 DR3). Multikinetic inverse thermal modeling Holocene salt lakes and Neogene Cambrian 2SP38 salt flats plutonic rocks sedimentary rocks of fi ssion track and (U-Th)/He ages was applied Alluvium undifferentiated volcanic rocks Paleozoic plutonic rocks to the three cobbles from the Arizaro basin and Upper Miocene-Pliocene Undifferentiated Ordovician: sedimentary TG190 andesites, dacites and basalts Cenozoic rocks, local volcanic rocks one sandstone from equivalent strata in the Salar Cretaceous Precambrian/Cambrian Geste Formation Salar de Arizaro Neogene ignimbrites Geste Formation sedimentary rocks plutonic rocks TG41 de Pastos Grandes basin (Figs. 1 and 3). (For Carboniferous 1SP32 Neogene basalts Precambrian/Cambrian 0m 0m sedimentary rocks sedimentary and igneous Cambrian- rocks 1SP0 details regarding analytical methods, see the Pleistocene-Holocene Late Paleozoic Precambrian undifferentiated volcanic rocks plutonic rocks Faults Study areas granites Ordovician Data Repository.) Figure 1. A: Map of central South America showing location of study area in northwestern RESULTS Argentina. B: Geological map of southern Central Andes modifi ed after Reutter et al. (1994). Apatite U-Pb ages of P1 and P2 grains are C: Stratigraphic column of Geste Formation in the Salar de Pastos Grandes (modifi ed after DeCelles et al., 2007) and Arizaro basins. New paleocurrent data, from this study, are shown almost exclusively between 500 Ma and 1000 to the right of the Arizaro stratigraphic column. Ma (Fig. 2A; Table DR1). Only a single grain from P1 yielded a Cenozoic age, but this age is much younger than the depositional age, sug- Early Devonian, a continental rift during the north-south–trending Macon Range, which is gesting signifi cant Pb loss. These late Precam- Early Cretaceous, and a foreland basin again composed of Cambrian and Precambrian grani- brian and early Paleozoic U-Pb apatite ages are during the Cenozoic (Jordan and Alonso, 1987; toid rocks (Fig. 1). slightly younger than zircon U-Pb ages from Isaacson and Díaz-Martínez, 1995; Rapela et the same samples (DeCelles et al., 2007), as al., 1998; Sempere, 1995; DeCelles and Horton, METHODS expected for a lower closure temperature. Dat- 2003; Carrapa and DeCelles, 2008). We selected 76 apatites, belonging to AFT ing of the same apatites by (U-Th)/He reveals The Salar de Pastos Grandes basin, in the populations P1 and P2 (43.7 ± 3.2 and 56.2 ± Eocene–late Miocene ages.