HR: 1340h
AN: T13A-1337    [Abstracts]
TI: Microstrutural analyses of an exhumed part of the San Andreas fault near the SAFOD site, California
AU: * Evans, J P
EM: jpevans@cc.usu.edu
AF: Department of Geology, Utah State University, Logan, UT 84341 United States
AU: Rymer, M J
EM: mrymer@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AU: Moore, D
EM: dmoore@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025 United States
AB: We examine the microstructures and textures of a suite of samples from an outcrop of the main trace of the San Andreas fault ~2.4 km north of the SAFOD drill site, in the area where the fault changes in slip behavior from locked to creeping. The fault at this exposure juxtaposes highly sheared, probably Jurassic to Cretaceous serpentinite, consisting of altered peridotite and tectonized antigorite within the fault, against Pliocene(?) brecciated sedimentary deposits on both sides of the fault. Serpentinite samples within 20 cm of the fault contact exhibit two subvertical foliations superposed on an earlier metamorphic foliation. Foliated zones 1-5 mm wide are subparallel to the SAF contact, and wavy to folded antigorite grains are cut by thin faults that consist of ultracataclasite. Foliation orientations are consistent with a right-side-up sense of shear. Samples from the sedimentary deposit exhibit Fe-filled brittle fractures and calcite veins in a random cataclastic texture within 50 cm of the fault. Within 10 cm of the fault the sedimentary rocks have a moderate foliation, and ultracataclasite fragments lie in a matrix of sheared sandstone. Beyond 50 cm of the fault the sedimentary rocks exhibit variable amounts of deformation ranging from regions of intense intragranular fracture to nearly undeformed grains. These observations suggest that at this site the SAF consists of a narrow zone of highly localized shear within the foliated host rock, and that seismic ruptures created a brittle damage zone on the SW side of the fault. Further analyses of these, and other exhumed parts of the SAF near SAFOD will provide constraints on the nature of the fault zone that will be encountered in the SAFOD drill hole.
DE: 8010 Fractures and faults
DE: 8030 Microstructures
DE: 8045 Role of fluids
DE: 5104 Fracture and flow
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting