Hi Joanie,
I think that people have avoided the topic because much has been published
on the matter over the past 30+ years and it is hard to distill what has
been written into a short communication.
My feelings are:
1. Passive (slip) and flexural (crumpling, buckling) are kinematic terms
that have largely been overtaken by more recent research.
2. Passive folding was said to have been achieved thru' inhomogeneous
simple shear (passive slip or flow) across kinematically INactive layering
- i.e. no significant viscosity contrast.
3. Flexural folding was said to have resulted from either inhomogeneous
simple shear with the slip vector parallel to kinematically active layering
(significant viscosity contrast) or inhomogeneous pure shear - Ramsay
called the former flexural flow (or shear in Hobbs Means and Williams) and
the latter tangential longitudinal strain. In relation to the latter he
developed Neutral surface folding and its implications.
4. Passive folding by the envisaged mechanism effectively results in an
increase of layer length, whereas in flexural processes the layer is of
approximately consistent length (depends on the amount of prefolding
homogeneous shortening and whether one is looking at the inner or outer
surface of the folded layer.
5. Passive folding was said to produce true similar folds, this being a
geometric term and involves all layer interfaces maintaining a constant
profile (= consistent changes of curvature), while flexural folding was
considered to form other fold geometries such as parallel and concentric in
which the radius of curvature decreased down the antiformal profile. Ramsay
(1967) and Ramsay and Huber (1984, 1987) deal with these concepts and, in
terms of dip isogons, similar folds are class 2 while other folds occupy
classes 1 and 3.
5. Glide-board folding is a form of 'shear folding' that can meet the
requirements of similar geometry and 'passive slip'. However, passive
folding and a truly similar fold-product are otherwise very uncommon or
non-existent! Much depends on how rigorously one applies the test for
similar geometry. I would argue that it has to be applied most rigorously
to avoid confusion between similar and modified ('flattened') parallel
folds. Indeed, excluding kinkbands, I would suggest that 99% (I would
actually say 100% but don't wish to be too confrontational!) of the folds
that are called similar (and, at least by implication for some people,
derived by 'passive' processes) are actually modified parallel systems with
an initial flexural component.
6. So do similar folds exist? I believe yes, for the non-rigorous observer,
for the 1% above, for kinkbands in slates with continuous cleavage, and for
certain types of 'shear fold'.
7. And do passive folds exist? I again believe yes, but most heavily
qualified! They probably exist in those types of shear zone where simple
shear is the predominant process. They could well exist in metamorphic
environments where layering is defined essentially by colour-banding
(negligible viscosity contrast) and the material is in the steady state
flow regime - this might be the case in glaciers, salt domes and thick
sequences of essentially homogeneous pelite. As with so much of geology,
scale of observation is fundamental - passive folding is likely to exist in
small fields of view in the types of environment indicated; in larger
fields of view, kinematically active layers will inevitably be encountered,
such that flexural processes excercise over-riding control.
This is longer than I intended. I hope it helps.
Brian.
>Dear All,
>
>Bit disappointed not to have generated more replies to this topic. I have
>seen the term 'passive folding' used for shear modification of an existing
>fold without generating a cleavage. As this is contrary to what I
>understand the term to mean, is there a structural concensus as to the
>term'passive folding'?
>
>Would welcome thoughts on my earlier email also.
>
>Thanks,
>Joanie Marion
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