IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS011 Oral Presentation 2101
Frequency distribution of seismic wavespeed as evidence for bodies of
distinct material in the lowermost mantle, and analogies with the
lithosphere.
Dr. Bernhard Steinberger
Center for Geodynamics Geological Survey of Norway IASPEI
Kevin Burke, Trond H. Torsvik, Mark A. Smethurst
Frequency or occurrence of shear-wave tomography models close to the CMB shows a distinct bimodal
distribution, with a larger peak at higher velocities and a smaller peak at lower velocities. The relative
size of the peak at higher velocities gets larger higher up in the mantle. Correspondingly, regions of
strong lateral gradient in shear wave speed frequently occur along approximately at the - 1 % shear
wave anomaly contour surrounding the Large Low Shear Velocity Provinces (LLSVPs) above the CMB.
This provides further evidence for bodies of distinct material in the lowermost mantle. They occupy a
surface area of ~ 20 % close to the CMB, which reduces to ~ 10 % 300 km above the CMB, and thus
contain about 2 % of mantle material. Large Igneous Provinces (LIPs), when reconstructed to their
eruption sites, cluster above the margins of these bodies at their base, which we term Plume Generation
Zones [PGZ]. Thus we find a correspondence between processes at the base of the mante and its top:
While subduction frequently occurs at the edges of continents, which are chemically distince and
positively buoyant, the return of material from the "slab graveyards" in the form of mantle plumes
occurs at the edges of LLSVPs, which appear to be also chemically distinct and negatively buoyant.
Different from continents, though, LLVPs appear not to have substantially moved or deformed --
possibly because their upper parts are at a depth with rather high viscosities of about 10**23 Pas. The
scenario proposed here has implications, which can be tested through seismic and geochemical
observations and geodynamic modelling.
Keywords: cmb, plumes, llsvps