Stress, faults, and folds - Tectonic Geomorphology - page 97

but rather the upthrown side varies along the
fault trace (Fig. 4.20). Whereas the vertical slip
can sometimes equal the horizontal displace-
ment, the ratio of vertical to horizontal slip is
much more commonly less than 25% on large
strike-slip faults (Beanland and Clark, 1994).
Over long time intervals, strike-slip faulting
typically leads to some well-known geomor-
phological features (Fig. 4.21). A linear trough
commonly forms along the principal displace-
ment zone, because structural blocks are
slipping past each other along this zone and
because the fractured materials are more read-
ily eroded along the fault zone. Recent numeri-
cal modelling suggests that simple deflection of
streams across strike-slip faults can create lin-
ear valleys along the fault trace in the absence
of any special “softening” due to brecciation
(Braun and Sambridge, 1997). Irrespective of
how the troughs are generated, within them,
sag ponds may form in low-lying regions.
Scarps can be preserved on either side of the
fault. Linear features like streams and ridges
become offset along the fault and can yield a
clear sense of slip directions. Some care must
be used, however, when inferring displacement
directions from offset streams. Offsets deter-
mined from streams that are displaced in an
uphill direction with respect to the local
hillslope gradients are more reliable than
downslope displacements because such offsets
could result from stream capture. When a ridge
that has been translated along the fault subse-
quently blocks a drainage, it is termed a shutter
ridge . On the downslope side of strike-slip
faults, beheaded stream valleys may be
preserved (Fig. 4.21A and C). These are aban-
doned valleys that have been rafted laterally
beyond the course of the stream that formerly
flowed through them (Keller et al. , 1982). On
both the upstream and downstream sides of a
fault, river terraces may be systematically off-
set. Commonly, streams crossing a strike-slip
fault will exit from a mountainous terrain into a
gentler one. Upstream, their valleys will have
been more confined, whereas downstream of
the fault, they may build alluvial fans in the
less confined topography. Horsts, grabens,
small-scale pull-apart basins (Fig. 4.21B), and

various thrusts and folds can also have clear
geomorphological expression and can often be
understood in the context of the imposed shear
couple and stress field (Fig. 4.18).

Normal faults

Normal faults form in settings where the
maximum compressive stress ( s 1 ) is vertical and
a deviatoric tensile stress in a horizontal orienta-
tion is present (Fig. 4.1A). Typically, normal
faults cut the surface at high angles ( ∼ 50-70 ° ).
In cross-section, coseismic displacements on
normal faults are commonly asymmetric with
respect to a horizontal datum, such that
subsidence of the hanging wall is typically
several times greater than uplift of the footwall.
Although down-dropped keystone blocks
(grabens) and uplifted horsts (blocks bounded
by normal faults on both flanks) were once
thought to typify regions of normal faulting,
unpaired normal faults creating half-grabens or
fault-angle depressions occur more commonly
(Fig. 4.22). The down-dropped hanging wall of
a normal fault commonly generates a basin
in which sediments accumulate, whereas the

footwall experiences uplift and, in terrestrial
settings, becomes a site of erosion. Thus, many
normal faults bound asymmetric ranges with
one steep, fault-bounded flank and a gently
sloping, largely unfaulted opposite flank
(Fig. 4.22). Secondary synthetic (dipping in the
same direction with similar sense of throw) or
antithetic (dipping in the opposite direction
with an opposite sense of throw) faults may
develop within the hanging-wall block.
Rather than amalgamating into a single fault
zone, displacement during extension can also be
transferred between adjacent faults by a variety
of structures (Gawthorpe and Hurst, 1993),
including (Fig. 4.23): (i) transfer faults , which
are oblique to the traces of the main faults, but
which link them together; (ii) relay ramps
(Fig. 4.9E), which may or may not be faulted and
which bridge between two faults facing the same
direction; (iii) antithetic interference zones ,
which are tilted (without or with faulting) ramps
that develop between normal faults facing each
other and dipping in opposite directions; and