This dissertation examines the development of topography during the construction of the northern part of the Tibetan Plateau, and the extension and dissection of the North American Cordillera using sedimentary, isotopic, and geochemical tools in sedimentary basins within these systems.
Chapter 1 uses Cenozoic oxygen isotope records of paleosol carbonates preserved in sedimentary basins in southwestern Montana to characterize its topographic and tectonic history. A decrease in oxygen isotope values between ∼50 and 47 Ma is interpreted to result from an increase in surface elevation of the drainage area of 2.5 to 3.5 km, most likely adjacent to the study area itself. We conclude that the southward sweep of associated oxygen isotope shifts, extension, and magmatism in this study area and others is most consistent with tectonic models of the removal of the mantle lithosphere from the Farallon slab.
Chapter 2 focuses on the Sage Creek Basin, part of the study area of Chapter 1, with the aim of understanding how the drainage area reaching the study area changed at ∼49 Ma. We used sedimentological records to characterize basin evolution, and obtained strontium and carbon isotope and bulk geochemical records. We better characterize the magnitude of the oxygen isotope shift, and fend coeval shifts in carbon and strontium isotope and bulk geochemical records, with no changes in sedimentary style.
Chapter 3 examines the hypothesis that the drainage reorganization event in southwestern Montana directed waters southward to the Greater Green River Basin, where isotopic and sedimentary records are interpreted to result from a freshening event at ∼49 Ma. This timing suggests that the same drainage reorganization event that affected both southwestern Montana and the Greater Green River Basin cut off the supply of waters to these two study areas, and eliminates these study areas as viable conduits of waters post ∼49 Ma. Thus any waters reaching the Greater Green River basin from high-elevation sources to the north must have flowed along strike through the Sevier fold-thrust belt, and entered the Greater Green River basin from the west.
Chapter 4 uses oxygen isotope records to examine the topographic influences on Miocene mammalian evolution. Because topographic change can isolate faunal communities, it has been hypothesized to be a driver for a mid-Miocene peak in mammalian evolution. Based on the spatiotemporal variability of oxygen isotope records, we find that there were no major changes in the segmentation of landscape during the study period.
Chapter 5 is focused on the evolution of the northern margin of the Tibetan Plateau. We see two overall trends in oxygen isotope values; a Paleogene decrease in mean values, and a Neogene increase in mean values. We interpret the Paleogene decrease to result from the integration of drainage systems and freshening of waters as the surface uplift of ranges occurred to the south of the study area. We interpret the Neogene increase in oxygen isotope values to result from increasing basin isolation as the study region became part of the Tibetan Plateau itself. The change in isotopic trajectories occurs with similar timing to a decrease in slip rate of the Altyn Tagh Fault, and thermochronologic indicators of rapid unroofing in this area, and so we interpret these changes to reflect a change from accommodation of convergence in this region between India and Eurasia through extrusion along the Altyn Tagh Fault to accommodation through distributed shortening in this region. (Abstract shortened by UMI.)