TECTONIC AND METAMORPHIC EVOLUTION

OF THE CENTRAL HIMALAYAN DOMAIN

IN SOUTHEAST ZANSKAR

by Pierre Dèzes

8.5 Discussion

We have chosen to present here only models where exhumation and extensional tectonics are associated with crustal shortening and thickening and not with crustal extension as proposed by Platt (1986) or Ruppel et al.(1988).

The effect of extrusion is to place the high-grade rocks of the HHCS in contact with the overlying low-grade sedimentary series of the Tethys Himalaya along the low-angle normal Zanskar Shear Zone but in a tectonic setting without any net extension of the overall system.

On the basis of these various models and from our own observation, we infer that ductile extention along the ZSZ and conjugated thrusting along the MCT are the result of the south-westward extrusion of the HHCS. The HHCS represents a slab of upper crustal material that was first underthrusted below a buttress formed by the Sedimentary series of the Tethys Himalaya (the Nyimaling-Tsarap nappe), until it reached a critical point where the downwards-pulling forces were counteracted by buoyancy. When this critical point was reached, the upper crust failed and the HHCS slab became detached from the subsiding lithosphere. Once detached, the HHCS was sandwiched beween the still subsiding lithosphere and the rigid buttress formed by the TH. The compressive forces exerted by the ongoing collision lead to the deformation of the HHCS through conjugated simple shear and pure shear.