LATE NEOGENE ENVIRONMENTAL CHANGES IN THE CENTRAL HIMALAYA RELATED TO TECTONIC UPLIFT AND ORBITAL FORCING

The linkage between tectonic forces and climate evolution remains a matter of much debate and speculation. Here we present high-resolution oxygen and carbon isotope data from late Miocene-Pliocene lacustrine strata exposed on the north slope of the central Himalaya in southern Tibet. These data, together with sedimentologic evidence, reveal major changes in drainage systems and depositional settings at ~7.2, ~5.5 and ~3.2 Ma. These environmental changes appear to be driven by regional-scale tectonics. The oxygen isotope record also reveals alternating wet and dry climates with periodicities of 24, 100 and 400 kyr that were likely controlled by orbital forcing. Paleo-temperatures, estimated using a fossil-based oxygen isotope “paleo-thermometer”, are 21+-6°C at ~7 Ma, which is ~19+-6 °C higher than the present-day mean annual temperature in the same area. The much warmer environment inferred here is consistent with fossil mammalian and pollen assemblages and sediment clay mineralogy as well as carbon isotope data from fossil tooth enamels and paleosol carbonates. The estimated temperature difference, if valid, would require the study area to have been raised by ~ 2-2.5 km since ~7 Ma to its current elevation of 4100-4500 m above sea level. This result can be interpreted as either indicating the presence of a low-altitude intermountain basin in the hanging wall of the already formed Main Central Thrust or a protracted development of a north-trending rift basin that has experienced changes in drainage system and depositional environment through time.