Gravity and Fault Structures, Long Valley Caldera, California

Abstract

The main and catastrophic phase of eruption in Long Valley occurred 0.73 m.y. ago with the eruption of over 600 km3 of rhyolitic magma. Subsequent collapse of the roof rocks produced a caldera which is now elliptical in shape, 32 km east west by 17 km north south. The caldera, like other large Quaternary silicic ash flow volcanoes that have been studied by various workers, has a nearly coincident Bouguer gravity low. Earlier interpretations of the gravity anomaly have attributed the entire anomaly to lower density rocks filing the collapsed structure. However, on the basis of many additional gravity stations an supporting subsurface data from several new holes, a much more complex an accurate picture has emerged of caldera structure. From a three dimensional inversion of the residual Bouguer gravity data we can resolve discontinuities that seem to correlate with extensions of pre caldera faults into the caldera dn faults associated with the ring fracture. Some of these faults are believed related to the present day hydrothermal up flow zone and the zone of youngest volcanic activity within the caldera.