ZIRCON (U-TH)/HE THERMOCHRONOMETRIC CONSTRAINTS ON THE EXHUMATION OF THE LESSER HIMALAYA OF NORTHWEST INDIA AND IMPLICATIONS FOR NEOGENE SEAWATER EVOLUTION

The uplift and exhumation of the Lesser Himalaya (LH) in the Uttarakhand-Himachal region of Northwest India remains a topic of debate. Two contrasting models have been proposed to explain the origin of the southward dipping Tons Thrust, which separates the LH into the inner LH (iLH) of late Paleo-Mesoproterozoic rocks and the younger outer LH (oLH) of low grade Cryogenian to Cambrian metasedimentary rocks. One model suggests that the Tons Thrust shared an original decollement with the South Tibetan Fault System and that the oLH is a far-traveled klippe emplaced against the iLH during the Eocene-Oligocene, followed by out-of-sequence activation of the Main Central Thrust (MCT). In contrast, a second model suggests that the oLH is a short-travelled, in sequence thrust sheet emplaced in the Late Miocene, which post dates movement along the MCT. Resolving the timing of LH exhumation may provide insight into the relationship between the weathering of chemically distinct Himalayan source rocks and the Neogene isotopic evolution of seawater osmium and strontium. Given a time discrepancy of at least 14 Ma between the long- and short-travelled oLH emplacement models, and broad constraints of the thermal history of the oLH, iLH, and MCT hanging wall, zircon (U-Th)/He (ZHe) can be used to effectively test these hypotheses. Newly generated ZHe data yield average cooling ages of 10.5 ± 3.6 Ma for the iLH, 16.2 ± 1.5 Ma for the middle oLH, and 23.0 ± 4.1 Ma for MCT hanging wall rocks in and around the Shimla and Mussoorie synclines of the frontal Himalayan system. These data provide support for the short-travelled kinematic model for emplacement of the oLH and are in agreement with predicted ages for the exhumation of black shale units enriched in ¹⁸⁷Os within the oLH, the weathering of which has been proposed to drive a global increase in seawater ¹⁸⁷Os/¹⁸⁸Os at ~16 Ma. With a higher spatial resolution of cooling ages along strike from the hinterland coupled with detrital zircon U-Pb-He double dating of foreland basin deposits, we can further constrain LH exhumation and more confidently link the weathering of the LH to the observed shifts in seawater compositions.