TECTONIC AND METAMORPHIC EVOLUTION

OF THE CENTRAL HIMALAYAN DOMAIN

IN SOUTHEAST ZANSKAR

by Pierre Dèzes

8. MODELS FOR SYN-OROGENIC EXTENSION

La cohérence séduisante des modèles constitue souvent un piège en canalisant la pensée vers une vision trop simple d'une nature infiniment complexe. H. Masson

8.1 Introduction

Our study reveals that the Zanskar Shear Zone accommodated tens of kilometres of displacement and that these extensional movements at the top of the HHCS were contemporaneous with the thrusting of this domain along the Main Central Thrust. The delimitation of the HHCS by these two major structures showing opposite sense of shear indicates that the HHCS formed a slice of crustal material that moved southwards relative to both India and Tibet while regional shortening and crustal thickening was still going on.

The Zanskar Shear Zone shares the same characteristics as most normal faults; a deep, hot footwall was exhumed from beneath a shallow, cold hanging wall, resulting in the close juxtaposition of high-grade metamorphic rocks and low-grade rocks. Early ductile fabrics within the shear zone are progressively overprinted by more brittle structures as the hot footwall is cooled during its ascent to shallower crustal levels.

The tectonic settings in Zanskar are however totally different from areas of regional extension such as the Basin and Range Province of the western United States (e.g. see Wernicke, 1984; Wernicke and Axen, 1988; Platt, 1993). These latter areas are marked by crustal extension and thinning, whereas in the Himalaya, extensional shearing along the South Tibetan Detachment System formed during regional shortening and crustal thickening. This indicates that the upper crustal extensional system must have been decoupled from the underlying system dominated by convergence.

The existence of syn-orogenic extension is a relatively new discovery, which was described only since the eighties and has received quite a lot of attention in the past ten years. The mechanism or even the kynemtics leading to the formation of orogen-parallel extensional structures are still little understood but several models have been proposed to cast some light on this intriguing phenomenon.

The aim of this chapter is to present and discuss several of these models.