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plunge of
fold axis
saddle in fold
(slip deficit)
forward
propagating
fault tip
laterally
Fig. 4.38 Lateral propagation of
blind thrust faults and surface
folds.
Displacement variations in the
subsurface fault are directly related
to the magnitude of rock uplift at
the surface. The noses of plunging
folds occur above fault tips, and
structural saddles mark zones of
fault linkage or slip deficits.
propagating
fold tip
fault plane
Slip Gradients on Blind Thrusts
and Fault-Related Folding
fold models have been modified to incorporate
the possibility of rounded hinges (Suppe et al. ,
1997), rather than relying exclusively on kink
bands (Fig. 4.35). Bedding dips in the forelimbs
of trishear folds tend to change continuously,
rather than abruptly. Therefore, kink bands
and  dip domains do not commonly occur in
such folds. These differences in the geometry
of  growth strata help to discriminate among
different fold models.
Owing to differences in the ways that beds
deform within folds of different types, the pat-
terns of rock uplift that are associated with each
fold model can vary considerably (Fig. 4.37) and
can be diagnostic of key characteristics of both
the underlying fault geometry and the way in
which shortening is accommodated, such as by
pure shear or simple shear. Depending on the
fold model, spatial variations in uplift rate that
result from an increment of shortening at any
given moment in the fold's history may or may
not mimic the pattern of total uplift over the life
of the fold (Fig. 4.37). For fault-bend folds, the
mismatch results from kink-band migration dur-
ing fold growth, which causes a continual evolu-
tion of the spatial pattern of rates. When it is
possible to document the changes in uplift rates
through time, such as through surveys of multi-
ple, deformed, and dated fluvial terraces, distinc-
tions can be drawn between a fold that is
undergoing limb rotation versus one growing by
kink-band migration: the former causes gradual
spatial changes in short-term uplift rates, whereas
the latter creates abrupt rate changes (Fig. 4.37).
More complex fault geometries, such as might
occur due to multiple wedge thrusts within lay-
ered rocks (Fig. 4.37B), or fault-bend folds with
rounded hinges (Fig. 4.35), can create both diverse
and intricate patterns of instantaneous uplift rates.
The greater the complexity, the more difficult it
becomes to confidently interpret subsurface fold
geometries from patterns and rates of rock uplift.
Lateral fold growth of folds as
three-dimensional features
Because faults grow by extending laterally, as
well as forward, fault growth in the subsurface
will cause lateral propagation of the tips of folds
at the surface (Fig. 4.38). Consider the case of a
single blind thrust fault. Folding at the surface
will wane above either end of the buried rup-
ture. The consequence of this decrease is to gen-
erate folds with doubly plunging terminations.
Variations in displacement along the underlying
thrust should also be reflected by the amplitude
of folding at the surface. Thus, the structural
crest of the fold should vary in height, and the
fold axis should plunge toward zones where
less displacement occurs on the underlying
fault. As illustrated previously (Fig. 4.12), indi-
vidual faults may simply lengthen, or they may
link together with adjacent faults. In the latter
case, if the linked structures are blind thrust
faults, variations in displacement along them
will create multiple plunging folds at the sur-
face, with structural saddles marking the zones
of linkage (Fig. 4.38).
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