Evidence for multiple boundary faults beneath the northwest moat of Long Valley Caldera: Magnetotelluric results

Abstract

The boundaries of Long Valley Caldera, in Eastern California, are sharply delineated by steeply dipping normal faults which, for the most part, are buried and obscured by recent volcanic and sedimentary cover. The only evidence for their presence is from their geophysical signatures on regional gravity, seismic and electromagnetic surveys. Recently, a surge of interest has focused studies in the western moat, where the volcanism is most recent, and where many workers feel that the geothermal potential is highest. A detailed, narrow‐band (20–80 s) magnetotelluric profile in that area has delineated a parallel set of buried high angle normal faults which have virtually no expression at the surface. The outermost fault has a throw of approximately 500 m, while the innermost has a throw of approximately 2 km; neither has an expression in the surface morphology. Moreover, based on a relatively low resistivity contrast between the basement and the basin fill, we infer a relatively high fracture porosity in the Sierran basement, which suggests that there may be considerable lateral and vertical migration of meteoric water; a result which has considerable significance to understanding the regional hydrology.

Magnetotelluric transect of Long Valley caldera: Resistivity cross‐section, structural implications, and the limits of a 2-D analysis

“…The Bishop Tuff, most of which is moderately to densely welded, appears resistive overall (cf. Hermance and Neumann, 1988).…”

“…If some off-profile coupling between the axial graben conductors and other caldera structures in the upper 2-3 km can be discovered to reproduce 5-10 degrees of anomaly in <Pxy at 100 s, then there seems no need to invoke a mid-crustal conductor to explain the long period TE data. However, simple channeling of currents from conductive fill in the northern or southern parts of the caldera (Hermance et al, 1988;Park and Torres-Verdin, 1988) should not be responsible and in fact should actually amplify Pxy (e.g., Jones, 1983).…”

“…That MT data was of variable quality, however, demonstrating the need for very precise observations to uncover any subtle, deeper structures of value. In the vicinity of our MT transect, principal axes of apparent resistivity presented by Park and Torres-Verdin and telluric ellipses given by Hermance et al (1988) also indicate a general north northwest preferred orientation to resistivity structure. This direction simply mimics the regional structural grain of the eastern front of the Sierra Nevada, which imposed the dominant control of many structures internal to the caldera (Bailey et al, 1976;Bailey, 1987).…”

“…Reconnaissance bipole-bipole apparent resistivities (Stanley et al, 1976) show the greatest upper crustal conductances in the east moat of the caldera, and in a confined area to the south of the resurgent dome, but with a general north northwest preferred trend to the contours. Analysis combining long-period (20 s) telluric ellipses and gravity data by Hermance et al (1988) revealed north northwest-elongate conductive bodies in both the east and west moats of the caldera, as well as the prior confined conductor in the south. Also interpreted by Hermance et al was a high-level resistive block cutting across the caldera center from southeast to northwest.…”

Electromagnetic Induction Studies

“…Long Valley caldera has seen extensive MT and active source EM work to explore for hydrothermal and magmatic energy resources. Hermance et al , [1988] and Hermance and Neumann [1988] have continued to interpret long period telluric results in terms of caldera fill conductance and possible boundary faults. Park and Torres‐Verdin [1988] carried out a fully 3D interpretation of industry‐donated MT data over the caldera, concluding that the data forbid highly anomalous deep crustal structure below the upper 2 km of variably conductive post‐caldera volcanics, but that more subtle structures were not resolvable given the limited data quality.…”