Mapping the geometry and geographic distribution of a very low velocity province at the base of the Earth's mantle

Abstract

We determine the geometry and geographic distribution of a very low velocity province (VLVP) at the base of the Earth's mantle beneath the South Atlantic and Indian Oceans, based on waveform modeling and travel time analysis of seismic data recorded in Africa. Our data sets consist of SH, ScS, SHdiff, SKS, and SKKS phases recorded at two temporary broadband PASSCAL seismic arrays in Africa, the Tanzania array (1994–1995) and the Kaapvaal array (1997–1999), and differential travel time residuals of the ScS‐SH phases recorded at the Global Seismographic Network (GSN). These seismic data constitute reasonably good sampling coverage for determining the geographic boundary of a very low velocity province in the South Atlantic and Indian Oceans. The boundary is well determined in the eastern, western, and southern portions but relatively poorly constrained in the northern portion because of the nature of the seismic data. The VLVP 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. It occupies an area of about 1.8 × 107 km2 at the core‐mantle boundary and a volume of about 4.9 × 109 km3. At least a 4% uncertainty exists in the area estimate, and a 20% uncertainty exists in the volume estimate. Waveform modeling and travel time analysis suggest that the VLVP has rapidly varying thicknesses from 300 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, consistent with previous results. These structural and velocity features unambiguously indicate that the VLVP is compositionally distinct.