Making the San Andreas Fault
at the Mendocino Triple Junction

A dense array of broadband seismographs are being used to investigate the structure of the Mendocino Triple Junction, the northernmost terminus of California's famed San Andreas fault system. The study area extends from the California-Oregon coast to 120o W longitude, and from 37 to 43o North latitude. The Mendocino Triple Junction, the site where the North America, Paclific and the Gorda plates meet, migrates northward with time relative to a fixed North American reference frame. A set of interacting processes associated with the migrating Mendocino Triple Junction, extending from the surface to asthenospheric levels, act to modify North America lithosphere from a subduction complex at Cascadia to the San Andreas transform plate boundary, leaving coastal California in its wake. The mantle imparts a dominant control on the processes occurring near the MTJ through its control on plate strength and kinematics and the flux of heat and mass from the asthenosphere to the North American plate.

Results of an extensive active source seismic study in 1993-94 provide excellent crustal control in the MTJ region. This experiment provides improved imaging of the structure, fabric, and seismic and aseismic deformation of the mantle and lower crust in the MTJ region.

Gorda crust and overlying sediment subduct and become incorporated with accretionary terranes from the Cascadia subduction zone as this mass flows into the emerging transform margin and becomes structured into the San Andreas strike-slip system. Simultaneously, slab gap opening south of the Gorda slab causes asthenospheric ascent and decompression melting, which magmatically underplates North America near the San Andreas fault. The reprocessed and inflated lithosphere thickens to create a small Cape Mendocino orogenic plateau, while erosion fluxes large fractions of the upper crust back to the subduction zone along tectonically controlled north-trending rivers. Nearby, Gorda-Juan de Fuca subduction results in the Cascade volcanoes, which also contribute to continental crust growth, segregation and recycling. The 3-D seismic velocity models of the crust and upper mantle, which will be developed as part of this project, will be included in event and strong ground motion characterization for northern California and southern Oregon, a site of potentially devastating great earthquakes.

Deployment photos