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We propose a simple slip line model to explain the faulting and seismicity patterns of southern California. We assume the Sierra Nevada-Great Valley (SNGV) block to be rigid, in view of its low seismicity, and other regions to undergo plastic deformation. From this point of view, the complex deformation patterns observed in southern California, including the Borderland region, result from the "plowing" of the eastern margin of the Pacific plate by the nondeforming SNGV block. A simple analogy can then be made between regional tectonics in southern California and two-dimensional flow solutions obtained in metallurgy for the extrusion of rigid-plastic metals. We use a simplified geometry based on the geology and seismicity of southern California to calculate a two-dimensional slip line field which predicts the shape of the San Andreas fault near the "big bend" surprisingly well. The largely aseismic, high-topography western Mojave can then be interpreted as an analogue to the nondeforming "dead metal" zone which arises for certain metal extrusion geometries. The slip field agrees quite well with the strikes and slip orientation of major dextral faults in southern California, including the San Jacinto, Elsinore, and Newport-Inglewood faults. It also predicts correctly the strikes and slip orientation of sinistral "cross faults" found in some areas of the region. In the Transverse Ranges, where thrust faulting dominates, the slip lines are no longer essential fault planes, but a simple argument predicts that they should lie at 45-degrees on the strikes of thrust faults, in fair agreement with the observations.