REFOLDING OF THIN-SKINNED THRUST SHEETS BY ACTIVE BASEMENT-INVOLVED THRUST FAULTS IN THE EASTERN PRECORDILLERA OF WESTERN ARGENTINA

A new model of active crustal deformation across the transition from thin- to think-skinned thrusting in western Argentina is developed for the eastern Precordillera northwest of San Juan, site of a devastating earthquake in 1944. Sierra de Villicum is a west-vergent thrust sheet bound on the northwest by the Villicum thrust, which juxtaposes Cambro-Ordovician carbonates in the hanging wall with Neogene red beds in the footwall. A series of Late Pleistocene fluvial terraces developed across the Villicum thrust show no evidence of active fold or fault deformation. In contrast, terraces are deformed by folds and faults in the middle of the southeastern flank of Sierra de Villicum, which comprises a southeast dipping panel of Cambro-Ordovician and Neogene strata that extends southeast to the Tulum Valley syncline and includes the La Laja fault on which roughly 6 km of surface rupture occurred in the 1944 earthquake. Surface rupture and fault dip indicate the La Laja fault is a flexural slip fault, a style of faulting related with fold growth. Folds in terraces have geometry consistent with kink-band models for fold growth. These observations indicate that surface deformation in the 1944 earthquake was dominated by folding, implying a blind thrust fault source of the earthquake. A southeast-facing, southwest-plunging monocline characterizes the Neogene red beds in the region of active folding. A blind basement reverse fault model of active deformation reconciles the zone of terrace deformation, coseismic surface rupture on the La Laja fault, refolding of the Villicum thrust sheet, a basement arch between the Precordillera and eastern Precordillera, and microseismicity that extends northwestward from a depth of ~5 km beneath Sierra de Villicum to ~35 km depth. Horizontal rates of shortening are estimated to be 2.6 to 4.6 mmyr-1 from the terrace fold model and correlation of the terraces with dated terraces located to the southwest of the study area. Basement rocks beneath Cerro Salinas, another eastern Precordillera thrust sheet to the southwest, are also characterized by an east-facing monoclinal geometry, which suggests that blind thrust faulting on east-vergent basement faults represents a significant, underappreciated seismic hazard in western Argentina.