A major, ongoing episode of continental rifting has formed a system of three intersecting grabens in the western Mexican volcanic belt. These structures may mark the initial stage of an eastward spreading-ridge jump beneath continental crust. Eastward ridge-segment jumps have been propagating northward along this part of the East Pacific Rise for the past 12 m.y., but this is the first plate-boundary reorganization event to involve continental crust since the separation of Baja California in the Miocene. Since Pliocene time, small volumes of unusual alkaline magmas have erupted in the graben areas in close association with the more abundant, subduction-related calc-alkaline magmas erupted from andesitic stratovolcanoes. The alkaline magmas are varieties found in zones of active rifting elsewhere in the world.

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