Seismic analysis of a slab tear in the Northeast Caribbean.
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Meighan, Hallie E.
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Active tectonic regions where plate boundaries transition from subduction to strike-slip can take several forms, such as triple junctions, acute and obtuse corners. The Caribbean-North American plate boundary is one such active margin, where subduction transitions from arc- to oblique-type off the northeast coast of Puerto Rico. Understanding mantle flow in this region will not only help determine the nature of tectonic activity and mantle dynamics that control these margins, but will also aid our understanding of the fate of subducting lithosphere. The forearc region of the northeast Caribbean plate north of Puerto Rico and the Virgin Islands has been the site of numerous seismic swarms since at least 1976. A six-month deployment of five ocean-bottom seismographs recorded two such tightly-clustered swarms, along with additional events. Focal mechanism solutions, calculated from a joint analyses of the OBS and land-based seismic data, reveal that the swarms generally have strike- and dip-slip mechanisms at depths of 50-100 km, while events at depths of 100-150 km have oblique mechanisms. A stress inversion reveals two distinct stress regimes, which supports the hypothesis that the subducting NOAM plate is tearing at or near these swarms. Global and well-documented slab tears associated with high rates of intermediate-depth seismicity are discussed here. Seismicity at these locations occurs, at times, in the form of swarms or clusters and various authors have proposed that each marks an active point of tear propagation. The swarms and clusters start at the top of the slab below the asthenospheric wedge and extend 30-60 km vertically downward within the slab. We interpret these swarms and clusters to be generated by dehydration embrittlement of mantle rocks. Focal mechanisms of these swarms generally fit the shear motion that is thought to be caused by the tearing process.