The Development of the Northern San Andreas Plate Boundary Fault System - Importance of Lower-crustal Ductile Shear in Producing Primary Plate Boundary Structure

The northward migration of the Mendocino Triple Junction (MTJ) drives a fundamental plate boundary transformation from convergence to translation; producing a series of strike-slip faults, that become the San Andreas plate boundary. How and why these faults develop where they do is enigmatic. We find that the 3-D structure of the Pacific plate lithosphere in the vicinity of the MTJ controls the location of San Andreas plate boundary formation. Recently developed, high-resolution seismic-tomographic imagery of northern California indicates that (1) the Pioneer Fragment, and extension of the Pacific plate beneath the western margin of North America occupies the western half of the slab window, immediately south of the MTJ; (2) the eastern edge of the Pioneer Fragment lies beneath the newly forming Maacama Fault system, which develops to become the locus for the primary plate boundary structure after approximately 6-10 Ma (eg. the present-day East Bay faults in the SF Bay region); and (3) the placement of the translating Pioneer Fragment adjacent to the asthenosphere of the slab window, and its coupling to the overlying North American crust generate a shear zone within and below the crust that develops into the  plate boundary faults. This plate boundary configuration has been operable since the initial formation of the transform plate boundary. As a result, the San Andreas plate boundary forms within the western margin of North America, approximately 100 km inboard of the western edge of North America, rather than at its western edge. One additional result of this is that blocks of North America lithosphere are detached and become terranes (such as the Salinian and Nacimiento (Franciscan) blocks) that are captured by and translate with the Pacific plate, producing the complex crustal architecture of coastal California.