Impact size and minor subduction arcs

It will be inferred from the arguments presented in this and the previous chapter that CFBs and OPBs have been studied, among other things, with the object of determining their date of emplacement. It has been established that these igneous features are usually emplaced within about a million years, though minor activity may continue for a further ten or so million years. Thus, the time of initiation and main development of these large basaltic features are established within the limits of error of the dating technique used.

When the sudden changes in track, whether they be in speed and or direction, have been established, it is often a relatively simple task to correlate the specific track change with a specific OFB. The magnitude of the effect of the inferred impact event can, in part, be estimated from the changes that the tracks reveal. However, we can also infer that the magnitude of change in these tracks is influenced and constrained by the type and size of plate, and also, possibly, the position of the impact in relation to the plate boundaries. As we have seen, prodigious changes may take place on one side of an event, while on the other, virtually nothing is recorded (e.g. the considerable change of track in S America compared with the lack of change of track of Africa following the 135 Ma Parana event). Hence, although we can correlate the time of a track change with the initiation of emplacement of a CFB, the size of the impact event can only be inferred from the volume of extruded rock that results, coupled with an assessment of the thickness of the lithosphere in which the impact took place.

As regards arcuate subduction features, the reverse situation exists. These features are mainly 'on-going' structures and have attracted relatively little investigation regarding their initiation. Dating of the volcanic activity associated with the subduction usually tells one little, if anything, of the date of initiation of the subduction event. The Amirante Arc appears to be no longer active, so is, of course, an exception to this statement.

The track changes, together with other evidence, are sufficiently specific to identify the time of impact of the Antilles, Amirante, Scotia and Banda features. However, the Mariana arcuate feature is somewhat ambiguous as regards the specific track change to which it can be attributed, but we conclude that it was probably the event that occurred at 175 Ma.

We have noted that arcuate thrusts developed as the result of the Snowball 500 ton TNT explosive experiments, and these were shown to crop out at the surface as far as twice the crater radius from GZ. Therefore, it is suggested that such thrusts brought about by a major oceanic impact could result in the development of an arcuate subduction zone. There is, however, the problem regarding the size of the impacting body that could possibly give rise to such arcuate features.

The radius of curvature of the Mariana Trench is about 600 km. Hence, if this natural feature also exhibited a relationship between crater diameter and outcrop diameter of the most distant thrust from GZ comparable with the 'Snowball' explosive experiment, it is necessary to postulate that the impacting body gave rise to a crater radius of about 300 km. The Lesser Antilles Arc in the Caribbean requires a crater of almost comparable size. We have also noted that the impact that gave rise to the ParaƱa could well have been associated with a crater of comparable diameter.

It can reasonably be postulated that one such 500 km diameter crater could possibly have developed in the latter half of the Phanerozoic; but to suggest that there are three such events may stretch the credulity of even the most open-minded reader. Let us, therefore, consider mechanisms of impact a little further.

The extent to which thrusts develop from GZ will be determined by the rate of attenuation of the pulse stress as it propagates outward from GZ. The outermost thrust occurs where the magnitude of the pulse stress falls to a value of differential stress, below which it is no longer able to cause the rock to fail in shear. Let us, therefore, consider the stress conditions which can give rise to arcuate thrust fault development.

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