Hypothetical impact model

As we have noted, plate movements tend to be reasonably constant for periods measured in millions of years. The relatively even, magnetic striping of the ocean floors is proof of such periods of near-constant speed. To explain how acceleration, deceleration, or sudden change in direction of motion of a plate can develop, we suggest it is necessary to have recourse to the mechanics of the early phase deformation induced by major impacts, discussed in the previous chapter. Consider a simple hypothetical plate as seen, in plan, in Figure 6.16a, in which an impact (IMP) has given rise to a high magnitude stress-wave at ground zero (GZ) which generates a simple crater, which we assume to be 300 km in diameter. The extremely high-magnitude of this initial compressive stress-pulse decays away from GZ and is reduced to a magnitude of 20 kb at a distance of about 10-30 times the radius of the crater (see Figure 5.31). This attenuating stress-wave, induced by the impact, gives rise to a degree of melting and thickening of the original LVZ. As a result, a circle of newly enhanced, low basal resistance at the LVZ layer extends over a very large area. This will result in a dramatic change in the balance of the driving and resistive forces acting in and adjacent to the plate by suddenly introducing a new, basal, low resistive-element over an area of perhaps several millions of square kilometres. It is this reduction of basal resistance of the plate that gives rise to a dramatic change in

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