Upper Mantle Discontinuity Structure Beneath the Western Atlantic Ocean and Eastern North America from SS Precursors

Monday, 14 December 2015
Poster Hall (Moscone South)
Nicholas C Schmerr1, Caroline Beghein2, Dimitrije Kostic1, Alice M Baldridge3, John D West4, Larry R Nittler5, Abigail Louise Bull6, Laurent Montesi1, Paul K Byrne7, Daniel R Hummer8, Jeffrey B Plescia9, Lindy T. Elkins-Tanton10, Vedran Lekic1, Britney E Schmidt11, Lynne J Elkins12, Catherine M Cooper13, Inge Loes ten Kate14, Douwe J J Van Hinsbergen14, Rita Parai8, Jennifer B Glass11, James Ni15, Nobuaki Fuji16, Francis M McCubbin17, Joseph R Michalski18, Chunpeng Zhao4, Ricardo D Arevalo Jr19, Paula Koelemeijer20, Anna M Courtier21, Heather Dalton22, Lauren Waszek23, Joan Bahamonde24, Ben Schmerr25, Nibbler Gilpin26, Ellen Rosenshein27, Kimberly Mach28, Lillian Rose Ostrach19, Razvan Caracas29, Robert Anthony Craddock30, Melissa M Moore-Driskell31, Wyatt L Du Frane32 and Louise H Kellogg33, (1)University of Maryland College Park, College Park, MD, United States, (2)University of California Los Angeles, Earth, Space, and Planetary Sciences, Los Angeles, CA, United States, (3)St. Mary's College of California, Environmental and Earth Science, Moraga, CA, United States, (4)Arizona State University, Tempe, AZ, United States, (5)Carnegie Inst Washington, Washington, DC, United States, (6)University of Oslo, Centre for Earth Evolution and Dynamics, Oslo, Norway, (7)North Carolina State University Raleigh, Marine, Earth, and Atmospheric Sciences, Raleigh, NC, United States, (8)Carnegie Institution for Science Washington, Washington, DC, United States, (9)Applied Physics Lab, Laurel, MD, United States, (10)University of California Davis, Davis, CA, United States, (11)Georgia Institute of Technology Main Campus, Atlanta, GA, United States, (12)University of Nebraska Lincoln, Earth and Atmospheric Sciences, Lincoln, NE, United States, (13)WSU, Pullman, WA, United States, (14)Utrecht University, Utrecht, Netherlands, (15)New Mexico State University Main Campus, Las Cruces, NM, United States, (16)Institut de Physique du Globe de Paris, Paris, France, (17)University of New Mexico Main Campus, Albuquerque, NM, United States, (18)Planetary Science Institute Tucson, Tucson, AZ, United States, (19)NASA Goddard Space Flight Center, Greenbelt, MD, United States, (20)ETH Swiss Federal Institute of Technology Zurich, Department of Earth Sciences, Zurich, Switzerland, (21)University of Wisconsin Whitewater, Whitewater, WI, United States, (22)Lunar and Planetary Institute, Houston, TX, United States, (23)University of Cambridge, Cambridge, United Kingdom, (24)Arizona State University, Mars Space Flight Facility, Tempe, AZ, United States, (25)Vandelay Industries, New York, NY, United States, (26)Nammo Talley Inc., Phoenix, AZ, United States, (27)Enlarged City School District of Middletown, Middletown, NY, United States, (28)Boeing Company Seattle, Seattle, WA, United States, (29)CNRS Lyon, Lyon, France, (30)Smithsonian Institution, Center for Earth and Planetary Studies, National Air and Space Museum, Washington, DC, United States, (31)University of North Alabama, Florence, AL, United States, (32)Lawrence Livermore National Laboratory, Livermore, CA, United States, (33)University of California - Davis, Davis, CA, United States
Seismic discontinuities within the mantle arise from a wide range of mechanisms, including changes in mineralogy, major element composition, melt content, volatile abundance, anisotropy, or a combination of the above. In particular, the depth and sharpness of upper mantle discontinuities at 410 and 660 km depth are attributed to solid-state phase changes sensitive to both mantle temperature and composition, where regions of thermal heterogeneity produce topography and chemical heterogeneity changes the impedance contrast across the discontinuity. Seismic mapping of this topography and sharpness thus provides constraint on the thermal and compositional state of the mantle.

The EarthScope USArray is providing unprecedented access to a wide variety of new regions previously undersampled by the SS precursors. This includes the boundary between the oceanic plate in the western Atlantic Ocean and continental margin of eastern North America. Here we use a seismic array approach to image the depth, sharpness, and topography of the upper mantle discontinuities, as well as other possible upper mantle reflectors beneath this region. This array approach utilizes seismic waves that reflect off the underside of a mantle discontinuity and arrive several hundred seconds prior to the SS seismic phase as precursory energy.

In this study, we collected high-quality broadband data SS precursors data from shallow focus (< 30 km deep), mid-Atlantic ridge earthquakes recorded by USArray seismometers in Alaska. We generated 4th root vespagrams to enhance the SS precursors and determine how they sample the mantle. Our data show detection of localized structure on the discontinuity boundaries as well as additional horizons, such as the X-discontinuity and a potential reflection from a discontinuity near the depth of the lithosphere-asthenosphere boundary. These structures are related to the transition from predominantly old ocean lithosphere to underlying continental lithosphere, as while deeper reflectors are associated with the subduction of the ancient Farallon slab. A comparison of the depth of upper mantle discontinuities to changes in seismic velocity and anisotropy will further quantify the relationship to mantle flow, compositional layering, and phases changes.