Crustal channel flows: 1. Numerical models with applications to the tectonics of the Himalayan-Tibetan orogen

Plane strain, thermal-mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens. In the models, radioactive self-heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large-scale horizontal flow. Channels can be exhumed and exposed by denudation focused on the high-relief transition between plateau and foreland. We interpret the Himalaya to have evolved in this manner. Channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws, and well developed if there is an additional reduction in viscosity to 10(19) Pa s. This reduction occurs from 700degreesC to 750degreesC in the models and is attributed to a small percentage of in situ partial melt ("melt weakening"). Model HT1 provides an internally consistent explanation for the tectonic evolution of many features of the Himalayan-Tibetan orogenic system. Erosional exhumation exposes the migmatitic channel, equivalent to the Greater Himalayan Sequence (GHS), between coeval normal and thrust sense ductile shear zones, corresponding to the South Tibetan Detachment and the Main Central Thrust systems. Outward flow of unstable upper crust rotates these shears to low dip angles. In the model both the GHS and the Lesser Himalayan Sequence are derived from Indian crust, with the latter from much farther south. Similar models exhibit a range of tectonic styles, including the formation of domes resembling north Himalayan gneiss domes. Model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau.