CONICET | Buscador de Institutos y Recursos Humanos

The South American subduction zone has extreme examples of active flat-slab subduction and is believed to be an analog for subduction that occurred during the Cretaceous-Eocene age ("Laramide") mountain building events in the Western U.S. This region is therefore ideal for gaining a better understanding of shallow slab subduction and its influences on deformation of the surrounding mantle and overriding crust. Shear wave splitting analysis is used to test a model for the direction of mantle flow beneath the Eastern Sierras Pampeanas (ESP) in the Sierras de Córdoba region of Argentina to better understand the dynamics of flat-slab subduction. This study may also contribute to our understanding of the role the slab plays in deforming the overriding crust. The results of Anderson et al. (2004) indicate that the seismic fast directions underlying Chile and western Argentina are oriented N-S, or trench parallel. To the east, under the Sierras Pampeanas and coincident with a segment of flat subduction, the seismic fast direction is E-W, or trench perpendicular. Anderson et al. formulated several hypotheses to explain this apparent heterogeneity in the anisotropy. One explanation is that the retrograde motion of the subducting slab, caused by the westward movement of the overriding slab, prohibits E-W mantle flow, thus causing an overall N-S flow direction and the observed N-S oriented fast directions. The E-W oriented fast directions would then result from anisotropy due to mantle material being drawn into the area vacated by the slab as it is flattened. If this is the case, E-W fast directions should only be measured at stations directly above the flat slab. As alternative interpretations, the detected heterogeneous anisotropy may be due to strong lithospheric anisotropy or differences in the hydration state in the mantle. In the case of hydration state, E-W fast directions would be expected in the backarc south of the flat slab segment, where normal subduction occurs but did not appear due to a lack of data in this area. Further data and analysis is necessary to test the current hypotheses. To test the retrograde slab hypothesis, one needs measurements both over the flat-slab segment, and to the north and the south of the segment. We therefore use the seismic data that has been collected from 12 broadband seismometers of the ESP network between -33° and -29° south and -64° and -67° west. The stations in this network have been strategically placed above the slab where it starts steeply plunging back down into the mantle, within the bedrock of the N-S trending Sierras de Córdoba, providing a densification and north- and southward extension of the data used in Anderson's research. We use the MatLab program SplitLab to examine the SKS waveforms for shear wave splitting and determine the seismic fast directions and lag times. The ESP network is still in place; yet it has already recorded several significant teleseismic earthquakes for our analysis, and we present preliminary results from this subset of the data.