Geometry and P and S velocity structure of the "African Anomaly"

Abstract

We constrain the geometry and P and S velocity structure of a low-velocity anomaly in the lower mantle beneath southern Africa (we term it the "African Anomaly") on the basis of forward traveltime and waveform modeling of seismic data sampling a great arc across the anomaly from the East Pacific Rise to the Japan Sea. Our collected data set consists of direct S, direct P, Sdiff, ScS, PcP, SKS, and SKKS phases recorded by three temporary broadband PASSCAL seismic arrays deployed in Africa between 1994 and 2002, the Tanzania seismic array (1994-1995), the Kaapvaal seismic array (1997-1999), and the Ethiopia/Kenya seismic array (2000-2002) for earthquakes occurring in the East Pacific Rise, Drake Passage, South Sandwich islands, Iran, Hindu Kush, Xinjiang, and the Japan Sea. The seismic data provide excellent sampling of the African Anomaly in the lower mantle along the specific great arc. In order to accurately account for the contributions from the African Anomaly, we relocate all the events using a global seismic shear velocity tomographic model and seismic data recorded by the Global Seismographic Network and correct for the contributions from the seismic heterogeneities outside the African Anomaly. The seismic observations suggest that the African Anomaly locally extends 1300 km above the core-mantle boundary beneath southern Africa (around -25°N, 27°E) and exhibits a "bell-like" geometry with both the southwestern and the northeastern flanks dipping toward its center with the lateral dimension of the anomaly increasing with depth. The base is about 4000 km wide extending broadly in both the southwestward and the northeastward directions. The seismic data can best be explained by a shear velocity structure with average velocity decreases of -5% in the base and -2% to -3% in the mid-lower mantle above the base, and a compressional velocity structure with a uniform S to P velocity perturbation ratio of 3:1 for the entire African Anomaly. These geometric and velocity features suggest that the mid-lower mantle portion of the African Anomaly is an integral component of the very low velocity province and the African Anomaly is compositionally distinct and geologically stable. Copyright 2007 by the American Geophysical Union.