HR: 1340h
AN: S53B-0221    [Abstracts]
TI: Geometry and P- and S- Velocity Structures of the "African Anomaly"
AU: * Wang, Y
EM: biphor@mantle.geo.sunysb.edu
AF: Department of Geosciences, SUNY at Stony Brook, Stony Brook, NY 11794 United States
AU: Wen, L
EM: wen@earth.geo.sunysb.edu
AF: Department of Geosciences, SUNY at Stony Brook, Stony Brook, NY 11794 United States
AB: Seismic evidence shows that the "African anomaly", a prominent low-velocity anomaly in the lower mantle beneath southern Africa, has a very-low velocity province (VLVP) in the lowermost 200-300 km of the Earth's mantle with rapidly varying geometries and a strong S-velocity reduction gradient from -2% (top) to -9% - -12% (bottom). It is now clear that the VLVP is compositionally distinct and can be best explained by partial melting driven by a compositional change produced in the early Earth's history (Wen, 2001; Wen et. al, 2001). However, the nature of the anomaly in the lower mantle remains unknown and controversial. Here, we investigate its geometry, and both P- and S- velocity perturbations along a specific great arc from the Drake Passage to the Hindu Kush region. The southwestern side of the anomaly is sampled by direct S, ScS, and SKS waves from earthquakes occurring in the South Sandwich islands and the East Pacific rise. Direct S waves from the South Sandwich islands earthquakes exhibit no travel time delay with respect to the preliminary reference Earth model (PREM) from 44$^{\circ}$~~to 59$^{\circ}$~~across the Kaapvaal array, but they are increasingly delayed from 3 s at 67.5$^{\circ}$~~to 12 s at 74$^{\circ}$~~across the Tanzania array and from 12 s at 74.5$^{\circ}$~~to 19 s at 89$^{\circ}$~~across the Kenya array. The ScS residuals have a simple trend increasing from 4.5 s at 44$^{\circ}$~~to 20.5 s at 74$^{\circ}$. The SKS waves for earthquakes occurring in the East Pacific rise show a uniform delay of about 3 s before 101$^{\circ}$, steeply increasing to a uniform delay of 7 s after 107$^{\circ}$. These observations indicate that the "African anomaly" extends about 1300 km above the core-mantle boundary (CMB) with its southwestern edge dipping towards its center and its basal layer near the CMB extends further southwest. The northeastern side of the anomaly is constrained by the direct S and ScS waves from three events in the Hindu Kush region, the SKS waves from an earthquake in the Drake Passage, and the SKS and SKKS waves from an earthquake in the Japan sea. Both S and ScS waves from earthquakes occurring in the Hindu Kush region do not show travel time delays with respect to PREM, placing bounds on the geographic extent of the anomaly on the northeastern side. The SKS residuals of the Drake Passage earthquake shows a decreasing trend from 7 s at 87$^{\circ}$~~to about 1 s at 93.5$^{\circ}$. The SKKS wave from an earthquake in the Japan sea is only delayed after 140$^{\circ}$~~whereas the SKS residuals are about 4 s between 129$^{\circ}$~~and 140$^{\circ}$. These observations suggest that, on the northeastern side, the anomaly also dips toward its center beneath southern Africa. Therefore, the "African anomaly" exhibits a "cusp-like" geometry in the lower mantle. The magnitudes of these travel time residuals can be best explained by a shear velocity structure, with average reductions of -5% for the basal layer and -2% - -3% for the portion in the lower mantle. With the "cusp-like" geometry, the P-wave velocity structure is constrained by the travel time of direct P, PcP and their differentials from the same events. P-wave data provide good sampling coverage for both the basal layer and the portion in the lower mantle. We test a series of P velocity models with uniform P to S velocity perturbation ratios from 1:1 to 1:7. We find that a ratio of 1:3 best explains the P wave data. The geometry and the P to S velocity perturbation ratio of the anomaly indicate that the "African anomaly" in the lower mantle likely, like the VLVP at its base, is compositionally distinct and has an origin of partial melting.
DE: 9305 Africa
DE: 7203 Body wave propagation
DE: 7205 Continental crust (1242)
DE: 7207 Core and mantle
DE: 1010 Chemical evolution
SC: Seismology [S]
MN: 2004 AGU Fall Meeting