Constraints on mantle flow at the Caribbean-South American plate boundary inferred from shear wave splitting

We measured shear wave splitting from SKS and SKKS data recorded by temporary stations deployed as part of the Broadband Onshore-Offshore Lithospheric Investigation of Venezuela and the Antilles Arc Region project and the national seismic network of Venezuela. Approximately 3000 station-event pairs yielded similar to 300 with visible SKS and/or SKKS phases. We obtained 63 measurements at 39 of the 82 stations in the network using the method of Silver and Chan (1991) and conventional quality criteria. We combined our results with previous measurements made by Russo et al. (1996). The most prominent feature in the data is an area of large (> 2.0 s) lag times with roughly east-west fast axes in northeastern Venezuela. Mineral physics models show split times this large are difficult to explain with horizontal foliation, but are more feasible with anisotropy characterized by a coherent vertical foliation and an east-west fast axis extending over most of the upper 250 km of the mantle. We interpret the large split times in northeastern Venezuela as a consequence of eastward translation of the Atlantic slab, which has left a strong vertical foliation in its wake parallel to the plate boundary. The peak split times correspond closely with the point the slab intersects the base of the anisotropic asthenosphere at 250 km. Away from this area of large split times the measured times fall to more standard values, but an east-west fast axis still predominates. We suggest this is linked to the rapidly varying strain field at the southern edge of the Atlantic which quickly disrupts the coherent strain field that causes the very large split times in northeastern Venezuela.