Cocos plate structure along the Middle America subduction zone off Oaxaca and Guerrero, Mexico: Influence of subducting plate morphology on tectonics and seismicity

Two new bathymetric and magnetic surveys are presented from which the history and recent tectonics of the Cocos plate off the Middle America subduction zone are determined. The East O'Gorman fracture zone, a previously proposed outer rise feature, is not present along the Oaxaca trench outer rise near the trench axis.; Several parallel ridges of seamounts are entering the subduction zone 15–20° from orthogonal to the trench axis. These ridges lie roughly parallel to the spreading direction and were created as off-axis volcanism. The southern Mexico trench outer rise exhibits reactivation of the inherited abyssal-hill and trench-parallel faults to accommodate extension from plate flexure. Plate boundary forces also produce a new unusual family of normal faults parallel to and flanking the seamounts. An anomalous increase in outer rise earthquakes accompany these faults indicating they are seismically active.; Trench-parallel extension related to the geometry of the trench axis bend and increasing plate convergence angle may contribute to their genesis. Unusual outer-rise faulting off of the Japan trench (Kobayashi et al., 1998) which is coincident with a trench axis bend is used to argue that specific conditions at subduction zones may activate inherited fracture-zone-parallel weakness. This controls fault orientation at the Japan and Mexico outer rises.; Seafloor morphology and seismicity evidence leads to a “slivered” ocean-crust model that is broken along the seamount-parallel faults at the subduction zone. This accounts for the consistent rupture geometry (40–100 km fault failure) and the uniformity in the character of the thrust waveforms. Limited magnitudes of shallow thrust earthquakes appear to be a consequence of the subducted slivering crust.; A review of regions whose structures and forces are similar to those present in the subducting Cocos plate off Oaxaca indicates that crustal slivering is not unique. Forces other than a trench axis bend must be present to furnish the entirely unique seamount-parallel outer-rise faulting. Either/both convergence direction and spreading-parallel inherited weakness orientation may determine outer-rise fault strike. Methods for resolving ambiguous data are proposed.