NEW GRAVITY AND AEROMAGNETIC MAPS REVEAL PLUTON, BASIN, AND FAULT GEOMETRY WITHIN AND AROUND THE SIERRA NEVADA, CALIFORNIA AND NEVADA

We present new gravity and aeromagnetic anomaly maps of the Sierra Nevada that, when combined with a new compilation of rock density and magnetic susceptibility data, provide information on the depth extents of plutons and basins within and around the Sierra Nevada as well as fault continuity and cumulative offset. Complete Bouguer and isostatic gravity maps were created from more than 28,900 gravity measurements from existing data sources and augmented by new data. The new aeromagnetic and derived pseudogravity maps were created from 57 surveys flown at various flight heights and spacings from 1951-2021.

These maps highlight the dense western part of the Sierra Nevada that reflects mainly Jurassic metavolcanic and ophiolitic rocks. Mafic plutons of the Jurassic Smartville Complex and Cretaceous gabbros in the Kings-Kaweah area are associated with prominent gravity and magnetic highs. Modeling of these mafic bodies indicates that they extend to depths of ~10 km or more. The western gravity highs are interrupted by three large, semicircular gravity lows, two of which coincide with the Early Cretaceous Bald Rock and Rocklin-Penryn plutons. Modeling of these bodies indicates that the plutons extend to 5-20 km depth. The lowest values of the third gravity low north of Fresno are not over exposed bedrock, but instead most likely result from a concealed early Cretaceous trondhjemite pluton. The concealed lobe of the pluton is separated from its exposed counterpart by a gravity ridge. This ridge (denser Jurassic slate) is aligned with the edge of the 100-km-long mafic Kings-Kaweah block to the southeast and with gradients that extend as far northwest as Lake Oroville. Strands of the Quaternary Foothills fault system coincide with some of these gradients and suggest reactivation of Mesozoic structures where favorably oriented.

The eastern Sierra Nevada is marked by magnetic highs and relatively low gravity values that are fit by density and magnetic properties that extend to depths of 10-15 km. Modeling of gravity lows east of the Sierra Nevada shows that Cenozoic basins may be up to 4-5 km deep, whereas the few Cenozoic basins within the Sierra Nevada are less than 1 km deep, with the possible exception of a pull-apart basin along the Mohawk Valley fault. The length of that basin suggests about 5 km of right-lateral offset.