Geologic Results from the Long Valley Exploratory Well

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As a deep well in the center of a major Quaternary caldera, the Long Valley Exploratory Well (LVEW) provides a new perspective on the relationship between hydrothermal circulation and a large crustal magma chamber. It also provides an important test of models for the subsurface structure of active continental calderas. Results will impact geothermal exploration, assessment, and management of the Long Valley resource and should be applicable to other igneous-related geothermal systems. Our task is to use the cuttings and core from LVEW to interpret the evolution of the central caldera region, with emphasis on evidence of current hydrothermal conditions ... continued below

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129-134

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McConnell, Vicki S.; Eichelberger, John C.; Keskinen, Mary J. & Layer, Paul W. March 24, 1992.

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Description

As a deep well in the center of a major Quaternary caldera, the Long Valley Exploratory Well (LVEW) provides a new perspective on the relationship between hydrothermal circulation and a large crustal magma chamber. It also provides an important test of models for the subsurface structure of active continental calderas. Results will impact geothermal exploration, assessment, and management of the Long Valley resource and should be applicable to other igneous-related geothermal systems. Our task is to use the cuttings and core from LVEW to interpret the evolution of the central caldera region, with emphasis on evidence of current hydrothermal conditions and circulation. LVEW has reached a depth of 2313 m, passing through post-caldera extrusives and the intracaldera Bishop Tuff to bottom in the Mt. Morrison roof pendant of the Sierran basement. The base of the section of Quaternary volcanic rocks related to Long Valley Caldera was encountered at 1800 m of which 1178 m is Bishop Tuff. The lithologies sampled generally support the classic view of large intercontinental calderas as piston-cylinder-like structures. In this model, the roof of the huge magma chamber, like an ill-fitting piston, broke and sank 2 km along a ring fracture system that simultaneously and explosively leaked magma as Bishop Tuff. Results from LVEW which support this model are the presence of intact basement at depth at the center of the caldera, the presence of a thick Bishop Tuff section, and textural evidence that the tuff encountered is not near-vent despite its central caldera location. An unexpected observation was the presence of rhyolite intrusions within the tuff with a cumulative apparent thickness in excess of 300 m. Chemical analyses indicate that these are high-silica, high-barium rhyolites. Preliminary {sup 40}Ar/{sup 39}Ar analyses determined an age of 626 {+-} 38 ka (this paper). These observations would indicate that the intrusions belong to the early post-collapse episode of volcanism and are contemporaneous with resurgence of the caldera floor. If they are extensive sills rather than dikes, a possibility being investigated through relogging of core from neighboring wells, they were responsible for resurgence. A {sup 40}Ar/{sup 39}Ar age of 769 {+-} 14 ka from Bishop Tuff at 820 m depth conforms with tuff ages from outside the caldera and indicates an absence of shallow hydrothermal activity (>300 C) persisting after emplacement. Work is proceeding on investigating hydrothermal alteration deeper in the well. This alteration includes sulfide+quartz fracture fillings, calcite+quartz replacement of feldspars, and disseminated pyrite in both the tuff and basement. Electron microprobe analysis of phases are being conducted to determine initial magmatic and subsequent hydrothermal conditions.

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129-134

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  • Proceedings, Geothermal Energy and the Utility Market - The Opportunities and Challenges for Expanding Geothermal Energy in a Competitive Supply Market; San Francisco, CA, March 24-26, 1992, Geothermal Program Review X

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  • Report No.: CONF-920378--23
  • Grant Number: None
  • Office of Scientific & Technical Information Report Number: 891923
  • Archival Resource Key: ark:/67531/metadc884147

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  • March 24, 1992

Added to The UNT Digital Library

  • Sept. 21, 2016, 2:29 a.m.

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  • Nov. 28, 2016, 6:33 p.m.

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McConnell, Vicki S.; Eichelberger, John C.; Keskinen, Mary J. & Layer, Paul W. Geologic Results from the Long Valley Exploratory Well, article, March 24, 1992; United States. (digital.library.unt.edu/ark:/67531/metadc884147/: accessed October 15, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.