The regional subsurface structure of Long Valley (California) caldera fill from gravity and magnetotelluric data

Abstract

Long Valley caldera, in eastern California, is a deep, basinlike feature bounded by steeply dipping normal faults having vertical offsets of at least several kilometers and is largely filled with effusive volcanics and lacustrine deposits. Interest in delineating its substructure has escalated over the past decade for a number of reasons; for example, there is increasing evidence that patterns of recent seismicity and volcanism may be controlled by pre-existing basement structures. In addition, the highly porous fill of the caldera basin forms the major hydrologic element in the region. For these and other reasons, characterizing the subsurface geometry of Long Valley caldera is an important objective of current geophysical studies. This paper reports a combined gravity and magnetotelluric interpretation which reveals a close similarity between a simple 3-D gravity model and a telluric thin sheet interpretation. This underscores the fact that both the gravity and the MT data are largely influenced by the same geologic elements: the caldera fill, basement topography, and the major boundary faults. Moreover, as in earlier MT interpretations, a conductive zone is seen beneath the southwest moat. In addition, however, because of new data, we now have evidence for a buried resistive ridge cutting across the main body of the caldera from the northwest to the southeast beneath the resurgent dome. Interpreting this resistive feature as a basement structural high (that is, a horst block ) is compatible with limited drilling data, and it appears to be the same feature that has been described as a central platform from previous gravity studies. The presence of such a horst may be fundamentally related to the structural evolution of the resurgent dome.