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.
The three institutions, Rice, UC Berkeley, and the University of Oregon, that conducted the Flexible Array Mendocino Experiment (FAME) deployed 75 broadband seismographs to record earthquakes in order to produce crust and upper mantle seismic velocity models of the Mendocino Triple Junction (MTJ) region. The MTJ is the northern terminus of the San Andreas Fault system, and the southern terminus of the Cascadia subduction zone, and is therefore a region of enhanced seismic hazard. It is also a tectonic novelty as it forms a migrating fault-fault-trench (FFT) triple junction between the North American, Pacific, and Gorda plates. This triple junction has migrated northward from roughly the latitude of Los Angeles since the San Andreas formed about 28 million years ago. Although this area has been studied previously using active source and earthquake imaging methods, FAME discovered a number of previously unknown aspects of the deeper processes associated with the Mendocino Triple Junction, in particular 1) We identified upper mantle sources of melt associated with migrating centers of basaltic and andesitic volcanism in the California Coast Ranges that migrate along with the triple junction a hundred to several hundred kilometers south of it. In particular we found the melt generation centers in the upper mantle for the Lake Pillsbury and Clear Lake regions. 2) We determined that the melt zone beneath the Clear Lake volcanic field is fed by a combination of asthenospheric sources, both upwelling into the slab-gap south of the MJT, and also from deeper hydrated mantle that was formerly the Cascadia mantle wedge, abandoned as the MTJ migrates northward. This explains some of the unique chemistry of the Clear Lake volcanic field. 3) We imaged upper mantle melt supply zones beneath the southernmost Cascades, Mts. Lassen and Shasta, as well as crustal magma chambers, identified as anomalously low shear velocity zones. 4) We added to the growing body of knowledge concerning upper mantle serpentinization in subduction zones that results from slab dewatering. 5) We produced a highly detailed 3D shear velocity model of the triple junction region that can be used in earthquake location studies.
