Mapping its boundary in the South Atlantic Ocean for a compositional anomaly at the base of the mantle

A seismic anomaly was recently discovered at the base of the mantle, extending from the South Indian Ocean to the South Atlantic Ocean [Wen et. al., 2001; Wen 2001, 2002]. This anomaly has steeply dipping edges, rapidly varying geometries and thicknesses (0 - 300 km), and a negative shear wave velocity gradient from -2% (top) to -9% to -12% (bottom). These structural and seismic characteristics unambiguously suggest it is a compositional anomaly. To further understand its dynamic consequences and its implications, it becomes important to map out the geographic extent of this compositional anomaly. In fact, this seismic anomaly has also been noted to coincide geographically with the geochemical DUPAL anomaly in both the South Atlantic Ocean [Wen et al., 2001] and the Indian Ocean [Wen, 2001]. In this study, we collect seismic data and constrain the geographic extent of this anomaly in the South Atlantic Ocean, where its exact geographic boundary remains sketchy. We search seismic data from those recorded in the global seismic network (1997 - 1999) and two broadband PASSCAL seismic networks deployed in Tanzania (1994 - 1995) and the Kaapvaal craton (1997-1999) in Southern Africa. Our selected dataset consists of tangential displacements for 6 events occurring in the South American subduction zone, travel time delays of SKS and SKKS phases, and differential ScS - S travel times. These observations constitute areasonably good sampling coverage for the lowermost mantle beneath the South Atlantic Ocean. The seismic observations consistently show little travel time delays for direct SH waves sampling 300 km above the core-mantle boundary and large travel time delays for ScS phases and large linearly increasing travel time delays for Sdiff phases (up to 15 seconds). Many SKS and SKKS phases exit acrossing the edge of the anomaly and show rapid variations of travel time delay (from 0 to 4.8 seconds) across small epicentral distances (about 460 km). We construct a map of the geographic boundary and geometry of this 300 km thick compositional layer based on waveform modeling of the tangential displacements for the 6 South American events and analysis of the travel time delays of SKS, SKKS and ScS - S phases. We also extensively explore trade-offs between geometry, seismic velocity reductions, and thickness of the seismic model in explaining the seismic data. For high-quality seismic data in the diffracted distance range, the waveform features and travel time delays place very tight constraints on the geometry, thickness, and negative seismic velocity gradient inside the anomaly. A negative shear velocity gradient from -2% (top) to -9% to -12% (bottom), a thickness of 300 km, and steeply dipping edges are required to fit the observations. A uniform velocity reduction of -3% cannot explain both those observed waveform features and travel times. At the pre-diffracted distances, significant trade-offs exist between velocity reduction and geometry in explaining the data. For those observations, we employ a shear velocity gradient of -10%, the velocity gradient obtained from waveform modeling the seismic data in the diffracted distance range, to constrain the location and geometry. The geographic locations can be constrained to within a horizontal distance of 200-400 km. The inferred locations of this boundary layer are also corroborated well with the ScS-S differential travel times.