[1] 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.

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International audienceNew GPS measurements in Chiapas (Mexico), Guatemala and El Salvador are used to constrain the fault kinematics in the North America (NA), Caribbean (CA) and Cocos (CO) plates triple junction area. The regional GPS velocity field is first analysed in terms of strain partitioning across the major volcano-tectonic structures, using elastic half-space modelling, then inverted through a block model. We show the dominant role of the Motagua Fault with respect to the Polochic Fault in the accommodation of the present-day deformation associated with the NA and CA relative motion. The NA/CA motion decreases from 18-22 mm yr−1 in eastern Guatemala to 14-20 mm yr−1 in central Guatemala (assuming a uniform locking depth of 14-28 km), down to a few millimetres per year in western Guatemala. As a consequence, the western tip of the CA Plate deforms internally, with ≃9 mm yr−1 of east-west extension (≃5 mm yr−1 across the Guatemala city graben alone). Up to 15 mm yr−1 of dextral motion can be accommodated across the volcanic arc in El Salvador and southeastern Guatemala. The arc seems to mark the northern boundary of an independent forearc sliver (AR), pinned to the NA plate. The inversion of the velocity field shows that a four-block (NA, CA, CO and AR) model, that combines relative block rotations with elastic deformation at the block boundaries, can account for most of the GPS observations and constrain the overall kinematics of the active structures. This regional modelling also evidences lateral variations of coupling at the CO subduction interface, with a fairly high-coupling (≃0.6) offshore Chiapas and low-coupling (≃0.25) offshore Guatemala and El Salvador

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