Vergence and facing patterns in large-scale sheath folds

The careful geometric analysis of minor structural detail elucidates the re lationships and evolution of associated large-scale curvilinear hinge geome tries, developed during WNW-directed Caledonian thrusting exposed in Neopro terozoic Moine psammites of the Moine Nappe. Reversals in the polarity of s tructural facing associated with minor folding, mark the position of major sheath folds which parallel transport. Upwardly convex sheaths (closing in the direction of thrust transport) cored by older gneissose basement inlier s are termed culminations, whilst those opening in the transport direction (and cored by Moine psammites) are termed depressions. Sheath folds are bis ected by transport parallel and foliation normal (culmination/depression) s urfaces which separate not only the reversals in facing, but also delineate zones of minor fold hinge obliquity into clockwise and anticlockwise domai ns relative to the transport direction. The sense of obliquity of minor Z a nd S folds is thus dependent on position with respect to the surfaces of cu lmination and depression and not the fold axial surfaces. Surfaces of culmi nation and depression may be superimposed on original overturned antiformal and synformal folds to produce a variety of dome (culmination on antiform) , saddle (depression on antiform), inverted saddle (culmination on synform) and basin (depression on synform) configurations. The curvilinear hinges o f minor folds may also be asymmetrical about the transport direction and wi thin the plane of the regional foliation to define patterns of Sold hinge-l ine vergence. Classical concepts of fold limb vergence may thus relate to l arger antiformal and synformal hinges, whilst the fold hinge-line vergence defines major curvilinear hinges associated with culminations and depressio ns. Major sheath folds may therefore be interpreted in terms of both minor fold hinge-line and limb vergence, coupled with fold axis obliquity and rev ersals in the polarity of structural facing. The ability to recognise consi stent and reliable structural relationships between facing and hinge obliqu ity at small scales indicates that the regional deformation process forms a linked and coherent system through several orders of magnitude. (C) 1999 E lsevier Science Ltd. All rights reserved.