Mapping tectono-metamorphic discontinuities in orogenic belts: implications for mid-crust exhumation in NW Himalaya

A newly identified high-temperature shear zone, the Badrinath shear zone (BSZ), is described within the metamorphic core of the Himalayan belt along the Alaknanda valley, Garhwal Himalaya (NW India). A multidisciplinary approach comprising fieldwork, microstructural analyses, petrology and in situ monazite geochronology was addressed to this high-strain zone to better understand its role in the Greater Himalayan Sequence (GHS) evolution along the study area. The Badrinath mylonite displays top-to-the-south thrust-sense of shear and affects sillimanite-bearing gneiss showing evidence of partial melting. The data integration allowed us to reconstruct the following model for BSZ development: the pre-mylonitic stage took place during prograde metamorphic path reaching conditions of 700–720 °C and 10 kbar during the time interval of 34 and 23 Ma, with incipient partial melting, followed by nearly-isothermal decompression triggered by the shear activity between 23 and 19 Ma, with exhumation rate of ±0.3 cm yr−1. Moreover, the rocks from the lower part of GHS, in the BSZ footwall, experienced metamorphic conditions of c. 660–700 °C and c. 10–11 kbar, the partial melting and exhumation took place ~3 Ma after the BSZ. Such findings led us to correlate the BSZ with the High Himalayan Discontinuity (HHD) of Central Himalaya. The BSZ is the first reported HHD branch in NW Himalaya (Garhwal). It corroborates the regional extent of the HHD accomplishing an important role during the GHS exhumation. In the Alaknanda valley, the BSZ activity accommodated deformation during early exhumation stages, 3 Ma before the MCTz and STDS coeval motion took place. Our observations point to a shift in time and place of the deformation responsible to trigger the GHS exhumation.