Seismological insights into heterogeneity patterns in the mantle

Both geophysical and geochemical results point to pervasive 3-D heterogeneity in the Earth's mantle. Geophysical evidence presents a snapshot of current structure, whereas geochemical data contain important information on age. A major source of information on heterogeneity within the Earth comes from seismic tomography, particularly when both P and S wave data can be exploited. A powerful tool for examining the character of heterogeneity comes from the comparison of images of bulk-sound and shear wavespeed extracted in a single inversion, since this isolates the dependencies on the elastic moduli. Such studies are particularly effective when common path coverage is achieved for P and S as, e.g., when common source and receiver pairs are extracted for arrival times of the phases. The relative behavior of bulk-sound and shear wavespeed can provide a useful guide to the definition of heterogeneity regimes. For subduction zones a large part of the tomographic signal comes from S wavespeed variations, but in the upper mantle and transition zone there can be significant bulk-sound speed contributions for younger slabs (<85 Ma), and in stagnant slabs associated with slab roll-back. The narrow segments of fast wavespeeds in the depth range 900-1500 km in the lower mantle are dominated by S variations, with very little bulk-sound contribution, so P images are controlled by shear. Deep in the mantle there are many fast features without obvious association with subduction in the last 100 Ma, which suggests long-lived preservation of components of the geodynamic cycle. Changes in the patterns of heterogeneity occur near 1200 km and 2000 km depth in the lower mantle and indicate the complexity of processes occurring in the current Earth.