Hangingwall bed rotation and the development of contractional and extensional structures around a thrust fault: geometric and experimental models

Tectonic inversion of normal faults, and development of extensional and contractional structures in the vicinity of thrust faults were studied using two models; (i) a geometric (or rigid) model, and (ii) an experimental (or brittle–ductile) model. The first model is based on simple trigonometric relationships between shortening, rotation of layers along a thrust, and dip of the thrust. The model may be used to understand the structures at upper levels of the Earthʹs crust in a predominant brittle regime. The second model is based on a series of experiments performed with clay analogues. It simulated the structures forming at a deeper level in a brittle–ductile regime and incorporates folding as an essential feature. The results of the study reveal that structures near a thrust fault depend on the thrust geometry and dip amount, the initial orientation of hangingwall layers, and their rheological properties. In the absence of frictional effects along a thrust surface, the rotation of layers and consequent displacement is directly proportional to the thrust dip. Since the listric faults are characterized by a decrease of dip with depth, the variation in the rotation of layers in a profile-section may result in formation of dilation spaces that may serve as potential sites for secondary mineral deposits or oil traps. A large rotation of hangingwall layers results in a reverse fault drag and a small rotation (caused by frictional effects along the fault) produces normal fault drag.