Structural Features and Velocity Structures of the “African Anomaly”
Structural Features and Velocity Structures of the “African Anomaly”
Credit:
Yi Wang, Lianxing Wen • SUNY at Stony Brook/IRIS Consortium
Description
(a) Geometry (green contour) and ray paths of the seismic phases used to constrain the geometry and S-velocity structure of the “African anomaly” in a 2-D cross section along the East Pacific Rise (EPR), Drake Passage (DP), South Sandwich Islands (SS), Iran (IR), and Hindu Kush (HK). Black and red traces represent propagation paths without and with observed travel time delays that can be attributed to the “African anomaly”, respectively. Black stars represent seismic events. Seismic arrays and earthquake locations are denoted at the top of the Earth’s surface. (b) Map view of great-circle paths (gray traces), locations of earthquakes (red stars) and seismic arrays (black triangles). The thick green dashed curve represents the 2-D cross section shown in (a) and the thick black contour is the geographic boundary of the base of the “African anomaly”shown in (c). (c) Three-dimensional views of structure features and velocity structure of the base of the “African anomaly” (vertical exaggeration: 5.55, viewed from N).
The IRIS PASSCAL experiments have supplied the seismology community with high-quality, freely available and spatially dense datasets. The high-quality broadband seismic data recorded in three PASSICAL experiments in Africa (the Tanzania, the Kaapvaal, and the Kenya/Ethiopia experiments) revealed a very-low velocity province occupying from the South Atlantic Ocean to the Indian Ocean in the lowermost mantle and locally extending 1300 km above the core-mantle boundary beneath southern Africa (we term it the “African anomaly”). The base of the “African anomaly” exhibits an L-shaped form changing from a north-south orientation in the South Atlantic Ocean to an east-west orientation in the Indian Ocean and occupies an area of about 1.8x107 km2 and a volume of about 4.9x109 km3. The dense seismic data and the development of hybrid method also revealed detailed structural features of the “African anomaly”. The base of the “African anomaly” 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 (Wen et al., 2001; Wen, 2001; Wen, 2002; Wang and Wen, 2004), Its extension into the mid-lower mantle beneath southern Africa exhibits a “cusp-like” geometry with both flanks dipping toward its apex and its lateral dimension increasing with depth. The average shear velocity decreases are –5% in the base and -2% to -3% in the portion in the mid-lower mantle, and an S- to P- velocity perturbation ratio (d InVS/d InVP) is 3:1 for the entire “African anomaly” (Wang and Wen, 2005). Our ability to constrain both the geometry and velocity structures of the “African anomaly” brought significant progress in understanding the origin and dynamics of the anomaly (Wen et al., 2001). The structural features and velocity structures of the “African anomaly” unambiguously indicate that the “African anomaly” is compositionally distinct and geologically stable. It has also been suggested that the velocity structure in the lowermost mantle can best be explained by partial melt driven by a compositional change produced in the early Earth’s history (Wen et al., 2001).
Wen, L., Seismic evidence for a rapidly varying compositional anomaly at the base of the Earth’s mantle beneath the Indian Ocean, Earth Planet. Sci, Lett., 194, 83 – 95, 2001.
Wen, L., P. Silver, D. James, and R. Kuehnel, Seismic evidence for a thermo-chemical boundary layer at the base of the Earth’s mantle, Earth Planet. Sci. Lett., 189, 141 – 153, 2001.
Wen, L., An SH hybrid method and shear velocity structures in the lowermost mantle beneath the central Pacific and South Atlantic oceans, J. Geophys. Res., 103, doi:10.1029/2001JB004999, 2002.
Wang, Y., and L. Wen, Mapping the geometry and geographic distribution of a very low velocity province at the base of the Earth’s mantle, J. Geophys. Res., 109, doi:10.1029/2003JB002674, 2004.
Wang, Y., and L. Wen, Geometry and P- and S- velocity structures of the “African anomaly”, submitted to J. Geophys. Res., 2005.
Photographer / Contributor: Yi Wang, Lianxing Wen • SUNY at Stony Brook
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