RAPID THRUST PROPAGATION AND EXHUMATION OF FORELAND BASIN FILL IN THE PRECORDILLERA THRUST BELT, FLAT-SLAB SEGMENT OF THE SOUTHERN CENTRAL ANDES, ARGENTINA

Rapid late Miocene advance of thin-skinned thrusting during shallowing of the subducting Nazca plate produced large-scale exhumation of foreland basin fill in the Precordillera of west-central Argentina. The structural evolution of the central segment of the Precordillera fold-thrust belt is better constrained by new analyses of clastic nonmarine deposits preserved in three intermontane regions between major east-directed thrust faults. We focus on the uppermost Oligocene–Miocene basin fill in the axial to frontal Precordillera at 31–32°S along the Río San Juan (Albarracín and Pachaco sections) and the flank of one of the leading structures (Talacasto section). These three successions record hinterland construction of the Frontal Cordillera, regional arc magmatism, and initial exhumation of Precordillera thrust sheets. Provenance shifts recorded by detrital zircon U-Pb age populations suggest that initial east-west shortening in the Frontal Cordillera coincided with an early Miocene shift from eolian to fluvial accumulation in the adjacent foreland basin. Upward coarsening of fluvial deposits and increased proportions of Paleozoic clasts reflect cratonward (eastward) advance of shortening into the Precordillera and resultant structural fragmentation of the foreland basin into isolated intermontane segments. Apatite (U-Th)/He thermochronometry of basin fill constrains to 12–9 Ma the most probable age of uplift-induced exhumation and cooling of Precordillera thrust sheets. This pulse of exhumation is evident in each succession, suggestive of rapid, large-scale exhumation by synchronous thrusting above a single décollement linking major structures of the Precordillera fold-thrust belt.