Geologic and SHRIMP U-Pb zircon studies in the Sierra San Pedro Mártir area of northern Baja California, Mexico, suggest that the western and eastern parts of the Peninsular Ranges batholith originated as separate arcs. They are now juxtaposed along a well-exposed ductile thrust (Main Mártir thrust) marked by an age discontinuity of at least 10 to 15 m.y., and sharp changes in rock type, metamorphic grade, and deformation history. The east-dipping thrust, and adjacent highly deformed rocks, were stitched by 108–97 Ma plutons that were generated in a region of crustal thickening formed during juxtaposition of the two arcs. Lack of chemical, isotopic, and geologic evidence for continentally derived rocks in the western arc may preclude an origin by rifting of the continental margin, or development of the arc on a prism of continentally derived sediments adjacent to the continental margin. Instead, the western arc may have originated as an island arc above a subduction zone outboard of North America. If so, a second, concurrent subduction zone along the North American margin must have driven convergence and suturing of the two arcs, and the Main Mártir thrust may mark a nonterminal suture within a wider convergence-related deformation zone that formed ca. 115–108 Ma.