Thermochronometric constraints on Cenozoic Himalayan exhumation in Uttarakhand, India

Abstract

To constrain the tectonic evolution of the Himalayan thrust belt, I generated new zircon (U-Th)/He (ZHe) thermochronometric data from samples collected in the Uttarakhand Indian Himalaya that span major lithotectonic zones and several prominent klippen. Integrating these new data with existing thermo- and geochronologic datasets from across western India demonstrates that: l) All Outer Lesser Himalayan (oLH) zone ZHe cooling ages are <20 Ma and fall within two distinct age ranges (from ~7 to 12 Ma, and from ~15 to 19 Ma), corresponding with their stratigraphic position within the Tons Thrust hanging-wall; 2) Thermochronometric ages of Inner Lesser Himalayan (iLH) rocks young from ~16 to <5 Ma with northwards proximity to the Main Central Thrust (MCT); 3) MCT hanging wall klippen within the Lesser Himalayan zone show an increase in ZHe cooling ages from ~12 to 24 Ma with greater proximity to the Main Boundary Thrust; and 4) Greater Himalayan rocks yield young cooling ages from multiple thermochronometers that range from 0 to ~5 Ma. The two oLH cooling age ranges suggest an exhumational history of emplacement by the Tons Thrust ~16 Ma, followed by exhumation through the ZHe closure temperature zone by motion on the Main Boundary Thrust from ~12 to 7 Ma. The older ages of the 2 southernmost MCT-hanging wall klippen demonstrate early (>20 Ma) emplacement of this material, with younger klippen ages to the north tracking exhumation in association with deformation along other Lesser Himalayan faults, notably those associated with the development of duplexing in the iLH. In the last ~5 million years, rapid exhumation around the modern MCT zone due to out-ofsequence thrusting has continued to exhume iLH and Greater Himalayan material to the surface.