The Main Central Thrust zone along the Alaknanda and Dhauli Ganga valleys (Garhwal Himalaya, NW India): Insights into an inverted metamorphic sequence

Abstract

New tectono-metamorphic data along the Alaknanda-Dhauli Ganga valleys in the Garhwal Himalaya (NW India) are presented focusing on sheared rocks of the Main Central Thrust zone (MCTz), one of the first order regional structures in the Himalaya. In regard of large hot orogens (LHOs), the MCTz corresponds to the base of the low-viscosity infrastructure. Our new findings support that the MCTz is a c. 4 km thick mylonitic zone, affecting different lithologies and encompassing a possible (previously defined) protolith boundary. Four selected mylonitic samples, at different structural position along the MCTz, were studied with a multi-disciplinary approach in detail, supporting that an inverted metamorphic field gradient, with temperature (T) and pressure (P) ranging from c. 500 up to c. 700 °C and c. 0.50–0.60 GPa up to >1.0 GPa, was present. This inverted P-T array is also discernable by the distribution of Al-rich minerals (e.g. chloritoid, staurolite, kyanite) in the intercalation of sheared quartzite. In situ U-(Th)-Pb monazite geochronology indicates that the high-T shearing in the upper part of the MCTz mylonites developed from c. 20 Ma on after a stage of prograde metamorphism that lasted until c. 23 Ma.The integration of our data with available structural, metamorphic and geochronological information along the MCTz profile, in the study area, sheds light on the development of the associated inverted metamorphic sequence (IMS). These data carry also empirical finite patterns to be tested against theoretical models investigating the possible roles of viscous heating and “tectonic overpressure” in the development of the IMS in the Himalaya, and possibly in LHOs in general. Both phenomena, if have been active at all, are not obvious in regard of the current state of geothermobarometric methods. On the contrary, a model for the tectonic assembly of the IMS is favored. The anomalous apparent P and T inverted field gradients are interpreted to be the result of the differential and diachronous transport along a shear zone experiencing heterogeneous vertical thinning linked to the non-coaxial flow.