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.

Abstract Three late Cenozoic rift systems—the northwest-trending Tepic-Zacoalco rift, the east-trending Chapala rift, and the north-trending Colima rift—intersect 50 km south-southwest of Guadalajara, Mexico to form a rift-rift-rift triple junction. These rifts structurally and topographically dominate the western part of the Mexican Volcanic Belt, a predominantly calc-alkaline continental arc. The Tepic-Zacoalco rift is the site of northwest-trending, late Cenozoic normal and right-lateral strike-slip faulting, whereas the Colima rift and its offshore extension (the Manzanillo Trough) overlies the subducting Rivera fracture zone, the boundary between the Rivera and Cocos plates. Together, the Colima and Tepic-Zacoalco rifts outline the Jalisco block. Associated with the rifts are 4.7 Ma to Holocene alkaline and peralkaline volcanic rocks, which are rarely found in continental volcanic arcs but are typical of continental rift zones. The alkaline volcanic rocks of western Mexico are thought to reflect rifting of the Jalisco block away from the Mexican mainland; the Jalisco block should eventually accrete to the Pacific plate. This rifting event represents the latest in a series of northward-propagating spreading-ridge jumps along the East Pacific Rise. Ongoing rifting in southwest Mexico may serve as a model for the rifting of Baja California, and for the origin of microplates and exotic terranes.

Colima volcano is situated at the western edge of the Mexican volcanic belt within the Colima rift zone. This contribution presents new geochemical and Sr-Nd-Pb-O isotope data for Colima volcano rocks and plutonic xenoliths found in prehistorical lava flows. Colima volcano magmas display strong subduction signatures (positive peaks of Ba, K, Pb, and Sr, and negative anomalies of Nb and Ti) and were generated in a depleted mantle source and emplaced at crustal levels (garnet-free zone), where they experienced fractional crystallization of plagioclase and pyroxene. Gabbroic and granitoid xenoliths found in prehistorical lava flows show evidence for partial melting and are considered to be representative of the basement beneath Colima volcano. At upper-crustal levels, Colima volcano magmas were contaminated by granitoids, like those of the nearby Cretaceous Manzanillo and Jilotlán Batholiths. Sr-Nd isotope ratios of these intrusives are nearly identical to those of Colima volcano lavas. For that reason assimilation of the granitic crust is not detectable in diagrams of these isotopic systems but can be clearly seen in a ϵ Nd versus δ 18 O plot. In comparison to other large Mexican volcanic belt stratovolcanoes, Colima volcano lavas display the least evolved geochemical and isotopic signatures of this arc.