Chronology of Andean Orogenesis from the Sedimentary Archive: Opportunities and Challenges Using Detrital Thermochronology
Julie Fosdick, University of Connecticut
Sedimentary rocks are a critically important chronological archive of structural processes during the tectonic growth and demise of convergent orogens. As technological advances in geochronology have revolutionized sedimentary provenance analysis, double (and triple) dating of minerals using multiple chronometers has generated a diverse toolset to constrain the timing of rock deformation, metamorphism, and exhumation. However, a persisting challenge in provenance analysis is to distinguish sediment derived directly from crystalline sources versus recycled source areas. This presentation discusses existing and new work from the southern Central Andes of the western margin of South America, a long-lived Cordilleran margin. Here, the paucity of Paleogene geologic records has been interpreted to reflect a period of neutral-stress/tectonic stasis and diminished plate coupling during subduction. Revised chronology on the timing of foreland basin deposition identifies a previously unrecognized phase of late Eocene sedimentation ca. 36-33 Ma. Double-dating of detrital zircons using U-Pb geochronology by LA-ICPMS and (U-Th)/He thermochronology from the Cenozoic Bermejo retroarc foreland basin yield zircon He dates between ca. 16 and 565 Ma (n = 80). Zircon U-Pb age spectra point to the High Andes (Frontal Cordillera) as sediment sources to the Eocene retroarc depocenter, consistent with eastward-directed paleocurrent indicators. Potential primary zircon sources in the High Andes include the Late Cretaceous and Cenozoic Andean Arc, Permo-Triassic Choiyoi Group rhyolitic volcanics, and the Carboniferous-Permian Elquí-Colangüil granitic batholiths. Notably, however, our detrital data show two distinct populations of Choiyoi volcanics: (1) grains with rapid post-emplacement cooling (Permo-Triassic He dates), and (2) grains with reset and more recent cooling (Eocene He dates). We suggest that these grains represent source areas within two different tectonic units of the Choiyoi Group characterized by different thermal histories. Alternative sources for the Choiyoi Group zircons include Permian and Mesozoic (Cretaceous) strata exposed in the Precordillera. These findings highlight challenges and opportunities to distinguish sediment derived directly from crystalline source areas versus recycled source areas. In the case of the Eocene Bermejo Basin, the syndepositional Eocene He dates are consistent with recently published bedrock apatite (U-Th)/He and fission track thermochronology from the Frontal Cordillera that suggests rapid cooling of Choiyoi Group by 35 Ma and a late Eocene constructional phase in the western and eastern limits of the High Andes. We interpret the rapidly exhumed Choiyoi grains in the Eocene basin-fill as sediment derived directly from the Frontal Cordillera during a renewed phase of hinterland unroofing.
Session 6: Advances in Geology, Geochronology, Geophysics