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Figure 7.16 Envisaged mode of breakthrough of Antarctic/Pacific plate into the S Atlantic, as the result of a major impact, with map of inferred impact showing stages in the development of the orocline and the movement of wedges into the crater area and the initiation of the Scotia Arc.

erected which required a certain element to exist. When that element was in fact found to exist, we were suitably encouraged.

It was shown in Chapter 5 that these thrusts crop out at a distance from ground zero (GZ) at a radius of about twice that of the crater-rim, depending upon the velocity of the impacting body. The radius of curvature of the Scotia Trench is approximately 300 km, so that, using the Snowball 500 ton explosive experiment as a guide, the impact crater could have had a diameter of 300 km.

The approximate location of the impact is, we suggest, as shown in Figure 7.14 and in greater detail in Figure 7.16. It is clear from this diagram that subduction took place using only one half of the thrust system that developed. Hence, the question arises as to why the subduction took place on the eastern part of the thrust array, rather than the west (or indeed any other direction).

The section through the suggested impact structure, shown in Figure 7.15, is parallel to the direction of S American plate motion. If we assume that this plate moved at a rate of about 2 cm/yr, then the rocks cut by the thrusts at the western end of the section are about 15 Ma older than the rocks at the eastern end. This means that they are older, colder and more dense, and therefore more likely to be subducted. However, this tendency will be a very small one, and could easily be overcome by a stronger, contrary influence.

It is further maintained that the position of impact was adjacent to S American and Graham Land landmasses, and that this point of impact was vital in determining the direction of subduction. If we assume that the impact GZ was in the Atlantic Ocean, about 200- 400 km to the east of the southern tip of S America, then GZ would have been about 2000 km from the S Atlantic spreading-ridge, but very close to S America. The elastic strain, which develops in the strong layer of this oceanic lithosphere of the S American plate as the result of ridge push, may easily approach an average value of about 0.5 per cent (see Chapters 2 and 3). Hence, the elastic strain stored in the oceanic plate to the east of GZ could be released shortly after impact, and a down-going slab, with an initial increment of movement of 10-30 km, could be initiated on the eastern side of the impact structure.

However, if, as indicated in Figures 7.14 and 7.16, the impact GZ is situated at about 350 km east from the tip of S America, the amount of strain release within the S. American plate to the west of the structure would result in under-thrusting of only a 100 m, or so. Thus, the side of the impact structures on which the down-going slab is initiated is, at first, controlled wholly by the amount of elastic strain energy stored in the oceanic element of the plate.

Let us now consider the changes in the balance of forces which take place as the result of the impact. In this area, as we have seen in Chapter 5, the impact causes massive changes in the rock properties in the crater area (c, in Figure 7.16). The changes near GZ involved considerable melting of the target rock, and certainly resulted in considerable minor 'Grady-Kipp' fracturing (Figure 5.30).

The changes in the material properties in the rocks in the outer area, beyond the crater rim, also involved a degree of melting of the LVZ. In addition, this outer area was greatly weakened by fractures, namely the arcuate thrusts and also the radiating vertical tensile fractures, which probably have extended outwards for hundreds of kilometres beyond the crater rim and may even have extended into (and possibly even through) the 'straight' continental area of S America. As we saw in Chapter 5, the rock-mass in the central area (c) is in part excavated, and what remains is, initially, relatively weak and lends little support to the 'outer area'. Consequently, inward migration by shear movements on the major, vertical, radiating tensile fractures can readily develop.

The elastic strain to the east of the structure is released by the initiation of a down-going slab. The de-stressing of the hachured area (Figure 7.14) to the west of the impact causes little deformation of the impact structure. The de-stressing, on the Atlantic side, however, leaves the force P of the Pacific/Antarctic plate almost completely unopposed. Breakthrough of a portion of the Pacific/Antarctic plate into the Atlantic element of the S American plate is, initially, unopposed and inevitable.

It may be thought that such a breakthrough of the Pacific/Antarctic plate would continue the down-going trend of this plate beneath the S Atlantic. However, this caveat applies to the slow processes normally associated with general plate motions. With the impact model proposed, we are dealing with far quicker rates of lithospheric movement than are usually associated with plate tectonics. It is suggested that the models described by Price and Audley-Charles (1983, 1987) are applicable here. They showed how gravity-glide from the ridge caused a buckle geometry of the lithosphere at the trench (Figure 7.17). The strains induced in the buckle can, in such circumstances, greatly exceed the elastic limit of the lithosphere (see

Figure 7.17 Break-up and separation from the horizontal plate of a down-going slab along a through-going fracture (after Price and Audley-Charles).

