Flat-slab subduction and continental deformation: an integrated geophysical and geological investigation of basement uplifts within the eastern Sierras Pampeanas, Argentina

RICHARDSON, T. J.; RIDGWAY, K. D.; MARTINO, R. D.; GILBERT, H. J.; ANDERSON, M. L.; ALVARADO, P. M.; CARIGNANO, C.; ENKELMANN, E.

San Francisco, California

Simposio; AGU Fall Meeting; 2009

American Geophysical Union

The Sierra de Córdoba in the eastern Sierras Pampeanas of north-central Argentina are the easternmost extent of crustal derformation in response to modern flat-slab subduction. We are combining geophysical evidence of the geometry and rate of active deformation on range-bounding faults with geological data on fault exposures and Quaternary basin sediments in order to more precisely constrain the geometry of faults controlling the development of these mountain ranges. The Sierra de Córdoba consists of a series of basement-involved uplifts bounded along their western margins by reverse faults. Geologic analysis shows that these faults juxtapose Paleozoic granitoids and gneisses over Pleistocene alluvial fan, fluvial, and aeolian strata. The faults have surface dips ranging from 22-58°, well developed fault gouge up to ~1m thick, and small asymmetric folds with westward vergence. Several of the fault scarps that we have studied are located ~6km from the main topographic front of the ranges, suggesting that the entire width of the fault zones are kilometers in scale. Backthrusts and tear faults also have an important role in the displacement budgets of these fault segments. The Eastern Sierras Pampeanas broadband seismic array, consisting of 12 seismometers spanning the Sierras de Córdoba, provides data that allows us to determine the geometry of the faults in the subsurface as well as the structure of the crust and upper mantle. These stations have detected an average of 10 local earthquakes each month in a region that has not previously been considered to be seismically active. Though the magnitudes of most events have not yet been determined, they are small enough that few appear in INPRES or USGS catalogs. Focal mechanisms for the larger earthquakes indicate that the stresses currently driving crustal deformation align with lineaments in the basement, not the orientation of surface faults. The geometry and location of surface faults may then result from pre-existing zones of weakness instead of current stresses. Preliminary correlations of earthquake locations with the surface expressions of the fault systems indicate at least three seismically active areas. One is located along the northern extent of the Sierra de Comechingones fault system while a second zone of seismicity aligns with an ancient lineament between the northern and central ranges. A third area of seismicity lies directly south of the city of Córdoba and is aligned with an unnamed fault system. Combining these geological and geophysical datasets will help to identify the surface expression of active flat slab subduction and determine how upper mantle processes in this region contribute to surface deformation.