Geology Reference
In-Depth Information
Fig. 5.24.
Cross section of second-order parasitic folds in a thin bed showing normal (buckle-fold)
sense of shear (
half arrows
) on the limbs of first-order folds.
Short thick lines
are local bedding atti-
tudes.
Dashed lines
labeled
a
are axial-surface traces of first-order folds.
Unlabeled dashed lines
are
cleavage, fanning in the stiff units (
s
) and antifanning in the soft units.
Large arrows
show directions
of boundary displacements
Fig. 5.25.
Interpretations of drag folds having the same sense of shear on both limbs of a larger fold.
a
Ob-
served fold with vertical axial traces.
b
Sense of shear interpreted as caused by a folded bedding-parallel
thrust fault.
c
Sense of shear interpreted as being the upper limb of a refolded recumbent isoclinal fold
thereby indicating the relative positions of anticlinal and synclinal hinges. The inclina-
tion of cleavage planes in the softer units (Fig. 5.24), in the orientation axial planar to the
drag folds, gives the same sense of shear information. Cleavage in the stiffer units is usu-
ally at a high angle to bedding, resulting in cleavage dips that fan across the fold.
Individual asymmetric higher-order folds are not always drag folds. The asymme-
try may be due to some local heterogeneity in the material properties or the stress
field, and the implied sense of shear could be of no significance to the larger-scale
structure. Three or more folds with the same sense of overturning in the same larger
