HR: 1330h
AN: T32B-0922    [PDF]
TI: Invisible Slabs: The Influence of Phase Boundary Deflection on Velocity Anomalies of Stagnant Slabs in the Transition Zone
AU: Pysklywec, R
EM: russ@geology.utoronto.ca
AF: University of Toronto, Department of Geology University of Toronto, Toronto, ON M5S 3B1 Canada
AU: * Chambers, K
EM: Kit.Chambers@earth.ox.ac.uk
AF: University of Oxford, Department of Earth Sciences Parkes rd, Oxford, OX1 3PR United Kingdom
AB: A primary constraint on the dynamics of subducting slabs comes from images produced by seismic tomography. The interpretation of the seismically fast anomalies in tomographic models as being cold bodies allows a comparison of behaviours in geodynamic models and the Earth's mantle. However, the inference of seismic anomalies as purely thermal in origin neglects the contribution of olivine phase changes in the transition zone to the seismic manifestation of subducted slabs. To demonstrate the potential importance of phase change effects, we use a numerical routine to simulate the sinking of subducted lithosphere in a convecting mantle. We pay particular attention to the behaviour of the slab around the endothermic phase change associated with the 660-km seismic discontinuity. The time-dependent temperature fields from the flow simulation are used to derive perturbations from a radially stratified seismic velocity model. We show that the positive velocity anomalies of the cold descending slabs may be significantly decreased at the phase change owing to the slab-induced downward phase boundary deflection and associated velocity discontinuity across the boundary. The slab may be completely or partially rendered seismically undetectable, depending on the amount of phase boundary deflection. Gaps in the seismic images of subducting slabs (van der Hilst [1995]) may reflect this effect. These results have implications for the observation and interpretation of the dynamics of subducted slabs in the transition zone.
DE: 3210 Modeling
DE: 7299 General or miscellaneous
SC: Tectonophysics [T]
MN: 2003 Fall Meeting