Stress, faults, and folds - Tectonic Geomorphology - page 108

accumulate displacement, they commonly
propagate toward the surface. Until they actually
break the Earth's surface, however, they are
termed blind faults . In basins with a thick
sedimentary fill, it is not uncommon for even
large earthquakes to fail to rupture the surface.
Instead, the highly localized strain that occurs
along a fault plane at depth is accommodated by
folding within the strata overlying the fault
tip. Because sedimentary beds typically deform
in predictable ways, patterns of folding can be
intimately linked to the underlying fault geom-
etry. Consequently, whenever a fold's geometry
and its evolution through time can be docu-
mented, much can be learned about the geom-
etry of faulting in the subsurface. This linkage is
particularly important in some urban areas, such
as Los Angeles and Seattle, where the seismic
risk is high, but where many of the faults capable
of causing destructive earthquakes are buried
beneath thick Quaternary sediments. Learning
to interpret the folds at the surface and to link
them to coseismic displacements represents
a significant, but worthwhile, challenge for
tectonic geomorphologists.
The geometry of the faults themselves is com-
monly influenced by the mechanical properties
of the rocks through which they rupture.
As might be expected, thrust faults in the sub-
surface exploit zones of mechanical weakness,
and, within sedimentary rocks, they often follow
bedding planes. At irregular intervals, they ramp
upward from an underlying bedding-plane
decollement to an overlying one. In contrast
to sedimentary rocks, igneous or metamorphic
rocks commonly display more isotropic mechan-
ical properties. Folding of formerly planar strata
in metamorphic rocks distorts mechanically
weak layers. As weak and strong layers become
more spatially disorganized, they have no
consistent orientation with respect to the
regional stress directions, and overall the rock
becomes more isotropic. Both planar and
curving ruptures occur within isotropic bedrock.
Kink-like changes in fault angle may occur
where rock bodies with contrasting mechanical
properties are juxtaposed.
The trajectory of a thrust fault within sedi-
mentary rocks is often visualized as following a

staircase-like pattern with long flats connected
by shorter ramps along which the thrust steps
upward through the stratigraphy. Thrusting of
a hanging wall along a fault comprising ramps
and flats causes uplift of the hanging wall above
the ramps, whereas rocks are commonly trans-
lated laterally without uplift above the flats. The
geometric consequence of this pattern is that
folds will be created above each ramp. Folds
also occur above faults that cut through more
mechanically isotropic rocks. Deformation
above the tip lines of faults and changes in the
angle of the fault cause differential uplift at the
surface and create folds.
In many cases, deposition occurs synchro-
nously with folding. Newly deposited strata that
are associated with an active fold are termed
growth strata , and they commonly provide the
best means of deciphering the history of fold
growth (Fig. 4.35). The reason that growth strata
are particularly useful is that we know their
initial geometry quite reliably: their upper
surfaces are essentially horizontal at the time of
deposition. Consequently, even when we do not
know the pre-folding geometry of the bedrock
that cores the fold, growth strata provide robust
markers that track deformation subsequent to
their deposition (Suppe et al. , 1992). When aggra-
dation is more rapid than the rate of uplift of the
highest part of the fold, growth strata will cover
the entire fold and will faithfully record the dips,
limb lengths, and geometry of its upper surface
(Burbank and Vergés, 1994; Vergés et al. , 1996).
Under these conditions, the fold may have no
topographic expression at the Earth's surface,
but the subsurface record will clearly show
changes in the fold shape through time. If the
rate of aggradation is less than the crestal uplift
rate, then the fold will become emergent, growth
strata will offlap the fold, and its uneroded
topographic shape may more clearly reflect the
geometric controls exerted by displacement
along an underlying ramp (Burbank et al. , 1996c).

Models of folding

From among the many types of folds that have
been described, only a few of the popular
models are described here. When rocks in the