HR: 08:15h
AN: S31D-02    [Abstracts]
TI: Pulverized Fault Zone Rocks Along the Mojave Section of the San Andreas Fault: Distribution and Mechanical Significance
AU: Dor, O
EM: dor@usc.edu
AF: University of Southern California, Department of Earth Sciences, 3651 Trousdale Pkwy #117, Los Angeles, CA 90089 United States
AU: * Ben-Zion, Y
EM: ybz@usc.edu
AF: University of Southern California, Department of Earth Sciences, 3651 Trousdale Pkwy #117, Los Angeles, CA 90089 United States
AB: We present field mapping of the San Andreas Fault zone along the Mojave section between Three Points and Pallet Creek (enclosing 60 km of the fault length), showing that pulverized crystalline rocks are a common component of the fault-zone structure, with asymmetric distribution across the fault. Studies of exhumed inactive fault zones show that faults have a damage zone with width of 100's of meters, and that shear is localized to a core fault zone layer typically only 10's of cm wide. Fracture density in the damage zone increases toward the fault core, in which highly deformed fault-rocks exhibit distinct textural and mineralogical characteristics (Schulz and Evans 2000; Chester and Chester, 1998). In addition, fault zone rocks along strands of the San Andreas Fault were found to be intensely pulverized (Brune, 2001). The grains of the pulverized rocks appear to have failed by tension and present extreme reduction in the original grain size. Shear in the pulverized rock is very limited and the texture of the protholite is preserved. Recent analysis of a 70 m wide belt of pulverized granite in Tejon Pass indicates fracturing in the sub-micron scale with non-fractal particle size distribution (Wilson et al., 2004). Our mapping shows that pulverization is restricted to crystalline rocks, and that these pulverized rocks are common within the fault zone, but their distribution across the fault is asymmetric. Crystalline rocks on the north east side of the fault are found to be pulverized wherever they are exposed, up to a distance of ~50 meters from the fault. This is followed by a transition zone ~10-20 meters wide of selective pulverization, beyond which the rocks are macroscopically fractured but not pulverized. Exposures on the south west side of the fault are much less common; where they are found, crystalline rocks adjacent to the fault show a mixed pattern that can range from full pulverization through selective pulverization and up to macroscopic fracturing. The observed pulverized crystalline rocks are likely to result from strong extensional dynamic strain associated with the passage of repeating earthquake ruptures. In particular, wrinkle-like ruptures along a material contrast provides a framework for strong dynamic extensional strain with preferred propagation direction (e.g., Weertman, 1980; Andrews and Ben-Zion, 1997; Ben-Zion and Huang, 2002) that may produce asymmetric distribution of tension-based rock damage across the fault. High resolution imaging of fault zone structures with seismic trapped waves (e.g., Ben-Zion et al., 2003; Peng et al., 2003; Lewis et al., 2004) show that the trapping structures of several major faults, associated with low velocity damaged rock, are limited to the upper 3-4 km. The lack of trapping structures below that depth suggests that the pulverized fault zone rocks may not extend below the top few kilometers.
DE: 8010 Fractures and faults
DE: 8020 Mechanics
DE: 8123 Dynamics, seismotectonics
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting