Dear All,
I have recently caught up on the discussion which has developed concerning
the orientation and geometry of folds relative to stretching lineations and
have a couple of thoughts to add below.
1) Although many fold hinges are described as "parallel to transport" -this
precise colinearity is rarely achieved and in many areas a small angle of
obliquity is commonly preserved between hinges and lineations. This may
reflect the incomplete rotation of hinges towards the transport direction
and as such indicates the sense of hinge rotation. Thus, relative obliquity
to transport lineations - coupled with the direction of fold facing allows
the location of culmination / depression surfaces which bisect major sheath
folds.
2) The vergence of S and Z minor folds will switch across both axial
surfaces and culmination / depression surfaces which bisect sheath folds.
Thus, within areas of curvilinear folding, reversals in the vergence of
minor folds alone does not permit the location of axial traces.
3) The hinge lines of major sheath folds may define asymmetrical curvature
towards larger culminations resulting in fold hinge-line vergence. Thus,
the hinge lines of major curvilinear sheath folds may not simply define
symmetrical "hairpin" geometries.
Further details in Alsop & Holdsworth 1999. Vergence and facing patterns in
large-scale sheath folds. J. Struc. Geol. 21, 1335-1349.
Dr. Ian Alsop
Lecturer in Structural Geology & Tectonics,
Crustal Geodynamics Group,
School of Geography & Geosciences,
University of St. Andrews,
St. Andrews, Fife, KY16 9AL UK.
Tel. (01334) 46 3937
FAX (01334 46 3849
e-mail [log in to unmask]
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