Anomalously low strength of serpentinite sheared against granite and implications for creep on the Hayward and Calaveras Faults

David A. Lockner; David A. Ponce
By: , and 

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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.

Additional publication details

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