Global Seismology - The Deep Earth

Studies of the Lowermost Mantle

DESMOND

A NERC funded consortium aimed at understanding the composition, chemistry, and dynamics of the deep Earth through an interdisciplinary study of the core-mantle and inner-outer core interfaces.

Anisotropy and Heterogeneity in the D" Region

The boundary layer known as D" occupies the lowermost few hundred kms of the mantle and can be viewed as the lower-mantle analog of the lithosphere. Convection simulations show that a thermal boundary-layer should exist in this region and strong arguments for a chemical boundary-layer come from evidence of heterogeneity on a wide range of length scales. In many areas the top of the D" layer is bounded by a seismic discontinuity which delineates dramatic variations in the thickness of the layer (260 km +/- 150 km). There is also mounting evidence for an ultra-low-velocity layer in the lowermost 5-40 km of D". Explanations for this heterogeneity include iron infiltration from the core, subducted material pooling at the core-mantle boundary (CMB) and primordial material from mantle differentiation. It has also been suggested that this region may be the site of another mantle phase transition. Recent evidence of seismic anisotropy in the lowermost mantle offers additional tantalizing insights into the nature of the D" region (Kendall and Silver, 1996). While these observations are very interesting from a seismological point of view, they are equally if not more valuable as constraints on the style of mantle dynamics, the mineral physics of the lowermost mantle and core- mantle coupling which may affect the Earth's magnetic field.

A map of the thickness of the D" layer as estimated from long-period S-waves (see Kendall and Shearer, 1994). Reflections from the top of this layer indicate dramatic lateral variability in its topography.

Publications

Kendall, J-M., 2000, Seismic anisotropy in the boundary layers of the Earth's mantle, in press invited chapter for Mineral Physics and Seismic Tomography, eds. S. Karato et al., AGU -- monograph.
Thomas, Ch, J-M. Kendall and M. W. Weber, 2000, The anomaly in D" beneath northern Siberia detected from different azimuths, in prep. for Phys. Earth Planet Int..
Kendall, J-M. and P. G. Silver, 1998, Investigating causes of D" anisotropy, invited paper in The Core Mantle Boundary Region, eds. Gurnis, Wysession, Knittle and Buffet , Geodynamics 28, AGU -- monograph, 97-118.
Kendall, J-M. and P. G. Silver, 1996, Constraints from seismic anisotropy on the nature of the lowermost mantle, Nature, 381, 409-412.
Kendall, J-M. and C. Nangini, 1996, D" beneath the Caribbean and Central America, Geophys. Res. Lett., 23, 399-402.
Kendall, J-M. and P. M. Shearer, 1995, Structure of the lowermost mantle beneath the southwest Pacific, southeastern Asia and Australasia, Phys. Earth Planet. Int., 92, 85-98.
Kendall, J-M. and P. M. Shearer, 1994, Lateral variations in D" thickness from long-period shear wave data, J. Geophy. Res., 99, 11575-11590.