Geology Reference
In-Depth Information
Fig. 1.21.
Common map symbols for folds. Fold trend is indicated by the
long line
, plunge by the
arrow-
head
, with amount of plunge given. The fold trend may be the axial trace, crest or trough line or a hinge
line.
a
,
b
Anticline.
c
Overturned anticline; both limbs dip away from the core.
d
,
e
Syncline.
f
Overturned
syncline; both limbs dip toward the core.
g
Minor folds showing trend and plunge of axis.
h
Plunging
monocline with only one dipping limb
1.5.3
Mechanical Origins
The fundamental mechanical types of folding are based on the direction of the causative
forces relative to layering (Ramberg 1963; Gzovsky et al. 1973; Groshong 1975), namely
longitudinal contraction, transverse contraction, and longitudinal extension (Fig. 1.22).
If the stratigraphy is without mechanical contrasts, forces parallel to layering produce
either uniform shortening and thickening or uniform extension and thinning. If some
shape irregularity is pre-existing, then it is amplified by layer-parallel shortening to
give a passive fold. If the stratigraphy has significant mechanical contrasts, then a
mechanical instability can occur that leads to buckle folding in contraction and pinch-
and-swell structure (boudinage) in extension. If the forces are not equal vertically, then
a forced fold is produced, regardless of the mechanical stratigraphy. Longitudinal
contraction, transverse contraction, and longitudinal extension are end-member bound-
ary conditions; they may be combined to produce folds with combined properties.
Buckle folds normally form with the fold axes perpendicular to the maximum prin-
cipal compressive stress,
σ
1
. The folds are long and relatively unchanging in geometry
parallel to the fold axis but highly variable in cross section. Buckle folds are character-
ized by the presence of a regular wavelength that is proportional to the thickness of the
stiff unit(s). A single-layer buckle fold consists of a stiff layer in a surrounding confin-
ing medium. The dip variations associated with a given stiff layer die out into the
regional dip within the softer units at a distance of about one-half arc length away
from the layer (Fig. 1.15). In the author's experience, buckle folds in sedimentary rocks
typically have arc-length to thickness ratios of 5 to 30, with common values in the range
of 6 to 10.
As buckled stiff layers become more closely spaced, the wavelengths begin to inter-
fere (Fig. 1.23) resulting in disharmonic folds. Once the layers are sufficiently closely
spaced, they fold together as a multilayer unit. A multilayer unit has a much lower
buckling stress and an appreciably shorter wavelength than a single layer of same
thickness (Currie et al. 1962). Stiff units, either single layers or multilayers, tend to
