HR: 11:20h
AN: T42C-05 INVITED    [Abstracts]
TI: Deformation of Sedimentary Rock Across the San Andreas Fault Zone: Mesoscale and Microscale Structures Displayed in Core From SAFOD
AU: * Chester, J S
EM: chesterj@geo.tamu.edu
AF: Texas A&M University, Dept. Geology & Geophysics, College Station, TX College St,
AU: Chester, F M
EM: chesterf@geo.tamu.edu
AF: Texas A&M University, Dept. Geology & Geophysics, College Station, TX College St,
AU: Kirschner, D L
EM: dkirschn@eas.slu.edu>
AF: Saint Louis University, Dept. Earth & Atmospheric Sciences, St. Louis, MO 63103,
AU: Almeida, R
EM: ralmeida@geo.tamu.edu
AF: Texas A&M University, Dept. Geology & Geophysics, College Station, TX College St,
AU: Evans, J P
EM: jpevans@cc.usu.edu
AF: Utah State University, Dept. Geology, Logan, UT 84322,
AU: Guillemette, R N
EM: guillemette@geo.tamu.edu
AF: Texas A&M University, Dept. Geology & Geophysics, College Station, TX College St,
AU: Hickman, S
EM: hickman@usgs.gov
AF: USGS Earthquake Hazards Team, 345 Middlefield Rd., Menlo Park, CA 94025,
AU: Zoback, M
EM: zoback@pangea.stanford.edu
AF: Stanford University, Dept. Geophysics, Stanford, CA 94305,
AU: Ellsworth, W
EM: ellsworth@usgs.gov
AF: USGS Earthquake Hazards Team, 345 Middlefield Rd., Menlo Park, CA 94025,
AB: Sedimentary rocks captured in cores taken at the San Andreas Fault Observatory at Depth (SAFOD) provide an unparalleled sampling of deformation in the transition zone between creeping and locked segments of a major transform fault at 2.5-3.1 km vertical depth. These samples provide the unique opportunity to study deformation processes and the development of brittle structures within porous and granular rocks that have been subjected to variable loading rates and chemically reactive fluids while residing at the top of the seismogenic zone. The samples provide a transect from relatively undeformed host rock through highly fractured and sheared rock, and capture the two prominent zones of active, aseismic slip. Core recovery was almost complete. Wrap-around 1:1 map tracings of the outer surfaces of all cores characterize the lithology and mesoscale deformation. Cores from 3056-3067 m and 3141-3153 m measured depth (MD) sample moderately deformed rock at the western boundary of the fault zone. The cores display massive to finely laminated, pebbly arkosic sandstones with lesser amounts of fine-grained sandstone and siltstone. Numerous shear fractures and cm-thick cataclastic shear zones form a conjugate geometry indicating contraction at high angles to the San Andreas fault. Both intervals display minor faults that juxtapose different lithologies consistent with meters or greater of slip. Fracture density is variable but tends to increase with proximity to the minor faults. Cross-cutting relationships between shear fractures and cataclastic zones indicate a general progression from early faulting along thicker shear zones to later, more localized slip within shear zones and along fractures. Microstructures provide ample evidence for densification of the sandstones through grain-scale fracture and crushing, as well as fluid assisted processes of crack-sealing, dissolution-precipitation, and alteration-neocrystallization. Grain-scale features are consistent with these processes having operated concurrently and cyclically. The microstructure of wear product along some slip surfaces, and associated mobilization and injections of wear microbreccia, are consistent with dynamic slip and local pore fluid pressurization. Cores from the actively creeping segments at 3194 m and 3301 m MD display a variety of moderately to highly sheared rocks. Disrupted bedding and cataclastic foliations oriented subparallel to the macroscopic orientation of the San Andreas fault are common. Protoliths of the fault rocks include interbedded sandstone, siltstone, shale, and serpentinite. Mesoscale calcite veins are abundant locally, yet are rare in the other cores. Zones of foliated cataclasites record a distributed shear within meters-thick zones, but sharp contacts between different types of cataclasite are also consistent with extremely localized slip having occurred at various stages of deformation. Zones of incohesive cataclasite and scaly fabrics could correlate with current locations of active, aseismic creep.
DE: 5112 Microstructure
DE: 8010 Fractures and faults
DE: 8025 Mesoscopic fabrics
DE: 8045 Role of fluids
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting