HR: 14:40h
AN: T23E-05 INVITED     [Abstracts]
TI: Structure and Composition of the San Andreas Fault Zone at Parkfield: Initial Results from SAFOD Phases 1 and 2
AU: * Hickman, S H
EM: hickman@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS977, Menlo Park, CA 94025 United States
AU: Zoback, M D
EM: zoback@pangea.stanford.edu
AF: Stanford University, Geophysics Department, Stanford, CA 94305 United States
AU: Ellsworth, W L
EM: ellsworth@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd., MS977, Menlo Park, CA 94025 United States
AB: The San Andreas Fault Observatory at Depth (SAFOD) is designed to directly measure the physical and chemical conditions under which fault movement occurs during both earthquakes and aseismic creep. The SAFOD borehole begins vertically 1.8 km SW of the surface trace of the fault and then angles through the fault zone until it passes beneath the surface trace at a depth of 3.2 km. During SAFOD Phases 1 and 2, continuous sampling of drilling cuttings and fluids; spot coring; optical mineralogy, XRD, elemental composition and isotope analyses; and a comprehensive suite of downhole measurements provide a profile of physical properties and mineralogy across the entire San Andreas Fault Zone. This talk summarizes the data obtained to date by members of the SAFOD science team, who present their results in more detail elsewhere in this session. Although pre-drilling site characterization studies predicted a relatively simple fault zone, with Salinian granite basement on the west side of the San Andreas and Franciscan meta-sedimentary rocks on the east side, the geology actually encountered during drilling was more complex. The vertical part of the hole penetrated Salinian granitic rocks as expected, but after deviating the hole toward the San Andreas Fault, SAFOD passed through 1.2 km of arkosic sandstones and conglomerates (with some shales) before terminating in over 800 m of shales, siltstones and fine sandstones. The presence of Cretaceous fossils in cuttings and core from the bottom of the hole and the absence of high-grade Franciscan metamorphic minerals in the cuttings indicate that SAFOD passed all the way through the San Andreas Fault Zone, terminating either in rocks of the Great Valley group or relatively unmetamorphosed Franciscan sedimentary rocks. The contact between the predominately arkosic sandstone/conglomerate Salinian rocks and Mesozoic siltstone/shale of the Franciscan and/or Great Valley appears to be the long-term "geologic" manifestation of the San Andreas Fault. This boundary is located approximately 600 m SW of the surface trace of the fault. On both sides of this geologic fault contact, several zones exhibit anomalous geophysical properties that may represent active shear zones. Some of the most dramatic of these are zones about 15 m wide exhibiting low resistivities and low P- and S-wave velocities at measured depths (MD) of 3.07 and 3.16 km. The shallowest of these zones was cored at the conclusion of Phase 1, and contains a 30-cm-thick, illite plus illite-smectite shear zone with low friction and abundant internally sheared surfaces. Other potential shear zones indicated by anomalous geophysical properties, drilling rate, changes in cuttings mineralogy or increases in mud gas content are located at 3.19, 3.33, and 3.41 km MD. The anomalous zone at 3.33 km is particularly interesting, in that it is a 13-m-wide zone associated with the sudden appearance of serpentinite, which is seen along surface exposures of the San Andreas Fault in central California and is thought to be important in controlling frictional strength and the stability of sliding. Which of these zones are currently active will be determined through repeat logging of SAFOD to identify casing shear and locating the SAFOD target earthquakes with seismic instruments placed directly within the fault zone at seismogenic depths.
DE: 7215 Earthquake source observations (1240)
DE: 7250 Transform faults
DE: 8034 Rheology and friction of fault zones (8163)
DE: 8110 Continental tectonics: general (0905)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Tectonophysics [T]
MN: Fall Meeting 2005