fault scarp
423
Structures Formed by Brittle Deformation
are associated with tensional forces that pull
the crust apart. This “pulling apart” can be
accomplished either by uplifting that causes
the surface to stretch and break or by
opposing horizontal forces.
REVERSE AND THRUST FAULTS. Reverse
faults are dip-slip faults in which the
hanging wall block moves up relative to
the footwall block (FIGURE 17.11B). Thrust
faults are reverse faults having dips less
than 45°, so the overlying block moves
nearly horizontally over the underlying
block (FIGURE 17.11C).
Whereas normal faults occur in tensional
environments, reverse faults result from strong
compressional stresses. Because the hanging
wall block moves up and over the footwall
block, reverse and thrust faults accommodate
horizontal shortening of the crust.
Most high-angle reverse faults are small
and accommodate local displacements in
regions dominated by other types of faulting. Thrust faults, on the other hand, exist
at all scales with some large thrust faults
having displacements on the order of tens
to hundreds of kilometers.
Thrust faulting is most pronounced
along convergent plate boundaries. Compressional forces associated with colliding
plates generally create folds as well as
Extension
Half
graben
Detachment fault
Graben
Horst
Horst
D I D Y O U K N O W ?
Many faults do not reach the
surface and hence do not produce
a fault scarp during an earthquake.
Faults of this type are termed blind
or hidden faults. The 1994
Northridge earthquake in the Los
Angeles area occurred along a
blind thrust fault.
thrust faults that thicken and shorten the crust to produce
mountainous topography (see Figure 17.21).
Strike-Slip Faults
A fault in which the dominant displacement is horizontal and
parallel to the trend or strike of the fault surface is called a
strike-slip fault
(FIGURE 17.11D). The
earliest scientific
records of strike-slip
faulting were made
following surface ruptures that produced
large earthquakes. One of the most noteworthy of these was the great San Francisco
earthquake of 1906. During this strong
earthquake, structures such as fences
that were built across the San
Andreas Fault were displaced as
much as
4.7 meters (15 feet). Because movement along the San Andreas causes
the crustal block on the opposite side
of the fault to move to the right as
you face the fault, it is called a rightlateral strike-slip fault (FIGURE 17.13).
The Great Glen Fault in Scotland is a
well-known example of a left-lateral strikeslip fault, which exhibits the opposite
sense of displacement. The total displacement along the Great Glen Fault is estimated to exceed 100 kilometers (60
miles). Also associated with this fault
trace are numerous lakes, including Loch
Ness, home of the legendary monster.
Some strike-slip faults cut through
the lithosphere and accommodate
motion between two large tectonic
plates. Recall this special kind of strikeslip fault is called a transform fault.
Numerous transform faults cut the
oceanic lithosphere and link spreadThis fault scarp formed north of Landers,
California, during an earthquake in 1992.
(Photo by Roger Ressmeyer/CORBIS)
FIGURE 17.12 Normal faulting in the
Basin and Range Province. Here,
tensional stresses have elongated and
fractured the crust into numerous
blocks. Movement along these
fractures has tilted the blocks,
producing parallel mountain ranges
called fault-block mountains. The
down-faulted blocks (grabens) form
basins, whereas the upfaulted blocks
(horsts) erode to form rugged
mountainous topography. In addition,
numerous tilted blocks (half-grabens)
form both basins and mountains.
(Photo by Michael Collier)
423
Structures Formed by Brittle Deformation
are associated with tensional forces that pull
the crust apart. This “pulling apart” can be
accomplished either by uplifting that causes
the surface to stretch and break or by
opposing horizontal forces.
REVERSE AND THRUST FAULTS. Reverse
faults are dip-slip faults in which the
hanging wall block moves up relative to
the footwall block (FIGURE 17.11B). Thrust
faults are reverse faults having dips less
than 45°, so the overlying block moves
nearly horizontally over the underlying
block (FIGURE 17.11C).
Whereas normal faults occur in tensional
environments, reverse faults result from strong
compressional stresses. Because the hanging
wall block moves up and over the footwall
block, reverse and thrust faults accommodate
horizontal shortening of the crust.
Most high-angle reverse faults are small
and accommodate local displacements in
regions dominated by other types of faulting. Thrust faults, on the other hand, exist
at all scales with some large thrust faults
having displacements on the order of tens
to hundreds of kilometers.
Thrust faulting is most pronounced
along convergent plate boundaries. Compressional forces associated with colliding
plates generally create folds as well as
Extension
Half
graben
Detachment fault
Graben
Horst
Horst
D I D Y O U K N O W ?
Many faults do not reach the
surface and hence do not produce
a fault scarp during an earthquake.
Faults of this type are termed blind
or hidden faults. The 1994
Northridge earthquake in the Los
Angeles area occurred along a
blind thrust fault.
thrust faults that thicken and shorten the crust to produce
mountainous topography (see Figure 17.21).
Strike-Slip Faults
A fault in which the dominant displacement is horizontal and
parallel to the trend or strike of the fault surface is called a
strike-slip fault
(FIGURE 17.11D). The
earliest scientific
records of strike-slip
faulting were made
following surface ruptures that produced
large earthquakes. One of the most noteworthy of these was the great San Francisco
earthquake of 1906. During this strong
earthquake, structures such as fences
that were built across the San
Andreas Fault were displaced as
much as
4.7 meters (15 feet). Because movement along the San Andreas causes
the crustal block on the opposite side
of the fault to move to the right as
you face the fault, it is called a rightlateral strike-slip fault (FIGURE 17.13).
The Great Glen Fault in Scotland is a
well-known example of a left-lateral strikeslip fault, which exhibits the opposite
sense of displacement. The total displacement along the Great Glen Fault is estimated to exceed 100 kilometers (60
miles). Also associated with this fault
trace are numerous lakes, including Loch
Ness, home of the legendary monster.
Some strike-slip faults cut through
the lithosphere and accommodate
motion between two large tectonic
plates. Recall this special kind of strikeslip fault is called a transform fault.
Numerous transform faults cut the
oceanic lithosphere and link spreadThis fault scarp formed north of Landers,
California, during an earthquake in 1992.
(Photo by Roger Ressmeyer/CORBIS)
FIGURE 17.12 Normal faulting in the
Basin and Range Province. Here,
tensional stresses have elongated and
fractured the crust into numerous
blocks. Movement along these
fractures has tilted the blocks,
producing parallel mountain ranges
called fault-block mountains. The
down-faulted blocks (grabens) form
basins, whereas the upfaulted blocks
(horsts) erode to form rugged
mountainous topography. In addition,
numerous tilted blocks (half-grabens)
form both basins and mountains.
(Photo by Michael Collier)