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Theses and Dissertations

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Browsing Theses and Dissertations by Subject "anisotropy"

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    Indications of complex lithospheric structure around the Cheyenne Belt in the western U.S. based on shear wave splitting analysis
    (Colorado State University. Libraries, 2014) Millikan, Ronald, author; Schutt, Derek, advisor; Hannah, Judy, committee member; Breidt, Jay, committee member
    This study employs the method of shear wave splitting using a modified Silver and Chan methodology to produce constraints on the character of the upper mantle below the Cheyenne Belt suture zone (CB), North America. The CB marks the boundary between the Proterozoic Yavapai and Archean Wyoming Provinces. A previously undetected regional anisotropic fossil fabric with north trending fast axis orientations was detected in topographically low areas and interpreted produced by Proterozoic strain created during the convergence that produced the CB. Anisotropy results sub-parallel to absolute plate motion (APM) were detected in the mountainous areas. This is interpreted as caused by local post-Laramide heating of the lithosphere, which has reset Proterozoic anisotropy. This interpretation explains the complexity of shear wave splitting noted in Colorado by other studies.
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    ItemOpen Access
    Using waveform modeling of seismic anisotropy to understand the process of Proterozoic lithospheric assembly
    (Colorado State University. Libraries, 2013) Solomon, Melinda A., author; Schutt, Derek L., advisor; Harry, Dennis, committee member; Hannah, Judy, committee member; Breidt, Jay, committee member
    In this study, SKS data from two long-standing broadband seismic stations located along the Proterozoic Trans Hudson Orogen is analyzed for the effects of complex anisotropy. The Trans Hudson Orogen is the lasting expression of collisional events that brought together Archean continental fragments to form the cratonic core of the North American continent. Split SKS waveforms observed at two GSN stations, FFC located on the Sask craton near Flin Flon, Manitoba, and RSSD located on the eastern margin of the Wyoming craton in the Black Hills of South Dakota, are analyzed for anisotropic layering of varying complexity. At FFC we find that a model with two flat layers of anisotropy is better able to explain the data than the simple model of one flat layer. A top layer shows anisotropy perpendicular to the convergence direction of the Trans Hudson Orogen that is similar to observations of anisotropy today at convergent margins, and is attributed to lithospheric deformation during convergence. RSSD shows more complex splitting that we interpret as sub-wavelength scale anisotropic heterogeneity resulting from convergence driven fragmentation of the pre-existing lithosphere into small, < ~40 km pieces. Previous studies show more coherent anisotropy to the east and west of RSSD, suggesting that anisotropic heterogeneity here is related to the convergence, and may be a longstanding feature of tectonic boundaries.