Dear all
Below is a contribution to the nappe discussion from Rob Twiss. He had
difficulty posting it himself and unfortunately I've been in the field this
week and so couldn't help him out until today. Apologies to all, but
especially Rob, if posting it now disturbs the chronology of the discussion.
Cheers
John
I have always understood that, at least in English usage, we
recognize two kinds of nappes: thrust nappes and fold nappes.
Ideally, a thrust nappe consists of a large allochthonous sheet of
rock that has been displaced kilometers (often many) from the
original location and is separated from the underlying authochthonous
rocks by a major low angle thrust fault, referred to as a décollement
(compliments again of our French colleagues!) or a sole fault; a fold
nappe consists of a large-scale (amplitude on the order of
kilometers), recumbant, close to isoclinal fold having a large aspect
ratio (amplitude/half-wavelength).
A thrust nappe may be complicated by smaller scale folds and
imbricate thrusts above the décollement. A fold nappe may be
complicated by higher order folds on its limbs and by having the
inverted limb sheared out so that it has some aspects of a thrust
nappe.
This seems relatively consistent with what Serge Perreault has said
about French usage, except that 'fold nappe' is simply a descriptive
term and includes no necessary implication of a gravity driven
deformation.
My general sense is that thrust nappes are more typical of lower
temperature deformation, and that fold nappes are more typical of
higher temperature deformation, although they clearly overlap, and
this is not a part of the definition of the terms.
Rob Twiss
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Robert J. Twiss email:
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Geology Department telephone: (530) 752-1860
University of California at Davis FAX: (530) 752-0951
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www.geology.ucdavis.edu/
Davis, CA 95616-8605, USA faculty/twiss.html
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