Fore-arc Motion and Cocos Ridge Collision in Central America

We present the first regional surface velocity field for Central America, showing crustal response to interaction of the Cocos and Caribbean plates. Elastic half-space models for interseismic strain accumulation on the dipping subduction plate boundary fit the GPS data well and show strain accumulation offshore and beneath the Nicoya and Osa peninsulas in Costa Rica but not in Nicaragua. Since large subduction zone earthquakes occur in Nicaragua, we suggest that interseismic locking in Nicaragua and some other parts of Central America occurs but is mainly shallow, depth, too far offshore to be detected by our on-land GPS measurements. Our data also show significant trench-parallel motion for most of the region, generally interpreted as due to oblique convergence and strong mechanical coupling between subducting and overriding plates. However, trench-parallel motion is also observed in central Costa Rica, where plate convergence is normal to the trench, and in the Nicaraguan fore arc, where trench-parallel motion is fast, up to 9 mm a−1, but mechanical coupling is low. A finite element model of collision (as opposed to subduction) involving the aseismic Cocos Ridge also fits the GPS surface velocity field, most significantly reproducing the pattern of trench-parallel motion. We infer that buoyant, thickened CNS-2-Cocos Ridge crust resists normal subduction and instead acts as an indenter to the Caribbean plate, driving crustal shortening in southern Costa Rica and contributing to trench-parallel fore-arc motion in Costa Rica and perhaps Nicaragua as a type of tectonic escape