HR: 16:45h
AN: T14B-04    [Abstracts]
TI: Seismic evidence for present-day plume upwelling at the core-mantle boundary
AU: * Rost, S
EM: srost@asu.edu
AF: Department of Geological Sciences, Arizona State University, Box 871404, Tempe, AZ 85283 United States
AU: Garnero, E J
EM: garnero@asu.edu
AF: Department of Geological Sciences, Arizona State University, Box 871404, Tempe, AZ 85283 United States
AU: Williams, Q
EM: qwilliams@es.ucsc.edu
AF: Department of Earth Sciences, University of California Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AU: Manga, M
EM: manga@seismo.berkeley.edu
AF: Department of Earth and Planetary Science, University of California Berkeley, 397 McCone Hall, Berkeley, CA 94720-4767 United States
AB: In recent years a broad range of seismic discoveries have painted a picture of Earth's core-mantle boundary (CMB) that is far more complex than a simple one-dimensional boundary between the molten outer core and the solid silicate mantle. Over the last ten years several anomalous regions of the lowermost mantle have been detected using a multitude of seismic phases and approaches. These regions are characterized by reductions of \textit{P}-wave and \textit{S}-wave velocities by at least 10%. These ultra-low velocity zones (ULVZ) are characterized as having strong variability at short scale length (e.g. ,$\le$ 100 km) and do not appear to be a global layer. Using 300 deep focus Tonga-Fiji subduction zone earthquakes recorded at the Warramunga array in central Australia, we detect an extremely localized low-velocity feature at the CMB that lies beneath low shear wave velocities in D$^{\prime\prime}$ south of New-Caledonia. An array analysis of \textit{ScP} reveals anomalous precursors that are most robustly explained by a dense ULVZ with the following characteristics: 8.5~km thick, $\sim$50~km wide, \textit{P}- and \textit{S}-wave reductions of 10 and 25%, respectively, and a density increase of 10% ($\pm$5%). These parameters are best explained by the presence of dense partial melt in the ULVZ. A model to keep the dense lens of partially molten material from spreading out along the CMB includes the entrapment of melt by intercumulus crystal growth after drainage from the anomalously hot overlying mantle. Therefore, this region may be closely related to thermal instabilities at the thermal boundary layer of the CMB that will influence the stability, genesis and persistence of mantle plumes.
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 7200 SEISMOLOGY
DE: 7207 Core and mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting