Abstract

The Tomochic caldera, located in western Chihuahua, is one of perhaps 350 calderas in the Sierra Madre Occidental volcanic field. Its resurgent dome, sedimentary moat, ring-fracture zone rhyolitic flow domes, and remnants of its structural margin are strikingly well preserved. Four widespread ash-flow formations are exposed around the structural and topographic caldera walls. The caldera appears to have formed ∼31 Ma in response to the eruption of the Rio Verde tuff, the youngest of these formations. An older caldera complex, exposed along the northern rim of the Tomochic caldera, may be the source for both the Vista and Aeropista tuffs, which erupted 33.5 and 34.5 Ma respectively. The source of the remaining formation (San Felipe tuff) is not known. The structure of the Tomochic caldera is similar to that of the Valles caldera, except that the resurgent dome is composed of post-caldera intrusive and extrusive porphyritic quartz latite rather than intracaldera tuff. The Tomochic caldera displays a resurgent style intermediate between that of calderas that resurge by doming of the caldera-fill tuff and those that respond to resurgence by extrusion of copious amounts of lava. Field evidence indicates that the Rio Verde tuff is the result of as many as 15 ash-flow eruptions. The multiple episodes of collapse following these eruptions may have fractured the intracaldera tuff and underlying rocks so extensively that they could not contain the magmatic pressure imposed during resurgence.

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