Reconstruction Of The Paleoelevation In The Gonjo Basin In The Southeastern Tibetan Plateau During The Paleocene-Eocene And Its Implications

The Tibetan plateau is the largest and highest land in the world. The majority of its boundaries are surrounded by high-relief and steep mountains. Contrasting with this features, the southeastern margin of the Tibetan Plateau has no sharp boundary like those elsewhere, instead with gentle topography linking the neighboring low lands. To explain the formation timing and dynamic mechanim for such a unusual topographic feature, two end-member models of "lower crust flow" and "block extrustion" have been proposed, which predict different evolution histories of the southeastern margin of the Tibetan Plateau. To test these two models require direct and quantitative study the amplitude of surface uplift in this area,which would help address the controversial issue on this subject.Paleoelevation reconstruction using stable isotopes provides a valuable perspective on the Cenozoic surface uplift of the Tibetan plateau. This thesis attempted to determine the magnitude of Eocene surface uplift of the Gonjo basin, which lies in the southeastern plateau. It is based on measuring the stratigraphic sections, analysis of the sedimentary facies, age dating and oxygen and carbon isotope measruement of pedogenic carbonates from the lower Ranmugou formation. The lithofacies associations indicate that the lower Ranmugou formation was deposited in typically fluvial environments. Age dated of these deposits is earlier than 43.2Ma based on U/Pb dating of the magmatic rock conformably overlying the middle Ranmugou formation. Paleoelevations are calculated using two diffrent methods:Regional empirical relationship and Rayleigh fractionation model. The regional empirical relationship is determined from a series of modern river samplings of small drainage catchments along the modern Lancang (Mekong) River. Calculated paleoelevations indicate the Gonjo basin had reached the minimum elevation of 2400+815/-850m as early as Paleocene-Eocene time.Combining with the data previously known in the Liming basin and Bamaoqiongzong-Heihuling area, this study suggest that the northernmost of the southeastern Tibetan plateau was probably a regional pre-existing highland or a part of a plateau in Eocene(-43Ma), rather than the result of local differential surface uplift controlled by the various sub-plates or faults. Moreover, The southeastern margin of Tibetan plateau once had a steep edge like that of the Longmenshan mountains, geographically coinciding with the Yalong-Yulong thrust belt to the north of Dali. This does not support the inference by the "lower crust flow model" on the uplift process in this region:after～13Ma, the sea surface began to tilt accompanied with regional tectonic uplift.