Relationship between Himalayan topography, crustal viscosity, and channel flow

Abstract:

The Himalayan orogen resulted from a continent-continent collision ~ 55 ma ago. This collision uplifted the Tibetan plateau and several large-scale in-sequence and out-of-sequence faulting happened with reverse slip sense. Additionally, a large-scale fault zone, the South Tibetan Detachment System, with first reverse- followed by normal sense motion is observed along the strike of the entire orogen. Channel flow is one of several proposed theories that explains the occurrence of this normal sense motion. The key requirements for channel flow are i) extruding mid-crust of low viscosity, and ii) excess gravitational potential due to topography.

We conducted 12 two-layer sandbox experiments to examine the relationship between the convergence rate, the amount of gravitational potential energy imposed by the Tibetan plateau, and the viscosity of the middle-crust. Our results show that the viscous middle crust extrudes when the strain rate imposed by the high topography of the plateau equals the India-Asia convergence rate. There is a direct relationship between the degree of upwelling of viscous material and the pressure gradient. Linking the excess topography, the viscosity and the convergence rate suggests that channel flow initiates with a mid-crustal viscosity higher than previously considered, questioning one of the fundamental requirements for channel flow.