HR: 0800h
AN: T21B-0472    [Abstracts]
TI: Dynamic Weakening of Ultracataclasite During Rotary Shear at Seismic Slip Rates
AU: Chester, J S
EM: chesterj@geo.tamu.edu
AF: Center for Tectonophysics, Department Geology & Geophysics, Texas A&M University, College Station, TX 77843-3115 United States
AU: * Kitajima, H
EM: hkitajima@geo.tamu.edu
AF: Center for Tectonophysics, Department Geology & Geophysics, Texas A&M University, College Station, TX 77843-3115 United States
AU: Chester, F M
EM: chesterf@geo.tamu.edu
AF: Center for Tectonophysics, Department Geology & Geophysics, Texas A&M University, College Station, TX 77843-3115 United States
AU: Shimamoto, T M
EM: shima@kueps.kyoto-u.ac.jp
AF: Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University, Kyoto, 606-8502 Japan
AB: Several mechanisms that can lead to a reduction in frictional strength at seismic slip rates have been identified in laboratory friction experiments, but at this time we are uncertain which mechanisms are most important in natural seismic faulting. To further investigate frictional behavior of natural faults, we have conducted high-speed friction tests of natural ultracataclasite collected from the Punchbowl fault, an exhumed, large-displacement fault in San Andreas system that juxtaposes arkosic sedimentary rock of the Punchbowl Formation and crystalline igneous and metamorphic rock of the San Gabriel basement complex. The ultracataclasite consists of extremely fine particles produced by comminution of host rock with some syn- and post-faulting alteration to zeolite and clay. Two samples of ultracataclasite are used in experiments: DP4F from near the contact with the Punchbowl Formation and DP189A from near the contact with the crystalline basement. Both samples contain smectite and zeolite, and were prepared for experiments by disaggregating to particle size less than 100 μm diameter. The disaggregated ultracataclasite was sheared between sawcut cylinders of granite in a high-velocity rotary apparatus at Kyoto University. Frictional strength was measured with displacement at a slip speed of 0.1, 0.7 and 1.3 m/s, normal stress of 0.2, 0.6, and 1.3 MPa, and after pre-compaction for times up to several hours. Samples were sheared to different total displacement, from 1.5 m to 80 m, to facilitate study of microstructural evolution. At 1.3 m/s, the friction coefficient rapidly increases to a peak about 1.2 followed by gradual decrease to 0.2 over a slip-weakening distance (Dc) of about 15m. At the lower speed of 0.1m/s, the coefficient friction is about 0.8 and there is little change in strength with slip. At the higher slip rates, Dc decreases with increase in normal stress and increase in slip rate. Precompaction tends to increase the initial peak frictional strength, but does not affect residual, steady-state strength. The behavior of both types of ultracataclasite are similar. That significant weakening is only observed at higher slip rates, and that the critical slip distance for weakening decreases with increase in normal stress and slip rate, implies weakening results from increase in temperature of the slipping surface. Moreover, slide-hold-slide tests show rapid strength recovery consistent with transient thermal effects. Current work is directed at correlating microstructure with frictional behavior and identification of weakening processes.
DE: 5104 Fracture and flow
DE: 7209 Earthquake dynamics (1242)
DE: 8012 High strain deformation zones
DE: 8034 Rheology and friction of fault zones (8163)
DE: 8118 Dynamics and mechanics of faulting (8004)
SC: Tectonophysics [T]
MN: Fall Meeting 2005