The elusive Rivera-Cocos plate boundary: not diffuse

Abstract

Abstract Inland and offshore gravimetric determinations in the Southern Colima rift in western Mexico allowed for the construction of a Bouguer anomaly map of the area. Four submarine canyons of tectonic relevance are located in the offshore area. Gravimetric models of the oceanic subducting slab were calculated from the residual Bouguer anomaly along six trench-parallel lines and three trench-perpendicular lines including the area of the canyons. The former lines show considerable distortion of the slab that we attribute to compression, while the latter show distinct dipping angles of the oceanic slab at distances of around 75 km from the trench: the westernmost line shows a dip angle of 55° between depths of 20 and 70 km, while the easternmost shows a dip angle of 32° at depths between 18 and 50 km. We submit that the former represents a section of the subducting Rivera plate, and the latter represents a section of the Cocos plate. Extracting coordinates of representative points on the surface of the nine slab models allowed for a reconstruction of the slab surface: the transition from the Rivera to the Cocos plate is marked by topographic gradients in the modelled slab surface suggesting the trajectory of the boundary between the plates. We propose a tectonic model that includes a transpression zone involving the marine platform and the Southern Colima rift and a transtension zone in the Northern Colima rift: the Colima Volcanic Complex is located in the transition zone between them.