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

Abstract

[1] 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 700°C to 750°C 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.

Geochemical variations in Miocene adakitic rocks from the western and eastern Lhasa terrane: Implications for lower crustal flow beneath the Southern Tibetan Plateau

“…Although Harrison (2006) thought that there exists insufficient evidence for a channel flow of partially molten crust from beneath the Tibetan Plateau during the formation of the Himalayas, the model of crustal channel flow (Beaumont et al, 2001(Beaumont et al, , 2004) is consistent with many observed features of the Himalayan-Tibetan system. For example, geophysical studies have shown that the partially molten middle crust is being extruded southward below Southern Tibet on the south side of the IYS (Nelson et al, 1996;Unsworth et al, 2005), which demonstrates the existence of crustal channel flow beneath Southern Tibet.…”

“…A model of channel flow in the middle crust has been proposed to explain these phenomena (Beaumont et al, 2001(Beaumont et al, , 2004Hodges, 2006;Hodges et al, 2001;Medvedev and Beaumont, 2006;Nelson et al, 1996;Unsworth et al, 2005). Subsequently, King et al (2007) proposed that the Kuday dykes are the result of partial melting of the Asian plate where it extends south of the IYS as a mid-crustal ductile channel structure beneath Southern Tibet.…”

“…Geophysical studies have revealed similar geophysical anomalies in the mid-crust of the Lhasa terrane (block) and the Himalaya range (Nelson et al, 1996;Unsworth et al, 2005), and a channel flow model has been proposed to explain this phenomenon (Beaumont et al, 2001(Beaumont et al, , 2004Clark and Royden, 2000;Hodges, 2006;Hodges et al, 2001;Klemperer, 2006;Medvedev and Beaumont, 2006;Nelson et al, 1996). Based on a study of the Kuday dykes emplaced on the south side of the Indus-Yalu suture (IYS), King et al (2007) proposed southward ductile flow of Asian crust beneath Southern Tibet.…”

Crustal–lithospheric structure and continental extrusion of Tibet

“…These geological constraints were subsequently simulated in the numerical models of Beaumont et al (2001Beaumont et al ( , 2004. In this model, partial melting occurs only in the middle crust beneath the seismogenic upper crust, but not in the lower crust, which is underthrust Indian Shield Precambrian granulites (Caldwell et al 2009).…”

“…Clark & Royden 2000;Haines et al 2003) and (b) the Himalayan mid-crustal 'channel flow' model (e.g. Beaumont et al 2001Beaumont et al , 2004Grujic et al 2002;Searle et al 2003Searle et al , 2006Searle et al , 2010bLaw et al 2004Godin et al 2006).…”