Thrust tectonics,thin skinned or thick skinned,and the continuation of thrusts to deep in the crust

The paper analyses the geometry of thin-skinned thrust zones, where the thrusts shallow out at depth and of thicker-skinned fault zones where much of the crust is involved and where the thrusts are frequently observed to become steeper downwards. In the interiors of many orogenic belts the steep dip of faults is not original but due to the folding above lower decoupling zones. The energy involved in the internal deformation of hanging-wall rocks may prohibit many faults becoming more shallow upwards. Such shallowing-upwards faults may occur in more ductile rocks to maintain compatibility between zones which have experienced different deformation intensities, but displacements on the faults are unlikely to be large.Another mechanism for producing faults which steepen downwards is proposed for the major thrusts which form the southern margin to the Himalayas. These carry large thicknesses (30 to 100 km) of crustal and upper mantle rocks to the south, causing flexuring and isostatic depression of the Indian plate. The steeply dipping thrusts are not footwall ramps; these may be some distance behind the steepened zone. This thrust-induced isostatic bending of the crust has important implications when considering regional seismic interpretations as well as thrust mechanics and kinematics.