The Evolution of the northeastern margin of the Tibetan Plateau : stratigraphy, paleoclimate, and tectonics

Thesis (Ph. D.)--University of Rochester. Dept. of Earth and Environmental Sciences, 2011.Tracking climate change through the use of paleoclimate proxies has the potential to reveal the spatial and temporal evolution of orogens and their associated plateaux. The development of topography associated with the Cenozoic evolution of the Tibetan plateau is coupled to climate, and feedbacks between climate and tectonics occur at all scales. Because climate changes resulting from topographic growth should scale with the amount, extent, and timing of surface uplift, this dissertation documents temporal and spatial changes in paleoclimate of the northeastern margin of the Tibetan plateau since ~30 Ma, in order to delineate when the modern climate pattern was achieved and, by inference, when the Tibetan plateau reached its current dimensions. Sedimentation in basins adjacent to the plateau margin likely began in a foreland basin setting that was later segmented by the growth of basin-bounding structures presumably during upward and outward growth of the plateau. This study focuses on a number of sub-basins in northeast Tibet, including Guide, Jian Zha, Xunhua, Tong Ren, Hualong, and Linxia, that span up to 30 myr in age and range up to 3 km thick. New lithologic, magnetostratigraphic, and stable isotope records from these basins suggest that topography began to develop in the Eocene and continued through the late Miocene before jumping ~100 km outward at ~8 Ma to the Liupan Shan and Haiyuan faults. Perturbations to local climate patterns resulted from the evolution of local topography and basin segmentation. These patterns are tracked through comparison of stable isotope compositions of calcareous basin fill materials. Similarity of isotopic compositions is interpreted to reflect the presence of integrated basins whereas distinct isotopic compositions reflect unique basin hydrologies. Within the study area, changes in isotope trends are observed throughout each record indicating the influence of local climate conditions on isotopic values. Synchronous isotope changes in multiple records are observed at 18.5, 16.3, and 9.4 Ma possibly corresponding with changes in regional scale climate. A time-transgressive trend of Miocene aridification in the lee of growing topography along the plateau’s northeast margin is roughly coeval with and spatially consistent with the blocking of vapor transport by west-to-east growth of eastern Tibet. Modern rainfall data collected from 2007-2009 across the Tibetan plateau’s northeastern margin show seasonal trends related to changes in air temperature and elevation, but not precipitation amount or relative humidity. To assess the spatial variability and thus the degree to which any one monitoring station is representative of a large geographic region, climate variables and rainwater isotope data from seven collection stations located across the study area were compared to each other and to the Global Network of Isotopes in Precipitation (GNIP) station data from Lanzhou. Annual mean and long-term mean isotope compositions of rainfall match each other and those from the GNIP station in Lanzhou suggesting that the long-term values for any one station are representative over relatively large regions. Trends in δ18O and d-excess indicate that source regions for summertime precipitation in northeast Tibet are consistent with increased soil moistening and local recycling of water vapor. When carbonates derived from modern rainfall are compared to time integrated paleo-carbonate samples, no significant change in regional climate are evident since at least 3-4 Ma. The results of this thesis demonstrate that changes in local climate are related to the onset of local deformation and that deformation and associated basin segmentation in NE Tibet initiated prior to uplift of eastern Tibet. However, the timing of regional-scale climate change, highlighted by a west-to-east pattern of aridification in northern and northeastern Tibet, is consistent with the systematic displacement of vapor pathways around a progressively eastward uplifting eastern Tibetan plateau. Once established, the upwind climate regime, and by inference the topographic framework of northeast Tibet, has remained stable since ~8 Ma