HR: 0800h
AN: S41B-0969    [Abstracts]
TI: Alteration of fresh fault gouge from focal depths of recent earthquakes in deep mines
AU: * Dewers, T
EM: tdewers@ou.edu
AF: School of Geology and Geophysics, University of Oklahoma, 100 East Boyd Street, Suite 810, Norman, OK 73019 United States
AU: Reches, Z
EM: reches@gcn.ou.edu
AF: School of Geology and Geophysics, University of Oklahoma, 100 East Boyd Street, Suite 810, Norman, OK 73019 United States
AB: Rock powder recovered from focal depths in South African gold mines has been used as an estimate of the surface energy released during earthquakes (Olgaard and Brace, 1983). Recent work (Wilson et al., this meeting) has shown the amount of new surface area generated by earthquakes can approach 100 $m^2/g$, representing a sizable fraction of earthquake energy budgets. Chemical alteration of gouge, including weathering reactions, simple dissolution, precipitation of new phases, and Ostwald ripening, will act to decrease specific surface area and thus mask estimates of released surface energy from a single earthquake with time post-event. To assess this question of gouge textural alteration/preservation, we examine alteration rates of quartzose fault gouge sampled at 2.7 km depth from the Bosman Fault, newly-borne during the 1997 M=3.7 earthquake in Hartebeestfontein gold mine, South Africa, which accumulated ~0.4 m of total slip. TEM analysis of the rock powder suggests no presence of glass or alteration mineralogy. Particle size distributions of starting material yield a broad range with mean of about 0.1 microns. Hydrothermal alteration experiments were performed at 50, 75, 100 and 150 degrees C at 0.7 MPa fluid pressure in a continuously stirred titanium pressure vessel. Notable changes in experimental products include an increase in mean size of distributions ($>$ 1 micron for longest run times) after weeks to months depending on temperature and a shift to fewer, larger particles in the distribution, consistent with Ostwald ripening. Aqueous silica concentrations from timed experiments at the higher temperatures show an initial increase above quartz solubility followed by asymptotic decay to saturation levels. We apply a particulate dynamics approach to assess the kinetics of this process with a PSD-dependent mineral-water reaction rate law, parameterized by observed textural and solution chemical changes. Inasmuch as this type of particulate growth mechanism can be recognized by attributes of the particle size distribution (Eberl et al., 1998), this model is then used to assess amount and mechanism of alteration in gouges from fault zones in other South African gold mines as well as an exhumed cataclasite zone of the San Andreas Fault at Tejon Pass, California USA. In the NELSAM project (Johnston et al., this meeting, session S11), fresh gouge will be recovered from drill core within months of an earthquake event to monitor total specific surface area produced and energy released, and to test the validity of a rapid alteration hypothesis.
DE: 8123 Dynamics, seismotectonics
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting