MANTLE FLOW AND MELTING BENEATH YOUNG OCEANIC LITHOSPHERE: SEISMIC STUDIES OF THE GALÁPAGOS ARCHIPELAGO AND THE JUAN DE FUCA PLATE by JOSEPH STEPHEN BYRNES A DISSERTATION Presented to the Department of Earth Sciences and the Graduate School of the University of Oregon in partial fulfillment of the requirements for the degree of Doctor of Philosophy June 2017 DISSERTATION APPROVAL PAGE Student: Joseph Stephen Byrnes Title: Mantle Flow and Melting beneath Young Oceanic Lithosphere: Seismic Studies of the Galápagos Archipelago and the Juan de Fuca Plate This dissertation has been accepted and approved in partial fulfillment of the requirements for the Doctor of Philosophy degree in the Department of Earth Sciences by: Douglas Toomey Chairperson Emilie Hooft Core Member Eugene Humphreys Core Member Alan Rempel Core Member Allen Malony Institutional Representative and Scott L. Pratt Dean of the Graduate School Original approval signatures are on file with the University of Oregon Graduate School. Degree awarded June 2017 ii © 2017 Joseph Stephen Byrnes iii DISSERTATION ABSTRACT Joseph Stephen Byrnes Doctor of Philosophy Department of Earth Sciences June 2017 Title: Mantle Flow and Melting beneath Young Oceanic Lithosphere: Seismic Studies of the Galápagos Archipelago and the Juan de Fuca Plate In this dissertation, I use seismic imaging techniques to constrain the physical state of the upper mantle beneath regions of young oceanic lithosphere. Mantle convection is investigated beneath the Galápagos Archipelago and then beneath the Juan de Fuca (JdF) plate, with a focus on the JdF and Gorda Ridges before turning to the off-axis asthenosphere. In the Galápagos Archipelago, S-to-p receiver functions reveal a discontinuity in seismic velocity that is attributed to the dehydration of the upper mantle. The depth at which dehydration occurs is shown to be consistent with prior constraints on mantle temperature. A comparison between results from receiver functions, seismic tomography and petrology shows that mantle upwelling and melt generation occur shallower than the depth of the discontinuity, despite the expectation of high viscosities in the dehydrated layer. Beneath the JdF and Gorda Ridge, low Vs anomalies are too large to be explained by the cooling of the lithosphere and are attributed to partial melt. The asymmetry, large Vs gradients, and sinuosity of the anomalies beneath the JdF Ridge are consistent with models of buoyancy-driven upwelling. However, deformation zone processes appear to dominate mantle flow over seafloor spreading beneath the Explorer and Gorda diffuse plate boundaries. Finally, S-to-p receiver functions iv reveal a seismic discontinuity beneath the JdF plate that can only be attributed to seismic anisotropy. Synthesis of the receiver function results with prior SKS splitting results requires heterogeneous anisotropy between the crust and the discontinuity. Models of anisotropy feature increasing anisotropy before the decrease at the discontinuity, but well below the base of the lithosphere, and a clockwise rotation of the fast direction with increasing depth. In these results and even in the SKS splitting results, additional driving mechanisms for mantle flow such as density or pressure anomalies are required. This dissertation includes previously published and unpublished co- authored material. v CURRICULUM VITAE NAME OF AUTHOR: Joseph Stephen Byrnes GRADUATE AND UNDERGRADUATE SCHOOLS ATTENDED: University of Oregon, Eugene, Oregon, USA Iowa State University, Ames, Iowa, USA DEGREES AWARDED: Doctor of Philosophy, Geological Sciences, 2017, University of Oregon Bachelors of Science, Physics and Mathematics, 2011, Iowa State University AREAS OF SPECIAL INTEREST: Seismic Imaging Techniques Processing of Oceanic Bottom Seismometer Data Mantle Dynamics of the Lithosphere-Asthenosphere System PROFESSIONAL EXPERIENCE: Research Staff, University of Minnesota, 2016-2017 Graduate Assistant, University of Oregon, 2011-2017 Undergraduate Researcher, Iowa State University, 2009-2011 GRANTS, AWARDS, AND HONORS: Johnson Award for Research, University of Oregon, 2012, 2013, and 2015 AGU Fall Meeting Outstanding Student Paper Award, 2014 NSF East Asia Pacific Scientific Institute (EAPSI) Grant, 2013 vi PUBLICATIONS: Byrnes, dynamics beneath the discrete and diffuse plate boundaries of the Juan de Fuca plate: Results fromJ. S., CascadiaD. R. Toomey, Initiative E. E. bodyE. Hooft, wave J. Nábělek, tomography, and J.Geochem. Braunmiller, Geophys. Mantle Geosys., accepted. Malony, A. D., M. A. H. Monil, C. Rasusen, K. Huck, J. Byrnes, and D. Toomey (2016), Towards scaling parallel seismic raytracing, IEEE International Conference on Computational Science and Engineering. Malony, A. D., S. McCumsey, J. S. Byrnes, C. Rasmusen, S. Keever, and D. R. Toomey (2016), A data parallel algorithm for seismic ray tracing, International Meeting on High Performance Computing for Computational Science (VECPAR). Byrnes, J.S., Hooft, E.E.E., Toomey, D.R., Villagómez, D.R., Geist, D.J., and Solomon, S.C. (2015), An upper mantle seismic discontinuity beneath the Galápagos Archipelago and its implications for studies of the lithosphere-asthenosphere boundary, Geochem., Geophys., Geosys., 16, 1070–1088, doi:10.1002/2014GC005694 vii ACKNOWLEDGMENTS I am deeply grateful for the time and patience of my advisors, Douglas Toomey and Emilie Hooft. I want to thank the other graduate students in the Hooft/Toomey labs, including Matt Beachly, Anne Wells, Kohtaru Araragi, Afshin Golamy, Ben Heath, Miles Bodmer, Brandon VanderBeek, Gillean Arnoux, Dan O’hara, and Eunyoung Kim. I would like to extend my thanks to Hitoshi Kawakatsu and Akiko Takeo for my summer at the Earthquake Research Institute at the University of Tokyo, and Max Bezada for my time at the University of Minnesota. I owe special gratitude to other members of this and other departments, in particular but not limited to, Eugene Humphreys, Leif Karlstrom, Alan Rempel, Allen Malony, Josh Roering, Edward Davis, David Schmidt, Becky Dorsey, Amanda Thomas, Jim Watkins, Greg Retallack, Mark Reed, and Paul Wallace. Jessica Lodweyk and Kasey Adherhold at IRIS were always helpful when working with Cascadia Initiative data. Craig Rassumesen and Rob Yelle wasted much of their time on my trivial computer issues. Finally, I am ever in debt to the office staff in the department, and would like to thank Marla Trox, Sharon Douglas, Sandy Thoms, Dave Stemple, and Vicki Arbeiter. viii For my partner, Linley Davidson, who keeps me whole ix TABLE OF CONTENTS Chapter Page I. INTRODUCTION .......................................................................................................................... 1 II. AN UPPER MANTLE SEISMIC DISCONTINUITY BENEATH THE GALAPAGOS ARCHIPELAGO AND ITS IMPLICATIONS FOR STUDIES OF THE LITHOSPHERE- ASTHENOSPHERE BOUNDARY ........................................................................................... 8 Introduction .............................................................................................................................. 8 Seismic Data and Receiver Functions ............................................................................. 13 Results ......................................................................................................................................... 20 Discussion .................................................................................................................................. 26 Spatial Relationship of Seismic Velocity Anomalies and the G Discontinuity ..................................................................................................................... 26 The G Discontinuity Marks the Base of Fully Dehydrated Mantle ............... 29 Potential Temperature of the Plume and Surrounding Mantle ..................... 31 A Scenario for Mantle Melting and Seismic Velocity Changes ........................ 33 The Amplitude of the G Discontinuity ..................................................................... 37 The G discontinuity is not the LAB............................................................................ 39 Conclusions ............................................................................................................................... 42 Bridge .......................................................................................................................................... 43 x Chapter Page III. MANTLE DYNAMICS BENEATH THE DISCRETE AND DIFFUSE PLATE BOUNDARIES OF THE JUAN DE FUCA PLATE: RESULTS FROM CASCADIA INITIATIVE BODY WAVE TOMOGRAPHY ...................................................................... 45 Introduction .............................................................................................................................. 45 Geological Setting ................................................................................................................... 48 Data and methods ................................................................................................................... 51 Seismic Experiments ...................................................................................................... 51 Measurement of Delay Times ..................................................................................... 53 Tomographic Method ..................................................................................................... 56 Results ........................................................................................................................................