SECONDARY DEFORMATION ASSOCIATED WITH ACTIVE BASEMENT-INVOLVED FOLDING; LATE QUATERNARY FLEXURAL-SLIP FAULTING AND KINK-BAND MIGRATION, SIERRA DE VILLICUM, SAN JUAN, ARGENTINA

Sierra de Villicum is an actively growing fold located between the thin-skinned Precordillera and basement-involved Sierras Pampeanas tectonic provinces of west central Argentina. Surface bedrock geology is characterized by an asymmetric anticline in the hanging wall of an east-dipping, west-vergent, range-front thrust fault. Neogene sedimentary strata unconformably overly Paleozoic carbonates on both limbs of the fold. A suite of abandoned Quaternary strath terraces with thin capping gravels is preserved on the eastern, backlimb of the fold. In contrast to the west-vergent surface structures, existing crustal seismicity suggest the presence of an east-vergent, thrust that extends from 8-35 km at depth within the basement. In 1944, the devastating San Juan earthquake (Ms 7.4), produced a 6 km-long fault scarp on the backlimb with 30 cm of coseismic and 30 cm of postseismic displacement. New mapping of the scarp reveals that it and other similar faults that crosscut the strath terraces are east-dipping and parallel to bedding within the folded Neogene bedrock. The relatively short scarp length, backlimb occurrence, and correlation between bedrock dip and fault dip suggest that the surface rupture is related to secondary flexural-slip faulting. In the hanging wall, east of the 1944 scarp, a ~0.5 m-high monoclinal fold was expressed in unconsolidated gravel by an inflexion between an uplifted, flat-lying panel and a ~12° east-dipping panel. Similar monoclinal flexures are present in surfaces uplifted ~1.5 m, ~6.0 m, and ~9.5 m above the modern channel from east to west, respectively. We interpret these flexures as growth axial surfaces bounding a growth triangle on the west, which is forming by eastward migration of an active axial surface to the east. If correct, this model suggests that the lowest and youngest monocline may have formed in association with the 1944 earthquake. Any model of active deformation and associated seismic hazard in the Precordillera-Sierras Pampeanas boundary region must reconcile coseismic faulting and fold growth as well as apparently discrepant shallow crustal bedrock structure and middle to lower crustal seismicity. Existing models do not satisfy these constraints and therefore do not adequately characterize the seismic hazard posed by active structures in the San Juan region.