Abstract

The south-trending Northern Colima Graben intersects two other grabens, the northwest-trending Zacoalco Graben and the east-trending Chapala Graben, to form a rift-rift-rift triple junction 50 km south-southwest of Guadalajara in southwest Mexico. The Northern Colima Graben represents the northernmost extension of the 190-km-long, 20- to 65-km-wide Colima Rift, which overlies the projection of the subducting boundary of the Rivera and Cocos plates and represents a rift of probable Pliocene-Pleistocene age of the east-trending Mexican Volcanic Belt (MVB). Associated with the northernmost 90 km of the rift, there are 4.6-m.y.-old to Holocene alkaline volcanic rocks rarely found in continental volcanic-arc environments and which are more typically associated with either continental intraplate rifting or oceanic islands. These alkaline lavas have erupted side by side with 10-m.y.-old to Holocene calc-alkaline lavas that represent typical arc volcanism. It is proposed that the grabens are associated with the initiation of a rifting of a portion of southwest Mexico away from the Mexican mainland, similar to the rifting of Baja California away from the Mexican mainland. The calc-alkaline K-Ar dates are consistent with post–mid-Miocene development of east-trending MVB volcanism.

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