COCOS RIDGE COLLISION: TECTONIC ESCAPE AND MOUNTAIN BUILDING IN CENTRAL AMERICA

We present the first regional surface velocity field for Central America, showing the crustal response to interaction of the Cocos and Caribbean plates. Our data show significant trench-parallel motion for most of the region, including central Costa Rica where plate convergence is perpendicular to the trench. Interseismic strain accumulation is observed in the outer forearc Nicoya and Osa Peninsulas, but not in the forearc of Nicaragua. However, large subduction zone earthquakes occur in Nicaragua (e.g., September 2, 1992, Mw 7.6). We propose that interseismic locking in Nicaragua and some other parts of Central America is mainly shallow, <20 km depth (most subduction seismogenic zones extend to ~50 km depth), too far offshore to be detected by on-land sensors. We present a collision (rather than subduction) model involving CNS-2 - Cocos Ridge crust; young, hot-spot thickened crust, and compare our model results and geodetic observations to geological and geophysical data for the region. CNS-2 – Cocos Ridge crust resists normal subduction, instead acting as an indenter to the Caribbean plate, driving crustal shortening in southern Costa Rica at rates of ~35 mm yr-1. The indenter, rather than oblique convergence, drives trench-parallel forearc motion in Costa Rica and Nicaragua at rates up to 14 mm yr-1.