FOLD INTERFERENCE STRUCTURES IN THRUST SYSTEMS

The accommodation problems associated with the generation of interference folds in thrust belts are examined. There are several ways in which interference folds may grow in thrust systems, such as moulding on to non-planar ramps, or via the interaction of non-coaxial structures. The geometries of interference folds from thrust belts in Spain and Pakistan are described, and form the basis of an analog model. The model is used to quantify the volume of rock which must be accommodated during the growth of these structures. Mechanisms which might solve the inherent space problem are discussed. The simplest geometrical solution would involve a mechanism which caused thickening in the fold hinges-but this was not observed in the field. Field evidence is used to construct a linked fold-and-fault model which reconciles the absence of thickening in the interference fold hinge with the volume of material which must be accounted for.