TRACKING THE NEOGENE TECTONIC EVOLUTION OF THE ACTIVE ANDEAN THRUST FRONT USING THE FORELAND BASIN RECORD AND DETRITAL GEO-/THERMOCHRONOLOGY, WESTERN ARGENTINA (31-32ºS)

The Neogene Bermejo Basin of west-central Argentina is a classic example of a seismically active broken foreland basin system associated with present day flat slab subduction tectonics. The region is comprised of a thin-skinned fold-thrust belt in the Central Precordillera and basement-involved uplifts of the Sierras Pampeanas that partition the once continuous foreland basin. The kinematic relationship between the Sierras Pampeanas, the Central Precordillera, and the enigmatic west-vergent Eastern Precordillera thrust front remains debated. End member tectonic models for Eastern Precordillera deformation include: (1) thin-skinned deformation at 8-6 Ma associated with the Central Precordillera fold-thrust belt to the west, and (2) thick-skinned deformation at <5 Ma linked to Sierras Pampeanas deformation to the east. Our study seeks to constrain the along-strike Neogene tectonics of the Eastern Precordillera by integrating new structural, stratigraphic, detrital zircon U-Pb geochronology and sediment provenance, and detrital apatite (U-Th/He) thermochronology datasets from the Bermejo Basin. New data from two ~1.7 km thick Neogene stratigraphic sections in southern Bermejo Basin constrain deposition between ~12-5 Ma with dominant fluvial-lacustrine mudstones, siltstones and sandstones that transition into fluvial/alluvial fan conglomerate facies tracking the eastward migration of Andean deformation and transition from foredeep to wedge-top depositional systems. Provenance data reveal an up-section unroofing sequence of the Central Precordillera demonstrated by greater input of Paleozoic sedimentary sources. Detrital apatite thermochronology ages from the base of the sections record a mean age of 6.5 Ma. When we compare new data with published data from northern Bermejo Basin, we observe (1) a north-south decrease in stratigraphic thicknesses from > 5 to < 2 km, and (2) along strike increase in exhumation ages from 2-5 Ma in the north to 6-8 Ma in the south. Our results suggest the southern segment of the Eastern Precordillera is kinematically linked to thin-skinned thrust structures in the Central Precordillera, whereas to the north structures appear to be linked to the thick-skinned Sierras Pampeanas, demonstrating complex along-strike variability of thrust-front evolution.