The Jura-Cretaceous Peninsular Ranges batholith (PRB) of southern and Baja California contains a remarkable example of a crustal transition between juxtaposed oceanic and continental floored arcs. Jura-Cretaceous deformation in the PRB is focused along this lithospheric discontinuity and defines a zone of contractional deformation that stretches at least 800 km along the batholith. The western edge of this zone is associated with a dramatic eastward increase in maximum recorded metamorphic pressures (from 2 to 6 kbar) and temperatures (from 650¿C). Elsewhere in the North American Cordillera, similar lithospheric boundaries are thought to accommodate various amounts of orogen-parallel displacements of outboard tectonic fragments. To test this proposal in the PRB, we examined an unusually well-preserved part of the transition zone in the southern Sierra San Pedro Martir (SSPM). Here a spectacular ~20 km wide, doubly vergent fan structure, which developed over a period >40 m.y. during both Jurassic and Cretaceous magmatism, occurs along the transition zone. This structure consists of moderately inward dipping, outward vergent, fold and thrust belts with widespread mylonitic fabrics that steepen toward the center where both mylonitic and magmatic fabrics occur in syntectonic, vertically sheeted plutons. Mineral lineations change from steeply pitching on the sides of the fan to moderately pitching in the center. A number of constraints on possible displacements, rotation of units, and duration of fabric formation limit the amount of orogen-parallel displacement to a few kilometers across the fan structure in the SSPM. Instead, we propose that the fan structure is a local feature that results from a combination of tectonic wedging caused by a cusp of rigid, pre-Mesozoic continental margin crust at this location and vise tectonics that arose from strong lateral rheological gradients resulting from extensive heating by the sheeted tonalite complex, focused deformation along the lithospheric discontinuity, and subsequent strain softening. We suggest that these mechanical controls on deformation during orthogonal convergence may be common in other orogens where similar fan structures have been attributed to transpression, emphasizing the need for caution in interpreting such features as zones of significant, orogen-parallel displacement.