HR: 0800h
AN: DI41A-1246    [Abstracts]
TI: Evidence for Positive Correlation of 400- and 670-km Discontinuity Topography Beneath the Central Pacific from SS Precursors
AU: * Schmerr, N
EM: nschmer@asu.edu
AF: Department of Geological Sciences Arizona State University, Box 871404, Tempe, AZ 85287-1404 United States
AU: Garnero, E
EM: garnero@asu.edu
AF: Department of Geological Sciences Arizona State University, Box 871404, Tempe, AZ 85287-1404 United States
AU: Stixrude, L
EM: stixrude@umich.edu
AF: Department of Geological Sciences University of Michigan, 1100 N. University Ave., Ann Arbor, MI 48109-1005 United States
AB: We image upper mantle discontinuity structure using precursors to the seismic phase SS, which occur as underside reflections at the midpoint of the SS path, and are sensitive to discontinuity depth and sharpness. Our dataset consists of ~ 4500 high-quality broadband seismograms collected from the Incorporated Research Institutions for Seismology (IRIS) and the Canadian National Seismic Network (CNSN) that densely sample the mantle beneath the majority of the Central Pacific, including the Hawaiian-Emperor chain, spanning 185 to 225 deg E and -1 to 40 deg N. We employ a new stacking method that considers the Fresnel zone structure of SS, and explore the dependence of solution discontinuity structure on a number of factors, including dominant period, crustal corrections, signal-to-noise ratio threshold, and effects of a mantle heterogeneity correction for several different tomography models. Retrieved topography is strongly dependent on dominant period; we separately analyze 10, 15, 20, and 25 second energy. Results are also shown to depend on the tomography model used to correct for mantle heterogeneity. We measure an average transition zone thickness of 241 ± 2 km, this value is similar to the average of 242 km found in global SS precursor studies. Topographic variation on each discontinuity is dependent on dominant period, at 25 seconds we measure ± 2 km of relief on each discontinuity, at 10 seconds ± 6 km of relief on the 670 and ± 3 km of relief on the 400. At long period (25 seconds), we find a positive correlation of topography on the 400- and 670-km discontinuities; contradicting mineral physics predictions of opposing Clapeyron slopes of the discontinuities. Possible explanations for correlated long-wavelength topography are lateral variation in bulk composition, or a 670-km discontinuity caused primarily by the garnet to perovskite transition, which may replace the ringwoodite to perovskite + magnesiowüstite transition in hot mantle due to the breakdown of ringwoodite to majorite + magnesiowüstite.
DE: 3621 Mantle processes (1038)
DE: 3900 MINERAL PHYSICS
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 9355 Pacific Ocean
SC: Study of Earth's Deep Interior [DI]
MN: Fall Meeting 2005