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Abstract
Serpentinized ophiolitic rocks are juxtaposed against quartzofeldspathic rocks at depth across considerable portions of the Hayward and Calaveras Faults. The marked compositional contrast between these rock types may contribute to fault creep that has been observed along these faults. To investigate this possibility, we are conducting hydrothermal shearing experiments to look for changes in frictional properties resulting from the shear of ultramafic rock juxtaposed against quartzose rock units. In this paper we report the first results in this effort: shear of bare-rock surfaces of serpentinite and granite, and shear of antigorite-serpentinite gouge between forcing blocks of granitic rock. All experiments were conducted at 250°C. Serpentinite sheared against granite at 50 MPa pore-fluid pressure is weaker than either rock type separately, and the weakening is significantly more pronounced at lower shearing rates. In contrast, serpentinite gouge sheared dry between granite blocks is as strong as the bare granite surface. We propose that the weakening is the result of a solution-transfer process involving the dissolution of serpentine minerals at grain-to-grain contacts. Dissolution of serpentine is enhanced by modifications to pore-fluid chemistry caused by interaction of the fluid with the quartz-bearing rocks. The compositional differences between serpentinized ultramafic rocks of the Coast Range Ophiolite and quartzofeldspathic rock units such as those of the Franciscan Complex may provide the mechanism for aseismic slip (creep) in the shallow crust along the Hayward, Calaveras, and other creeping faults in central and northern California.
| Publication type | Conference Paper |
|---|---|
| Publication Subtype | Conference Paper |
| Title | Anomalously low strength of serpentinite sheared against granite and implications for creep on the Hayward and Calaveras Faults |
| Volume | 219 |
| Year Published | 2010 |
| Language | English |
| Publisher | California Geological Survey |
| Publisher location | Sacramento, CA |
| Contributing office(s) | Earthquake Science Center |
| Larger Work Type | Conference Paper |
| Larger Work Subtype | Conference Paper |
| Larger Work Title | Proceedings of the Third Conference on Earthquake Hazards in the Eastern San Francisco Bay Area : science, hazard, engineering and risk: California Geological Survey Special Publicatoin 219 |
| First page | 101-113 |
| Conference Title | Third Conference on Earthquake Hazards in the Eastern San Francisco Bay Area : science, hazard, engineering and risk |
| Conference Date | October 22-24, 2008 |
| Online Only (Y/N) | N |
| Additional Online Files (Y/N) | N |
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