Implication of Dynamic Recrystallization Mechanism for the Exhumation of Lower Crustal Rocks: A Case Study in the Shear Zones of the Ambaji Granulite, NW India

Abstract

Abstract Shear zones are important channels for the exhumation of lower crustal rocks. The Ambaji granulite of the Aravalli-Delhi mobile belt (ADMB) has been exhumed along several shear zones, and earlier studies have shown a two-stage exhumation process during a continuous compressional tectonic event, consisting of an initial phase of vertical flow that brought the granulites to the brittle-ductile transition zone and a successive phase during which the granulite underwent a lateral flow. In this contribution, we studied the microtectonics of granulites by analyzing the dynamic recrystallization behaviour of quartz, while the granulite was passing through the vertical flow regime to the horizontal flow regime. We show that the dynamic recrystallization process assists the flow pattern at different levels of exhumation. The vertical flow is dominated by grain boundary migration (GBM), registering high temperatures for recrystallization between 490 and 600°C and low flow stresses of 12-15 MPa. The horizontal flow at the brittle-ductile transition (BDT) is characterized by bulging (BLG) and subgrain rotation (SGR), which occurred at low temperatures of 390-490°C and high flow stresses of 18-26 MPa. Strain rates are between 1.20×10−12 and 7.26×10−14/s. For the ductile exhumation of the granulite, we suggest that at depths of ~22 km, the granulite exhumed in a vertical direction facilitated by GBM. Once the granulite reached the BDT, at ~16 km depth, the material flowed laterally assisted by BLG and SGR. Once an exhuming body reaches the BDT, the deformation mechanism changes to BLG-SGR, and the only direction in which the material can move further is in the horizontal plane.