HR: 1340h
AN: T33C-1497    [Abstracts]
TI: Coupling between brittle fracture and anticrack-vein pressure solution at asperities along a small-displacement thrust fault in limestone
AU: * Fletcher, R C
EM: rfletche@geosc.psu.edu
AF: The Pennsylvania State University, Department of Geosciences, University Park, PA 16801, United States
AU: * Fletcher, R C
EM: rfletche@geosc.psu.edu
AF: University of Oslo, Center for Physics of Geological Processes, Oslo, NO-0316, Norway
AU: Savage, H M
EM: hsavage@geosc.psu.edu
AF: University of California, Santa Cruz, Department of Earth Science, Santa Cruz, CA 95064, United States
AB: A significant fraction of displacement on a fault may take place by aseismic creep, but specific mechanisms of creep are not well-documented. In addition, seismic events are often nucleated at asperities, with associated damage tending to smooth the fault. Alternatively, aseismic creep processes such as pressure solution dissolve material on one side of asperities and precipitate material on the opposite side, thereby maintaining fault waviness. Depending on the tectonically-imposed displacement rate, slip by aseismic creep will take place if asperity roughness is sufficiently small, and roughness will be preserved. To study these processes, we mapped a wavy fault zone in limestone. Our observations are consistent with creep by pressure solution mechanisms and asperity reduction by brittle failure. The decimeter-scale thrust fault studied is in a complexly-faulted tabular region near the axial surface of a kilometer-scale syncline in Ordovician carbonates, Valley & Ridge Province, State College, Pennsylvania. The fault was selected because of excellent exposure, known displacement, and the presence of asperities. Dissolution and precipitation, with transport across asperities, may be inferred from local concentration of calcite precipitation at the fault surface. This is the first mechanism associated with slip across an asperity. Brecciation is also observed, which acts to smooth the fault, whereas the pressure solution mechanisms allow slip without smoothing. The latter requires small fault creep rate, whereas brecciation implies that pressure solution could not accommodate the slip rate required by the tectonic loading rate. Within the wall rock, anticracks (tectonic solution seams) & veins are strongly concentrated at undulations with wavelengths of 10s of cm to a few meters and amplitudes of approximately 10 cm. If maximum dissolution is approximately 0.03 anticrack length, as measured elsewhere, vein precipitation roughly balances dissolution locally. This local strain of a few percent is associated with slip of about 1 m. Estimates of asperity wavelength and amplitude, localization of pressure solution, relative proportions of brecciation and mass transport across asperities have been used to constrain a mechanical model of the coupled processes. Condition for a transition between aseismic slip and seismic, brittle failure can be established.
DE: 5120 Plasticity, diffusion, and creep
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8010 Fractures and faults
DE: 8020 Mechanics, theory, and modeling
SC: Tectonophysics [T]
MN: 2007 Fall Meeting