HR: 18:05h
AN: S44A-08    [Abstracts]
TI: Evidence from seismic tomography for a possible geodynamic link between the active and passive margins of South and North America
AU: * van der Lee, S
EM: suzan@earth.northwestern.edu
AF: Dept. Earth & Planetary Sciences, Northwestern University, Evanston, IL 60208, United States
AU: Assumpcao, M
EM: marcelo@iag.usp.br
AF: Dept of Geophysics, University of Sao Paulo, Sao Paulo, Brazil
AU: Bedle, H
EM: heather@earth.northwestern.edu
AF: Dept. Earth & Planetary Sciences, Northwestern University, Evanston, IL 60208, United States
AU: Feng, M
EM: anmeijian@yahoo.com.cn
AF: Inst. of Geomechanics, Chinese National Academy of Sciences, Beijing, China
AB: South and North America each have a seismically active western margin and a seismically passive eastern margin bordering a seismically stable Precambrian Craton. Regional seismic waveform imaging of the upper mantle beneath these two continents relies on this seismic activity, as recorded at seismic stations across the continents. Imaged upper-mantle structure beneath the active margins of both continents is remarkably similar and characterized by very low S-wave velocities. The low S-velocity regions are adjacent to or above high S-velocity regions. In South America the high-velocity region likely represents subducting lithosphere of the Nazca Plate, dipping under the low-velocity zone. In North America the high-velocity region likely represents subducted lithosphere from the Farallon Plate, sunk to the deep upper mantle. East of these striking subduction-related structures, both the South American and North American upper mantles display a strong, thick, high-velocity lithosphere beneath the Precambrian Cratons. However, in South America this lithosphere is significantly thinner and weaker than that in North America. Furthermore, sub-lithospheric structure appears different as well. In North America, low velocities are imaged in a large depth range beneath the eastern passive margin. These low velocities might represent an upwelling of a somewhat hydrous mantle. This hydrous mantle might be the result of Farallon subduction beneath the western margin, which could have carried small quantities of water into the top of the lower mantle and transition zone. This hydrous, upwelling mantle might act to weaken the passive- margin lithosphere and allow subduction of Atlantic lithosphere in the near geologic future.
DE: 9350 North America
DE: 9360 South America
SC: Seismology [S]
MN: 2007 Joint Assembly