Seismic imaging of the Cocos plate subduction zone system in central Mexico

Abstract

Broadband data from the Meso-America Subduction Experiment (MASE) line in central Mexico were used to image the subducted Cocos plate and the overriding continental lithosphere beneath central Mexico using a generalized radon transform based migration. Our images provide insight into the process of subducting relatively young oceanic lithosphere and its complex geometry beneath continental North America. The converted and reverberated phase image shows complete horizontal tectonic underplating of the Cocos oceanic lithosphere beneath the North American continental lithosphere, with a clear image of a very thin low-velocity oceanic crust (7–8 km) which dips at 15–20 degrees at Acapulco then flattens approximately 300 km from the Middle America Trench. Farther inland the slab then appears to abruptly change from nearly horizontal to a steeply dipping geometry of approximately 75 degrees underneath the Trans-Mexican Volcanic Belt (TMVB). Where the slab bends underneath the TMVB, the migrated image depicts the transition from subducted oceanic Moho to continental Moho at ∼230 km from the coast, neither of which were clearly resolved in previous seismic images. The deeper seismic structure beneath the TMVB shows a prominent negative discontinuity (fast-to-slow) at ∼65–75 km within the upper mantle. This feature, which spans horizontally beneath the arc (∼100 km), may delineate the top of a layer of ponded partial melt