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Tectonic Evolution of the Northeastern Tibetan Plateau

  • Author(s): Zuza, Andrew Vincent
  • Advisor(s): Yin, An
  • et al.
Abstract

How the Tibetan Plateau was constructed and evolved in response to ongoing India-Asia convergence since 65-55 Ma is fundamental in understanding processes of continental tectonics. Furthermore, the kinematics and mechanisms of plateau formation and continental deformation have implications for the complex interactions between tectonics, erosion, and climate change in Earth’s most recent history. To provide insights into these processes, my research is focused on the development of the northern margin of the Tibetan Plateau, which is defined by the 350-km-wide and 1300-km-long Cenozoic Qilian Shan-Nan Shan thrust belt. This active fold and thrust system overprinted a region that has a complex pre-Cenozoic tectonic history involving multiple phases of Proterozoic basement deformation and early Paleozoic orogeny. In this work, I integrate geologic mapping, balanced cross section construction and restoration, seismic reflection interpretation, geochronology, thermobarometry, geodetic data analysis, and analogue modeling to investigate the tectonic development of northern Tibet over a range of timescales, from the Proterozoic evolution of central Asian cratons to the active deformation that is shaping the northern margin of the Tibetan Plateau.

The magnitude, style, and distribution of Cenozoic shortening strain across northern Tibet can be used to test competing models of continental deformation. The shortening distribution across the Qilian Shan-Nan Shan thrust belt, derived from surface mapping and subsurface seismic reflection profiles, suggests that the modern thickness and elevation of the northern plateau has developed as a result of southward continental underthrusting of Asia beneath Tibet and distributed crustal thickening. The thrust systems in northern Tibet link to the east with > ~1000-km-long parallel left-slip strike-slip faults (i.e., the Haiyuan, Qinling, and Kunlun faults). The along-strike variation of fault offsets and pervasive off-fault deformation along these strike-slip faults create a strain pattern that departs from the expectations of the classic plate-like rigid-body motion and flow-like distributed deformation models of continental deformation. Instead, I propose that the major strike-slip faults formed as a non-rigid bookshelf-fault system where clockwise rotation of northern Tibet drives left-slip bookshelf faulting and related off-strike-slip fault deformation. In addition, I employ a stress-shadow model that uses the characteristic spacing of strike-slip faults and seismogenic-zone thickness estimates across northern Tibet and central Asia to estimate fault strength and the regional stress state. The strike-slip faults in Asia have a low coefficient of fault friction (~0.15), which may explain why deformation penetrates more than 3500 km into Asia from the Himalayan collisional front, and why the interior of Asia is prone to large (M > 7.0) devastating earthquakes along major strike-slip faults.

A well-constrained understanding of Cenozoic deformation across northern Tibet allows for better reconstructions of the Proterozoic and Paleozoic tectonics. Field relationships and geochronologic studies reveal that the early Paleozoic Qilian suture, which bounds the southern margin of the North China craton, records the Ordovician-early Silurian closure of the Qilian Ocean via south-dipping subduction beneath the Qaidam continent. The evolution of this ocean and North China’s southern margin has implications for reconstructions of Neoproterozoic and Paleozoic Earth, including the development of the Tethyan and Paleo-Asian Oceanic Domains. By restoring the Phanerozoic deformation along the northern and southern margins of the Tarim and North China cratons, I propose and test a hypothesis that these cratons once stretched westward across present-day Asia, possibly as far west as Baltica, as a continuous Neoproterozoic continent.

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