Devonian and Lower Mississippian volcano-plutonic complexes and related basinal strata that extend from California to British Columbia lie outboard of coeval North American shelf rocks. Remarkable similarities between these complexes indicate that they form the remnants of a mid-Paleozoic east Pacific fringing arc system. Arc basement was composite. It is characterized in part by high initial Sr ratios, radiogenic Pb, and −ɛ_Nd values, and in part by low initial Sr ratios, various initial Pb ratios, and +ɛ_Nd values. These distinctive isotopic signatures, together with the presence of Proterozoic average ages for detrital zircon in ultimately continent-derived sedimentary rocks and Proterozoic upper-intercept (inheritance) ages on zircon from arc- and rift-related magmatic rocks, imply that basement was composed of both continental crust and transitional or oceanic crust overlain by continent-derived sedimentary rocks. These data suggest that the arc system formed adjacent to a continental landmass. The initial geometry of the arc has been highly modified by subsequent crustal thickening, extension, associated uplift, and transport along strike-slip faults.