HR: 17:30h
AN: T54A-07    [Abstracts]
TI: Fluid Dynamic Evidence for Extremely Low Viscosity Coseismic Fault Fluids
AU: * Brodsky, E E
EM: brodsky@es.ucsc.edu
AF: Dept. of Earth & Planetary Sci UC Santa Cruz, 1150 High St, Santa Cruz, CA 95060, United States
AU: Meneghini, F
EM: meneghini@dst.unipi.it
AF: Dipartimento di Scienze della Terra, Università di Pisa, Pisa, 43 5612, Italy
AU: Rowe, C D
EM: crowe@pmc.ucsc.edu
AF: Department of Geological Sciences, University of Cape Town, Rondebosch, Cape Town, 7707, South Africa
AU: Moore, J C
EM: casey@pmc.ucsc.edu
AF: Dept. of Earth & Planetary Sci UC Santa Cruz, 1150 High St, Santa Cruz, CA 95060, United States
AB: We combine geological observations of fault rock textures with fluid mechanics to constrain the mechanics of a fault zone during a subduction earthquake. We analyze buoyant intrusive features in a fault rock that formed at 12- 14 km depth in a large-scale thrust fault embedded in a paleo-accretionary prism in Kodiak Island, AK. The fault rock can been interpreted as either a pseudotachylyte or fluidized ultracataclasite. The intrusive structures provide new, direct evidence on the coseismic rheology of the fault. The asymmetric buoyant intrusions are most readily understood as Rayleigh-Taylor instabilities with an unusually short wavelengths relative to the thickness of the layer. The geometry requires a moderately high Reynolds number flow (Re~1-10) in order to produce the observed wavelength to thickness ratio. The resulting rise velocity under these conditions is ~40 cm/s. Since the shear strain in the layer is over order 1 and the deformation is continuous, the rise velocity must be comparable to the horizontal shear velocity during emplacement. Thus, the geometry alone requires that the fault rocks were intruded coseismically. Furthermore, the Reynolds number constraint combined with the computed rise velocity provides a maximum bound on the viscosity of the fluid during emplacement. The coseismic fault fluid at this locality must have had a viscosity of \ll 10 Pa-s. This viscosity constraint is compatible with the viscosity of the silicate melt of the observed composition at 1300-1400°, which is consistent with the temperature constraints imposed by the absence of plagioclase survivor grains. In summary, both the fluid dynamical and geological evidence points to an extraordinarily low viscosity fluid in the fault zone during rupture and hence extremely low local stress in the fault during an earthquake.
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 7209 Earthquake dynamics (1242)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting