THE GEOMETRY AND KINEMATICS OF SHEATH FOLDS

Sheath folds are clasically depicted as displaying symmetrical geometries about two orthogonal mirror planes centred along the (X-Y) axial surface and the (X-Z) medial (culmination/depression) suface whch bisects the fold nose. Detailed topological analyses of minor folds and fabrics associated with major curvilinear dome and basin sheath folds in the Caledonides of N Scotand reveals however, that sheath folds display distinct and predictable asymmetries across both axial and medial surfaces. 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 consistently 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 ccurs on the lower limbs on long hinge-line segments. Major medial surface 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 direction 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.