The terms 'passive' and 'flexural' folding are old concepts
that try to relate the geometry of the fold to the rheology of the
layers involved (see Donath and Parker 1964, Geol. Soc. Am. Bull.).
In general, similar folds were proposed to be the result of 'passive
flow' in rock layers that resulted from a low effective-viscosity
(low competence) and low contrast in effective-viscosity (low
competence contrast) between adjacent layers. Parallel folds were
proposed to be the result of flexural folding (buckling) in rock
layers having a moderate to high mean effective-viscosity and high
effective viscosity contrast between adjacent layers. These ideas
are discussed in chapter 12 of Twiss and Moores (1992, 'Structural
Geology') (plug plug!!)
Current understanding of the relation between fold geometry
and rheology is much more sophisticated and makes the ideas of
'passive' and 'flexural' folds very over-simplified concepts. The
best thing I have seen recently on fold development is the following
paper:
Fold geometry and constitutive behaviour.
Hobbs B., Mülhaus H., Ord A., Zhang Y. & Moresi L.
It can be seen on line at
http://virtualexplorer.com.au/VEjournal/Volume2/www/intro/index2.html
Rob Twiss
> >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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Robert J. Twiss email: [log in to unmask]
Geology Department telephone: (530) 752-1860
University of California at Davis FAX: (530) 752-0951
One Shields Ave.
Davis, CA 95616-8605, USA
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