HR: 0800h
AN: T41B-0579    [Abstracts]
TI: Mantle-derived fluids and their potential role in weakening the San Andreas Fault
AU: * Fulton, P M
EM: pfulton@geosc.psu.edu
AF: Dept. of Geosciences, Pennsylvania State University, University Park, PA 16802, United States
AU: Saffer, D M
EM: dsaffer@geosc.psu.edu
AF: Dept. of Geosciences, Pennsylvania State University, University Park, PA 16802, United States
AB: Many plate boundary faults, including the San Andreas Fault (SAF), have been interpreted to slip at shear stresses considerably less than expected for typical rock and fault gouge friction coefficients and hydrostatic fluid pressures. The lack of a heat flow anomaly from frictional heating as expected near the SAF and maximum horizontal principal stress directions inferred to be at a very steep angle with the SAF imply that the fault is weak in both an absolute sense and considerably weaker than the surrounding crust. One explanation for this fault weakness is the presence of near-lithostatic fluid overpressures localized within the fault zone. Here, we use numerical models of fluid flow and heat transport to investigate the potential role of mantle- derived fluids in generating and sustaining overpressures localized within the fault zone. In cross-sectional models of seismogenic crust perpendicular to the SAF, we consider two types of basal boundary conditions representing the influence of mantle fluids. First, we consider a lithostatic pressure boundary extending 25 km or more along the model base. In a separate set of scenarios, we treat a portion of the base as a flux boundary for fluids produced within the mantle. We consider a large, yet geologically reasonable mantle flux of water of 1.7x10^-9 kg s^-1 per m² of basal area, equivalent to a 10 km thick mantle wedge serpentinized up to 50% during subduction prior to the creation of the SAF, dehydrating over 25 Myr. We evaluate the sensitivity of the location of both basal boundary conditions by centering them directly beneath the fault zone as well as at positions further east. We evaluate two different permeability architectures hypothesized to localize overpressures generated from a large, prolonged regional source onto a near-vertical fault. First, we evaluate a permeable fault zone surrounded by low permeability crust, both with pressure-dependent permeability (e.g., Rice, 1992). Second, we investigate an alternative model in which hydraulically conductive, east-dipping faults and fractures within the Franciscan Assemblage, truncated against the SAF to the west, focus overpressures on the near-vertical fault zone through flow focusing (i.e., the centroid effect). This model allows for crust more permeable than the fault zone, and may more easily account for peak mantle sources located east of the fault as suggested by geochemical data. We find that pressure-dependent permeability has little influence on localizing pressures within the fault zone; resulting pore pressures are similar to those for depth-dependent and depth-invariant permeability. Through sensitivity analysis, we show that large overpressures can be localized within the fault zone with both basal conditions as long as the surrounding crust is at least 2 orders of magnitude less permeable than the fault zone. Models that incorporate dipping permeable faults within the Franciscan result in a broad zone of overpressure near the SAF, with pressures greatest near the fault zone if it acts as a barrier to fluid flow. Our results also suggest that for either permeability architecture to be viable, crustal permeability ≤ 10^-19 m² is required in order to prevent a large near-fault heat flow anomaly from advection in simulations with a basal pressure boundary condition. There is little effect on heat flow in our models with a fluid flux boundary condition, but these models require low crustal permeabilities (<10^-19 m²) in order to generate large overpressures. Both permeability architectures may provide a mechanism for localizing overpressures within the fault zone, and could act in conjunction with low-friction fault gouge to explain a weak SAF.
DE: 8020 Mechanics, theory, and modeling
DE: 8045 Role of fluids
DE: 8150 Plate boundary: general (3040)
DE: 8163 Rheology and friction of fault zones (8034)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting