Three-dimensional vorticity and time-constrained evolution of the Main Central Thrust zone, Garhwal Himalaya (NW India).

Montemagni, Chiara; Carosi, Rodolfo; Fusi, Nicoletta; Iaccarino, Salvatore; Montomoli, Chiara; Villa, Igor M.; Zanchetta, Stefano (2020). Three-dimensional vorticity and time-constrained evolution of the Main Central Thrust zone, Garhwal Himalaya (NW India). Terra nova, 32(3), pp. 215-224. Wiley 10.1111/ter.12450

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The use of porphyroclasts analysis for vorticity estimates is limited by the extrapolation to three dimensions of the two dimensional thin-section analysis of rigid clasts. We propose an alternative approach based on the use of X-ray micro computed tomography that offers a 3D view and thus considerably decreases the limitations of the method. Two mylonitic orthogneisses from the Munsiari Thrust, the lower boundary of the top-to-the-SW km-thick Main Central Thrust zone in the Indian Himalaya, containing K-feldspar porphyroclasts yield a kinematic vorticity ranging between 0.49 and 0.57. 40Ar/39Ar dating constrains the biotite growth on the main foliation at 4.8 ± 0.02 Ma.
Adding as much information as possible to a shear zone and relating them to different pressure and temperature conditions of shear episodes recorded besides by age variations of micas is a key point to keep in mind when debating the non-cylindrical nature of Himalayan orogen.

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