Earthquake focal-plane mechanisms and well-bore breakouts in the Long Valley caldera, California, indicate that the resurgent dome and caldera south moat are characterized by a northeast extensional stress field, consistent with geodetically determined extensional strain within the caldera. Similar data from the western caldera indicate that it is characterized by a markedly different, northwest-trending, extensional stress field. We hypothesize that this localized rotation of the stress field is possible because of near-lithostatic pore pressure at depth. Because an east-west extensional stress field appears to have existed in the western caldera at the time of emplacement of the Inyo volcanic deposits (500-1000 yr ago), the state of stress in the Long Valley caldera appears to be both spatially and temporally heterogeneous, most likely as a consequence of intracaldera processes related to magmatic resurgence.

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