Seismic anisotropy and its implication for the formation and evolution of cratons
Creators: name: Chen, Xiaoran
Publisher: name: No Publisher Supplied
Year: 2020

description: My research focuses on understanding seismic anisotropy from the perspective of theoretical development and exploring the upper mantle anisotropic structures beneath the Superior craton and the Yilgarn craton to set constraints to the craton formation processes. In Chapter I, I performed shear wave splitting measurements to explore the anisotropic structure in the upper mantle in Eastern North America. For Chapter II, I did theoretical development and built up a Matlab package to predict the seismic body wave propagation through horizontally stratified anisotropic media using the most general parameterization of the elastic tensor. Based on that package, I was able to explore the S-P and P-S conversions at the anisotropic boundaries and shear wave splitting in a medium composed of pure olivine. Chapter III is built up on the first two chapters, where I included part of the data analysis results from Chapter I and adopted the Matlab code package from Chapter II. Thus, I am able to carry out a comparative study between the Superior craton in North America and the Yilgarn craton in West Australia using a combination of two different groups of techniques and figure out the implications for the craton formation process.For the array from James Bay to Fundy Basin in eastern North America, I find the fast polarizations concentrate between N60°E and N90°E with an average of N80°E and change systematically with backazimuth. In addition, I observed a lateral increase in delay time from 0.56 ± 0.25 s at the NW end of the array to 0.90 ± 0.41 s at the SE end. The location of lateral change in delay time does not match geological boundaries on the surface but seems to match the geophysical boundary at depth of 160 km. I interpret this boundary in splitting values to be the edge of cratonic lithosphere at depth. The observations suggest that the anisotropic structure beneath my study area is complex and possibly both multilayered and laterally variable.Based on a comparative study between the Superior craton and Yilgarn craton, I find that the cratonic lithospheres at both cratons are not homogenous domains, instead, they are different from site to site within the same cratons. At all the sites in both cratons, I find there are back azimuthal variations of the fast polarizations and NULL measurements from nearly all the back azimuths where the measurements are made. Besides, I can identify multiple anisotropic boundaries in the lithosphere at all the sites in both cratons, most of which are mainly above ~ 100 km with the rest between 120 km and 160 km. However, no consistent anisotropic patterns are observed between two cratons. Thus, I consider cratons are different from each other and each craton is composed of different building blocks. Before building blocks came together, they were far apart and went through different kinds of tectonic evolution, through which they obtained different anisotropic fabrics. After this, those building blocks came together to form the craton. After the formation of the craton, the anisotropic fabrics got preserved in the lithosphere without further modification. Thus, different fabrics preserved in the craton now are actually those obtained before the formation of the current craton.