Seismic anisotropy beneath the northeastern Caribbean: implications for the subducting North American lithosphere

Active plate boundaries in the Caribbean form a complex tectonic environment that includes transform and subduction zones. 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 existence of tears, windows, and gaps in subducting slabs has been proposed at various locations around the world but few have been confirmed. Since mantle flow and crustal deformation are believed to produce seismic anisotropy in the asthenosphere and lithosphere, searching for changes in, for example, SKS splitting parameters can help identify locations at which subducting slabs have been disrupted. Several lines of evidence support the notion of a slab tear within the subducting North American plate at this transition zone, including the counter-clockwise rotation of the Puerto Rico microplate over the past ~10 Ma, clusters of small seismic events, and trench collapse initiating ~3.3 m.y. Here we present results from a detailed investigation of seismic anisotropy from 28 stations across six networks in the Northeast Caribbean that support the hypothesis of a significant slab gap in the vicinity of the U.S. and British Virgin islands. A regional synthesis of our results reveals fast shear wave polarizations that are generally oriented parallel to the plate boundary with intermediate to high SH-SV delay times. For example, polarization directions are oriented roughly NE-SW along the bulk of the Lesser Antilles, E-W along the Puerto Rico trench and the northern Lesser Antilles, and NW-SE beneath Hispaniola. Beneath the U.S. and British Virgin Islands, however, the fast polarization direction differs markedly from the regional pattern, becoming almost perpendicular to the plate boundary. Stations on Anegada, British Virgin islands and St. Croix, U.S. Virgin islands show a fast polarization direction that is oriented nearly NNE-SSW and smaller delay times than surrounding stations. These results suggest that mantle flow is redirected NE-SW at this location through a gap in the subducted lithosphere of the North American plate.