ROLE OF 3-D LITHOSPHERE GEOMETRY IN THE DEVELOPMENT OF THE NORTHERN SAN ANDREAS PLATE BOUNDARY FAULT SYSTEM

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. As our understanding of deformational processes in the vicinity of the MTJ has improved, it is clear that spatial variation in thermal structure alone is not adequate to drive the fault development and localization. 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. At the time of initiation of the Pacific-North America plate boundary (~ 30 Ma), the sequential interaction with the western margin of North America of the Pioneer Fracture Zone (PFZ), soon followed by the Mendocino Fracture Zone (MFZ) led to the capture of a small segment of partially subducted Farallon lithosphere (between the PFZ and MFZ) by the Pacific plate, termed the Pioneer Fragment (PF). Since that time, the PF has translated with the Pacific Plate along the western margin of North America.

Recently developed, high-resolution seismic-tomographic imagery of northern California indicates that (1) the PF is extant, occupying the western half of the slab window, immediately south of the MTJ; (2) the eastern edge of the PF 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 PF adjacent to the asthenosphere of the slab window, and PF 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, rather than at its western edge; and 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.