WHEN SHEARING IS A CAUSE OF FOLDING

Abstract

Numerous publications on shearing as a cause of folding do not refute the idea of buckling being the main process responsible for the formation of fold structures. Deformations of other types may either produce conditions for layer-parallel shortening needed for buckling or modify fold shapes, but do not generate new fold structures on any scale.The origin of similar folds, commonly considered to be a result of inhomogeneous shearing across layering, may be explained by buckling of a multilayer where layers have uniform favourable rheological properties. Tightening and amplification of folds near fault surfaces is not necessarily a result of shearing caused by friction at fault surfaces. On the contrary, in these cases, faulting is a result of strain localization during folding. Recumbent isoclinal folds, so common in metamorphic rocks, originate mainly by buckling of subvertical planar structures. Intrafolial folds are usually very flattened relics of former folds.The idea of sheath folds being produced mostly by shearing is not confirmed by the available geological data. Sheath folds that were not produced by shearing are rather common. More likely, they are formed by constriction or flattening of folds with curved hinges; the latter are a result either of fold interference or of the constriction in the layering plane.