The coastal region of northern California from latitude of 38o to 41oN represents a region in which the present tectonic regime changes abruptly from subduction to transform motion at the Mendocino Triple Junction (MTJ). Because the triple junction is migrating north along the coast of North America, an understanding of the crust and upper mantle structure in this region will provide insights into the tectonic evolution of the entire U.S. west coast continental margin as well as other convergent margins worldwide. This project involves an integrated seismic experiment to image the structure of the crust and upper mantle immediately before and after passage of the MTJ. This study represents an important part of a long-term, multi-disciplinary effort on the part of many investigators to exploit this well defined system as an in-situ laboratory to study physical and chemical processes associated with mantle upwelling, melt segregation and migration, tectonic accretion, and crustal deformation in both transform and convergent tectonic settings, and to evaluate the implications of these processes for continental growth and evolution. The study encompasses two field seasons. One season involves acquisition of eight multichannel seismic (MCS) profiles designed to image deep crustal structure beneath: 1) Pt. Arena basin, San Andreas fault, and extinct trench south of the MTJ; 2) the Mendocino Transform fault; and 3) the Eel River basin and Cascadia subduction zone north of the MTJ. The profiles intersect wells both north and south of the MTJ to provide stratigraphic control. The MCS profiles will also be recorded on ocean-bottom hydrophones and seismographs and on REFTEK seismographs deployed onshore to provide complementary large aperture data. The other season involves acquisition of three onshore explosive-source refraction profiles, two of which extend onshore/offshore transects shot north and south of the MTF, in order to cross the fore-arc and slabless window. The third refraction profile will follow the trend of the Franciscan terrane in order to cross the southern edge of the Gorda plate, map possible along-strike variations in crustal structure and tie together the two primary onshore-offshore profiles. All three land profiles will be densely instrumented with REFTEK and SGR seismographs.
