Mantle d layer

Abstract

The diverse seismic properties of the D region appear to demonstrate the presence of heterogeneity at the base of the mantle associated with thermal and chemical boundary layers of a complex dynamical system. There is a predominance of large-scale structures in D, and the strength of heterogeneities at intermediate and large-scales appears to be significantly greater than in the overlying lower mantle. Large low shear velocity provinces (LLSVPs), ultra-low velocity zones (ULVZs), multiple seismic discontinuities, a major phase change, and seismic anisotropy are all fundamental attributes of the D region. The present day configuration of D structures represents a “snap-shot” of an evolving system, with some aspects reflecting very long time-scale processes (LLSVPs may have been in place for at least hundreds of millions of years; ULVZs may be the last remnant of an extensive magma ocean that dates back to core formation) and much shorter time scales (post-perovskite lenses may be found in recently downwelled slab materials and seismic anisotropy may be sustained by present day dynamic shear flows). Together with the ongoing role of D as a thermal boundary layer that regulates cooling of the core and as the site of electromagnetic-mechanical coupling of rotation between the core and mantle, the significance of this region for Earth dynamics appears more evident than ever. While debate continues regarding the extent to which mantle material is fluxing between the shallow and deep mantle, evidence has grown to support a significant feed-back/control of D structures on geological processes at Earth’s surface.