A core hole in the southwestern moat of the Long Valley Caldera: Early results

Wollenberg, H.A.; Sorey, M.L.; Farrar, Christopher D.; White, Art F.; Flexser, S.; Bartel, L.C.

doi:10.1029/eo068i020p00529-02

Cited by 23 publications

(7 citation statements)

References 6 publications

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“…Lookout Mountain in the northeast. Evidence from drilling [Wollenberg et al, 1986;Suemnicht, 1987] and modeling studies [Sorey, 1985] suggest that circulation in the caldera's geothermal system is fracture related and depends on deep circulation in the precaldera basement and intracaldera fill. In the western caldera moat, fault intersections of northeast trending Discovery faults and northwest trending resurgent dome faults provide a promising setting.…”

Section: Exposuresmentioning

confidence: 99%

“…This region is characterized by unusually deep MT resistivity lows which may be related to the upwelling part of the caldera's geothermal system [Nordquist, 1987]. Peripheral drilling [Wollenberg et al, 1986;Suemnicht, 1987] establishes this area as one of the hottest parts of the entire caldera. The inherited deep basement structures of the western caldera may provide the high fracture density and deep permeability necessary for circulation in the present geothermal system in Long Valley.…”

Section: Exposuresmentioning

confidence: 99%

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

Suemnicht¹

1988

J. Geophys. Res.

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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.

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Section: Exposuresmentioning

confidence: 99%

Section: Exposuresmentioning

confidence: 99%

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

Suemnicht¹

1988

J. Geophys. Res.

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“…The active involvement of mafic magma in the 600-year-old Inyo Craters rhyolitic eruptions suggests that more heat was delivered to the shallow crust beneath the west moat of Long Valley caldera than might be expected from the rhyolite alone. The recency of this thermal replenishment may explain the relatively high temperatures encountered in drill holes in the west moat of Long Valley [Suemnicht, 1987;Wollenberg et al, 1987], as well as some of the seismic and geophysical anomalies encountered there [Dawson et al, 1987, Nordquist, 1987. The diversity of magmas involved in the 600-year-old eruption attests to the complexity of the magmatic plumbing system in the west moat of Long Valley caldera.…”

Section: Discussionmentioning

confidence: 99%

Evidence for 600 year‐old basalt and magma mixing at Inyo Craters Volcanic Chain, Long Valley Caldera, California

Bailey¹,

Suemnicht²

1990

J. Geophys. Res.

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Andesite inclusions are found within the vent areas of two 600‐year‐old rhyolite domes in the Inyo Craters volcanic chain, eastern California. Both domes lie within the present northwestern sector of 0.73 Ma Long Valley caldera. Inclusion morphology indicates that they were partially molten and ductile during incorporation within coarsely porphyritic rhyolite. The xenocryst assemblage within the inclusions (plagioclase + hornblende + biotite + quartz ± sanidine) is identical to the phenocryst assemblage within the host rhyolite. Microprobe analyses of xenocrysts within the andesite inclusions show that they have compositions similar to phenocrysts in the host rhyolite. These data suggest that the andesite inclusions are the result of mixing of the coarsely porphyritic rhyolite with a more mafic end member. Whole rock geochemistry of the inclusions generally supports a simple mixing model between the coarsely porphyritic rhyolite of the Inyo domes and typical postcaldera mafic lavas found throughout the western caldera moat. The inclusions provide the first evidence for involvement of basaltic magma in the 600‐year‐old rhyolite eruptions of the Inyo Craters volcanic chain and suggest the possibility that the rhyolitic eruptions were initiated by injection of basalt into the lower part of the silicic magma system. Periodic thermal replenishment of the magmatic system through basaltic injection may explain the relatively high temperatures encountered in drill holes within the western caldera moat. The inclusions further suggest that although rhyolite eruptions are statistically more likely from the Inyo Craters volcanic chain, future basaltic eruptions cannot be ruled out.

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“…Low resistivities in the Early Rhyolite units may be explained by the increase in clay content with depth (Nordquist, 1987).Moreover, high porosities and low resistivities have been measured directly in the tuffs (Smith and Rex, 1977). Pleistocene Long Valley Lake was at its peak during Early Rhyolite emplacement (Bailey et al, 1976) and lacustrine clays of low resistivity may be important contributors also in this interval (Wollenberg et al, 1987;Park and Torres-Verdin, 1988). The Bishop Tuff, most of which is moderately to densely welded, appears resistive overall (cf.…”

Section: Structural Implications and Geothermal Significancementioning

confidence: 99%

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

et al. 1991

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Twenty-four magnetotelluric (MT) soundings have been collected in an east-west profile across the center of Long Valley caldera. The average station spacing is approximately I km and appears adequate to sample the important features of the upper crustal and deeper resistivity structures. Additional control on the shallowest resistivity is provided by a continuous profile of time domain electromagnetic soundings coincident with the western portion of the MT line. Our MT data set reveals numerous resistivity structures which illuminate the evolution and present state of the Long Valley system. Many of these have been quantified through two-dimensional (2-D) finite element modeling emphasizing the transverse magnetic (TM) mode. Important structural components include low-resistivity layers 0.5-1.5 km deep under the eastern half of the caldera, beneath the axial graben of the resurgent dome, and under the west caldera moat. Most of this layering appears to lie in post-caldera Early Rhyolite tuffs, and the uppermost unwelded Bishop Tuff. These rhyolite units have been observed to be porous and highly altered and to commonly contain Pleistocene intercalated lacustrine clays. The remainder and majority of the Bishop Tuff appears highlyresistive. A low resistivity layer also occurs below the axial graben near the base of the Bishop Tuff (1.5 km). Hydrothermal fluids or alteration in precaldera volcanic strata or, less likely, carbonaceous metasediments may be the cause of this. Resistive, probably crystalline basement at high levels is apparent beneath the center of resurgence. Low resistivities are modeled at a depth around 5 km below the entire west moat and central graben and may represent a zone of hydrothermal fluids released from magma crystallization, with potential magmatic contributions at greater depths. The correspondence between this low resistivity and teleseismic delay and low density zones found in other studies is quite striking. A subtle anomaly in the transverse electric (TE) mode impedance is weakly suggestive of a midcrustal conductive axis centered beneath the central graben and resurgent dome. However, it cannot be simulated by two-dimensional transverse electric calculations and requires a full three-dimensional evaluation to ensure that the anomaly does not represent resistivity complexity in just the upper few kilometers. A fundamental, caldera-wide 3-D effect is documented by comparison of observed and computed TE impedance and vertical magnetic field data. The abrupt termination of conductive caldera sediments less than 10 km north and south of our profile greatly depresses the observed TE apparent resistivity and vertical magnetic field relative to the model calculations for periods greater than 0.3 s for the central and eastern caldera. Analysis of the TE mode data also suggests that a similar finite-strike effect lies in the response at periods greater than 3 s due to the mid-crustal west moat conductor. The TM mode measurements are judged to also contain some large-scale departure from the ...

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A core hole in the southwestern moat of the Long Valley Caldera: Early results

Cited by 23 publications

References 6 publications

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

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

Evidence for 600 year‐old basalt and magma mixing at Inyo Craters Volcanic Chain, Long Valley Caldera, California

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

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