TECTONIC CONTROLS ON MAGMATISM IN THE GEYSERS-CLEAR LAKE REGION - EVIDENCE FROM NEW GEOPHYSICAL MODELS

Citation
Wd. Stanley et al., TECTONIC CONTROLS ON MAGMATISM IN THE GEYSERS-CLEAR LAKE REGION - EVIDENCE FROM NEW GEOPHYSICAL MODELS, Geological Society of America bulletin, 110(9), 1998, pp. 1193-1207
Citations number
46
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Geosciences, Interdisciplinary
ISSN journal
0016-7606
Volume
110
Issue
9
Year of publication
1998
Pages
1193 - 1207
Database
ISI
SICI code
0016-7606(1998)110:9<1193:TCOMIT>2.0.ZU;2-U
Abstract
In order to study magmatism and geothermal systems in The Geysers-Clea r Lake region, Ne developed a detailed three-dimensional tomographic v elocity model based on local earthquakes. This high-resolution model r esolves the velocity structure of the crust in the region to depths of approximately 12 km. The most significant velocity contrasts in The G eysers-Clear Lake region occur in the steam production area, where hig h velocities are associated with a Quaternary granitic pluton, and in the Mount Hannah region, where low velocities occur in a 5-km-thick se ction of Mesozoic argillites, In addition, a more regional tomographic model was developed using traveltimes from earthquakes covering most of northern California. This regional model sampled the whole crust, b ut at a lower resolution than the local model. The regional model outl ines low velocities at depths of 8-12 lan in The Geysers-Clear Lake ar ea, which extend eastward to the Coast Range thrust. These low velocit ies are inferred to be related to unmetamorphosed Mesozoic sedimentary rocks. In addition, the regional velocity model indicates high veloci ties in the lower crust beneath the Clear Lake volcanic field, which w e interpret to be associated with mafic underplating. No large silicic magma chamber is noted in either the local or regional tomographic mo dels. A three-dimensional gravity model also has been developed in the area of the tomographic imaging. Our gravity model demonstrates that all density contrasts can be accounted for in the upper 5-7 lan of the crust. Two-dimensional magnetotelluric models of data from a regional , east-west profile indicate high resistivities associated with the gr anitic pluton in The Geysers production area and low resistivities in the low-velocity section of Mesozoic argillites near Mount Hannah. No indication of midcrustal magma bodies is present in the magnetotelluri c data. On the basis of heat dow and geologic evidence, Holocene intru sive activity is thought to have occurred near the Northwest Geysers, Mount Hannah, Sulphur Bank Mine, and perhaps other areas. The geophysi cal data provide no conclusive evidence for such activity, but the det ailed velocity model is suggestive of intrusive activity near Mount Ha nnah similar to that in the ''felsite'' of The Geysers production area The geophysical models, seismicity patterns, distribution of volcanic vents, heat now, and other data indicate that small, young intrusive bodies that were injected along a northeast trend from The Geysers to Clear Lake probably control the thermal regime.