The mechanics of continental transforms: An alternative approach with applications to the San Andreas system and the tectonics of California

Displacement on intracontinental transforms is commonly distributed over a zone several hundred km wide, and may incorporate large regions of transtensional and transpressional strain, but no consensus exists on what controls the distribution and style of this deformation. We model the transform boundary as a weak shear zone of finite length that exerts shear stress on the deformable continental lithosphere on either side. Strain-rate decreases away from the shear zone on a scale related to its length. Force balance in this system requires lateral gradients in shear strain-rate to be balanced by longitudinal gradients in stretching rates, which create zones of lithospheric thickening and thinning distributed anti-symmetrically about the shear zone. Simple analytical estimates, two-dimensional (2D) spectral models, and 2D/3D numerical models are employed to study the spatial scales and magnitudes of the zones of uplift and subsidence. Using reasonable parameter values, the models yield geologically relevant rates. Strain-rate components inferred from the GPS-determined 2-D velocity field, and analysis of seismicity using Kostrov's method, taken together with the geological data on the distribution of active faults, uplift, and subsidence, suggest that the distribution and rates of active deformation in California are consistent with our predictions. This validates the assumptions of the continuum approach, and provides a tool for predicting and explaining the tectonics of California and of other intracontinental transform systems.