Active deformation in the Sierras de Córdoba: Observations from the Eastern Sierras

GILBERT, H. J.; RICHARDSON, T. J.; ANDERSON, M. L.; ALVARADO, P. M.; MARTINO, ROBERTO D.; BECK, S.L.; ZANDT, G.; GANS, C.

Foz de Iguazú, Brasil

Congreso; The Meeting of the Americas 2010; 2010

American Geophysical Union (AGU)

The Sierra de Córdoba in central Argentina is the easternmost extent of crustal deformation in the Sierras Pampeanas. This mountain range consists of a series of basement-involved uplifts bounded along their margins by thrust faults that juxtapose Paleozoic granitoids and gneisses over Pleistocene sedimentary strata. Questions regarding the development of these ranges include whether or not the mountains are actively deforming, if they are deforming, how that deformation is being accommodated, and if the deformation relates to the shallowly subducting Nazca plate. The two-year deployment of the Eastern Sierras Pampeanas (ESP) array of 12 broadband seismometers, which began in 2008, is designed to investigate the origin of the Sierra de Córdoba. The seismometers have recorded over 400 earthquakes within the crust of Sierra de Córdoba during the first year of deployment. This is notable for a region that has not previously been known to be seismically active. Though the magnitudes of most events have not yet been determined, they are small enough that only a few of which appear in either the INPRES or USGS catalogs. The distribution of seismicity aligns near the surface traces of several of the major east dipping thrust faults, which have accommodated over one kilometer of uplift within the Sierra de Córdoba. Toward the eastern margin of the Sierras de Córdoba, earthquakes occur within the five to 10 km depth range. Seismicity deepens further west and clusters at depths between 20 and 25 km. Earthquakes do not appear to extend to greater depths within the crust of the central portion of the Sierras de Córdoba. Focal mechanisms for the larger earthquakes indicate that the stresses associated with seismicity align along inherited lineaments in the basement, and not the orientation of surface faults, which formed more recently. This indicates that the orientation of stresses and the orientation of faults may not be related and suggests that the surface faults may result from pre-existing zones of weakness in the crust instead of current stresses. In the western portion of the Sierras de Córdoba, earthquakes cluster at mid-crustal levels, near the 20-25 km depth where, we also detect a converted phase on receiver functions calculated from ESP data. This mid-crustal phase may correspond to a detachment surface that extends further west into the western Sierras Pampeanas. Stresses could then be transmitted along this surface and drive distant shortening in the eastern Sierras Pampeanas. This suggests that while the shallowly dipping Nazca slab may not be coupled with the lithosphere of the eastern Sierras Pampeanas, a detachment zone in the mid-crust could transmit stresses from further west into the Sierra de Córdoba.