During the middle and late Tertiary, several prominent topographic scarps, the Farallon plate equivalents of present-day fracture zones in the Pacific plate, were subducted under the western margin of North America. If subduction occurred at a shallow angle so that the underthrust Farallon plate was in isostatic balance with the overlying North American plate, then differential uplift of the North American plate would have occurred. The Mendocino fracture zone was the tallest of the subducted scarps, and isostasy calculations indicate that the surface of the North American plate south of the trace of the subducted fracture zone should have been 500 to 1000 m higher than the surface north of the trace. Flexure calculations in which the North American plate is treated as a thin slab predict that this uplift should have been spread over a north-south distance of 70 to 120 km, yielding an average slop of around 0.5¿. A regional slope of this magnitude would have been great enough to account for observed late Oligocene and Miocene disruptions of drainage and sedimentation in southern California. Calculated stresses in the upper part of the flexed plate are 50 to 80 MPa. These stresses exceed the tensile failure strength of crustal rocks and suggest that the plate should have failed by tensile cracking. Tensile fracture would decrease the elastic thickness of the plate, enhance bending, and perhaps lead to failure by normal faulting. This mechanism may explain the formation of east-west faults in southern California.