HR: 09:15h
AN: T21D-05    [Abstracts]
TI: High-velocity Frictional Behavior of Dunite, Biotite Gneiss, Phyllite and Coal Show Evidence for Melting and Thermal Degasing
AU: * O'Hara, K D
EM: geokoh@uky.edu
AF: Department of Geological Sciences, University of Kentucky, Lexington, KY 40506 United States
AU: Mizoguchi, K
AF: Department of Geology and Mineralogy, University of Kyoto, Kyoto, Hon 606-8502 Japan
AU: Shimamoto, T
EM: shima@kueps.kyoto-u.ac.jp
AF: Department of Geology and Mineralogy, University of Kyoto, Kyoto, Hon 606-8502 Japan
AB: We conducted high-velocity frictional experiments on dunite, biotite gneiss, phyllite gouge and coal gouge at Kyoto University using a rotary high-velocity frictional testing machine. The purpose was to examine the effect of frictional melting in various rock types and to explore the effect of thermal degassing using coal as an analogue for a volatile fault zone. Experiments were conducted dry at equivalent slip rates of 1 m/s (1200 rpm) at normal stresses of 0.6-16 MPa for distances up to 90 m. Solid cylinders (25 mm diameter) of dunite and biotite gneiss were sheared with aluminum-alloy jackets at high stress, whereas phyllite and coal gouges were sheared with Teflon sleeves at low stress. The metal jackets allow high stress experiments to be performed and are inferred to melt before rock melting occurs. Dunite sheared at 10-16 MPa shows a weakening-strengthening followed by second weakening on melting, similar to previous experiments on gabbro without a metal jacket. Dunite melting is confirmed by, as yet unidentified, dendritic microlites, and a rapid reduction of steady-state frictional strength to 0.15. Under similar conditions, biotite gneiss shows apparent melting, but undergoes continuous strengthening without reaching steady state. Bituminous coal gouge sheared at 0.6 MPa undergoes a highly reproducible rapid weakening from 0.75 to 0.2, with odorous white gas emissions, sometimes accompanied by liquid hydrocarbons. Shear stress decreases prior to gasification and rapidly oscillating sample shortening/elongation occurs during gas emission. A slowly sheared sample (15 rpm) did not show weakening or gas emission. This is the first experimental demonstration of weakening associated with devolatilization during rapid slip. Vitrinite reflectance measurements on sheared coal samples may provide constraints on the temperature during gasification. Phyllite gouge sheared under the same conditions shows a gradual weakening to a steady-state strength of about 0.2; the weakening mechanism is not yet understood. Video presentations with mechanical and petrological data will illustrate these different behaviors.
DE: 7230 Seismicity and seismotectonics
DE: 5104 Fracture and flow
DE: 5134 Thermal properties
DE: 7209 Earthquake dynamics and mechanics
DE: 3630 Experimental mineralogy and petrology
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting