Depth-dependent anisotropy in the Caribbean

The tectonics of the Caribbean plate is controlled by its interaction with four other plates, the North American (NA), the South American (SA), the Nazca and the Cocos. Deformation along the east and the west boundaries is controlled by subduction - the Atlantic oceanic plate subducts beneath the Lesser Antilles trench in the east and the Cocos plate subducts beneath the Middle America trench in the west. The northern boundary is characterized by dextral strike-slip faulting to the east and a minor spreading center, the Cayman trough, in the west. The southern boundary zone is a complex and wide deformation zone consisting of a right-lateral transpressional fault connecting trench zones at either end where the Caribbean plate subducts beneath continental South America in the west and oceanic South America subducts beneath continental South America in the east. Our aim is to use observations of anisotropy to help characterise the style of mantle flow in the Caribbean region. We present preliminary estimates of seismic anisotropy obtained from shear-wave splitting analyses of SKS, SKKS and local S-phases. SKS and SKKS splitting has been analysed at the Caribbean IRIS stations, SDV (Santo Domingo, Venezuela), JTS (Costa Rica) and BOCO (Bogota, Colombia). The polarisation of the fast shear-wave correlates well with surface structural features, suggesting at first glance a continuity in mantle and crustal structures. However, a 90 degree periodicity in shear-wave splitting parameters with backazimuth is clearly observed at SJG, and to a lessor extent at SDV, implying depth-dependent variation in mantle anisotropy. Forward modelling suggest a 2-layer model of anisotropy with polarization directions of φ _1=246^o and φ _2=270^o and delay times of δ t1=1 s and δ t2=0.5 s. The lower layer aligns with the APM direction and the upper layer with the surface geology at the station. To gain further insight into variations in anisotropy with depth we analyse S--phases recorded on the Venezuelan Seismological Network in the northern eastern part of Venezuela. Given the limited number of stations but an abundance of seismicity, many events lie outside the shear-wave window. To better exploit this dataset we use a predictions of near-surface anisotropy and the associated free--surface effects to guide the interpretation of S-phases splitting outside the shear-wave window. Preliminary analysis suggests an average polarisation direction of 81 degrees, which corresponds well with the major structural features present in the area (e.g., the El Pilar fault) and with previous studies of SKS and SKKS phases done by Russo et al. (1993). The results to date suggest that the crust and mantle are coupled during deformation, but that deeper anisotropy may reflect current mantle flow patterns.