Collaborative Research: Making the San Andreas Fault at the Mendocino Triple Junction: Part II

William Marsh Rice University, Houston TX

Investigators

Abstract

Collaborative Research: Making the San Andreas Fault at the Mendocino Triple Junction: Part II A dense array of broadband seismographs have been deployed 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 degrees to 43 degrees North latitude. The Mendocino Triple Junction, the site where the North America, Pacific and 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 (> 100 km depth), act to modify North America lithosphere from a subduction complex at the Gorda subduction zone to the San Andreas transform plate boundary. 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. 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. To the east, Gorda-Juan de Fuca subduction results in the Cascade volcanoes, which contribute to continental crust growth, segregation and recycling. Results of an extensive active source seismic study in 1993-94 provide excellent crustal control in the MTJ region. The broadband seismograph array now deployed will remain in place through April 2009, to provide a dataset that will enable us to image the structure, fabric, and seismic and aseismic deformation of the mantle and lower crust in the MTJ region and the southernmost Cascades. Using a variety of analysis techniques we will produce a 3-D seismic velocity model of the crust and upper mantle that will be included in event and strong ground motion characterization for northern California and southern Oregon, a site of potentially devastating great earthquakes.