Variation in mechanism of dynamic recrystallization and differential stress across the Chiplakot Crystalline Belt, Kali River Valley, Kumaun Himalaya: Implications for exhumation of basement rocks in a 'critical taper wedge' setting

Abstract

In this study, we carried out mesoscopic and magnetic fabric analysis along with differential flow stress based on quartz piezometer and fractal dimension (D) of quartz grain boundaries from area–perimeter method to understand variation in fabric and deformation intensity across the Chiplakot Crystalline Belt (CCB) from the Kumaun Lesser Himalaya of Kali River Valley, to understand strain variation in relation syn-Himalayan deformation and the mechanism of exhumation of the CCB. The CCB is separated from the low grade metasedimentary rocks that include siliciclastics and carbonates of the Lesser Himalaya by binding thrust zones. The northern and southern contacts of the CCB are demarcated by North Chiplakot Thrust (NCT) and South Chiplakot Thrust (SCT) and the core part of the CCB is also demarcated by a thrust called the Central Chiplakot Thrust (CCT). Both mesoscopic and magnetic fabrics are concordant and follows the NW dipping attitude of all the major thrust zones, including the Munsiari Thrust and the Main Central Thrust of this area. High temperature dynamic recrystallization texture such as Grain Boundary Migration (GBM) is more prominent in and around the CCT and progressively this texture is replaced by Sub Grain Rotation (SGR) towards the north nearing the NCT and Grain Boundary Bulging (BLG) and Sub Grain Rotation (SGR) at SCT. Differential flow stress (σ) decreases exponentially from the CCT to the SCT with increasing grain sizes. Similarly, towards the north from the CCT, value of σ gradually decreases with sudden increment within the NCT zone. We infer that the intensity of brittle-ductile shearing is more prominent along the CCT compared to the NCT. On the basis of our findings, we envisaged a 'critical taper wedge' scenario where internal deformation and forward propagation of thrusts are the key mechanisms of late stage exhumation. We opine that the CCT is an out of sequence thrust formed due to internal deformation and has played a pivotal role in the exhumation of the Chiplakot Crystalline Belt. © 2019, Wadia Institute of Himalayan Geology. All rights reserved.