Seismic structure beneath the Rivera subduction zone from finite-frequency seismic tomography

The subduction zone of western Mexico is a unique region on Earth where microplate capture and overriding plate disruption are occurring today. The young, small Rivera plate and the adjacent Cocos plate are subducting beneath the Jalisco block of Mexico. Here, we present a P wave tomographic model of the upper mantle to 400 km depth beneath the Jalisco block and surrounding regions using teleseismic P waves recorded by the Mapping the Rivera Subduction Zone (MARS) and Colima Volcano Deep Seismic Experiment (CODEX) seismic arrays. The inversion used 12,188 P wave residuals and finite-frequency theory to backproject the 3-D traveltime sensitivity kernels through the model. Below a depth of 150 km, the tomography model shows a clear gap between the Rivera and Cocos slabs that increases in size with depth. The gap between the plates lies beneath the northern part of the Colima graben and may be responsible for the location of Colima volcano. The images indicate that the deep Rivera plate is subducting more steeply than does the adjacent Cocos plate and also has a more northerly trajection. At a depth of about 100 km, both the Rivera and Cocos slabs have increased dips such that the slabs are deeper than 200 km beneath the Trans-Mexican Volcanic Belt (TMVB). It is also found that the Rivera plate is at roughly 140-km depth beneath the young central Jalisco Volcanic lineament. Our images suggest that the Rivera plate and westernmost Cocos plate have recently rolled back toward the trench. This scenario may explain the unusual magmatic activity seen in the TMVB.