Abstract

Seismic tomographic images from the MARS experiment (Mapping Rivera Subduction zone) show that below a depth of 160 km there exists a gap between the Rivera and Cocos slabs that increases in width with depth. Teleseismic SKS and local S wave splitting parameters indicate that the main source of anisotropy resides in the entrained mantle below the young and thin Rivera plate. The curved pattern of fast SKS split directions in the western Jalisco block and the Rivera-Cocos slab gap indicates 3-D toroidal mantle flow around the northwestern edge of the Rivera slab and the Rivera-Cocos gap. Both the tomographic images and shear wave splitting results support the idea that the Rivera and western Cocos plates not only moved in a down-dip direction but also have recently rolled back toward the trench and that the Colima rift is intimately related to the tearing between the Rivera and Cocos plates.