HR: 1340h
AN: MR23A-0190    [Abstracts]
TI: The "African Anomaly" and the "Pacific Anomaly" in the Lower Mantle: Similarities and Differences
AU: * Wen, L
EM: Lianxing.Wen@sunysb.edu
AF: State University of New York at Stony Brook, Department of Geosciences, Stony Brook, NY 11794 United States
AU: He, Y
EM: ymhe@mail.igcas.ac.cn
AF: Chinese Academy of Sciences, Institute of Geology and Geophysics, Beijing, 10029 China
AU: Wang, Y
EM: biphor@mantle.geo.sunysb.edu
AF: State University of New York at Stony Brook, Department of Geosciences, Stony Brook, NY 11794 United States
AB: Seismic results have consistently shown two prominent low-velocity anomalies in the lower mantle, with one beneath southern Africa and the other beneath Pacific. For convenience, we refer them here the "African anomaly" and the "Pacific anomaly". The African anomaly was constrained mostly by the SH, ScS, SHdiff, SKS, SKKS, P, Pdiff and PcP phases recorded in three PASSCAL seismic arrays in Africa: the Tanzania array (1994 - 1995), the Kaapvaal array (1997 - 1999) and the Kenya array (2001-2002). These seismic observations indicate that the African anomaly has a very-low velocity province (VLVP) at its base, extends at least 1300 km above the core-mantle boundary with its edges in the lower mantle dipping toward its center and has a P to S velocity perturbation ratio of 1:3. The VLVP exhibits an "L-shape" changing from a north-south orientation in the South Atlantic Ocean to an east-west direction in the Indian Ocean, and has rapidly varying thicknesses from 300 km to 0 km, steeply dipping edges and a linear gradient of shear velocity reduction from -2% (top) to -9% to -12% (bottom) relative to the preliminary reference Earth model. These structural and velocity features unambiguously indicate that the VLVP, and likely the whole African anomaly, is compositionally distinct. The Pacific anomaly is studied using the ScS and SH waves recorded in the F-net in Japan, the China National Digital Seismographic Network and several dense seismic arrays in the Northern China Interior Structure Project, and the PKP precursors and SKS-SPdKS phases recorded in several WWSSN stations. Our ScS-S differential travel times confirm the early tomographic results that the the base of the Pacific anomaly is broad, but suggest presence of larger shear velocity reductions. The ScS-S differential travel times show residuals as large as 10 seconds when the ScS phases sample the eastern part of the base, which would correspond to an average shear velocity reduction of -5% for a 300-km-thick base.Our ScS-S data also provide good sampling coverage for determining the northern, western and southern boundaries beneath the western Pacific. The ScS-S travel time residuals exhibit rapid changes across the boundaries over small epicentral distances (1$^{\circ}$), suggesting that the lateral transitions of the Pacific anomaly occur over small distances (about 50 km). Different from the African anomaly which appears to lack internal small-scale heterogeneities and has relatively small magnitude of P velocity reductions, the Pacific anomaly is characterized by the presence of significant internal small-scale heterogeneities with large P-velocity reductions and length-scales from tens kilometers to hundreds kilometers. These two anomalies, although characteristically different to some extent, could represent similar compositional anomalies produced in the early Earth's history, with different length scales and different degrees of melt.
UR: http://geophysics.geo.sunysb.edu/wen/
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8125 Evolution of the Earth
DE: 7200 SEISMOLOGY
DE: 7203 Body wave propagation
DE: 7207 Core and mantle
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting