North Atlantic. White and McKenzie (1989) site the plume beneath Kangerlussuaq, E Greenland. The extent of the plume-head and inferred limits of updoming are determined by the position of extrusive volcanic rock (shaded black in Figure 4.7) and the extent of early Tertiary igneous activity in the region (diagonal hatching).
Figure 4.7 Greenland/Iceland/Faroes Ridge.
White and McKenzie have suggested that, except for the Antarctic CFBs, which they did not discuss, the CFBs listed in Table 4.3 are the result of plumes. They applied the concepts of plumes coupled with lithospheric thinning, in an attempt to explain the development of specific continental flood basalts. They also suggest that the splitting of a super-continent can result from the topographical elevation of the continental lithosphere attained as the result of the plume. Of the CFBs discussed by them, it is the application of plume development to the Deccan Traps which is most well known and which we shall consider first. The Deccan Traps is, perhaps, the best studied example of continental flood basalts to have occurred in the last 250 Ma. The eruption occurred approximately 65 Ma ago. The basalts attain a maximum thickness of 3.5 km and extend over an area equal to one sixth of the sub-continent. The total amount of extruded material is somewhat in excess of 106 km3 and was emplaced in a period of, possibly, less than 106 years.
This event, White and McKenzie suggest, can be attributed to the development of a major plume below India. Indeed, they further claim that this plume was so important, that not only did it give rise to the Deccan Traps but also to a contiguous massive out-pouring of basalts which currently form the Seychelles and Mascarene Banks. (We will comment upon this conclusion in Chapter 6.)
These authors did not consider the Antarctic bodies in their paper, but suggested that the other CFBs listed above could be attributed to the arrival beneath the lithosphere of a powerful plume. They noted that, with the exception of the Siberian and Columbia River flood basalts, the other bodies, they consider, were all associated with continental break-up.
Just how a hypothetical plume gave rise to the opening of the N Atlantic is not very clear. White and McKenzie indicate that the elevation of the central part of the dome, which results from the assumed upwelling plume, relative to the limits of the plume-head, permits a gravitational glide potential, which can cause rupture in a favoured orientation which could eventually give rise to ridge spreading. We have already indicated, in Chapter 3, that the magnitude of the stresses engendered by such sliding is insufficient to give rise to the break-up of a continent.
However, when dealing with the North Atlantic Igneous Province, Holm et al. (1992) attest that the inferred plume caused no thermal doming of the lithosphere. Consequently, rifting in E Greenland was not generated by a gravitational slide off a plume-initiated thermal bulge. Indeed, Skogseid et al. (1992) argue that the spatial correlation between tectonic activity, magmatism and subsidence permits one to infer that rifting occurred before the assumed plume reached the lithosphere. Possibly with these points in mind, Gill et al. (1992) mention that the plume which is held to have given rise to the Greenland-Icelandic features was a plume with a 'ridge-like configuration'. There is the further problem that the influence of the hypothetical plume appears to become apparent so suddenly, that it precludes a prelude of volcanic activity. Thus, the plume concept appears to be at variance with the geological evidence adduced by several geologists. However, Iceland, which now sits astride the N Atlantic spreading-ridge, marks the site of a hotspot that has been in existence for at least 60 Ma, so that the situation is not clear-cut. Let us therefore turn to the tomographic evidence.
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