Reference 1238

Abstract

The Mexican Volcanic Belt is a volcanic arc that is aligned 150 degrees counterclockwise from the orientation of the Middle American Trench instead of parallel to the trench as in typical Chilean margins. Seismicity within the arc is much lower in frequency and shallower in depth than seismicity in a typical subducting slab. A Harvard moment tensor from an earthquake M sub b = 5.4 indicates that extensional tectonism is active at a depth of about 15 km within the central Mexican volcanic belt. At least three distinct periods of volcanism and one intrusive period have been recognized in the Jungapeo area located in the central Mexican Volcanic Belt. The earliest period produced andesitic lava flows that contain pillow structures and are extensively fractured and intruded by dikes. Younger, abundant extrusions of andesitic and basaltic lava flows have fission track and whole rock K-Ar ages of 4.2+ or -0.5 Ma and 6.0+ or -0.7 Ma. The latest episode of volcanism is represented by cinder cones that are prevalent in the Jungapeo area and are thought to be of Late Pliocene to Holocene age. Three major fracture orientations N-S, NE-SW, and NW-SE have been recognized.

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