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Source: Reproduced from Coffin and Eldholm (1992), by permission of the Geological Society Publishing House. Notes a At 15-30 per cent partial melting.

b Assumes crustal thickness from seismic refraction experiments. c Minimum and maximum volumes assume off- and on-ridge emplacement, resp. d Timescale of Harland et al. (1982). e Timescale of Harland et al. (1990).

Large igneous Area (106 km2) Volume (106 km3) Age range (Ma) Emplacement Spherical province rate (km3 yr-1) diameter" (km)

f First range, minimum volume (pre-existing crust, maximum and minimum age range; second renge, maximum volume (no pre-existing crust), maximum and minimum age range. g Assumes crustal thickness from satellite altimetry data. h Extrusive component.

j Assuming 0.5-m.y. eruption period for two-thirds of the basalt. j For 80 per cent of the lavas.

Figure 4.1 Map of large igneous provinces (after Coffin and Eldholm, 1992).

Figure 4.1 Map of large igneous provinces (after Coffin and Eldholm, 1992).

Of the eruptive features listed and described by Coffin and Eldeholm, we shall first consider those associated with hotspots. We present this group of features first, because their characteristics are reasonably well defined, and the mechanism by which these relatively small to medium-sized bodies are emplaced is thought to be reasonably well understood in terms of plumes of relatively hot mantle material, which may originate at the core/mantle boundary, or possibly in the vicinity of the transition zone. The amount of melt that can be produced by such plumes has been assessed by mathematical modelling. The volume of melt that such plumes can deliver at the base of the lithosphere is, however, somewhat limited and is only sufficient to give rise to single islands, seamounts, or chains of such bodies.

In the second part of this chapter we deal with the emplacement of continental flood basalts and oceanic plateau basalts (CFBs and OPBs). The mechanisms which are held to be responsible for the emplacement of these large basaltic provinces are contentious, and hotly debated. Initially, the concept of an extremely active plume, i.e. a super-plume, was invoked. Alternatively, some other possibly additional mechanism was thought to be required. This second mechanism, it has been suggested, relates to lithospheric thinning, brought about by stretching. However, a high degree of stretching is required to give rise to the volume of melt-rock at the base of the lithosphere that is necessary to generate major CFBs and OFBs. We argue that the degree of stretching required far exceeds what can feasibly be expected.

It has been further suggested in the literature that plumes, coupled with stretching of continental lithosphere, can give rise to the splitting of super-continents. This leads us to discuss how stretching of continental lithosphere may be induced by strike-slip faulting, and how some rift valleys form. We indicate how a relatively small degree of stretching may give rise to some, but possibly not all, rift valleys. Moreover, we point out that the generation of such faults does not give rise to a general tensile stress in the continental crust.

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