Stress, faults, and folds - Tectonic Geomorphology - page 92

system, four different types of segment boundaries
(Machette et al. , 1992a) are recognized (Fig. 4.16).
The most commonly observed boundary occurs
where bedrock spurs or salients extend into the
adjacent basin and major range-front faults
terminate against them. Some of these spurs are
bounded by Quaternary faults that are active, but
have less displacement than the range-front faults.
Other segment boundaries are delineated by an
en echelon step in the range-bounding faults. Still
others occur where the range-front faults are
intersected by a cross-fault that trends at a high
angle to the range front. Finally, some segment
boundaries are simply defined by the absence of
a rupture between two active range-front faults
(Machette et al. , 1992a). Several of the segment
boundaries display a combination of spurs, cross-
faults, and en echelon offsets. If the positions of
the terminations persist through time and if entire
segments typically rupture in a single seismic
event, each segment might be typified by
characteristic earthquakes (Schwartz and
Coppersmith, 1984). At present, however, neither
the persistence of the terminations nor the
tendency for an entire segment to rupture
“characteristically” is well established. Moreover,
recent studies suggest that no widely applicable
rule yet exists to predict whether or not an
earthquake will rupture across segment
boundaries. A compilation of strike-slip
earthquakes suggests that ruptures terminate
when segments are separated by more than
3-5 km (Wesnousky, 2006), whereas a study of
large earthquakes on continental thrust faults
reveal that segments separated by up to 10 km can
rupture in the same earthquake (Rubin, 1996),
although this coeval rupture does not require a
surface rupture to link the segments.

Nonetheless, it is useful to describe the struc-
tures and geomorphic features that are
expected for the primarily dip-slip and strike-
slip end-members, because their contribution
to a given natural fault setting can then be
more readily recognized. Experimental results
using homogeneous materials with known
physical characteristics often depict an ideal-
ized array of structures that are associated
with a particular stress field. Such structures
are described in the following paragraphs.
Natural heterogeneities in rocks, however, dic-
tate that they will not deform uniformly, so
that few natural situations exactly duplicate
model predictions.
We typically envision a fault as an irregular,
but singular, surface dipping into the crust. In
fact, at the scales of less than 10 km, many
faults consist of a tabular volume of typically
unconnected or anastomosing smaller faults
(Scholz, 1998). These component faults will
span a broad spectrum of sizes, ranging from
a few meters to several kilometers. Thus,
whereas some faults appear to slip along a
single plane at depth with a compact damage
zone surrounding it (Fig. 4.17A), others display
highly fractured zones hundreds of meters
thick with multiple actively slipping fault
surfaces (Fig. 4.17B) (Faulkner et al. , 2003).
Even at the surface, however, the complexity
of many fault systems is commonly difficult to
delineate. Where fault traces on the ground
are obscured by deposition in basins, erosion
on hillslopes, or vegetation, remotely sensed
data may serve to illuminate fault systems
more completely. For example, aerial imagery
can detect subtle changes in vegetation that
are responses to variations in the water table
or soil types due to faulting. In actively
aggrading basins, high-resolution aeromagnetic
surveys (Grauch, 2001) have been successfully
used to expose intricate fault systems that
were previously unknown (Fig. 4.17C and D).
The complexity of surface deformation,
therefore, is not solely attributable to the
heterogeneous materials of the crust, but
also to the fact that, during an earthquake,
hundreds of small rupture surfaces actually
accommodate the total displacement. Over

Geomorphic expression of faults

Faults in which the hanging wall moves only
vertically, that is, directly up or down the fault
plane, are termed dip-slip faults, whereas pure
horizontal motion results in strike-slip
faults. Few faults, however, are purely dip-slip
or strike-slip. Most have some component
of both horizontal and vertical motions.