Deformation mechanisms and inverted thermal profile in the North Almora Thrust mylonite zone, Kumaon Lesser Himalaya, India

The thermal structure of a 2.2 km thick mylonite zone from the Kumaon Lesser Himalaya (India) is interpreted based on deformation mechanisms and microstructures. The mylonite zone lies along the North Almora Thrust (NAT), a S-dipping thrust that marks the northern flank of a large synformal crystalline klippe (Almora klippe), approximately 17 km north of the town of Almora. The mylonite zone evolved during the upper Eocene emplacement of the Almora klippe (part of a larger Munsiari thrust sheet), following the collision of India and Eurasia in lower-middle Eocene. Mylonitic rocks near the top of the zone show dynamic recrystallization of both feldspar and quartz, indicating deformation at amphibolite grade (500-600 degrees C). Near the center of the mylonite zone, the feldspars are fractured and quartz is dynamically recrystallized though to a lesser degree than near the top of the zone. This evidence, combined with the occurrence of secondary epidote + chlorite, biotite and calcite in these rocks, suggests that mylonitization occurred at greenschist grade (500-400 degrees C). Near the base of the mylonite zone, biotite is present, feldspars are fractured and altered to sericite, and quartz shows limited plastic deformation in the form of undulose extinction and subgrain formation, indicating that temperatures of 400 degrees C or less were reached during deformation. Deformation mechanisms and metamorphic reactions in the mylonite zone rocks thus point to a decrease in the temperature of deformation from 500-600 degrees C near the top of the thrust zone to about 400 degrees C near its base, suggesting an inverted thermal profile. We interpret this inverted thermal profile in the mylonite zone to have resulted from gradual cooling of the zone by heat transfer to a cold footwall at the time of thrusting.