COUPLED SURFACE PROCESSES AND TECTONICS IN LARGE HOT OROGENS: MELTING, CRUSTAL CHANNEL FLOWS AND THE DENUDATIONAL EXTRUSION OF THE GREATER HIMALAYAN SEQUENCE

Evidence for melting in the thick crust beneath modern orogenic plateaus, for example beneath the Tibetan plateau, is complemented by the exposure of mid- to lower crustal migmatitic gneiss terranes in ancient orogens. Crust containing even a small percentage of in situ partial melt is much weaker, i.e, has a lower effective viscosity, than normal cold or hot melt-free crust. We show, using plane strain coupled thermal-mechanical finite element models of orogens, that gravitationally driven, mid-crustal channel flows may spread laterally outward from beneath plateaus, and inject and inflate the surrounding crust causing the plateau to expand. The flow velocity is approximately 1 cm/y, even when their effective viscosity is as large as 10**19 Pa.s., probably a conservative upper bound.

When linked to the surface by denudation, these channel flows form a dynamically coupled surface processes-tectonic system with several possible modes. In one mode exhumation leads to extrusion of the channel at the surface in the form of a 'gneiss fountain' with similar mechanics to 'salt fountains'. Such exposures are most likely to occur at steep windward plateau-foreland transitions where climate-coupled orographically charged fluvial and glacial denudation rates are a maximum.

This mechanism provides a credible, testable explanation of the denundational, structural and metamorphic framework of the Greater Himalayan Sequence during its Early Miocene evolution. An important question concerns the Late Miocene evolution when inferred intensification of the monsoon should have enhanced extrusion, yet thermochronological rate data imply reduced exhumation rates. Did climate or tectonics control the Himalaya during this time?