Estimating the viscosity and Prandtl number of the Tso Morari crystalline gneiss dome, Indian western Himalaya

The Tso Morari crystalline (TMC) gneiss dome in the Indian Himalaya extruded from a depth of ~120 km through an inclined subduction channel of sub-elliptical cross-section at the leading edge of the Indian plate. The velocity profile of this gneiss dome is derived after (1) presuming its incompressible Newtonian rheology, (2) finding the "best fit" of the outcrop of the gneiss dome to an ellipse, (3) taking into account different lithologies to have existed at the top of the extruding gneiss body, (4) considering the extrusion to have been driven by the buoyant push of the denser mantle beneath the lighter gneiss, and (5) assigning a range of plausible densities for different litho-units. Fitting the known rates of extrusion-from a few centimetres up to about one-hundredth of a millimetre per year-from ~53 Ma onwards of this gneiss dome to its velocity profile constrains its maximum possible viscosity to ~7. 5 × 10 ²² Pa s. This magnitude is 10 ²-10 ⁴ times higher than previous estimates for gneisses and granites. Alternative explanations of our data are the following: (1) There was a fall in extrusion rates of the TMC gneiss from 53 to <30 Ma because of an increase in the estimated maximum viscosity from 6. 2 × 10 ²⁰ to 7. 5 × 10 ²² Pa s, possibly indicating a fall in temperature and/or compositional change of the TMC gneiss. (2) Lower the extrusion rates, higher are the estimated viscosities. (3) The TMC gneiss was more viscous probably due to its eclogite content. (4) The estimated maximum viscosity is ~10 ² times higher than that in collision zones and 10 ²-10 ⁴ times than that in the Tibetan lower crust, but broadly conforms to that for the crustal channel, and average lithospheric and asthenospheric values. The high magnitude of maximum possible Prandtl number of ~10 ²⁸ of the TMC gneiss might be related to isothermal decompression of the gneiss during its extrusion.