Dear All,
Highlighted recently at TSG, there seems to be some discrepancy of
understanding between passive and flexural folding.
My understanding of passive folding is that pressure solution modifies an
original fold shape (ie. buckling first, then intense pressure solution).
This would produce the deck of cards effect (ie. regular thickened hinge,
thinned limbs). The limb volume of a layer is not reduced by shear along
the cleavage plane (which I further understand cannot happen), but by
material loss, therefore rotation is an irrelevant issue. Is this the
correct description of the term 'passive folding'? If not, what is.
Flexural folding, as I understand it, occurs by shear between or within
layers (slip/flow) and the original volume is maintained - the buckling
effect. If this is so, cleavage within a flexural fold that shows no sign
of bedding misalignment between cleavage surfaces would need to have
developed prior to the buckling. Further, any fold containing a cleavage
that does not show bedding misalignment between cleavage surfaces must be
deemed to be flexural? Comments welcome please.
As an aside, does anyone know the upper temperature limit of cleavage
development.
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