Chapter 3). Normal faults will develop at the upper levels of the anticlinal lithosphere and thrusts develop on its lower side, beneath the neutral surface. At relatively fast strain-rates, one or more such thrusts and normal faults would tend to extend, and occasionally join up, to form a through-going fracture, which would facilitate over-riding of the down-going portion of the slab. In the over-riding element of the Pacific/ Antarctic, slab-accelerated motion results from the loss of resistance caused by the separation along this thrust plane. In turn, this sudden release would enable stored strain-energy to give rise to a relatively rapid (unbending) strain-rate in the curved lithosphere above the thrust plane of the down-going slab. This promotes break-away along an upwardly convex surface of the through-going fracture.

This breakthrough results in the relatively rapid uplift of the upper portion of the hitherto down-going slab. Such recovery, and detachment from the down-going slab, within a period of about 104-5 years, would return the Pacific/Antarctic ocean floor (in this locality, opposite the impact site) to the horizontal.

This part of the lithosphere would have a ski-like, upward-curving front, which would remove any tendency for further down-warping of the Pacific element of the Pacific/Antarctic plate as it broke through to the Atlantic.

The uplift and straightening of this part of the lithosphere at the front end of the Pacific/ Antarctic plate would increase the horizontal extent of that portion of the plate, opposite the impact de-stressed released zone, by about 10 km. Hence, this extension, together with the elastic recovery of the compressive strains in that segment of the Pacific/Antarctic floor element, would result in a very rapid initial incursion of this element of the Pacific plate into the landmass of S America and part of the Antarctic Peninsula by 20-50 km. This incursion could have been exacerbated by movement along the radial tension fractures induced by the impact.

This eastward movement of the plate in the Pacific would cause major fracturing of the continental rocks, thereby forming minor elements from which the islands, which now flank the current Scotia plate, are derived. The mode of breakthrough which is envisaged is indicated in Figure 7.16. The relatively weak central area of the impact (c) is deformed by wedges of outer and stronger rock, which are defined by the radial tension fractures (RTF) that are a feature of the Snowball explosion experiment.

The breakthrough from the Pacific side into the Atlantic would probably have taken place within 100,000 years of the impact event (i.e. almost immediately in terms of normal plate motions), as the result of elastic rebound of the Pacific/Antarctic ocean floor and enhanced by the isostatic recovery of that plate at the breakthrough area.

Figure 7.18 Block diagram representing subduction of S Atlantic lithosphere beneath the Scotia Plate.

The forces at the appropriate ridges would gradually build up the compressive stresses in the de-stressed portions of the floors of both the Pacific/Antarctic and S Atlantic. This requires regeneration of the original elastic strains induced in the oceanic plates by ridge-slope gravity-glide. The rate of stress build-up would initially be relatively fast, because the horizontal stresses would be reduced to that which can be induced by gravitational loading. Continued build-up of the stresses would become progressively slower, as the elastic stresses induced by gravity-glide became larger and reached about 0.5 per cent. The time required to 'replace' the original elastic strains will depend upon the length of the oceanic plate from ridge to impact, and upon the average rate of plate movement. Thereafter, there would be a slower but inexorable incursion of the Pacific/Antarctic oceanic lithosphere into the Atlantic, at the rate of the relative motions of the two opposing plates. This incursion would decelerate and eventually cease, when the resistive elements along the transform faults, which border the Antarctic Plate incursion, approached equality with the driving forces of the East Pacific Ridge.

Figure 7.18 Block diagram representing subduction of S Atlantic lithosphere beneath the Scotia Plate.

The forces at the appropriate ridges would gradually build up the compressive stresses in the de-stressed portions of the floors of both the Pacific/Antarctic and S Atlantic. This requires regeneration of the original elastic strains induced in the oceanic plates by ridge-slope gravity-glide. The rate of stress build-up would initially be relatively fast, because the horizontal stresses would be reduced to that which can be induced by gravitational loading. Continued build-up of the stresses would become progressively slower, as the elastic stresses induced by gravity-glide became larger and reached about 0.5 per cent. The time required to 'replace' the original elastic strains will depend upon the length of the oceanic plate from ridge to impact, and upon the average rate of plate movement. Thereafter, there would be a slower but inexorable incursion of the Pacific/Antarctic oceanic lithosphere into the Atlantic, at the rate of the relative motions of the two opposing plates. This incursion would decelerate and eventually cease, when the resistive elements along the transform faults, which border the Antarctic Plate incursion, approached equality with the driving forces of the East Pacific Ridge.

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