Tomographic evidence

If a continent passes over a hotspot it will be heated to an abnormally high temperature in the lower regions of the continental lithosphere. Consequently, it is reasonable to suppose that a large and relatively recent thermal event must leave a readily detectable heat-signature. However, as regards the Deccan Traps, Anderson et al. (1992) state that no such signature exists. This conclusion is based on their study of highresolution upper-mantle tomographic models in terms of plate tectonics, hotspot and plume theories. They state that if this body of plateau basalts were the result of a major plume, one would expect to detect a large area of thin lithosphere and very slow seismic velocities under most of India. However, the tomography shows fast seismic velocities under the Indian subcontinent, down to a depth that includes the full thickness of the lithosphere of India, which indicates a thick, cold lithosphere and a cold asthenosphere. Therefore, they state, there is no evidence for plume head or lithospheric thinning under this major flood basalt.

Let us now extend the remarks by Anderson et al. (1992) to other continental flood basalts that have been attributed by White and McKenzie to be the result of plumes.

As regards the Parana flood basalts (135 Ma), Anderson et al. comment that 'the time since the conjectured plume head.. .event.. .is too short for the lower lithosphere or (upper) asthenosphere to cool off substantially... The regions of S America and Africa which should have been affected by the plume head do not differ from the surrounding continental areas. The seismic evidence is therefore not favourable to the plume-head, lithospheric stretching .hypothesis.'

Both the Karoo (178 Ma) and the Siberian (248 Ma) events have also been attributed by White and McKenzie to major plume heads. Both these provinces are sufficiently old to have experienced some cooling of the affected lithosphere. However, the tomographic evidence shows that these regions are no different from other areas in the vicinity, which were not affected by the hypothetical plume head. So once again, for these two older events, Anderson et al. infer that there is no evidence to support the plume mechanism.

In their discussion, Anderson et al. state that tomographic results contradict the premises of all currently popular plume and flood basalt scenarios. Thus, we suggest, the fact that no plume signature can be found related to the plateau basalts to which White and McKenzie applied their model, renders their application to the several examples of plateau basalt development more than a little suspect.

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