Results from High Resolution Global Tomography

Recently, considerable progress has been made in the field of global travel-time tomography. Global models by Van der Hilst et al. (Nature, 1997) and Zhou (JGR, 1996) now show upper mantle small-scale structure that approaches the detail imaged in regional mantle studies. In the lower mantle these models show clear images that can be interpreted in terms of subducted (Mesozoic) oceanic basins.
We take the improvement one step further, mapping upper mantle structure with detail (60-100 km) directly comparable with that obtained in high resolution regional mantle studies. This is achieved through the implementation of a model parametrization with cells of variable sizes, which (contrary to Zhou) reduces the number of unknowns with 90%. The imaged lower mantle structure resembles that of the aforementioned studies closely. The new model makes it possible to study areas that have never been imaged with such detail before and compare areas in which similar processes take place and put them in a global perspective.
Some of the features of the model include: subduction of slab either directly into the lower mantle or (currently) flattening in the transition zone, indicating that the lower mantle resists, but may not prevent, whole mantle convection; remnants of past subduction in the lower mantle (Tethys, Pacific and Farallon plates), sometimes connected to heterogeneity at the CMB; deep roots of cratons underneath, e.g. Australia and Scandinavia; rift zones in Africa and hot upwellings under Iceland. In general, we find high amplitude (up to 5%) and small-scale heterogeneity in the upper mantle that gradually turns into longer wavelength and lower amplitude (but up to 1.5%) anomalies in the mid-lower mantle (around 2000 km), from where both amplitude and scale increase to the CMB.