Oral Presentation

The Pacific-North America transform plate boundary fault system spans the length of California, from the Gulf of California rift to the Cascadia subduction zone. The lithosphere traversed and modified by this system reflects over 250 Myr of prior subduction-related magmatism and forearc deformation. To first order, the geologic framework of California exhibits the classic architecture of a continental subduction margin: accretionary prism, forearc basin, magmatic arc, and mobile back-arc. In this talk I briefly summarize four major events that disrupted this margin and impact the present distribution of active faults and fault behavior: (1) Late Jurassic trench rollback and forearc extension that led to development of the Great Valley ophiolite body. This strong, mafic crust forms the western third of the tectonically stable Sierra Nevada-Great Valley (SNGV) microplate and also underlies the outer borderland block offshore southern California. (2) Early Cretaceous underplating of accretionary prism-derived rocks beneath the Klamath mountains terrane, commensurate with trenchward migration of arc-modified upper crust. Today this remote region hosts diffuse, active deformation between the more rigid SNGV microplate and the Oregon Coast block, transferring strain between the Walker Lane and Mendocino Triple Junction. (3) A second episode of underplating in the Late Cretaceous and Early Cenozoic in southern California, again resulting in trenchward migration of arc-modified upper crust. This event modified much of the lithosphere of southern California and set up the conditions leading to the complex transpressional fault network active today. (4) Late Cenozoic impingement of the Pacific-Farallon spreading center against the subduction zone, initiating the transform plate boundary and leading to extreme crustal thinning and rotation of crustal blocks to form the inner Continental Borderland. This event further modified the lithosphere of the central and southern California, as well as stranding a fragment of oceanic lithosphere beneath the margin that has subsequently affected the development of the multi-stranded transform fault-system in northern California. Consequent features of the San Andreas fault system that reflect this inherited lithospheric architecture include the heterogeneous distribution of active faults, the locations of creeping faults within central and northern California, and, arguably, the time-dependent behavior of earthquake hazard.