>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.
>
Dear Joanie,
the questions you raise seem of primary interest, and to my knowledge there
has been much debate on whether natural folding is passive, or flexural, or
both.
Although I am not updated with the most recent advances in the field, I am
aware of two recent contributions that contain discussions aiming to a
clearer understanding of the folding process and the cleavage-folding
relationships within natural folds.
So, try with the followings:
-Luneburg & Lebit (1998) "The development of a single cleavage in an area
of repeated folding" - Journal of Structural Geology, 20, 11, 1531-1548.
-Treagus, Hudleston & Labao (not necessarily in this order) (1996? or 1997)
"Flexural-slip folding: does it exist in nature?" Geology (must be 1996 or
1997).
I hope you find these references of use.
Best wishes
Enrico Tavarnelli
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Enrico Tavarnelli
Dipartimento di Scienze Geologiche,
Universita' della Basilicata,
Campus Macchia Romana, 85100, Potenza (Italy),
phone: ++39-(0)971-206 179; fax: ++39-(0)971-206 077;
e-mail: [log in to unmask]
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