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

Abstract

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.

Slab tears and intermediate‐depth seismicity

“…The North America plate is obliquely subducting beneath the Caribbean plate along the obtuse northern corner of the Lesser Antilles arc, northeast of Puerto Rico and the Virgin Islands. Subduction under an obtuse corner is expected to create geometric complications and as some have suggested, segmentation of the slab [ ten Brink and López‐Venegas , ; ten Brink , ; Dillon et al ., ; McCann and Sykes , ; Meighan and Pulliam , ; Meighan et al ., ]. An actively propagating slab tear is suggested to be the dominant process affecting local tectonics, based on the trench's bathymetry and negative gravity anomaly, stress changes along the trench corner related to seafloor asperities, timing of the trench collapse, seismic anisotropy, continuous Global Positioning System (GPS) vector analyses, and seismic swarm activity [ ten Brink and López‐Venegas , ; ten Brink , ; Meighan and Pulliam , ; Meighan et al ., ].…”

“…Furthermore, seamounts are resistant to subduction [ Gutscher et al ., ] and stress modeling has confirmed that large tensile stresses have developed within the slab, down dip of the seamount [ ten Brink , ; Toda and Stein , ]. Mantle flow patterns inferred from crustal seismic anisotropy studies, generally parallel the Caribbean plate boundaries [ Meighan and Pulliam , ; Piñero‐Feliciangeli and Kendall , ; Russo et al ., ; Russo and Silver , ]. One exception was observed off the NE corner of Puerto Rico where flow (inferred from crustal anisotropy) is trench normal and suggests passage of mantle material through a gap in the slab [ Meighan and Pulliam , ].…”

“…Mantle flow patterns inferred from crustal seismic anisotropy studies, generally parallel the Caribbean plate boundaries [ Meighan and Pulliam , ; Piñero‐Feliciangeli and Kendall , ; Russo et al ., ; Russo and Silver , ]. One exception was observed off the NE corner of Puerto Rico where flow (inferred from crustal anisotropy) is trench normal and suggests passage of mantle material through a gap in the slab [ Meighan and Pulliam , ]. Counterclockwise rotations inferred from GPS observations and the occurrence of historic and recent seismic swarms off the NE corner of Puerto Rico imply that the subducted portion of the North American plate is actively tearing at this location [ ten Brink and López‐Venegas , ; Meighan et al ., ].…”

Dip variations of the North American and North Caribbean Plates dominate the tectonic activity of Puerto Rico–Virgin Islands and adjacent areas

“…The locations of the North American subduction slab and North Caribbean subducting slab are separated discussions in previous studies (McCann, ; Mendoza and McCann, ; Meighan and Pulliam, ; Meighan et al ., ; ten Brink, ). However, we consider the tectonic activity in this area to be controlled by both subduction zones.…”

“…ten Brink and Granja Bruña () proposed that bivergent thrusting exists on both sides of the PRVI. In addition, previous research (ten Brink, ; Meighan and Pulliam, ) on the southward subduction of the North American Plate suggested that the North American subduction slab splits in the eastern PRVI. So far, however, no models have been attempted to include the influence of the North Caribbean subducting slab because of few evidences, although the North Caribbean subduction zone and North American subduction zone could both have affected the tectonic evolution of the PRVI.…”