| The channel flow extrusion of the Higher Himalayan Shear Zone (HHSZ) involved a
top-to-SW simple shear in combination with a pressure gradient induced flow against gravity.
Presuming a Newtonian incompressible rheology of the HHSZ with parallel inclined
boundaries- the Main Central Thrust-Lower (MCTL) and the South Tibetan Detachment
System-Upper (STDSU), the viscosity of the HHSZ along the entire Himalayan chain within
India, Nepal and Bhutan is estimated to vary between ~ 1016-1023Pa s, and its Prandtl
number within ~ 1021-1028. The parameters specifically for the HHSZ in the Sutlej section
(India) are calculated to be ~1017-1023 Pa s and ~ 1022-1028. These estimates utilized
ranges of known thickness (6-58 km) of the HHSZ, and that of its top sub-zone of
reverse ductile shear (STDSU: 0.35-9.4 km), total rate of slip of its two boundaries
(0.7-0.69 mm y-1), pressure gradient (0.2-6 kb km-1), density (2.2-3.1 g cm-3) and
thermal diffusivity (0.5Ã10-6-2.1Ã10-6m s-2) along the studied orogenic trend.
The deduced magnitudes are in conformity with a strong Tibetan mid-crust, and
range within those for its constituent main rock types, partly for the superstructure
and partly for the infrastructure. The estimated magnitude of viscosity will help
to build dynamically-scaled analogue models of the evolution of the Himalaya. |