Mesozoic history of the southeastern Tibetan plateau: Sediment provenance, paleoclimate, and surface elevation history

This dissertation presents new results of a multi-disciplinary investigation of the linkages between erosion, uplift and regional climate change associated with the Mesozoic accretionary tectonism in southeastern Tibet. We studied sedimentary record preserved in a robust and strategically-located geologic archive---the Qamdo basin system. Since tectonics can greatly influence the geomorphic and stratigraphic development of sedimentary basins, studying evolution of the Qamdo basin could provide useful insights into the tectonic information of the adjacent areas. The following questions are addressed in the three chapters of this thesis: • What are the sediment sources for the Mesozoic Qamdo basin? • Is there a link between provenance of Qamdo sediments and Mesozoic accretionary tectonism of southeastern Tibet, such as the closure of the Paleo-Tethyan and Meso-Tethyan oceans? • What impact did Cretaceous global climate change have on local climate of southeastern Tibet? • What impact did Cretaceous orogenesis have on local climate of southeastern Tibet? • Can we estimate the amount of Cretaceous surface uplift in southeastern Tibet by distinguishing between effects caused by global climate and local orogenesis?;Chapter 1 provides an overview of the sedimentology, stratigraphic characterization and sandstone provenance of the Mesozoic Qamdo basin. Study revealed that the Qamdo basin was sourced primarily from erosion of Triassic turbidite of the Yidun Group, metasedimentary rocks of the Zhongza massif, igneous rocks of the Jomda-Weixi arc and Bangong arc, and recycled Upper Triassic Qamdo strata. Upper Triassic sediments of the Qamdo basin were sourced primarily from the inverted Yidun Group as a result of the closure of the Paleo-Tethyan Ganzi-Litang ocean and development of a widespread fold-thrust belt in northeastern Tibet during Late Triassic time. Three distinct depocenters developed in the Qamdo basin by the beginning of Early Jurassic time, as indicated by a spatial diversification of detrital zircon age signatures.;Chapter 2 employs carbonate clumped-isotope thermometry in combination with paleosol CO2 barometry to reconstruct mid-Cretaceous terrestrial climate conditions of southeastern Tibet. The mid-Cretaceous (Cenomanian-Turonian) super-greenhouse world of ∼100 million years ago was characterized by high levels of atmospheric CO2 driven by intense volcanic activity, and characterized by high sea surface temperatures (SSTs) and a small equator-to-pole temperature gradient. Although it is commonly accepted that the extreme warmth of the mid-Cretaceous was driven by increased atmospheric pCO 2, large discrepancies exist between climate models and proxy data, as models often fail to simulate SSTs unless atmospheric pCO 2 reaches implausibly high levels (\u3e 3,500 ppm) when using a standard range of model climate sensitivities. We reconciled this discrepancy using new temperature and pCO2 reconstructions that indicate climate sensitivity may have been underestimated for greenhouse conditions. We use carbonate clumped isotope thermometry to constrain soil temperatures, meteoric water delta18O ratios, and pCO 2 levels for the Cenomanian-Turonian interval (∼101--90 Ma) from paleosol carbonates in southeastern Tibet, which was located at ∼15° N during this time.;Chapter 3 reports results from clumped isotope, strontium isotope and sedimentological analyses of the entire succession of Late Cretaceous (Cenomanian-Santonian) strata preserved in the Qamdo basin, accompanied by results of Late Cretaceous climate modeling. This chapter provides evidence of an episode of surface uplift in southeastern Tibet during Late Cretaceous (Coniacian-Santonian) time. We examined two stratigraphic sections in the Cenomanian-Santonian strata of the Qamdo basin and find that petrography, burial history and elemental composition of paleosol carbonates are consistent with preservation of original isotopic signatures and minimal influence from solid-state reordering. Meanwhile, the development of growth strata and occurrence of thick-bedded conglomerate layers within the Late Cretaceous strata all implying active tectonics during deposition. Together, out data suggest that a moderate topography with elevation \u3e1.5 km was probably established by Late Cretaceous time in SE Tibet. (Abstract shortened by UMI.)