Stress, faults, and folds - Tectonic Geomorphology - page 111

(Fig. 4.36A). In the fault-bend fold model, axial
surfaces are defined by kink bends in the
deforming hanging-wall strata. Between any two
adjacent axial surfaces, beds remain parallel that
were parallel to each other prior to folding, and
these parallel beds define a dip domain .
Although fault-bend folding due to thrusting is
emphasized here, folds obeying similar geomet-
ric “rules” have also been modeled for ramps
along normal faults.
In a fault-propagation fold , a blind thrust
creates a new ramp by progressively propagat-
ing upward toward the surface (Suppe and
Medwedeff, 1990). The thrust never needs to
reach the surface, but, through time, it
accumulates more and more displacement.
Folding occurs because there is a gradient in
the amount of displacement along the ramp
(Fig. 4.36B); at the tip of the thrust, no displace-
ment occurs, whereas maximum displacement
occurs at the base of the ramp. In both the fault-
bend and fault-propagation fold models (Suppe,
1983; Suppe and Medwedeff, 1990), the dip of
the forelimb is established during the first
increment of folding, and it retains this dip
throughout subsequent growth. Note that, for
both models, the geometry of the growth strata
changes with each increment of folding (Suppe
et al. , 1992) and can become very complex
(Fig. 4.36). If smaller-scale ramps and flats were
incorporated into the overall ramp in either
model, more axial surfaces and dip domains
would be introduced and would create an
increasingly complicated folding geometry.
Commonly, length and thickness of beds do
not remain constant during folding. If the
requirement for constant bed length is relaxed
and only bed area is preserved, then displace-
ment-gradient folds (Wickham, 1995) can be
defined (Fig. 4.36C). As in fault-propagation
folds, the amount of displacement varies system-
atically along the ramp. Because bed lengths
change, however, the forelimb is allowed to
rotate. Even if the thrust tip does not propagate,
such folds can continue to amplify by accumu-
lating more displacement along the ramp behind
the thrust tip. In the absence of growth strata,
the final geometries of fault-propagation and
of displacement-gradient folds may be nearly

indistinguishable. Growth strata, however, can
reveal whether the forelimb rotated or was fixed
and will display distinctive differences both over
the crest of the fold and in its forelimb (compare
Fig. 4.36B and C).
A detachment fold forms by buckling above
a fault that is subparallel or parallel to original
layering (Fig. 4.36D). No ramp is needed to
create such folds: they grow simply because
displacement dies out toward a fault tip. At some
stage, the fault may propagate upward through
the overlying detachment fold, and the resulting
final shape may be geometrically similar to a
fault-propagation fold. Detachment folds are
commonly associated with readily deformed
strata, such as evaporites or shales, because
such strata provide weak horizons along which
the detachment can propagate, and these weak
strata flow readily into the cores of growing
folds. Limb rotation during folding is commonly
observed in detachment folds, whereas dip
domains and linear axial surfaces are typically
difficult to define due to the more continuous
curvature of the beds on the flanks of the fold
(Hardy and Poblet, 1994; Poblet et al. , 1998;
Vergés et al. , 1996).
A trishear fold (Fig. 4.36E) develops when a
single fault at some depth expands outward to
form a triangular zone of distributed shear
(Allmendinger, 1998; Erslev, 1991; Hardy and
Ford, 1997). This triangular zone is symmetrical
with respect to the dip of the fault. Within this
trishear zone, slip varies systematically in both
orientation and magnitude. At its top, slip vectors
match the slip of the hanging wall and are paral-
lel and equal to that of the master fault. At the
base of the trishear zone, the slip decreases to
zero. In between these two boundaries of the
trishear zone, the slip vectors systematically
decrease in magnitude and rotate away from the
hanging-wall slip direction toward parallelism
with the lower (no-slip) boundary of the trishear
zone. The shearing that results from differential
slip causes both bed thickness and forelimb dips
to change as the fold grows. The ratio between
the rate at which the tip line of the fault
propagates and the amount of slip on the fault
itself controls the geometry of the forelimb. Low
values of propagation-to-slip ratio cause the