Mega ultra low velocity zone and mantle flow

Mantle flow in Earth's interior has been inferred from a variety of geo-disciplines. Two continental-scale, nearly antipodal, large low shear velocity provinces (LLSVPs) at the base of the mantle, thought to be dense and chemically distinct likely play a significant role in mantle dynamics and plume generation, and hence are targeted in a high-resolution seismic study. We analyze broadband SPdKS waveforms using a 2.5D axi-symmetric finite difference wave propagation algorithm PSVaxi. Here we find patches of greatly reduced seismic wave speeds at the core-mantle boundary, i.e., ultra-low velocity zones (ULVZs), within the Pacific LLSVP, including the largest ULVZ detected to date, roughly 250 x 800 km in lateral dimension and 10-15 km thick, in an apparent hole in the LLSVP. The presence of this ULVZ in the LLSVP hole is well explained by dynamically merging, chemically-distinct piles containing ULVZs at their margins. The consequence of these merging piles may be to initiate anomalously large, infrequent plumes, as well as to provide a means to transfer isotopes to the surface. (C) 2013 Elsevier B.V. All rights reserved.