The origin of recumbent fold nappes: The Lochalsh fold as the main example

This thesis describes structural studies of the Kishorn and Balmacara Nappes of Northwest Scotland and the Mellene Nappe of southern central Norway. These studies have involved the mapping of the orientation, age and distribution of cleavages related to recumbent folding. Using a variety of techniques, including grain shape analysis, measurement of conglomerate pebbles, magnetic anisotropy and palaeomagnetism strain patterns have been obtained for these nappes. Different rock types control the style of folding, and, in part, the sequence of structures developed. The development of. each recumbent fold is a multistage process and prior to each change of mechanism the rate of the current meahanism declines. It is suggested that each change of mechanism is an attempt by the developing structure to maintain the displacement rate required by the thrust belt as a whole. The suggested model for the development of the bedding parallel fabric seen in these nappes indicates the dominance of simple shear deformation on the overturned limbs of recumbent folds. In recumbent fold nappes the passage of the thrust tip into undeformed strata is preceded by folding. The relative ease of lateral propagation may be indicated by the presence or absence of extension parallel to the fold axis or, the presence of layer normal shear strains depending upon the style of folding. The Skarvemellen Anticline or the Mellene Nappe is a non-cylindrical fold with a marked extension lineation parallel to the fold axis. It is thought that this extension lineation results from restriction to the lateral propagation of the fold and thrust. There is an average of 40% extension parallel to the fold axis of the Skarvemellen Anticline in contrast to the Lochalsh Syncline which is a relatively cylindrical fold with relatively little extension parallel to the fold axis, less than 10%.