HR: 1340h
AN: S43B-1003    [Abstracts]
TI: Investigating topography of transition zone discontinuities in the Southwest Pacific using a migration method
AU: * Thomas, C
EM: tine@liv.ac.uk
AF: University of Liverpool Department of Earth and Ocean Sciences, 4, Brownlow Street , Liverpool, L37 3HD United Kingdom
AU: Billen, M I
EM: billen@geology.ucdavis.edu
AF: University of California Davis, Department of Geology One Shields Avenue , Davis, CA 95616 United States
AB: Due to the temperature dependence of the phase transitions of upper mantle minerals the upper mantle discontinuities hold insight into Earth dynamics and can give information on temperatures in and around of regions of slabs. In regions of cold downwelling (i.e. slabs) the discontinuity at 410 km depth is expected to be shallower whereas the 660 km discontinuity should be deeper. The behaviour of the discontinuity at 520 km depth has yet to be analysed in more detail. We employ a migration method and underside reflections of PP off these discontinuities to investigate their topography in the upper mantle beneath the Southwest Pacific. The events are reflected in regions near and within slabs and the grid of potential reflectors spans from 115 to 150deg E and 0 to 30 deg N in 1degree steps and layers in steps of 10 km from the surface to 730 km. With this method the existence of the 520 km discontinuity is confirmed in this region although the reflector appears to be much deeper (at 560 km depth). In addition discontinuities at 220 km and 350 km are observed. The deflections of 660 and 410 km are studied in more detail. The underside reflection P660P that was recently considered to be absent is visible with the migration method in these regions. The results show that the 660 km discontinuity seems to be deeper up to 40 km whereas the 410km discontinuity seems to be only slightly shallower near subduction areas and the data show laterally varying topography on all observed discontinuities. Using the depths of the discontinuities we are attempting to constrain the temperature and Clapeyron slope within and in the vicinity of the slabs.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting