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 a e 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;

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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.