PULLING THE RUG OUT FROM CALIFORNIA: SEISMIC IMAGES OF THE MENDOCINO TRIPLE JUNCTION REGION

additional authors: LEVANDER, Alan, Dept. of Earth Sci., Rice Un., Houston TX 77005, alan@rice.edu; MELTZER, Anne S., Dept. of Earth and Envir. Sci., Lehigh Un., Bethlehem PA 18015, ameltzer@lehigh.edu, MOONEY, W., USGS, Menlo Park CA 94025, mooney@usgs.gov

The Mendocino Triple Junction (MTJ), currently located near Cape Mendocino in northern California, has been migrating northward along the western margin of North America since the Oligocene, affecting the geologic evolution of California from Los Angeles to Oregon. A simple plate tectonic model of triple junction migration predicts abrupt thinning of the lithosphere and upwelling of aethenospheric mantle as the MTJ migrates to the north, bringing with it the Gorda plate and leaving the thin, wedge-shaped lithosphere of subduction zone forearc in its wake. The MTJ Seismic Experiment (MTJSE), which was the first of a series of NSF-funded Continental Dynamics projects to investigate the continental margins of North America using integrated onshore and offshore seismic techniques, was designed to test this model and evalute the impact of triple junction migration on crustal structure and seismic hazards.

Data for the MTJSE were collected during two field seasons in 1993 and 1994, resulting in a network of seismic reflection and refraction profiles covering the entire active plate boundary zone from 39-42oN and 121-126oW. Results from the seismic imaging include: 1) detailed constraints on the geometry of the subducting plate and its relationship to seismicity north of the triple junction; 2) indications that the strike-slip faults south of the triple junction penetrate through the entire crust, including a highly reflective lower crustal layer that is interpreted to result from aqueous fluids and/or magmatic underplating; and 3) correlation of intraplate deformation with plate age. While the first order plate tectonic model remains valid, the high resolution images of crust and upper mantle structure obtained from our seismic experiment have led to revisions of the simple plate tectonic model of triple junction migration to include generation of an ephemeral welt of thickened crust in the vicinity of the triple junction,evolution of thrust faults into strike-slip faults in advance of the triple junction, and the delayed impact of fluid processes in the subduction zone on later magmatism in the transform regime.