AEROMAGNETIC SIGNATURES REVEAL STRATIGRAPHIC AND STRUCTURAL RELATIONS IN THE FRANCISCAN COMPLEX EAST OF THE SAN ANDREAS FAULT, NORTHERN CALIFORNIA

Detailed aeromagnetic data integrated with geologic mapping reveals important details of the Franciscan Complex east of the northern San Andreas Fault between Fort Ross and Pt. Arena. Contrasts in rock magnetism help define the shape of rock bodies within mélange and broken formation of the Mesozoic and Tertiary Coastal and Central belts of the Franciscan Complex. These contrasts appear to reflect the distribution of serpentinite and mafic rocks in the Central belt and metavolcanic detritus in coherent sandstone bodies within the Coastal Belt. Those sandstone bodies in the Coastal belt that are spatially coincident with linear aeromagnetic anomalies, which are continuous over tens of kilometers, yield field- measured magnetic susceptibilities in the range of 5 – 20 SI units, compared to susceptibilities of < 1 SI for nonmagnetic sandstones.

The variation in sandstone susceptibility is caused by variation in the amount of magnetite (extracted with a hand magnet from pulverized sandstone samples). Petrographic study indicates that the magnetic susceptibility is, to a first approximation, proportional to the percentage of andesitic volcanic detritus in the sandstones. The volcanic grains within the magnetic sandstones show primary grains of hornblende and euhedral to subhedral magnetite. Pyroxene phenocrysts in the volcanic detritus are extensively replaced by chlorite, epidote-clinozoisite and pumpellyite, however, indicating the volcanic source terrane has undergone uralitization and low greenschist grade metamorphism.

The correspondence between magnetic susceptibility and framework-grain composition allows the mapping of stratigraphy through extensive areas of poor exposure, the modeling of rock-body geometries in the subsurface, and delineation of structural discontinuities. The magnetic sandstones of the Coastal belt thus serve as tectonostratigraphic fabric markers, which will provide new information on the complex structure of the Coastal belt and its contact relations with the Central belt. These fabric markers, irresolvable by conventional geologic mapping techniques, are clearly revealed in the new high-resolution aeromagnetic data.