Mechanics of thin-skinned fold-and-thrust belts

Abstract

The essential characteristics of thin-skinned fold-and-thrust belts include the following: a wedge-shaped deforming region, thicker at the back end from which the thrusts come; a weak layer at the base of the wedge; and large amounts of shortening and thickening within the wedge. All these characteristics are incorporated into an analytical model of a perfectly plastic wedge, underlain by a weak basal layer and yielding in compressive flow. The model is sufficiently flexible to be applied to a variety of tectonic situations. In each situation the model shows that shortening of the wedge and sliding over its base can occur for reasonable values of the yield stress of the wedge and of the weak basal layer, of the surface topographic slope, of the back slope of the basal layer, and of the thickness of the wedge. The present model differs markedly from the conventional concept of gravity gliding in that it shows that horizontal compressive stresses can play a major role in overcoming the resistance to basal sliding. In addition, the model suggests that the topographic slope, when necessary, can be a result of the compression within the wedge itself; no additional outside agency for producing the topographic slope need be postulated. Thus, the model suggests that a surface topographic slope is in general neither a necessary nor a sufficient condition for the formation of a thin-skinned fold-and-thrust belt.