Relationship between channel flow initiation and crustal viscosity in convergent settings: an analog modeling approach

Channel flow has been proposed as a mechanism to explain the formation of the Greater Himalayan Sequence that is bounded by normal sense ductile shear along the Himalayan orogen. The key requirements for channel flow are: (i) extruding middle-to-lower crust of low viscosity, and (ii) excess gravitational potential due to topography. We present scaled two-layer physical models where the effect of the gravitational potential with respect to the plate convergence rate is investigated. Viscous middle crust starts moving towards the surface where the strain rate imposed by the convergence is 30% of that arising from the lateral pressure gradient. How efficiently the low-viscosity crust extrudes is directly linked to the imposed pressure gradient. A simple correlation between the extruding rock's viscosity, the convergence rate, and the topography imposing the pressure gradient is established. The upward motion of viscous material is expected already for a mid-crustal viscosity of 10(21) Pa s. This is significantly higher than previously expected, suggesting that one of the fundamental requirements for channel flow might not be necessary.