Constructing a paleo-viscosity profile for the Himalayan middle crust using mineral microstructure

The viscosity response of continental lithosphere to heating and partial melting has been a key focus of tectonics research over the past two decades. Much of this research has focused on the Himalayan orogen because it provides a canonical example of lithospheric behavior during continental convergence, involving widespread partial melting. The Himalayan mid-crust, now partially exposed as the Greater Himalayan Sequence, is widely interpreted to have been a low-viscosity, melt-bearing zone that accommodated ductile flow during the early Miocene. However, few studies have attempted to quantify the paleo-viscosity of Greater Himalayan Sequence rocks. To address this gap, we applied muti-mineral subgrain-size piezometry and titanium-in-quartz thermometry to specimens from across the Greater Himalayan Sequence. The specimens record stresses ranging from ~5–25 MPa and melt proportions ranging from ~0–20 modal percent, and include an example in which the final strain event occurred at supra-solidus conditions. By integrating the stress values and their corresponding temperatures with quartz flow laws, we calculate viscosities on the order of 10¹⁸Pa·s. Similarity in our stress and viscosity values, regardless of the proportion of melt present during final strain, indicate that the high metamorphic temperatures alone led to the effective crustal viscosities required for gravity-driven ductile extrusion of the Himalayan mid-crust, and decompressive melting was a merely a result of, rather than cause of, ductile flow.