GARNET ZONING IMPLIES SPATIAL AND TEMPORAL MIGRATION OF THE MAIN CENTRAL THRUST ZONE, SUTLEJ VALLEY, NW INDIA

We present integrated microstructural and petrologic analyses to infer the burial, deformation, and exhumation history of a High Himalayan Crystalline Series (HHCS) schist from the Main Central Thrust (MCT) zone, eastern Sutlej Valley, NW India. Microstructural and textural analyses of specimens within ~1.5 km structurally above the MCT indicate that garnet overgrew an early S1 foliation that was subsequently folded and transposed into the dominant S2 transposition foliation during the main D2 deformation phase associated with Miocene top-SW, reverse-sense motion on the MCT. Waning stages of D2 deformation outlasted porphyroblast growth, and late top-NE shear bands related to extension in the MCT hanging wall locally overprint S2.

Compositional zoning patterns of large garnet porphyroblasts in metapelites were used to infer a more detailed P-T-D history of MCT zone rocks. Growth zoning is largely concentric and continuous, and has not been significantly modified by diffusion. This interpretation is based on observed zoning characteristics, and is further supported by results of forward modeling incorporating the effects of simultaneous fractional crystallization and intracrystalline diffusion. The P-T path extracted from MCT zone rocks consists of an early phase of burial associated with modest heating (steep dP/dT), followed by continued heating during initial unloading, and finally cooling and continued exhumation. The path is best explained by initial burial of MCT footwall rocks, followed by their incorporation into the hanging wall of the southward-migrating MCT deformation front.

Distinct P-T path geometries such as that inferred for the Sutlej Valley provide a valuable tool to test geodynamic model predictions for the burial and exhumation history of rocks expected to be currently exposed in similar structural settings.