A general model for tectonic control of magmatism: Examples from Long Valley Caldera (USA) and El Chichón (México)

The relationship of volcanoes to regional tectonic setting and earthquake dynamics is intimate. We know that eruptions are often triggered by earthquakes, and that volcanoes generally lie along or near major faults or within faulted provinces. It has been generally found that bimodal basaltic-rhyolitic volcanism is associated with extensional settings, presumably because of the creation of accommodating space. For intermediate arc volcanoes, tectonic settings are generally compressional or transpressional. The spatial pattern of faulting indicates that Long Valley was focussed by a releasing bend in the transten-sional, Sierran range-front fault system. The temporal pattern of offset rates suggests that the zone of greatest activity has migrated to the NW through time, and is now focussed at the Mono-Inyo Craters. The southern Mexi-can volcanic arc presents an example of the coexistence of regional compressional and transpressional structures with volcanoes. On an event basis, slip on regional structures creates opportunities for magma movement and eruption, in a type of dynamic fault pumping of fluids. Both kinematically and dynamically, volcanic activity may be completely dependent on tectonic factors for accumulation, storage and eruption of magma.