Geoscience Reference
In-Depth Information
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(a)
(b)
Figure 8.24
Information from fold asymmetry. (a) Schematic cross-section showing an example of how fold
asymmetry (and vergence) changes across a fold axial plane (red line). (b) Isolated asymmetric fold in
calcareous mylonite, implying dextral (top-to-the-right) shear, Switzerland. (Tom W. Argles, The Open
University, UK.)
the fold axial surface, defi nes whether a fold is upward- or
downward-facing. If you fi nd the latter, this indicates that the
fold has been affected by at least two distinct deformation
events, a valuable deduction from an isolated exposure.
Symmetric folds have limbs of equal length that are mirror
images either side of the hinge, but many folds are asymmetric
(Figure 8.24). Fold asymmetry may help constrain the
following:
for minor folds, their asymmetry changes across a major
fold hinge - a useful aid in mapping (Figure 8.24a);
asymmetric shear-related folds may be good kinematic
indicators for shear zones (Figure 8.24b).
However, it is the
orientation
of folds (Appendix A8, Figure
A8.4) that relates directly to either local or regional tectonic
stresses, which raises the question of what exactly to measure.
You should measure the following elements if possible (using
your preferred notation; Table 8.1):
Fold axial plane
dip and dip azimuth
e.g. 83/147
Fold axis
(hinge line)
plunge and plunge azimuth
e.g. 11/056
How are these theoretical elements measured? Fortunately, an
associated axial-planar cleavage will be roughly parallel to the
