CRUSTAL STRUCTURE OF THE SIERRA NEVADA, CALIFORNIA: INSIGHTS FROM MAGNETIC AND GRAVITY DATA

A new compilation of magnetic and gravity data of the Sierra Nevada reveals regional crustal structures that can be mapped across Cenozoic cover and Cretaceous plutons. Earlier studies noted a correspondence of magnetic properties with initial strontium ratios at the latitude of Yosemite Valley, borne out by magnetic highs filtered to enhance voluminous magnetic sources that occur in areas where the initial strontium ratio in Mesozoic plutons is greater than 0.706. The new compilation shows that the correspondence is not so simple, however, as 40 km to the northwest, the magnetic highs terminate south of the northern extent of the .706 line. The eastern margin of the magnetic high does not follow the .706 line to the southeast.

The rest of the Sierra Nevada is characterized by magnetic highs associated with the northeastern part of the Northern Sierra terrane, the Kings River ophiolite and Kaweah serpentinite mélange, the Feather River Peridotite, and mafic and ultramafic bodies in the Western Metamorphic Belt, separated from each other and from the Great Valley magnetic highs to the west by prominent magnetic lows. The southern boundary of the Northern Sierra terrane magnetic high cuts across exposures of the Lake Tahoe intrusive suite and locally coincides with the Quaternary Dog Valley fault. Locally along the Kings-Kaweah western margin are concealed faults that project towards a zone of linear N-NW-trending magnetic and gravity gradients that can be mapped along the entire length of the Sierra Nevada. These gradients coincide locally with strands of the Quaternary Foothills fault system. The gradients appear to extend across circular gravity and magnetic lows whose eastern parts coincide with the Cretaceous Penryn and Rocklin plutons. Southeast of Auburn, Quaternary faults step east around prominent magnetic and gravity anomalies of the mafic Pine Hill intrusive complex. The magnetic data show that the Feather River peridotite dips steeply NE and extends to mid-crustal depths where it coincides with the Meadow Valley fault zone. The magnetic data can be used to map the peridotite beneath Cenozoic volcanic cover for about 25 km. Combined with surface geology, potential-field data suggests that some of the faults marked by magnetic and gravity anomalies appear to influence pluton architecture and Cenozoic faulting.