HR: 09:00h
AN: T31D-05    [Abstracts]
TI: Shear-Wave Splitting Tomography in the Central American Mantle Wedge
AU: * Abt, D L
EM: David_Abt@brown.edu
AF: Department of Geological Sciences, Brown University 324 Brook St. Box 1846, Providence, RI 02912 United States
AU: Fischer, K M
T31D-05 AF: Department of Geological Sciences, Brown University 324 Brook St. Box 1846, Providence, RI 02912 United States
AU: Martin, L
T31D-05 AF: Department of Geological Sciences, Brown University 324 Brook St. Box 1846, Providence, RI 02912 United States
AU: Abers, G A
T31D-05 AF: Department of Earth Sciences, Boston University 685 Commonwealth Ave., Boston, MA 02215 United States
AU: Protti, J M
T31D-05 AF: Universidad Nacional, OVSICORI, Heredia, 86-300 Costa Rica
AU: Gonzalez, V
T31D-05 AF: Universidad Nacional, OVSICORI, Heredia, 86-300 Costa Rica
AU: Strauch, W
T31D-05 AF: Geophysics, INETER, Managua, 2110 Nicaragua
AB: We are constraining the three-dimensional distribution of anisotropy in the Central American subduction zone using shear-wave splitting measurements from local and teleseismic shear phases recorded by the TUCAN seismic array. Our objectives are to determine the processes responsible for producing anisotropy and to better understand subduction zone melting and its relationship to mantle flow. The TUCAN array comprises 48 broadband IRIS/PASSCAL seismometers located across the arc and back-arc in Nicaragua and Costa Rica. The array, part of the NSF MARGINS program, was deployed in July and August of 2004 and will remain in place until March of 2006. Recent high pressure and temperature deformation experiments suggest that the relationship of olivine lattice preferred orientation to strain depends on pressure, stress, and melt or volatile content, specifically that the fast (100) axis of olivine may not align parallel to the overall flow direction. Several subduction zones (e.g. Tonga, South America, Japan) display both arc-normal and arc-parallel fast directions, indicating either complex three-dimensional flow in the mantle wedge or heterogeneous development of anisotropy most likely related to slab dehydration and mantle melting. Beneath the TUCAN array, over 120 splitting measurements made from local S phases recorded in the first 6 months of the experiment exhibit predominantly arc-parallel fast directions, but there also exist some areas with very rapid changes in fast direction. Local S splitting times range from 0.04-1.14 s with an average of 0.30 s. SKS splitting measurements display more consistent fast directions (all nearly arc-parallel) and larger split times (1.5-2.0 seconds). These results require the presence of arc-parallel anisotropy beneath the slab or the deep back-arc. To better resolve the spatial variation of anisotropy throughout the mantle wedge, we have developed a method to invert for anisotropic structure using an iterative, linearized approach. One-dimensional inversions converge well on a solution, and the resulting splitting residuals indicate that inversions in two and three dimensions should resolve meaningful variations in anisotropy. The sampling provided by splitting measurements obtained thus far suggests the complete data set will allow for resolution of anisotropy at scales of ~25 km in several areas. Our ultimate goal is to integrate these results with attenuation, absolute velocities, and geochemical data to map the pattern of deformation and the distribution of melt and volatiles within the mantle wedge.
UR: http://www.geo.brown.edu/geopeople/grads/abt/html/CASWS/CAgeneral.htm
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8104 Continental margins: convergent
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8180 Tomography (6982, 7270)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Tectonophysics [T]
MN: Fall Meeting 2005