As we have seen in Chapter 5, the development of a crater and its attendant structures is a complex affair. The surface, or near-surface, features are reasonably well documented. Movements set at some depth below the crater can only be described in general terms. Moreover, precise definition of the limits of such movements and the time necessary for them to reach completion have not yet been definitively ascertained.
Let us now attempt to indicate these limits in general terms, starting with the development of the transient crater (Figure 6.17). Such craters have a simple cross-sectional geometry. The profile approximates to a parabola, and the ratio of the depth to diameter of the crater is approximately 1-3.
We have noted that continental flood basalts are much smaller in volume than the largest oceanic plateau basalts. This relationship, coupled with the tremendous quantities of melting associated with the development of the larger OPB, may present traditional theorists with problems. We shall, therefore, begin with a discussion of the effect of impacts leading to the development of the largest of oceanic plateau basalts, namely, the Ontong-Java feature.
The area of this massive eruption is approximately one third of the area of the coterminus USA, and its age was originally estimated to be approximately 119 Ma (Coffin and Eldholm, 1992), while much of the surrounding ocean floor dates from 130-140 Ma. More recently, Gladczenko et al. (1997) updated the age of this PB to 122.4+/-0.8 Ma.
Because the Ontong-Java PB is only a little younger than the surrounding ocean floor, we can infer that this feature developed in and extruded onto quite young and hence relatively thin oceanic lithosphere.
Parsons and Sclater (1977) modelled the N Pacific Plate and concluded (see Figure 2.5) that for the period 20-30 Ma, the 500°C contour had a depth from 15-20 km. As the 500°C contour will approximate to the lower limit of the strong part of the oceanic lithosphere, the strong layer will have a thickness of about 17.5 km, on average, while the junction between the oceanic lithosphere and the asthenosphere for the period 20-30 Ma will be at about 45-55 km. The oceanic lithosphere, below 17.5 km, rapidly becomes progressively weaker and approaches zero strength (at normal slow geological strain-rates) near the LVZ.
Let us assume that these thicknesses of strong lithosphere are typical of the young oceanic lithosphere, into which a large comet strikes and generates a transient crater 100 km deep and 300 km in diameter (Figure 6.18a).
Upon impact, the lithosphere around GZ instantaneously (or at least in a matter of tens of seconds) goes to zero thickness (i.e. P, in the McKenzie model, is infinite). Here we have the situation that McKenzie (1978) has sought, but not in the manner he envisaged. This process is not attained by stretching, but by b a o
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