HR: 0830h
AN: U51B-0008    [PDF]
TI: Lower-Mantle Seismic Discontinuities and the Thermal Structure of Subducted Slabs.
AU: * Thomas, C
EM: tine@liv.ac.uk
AF: University of Liverpool, Department of Earth and Ocean Sciences Herdman Laboratories 4, Brownlow Street, Liverpool, L37 3HD United Kingdom
AU: Kendall, J M
EM: kendall@earth.leeds.ac.uk
AF: University of Leeds, Department of Earth Sciences, Leeds, LS2 9JT United Kingdom
AU: Lowman, J
EM: j.lowman@earth.leeds.ac.uk
AF: University of Leeds, Department of Earth Sciences, Leeds, LS2 9JT United Kingdom
AB: A long-standing debate in geophysics has revolved around the degree of mass exchange between the upper and lower mantle. Tomographic images show slab structures extending well into the lower mantle, and in some places to the core-mantle boundary (CMB), thereby supporting the whole-mantle convection paradigm. However, it is that tomography results have also formed the basis for arguments for a compositionally distinct layer at the base of the mantle, which could explain the source for the primordial geochemical signature observed in oceanic island basalts. The nature of the D" region, the lowermost few 100kms of mantle, holds insights into these issues. The complexity of this region has often been used as an argument for chemical heterogeneity at the base of the mantle. Here we investigate the fine structure of this region beneath Eurasia using seismic migration of shear-waves reflecting from velocity discontinuities in the lowermost mantle and recorded by a dense network of European seismic stations. Two main features are found. One structure exhibits large-scale topography and can be explained by a sharp increase in velocity 206-316 km above the CMB. The second structure lies 55-85 km above the CMB and marks a sharp reduction in seismic velocity. To explain the origin of these discontinuities we appeal to recent numerical simulations of mantle convection, which show that a retained thermal anomaly in subducting slabs will produce such upper and lower discontinuities, the former with considerable topography. The results suggest that, in places, complexity in D" discontinuity structure may be due to the thermal structure of cold slabs accumulating at the CMB.
DE: 7207 Core and mantle
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8124 Earth's interior--composition and state (old 8105)
SC: U
MN: 2003 Fall Meeting