Major intracontinental strike-slip faults and contrasts in lithospheric strength

A large fraction of major intracontinental strike-slip faults, defined here as those slipping at rates of similar to 10 mm/yr or more, lie adjacent to relatively strong regions, such as oceanic lithosphere or Precambrian shields. We suggest that such faults form adjacent to discontinuities in strength, because strain in a continuous medium must concentrate near such strong objects. Concentration in strain is enhanced in deforming continua where viscosity is non-Newtonian, and hence where strain rates vary with deviatoric stress raised to a power greater than one, as is the case for rock-forming minerals at low temperatures characteristic of the lithosphere. Conversely, in regions where deformation is spread over a wide region, such as in the Basin and Range Province of the western United States or in the high interior of the Tibetan Plateau, the absence of strong objects may allow deformation to be diffuse without the development of rapidly slipping faults.