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California Has Its
Faults
From
January/February 1992 issue of
California Geology magazine
A fault is a fracture
along which there is movement. Some
faults are actually composed of several
fractures called fault branches.
Collectively the branches are a fault
zone (see map).
California's diverse landscape and complex geology can be attributed to faulting. Many
of the State's valleys, mountain ranges, and desert areas show the effects of faulting.
Faults create underground traps in which valuable reservoirs of petroleum form, and spaces
in which underground waters deposit valuable metals in the form of veins and masses of
ore.
Faults are distinguished by abrupt changes in rock structure or composition. Sometimes
a fault can be recognized by the displacement of a particular feature such as a bed or a
vein.
The best places to observe faults are usually in roadcuts, quarries, and sea cliff
exposures.
Faults and fault
zones are classified by how the rocks on
each side of the fault or fault zone
move past each other. There are two main
types of movement along faults: 1) a
sideways movement called strike slip,
and 2) an up or down movement called dip
slip.
Strike-Slip Faults
 The
movement along a strike-slip fault is approximately parallel to the
strike of the fault, meaning the rocks move past each other horizontally.
The San Andreas is a
strike-slip fault that has displaced
rocks hundreds of miles. As a result of
horizontal movement along the fault,
rocks of vastly different age and
composition have been placed side by
side. The San Andreas fault is a fault
zone rather than a single fault, and
movement may occur along any of the many
fault surfaces in the zone. The surface
effects of the San Andreas fault zone
can be observed for over 600 miles
(1,000 km).
Dip-Slip Faults
Dip-slip faults are
faults on which the movement is parallel
to the dip of the fault surface. Normal
faults are dip-slip faults on which the
hanging wall (the rocks above the fault
surface) move down relative to the
footwall (the rocks below the fault
surface). Normal faults are the result
of extension (forces that pull rocks
apart).
Where the dip of a
normal fault's surface is steep, it is
called a high-angle normal fault, or
simply a normal fault. The Owens Valley
and the Sierra Nevada fault zones are
examples of high-angle normal faults.
Together they produce a down-dropped
block which forms the Owens Valley. This
type of fault-bounded valley is called a
graben. A fault-bounded ridge is
called a horst.
Where the dip of a normal fault's
surface is very gentle or almost
flat, it is referred to as a
detachment fault or low-angle
normal fault. Detachment faults are
common in the desert areas of
California.
 Reverse faults are dip-slip faults in
which the hanging wall moves up relative
to the footwall. Reverse faults are the
result of compression (forces that push
rocks together).
The Sierra Madre
fault zone of southern California is an
example of reverse-fault movement. There
the rocks of the San Gabriel Mountains
are being pushed up and over the rocks
of the San Fernando and San Gabriel
valleys. Movement on the Sierra Madre
fault zone is part of the process that
created the San Gabriel Mountains.
A thrust fault
is a reverse fault with a gently-dipping
fault surface. Thrust faults are very
common in the Klamath Mountains of
northern California.
Notes:
The terms normal and
reverse were first used by English coal
miners to describe faults. When working
a flat coal bed where it was dislocated
by a normal fault, the miners
continued the workings either upward or
downward on the fault surface in the
same, or normal, direction. The workings
in a seam dislocated by a reverse
fault were also continued upward or
downward on the fault, but in the
opposite, or reverse, direction (Ojakangas,
1991).
The terms hanging
wall and footwall are also old mining
terms. These terms were originally used
in inclined underground passageways to
refer to the rock "hanging" overhead
(the hanging wall) and the floor beneath
the miners' feet (the footwall) (Ojakangas,
1991).
References Cited
Ojakangas, R.W.,
1991, Schaum's outline of theory and
problems of introductory geology:
McGraw-Hill, Inc., New York, 294 p.
Credits: "California
has its faults..." by Cindy Pridmore,
1992
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