CHRONOLOGY OF MOTION IN A COMPLETE THRUST BELT - THE PRECORDILLERA, 30-31-DEGREES-S, ANDES MOUNTAINS

In the Precordillera thrust belt of west-central Argentina, faulted, rotated, and onlapping relations of piggyback-basin units and foreland basin-units yield an accurate but imprecise chronology of thrust motion. Addition of data from provenance history, strata accumulation rates and presence of thick conglomerate units in the Bermejo foreland basin results in a thrust chronology that is more precise, but not entirely independent of assumptions about foreland basin dynamics. Seven new radiometric dates allow substantial revision of the interpreted age of first deformation in the Precordillera at about 30-degrees South. Units previously assigned to the Paleozoic and Mesozoic are of Oligocene and Miocene ages. A redbed unit that began before 21.6 +/- 0.8 Ma appears to have pre-dated thrusting. Deposits in the early Bermejo foreland basin were dominated by eolian sandstones, unlike the younger fluvial and alluvial units. An andesitic volcanic center within the Miocene foreland basin had long-term effects on subsidence and local paleogeography. All the Precordillera's six major E-verging thrust faults moved between about 20 Ma and present. The two westernmost faults moved before 19.5 Ma and continued to move until, or moved again episodically after, 19.5 Ma; one moved after approximately 14 Ma. The middle thrusts probably moved initially by 11 Ma. The second fault from the east began to move at approximately 10 Ma, and the easternmost by 5 Ma. These data imply that simultaneous motion on several thrusts maintained the wedge taper of Coulomb wedge deformation models. Most of the uncertainty in thrust ages in the Precordillera is due to in incomplete preservation of growth strata relations and paucity of datable material.