Stress, faults, and folds - Tectonic Geomorphology - page 105

basin is commonly comparable to the bedrock
uplift of the hanging wall (Fig. 4.28B). Thus, the
preserved geological structure can be concep-
tualized as resulting from both coseismic and
interseismic deformation (Fig. 4.30). The rela-
tive importance of the coseismic versus the
interseismic deformation in creating the pre-
served geological structure depends on the size
of the structure with respect to the strength of
the crust and on the efficiency of surface pro-
cesses in eroding and redistributing the load. If
the load is small compared to the crustal rigid-
ity, the isostatic response to the load will be
less, and additional footwall subsidence due to
sediment loading of the flexural depression will
be limited. Conversely, if erosional processes
are highly efficient, they will tear down the
uplifted hanging wall and cause enhanced
sediment loading (and subsidence) on the
footwall.
Commonly, the amount of displacement on a
thrust fault dies out toward the thrust tip,
causing the hanging wall to fold adjacent to the
thrust tip. The coseismic displacement during
the Kern County earthquake displays this folding
whereby the greatest vertical change occurs
8-10 km from the fault trace (Fig. 4.28A).
Repetitions of this pattern of deformation
through many successive earthquakes would
generate a hanging-wall anticline.
In the arid terrain of northern Algeria
where the 1980 El Asnam ( M = 7.3) earthquake
occurred, patterns of hanging-wall folding and
surface rupture are well illustrated by coseismic
displacements (Philip and Meghraoui, 1983).
Across the crest of hanging-wall anticlines,
grabens due to bending-moment faulting (see
description in next subsection) opened in sev-
eral sites. The obliquity of the shortening vector
with respect to the trace of the thrust can be
judged by the orientation of these crestal gra-
bens. Where the trend of the thrust trace and of
the normal faults bounding the grabens are
parallel, shortening occurred approximately
perpendicular to the trace of the thrust (Fig.
4.31A and C; see also Fig. 4.15 for analogous
graben orientation on the Ostler Fault). On the
other hand, where the grabens trend obliquely
to the anticlinal crest and to the thrust trace,

shortening is inferred to have been oblique to
the thrust trace (Fig. 4.31B and D). This obliq-
uity of the shortening vector is also consistent
with the orientation of the conjugate shears that
develop along the thrust front. In each case, the
locally observed strain can be seen to result
from the orientation of the local stress field (in
the case of the grabens or conjugate shear
zones) or regional stress field (in the case of the
thrusts) with respect to the orientation of the
fault surfaces (Fig. 4.32).

Flexural-slip and bending-moment faults

In reality, fault geometries are often far more
varied than might be expected with simple mod-
els related to regional stress fields (Fig. 4.1). In
part, this complexity is due to inhomogeneities
in rocks: they have variable strengths, may be
bedded, and may have had a diverse deforma-
tional history prior to the current deformation.
The orientation of weaknesses within rocks that
are subjected to stresses can exert a strong con-
trol on how they deform. Anyone who has
observed topics tilted on a shelf can recognize
that tilting of rigid entities (like rock strata)
requires slip between the rigid blocks. Thus,
faulting along bedding planes is common
whenever relatively rigid strata are tilted.
Similarly, folding of a deck of cards results in
relative slip between each of the cards. When
strata are folded, if the initial length of each bed
is preserved, then flexural-slip faults develop
along bedding planes to accommodate the
differential motion between adjacent beds.
When strata are folded, local stresses are
created because the convex side of a folded bed
is lengthened, whereas its concave side is
shortened. The folding can be considered analo-
gous to bending an elastic plate around a fold
axis, such that equal and opposite moments are
applied at the ends of the plate. The faults that
result from the tensile stresses along the convex
regions of the folded plate or from compressive
stresses in the concave regions are called bend-
ing-moment faults (Fig. 4.33). Thus, normal
faults are expected to form across the convex
regions in order to accommodate length changes
along these surfaces, whereas thrust faults will