Inferring the Thermomechanical State of the Lithosphere Using Geophysical and Geochemical Observables by William Joseph Shinevar B.Sc. in Geology - Physics/Mathematics and B.A. in German Studies Brown University, 2015 Submitted to the Department of Earth, Atmospheric, and Planetary Sciences in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Geophysics at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY and WOODS HOLE OCEANOGRAPHIC INSTITUTION September 2021 © 2021 Massachusetts Institute of Technology. All rights reserved. Author …………………………………………………………………………………...… William Joseph Shinevar MIT-WHOI Joint Program in Oceanography/Applied Ocean Science and Engineering Massachusetts Institute of Technology & Woods Hole Oceanographic Institution July 27, 2021 Certified by ……………………………………………………………………………...… Dr. Oliver Jagoutz Associate Professor of Geology, Department of Earth and Planetary Sciences Massachusetts Institute of Technology Thesis Supervisor Certified by ……………………………………………………………………………...… Dr. Mark D. Behn Associate Professor in Earth and Environmental Sciences Boston College Thesis Supervisor Accepted by ……………………………………………………………………………….. Dr. Oliver Jagoutz Associate Professor of Geology, Department of Earth and Planetary Sciences Massachusetts Institute of Technology Chair, Joint Committee for Marine Geology & Geophysics 2 Inferring the Thermomechanical State of the Lithosphere Using Geophysical and Geochemical Observables by William Joseph Shinevar Submitted to the MIT-WHOI Joint Program in Oceanography and Applied Ocean Science and Engineering on July 27, 2021 in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in Geophysics. Abstract This thesis focuses on interpreting geophysical and geochemical observables in terms of the thermomechanical state of the lithosphere. In Chapter 1, I correlate lower crustal rheology with seismic wave speed. Compositional variation is required to explain half of the total variability in predicted lower crustal stress, implying that constraining regional lithology is important for lower crustal geodynamics. In Chapter 2, I utilize thermobarometry, diffusion models, and thermodynamic modelling to constrain the ultra- high formation conditions and cooling rates of the Gore Mountain Garnet Amphibolite in order to understand the rheology of the lower crust during orogenic collapse. In Chapter 3, I interpret geophysical data along a 74 Myr transect in the Atlantic to the temporal variability and relationship of crustal thickness and normal faults. In Chapter 4, I constrain the error present in the forward-calculation of seismic wave speed from ultramafic bulk composition. I also present a database and toolbox to interpret seismic wave speeds in terms of temperature and composition. Finally, in Chapter 5 I apply the methodology from Chapter 4 to interpret a new seismic tomographic model in terms of temperature, density, and composition in order to show that the shallow lithospheric roots are density unstable. Thesis Supervisor: Dr. Oliver Jagoutz Title: Full Professor in Earth, Atmospheric, and Planetary Sciences Massachusetts Institute of Technology Thesis Supervisor: Dr. Mark D. Behn Associate Professor in Earth and Environmental Sciences Boston College 3 Acknowledgements This thesis would not be possible without the vast support of friends, coworkers, mentors, and all the helpful hands (and paws) over the last 6 years. First off, I want to thank both of my thesis advisors, Mark Behn and Oli Jagoutz, who have greatly helped develop me as a scientist. Their focus on testable, quantifiable hypotheses using a diverse range of observables and models has greatly influenced and improved my own science. Their patience with and encouragement of my detour-prone academic journey helped me begin to grasp the intricate and deeply interdisciplinary nature of geology. I will always appreciate their healthy skepticism of themselves and others as well as them pushing me to go further in my science. I thank Oli for teaching me how to sledge rocks and Mark for the many cups of espresso throughout my graduate program. I thank my parents for always encouraging me to learn. Without their support, I would have never gained my scientific curiosity. I thank my husband, Yu-Kang, who I’ve grown so much with over the last six years. His supportive smile, understanding of the stresses of a graduate program, and willingness to listen to my rants on figure-making have been helpful, especially throughout COVID lockdown. I thank the members of my thesis committee, Matej Pec, Brad Hager, and Dan Lizarralde, who have taken time to deeply consider, criticize, and improve the science in this thesis. I thank previous advisors and teachers who inspired me to continue studying and researching geology after university, including Greg Hirth, Marc Parmentier, Don Forsyth, Karen Fischer, Jan Tullis, and Reid Cooper. I thank all the scientists who have helped me learn methodologies or given me access to scientific instruments, including but not limited to Tim Grove for help with the rock saw, microscope, and microprobe, Neel Chatterjee for help with the microprobe, Jake Setera for help with the Rutgers LA-ICPMS, Kristen Bergmann for access to her microscope, Jahan Ramezani with the help with mineral separation, Meghan Jones for advice using MBSystem, Jay Ague and J. O. Eckert, Jr for analyses at the Yale microprobe, Seth Kruckenberg with help with EBSD measurements, Maurice Tivey, John Greene, and Masako Tominaga for advice on processing magnetic datasets. I also thank Bonnie Barton for access to samples at Gore Mountain. I thank coworkers besides my advisors who have challenged, inspired, and improved my scientific work like Eva Golos, Jill VanTongeren, and the SCARF cruise people and co-authors. Special thanks to Jean-Arthur Olive who correctly supported naming the discovered oceanic core complex the Kafka Dome. I thank my office mates, Fiona Clerc, Stephanie Krein, Patrick Beaudry, and Max Collinet, who were always willing to chat about geology and more, keeping me motivated through the slow times. I also thank my lab members and fellow graduate students who suffered my complaints throughout my Ph.D. and helped me keep captivated by all the amazing fields of work. Lab mates like Craig Martin, Ben Klein, Zach Molitor, Joshua Murray, and Hannah Mark were always open to my intrusions into their offices to distract us all from work with coffee or lunch, and to tell me no matter how many times I asked, that I did need to write the Gore Mountain paper myself. When those intrusions failed, the office of I thank the office of Marjorie Cantine, Maya Stokes, and Sam Goldberg for being similarly 4 dismissive to my complaints. I thank Ben Urann and all others who were happy to go do field work me for a weekend. I also thank all my friends who have supported me throughout my time as a graduate student: going to the gym, grabbing coffee and pastries, eating a wonderful variety of foods, listening to my rants whether science-related and not, and suffering my cocktail inventions. Special thanks to Jake and Ryan for attempting to untilt me in DotA. Without good friends keeping me cheerful, especially during COVID, this thesis would have never reached fruition. I’d like to thank all the MIT and WHOI administrators who helped me along the way including Roberta Allard, Megan Jordan, Lea Fraser, Julia Westwater, Annora Borden, Yanick Pierre, Brandon Milardo, Jennifer Fentress, Kris Kipp, Daisy Caban, Brenda Carbone, Christine Charette, Sally Houghton, Kelly Servant, and Mira Parsons. Funding for this research was provided by an MIT Presidential Fellowship, MIT Student Research Funds, the National Science Foundation Division of Earth Sciences (EAR) and Ocean Sciences (OCE) grants EAR-16-24109, EAR-17-22932, EAR-17- 22935, OCE-14-58201, and SCEC Awards 16106 and 17202., SCEC, Geological Society of America Graduate Student Research Fellowship, WHOI Ocean Venture Fund, and the WHOI Academic Programs Office. 5 Table of Contents: Abstract 3 Acknowledgements 4 List of Figures 9 List of Tables 11 Introduction 13 Chapter 1: Inferring crustal viscosity from seismic velocity: Application to the lower crust of Southern California 18 Abstract 19 Introduction 20 Methods 21 Application to Southern California 28 Discussion 31 Conclusions 34 Acknowledgements 35 Figures 36 Tables 43 Chapter 2: Gore Mountain Garnet Amphibolite records UHT Conditions: Implications for the Rheology of the Lower Continental Crust During Orogenesis 44 Abstract 45 Introduction 46 Background 47 Field Relations 50 Lithologic Descriptions 51 Analytical Methods 55 Whole-Rock Chemistry 59 Mineral Chemistry 61 Geochronology 64 Thermometry 66 Garnet Diffusion Modelling 68 Thermodynamic Modelling 71 Discussion 73 Conclusion 79 Acknowledgments 80 6 Figures 81 Tables 104 Chapter 3: Causes of oceanic crustal thickness oscillations along a 74-Myr Mid- Atlantic Ridge flow line 107 Abstract 108 Introduction 109 Geologic Setting 111 Observations: Underway Geophysical Data Collection 112 Time-Series Analysis 117 Discussion 121 Conclusion 127 Acknowledgements 128 Figures 129 Chapter 4: WISTFUL: Whole-Rock Interpretative Seismic Toolbox for Ultramafic Lithologies 142 Abstract 143 Introduction 144 Previous Work 145 Uncertainty in Seismic Wave Speed Forward Calculations 148 WISTFUL 156 General User Interfaces 160 Summary 162 Figures 165 Tables 177 Chapter 5: Mantle Thermochemical Variations beneath the Continental United States Through Petrologic Interpretation