Subduction seismicity and tectonics in the Lesser Antilles arc.

We have studied the mechanisms of 17 earthquakes along the Lesser Antilles subduction zone to examine a site where very old lithosphere subducts at a slow convergence rate. No large thrust earthquakes occurred during the 1950-1978 study period; the three large (magnitude seven) events are all normal faults. One is a normal faulting event seaward of the trench. A second indicates extension within the slab at depth. These observations suggest that subduction in this region is primarily decoupled and aseismic unless the time interval studied is unrepresentative. The third normal fault earthquake occurred within the upper plate with fault planes perpendicular to the arc and trench. This unusual geometry may represent a flexural response to the subduction of the Barracuda Ridge, a major bathymetric high with uncompensated excess mass at depth which seems analogous to flanking ridges found along some Mid-Atlantic Ridge fracture zones. Thus, the Barracuda Ridge is not buoyant and does not affect Benioff zone dip. Strike slip faulting occur at depth in the subduction zone along a concentration normal to the arc and may indicate a fossil fracture zone. There is no direct evidence in the shallow seismicity for the hypothetical N America-S America-Caribbean triple junction.-from Authors