Pressure-temperature-time-deformation history of the Lahul Valley, NW Indian Himalaya

Abstract

The central Himachal crystalline of the NW Himalaya is a unique region characterized by broad shear along a decentralized section of the South Tibetan Detachment (STD). New 40Ar/39Ar in Ms and Bt age constraints for migmatization along the footwall of the STD through the Lahul valley (between Rhotang pass and Batal) combined with SHRIMP U-Pb in zircon, Ti-in-Bt and Ti-in-quartz thermometry, and thermodynamic modeling from GHS and THS samples provides new constraints for Pressure-Temperature-time-deformation (P-T-t-d) histories for the central Himachal crystalline. Peak metamorphism -700 °C at 8 kbars near 34 Ma followed by slow extrusion (l-2km/Ma) of GHS migmatites and high-grade gneisses to below 300 °C by -20 Ma suggest modest exhumation and cooling rates along a late Oligocene to early Miocene STD. Late Oligocene migmatization synonymous with peak metamorphism at shallow and deep structural levels of the GHS suggest anatexis may have facilitated in the extrusion of the GHS through rheologic weakening, similar to other locations in the NW Indian Himalaya. Slow cooling rates (~25-30 °C/Ma), limited duration at peak conditions (~5 Ma), and increasing temperatures with structural level of the GHS favor critical taper models for orogenic development compatible with limited structural disconnect and a broad zone of shear for the STD through Lahul.