Sub-slab mantle flow parallel to the Caribbean plate boundaries: Inferences from SKS splitting

Upper-mantle deformation near the margins of the Caribbean plate is investigated using observations of shear-wave splitting in teleseismic and local shear phases. The Caribbean plate is almost stationary in the hot-spot reference frame and is wedged between the North America, South America, Nazca and Cocos plates; collisional belts and major shear zones encircle the plate. Data from seismic stations operated by IRIS, GEOSCOPE, the Venezuelan Seismological Network, and the British Geological Survey have provided nearly 2000 seismic records for analysis. Analysis of shear-wave splitting in teleseismic core phases (e.g., SKS) at stations reveals fast shear-wave polarisations that are conformal to the plate boundary, paralleling the major structural features. The magnitude of the splitting is in general quite large (1.2–2.1 s). In northern Venezuela, the magnitude of splitting increases towards the Caribbean–South American collisional front. Local shear phases from earthquakes up to 200 km deep beneath northeastern Venezuela show very small amounts of splitting (0.1–0.3 s). Analysis of the depth dependence in the magnitude of the splitting suggests that most of the upper-mantle wedge is isotropic and the splitting in the local phases is mostly accrued in the crust and uppermost mantle. In NE Venezuela fast shear-wave polarisations in both the local and teleseismic phases closely parallel the fault systems in the region, suggesting that the crust and mantle are coupled in this area. Stations on the Island of Montserrat and near Bucaramanga, Columbia, also show very small magnitudes in local S-wave splitting, but large amounts of SKS splitting. In general, the small magnitude of the local S-phase splitting suggests that the teleseismic phases accrue considerable splitting beneath the top of the slab. We interpret the bulk of the splitting in terms of sub-slab flow that is forced around the Caribbean plate due to the convergence of the surrounding plates.