Quick Search:    advanced search

GSW Home   GeoRef Home   My GSW Alerts   Contact GSW   About GSW   Journals List   Help

This Article Figures Only Full Text Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (7) Citing Articles via Google Scholar Google Scholar Articles by Monastero, F.C. Articles by Richards-Dinger, K. Search for Related Content GeoRef GeoRef Citation

The Coso geothermal field: A nascent metamorphic core complex

F.C. Monastero^,1, A.M. Katzenstein¹, J.S. Miller², J.R. Unruh³, M.C. Adams⁴ and Keith Richards-Dinger⁵

¹ Geothermal Program Office, Naval Air Weapons Station, China Lake, California 93555-6108, USA
² Department of Geology, San Jose State University, San Jose, California 95192-0102, USA
³ William Lettis & Associates, Walnut Creek, California 94596, USA
⁴ Energy & Geosciences Institute, University of Utah, Salt Lake City, Utah 84108, USA
⁵ Geothermal Program Office, Naval Air Weapons Station, China Lake, California 93555-6108, USA

Investigation of the Coso Range using seismicity, gravity, and geochemistry of rocks and fluids, supports the interpretation that the structure hosting the geothermal resource is a nascent metamorphic core complex. The structural setting is a releasing bend in a dextral strike-slip system that extends from the Indian Wells Valley northward into the Owens Valley. This tectonic setting results in NW-directed transtension, which is accommodated by normal and strike-slip faulting of the brittle upper 4–6 km of the crust, and shearing and ductile stretching below this depth, accompanied by shallow igneous intrusions. Focal mechanisms of some small earthquakes that have occurred from 1996 to the present beneath the Coso Range exhibit depth-dependent rotation of seismic P and T axes, indicating that the local orientations of the principal stresses likely favor resolved shear stress on low-angle faults. These small earthquakes occur near the base of seismicity, which we interpret as coincident with the brittle-ductile transition. Geochemical results show a significant asthenospheric influence in the isotopic composition of rocks and fluids, indicating that the crust is thinned within the Coso structure. Thinned upper crust is underlain by a more dense mafic body at depths of 10 km or less. This is consistent with observed gravity anomalies and models. The mafic body may represent cumulates left over from the fractional crystallization of rhyolite, which occurs as endogenous domes at Coso, or it could be a sheeted-dike complex in the upper mid-crustal area. Transtension began at 2–3 Ma, and continues today. Using a long-term crustal deformation rate of 2 mm/yr, we infer that the basal detachment fault commonly observed in fully exhumed metamorphic core complexes will reach the surface in two to four million years.

Key Words: metamorphic core complex • brittle-ductile transition • geothermal • eastern California • transtension

This article has been cited by other articles:

				S. N. Bacon and S. K. Pezzopane A 25,000-year record of earthquakes on the Owens Valley fault near Lone Pine, California: Implications for recurrence intervals, slip rates, and segmentation models GSA Bulletin, July 1, 2007; 119(7-8): 823 - 847. [Abstract] [Full Text] [PDF]