The Jurassic-Cretaceous Peninsular Ranges batholith of southern and Baja California exhibits several along-strike variations that are most pronounced across the Agua Blanca fault. These variations include differences in the age of magmatism, degree of inheritance in zircons, depositional environment of volcanic strata, extra-arc basin geography, lateral extent of continentally derived flysch strata, and the structures attending juxtaposition of the two segments with the North American continental margin and with each other. We propose that these variations, when taken together, imply that the two segments of the western zone currently juxtaposed by the active Agua Blanca fault were accreted to North America diachronously and did not share a common history prior to the late Early Cretaceous. The Santiago Peak arc segment north of the fault developed on oceanic lithosphere juxtaposed with the continental margin prior to and during arc magmatism. Conversely, the Alisitos arc segment developed on oceanic lithosphere exotic to North America prior to accretion at 115–108 Ma. If this model is correct, it implies that the Agua Blanca fault initiated as a transpressional continuation of the suture joining the latter arc segment to the continent and that a large portion of the forearc that existed between them was subducted.