HR: 14:10h
AN: T13G-03    [Abstracts]
TI: Preliminary Results from SAFOD Phase 3: Implications for the state of stress and shear localization in and near the San Andreas Fault at depth in central California
AU: * Zoback, M D
EM: zoback@pangea.stanford.edu
AF: Department of Geophysics Stanford University, Mitchell Building Stanford University, Stanford, CA 94305,
AU: Hickman, S H
EM: hickman@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, MS977, Menlo Park, CA 94025,
AU: Ellsworth, W
EM: ellsworth@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, MS977, Menlo Park, CA 94025,
AU: Kirschner, D
EM: dkirschn@gmail.com
AF: St. Louis University, Dept. of Earth and Atmospheric Sciences St. Louis University, St. Louis, MO 63103,
AU: Pennell, N B
EM: Naomi.Boness@chevron.com
AF: Chevron Energy Technology Company, Chevron Energy Technology Company, San Ramon, CA 94583-0719,
AU: Chery, J
EM: jean@dstu.univ-montp2.fr
AF: University of Montpellier, Laboratoire Dynamique de la Lithosphere University of Montpellier, Montpellier, cc060, France
AU: Sobolev, S
EM: stephan@gfz-potsdam.de
AF: GeoForshungZentrum, GeoForshungZentrum Telegrafenberg A-34, Potsdam, D-14473, Germany
AB: Strain localization along the San Andreas fault system in central California appears to result from both a thermally-weak lower crust and upper mantle (reflecting northward migration of the Mendocino triple junction and its associated slab window) and a fault zone in the upper brittle crust that is distinctly weaker than the surrounding crust. Geophysical logs and cuttings analyses from SAFOD Phase 2 (completed in 2005) revealed the San Andreas Fault Zone at approximately 2.7 km depth to be relatively broad (about 250 m), with several discrete, localized zones only 2-3 m wide with very low P- and S-wave velocities and low resistivity. Since 2005, fault creep at two of these localized zones has deformed the casing and thus demonstrates that these zones are actively creeping faults. During SAFOD Phase 3, continuous cores were obtained across these two actively creeping faults. Another core was obtained near the geologic boundary between the Salinian terrane (Pacific plate) and Great Valley/Franciscan terrane (North American plate). Each set of cores reveal zones of profound strain localization and probable weakening. These include ultracataclasites, highly-foliated shear zones (some containing veined serpentine) and intervals that appear to be cohesionless, compacted fault gouges which are likely composed of minerals with low frictional strength. No evidence of significantly elevated fluid pressure is observed within the fault zone. Information about the state of stress in the fault zone and adjacent crust comes from observations and modeling of wellbore failures, direct measurements of the magnitude of the least principal stress and the direction of stress-induced shear wave velocity anisotropy. Observations made after rotary drilling through the fault in 2005 indicate that the San Andreas is a weak fault imbedded in a strong crust. These observations made within about 100 m of the active fault zone at 2.7 km include i) stress orientations that are nearly perpendicular to the strike of the San Andreas, ii) very small differences in the magnitudes of the three principal stresses, and iii) magnitudes of all three principal stresses that are significantly above lithostatic. In contrast, high stress differential stresses in the crust outside the fault were observed in the SAFOD pilot hole at a distance of 1.8 km from the San Andreas, consistent with Byerlee's law and hydrostatic fluid pressure. At the time of this writing, additional geophysical logs are about to be obtained to extend this information directly into and across the zones of active deformation at depth.
UR: http://safod.icdp-online.org
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8122 Dynamics: gravity and tectonics
DE: 8123 Dynamics: seismotectonics
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8168 Stresses: general
SC: Tectonophysics [T]
MN: 2007 Fall Meeting