Basement Structure and Implications for Hydrothermal Circulation Patterns in the Western Moat of Long Valley Caldera, California

Abstract

Detailed surface mapping, subsurface drill hole data, and geophysical modeling are the basis of a structural and hydrothermal model for the western part of Long Valley caldera. Six fault zones are recognized in the western caldera with dominant orientations of north, northwest, and northeast, sub‐parallel to regional fault trends in the surrounding Sierran basement. The internal structural geometry of the cores of 12 exogenous domes inside and outside the caldera suggests that the domes erupted at the intersections of these principal fault trends rather than along the axis of a single dike. Gravity modeling and subsurface data from deep geothermal wells indicate that the floor of the caldera is segmented into a number of discrete fault blocks with varying offsets. One of the northeast trending fault zones, designated Discovery fault in this paper, appears to be part of the original Sierran embayment that existed before caldera collapse. Recent hydrothermal alteration occurs along Discovery fault strands and composite vertical offset of intracaldera volcanic units across the entire fault zone may be as much as 400 m. Field relationships, geophysical interpretations, and interpretaions of drill hole data suggest that this fault is a fundamental flaw in the western caldera. The preexisting tectonic framework of the basement rocks controlled the configuration of the western caldera floor and, through it, the location of postcollapse eruptive centers. These deep basement structures may also provide the high fracture density which controls circulation in the present geothermal system of Long Valley.

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

“…Beneath the site of the Unocal Clay Pit well (Figure 1), the conductive layer is only 100-200 m in depth, but this corresponds well with Early Rhyolite tuffs also according to outcrop and the well cuttings (Bailey et al, 1976;Suemnicht and Varga, 1988). The conductance of the layer overlying this uplifted block is substantially less than in the west moat, and the east one for that matter, possibly because Pleistocene Long Valley Lake may have dwelt relatively briefly at this upper elevation during early resurgence and deposition of Early Rhyolites (Bailey et al, 1976).…”

“…The upper approximately 400 m of the west moat with high resistivities of several hundred ohm-m meters is comprised of Late Quaternary basalt flows and the underlying upper Early Rhyolite flows, with local moat rhyolites (Bailey et al, 1976;Suemnicht and Varga, 1988;Eichelberger et al, 1988). Eastward over the central graben of Smokey Bear Flat, such high resistivities are more restricted laterally and in depth and appear to coincide with outcropping Early Rhyolite flows also (Bailey et al, 1976).…”

“…Nordquist (1988) was able to show that low resistivities persisted to unusually deep levels beneath the axial graben, but the high quality remote reference data taken for this study have allowed resolution of upper and lower members. The depth of the lower conductor is near the base of the Bishop Tuff in this area (Suemnicht and Varga, 1988). Candidates to explain it may include precaldera volcanics such as seen in Unocal well 44-16 or perhaps Paleozoic carbonaceous metasediments represented by the Mount Morrison roof pendant observed to be conductive in the Mammoth-l hole some 4 km south (Park and TorresVerdin, 1988).…”

“…This is Downloaded 05/15/15 to 130.132.123.28. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/ The west moat conductive layer over the depth interval 400-700m in the model is too shallow to lie within the Bishop Tuff, although it may include the uppermost unwelded units of the Tuff where present (Suemnicht and Varga, 1988;Eichelberger et al, 1988). Instead, most of the layer appears to lie overhead in Early Rhyolite tuffs.…”

“…The conductance of the layer overlying this uplifted block is substantially less than in the west moat, and the east one for that matter, possibly because Pleistocene Long Valley Lake may have dwelt relatively briefly at this upper elevation during early resurgence and deposition of Early Rhyolites (Bailey et al, 1976). The high resistivities modeled at greater depth are from welded Bishop Tuff, Sierran plutonic rocks, and possibly a post-caldera intrusive stock (Suemnicht and Varga, 1988).…”