Recent global travel time tomography studies by Zhou [1996] and Van der Hilst et al. [1997] have been performed with cell parameterizations on the order of those frequently used in regional tomography studies (i.e. with cell sizes of 1-2 degrees). These new global models constitute a considerable improvement of previous results that were obtained with rather coarse parameterizations (5 degree cells). The inferred structures are however of larger scale than is usually obtained in regional models and it is not clear where and if individual cells are actually resolved.
This study aims at resolving lateral heterogeneity on a smallest scale of 0.6 degrees in the upper mantle and of approximately 1.2-3 degrees in the lower mantle. This allows for the adequate mapping of expected small-scale structures induced by, e.g. lithosphere subduction, hotspots, and mid-ocean ridge upwellings. There are three major contributions that allow for this advancement. First, we employ an irregular grid of non-overlapping cells adapted to the heterogeneous sampling of the Earth's mantle by seismic waves. Second, we exploit a totally reprocessed version of the global data set of the International Seismological Center. Finally, we combine all employed data (P , pP and pwP phases) in nearly 5 million ray bundles with a limited spatial extent such that averaging over large mantle volumes is prevented while the signal-to-noise ratio is improved.
In the approximate solution of the huge inverse problem we obtain a variance reduction of 57.1%. Synthetic sensitivity tests indicate horizontal resolution on the scale of the smallest cells (0.6-1.2 degrees) in the shallow parts of subduction zones decreasing to approximately 2-3 degrees resolution in well-sampled regions in the lower mantle. Vertical resolution can be worse (up to several hundreds of km) in subduction zones with rays preferentially travelling down dip. Important features of the solution are: 100-200 km thin high velocity slabs beneath all major subduction zones, sometimes flattening in the transition zone, sometimes directly penetrating into the lower mantle; large high velocity anomalies in the lower mantle that have been attributed by Van der Hilst et al. [1997] and Grand et al. [1997] to subduction of the Tethys ocean and the Farallon plate; low velocity plumes continuing across the 660 km discontinuity to hotspots at the surface under Iceland, East Africa, the Canaries, Yellowstone, and the Society Islands. Our findings indicate that the 660 km boundary may resist but not prevent (present day) large-scale mass transfer from upper to lower mantle or vice versa.