Untangling the complex kinematic and geochronological history of a crustal-scale shear zone: an example from the Main Central Thrust (Garhwal Himalaya, NW India)

Chiara Montemagni
Dipartimento di Scienze dell'Ambiente e della Terra, Università degli Studi di Milano – Bicocca, Milano, Italy. Corresponding author e-mail: chiara.montemagni@unimib.it

This contribution focuses on geochronological and kinematic data on the Main Central Thrust zone (MCTz) in the Garhwal Himalaya (NW India). A multidisciplinary approach combining detailed 2D and 3D microstructural observations with a new interpretation key of geochronological data on micas is here applied to reconstruct the polyphase evolution of a crustal-scale shear zone. The Ar Differential Release Plot and the use of X-ray micro-Computed Tomography allow to disentangle misleading Ar/Ar spectra of micas from mylonites and to avoid methodological limitation in vorticity estimates, respectively. This new approach allows to unravel the microstructural and petrological complexities linked to the temporal and kinematic evolution of the MCTz. The obtained data indicate a structurally downward shift of deformation, i.e. towards the foreland, across the MCTz, with an increasing pure shear component mirrored to a progressive rejuvenation of the mica age of structures.

Shear zones, Ar/Ar dating, kinematic vorticity, MCT, Himalaya.