TECTONIC PROCESSES AND LANDFORMS - Discovering Physical Geography - page 351

and strike-slip faults is that transform faults are associated with
large, tectonic plate boundaries, whereas strike-slip faults occur
where small rock blocks move horizontally relative to one another.
As indicated previously, one of the best-known transform
faults is the San Andreas Fault in California, which occurs
along the boundary of the Pacific and North American plates.
The Pacific plate is moving in a northwesterly direction along
this boundary relative to the North American plate at an aver-
age rate of about 5 cm/year (2 in./year). Figure 13.24 shows

Oregon

Cape Mendocino

Figure 13.23 The Sierra Nevada. The Sierra Nevada forms the
western boundary of the Basin and Range Province. Uplift and
westward tilting of the horst has formed an approximately 2700-m
(~9000-ft) escarpment into the Owens Valley graben (foreground).

California

Nevada

slip up relative to the fault plane as an upthrown block (called a
horst ), while the other slips down as a downthrown block (called
a graben ). The exposed side of the upthrown block forms a cliff-
like feature known as a fault escarpment (or scarp for short). A
fantastic example of a landscape created by normal faulting is the
awesome Sierra Nevada in eastern California. As you approach
this mountain range from the east, the horst forms an abrupt es-
carpment that rises over 2700 m (~9000 ft) above Owens Valley,
the graben to the east (Figure 13.23). Given that the Sierra horst is
tilted to the west, the slope toward the west is much more gradual.
Another kind of fault is a reverse fault (Figure 13.21b).
Although a reverse fault looks very similar to the normal fault,
the cause and nature of the movement differ. Whereas a nor-
mal fault entails movement of blocks away from each other, a
reverse fault results when rock blocks move toward each other,
causing one block to ride up steeply over the other.
As noted previously, overthrust faults result when one rock
body is thrust up and over another, usually in association with
folding. These faults differ from normal and reverse faults because
their fault planes are usually at a shallower angle in comparison.
Of the four types of faults illustrated in Figure 13.21, three
are directly associated with some kind of uplift—specifically the
normal, reverse, and overthrust faults. In contrast to these types
of faults, the strike-slip fault entails purely horizontal movement
of the two plates past each other. Strike-slip faults and transform
faults are closely related because they share the same kinds of
horizontal movement. The primary difference between transform

North
American
Plate

San Francisco

Hayward
Fault

San Andreas
Fault

Pacific
Plate

Garlock
Fault

Los Angeles

Pacific Ocean

0 50 100 km

0 50 100 mi

Figure 13.24 California fault systems. The San Andreas Fault
is a transform fault that forms the boundary between the North
American (to the east) and Pacific (to the west) plates. Arrows
on either side of the fault reflect the relative movement of plates.
Note the various subsidiary faults in the system.

Horst An upthrown block of rock that lies between two steeply
inclined fault blocks.

Reverse fault A steeply inclined fault in which the hanging
rock block moves relatively upward.

Graben A downthrown block of rock that lies between two
steeply inclined fault blocks.

Strike-slip fault A structural fault along which two litho-
spheric plates or rock blocks move horizontally in opposite
directions and parallel to the fault line.

Fault escarpment A step-like feature on the Earth's surface
created by fault slippage.