Crustal structure of the central Lesser Antilles island arc : seismic near-vertical and wide-angle profiling
In the context of the EU funded THALES project active seismic investigations were conducted along the Lesser Antilles subduction zone, using both, wide-angle and near-vertical techniques. The structure of the Caribbean plate and the subducting Atlantic plate was determined through a combined analysis of ocean bottom seismometer and multi-channel seismic data along a transect between 15°N and 17°N. A detailed analysis of a priori and a posteriori information of a Monte Carlo based tomography scheme with a subsequent structural evaluation by forward modeling are applied to a 2-d seismic profile south of Guadeloupe. The island arc crust of the Caribbean plate has an average thickness of 28±2 km and overrides the approximately 8 km thick late cretaceous (80 Ma) Atlantic crust. The island arc crust shows two distinct reflections, which are interpreted as an intracrustal and Moho discontinuity, respectively. To a depth of 40 km the Atlantic lithosphere subducts at an average angle of 40°. The accretionary prism gains a thickness of more than 10 km and shows high energy attenuation in the middle prism. A backstop structure is imaged by a strong lateral increase of the vertical velocity gradient superimposed on structural heterogeneities. The decollement zone reaches a thickness of up to 1.8 km and shows no seismic phase polarity change. The slow convergence rate and the subduction of the Tiburon and Barracuda ridges make this subduction zone a prime candidate for major earthquakes. Between 15°N and 17°N, the seismogenic rupture area is approximated by the contact zone between the backstop and the mantle wedge. Accordingly, this area is capable of triggering an earthquake of a magnitude of Mw 7.6 and indeed makes this active European subduction zone a place of major telluric risk.