CRUSTAL STRUCTURE OF THE NORTHWESTERN VIZCAINO BLOCK AND GORDA ESCARPMENT, OFFSHORE NORTHERN CALIFORNIA, AND IMPLICATIONS FOR POSTSUBDUCTION DEFORMATION OF A PALEOACCRETIONARY MARGIN

The Vizcaino block is an anomalously shallow region of the western U.S . continental margin located southwest of the Mendocino triple junctio n. It originated as part of the accretionary prism of the North Americ a plate and was transferred to the Pacific plate in the Miocene as the Pacific-North America plate boundary migrated similar to 130 km eastw ard, forming the Gorda Escarpment at its northern boundary. We present hybrid crustal models for the northwestern part of the Vizcaino block derived from marine seismic and gravity data. The velocity and densit y structure of the northwestern Vizcaino block are compatible with pal eoaccretionary complex material similar to San Simeon/Patton terrane o verlying oceanic crust or a mafic layer. The most significant result o f our modeling is an abrupt increase in Moho dip from similar to 5 deg rees to similar to 20-30 degrees beneath the western edge of the Ocono stota ridge along the northwestern margin of the Vizcaino block. This Moho dip is steeper than observed anywhere along the Cascadia subducti on zone, indicating postsubduction deformation. We suggest that the pa leotrench was deformed by compression, which reactivated preexisting t hrust faults in the upper crust and thickened the crust within this ap parent weak zone. At least part of the deformation predates late Plioc ene Pacific-North America plate convergence and may result mainly from north-south compression between the Pacific-Juan de Fuca plates acros s the Mendocino transform fault. North-south compression continues tod ay and may dynamically support the uplifted northern margin of the Viz caino block, although the primary locus of deformation shifted to the relatively weak Gorda plate sometime prior to 3 Ma.