HR: 14:30h
AN: T33E-04    [Abstracts]
TI: Evidence of long-term weakness on faults in western North America from dynamic modeling
AU: * Klein, E C
EM: elliot@mantle.geo.sunysb.edu
AF: Stony Brook University, Department of Geosciences, Stony Brook, NY 11794 United States
AU: Holt, W E
EM: wholt@mantle.geo.sunysb.edu
AF: Stony Brook University, Department of Geosciences, Stony Brook, NY 11794 United States
AU: Flesch, L M
EM: flesch@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015 United States
AU: Haines, A J
EM: haines@ese.cam.ac.uk
AF: University of Cambridge, Bullard Laboratories, Cambridge, CB3 0EZ United Kingdom
AB: Dynamic models of the lithosphere resulting from thin sheet approximations provide estimates of the total strength of the lithosphere but only to a maximum thickness that is governed by the degree of mechanical coupling between rheologically stratified layers. The brittle-plastic transition (BPT) within the crustal portion of the lithosphere divides the crust into two distinct rheologic layers, a mechanically strong elastic upper crust and a mechanically weaker plastic lower crust. Over geologic time, the elastic upper crust is subject to deformation by brittle fracture and faulting while the lower crust beneath the BPT is ductile and flows plastically. The cut-off depth for shallow earthquakes occurring in tectonically active regions of upper crust often delineates the depth at which there is a transition from a brittle, seismically active elastic upper crustal layer to a plastic, aseismic lower crustal layer [\textit{Bonner et al.}, 2003]. These observations suggest that the BPT depth is highly variable, ranging from 7 to 40 km, where the actual depth at a given location is a consequence of the local fault regime and temperature gradient. Due to the large rheological contrast between the upper and lower crust, we regard the upper crust as sufficiently mechanically de-coupled from the lower crust such that the upper crust cannot sustain long-term shear stress at its base. In this study, we seek to match the long-term styles and directions of the deviatoric stress field associated with active faulting within the seismogenic layer of the plate boundary zone of western North America. We use a thin sheet approximation that neglects the small shear stresses at the base of the seismogenic layer as well as stresses due to flexure. We vertically integrate the force balance equations, incorporating spatially varying vertically integrated vertical stresses and stress field boundary conditions [\textit{Flesch et al.}, 2001], to provide estimates of the total strength of a variably thick upper crustal seismogenic layer. In addition, the BPT for these models are assumed to occur at crustal depths where the temperature ranges from 300 to 350$^\circ$C. Our BPT depths are based on interpolated geothermal gradients constrained by heat flow observations. Our modeling process begins by development of a long-term kinematic strain rate and velocity field model based on interpolation of Quaternary strain rates and to a lesser extent GPS velocity vectors. We assume that the relationship between deviatoric stress directions and kinematic strain rate directions is isotropic and the directions and style of the principal axes of kinematically defined strain rates are the appropriate stress field indicators. We assume that over geologic time the upper crustal elastic layer fails by Byerlee's rule [\textit{Kohlstedt et al.}, 1995] and that our estimates of the absolute magnitudes of the total strength of the upper crust, which range from $0.5$ -- $1.5$ $\times$ $10^1$$^2$ Nm$^-$$^1$ are supported by long-term values of the coefficient of friction ($\mu$). Initial results for western North America indicate that long-term values of $\mu$ that support the stresses in the upper crust are low, ranging from 0.05 to 0.45, under hydrostatic conditions. The disparity between $\mu$ values inferred from laboratory based experiments versus those inferred here suggest that $\mu$ may be lowered significantly during the rupture process, when most of the work is done within the seismogenic layer.
DE: 8110 Continental tectonics--general (0905)
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8122 Dynamics, gravity and tectonics
DE: 8164 Stresses--crust and lithosphere
DE: 5418 Heat flow
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting