Folds and folding - Introducing Tectonics, Rock Structures and Mountain Belts - page 54

Fault-induced folding
Many folds are directly caused by fault
movements, as we saw in Chapter 5.
Such folds are confined to the upper
brittle layer of the crust and are of two
main types - active bending induced
by the upward or downward move-
ment of a layer under compression,
and passive bending of a layer under
extension. Thrust faulting of layered
rocks produces hangingwall synforms
at the cut-off point at the foot of a ramp
and antiforms where the thrust sheet
rises over the top of a ramp (Figure
6.10A). More complex shapes result
from multiple ramp/flat arrangements
or from movements on lower thrust
systems (e.g. see Figure 5.7). An impor-
tant feature of the folds produced in
this way is that the strain produced

around the hinge area moves progres-
sively along the thrust sheet as the
sheet itself is transported. In conse-
quence, each part of the layer is suc-
cessively subjected to episodes of both
extensional and compressional strain
that will inevitably affect the micro-
structural fabric of the whole layer.
Passive bending folds may form as
a consequence of extensional fault-
ing. Two examples are shown in Figure
6.10. An unfaulted layer may form a
drape fold , which has both antiformal
and synformal components, above a
normal fault (Figure 6.10B); such folds
are gravity driven and are primarily
extensional at the antiformal bend
and compressional on the synformal
bend. Another common class of passive
bending structures, discussed in the
previous chapter, are the hangingwall
antiforms and footwall synforms associ-
ated with extensional faulting (Figure
6.10C).

generally be localised and have a limited
effect on the overall strain pattern.
Where two ductile deformations are
superimposed, a series of rather strange
shapes can be formed as a result (Figure
6.11). The geometric consequence
of such a superimposition is known
as an interference structure . These
structures are typically formed under
conditions of elevated temperature
resulting in flow folds of similar-type
geometry, as in Figure 6.11A. The earlier
set of folds in such a structure need not
necessarily have formed under these
metamorphic conditions, but in order
for penetrative ductile behaviour to
be imposed during the later deforma-
tion, the rocks must either have been
retained at deep level or re-buried.
A wide variety of complex shapes
are possible with superimposition,
depending on the relationship between
the fold axes and axial planes of the
two systems, and their orientation with
respect to the viewer. The examples of
Figures 6.11A and B are obvious, but
more enigmatic shapes are possible.
Two common types are shown as exam-
ples: Figure 6.11C shows the outcrop
pattern produced by two superimposed
upright folds with vertical axial planes,
whereas in Figure 6.11D, the F1 folds
are tight with an inclined axial plane.

thrust
fault

Superimposed folding and
interference structure
The core regions of mountain belts typi-
cally contain metamorphic complexes,
originally formed at deep crustal levels
but now exposed at the surface. These
may have experienced several episodes
of deformation; indeed, structural geol-
ogists have been known to record up to
nine such episodes in a single metamor-
phic complex! However, cases of meta-
morphic terrains having experienced
more than two episodes of penetrative
ductile deformation that have affected
the whole of the terrain would appear
to be uncommon. There may have been
several episodes of deformation after
the last period of thoroughgoing ductile
deformation in a region, but these will

A

normal fault

B

normal fault

Shear zones
At deep levels in the crust and upper
mantle, zones of ductile displacement,
termed shear zones, take the place
of faults at higher levels . These zones
of relatively high strain are confined
between blocks on each side where
the strain is either absent or appreci-
ably lower. They are formed when
the boundary blocks or 'walls' of the

C

Figure 6.10 Fault-induced folding. A. Antiforms
and synforms in the hangingwall of a thrust sheet
caused by the movement of the sheet. B. Drape
fold formed above a normal fault. C. Hangingwall
antiform and footwall synform formed as a
result of gravitational accommodation to the
fault movement. See Chapter 5 for further
explanation.