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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.