Dynamic Earth: Crustal and mantle heterogeneity

The dynamic processes within the Earth leave their record in geophysical and geochemical variation about the general stratification with depth. A snapshot of current structure is provided by geophysical evidence, whereas geochemical information provides a perspective over the age of the Earth. The combination of information on the distribution of heterogeneity from geophysical and geochemical sources provides enhanced insight into likely geodynamic processes. A variety of techniques can be used to examine crustal structure, but the major source of information on seismic heterogeneity within the Earth comes from tomographic studies, exploiting surface waves for the lithosphere and body waves for the bulk Earth. A powerful tool for examining the character of mantle heterogeneity is the comparison of images of bulk-sound and shear-wave speed extracted in a single inversion, since this isolates the dependencies on the elastic moduli. Such studies are particularly effective when a common path coverage is achieved for P and S as, for example, when common source and receiver pairs are extracted for arrival times of the phases. The relative behaviour of the bulk-sound and shear-wave speeds allows the definition of heterogeneity regimes. For subduction zones, a large part of the imaged structure comes from S-wave speed variations. The narrow segments of fast wave speeds in the lower mantle, in the depth range 900 - 1500 km, are dominated by S variations, with very little bulk-sound contribution, so images of P-wave speed are controlled by shear-wave variability. Deep in the mantle, there are many features with high seismic-wave speed without an obvious association with subduction in the last 100 million years, which suggests long-lived preservation of components of the geodynamic cycle. The base of the Earth's mantle is a complex zone with widespread indications of heterogeneity on many scales, discontinuities of variable character, and shear-wave anisotropy. Discordance between P- and S-wave speed anomalies suggests the presence of chemical heterogeneity rather than just the effect of temperature.