New geologic mapping and SHRIMP U-Pb zircon data in the Peninsular Ranges batholith, Baja California, Mexico: Evidence for a suture?

Geologic and SHRIMP U-Pb zircon studies in the Sierra San Pedro Martir 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 Martir thrust) marked by an age discontinuity of at least 10 to 15 m.y., and sharp change s in rock type, metamorphic grade, and deformation history. The east-dippin g thrust, and adjacent highly deformed rocks, were stitched by 108-97 Ma pl utons that were generated in a region of crustal thickening formed during j uxtaposition of the two arcs. Lack of chemical, isotopic, and geologic evid ence for continentally derived rocks in the western arc may preclude an ori gin by rifting of the continental margin, or development of the arc on a pr ism of continentally derived sediments adjacent to the continental margin. Instead, the western arc may have originated as an island arc above a subdu ction zone outboard of North America. If so, a second, concurrent subductio n zone along the North American margin must have driven convergence and sut uring of the two arcs, and the Main Martir thrust may mark a nonterminal su ture within a wider convergence-related deformation zone that formed ca. 11 5-108 Ma.