3D teleseismic travel time tomography along the Lesser Antilles subduction zone

Abstract

This study presents a 3D teleseismic P-wave travel time tomography model along the Lesser Antilles subduction zone. Located on the eastern boarder of the Caribbean plate the Lesser Antilles island arc formed during the last 40 Ma as a consequence of subduction of the relatively westwards moving North American and South American plates underneath the Caribbean plate. The location of the hereby generated triple junction is debated in current research and will be subject of this study. A joint inversion methodology is applied combining global and regional tomographic approaches. As an initial model the entire Earth’s mantle in discretized into spherical blocks with smaller block sizes for the area of investigation. Ray paths are calculated using a 3D pseudo-bending ray tracing algorithm yielding absolute travel times. They are correlated with absolute velocities of the subsurface by performing a LSQR inversion. The data consists of a filtered version of the global EHB catalogue and a manually processed set of 2604 travel times recorded on OBS stations deployed during the V oiLA project as well as on permanent land stations in the Caribbean region. Resolution of the model space is assessed by checkerboard tests and reconstruction of a synthetic slab model. The tomographic image of the upper mantle shows a continuous arcuate high velocity zone ranging from Puerto Rico along the Lesser Antilles island arc to northeastern Venezuela. This is attributed to the westward subduction of Atlantic lithosphere beneath the Caribbean plate. In contrast to previous studies the model does not show a tear in the subducting plate beneath the Lesser Antilles. Dip angles of the descending plate range from almost vertical in the southern Lesser Antilles to 40 ◦ - 50 ◦ along the bent of the island arc to 60 ◦ in the northwestern part. Extensive areas of increased velocity are mapped in the mantle-transition zone beneath the Caribbean Sea and are attributed to remnants of Proto-Caribbean crust.