As rifts propagate through oceanic plates, they must occasionally intersect subduction zones, unless subduction-related stresses are sufficient to terminate propagation. If propagators do reach subduction zones, there are possible consequences for continental margin evolution, depending on the geometry of propagation. For example, if the left-offset propagators that approached the western North American continental margin reached the Farallon–North America subduction zone and continued to propagate down the subducting Farallon plate, they would have transferred already-subducted Farallon lithosphere to the Pacific plate. Basal shear stresses between the resulting northwest-moving parts of the subducted slab and the overriding continent could have led to rifting of the continental margin. The intersections of the only two left-offset propagators that appear to have reached this margin correlate spatially and possibly temporally with the initiation of the two most unique Neogene geologic structures of the western North American continental margin, the Gulf of California and the Transverse Ranges, suggesting a possible causal relationship.