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| TRANSECTED FOLDS, CLEAVAGE
When there is a time lag between the inception of
folding and development of cleavage, the cleavage may not be strictly
axial planar to folds but may have an angular relationship with fold
elements and sometimes, this cleavage, which is genetically
related to that very fold forming event may cross cut both the limbs of a
fold. Such folds are called transected folds and were first described by
Powell (1974) from Tasmania. Borraidale divided the types of transection
into Type 1 when the cleavage seems to be parallel to axial plane in fold
profile but cuts the hinge. The type 2 transection is such that the
cleavage is not axial planar in fold profile but makes some small angle
with the axial plane of the fold. But its trace is parallel to the fold
hinge (see figures). The third type of transection is the combination of
both type 1 and type 2, hat is the cleavage bears angular relationship with both the fold hinge
and the fold axial plane. It was earlier believed that the transected
folds may be produced only under a noncoaxial strain history
(heterogeneous simple shear, but Borraidale showed by theoretical analysis
that even a coaxial strain history can produce transected folds. For example if a layer is inclined asymmetrically with all three
principal planes then, the cleavage which will always develop parallel to
the XY principal plane of the finite deformation ellipsoid, will cut
across the hinges and axial planes of the asymmetrical S or Z folds that
are produced and the minor or mesoscopic outcrop scale folds will appear
to be of of the transected type. A more detailed discussion on these will
be dealt with on Fabrics when this also goes on line.
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