A P-T pseudosection modelling approach to understand metamorphic evolution of the Main Central Thrust Zone in the Alaknanda valley, NW Himalaya

The Main Central Thrust Zone (MCTZ) in the Alaknanda valley, NW Himalaya, affected the Lesser Himalayan Crystalline Sequence and has a gradual transition to the structurally overlying Higher Himalayan Crystalline Sequence (HHCS). This boundary is defined on the basis of the following petrographic features in pelitic rocks at the base of the HHCS: (i) first appearance of microscopic needles of kyanite upon moving up-section along the transect; (ii) garnet porphyroblasts with random inclusion pattern and non-rotational growth history, which contrast with the garnet porphyroblasts in MCTZ rocks with spiral inclusion trails; and (iii) chemically homogeneous garnet porphyroblasts as opposed to the growth-zoned garnets in MCTZ rocks. Pseudosection modelling and garnet isopleth thermobarometry of pelitic rocks yield peak metamorphic conditions of 6.3-7.5 kbar and 550-582 degrees C in the MCTZ, and 8.0-10.0 kbar and 610-650 degrees C in the basal part of the HHCS. The results indicate continuity in the P-T field gradient across the contact between the MCTZ and HHCS. The MCTZ shows an inverted metamorphic sequence from biotite to garnet zones. Metamorphism in the basal part of the HHCS is in the kyanite zone, which is continuous with the inverted metamorphic sequence. Both P and T increase up-section, peak in the lower HHCS and then decrease higher up in the HHCS unit. The observations are consistent with predictions of a recently proposed thermomechanical model in which temperature in the shear zone rises due to viscous heating and pressure rises as a result of weakening of the rocks.