THE SAN ANDREAS TRANSFORM AS A LITHOSPHERIC SHEAR ZONE

Pacific – North America displacement in California is distributed over a zone 400 km wide, and incorporates large regions of transtensional and transpressional deformation. This pattern of deformation is not easily explicable in terms of Coulomb failure of brittle upper crust, which should localize deformation on to a single fault. The most likely control on the distribution of deformation is ductile flow in the upper lithospheric mantle, with a bulk rheology approximating power-law creep. Two processes with different characteristic length scales control the distribution of velocity in the shear zone. One is the diffusion of velocity within the fluid away from a boundary, such as a plate boundary with strong oceanic lithosphere, as set out in the length-scales concept of England et al. (1985). The San Andreas shear zone, however, cuts continental lithosphere, and there is no obvious hard boundary. The other concept is localization of deformation by microstructural softening. Power-law creep in a crystalline solid is commonly accompanied by dynamic recrystallization, decreasing grain-size and increasing the contribution to the strain-rate from grain-size sensitive creep. This effect may be enhanced by the development of crystallographic fabrics and phase transformations associated with deformation and hydration. Velocity diffusion away from a zone shear localized in this way may explain observed velocity distributions.

Both observed and theoretically predicted velocity distributions across the San Andreas zone involve lateral gradients in shear-strain rate. Force balance in a deforming thin viscous sheet requires lateral gradients in shear-strain rate to be balanced by longitudinal gradients in elongation rate. The latter will be expressed as zones of lithospheric thickening and thinning distributed anti-symmetrically about the shear zone. Lithospheric thickening in the Transverse Ranges and the Klamath Mountains, and thinning in the Eastern California shear zone and the San Francisco Bay area, correspond reasonably well to these predictions, and suggest that a description of the San Andreas transform in terms of velocity diffusion in a power-law fluid is reasonable.

Reference: England et al. (1985). J. Geophys. Res. 90, 3551-3557.