Structure and hydration of the subducting plate offshore Nicaragua

In summer 2003 a seismic pilot experiment was conducted offshore Nicaragua across the Middle American Trench, where the Cocos plate subducts under the Caribbean plate, in order to investigate the influence of subduction related bend-faulting on the hydration and serpentinization of the incoming lithosphere. Seismic wide-angle data were collected along a transect which extends from the outer rise region not yet affected by subduction into the trench northwest of Nicoya Penninsula, in the region cut by normal faults. A tomographic inversion of the P-wave velocities reveals an anomalously low seismic velocity zone in the lower crust and uppermost mantle at the trench. Crustal velocities are reduced by 0.2-0.5 km/s compared to normal mature oceanic crust. Seismic velocities of the uppermost mantle are 7.6-7.8 km/s, what is 5-7 % lower than the typical velocity of mantle peridotites. These systematic changes in P-wave velocity from the outer rise towards the trench axis indicate an evolutionary process that is consistent with percolation of seawater through the faulted and fractured litosphere and serpentinization of mantle peridotites.