The Greater Himalayan Sequence (GHS), constituting the anatectic core of the Himalaya, is generally modelled as a mid-crustal southward extruding channel or wedge. Movements along the Main Central Thrust (MCT) in the south and the South Tibetan Detachment System (STDS) in the north and exhumation along the Himalayan front played an important role in the extrusion of the GHS from beneath the Tibetan plateau during the Miocene. To understand the kinematics of these orogen-scale shear zones, it is important to constrain the percentage of pure shear associated with them. In this paper, we present the kinematic vorticity data from the Main Central Thrust Zone (MCTZ), Alaknanda and Dhauli Ganga Valleys (Garhwal), Uttarakhand Himalaya. The mean kinematic vorticity number (Wm), which can be used to calculate the percentage of pure shear, has been estimated by analysing the rotational behaviour of rigid grains in a ductile matrix. The analysis reveals that pure shear provides significant contribution (30–52%) to the deformation associated with southward ductile shearing along the MCT, with the highest mean kinematic vorticity number (Wm) values close to the MCT. The results provide important quantitative constraints for the boundary conditions in the extrusion models. The Wm values from within the anatectic core have not been reported as most of the vorticity gauges fail due to increased deformation temperatures in this region.
Orogen-scale mid-crustal southward extruding channel or wedge models deformation of the Great Himalayan Sequence (GHS) of the anatectic core, whose kinematics is to be understood by constraining the percentage of pure shear.
Vorticity estimation near the Main Central Thrust Zone (MCTZ) is performed along the Alaknanda–Dhauli Ganga Valleys, Uttarakhand Himalaya along with critical analysis of published vorticity data from the other areas.
Mean kinematic vorticity number (Wm), a quantitative estimator of pure shear percentage during non-coaxial deformation in a shear zone, varies between 0.675 and 0.875 within the MCTZ, corresponding to a pure shear percentage between 30% and 52%.
A general trend of decreasing pure shear component towards the channel boundaries is explained by velocity profile within an extruding channel of hot and low-viscosity mid-crustal rocks and observed from the compiled vorticity data from other Himalayan traverses.
Our results agree with the channel flow conceptual model and provide quantitative constraints on the percentage of pure shear associated with deformation within the GHS.
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LK is grateful to Professors A K Saraf, Ex-Head and R Krishnamurthy, Department of Earth Sciences, IIT Roorkee, for their support and facilities during the M.Tech thesis. Thanks are also due to Dr D P Kanungo, CSIR-Central Building Research Institute, Roorkee for providing facilities for petrographic studies, Dr P K Mukherjee for providing the samples from the Malari–Niti region, Mrinal Shreshtha and Puneet Seth for their help and fruitful discussions. Comments by two reviewers have been extremely useful in improving the manuscript. Financial assistance to A K Jain by the Indian National Science Academy (Grant No. SP/HIS/2016 dated 13.6.2016), New Delhi made it possible to run the field excursions, while this work was completed during a research project from the Ministry of Earth Sciences [MoES/P.O.(Geo.)/101(9)/2016]. Research facilities by the CSIR-CBRI, Roorkee are duly acknowledged.
Communicated by Somnath Dasgupta
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Kanyan, L., Jain, A.K. & Singh, S. Vorticity patterns along the Main Central Thrust Zone, Alaknanda–Dhauli Ganga Valleys (Garhwal), Uttarakhand Himalaya. J Earth Syst Sci 130, 31 (2021). https://doi.org/10.1007/s12040-020-01539-1
- Main Central Thrust (MCT)
- Greater Himalayan Sequence (GHS)
- South Tibetan Detachment System (STDS)
- vorticity measurements
- tectonic models