HR: 1330h
AN: T22B-0519    [PDF]
TI: The San Andreas Fault-Zone at Tejon Pass: Internal Structure and Grain-Size Distribution
AU: * Wilson, B
EM: brentwilson@ou.edu
AF: University of Oklahoma, 100 East Boyd Street, Suite 810, Norman, OK 73069 United States
AU: Mohamed, Y
EM: moha6730@ou.edu
AF: University of Oklahoma, 100 East Boyd Street, Suite 810, Norman, OK 73069 United States
AU: Dewers, T
EM: tdewers@ou.edu
AF: University of Oklahoma, 100 East Boyd Street, Suite 810, Norman, OK 73069 United States
AU: Reches, Z
EM: reches@pangea.Stanford.EDU
AF: University of Oklahoma, 100 East Boyd Street, Suite 810, Norman, OK 73069 United States
AU: Brune, J
EM: brune@seismo.unr.edu
AF: University of Nevada Reno, MS 172, Reno, NV 89557
AB: The exhumed fault-zone of the San Andreas near Tejon Pass, the Big Bend region of California, displays exceptional exposures of the gouge material. The main segments of the San Andreas Fault in this region bound a few hundred meter wide zone in which the most conspicuous gouge lithology is the pulverized Tejon Lookout granite of Cretaceous age. This body forms a 60-120 m wide zone that extends for more than 2 km along the San Andreas. We have studied the large scale structure of this fault-zone (Dewers et al., 2002, AGU Fall meeting), and we discuss here the small-scale maps (1:5-1:10) of portions of the pulverized granite within the fault-zone and the particle-size-distribution (PDS) of the gouge. The small-scale maps reveal a multitude of shear surfaces with 10's of cm of slip and sub-meter spacing. The dominant transport direction along these surfaces is normal to the trend of the San Andreas, displaying either normal or reverse dip-slip motion. There is a striking lack of shear surfaces and slip striations parallel to the San Andreas Fault. We used a laser particle size analyzer (range 0.04-2000 micron) to measure the PSD of tens of samples of the granitic gouge collected along a 70 meter long traverse across the fault-zone and at the sites of the small-scale mapping. The PSD measurements were conducted by running the laser particle size analyzer for extended periods up to 3 days (Dewers et al., 2003, AGU Fall meeting). The gouge systematically disaggregates during the long PSD runs and its intrinsic mean grain size is smaller than one micron. Further, granitic gouge zone displays relatively uniform PSD from 0.1 m scale to its entire sampled width (70 m). These macro- and micro- properties of the San Andreas gouge at Tejon Pass indicate that (1) the gouge did not form by shear parallel to the San Andreas, (2) the pervasive pulverization could account for a large portion of the earthquake energy balance.
DE: 3250 Fractals and multifractals
DE: 3902 Creep and deformation
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics and mechanics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting