Influence of plate subduction on isostatic compensation in northern California

Abstract

The isostatic gravity map for northern California calculated by the U.S. Geological Survey on the assumption of Airy compensation reveals a residual correlation between gravity and topography. Peaks in the Klamath and Siskiyou Mountains west of 122.25°W and north of 40°N are associated with isostatic gravity highs in excess of 100 gu (gravity units), while to the east the Cascade volcanoes Lassen and Shasta display gravity lows less than −300 gu. Admittance calculations which determine the correlation between Bouguer gravity and topography demonstrate that some of the difference in isostatic gravity signature for the Klamaths and Cascades can be attributed to density variations in surficial rock types. The admittance estimates at wavelengths less than 50 km suggest that the mafic and metamorphosed units in the Klamath block are at least 80 kg/m³ more dense on average than the standard Bouguer reduction density of 2670 kg/m³. In addition, longer wavelength response estimates require some differences in the nature of the isostatic compensation for features in eastern versus western sections of northern California. Interpretations of the response functions in addition to direct modeling of a representative east‐west cross‐section indicates that the Klamath Mountains are rootless structures supported by the rigidity of the underlying Gorda plate, while the Cascades are locally compensated by shallower, low‐density material within the North American plate. Seismicity studies, heat flow data, and gravity structures south of this region are also consistent with this model.