Geology Reference
In-Depth Information
layers is shortening approximately perpendicular to the axial surface. The strain in
thin soft layers between stiff layers may be close to layer-parallel shear strain. The pure
bending strain distribution is usually more closely approached in transverse contrac-
tion folds, for example above a salt dome, than in buckle folds.
Cleavage planes and tectonic stylolites in a fold can indicate the mechanical origin
of the fold because they form approximately perpendicular to the maximum shorten-
ing direction by processes that range from grain rotation to pressure solution (Groshong
1988). Cleavage in a buckle fold is typically at a high angle to bedding (Fig. 1.25), being
more nearly perpendicular to bedding in stiff units and more nearly parallel to the axial
surface in soft units. Cleavage that is approximately perpendicular to bedding produces
a cleavage fan across the fold. The line of the cleavage-bedding intersection is approxi-
mately or exactly parallel to the fold axis and can be used to help determine the axis.
Folds produced by an unequal distribution of forces in transverse contraction
(Fig. 1.22) are termed forced folds (Stearns 1978). Forced folds tend to be round to
blocky or irregular in map view. The major control on the form of the fold is the rhe-
ology of the forcing member (Fig. 1.26). A stiff and brittle forcing member (i.e., crys-
talline basement) leads to narrow fault boundaries at the base of the structure and
strain that is highly localized in the zone above the basement fault. A soft unit between
a stiff forcing member and the cover sequence will cause the deformation to be dishar-
monic. A soft forcing member (like salt) typically produces round to elliptical struc-
tures with deformation widely distributed across the uplift.
Little strain need occur in the uplifted or downdropped blocks associated with a
stiff forcing member. Nearly all the strain is localized in the fault zone between the
Fig. 1.25.
Cleavage pattern in a buckle
fold. The gently dipping sur-
faces are bedding and the
steeply dipping surfaces rep-
resent cleavage or stylolites.
Arrows
show directions of
the boundary displacements
Fig. 1.26.
Effect of mechanical stratigra-
phy on drape folds. The lowest
unit is the forcing member
