Weak faults in a strong crust: Geodynamic constraints on fault strength, stress in the crust, and the vertical distribution of strength in the lithosphere.

HR: 0800h
AN: V21A-0597 [Abstracts]
TI: Weak faults in a strong crust: Geodynamic constraints on fault strength, stress in the crust, and the vertical distribution of strength in the lithosphere.
AU: * Fay, N
EM: nfay@uoregon.edu
AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403 United States
AU: Humphreys, G
EM: gene@newberry.uoregon.edu
AF: University of Oregon, Department of Geological Sciences, Eugene, OR 97403 United States
AB: We present results of steady-state dynamic finite element numerical models for the state of stress and strain rate in the crust and upper mantle in the vicinity of a transform fault. Model rheology is elastic-viscous-plastic where plastic mechanical behavior is used as a proxy for pressure-dependent friction of the seismogenic crust. Viscous flow is incorporated as temperature dependent, power-law creep. We assume that the crust outside the fault zone is at or near its frictional yield strength (i.e., `strong', e.g., Townend and Zoback, 2001) predicted by Byerlee's Law and aim to determine the acceptable range of fault strength and viscosity distributions that satisfy the observations that seismic faulting extends to typically 15 km and that the tectonic strain rate of fault-bounding blocks is small. Assuming the traditional `christmas-tree' strength distribution of the lithosphere (e.g., Brace and Kohlstedt, 1980), our primary results are the following. The upper limit of fault strength is approximately 30 MPa (averaged over 15 km), the majority (>50%) of the vertically integrated strength of the lithosphere resides in the uppermost mantle, and the depth to which frictional deformation occurs in the crustal blocks depends on the strength of nearby faults and viscosity of the lower crust and should not exceed approximately 6-9 km, below which relatively low strain rate viscous creep is the dominant deformation mechanism. We also incorporate the thermal feedback effects of viscous and frictional shear heating and explore the controls on the distribution of shear strain in the lower crust and upper mantle.
DE: 8106 Continental margins: transform
DE: 8111 Continental tectonics: strike-slip and transform
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8159 Rheology: crust and lithosphere (8031)
DE: 8164 Stresses: crust and lithosphere
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005