"Crust and Mantle Structure beneath the Eastern Margin of North America" by Yantao Luo

Date of Award

Spring 2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Long, Maureen

Abstract

Plate tectonics is a vital factor to the uniqueness and habitability of the Earth. Fundamental plate tectonic processes including subduction, terrane accretion, and continental rifting not only shape the surface geologic features, but also modify the crust and mantle structure deep beneath the surface. Correlating the surface geologic information with the crust and mantle structure resolved by geophysical methods can further our understanding of key tectonic processes that have operated in the past and are still operating on this planet. In this dissertation, I investigate the crust and mantle structure beneath the eastern margin of North America using various seismic methods, which yield different constraints on subsurface features at a wide range of depths. I combine the seismic imaging results with surface geologic observables to shed lights on the tectonic evolution of this margin. Along the eastern margin of North America, southern New England presents a unique example of pronounced variations of surface geology with numerous tectonic boundaries situated in a compact region. To investigate the subsurface structure and unravel the complicated tectonic history of this region, I analyze the data from the SEISConn (Seismic Experiment for Imaging Structure beneath Connecticut) array, a dense array with ~10 km station spacing deployed across northern Connecticut. I apply traditional and anisotropic receiver function (RF) analyses and a scattered wavefield migration technique. Radial stacks of RFs show a drastic Moho depth offset located closely beneath the surface boundary between Laurentia and Gondwana-derived terranes. Common-conversion-point (CCP) stacking of RFs also reveals several dipping discontinuities in the crust and upper mantle that are likely associated with subduction and terrane accretion episodes during Paleozoic Appalachian orogenesis. The scattered wavefield migration technique further resolves the complicated 2-D geometry of the Moho depth offset and refines the image of a slab-like feature in the upper mantle, which may be a relict slab subducted during the closure of the Rheic Ocean. The anisotropic RF analysis reveals layered anisotropic features beneath southern New England at varied depths and scales, from a highly localized shear zone beneath the Hartford basin to a regional scale change of asthenospheric flow direction associated with the Northern Appalachian Anomaly (NAA). To investigate whether the observed Moho depth offset beneath southern New England is typical along the easter margin of North America and if not, how the Moho geometries vary along the Appalachian orogen, I conduct scattered wavefield migration analysis on dense seismic arrays deployed across the orogen at different latitudes. In contrast to the Moho offset beneath southern New England, the Moho depth gradients beneath the central and southern Appalachians are smooth, even though the total amount of the Moho depth change is similar beneath each profile. This discrepancy in Moho geometries supports the presence of a major boundary near the New York Promontory that marks a first-order transition in the crustal structure, which infers drastically different tectonic settings in the southern and northern segments of the Appalachian orogen before and during Paleozoic Appalachian orogenesis. One of the most intriguing features beneath the present-day passive margin of eastern North America is the presence of low velocity anomalies in the upper mantle, notably the Central Appalachian Anomaly (CAA) and the Northern Appalachian Anomaly (NAA). I investigate the spatial extents of these anomalies and the mechanisms behind them by examining their influence on the mantle transition zone (MTZ) beneath. I greatly extend the functionality of the scattered wavefield migration algorithm to resolve MTZ geometry variations beneath the eastern US, utilizing the USArray data. Migration images reveal a significantly thinned MTZ beneath the NAA and a moderately thinned MTZ beneath the CAA. These observations suggest that the upwelling mechanisms associated with the NAA and the CAA likely extend into the MTZ. Furthermore, the uplifted 660 discontinuity and the relatively constant 410 discontinuity imply a predominantly thermal effect at the 660 and a combined effect of temperature variations and hydration at the 410, which can be achieved by a relatively dry deep mantle upwelling penetrating a wet MTZ.

Download

Share

COinS