Origin and Evolution of the Northern Appalachian Anomaly

Date of Award

Fall 1-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Long, Maureen

Abstract

Plate tectonics is a foundational theory widely accepted in the Earth Sciences for the last ~60 years, and continental lithosphere represents a fundamental component of the plate tectonic system. Broad mechanisms of continental lithospheric evolution are known, though not entirely understood in detail. These include terrane accretion, orogenesis, subduction, rifting, magmatic activity, lithospheric delamination, Rayleigh-Taylor instabilities, ablation, edge-driven convection, and plumes. The northeast North American margin experienced multiple episodes of orogenesis and terrane accretion between ~450-320 Ma of several terranes on the margin of Laurentia, the formation of Pangea via continental collision by ~250 Ma, the rifting of Pangea ~200 Ma, and intraplate magmatic activity from ~190 through ~125 Ma. Beneath present-day New England, there is a ~400 km wide slow velocity region in the upper mantle known as the Northern Appalachian Anomaly (NAA), whose origins remain enigmatic. The New England region thus presents itself as an ideal tectonic setting for considering questions of continental lithospheric evolution, which contributes to our knowledge framework of plate tectonics. This thesis presents new geophysical investigations of the NAA, mainly based on recently collected seismic data from dense broadband experiments. To achieve an understanding of lithospheric evolution in the NAA region, we first constrain the present-day lithospheric thickness. We use Ps receiver function analysis and data from the New England Seismic Transects (NEST) array, with ~25 km station spacing. We apply a harmonic decomposition approach to modeling our study region. Next, we make XKS splitting measurements using SKS, SKKS, and SKIKS phases to evaluate mantle anisotropy and receiver function data in the NAA region. We utilize data from three dense arrays in New England: NEST, the GEology of New England via Seismic Imaging Studies (GENESIS) array with ~5 km station spacing, and the Seismic Experiment for Imaging Structure Beneath Connecticut (SEISConn) array with ~10 km station spacing. We also use data from other networks in our study region, including from USArray and regional networks. Third, we present the first tomography model centered on New England to constrain the structure, geometry, and depth of the NAA by incorporating data from dense arrays above it. This tomography model includes the three local dense temporary broadband stations and other permanent and long-running broadband stations. In our final contribution, we integrate four different geophysical datasets to evaluate possible models for the origins of the NAA. This includes a previously published seismic tomography model, gravity anomalies, topography, and estimated Curie depths in the region of the Appalachian anomalies. In summary, this dissertation considers processes of lithospheric evolution, modification, and destruction in the NAA region, as well as the origins of the NAA itself. By applying a suite of seismic analyses to newly collected data from dense seismic arrays, in combination with other geophysical observations, we aim to gain new insights into crustal and upper mantle structures beneath New England. The new observations developed in this dissertation contribute to our understanding of seismic structure beneath the New England Appalachians, mechanisms of continental lithospheric evolution and modification, and the origin and evolution of the Northern Appalachian Anomaly.

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