Abstract
Scaling of viscous shear zones with depth-dependent viscosity and power-law stress–strain-rate dependence
Highlights
SUMMARYOne of the unresolved questions concerning fault deformation is the degree and cause of localization of shear at depth beneath a fault
We observe faults as quasi-planar structures at the Earth’s surface, but what governs the distribution of shear at depth? Geologic observations of exhumed shear zones (Poirier 1980; White et al 1980; Rutter 1999; Norris & Cooper 2003) indicate that while the motion is no longer planar, it can still be localized near the downdip extension of the fault; the degree of localization is still uncertain (Wilson et al 2004; Burgmann & Dresen 2008)
We have investigated the interplay of three physical mechanisms responsible for the localization of shear at depth beneath a strike-slip fault: depth-dependent viscosity, a non-linear stress–strain relation and thermomechanical coupling driven by shear-stress heating
Summary
One of the unresolved questions concerning fault deformation is the degree and cause of localization of shear at depth beneath a fault. We find that the primary control on the shear-zone width is the depth dependence of viscosity that arises from the temperature dependence of viscosity and the increase in temperature with depth As this relationship is exponential, scaling relations give a dimensional half-width that scales approximately as δw ≈ T1 2. (J mol−1) the activation energy and β (K km−1) the geothermal gradient This relation predicts the numerical results for the parameter range consistent with continental rheologies to within. While the width of the shear zone may not decrease significantly, local temperature increases from shear-stress heating range from 50 to 300 ◦C resulting in a reduction in viscosities beneath the fault of several orders of magnitude and a concomitant reduction in the stresses needed to drive the motion
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