HR: 0800h
AN: U21A-0009    [Abstracts]
TI: Thermo-Petrologic Evolution of Slabs in the Transition Zone Controls Deep Subduction
AU: * Brudzinski, M R
EM: brudzimr@muohio.edu
AF: Miami University, 114 Shideler Hall, Oxford, OH 45056, United States
AU: Chen, W
EM: wpchen@uiuc.edu
AF: University of Illinois, Dept. of Geology, Urbana, IL 61801, United States
AU: Green, H
EM: hgreen@ucrac1.ucr.edu
AF: University of California-Riverside, Institute of Geophysics and Planetary Physics, Riverside, CA 92521, United States
AU: Pillet, R
EM: pillet@geoazur.unice.fr
AF: UMR GéoScience Azur, 250 rue Albert Einstein, Sophia-Antipolis, 06560, France
AB: The interaction between subducted lithosphere and the transition zone of the mantle is a key issue in assessing the scale of mantle convection. However, interpretations of seismic tomography seem to indicate a puzzling mix of slab-like anomalies in the lower mantle as well as large-scale remnants of slab within the transition zone. High- resolution studies of the Tonga subduction zone, where the strongest thermal anomaly associated with subduction is expected, reveal that an anisotropic petrologic anomaly exists at the deep leading-edge of the Wadati-Benioff zone (WBZ) over distances of about 300 km. Strong radial anisotropy (~1%) and lower than expected seismic wave speeds (~3%) are evident from analysis of triplicate P and S-waveforms recorded by the CAVASCOPE broadband array. Similar evidence for a petrologic anomaly also exists over a length-scale of about 1,000 km in a seismically active, detached remnant of slab immediately to the west of the WBZ. Surrounding the anisotropic region of the remnant slab, elevated seismic wave speeds gradually diminish laterally, indicating that the petrologic anomaly is the center of a large, diffuse thermal anomaly.

The only candidate for the petrologic anomaly that satisfies all available observations is metastable olivine -- a buoyant material when present in the transition zone and therefore acting as a barrier to slab penetration. On a global scale, similarly rapid subduction of cold lithosphere is pervasive in the western Pacific where large-scale remnants of slabs are common in the transition zone. We interpret these observations as a natural consequence of how lithosphere sinking into the transition zone generates a buoyant thermo-petrologic anomaly that evolves over time into a negatively-buoyant, pure thermal anomaly. As the slabs evolve, the effect of buoyancy from the petrologic anomaly would be important for retaining subducted material as subhorizontal remnants which, in turn, would continue to serve as an effective heat sink, lowering temperature over broad regions
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Union [U]
MN: 2007 Fall Meeting