HR: 0800h
AN: T41F-1285    [Abstracts]
TI: Fluid overpressures on the San Andreas Fault following the passage of the Mendocino Triple Junction
AU: * Fulton, P M
EM: pfulton@uwyo.edu
AF: Deptartment of Geology and Geophysics, University of Wyoming, P.O. Box 3006, Laramie, WY 82071 United States
AU: Saffer, D M
EM: dsaffer@uwyo.edu
AF: Deptartment of Geology and Geophysics, University of Wyoming, P.O. Box 3006, Laramie, WY 82071 United States
AU: Bekins, B A
EM: babekins@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025 United States
AB: Fluid pressures significantly greater than hydrostatic have been hypothesized to account for the weak nature of many large plate-boundary faults. However, on the San Andreas Fault, the hypothesized subsurface processes which could create, sustain, and potentially localize such pressures over millions of years are not well understood. In this study, we use two-dimensional finite element models of coupled fluid flow and heat transport perpendicular to the fault to evaluate hypothesized mechanisms for generating elevated pore pressure. The models account for transient changes in crustal geotherm and thickness of the seismogenic crust in response to the passage of the Mendocino Triple Junction. Theoretical curves of whole-rock fluid content as functions of pressure and temperature allow us to calculate fluid sources due to metamorphic dehydration within the Franciscan m\'{e}lange as a function of depth and thermal history. Average fluid sources in the seismogenic crust range from 10$^{-18}$ to 10$^{-16}$ s$^{-1}$ over the 15 Myr spanned by our models. We consider a variety of permeability distributions within the models, including a range of homogenous permeability and depth-dependent permeability. We also consider heterogeneous permeability distributions reflecting fault properties and geologic features such as serpentine sills. Our results show that over 15 Myr, thermal expansion of pore fluids due to initial burial, followed by additional heating during exhumation, can create significant overpressures. In addition, models which include fluid sources from metamorphic dehydration of the Franciscan m\'{e}lange result in pore pressures approaching a significant fraction of lithostatic. Generally, all model results show overpressures extending several kilometers to each side of the fault. Due to the continual nature of many of these processes, overpressures are sustained for millions of years without the need for complex and/or extremely low-permeability seals. Models which include geologic structures such as serpentine sills and fault core and damage zones allow only limited localization of overpressures within the fault zone, thus offering a possible mechanism for regional crustal weakness, but not for localized fault weakness.
DE: 8045 Role of fluids
DE: 8123 Dynamics, seismotectonics
DE: 8150 Plate boundary--general (3040)
DE: 8159 Rheology--crust and lithosphere
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting