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Author ORCID Identifier
Open Access Dissertation
Doctor of Philosophy (PhD)
Year Degree Awarded
Month Degree Awarded
Geophysics and Seismology
The impact of past tectonic events on formation and modification of continental lithosphere over the course of Earth’s history remains as an open question of fundamental importance. Physical properties of continental crust and mantle lithosphere, such as their age, thickness, composition, temperature, and velocity, contain crucial information for informing this question. Eastern North America provides at least two complete records of supercontinent assembly and breakup over the past 1.3 Ga, serving as a natural laboratory for our understanding of continental lithosphere evolution and for integrating geologic and geophysical observations. In this thesis, I have investigated the seismic properties of crust and upper mantle beneath eastern North America with different seismological methods. I first used teleseismic P- wave receiver function analysis to investigate the crustal thickness distribution beneath eastern North America, in order to explore the possible linkage between geologically- defined boundaries and the variation of crustal thickness. The results show a significant variation in the depths of the Moho and intra-crustal layers within and across the major tectonic units. Specifically, there are distinct differences in crustal thickness between the northern and southern Grenville Province. A dipping intra-crustal feature can be seen within the central Grenville Province, with the depth increasing eastward from 5 to 27 km. The Moho depth decreases southeastward across the Grenville‐Appalachian boundary, with a sharp Moho offset of up to 12-15 km in the central segment and a more gradual variation to the north and south. The thickness difference between the southern and northern Grenville‐aged crusts suggests different tectonic and/or exhumation histories during and after the Grenville Orogeny. The low‐angle eastward dipping crustal feature is interpreted to be a Grenville‐aged collisional structure. Differences in the steepness of the Moho offset along the strike of Appalachians probably reflect variation of the steepness of the subsurface boundary between Laurentia and accreted terranes with different intensities of post-orogenic modification. The observed spatial relation between the geologically defined tectonic boundaries and crustal thickness variations provides new constraints on the depth extent of the tectonic units within the crust.
I then used full-wave ambient noise tomography to construct a high-resolution velocity model beneath the southern part of eastern North American margin. The goal is to understand the impact of rifting-related tectonism on the modification of lithospheric structure. The velocity model demonstrates a clear crustal thickening over a transitional zone from ocean to continent, a lower-than-average mantle lithospheric velocity underlying the transitional crust, and a nearly vertical low-velocity column in the uppermost mantle beneath the Virginia volcanoes. I propose that the initial rifting formed the transitional oceanic-continental crust, and the underlying mantle lithosphere had been partially melted or thermally and chemically modified during and after rifting. A rifting-induced mantle convection flow, together with possible lithospheric delamination, may explain the low- velocity column and volcanism observed within the continental interior.
My study provides tight constraints on the crustal thickness distribution and lithospheric velocity variation beneath eastern North America. These seismic characteristics, together with constraints from other geological and geodynamic studies, have significant insights on the formation and modification of the eastern North American lithosphere through geologic time.
Li, Cong, "SEISMIC CHARACTERISTICS OF THE EASTERN NORTH AMERICAN CRUST AND UPPER MANTLE: THE FORMATION AND EVOLUTION OF CONTINENTAL LITHOSPHERE" (2021). Doctoral Dissertations. 2118.
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