Shear-wave splitting in northeast Venezuela, Trinidad, and the eastern Caribbean

Abstract

We present new measurements of splitting of core shear-wave phases (SKS, SKKS, PKS) recorded at seven stations in the eastern Caribbean region. Six stations in northeastern Venezuela and Trinidad constituted the SECaSA92 temporary broadband array. The seventh station, SJG, is installed on Puerto Rico. We also attempted to measure source-side splitting of S waves from earthquakes in the Lesser Antilles subducted slab, but with little success. Mean splitting parameters, fast polarization direction, φ, and delay time, δt, at the stations are consistently east-west trending and over 1 s, respectively. Delay times at the SECaSA92 array are amongst the highest yet measured, reaching 2.1 s and generally over 1.5 s. We note an approximately linear decrease in delay times here from maxima at the northernmost stations to a minimum at the southernmost. The SECaSA92 array is situated in the wide South America-Caribbean (SA-Ca) plate boundary zone, and SJG is within the North America-Caribbean (NA-Ca) plate boundary zone. In both cases, φ trends are parallel to the plate boundary zones, locally. We interpret these results to indicate that the SA-Ca plate boundary deformation extends to depths of at least 200 km, and that South American crust, lithospheric mantle, and asthenosphere are coherently deformed. The extreme delay times we measure are probably the result of transpression in the SA-Ca plate boundary and overriding of the Lesser Antilles slab by South America: compressional flattening results in extremes of foliation development, and dextral shearing yields a strong lineation in upper-mantle olivine. By contrast, the delay time at SJG, 1.2 s, is near the global average and reflects the fact that the NA-Ca plate boundary zone is purely sinistral or includes minor transtension. We note that recent counterclockwise rotation of Puerto Rico does not apparently affect upper-mantle deformation beneath the island. We suggest that this rotation is crustal, driven by far-field sinistral shear, and partly decoupled from the mantle shear. Our results are consistent with the trench-parallel mantle flow hypothesis (Russo and Silver, 1994, Science, 263: 1105–1111) and indicate that asthenospheric mantle flow beneath the Caribbean plate is eastward, and probably drives the plate's eastward motion.