AXISYMMETRY OF MANTLE ASPHERICAL STRUCTURES

Recent seismic tomographic models have revealed broad, low velocity an omalies in the lower mantle beneath Africa and the central Pacific tha t correspond to the poles of the surface geotectonic reference network . We test the hypothesis that the tomographic models are axisymmetrica l with respect to axes passing near the geotectonic poles. The symmetr y is apparent at the bottom of the lower mantle as well as in the tran sition zone in the upper mantle. This supports the idea of massive flu shing of cold upper mantle material into the bottom part of the lower mantle and points to a stabilizing role of the lower mantle megaplumes on whole mantle processes.