THE NEOGENE ELEVATION OF SOUTHERN TIBET AS DETERMINED BY FOLIAR PHYSIOGNOMY

The significance of uplifting the 2,000 km-wide Tibetan plateau to an average elevation of 5 km and the associated modification of global climate is widely recognised and a range of studies have linked the elevation of the plateau with changes in monsoon intensity. Moreover mechanical and thermal models for homogeneous thickening of the lithosphere make specific predictions about uplift rates of the Tibetan Plateau that have yet to be confirmed by empirical observation. In this study we have derived altitude constraints and precise depositional ages (15.10±0.49 to 15.03±0.11 Myr) from well-preserved fossil leaf assemblages recovered from the Namling basin, southern Tibet. Laser stepped heating on mineral separates from tuffs below the plants and basalts above gave ages of 15.10±0.49 and 15.03±0.11 Myr respectively.

The leaf material, deposited in lacustrine waterlain tuffaceous silts, possesses taphonomic characteristics that suggest rapid burial, minimal pre-depositional transport and no species-dependant preferential preservation. Palaeoaltitude data were derived using leaf physiognomic analysis (CLAMP) combined with enthalpy fields derived from coupled ocean/atmosphere modelling for the mid Miocene and validated by CLAMP analysis of coeval sea-level floras. Altitudes derived from two CLAMP datasets (PHYSG3ar and PHYSG3br) suggest no significant change in altitude.

These data imply that the elevation of the southern plateau has remained unchanged over the past 15 Myrs. The results, coupled with other geological indicators also suggest a strongly diachronous nature of plateau uplift, thereby precluding simple links between local elevation histories and intensification of the monsoon system.