Lithospheric deformation overlying a shallowly subducting slab: insights from the Eastern Sierras Pampeanas seismic array (Invited)

Deformation in the overriding plate above areas of shallow subduction extends well away from the plate margin. In the case of the Sierras Pampeanas in central Argentina, the pattern of deformation is the culmination of stresses originating from the shallowly subducting Nazca plate and rheological variations of South American lithosphere. The boundaries of several terranes that combined to form South America divide this part of the continental crust. At greater depth, the Nazca slab subducts horizontally at a depth close to 100 km before resuming its descent into the mantle at a steeper angle. Over 200 km to the east of the point where the Nazca slab steepens, the Sierras de Córdoba marks the easternmost extent of crustal deformation, leading to questions regarding the origin of the forces responsible for driving their uplift. The Sierras de Córdoba consist 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 the Sierras de Córdoba include whether or not the mountains are actively deforming, and if they are deforming, how that deformation is being accommodated. Identifying the means of accommodation may provide insight into the origin of the mountain range. The Eastern Sierras Pampeanas (ESP) array of 12 broadband seismometers, which was deployed from 2008-2010 in the Sierras de Córdoba, has provided data for locating over 400 earthquakes within the crust of Sierras de Córdoba during the first year of deployment. Though the magnitude of the majority of these earthquakes are small enough that they do not appear in regional or global catalogues, they do indicate ongoing deformation within the mountain range. The distribution of seismicity aligns near the surface traces of several of the major east dipping thrust faults on which over one kilometer of uplift has occurred. Toward the eastern margin of the Sierras de Córdoba, earthquakes occur within the five to 10 km depth range. In the western portion of the Sierras de Córdoba, earthquakes cluster at mid-crustal levels, near the 20-25 km depth range, which is near the depth of a converted phase on receiver functions calculated from ESP data. Earthquakes do not appear to extend to greater depths within the crust of the Sierras de Córdoba. The seismicity in the 20 km depth range, along with the mid-crustal receiver function arrival, appears to correspond to a detachment surface that extends further to the west. Stresses could be transmitted along this surface and drive shortening in the Sierras de Córdoba. 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 transmits stresses from further west, where the slab is shallower, into the Sierras de Córdoba. The distribution of deformation in the South American plate may then reflect terrane boundaries with a strong terrane to the west of the Sierras de Córdoba transferring stresses eastward while resisting deformation.