AN ABSTRACT OF THE DISSERTATION OF Benjamin S. Murphy for the degree of Doctor of Philosophy in Geology presented on May 28, 2019. Title: Magnetotelluric Constraints on Lithospheric Properties in the Southeastern United States Abstract approved: _______________________________________________________ Gary D. Egbert By inverting EarthScope long-period magnetotelluric (MT) data from the southeastern United States (SEUS), we obtain electrical conductivity images that provides key insights into the geodynamics of this region. Significantly, we resolve a highly electrically resistive block that extends to mantle depths beneath the modern Piedmont and Coastal Plain physiographic provinces. As high resistivity values in mantle minerals require cold mantle temperatures, the MT data indicate that the sub- Piedmont thermal lithosphere must extend to greater than 200 km depth. This firm bound appears to conflict with conclusions from seismic results; tomography shows that velocities in this region are generally slightly slow with respect to references models. This observation has led to a seismically-informed view of relatively thin (<150 km), eroded thermal lithosphere beneath the SEUS. However, resolution tests demonstrate that our MT constraints are robust. Furthermore, narrow-band biases in MT transfer functions from the SEUS due to geomagnetic pulsations associated with field-line resonances support the presence of bulk resistive lithosphere in this region. We show that, by considering anelastic prediction of seismic observables, MT and seismic (tomography, attenuation, receiver function) results are in fact consistent with thick (~200 km), coherent thermal lithosphere in this region. Our results demonstrate the danger of interpreting seismic results purely in terms of reference models and the importance of integrating different geophysical techniques when formulating geodynamic interpretations. © Copyright by Benjamin S. Murphy May 28, 2019 All Rights Reserved Magnetotelluric Constraints on Lithospheric Properties in the Southeastern United States by Benjamin S. Murphy A DISSERTATION submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Presented May 28, 2019 Commencement June 2020 Doctor of Philosophy dissertation of Benjamin S. Murphy presented on May 28, 2019 APPROVED: Major Professor, representing Geology Dean of the College of Earth, Ocean, and Atmospheric Sciences Dean of the Graduate School I understand that my dissertation will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my dissertation to any reader upon request. Benjamin S. Murphy, Author ACKNOWLEDGEMENTS The research presented here was supported by the NSF Graduate Research Fellowship Program under Grant 1314109-DGE to Oregon State University. Thanks to Anne Fulton and Bug, Moo, Peanut, and Cinder Fluffton for all their support. Chapter 1: We thank Emily Hart, Kyle Jones, Allen Hooper, Kyle McDonald, Taryn Bye, Eleanor Kester, Lana Erofeeva, Brady Fry, Lu Pellerin, and Adam Schultz for their work in collecting and processing these data. Bo Yang and Naser Meqbel assisted greatly with use of ModEM in this project. Conversations with and input from Nikki Moore, Phil Wannamaker, Dave Graham, Gene Humphreys, Steve Constable, Paul Bedrosian, and Eric Kirby greatly benefited this work. We thank Rob Harris and Brandon Schmandt for valuable feedback on a preliminary version of this paper. We also thank Kate Selway, Ian Ferguson, and Peter Shearer for very helpful reviews of the manuscript. This research was supported by NSF Grant EAR1053628 and by the NSF Graduate Research Fellowship Program under Grant 1314109-DGE. (Copyright for this chapter is held by Elsevier B.V.; it is reproduced here under the Author’s Right to Personal Use.) Chapter 2: We are grateful to Robert McPherron for helping us to understand the physics of geomagnetic pulsations and field-line resonances. We also thank Maxim Smirnov for his help with the robust array processing program. We thank two anonymous reviewers and editor Juanjo Ledo for a helpful review of the manuscript. This research was supported by the NSF Graduate Research Fellowship Program under Grant 1314109-DGE and by NSF Grant EAR1447109. Chapter 3: We thank Brandon Schmandt and Derek Schutt for their valuable insights that inspired and greatly benefited this work. We also thank Uli Faul, Emily Hopper, Jeff Park, Shun Karato, and Tolu Olugboji, who all provided helpful feedback on these calculations and interpretations. Jeff Park inspired the idea that the Piedmont Resistor may possibly represent a Mesozoic example of craton formation. We acknowledge Lara Wagner, Fred Pollitz, and Brandon Schmandt for graciously providing their surface wave tomography models to us for these analyses. We thank Stephan Thiel, an anonymous reviewer, and editor Uli Faul for a helpful and constructive review of this manuscript. This work was supported by the NSF Graduate Research Fellowship Program under Grant 1314109-DGE and by NSF Grant EAR- 1820688. TABLE OF CONTENTS Page Introduction ............................................................................................................................... 1 Chapter 1: Electrical Conductivity Structure of Southeastern North America: Implications for Lithospheric Architecture and Appalachian Topographic Rejuvenation .................................. 3 1.1 Abstract ........................................................................................................................... 4 1.2 Introduction ..................................................................................................................... 4 1.3 Data and Methods ............................................................................................................ 6 1.4 Results ............................................................................................................................. 7 1.5 Discussion ..................................................................................................................... 10 1.5.1 Well Resolved Shallow Structures ......................................................................... 10 1.5.2 Poorly Resolved Structures .................................................................................... 13 1.5.3 Deep Lithospheric Conductivity Contrast and the Piedmont Resistor ................... 14 1.5.3.1 Constraints from the Magnetotelluric Data: Thick Sub-Piedmont Lithosphere14 1.5.3.2 Comparison to Seismic Images: Minor Agreement and Major Conflict.......... 16 1.5.3.3 Possible Explanation: A Metasomatized Lithospheric Root that Regrew after Delamination ................................................................................................................ 18 1.5.3.4 Implications for Appalachian Topographic Rejuvenation ............................... 20 1.6 Conclusions ................................................................................................................... 21 1.7 References ..................................................................................................................... 22 Chapter 2: Source Biases in Mid-Latitude Magnetotelluric Transfer Functions due to Pc3-4 Geomagnetic Pulsations .......................................................................................................... 29 2.1 Abstract ......................................................................................................................... 30 2.2 Introduction ................................................................................................................... 30 2.2.1 The Quasi-Uniform Source Assumption in the Magnetotelluric Method .............. 32 2.2.2 Geomagnetic Pulsations and Field-Line Resonances ............................................ 32 2.3 Data and Methods .......................................................................................................... 34 2.4 Results ........................................................................................................................... 35 2.5 Discussion ..................................................................................................................... 38 2.5.1 Pc’s and the 3D Earth ............................................................................................ 39 2.5.2 Implications for MT Data Inversions ..................................................................... 39 2.5.3 Implications for Earth Structure ............................................................................. 40 2.6 Conclusions ................................................................................................................... 40 2.7 References ..................................................................................................................... 41 TABLE OF CONTENTS (cont.) Page Chapter 3: Synthesizing Seemingly Contradictory Seismic and Magnetotelluric Observations in the Southeastern United States to Image Physical Properties of the Lithosphere ............... 45 3.1 Abstract ......................................................................................................................... 46 3.2 Introduction ................................................................................................................... 46 3.2.1 Geologic setting ....................................................................................................