Browsing by Author "Liu, Kaijian"
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Item Imaging lithospheric structure beneath the Colorado Plateau and its adjacent regions using Rayleigh wave tomography(2010) Liu, Kaijian; Levander, Alan R.This thesis presents a new 3-D shear velocity (Vs) model of the lithospheric structure beneath the Colorado Plateau and its surrounding regions to understand the complicated lithospheric modifications caused by major Cenozoic tectonic and magmatic activities. Prior to this work, lack of an overall velocity model had prevented a comprehensive understanding of the current encroachment patterns near the margins, and of the perplexing crust/mantle boundary beneath the plateau. Using the new USArray data, I have inverted the isotropic Vs model from the Rayleigh wave dispersion curves obtained by a modified two-plane wave method. The resulting Vs structures not only clearly image the sublithospheric low-velocity channels, but also identify the high velocity anomalies underlying the western plateau for the first time in surface wave tomography to support the recent delamination hypothesis. This work also provides a high-resolution velocity model for geodynamic modeling on lithosphere-asthenosphere interactions.Item Teleseismic Imaging of the Crust and Upper Mantle in the Western United States(2012-09-05) Liu, Kaijian; Levander, Alan R.; Niu, Fenglin; Lee, Cin-Ty A.; Alexander, DavidHigh-resolution seismic images of lithospheric structures allow us to infer the tectonics that modified the lithosphere. We apply such methods to understand Cenozoic modification of the lithosphere by tectonic and magmatic processes in the tectonically active western United States. Using USArray Transportable and Flexible Array data, we present high-resolution images for three regions in this thesis. (1) In the Mendocino triple junction, we use a joint inversion of Rayleigh-wave dispersion data and receiver functions to obtain a new crust and upper Vs model to ~150km depth. The model shows four distinct, young lithosphere-asthenosphere boundary systems. A low-Vs anomaly beneath the Great Valley-Sierra Nevada reconciles existing slab window models with the mantle-wedge geochemical signatures in Coast Range volcanics, and explains the ~3 Myr delay of the onset of volcanism after slab removal. Uppermost mantle low velocities provide evidence for forearc mantle serpentinization extending along the Cascadia margin. (2) In the Colorado Plateau, a Rayleigh wave tomography model sheds light on the volcanism along the margins and plateau uplift. Strong upper mantle heterogeneity across the plateau edge results from the combined effect of a ~200-400 K temperature difference and ~1% partial melt. A ring of low velocities under the plateau periphery suggests that the rehydrated Proterozoic lithosphere is progressively removed by convective processes. Particularly, a high-Vs anomaly imaged beneath the western plateau adds evidence for a downwelling/delamination hypothesis [Levander et al., 2011]. Thermo-chemical edge-driven convection causing localized lithospheric downwelling provides uplift along the margins and magmatic encroachment into the plateau center. (3) In the final study, we developed a 3-D teleseismic scattering wave imaging technique based on the Kirchhoff approximation and 3-D inverse Generalized Radon Transform. Synthetic tests demonstrate higher resolution imaging for continuous, irregular interfaces or localized scatterers, in comparison to conventional methods. Applied to the High Lava Plains dataset, the transmission coefficient structure shows a deepening Moho near 117.6°W and three negative events that correlate well with the Rayleigh wave low-Vs zones. Images made with the Mendocino data clearly show rapidly decreasing lithosphere-asthenosphere boundary depths from the subduction to transform regime.