Channel flow extrusion model to constrain dynamic viscosity and Prandtl number of the Higher Himalayan Shear Zone

摘要

Constraining magnitudes of mechanical and thermo-mechanical parameters of rocks and shear zones are the important goals in structural geology and tectonics (Talbot in J Struct Geol 21:949-957, 1999). Such parameters aid dynamic scaling of analogue tectonic models (Ramberg in Gravity, deformation and the Earth's crust in theory, experiments and geological applications, 2nd edn. Academic Press, London, 1981), which are useful to unravel tectonics in further details (Schultz-Ela and Walsh in J Struct Geol 24:247-275, 2002). The channel flow extrusion of the Higher Himalayan Shear Zone (HHSZ, = Higher Himalaya) can be explained by a top-to-S/SW simple shear (i.e. the D_2 deformation) in combination with a pressure gradient induced flow against gravity. Presuming its Newtonian incompressible rheology with parallel inclined boundaries, the viscosity (μ) of this shear zone along a part of the Himalayan chain through India, Nepal and Bhutan is estimated to vary widely between ～ 10~(16) and 10~(23) Pa s, and its Prandtl number (P_r) within ~ 10~(21)-10~(28). The estimates utilized ranges of known thickness (6-58 km) of the HHSZ, that of its top subzone of ductile shear of normal shear sense (STDS_U: 0.35-9.4 km), total rate of slip of its two boundaries (0.7-131 mm year~(-1)), pressure gradient (0.02-6 kb km~(-1)), density (2.2-3.1 g cm~(-3)) and thermal diffusivity (0.5 × 10~(-6)-2.1 × 10~(-6) m s~(-2)) along the orogenic trend. Considering most of the parameters specifically for the Sutlej section (India), the calculated viscosity (μ) and the Prandtl number (P_r) of the HHSZ are deduced to be μ: ～ 10~(17)-1023 Pa s and P_r ～ 10~(22)-10~(28). The upper limits of the estimated viscosity ranges are broadly in conformity with a strong Tibetan mid-crust from where a part of the HHSZ rocks extruded. On the other hand, their complete ranges match with those for its constituent main rock types and partly with those for the superstructure and the infrastructure. The estimated mechanical and thermo-mechanical parameters of the HHSZ will help to build dynamically scaled analogue models for the Himalayan deformation of the D_2-phase.