Exhumation of the Lesser Himalaya of Northwest India : Zircon U-Pb and (U-Th)/He constraints and implications for the Neogene seawater evolution
The kinematic and exhumational evolution of the Lesser Himalayan (LH) of Northwest India remains a topic of debate; the resolution of which may provide insight into the relationship between the weathering of chemically distinct Himalayan source rocks and observed shifts in global seawater chemistry. Two contrasting models have been proposed to explain the origin of the now southward dipping Tons Thrust, which separates the LH into the inner Lesser Himalaya (iLH) of late Paleo-Mesoproterozoic rocks and the younger outer Lesser Himalaya (oLH) of low grade Cryogenian to Cambrian metasedimentary rocks. The initial exhumation of Cambrian black shales of the oLH anomalously enriched in 187Os has been proposed to drive a rapid increase in global seawater 187Os/188Os at ~16 Ma, thus testing these contrasting tectonic models is critical to elucidate this proposed relationship. While 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 prior to out-of-sequence activation of the Main Central Thrust (MCT), a second model suggests that the oLH is a short-traveled, in sequence thrust sheet emplaced in the Late Miocene, post-dating movement along the MCT. Given the temporal discrepancy for oLH emplacement of at least 14 Myrs and broad constraints on the thermal history of the oLH, iLH, and MCT hanging wall, zircon (U-Th)/He (ZHe) thermochronology can effectively be used to test these hypotheses and reconstruct the exhumational evolution of the LH. New bedrock and foreland basin ZHe data support the short-traveled oLH model and provide direct evidence for rapid exhumation and southward advancement of thrusting away from the Main Central Thrust (MCT) to the Tons Thrust starting at ~16 Ma, resulting in a shift in exhumation to LH strata highly enriched in Os and relatively less in Sr compared to MCT hanging-wall rocks. This shift in exhumation directly corresponds to coeval shifts in global 187Os/188Os and 87Sr/86Sr seawater records. While these seawater records are commonly utilized as deep-time proxies to track global silicate weathering intensity responsible for CO2 drawdown and climatic cooling, our data, coupled with mass balance box-modeling, indicates that regional weathering of isotopically unique source rocks can drive these seawater records independently from shifts in global-scale weathering rates, consequently hindering this utility of these records.