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