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SOURCES OF GRAVITY AND
MAGNETIC ANOMALIES

Conspicuous features of the gravity field over the San Andreas fault system are linear highs and lows that trend subparallel to the major faults in the system. Highs (>=10 mGal) generally occur over exposed crystalline rocks of the Salinian block southwest of the San Andreas fault, over mafic granitic and metamorphic rocks of the Sierra Nevada and the Mojave Desert, and over Mesozoic and Tertiary layered rocks of the Franciscan assemblage, particularly in areas containing large amounts of mafic volcanic rocks (generally part of an ophiolite belt) or high-pressure metamorphic-mineral facies. Most of the deepest lows are caused by thick accumulations of low-density Cenozoic sedimentary rocks that fill tectonic basins adjacent to the faults and in the surrounding areas. Shallower lows occur over certain large serpentinite bodies within the Franciscan assemblage, over felsic plutons in the granitic terranes of California, and over a young concealed granitic pluton associated with the Geysers geothermal area at lat 39� N., long 122�45' W. (Chapman, 1975; Isherwood, 1976).

Magnetic anomalies in the vicinity of the San Andreas fault system typically are caused by any one of three different rock types. The strongest anomalies generally reflect tabular bodies of serpentinite associated with the Franciscan assemblage and may also reflect the ophiolitic rocks, especially serpentinite, that locally lie above it. Mafic plutonic rocks, such as those exposed in the western Peninsular Ranges and along the west edge of the southern Sierra Nevada, can produce moderate to strong magnetic anomalies. Plutonic sources, not necessarily mafic only, probably account for most of the anomalies in the Salinian block, southwest of the San Andreas fault. Although younger volcanic rocks, in particular the mafic varieties, commonly are highly magnetic, such rocks do not cause significant magnetic features near the San Andreas fault as shown on our magnetic anomaly map (fig. 9.3) because magnetic volcanic rocks are volumetrically unimportant at the scale of this map.

In most areas, sedimentary rocks are considered nonmagnetic because they fail to cause aeromagnetic anomalies. Along the San Andreas fault system, however, several sedimentary-rock units cause magnetic anomalies as large as 150 nT. These units include rocks of Mesozoic and Tertiary age; other units composed primarily of detrital serpentinite also produce anomalies of this magnitude. None of these sedimentary units are areally large enough to produce magnetic anomalies visible at the scale of our magnetic anomaly map (fig. 9.3).

Near the north end of the San Andreas fault, several magnetic anomalies project landward from the linear pattern of anomalies that characterizes the oceanic crust. These anomalies reflect remanent magnetization in the oceanic crust; the source rocks are primarily basaltic volcanic rocks.