Pacific - North America displacement in California is distributed over a zone of intracontinental deformation 400 km wide, and incorporates large regions of transtensional and transpressional deformation. This pattern of deformation is not easily explicable in terms of brittle Coulomb failure, which should localize deformation on to a single fault. There is no consensus at present on what controls the width of this zone or the distribution of strain within it. We model the transform as a weak ductile shear zone, terminating at either end in an effectively stress-free boundary. The shear zone exerts a shear-stress boundary condition on the stronger but deformable continental lithosphere either side. Stress and strain-rate decrease away from the shear zone because of its limited length in relation to the scale of the plates. Force balance in a sheet of deformable material with free upper and lower surfaces requires lateral gradients in horizontal shear-strain rate to be balanced by longitudinal gradients in horizontal stretching rate. Analytical estimates and 3D numerical modeling demonstrate that these gradients will create 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. This provides a test for the length- scales concept, and a powerful predictive tool for understanding the tectonics of California and other intracontinental transforms.