Abstract:
We have investigated the relationship between the lateral seismic structure in the mantle and the geological features associated with major mantle upwellings, the oceanic ridge system and hotspots. Using the technique of correlation analysis we have identified that these features extend quite deeply into the mantle to a depth of around 1300 km. This finding is surprising for the ridges because it indicates that the style of upwelling is rather active and is somewhat two-dimensional in character. The confidence level of the correlation between ridges and slow seismic regions is maintained at a level of greater than 90% to a depth of 1300 km. This is followed by a sharp decrease to 50% at 1700 km. A similar correlation pattern is found for the hotspots. In both cases the correlation maxima lie close to a depth of 1000 km, where remnants of old slabs have recently been revealed by correlation analysis. These findings suggest that dynamical processes at a depth of around 1000 km can be linked with surface tectonic activity. This correlation disappears at a depth of around 1500-1700 km. Certain features of the surface tectonic pattern can again be recognized around a depth of 2000 km and in the deepest mantle where major hotspot plumes and the remnant old lithosphere are the dominant features. There may be some dynamical relationship between the ridge system and the two megaplumes. We have proposed a physical mechanism based on viscous heating within megaplumes to explain the two peaks in the correlation between the hotspots and the seismic heterogeneities.