Continental flood basalts

As we have seen in Chapter 4, the location, extent and volumes of erupted rocks and the age and duration of emplacement have been widely studied (Coffin and Eldholm, 1992, and the many authors cited by these collators). The major continental flood basalts are known to have a volume which may range from about 0. 2*106 km3 to well in excess of 106 km3.

Although eruptive activity, at any one site, may extend over a period of up to 10 Ma, it is generally held that the majority of these various flood basalts were emplaced in a period of 1.0 Ma, or even less (Gallett et al., 1989; Baksi, 1994; Courtillot, 1990). Thus, the rate at which melt is brought to the surface in CFBs and OFBs is as much as 3 to 4 orders of magnitude faster than the rate of supply of melt at a hotspot/plume.

The ready acceptance of the Chicxulub structure as being the result of an impact was predicated on the finding of small particles of iridium and stress-metamorphosed minerals in the sediments, of near K/T age, around the world. Such material is prime evidence pointing to the existence of meteoritic impact. However, the distribution of such material near the K/T boundary is so extensive that, it has been argued, there is almost certainly more than one source of this exotic material.

A prima facie case can be put that the Deccan Traps (and possibly certain other features that we shall discuss in the next chapter) which developed about 65-67 Ma ago, are the result of impacts; and one can reasonably conclude that rapid changes in speed and/or direction of movement of plates can be attributed to major impacts.

The major continental flood basalts that have developed in the last 250 Ma are listed in Table 6.1. In addition to the name, the location and approximate age of the main continental flood basalts (CFB) which have developed within the past 250 Ma, are also listed in Table 6.2.

As pointed out in Chapter 4, if a continent passes over a major 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 have left a readily detectable heat-signature. Anderson et al. (1992) state that no such signature exists for six of the eight listed CFBs. (These authors did not include the Antarctic features in their study.) Their conclusion is based on their study of high-resolution, upper-mantle tomographic models in terms of plate tectonics, hotspot and plume theories. They state that there is no evidence for plume head or lithospheric thinning under any of the major continental flood basalts listed in Table 6.2.

Anderson et al. further state that tomographic results contradict the premises of all currently popular plume and flood basalt scenarios. Moreover, the fact that no plume signature

Table 6.2 Location and age of continental flood basalts.



Age (Ma)

Columbia River

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