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The continental slope and plateau off northern California are underlain by a well-defined and extensive acoustic reflector that crosses other reflectors and mimics the surface of the sea floor. This bottom-simulating reflector (BSR) lies at an average subsurface depth of 250 m and is laterally continuous beneath the Klamath Plateau (water depths of 800-1,200 m) off northernmost California. Limited data show that it extends northward into offshore Oregon and seaward at least to the base of the slope (3,000 m water depth). The BSR has been mapped along the continental margin for a distance of over 130 km and underlies an area of at least 3,000 km2.
The water depths and sediment depths of the BSR, as well as its pervasiveness, all suggest that it represents the base of a natural-gas hydrate. Using standard phase-boundary diagrams for hydrate stability, we estimate that the geothermal gradient in this area is about 50°/km. Such a gradient is higher than found along most subduction margins, possibly because the subducting oceanic crust in this region is young. This acoustically inferred gas hydrate is the first mapped along the western United States conterminous continental margin.
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