The geometry and kinematics of sheath folds
G.I. Alsop1 and R.E. Holdsworth2
1CGG, School of Geography & Geosciences, University of St. Andrews, KY16 9AL UK
2
RRG, Department of Geological Sciences, University of Durham, DH1 3LE UKgia@st-andrews.ac.uk
Sheath folds are classically depicted as displaying symmetrical geometries about two orthogonal mirror planes centred along the (X-Y) axial surface and the (X-Z) medial (culmination/depression) surface which bisects the fold nose. Fabric Topology Plots (FTP’s) of minor folds and fabrics associa-ted with major curvilinear dome and basin sheath folds in the Caledonides of N Scotland reveal however, that sheath folds display distinct and predictable asymmetries across both axial and medial surfaces. This area is ideal for such a study as heterogeneous shear preserves structural fabrics and younging evidence within sheath folds at varying stages of development, which therefore allows an analysis of the evolution of such structures from tongue folds to more extreme tubular folds. Seven robust geometric parameters are identified which provide an effective means of monitoring planar and linear fabric rotations with increasing deformation within any system of coherent shear. They consis-tently display systematic variation from regions of lower to higher strain on passing from upper to lower fold limbs across major axial surfaces, and on crossing medial surfaces from short to long hinge-line segments related to fold hinge-line vergence. Axial and medial surfaces effectively divide major sheath folds in to quadrants with different amounts, senses and combinations of planar and linear fabric rotation within each domain. Only limited rotations occur on the upper limbs within short hinge-line segments, whilst the greatest deformation occurs on the lower limbs on long hinge-line segments. Major medial surfaces separate hinge rotations of opposing sense and hence effectively delineate the trend of tectonic transport even in areas where lineations are poorly preserved. The sense of rotation of minor fold axial planar strike towards the shear plane (marked by the foliation) is governed by minor fold (Z/S) geometry and hence location relative to the major axial surface which separate minor axial plane rotations of an opposite sense. Minor fold hinges and axial planes may thus rotate in the same (synthetic) sense or in opposing (antithetic) directions depending on position relative to major axial and medial surfaces. Bedding/cleavage intersections are developed at greater angles to the transport direc-tion than fold hinges which they transect in a consistent and predictable sense thereby confirming the direction of fold rotation even in areas which lack information on fold facing. Sheath folds display larger apical angles on upper limbs which may be asymmetrically bisected by the medial surface and transport lineation, whilst the lower limbs define more acute apical angles which are symmetrically disposed about the transport direction, thus confirming that pronounced fold rotations have generated the observed sheath morphology. In cross section, asymmetric tear-drop eyes indicate the sense of fold hinge-line vergence, whilst fold limb and hinge-line asymmetry may be combined on 3-dimensional Vergence, Ranking and Rotation Grids which allow location and relative strain states to be accurately predicted within the overall sheath fold framework.
Schematic 3-D cartoon illustrating asymmetric fold hinge-line vergence and increasing deformation towards a major culmination on the right of the sketch. Increasing deformation also results in pronounced attenuation on the lower fold limb and reduction in the apical angle of the sheath fold from tongue folds on the upper limb to tubular folds on the lower limb.