Cause and effect among thrust and normal faulting, anatectic melting and exhumation in the Himalaya

Abstract

The transition from compression to extension in a regime of overall convergence in a narrow mountain belt requires a change with time in at least one of the following: horizontal normal stress applied to the belt by its surroundings; the distribution of shear traction on the base of the belt; the constitutive relation between stress and strain rate of material in the belt itself; the surface height of the belt. The following processes could have caused one or other of these changes in the Himalaya in Miocene time and thus could have triggered the onset of extension by normal faulting on planes parallel to thrust faults that were active at the same time. The shear stress on the thrust fault at the base of the Himalaya could have decreased, for instance because of melting on the fault plane. Alternatively, the creation of a new deeper thrust fault, and hence a new locus of convergence, could have initiated extensional faulting if the distribution of shear stress on the new thrust fault were unable to support the Himalaya above it. The Himalaya could have been weakened by an increase in pore fluid pressure, for instance owing to metamorphic dehydration reactions within the wedge. The Himalaya could have been elevated, for instance following the detachment of a tongue of mantle lithosphere from beneath it, or by the underthrusting of a lens of material beneath its base. The apparent simultaneity of thrust and normal faulting on nearly parallel planes, of anatexis and of exhumation in the Himalaya in Miocene time can be explained if normal faulting was responsible for most of the exhumation and, through decompression melting, for anatexis. Existing data suggest that the most probable cause for Miocene normal faulting was either uplift of the Himalaya or the cessation of slip on the Main Central Thrust at the base of the orogenic wedge and the creation of a new, deeper thrust fault and a new base to the deforming Himalaya. © 1993 The Geological Society.