The timing and paleogeography of early Andean mountain building are topics of ongoing debate. We track development of the early central Andes in Bolivia and NW Argentina by studying widespread Tertiary deposits in the Altiplano-Puna and Eastern Cordillera. These deposits accumulated on top of Late Cretaceous post-rift marginal marine facies and Precambrian-Paleozoic basement. Over a north-south distance of 1000 km, the Paleocene-Oligocene succession consists of three stratigraphic assemblages: (1) Paleocene-Eocene fluvial- lacustrine siltstone and marl, up to ~200 m thick; (2) a 10-100 m thick zone of stacked Eocene paleosols, including stage III-IV Calcisols, Vertisols, and strongly reduced Gleysols; and (3) an upward coarsening, several km-thick sequence of fluvial to alluvial fan deposits. Locally, upper Eocene-Miocene rocks consist of thick (>2 km), proximal alluvial fan deposits containing growth structures. Modal petrographic data indicate derivation from metasedimentary and plutonic source terranes, and paleocurrent data show eastward sediment transport. Detrital zircon U-Pb ages indicate derivation from Paleozoic low-grade metasedimentary and igneous source rocks. Eocene detrital AFT ages from apatite grains that yield early Paleozoic U-Pb crystallization ages require rapid exhumation (0.4 mm/yr to >1mm/yr) of western source areas in neighboring regions such as the Eastern Cordillera and indicate a constructional orogen. Combined with previous structural studies, our data are consistent with a Late Cretaceous-Eocene thrust belt in the Cordillera de Domeyko in northern Chile, flanked to the east by a several hundred km wide foreland basin system. Flexural subsidence dominated the proximal region, and back-bulge and post-rift thermal subsidence may have operated in the distal eastern part of the basin. Approximately 500 km of eastward migration of the foreland basin system produced the vertical succession preserved in the Paleogene of the Puna and Eastern Cordillera. Shortening estimates for NW Argentina combined with simple flexural modeling raise the issue of the possible whereabouts of a roughly 300 km long slab of underthrust South American lower crust and lithosphere.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 1213 Earth's interior: dynamics (1507;
- 8102 Continental contractional orogenic belts and inversion tectonics;
- 8110 Continental tectonics: general (0905);
- 8169 Sedimentary basin processes