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Apparent polar wandering and its implications for past plate motions

  • Author / Creator
    Wu, Lei
  • Paleomagnetism is the study of ancient geomagnetic field vectors recorded by rocks and sediments. Based on the latitudinal and azimuthal constraints from the paleomagnetic inclinations and declinations, many important discoveries about the horizontal movements of continental blocks, such as the accretion of Eurasia and the breakup of Gondwana in the Mesozoic, had been made in this period. More than half a century later, the golden paleomagnetic latitudes and azimuths now have very limited values for most quantitative plate tectonic research because paleomagnetic reconstructions cannot provide rigorous paleolongitude constraints. To address this challenge, this dissertation is targeted at deriving a new method to calculate paleolongitudes from paleomagnetic data. Contrary to the traditional paleomagnetic reconstructions based on individual paleomagnetic pole positions with respect to a tectonic plate, here we treat paleopoles as an integral sequence and investigate the geometry of paleomagnetic polar wandering trajectory or apparent polar wander path (APWP) for the kinematic implications. Specifically, a studied APWP needs to be divided into different segments, to which circle parameterizations are applied to estimate their stage rotation poles and angles. To correct the errors arising from the fact that all resultant reconstructions are artificially restrained along circle arcs centering at the rotation poles, which renders paleocolatitudes determined from paleomagnetism void, we force the spherical distances between reconstructions and paleo-poles to be the observed paleocolatitudes. To help readers implement this new method, we present a MATLAB-based toolbox PMTec, which can be downloaded from http://www.ualberta.ca/~vadim/software.htm. Using our new method, APWP geometric parametrization (APWPGP), we reevaluate the separation history of the East Gondwanaland since 140 Ma. Our reconstructions indicate that the affinity between India and Australia-East Antarctica was broken down around 130-120 Ma, and that Australia and East Antarctica were separated at around 60-40 Ma. We compare our reconstructions against those from other absolute plate motion (APM) models, which reveals tremendous discrepancies among the different APM predictions. There is a trend to dynamically model mantle thermal structures especially those related to sinking slabs, from which they evaluate the quality of various APM models. We argue that this type of research is a round reasoning and it provides little help in the quality discrimination, because the current interpretations of slab-like structures imaged from seismic tomography models were made from plate reconstructions in the first place. The most effective way to prove the quality of our APWPGP method is to reconstruct some collisional tectonic processes and then compare the resultant APM reconstructions with the independent geophysical and geological observations. Using the APWPGP method, we reconstruct the closing of the Mongol-Okhotsk Ocean (MOO) in the Mesozoic. The APWPs of North China-Amuria (NCA), Siberia and Europe for the last 260 Myr are parameterized for rotation parameters. A new global APM model is constructed in the coordinates of stable Europe, from which APMs of other major continental plates are predicted through the relative plate motion (RPM) models of Torsvik et al. [2012]. Three stages including 260-200 Ma, 200-150 Ma and 150-120 Ma, are discriminated from the temporal variations in the approaching speed of NCA and Siberia and the depth-varying slab volumes. The derived surface kinematics are used to interpret the slab-like high seismic velocity structures related to the subduction of the MOO. The interval of 150-120 Ma witnessed the closing of the remnant paleo-oceanic basin confined between Siberia and NCA. This is supported by a southwestward transition of slabs distributed at the depth of 1800-1300 km, which corresponds to the subduction zones at 140-120 Ma, with an upward reduction in slab volume predicted by the shear-wave velocity model GyPSuM. We find a similarly southwestward trend in the downward geoid undulation in the India-Eurasia geoid low (IEGL). There is a remarkably strong linear correlation of -0.92, with the associated coefficient of determination of 0.84, between the surface geoid height in the IEGL and the underlying slab volume at the depth of 1200-2800 km. Combined with the seismically constrained boundary topographies of the lower mantle, we propose that the mass deficit reflected from the IEGL is mainly caused by the negative density contrast between the sinking slabs and the ambient lower mantle.

  • Subjects / Keywords
  • Graduation date
    2017-06:Spring 2017
  • Type of Item
    Thesis
  • Degree
    Doctor of Philosophy
  • DOI
    https://doi.org/10.7939/R3K931J7B
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.
  • Language
    English
  • Institution
    University of Alberta
  • Degree level
    Doctoral
  • Department
    • Department of Physics
  • Specialization
    • Geophysics
  • Supervisor / co-supervisor and their department(s)
    • Vadim Kravchinsky (Physics)
    • David Potter (Physics)
  • Examining committee members and their departments
    • Mark Freeman (Physics)
    • Vadim Kravchinsky (Physics)
    • David Potter (Physics)
    • Claire Currie (Physics)
    • Aleksey Smirnov (Michigan Technological University)
    • Karlis Muehlenbachs (Earth and Atmospheric Sciences)