MIOCENE PALEOALTIMETRY OF THE SOUTHEAST MARGIN OF THE TIBETAN PLATEAU: EVIDENCE FROM CARBONATE STABLE ISOTOPE COMPOSITIONS, YUNNAN PROVINCE, SE CHINA

The evolution of topography along the SE margin of the Tibetan Plateau is a crucial for understanding the mechanisms by which Tibetan plateau formed and expanded, as well as the linkage between plateau growth and regional Neogene climate and paleoenvironmental change. In this study we present stable O and C isotopic data from lacustrine carbonates and aragonitic shells from two Cenozoic basins (Jianchuan basin and Xiaolongtan basin) in Yunnan Province of China to provide more constrains on the timing and magnitude of uplift of this region. We also utilize SEM and XRD data to investigate effects of diagenesis on carbonate stable isotopic compositions to assess the validity of each sample on interpreting paleoelevation. With these data, we are able to model the Miocene paleoelevation along the SE margin of the Tibetan Plateau.

Lacustrine carbonates and shells sampled from the Jianchuan and Xiaolongtan basins are early Miocene (~23-16 Ma) and late Miocene (~10 Ma) in age, respectively. Stable isotopic analyses yielded d¹⁸O values (VPDB) ranging from -14.83‰ to -9.53‰ for the Jianchuan samples and from -14.55‰ to -7.64‰ for the Xiaolongtan samples. SEM and d¹³C compositions, however, indicate that the Xiaolongtan lacustrine carbonates have been modified by extensive sulfate reduction and methanogenesis associated with organic matter decomposition. Unaltered aragonitic mollusc shells from the Xiaolongtan basin yield d¹⁸O values between -11.12‰ to -7.64‰. Rayleigh-based distillation modeling results produce an average paleoelevation of ~2600 m for the early Miocene Jianchuan basin, and ~1400 m for the late Miocene Xiaolongtan basin, both of which are indistinguishable from the present elevations of the sample localities. Constant elevations of Jianchuan and Xiaolongtan since late Miocene suggest the establishment of a relatively gentle slope along the SE Plateau margin by ~10 Ma and that deep crust flow, as suggested by previous thermal-mechanical numerical models, may have influenced the topographic elevation of the region since the early Miocene.