Near-surface hydrothermal regime of Long Valley caldera

Journal Article · Tue Feb 10 04:00:00 EST 1976 · J. Geophys. Res.; (United States)

DOI:https://doi.org/10.1029/JB081i005p00763· OSTI ID:7196163

Lachenbruch, A H ^[1]; Sorey, M L; Lewis, R E; Sass, J H

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1. Geological Survey, Menlo Park, CA

Temperatures at the 5- to 10-m depth from 29 shallow holes in Long Valley caldera can be contoured systematically; they correlate well with the character of the thermal gradient to 30 m. Where the temperature at a depth of 10 m is less than 11/sup 0/C (group I), the gradients to 30 m are practically zero; where the 10-m temperature is between 11/sup 0/C and 16/sup 0/C (group II), the gradients are 200/sup 0/-400/sup 0/C/km and uniform corresponding to conductive heat flows of 4-8 HFU (1 HFU = 1 x 10/sup -6/ cal/cm/sup 2/s). Where the 10-m temperatures exceed 16/sup 0/C (group III), gradients are larger and irregular, with local heat flows to 50 HFU. Thermal considerations suggest that the first group is characteristic of regions of hydrologic recharge, that the second group is probably characteristic of regions with conductive regimes to substantial depth, and that the third group is characteristic of regions of hydrologic discharge. This interpretation is supported by limited drilling to depths up to 300 m. Regimes in group I occur in the peripheral portion of the caldera, suggesting that this is an area of recharge. The hot springs discharge in a fault zone characterized by near-surface regimes in groups II and III; chemical evidence indicates that their source reservoir is at about 200/sup 0/C. Evidently, the springs are fed by local fractures; if the background regime is conductive, their reservoirs are probably less than 1 km deep. Hydrologic and isotopic data indicate that gross circulation in the hydrothermal system is from west to east, suggesting that the hot springs gain their heat in the western caldera. The large estimates of heat being removed from the caldera by flowing water and the geologic inference that hydrothermal activity was more intense in the past support the view that the Long Valley system was resupplied with heat from deep magmatic sources during its eruptive history.

• OSTI ID: 7196163
• Journal Information: J. Geophys. Res.; (United States), Journal Name: J. Geophys. Res.; (United States) Vol. 81:5; ISSN JGREA
• Country of Publication: United States
• Language: English

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