HR: 0800h
AN: DI41A-1262    [Abstracts]
TI: Connecting Active Subduction to Aseismic Remnant Slabs: Evolution of Petrologic Anomalies in the Mantle Transition Zone
AU: * Brudzinski, M R
EM: brudzimr@muohio.edu
AF: Miami University, Department of Geology, Oxford, OH 45056 United States
AU: Chen, W
EM: wpchen@uiuc.edu
AF: University of Illinois, Department of Geology, Urbana, IL 61801 United States
AU: Pillet, R
EM: pillet@noumea.ird.nc
AF: Institut de Recherche pour le Developpement (IRD), Geosciences Azur, Noumea, 98848 New Caledonia
AU: Tseng, T (
EM: tseng1@uiuc.edu
AF: University of Illinois, Department of Geology, Urbana, IL 61801 United States
AB: The extent of slab penetration through the mantle transition zone (TZ) is a key issue in mantle convection and geochemical mixing. For the most part, both proponents and opponents of deep slab penetration have relied upon the detection of high seismic wave speeds as evidence for regions of low temperature, a proxy for subducted material. For instance, under the Northern Philippine Sea, a small contrast in both P- and S-wave speeds (V_p and V_s) across the 660-km discontinuity, indicating high V_p and V_s at the bottom of the TZ, and a large (~8%) contrast in density across the 660-km discontinuity adjacent to this anomaly are key evidence for a stagnant, aseismic remnant slab originated from the Izu-Bonin subduction zone. If one considers the entire process of slab evolution, very cold remnant slabs would have a precursor in which the temperature was low enough to retain a buoyant petrologic anomaly due to the presence of volatiles and/or meta-stable minerals. Beneath the back-arc of the Tonga-Kermadec region, evidence for a prominent petrologic anomaly includes: outboard earthquakes that occur up to 1,000 km away from a well-defined Wadati-Benioff zone, a lack of the typical regional pattern of strain for deep earthquakes, strong seismic anisotropy but an absence of high V_p and V_s in the core of the expected thermal anomaly, and gradually fading high V_p and V_s in regions surrounding the core. Since a remnant slab must have been part of active subducting lithosphere, a similar petrologic anomaly should exist along the leading edge of an active Wadati-Benioff zone in the TZ. This prediction is borne out by new broadband seismic array data in the western Pacific. In particular, we present a collection of evidence to show that over a distance of 300 km, the subhorizontal leading-edge ("toe") of the Tonga Wadati-Benioff zone bears the same characteristics as the large (over 1,000 km in length) remnant of a detached slab farther to the west. In essence, these new observations constitute the missing-link between active and remnant slabs. In summary, seismic data from the western Pacific now have detected all three main stages of slab stagnation in the transition zone: from the leading edge of active subducting lithosphere, to a buoyant petrologic anomaly that accompanies a strong thermal anomaly in a remnant slab, ultimately to an aseismic thermal anomaly where the petrologic anomaly has been assimilated. This sequence of evolution illustrates how and why deep slab penetration is not ubiquitous.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 7203 Body waves
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 8125 Evolution of the Earth (0325)
SC: Study of Earth's Deep Interior [DI]
MN: Fall Meeting 2005