HR: 14:40h
AN: T43C-05 INVITED    [Abstracts]
TI: Mantle Structure, Melting and Flow in the Nicaragua-Costa Rica and Izu Bonin-Marianas Subduction Zones
AU: * Fischer, K M
EM: Karen_Fischer@brown.edu
AF: Department of Geological Sciences, Brown University, Providence, RI 02912, United States
AU: Abers, G A
EM: abers@bu.edu
AF: Department of Earth Sciences, Boston University, Boston, MA 02215, United States
AU: Plank, T
EM: tplank@bu.edu
AF: Department of Earth Sciences, Boston University, Boston, MA 02215, United States
AU: Wiens, D A
EM: doug@kermadec.wustl.edu
AF: Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, United States
AU: Syracuse, E M
EM: syracuse@bu.edu
AF: Department of Earth Sciences, Boston University, Boston, MA 02215, United States
AU: Rychert, C A
EM: crychert@ucsd.edu
AF: Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093, United States
AU: Abt, D L
EM: David_Abt@brown.edu
AF: Department of Geological Sciences, Brown University, Providence, RI 02912, United States
AU: Pozgay, S H
EM: spozgay@wustl.edu
AF: Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, United States
AB: Recent broadband seismometer experiments in the Nicaragua-Costa Rica and Izu Bonin-Marianas subduction zones (the NSF MARGINS Subduction Factory focus sites) have led to enhanced imaging of mantle wedge, slab and upper plate structure. Here we relate results from the TUCAN Experiment (Nicaragua-Costa Rica) to mantle temperature, hydration, flow, melting and melt transport, and compare them to findings from the MARIANA array. Velocity and attenuation tomography based on TUCAN data reveal a high velocity, low attenuation subducting slab, a shallow wedge corner with intermediate attenuation, and a slower, more highly attenuating mantle wedge beneath the arc and back-arc. However, velocity and attenuation structures also contain strong regional variations, and these results correlate with arc geochemical data that suggest higher concentrations of slab- derived fluids and greater extents of melting beneath Nicaragua. Beneath Nicaragua, the shallow slab is slower and more attenuating than the slab beneath Costa Rica, consistent with greater slab hydration. In addition, the mantle wedge at depths of 60-100 km is more highly attenuating in Nicaragua than in Costa Rica. Comparison of shear attenuation values with mantle temperatures inferred from arc magmas suggests that much of the difference in attenuation between the Nicaraguan and Costa Rican wedges can be explained by greater water concentrations beneath Nicaragua. A particularly intriguing finding is a column of high Vp/Vs (P-wave velocity/S- wave velocity) that rises from the slab interface directly beneath the arc in Nicaragua. This anomaly could reflect the presence of melt. In three-dimensional models of anisotropy obtained by tomographically inverting shear-wave splitting measurements from local events recorded by the TUCAN array, olivine a-axes are predominantly arc-parallel in the mantle wedge beneath the arc and back-arc at depths of 50 to 150 km (except in northern Nicaragua). The arc-parallel a-axes extend into mantle wedge well beyond the cold, shallow wedge corner where B-type olivine fabric may occur. The observed anisotropy cannot be explained by simple two-dimensional arc-normal corner flow, and instead suggests significant arc-parallel flow. In SKS splitting measurements, fast directions are roughly arc-parallel, and large SKS splitting times indicate that arc-parallel-fast anisotropy is also present beneath the subducting plate. Comparisons of mantle models between the TUCAN and MARIANA experiments are still underway, but a few initial points are apparent. First, as in Nicaragua-Costa Rica, the mantle wedge beneath the arc in the Mariana system is dominated by anisotropy with a fast symmetry axis parallel to the arc. This result suggests the presence of arc-parallel flow in both subduction zone mantle wedges, despite their many tectonic differences (an older, steeper, more tightly curved slab and the presence of active back-arc spreading in the Marianas). Second, while both subduction zones contain a zone of high attenuation in the wedge beneath the arc, the Mariana anomaly is shallower, and the Mariana wedge also contains a second distinct and deeper volume of high attenuation beneath the active spreading center.
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7270 Tomography (6982, 8180)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting