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Figure 6.27 (a1) Shows the early stage of the pushing aside of the dyke walls to generate the initiation of a spreading-ridge. The width of the initial dyke is determined by the ease with which the continental lithosphere can be pushed back. This is largely determined by the extent to which the 'viscosity' of the LVZ has been reduced by the impact. (a2) It will be seen that the section of aa and bb, the low viscosity LVZ about the impact site are extensive, so that the dyke will be wide. (b) Shows three stages (1, 2 and 3) in the early development of the spreading-ridge. The generation of the sloping lithospheric/ asthenospheric interface is in part the result of high lateral asthenospheric pressure on the weaker layers of the lithosphere and the addition of rising asthenosphere diapirs (D) giving rise to under-plating (UP). (c) By the time the base of the asthenosphere has parted to about 200 km, the athenosphere will be comparable to the early conditions shown here. (d) As an approximation this diagram indicates the horizontal stress that can be generated as the result of gravity-glide, to which must be added the influence of the asthenospheric pressure (Pa).

Figure 6.27 (a1) Shows the early stage of the pushing aside of the dyke walls to generate the initiation of a spreading-ridge. The width of the initial dyke is determined by the ease with which the continental lithosphere can be pushed back. This is largely determined by the extent to which the 'viscosity' of the LVZ has been reduced by the impact. (a2) It will be seen that the section of aa and bb, the low viscosity LVZ about the impact site are extensive, so that the dyke will be wide. (b) Shows three stages (1, 2 and 3) in the early development of the spreading-ridge. The generation of the sloping lithospheric/ asthenospheric interface is in part the result of high lateral asthenospheric pressure on the weaker layers of the lithosphere and the addition of rising asthenosphere diapirs (D) giving rise to under-plating (UP). (c) By the time the base of the asthenosphere has parted to about 200 km, the athenosphere will be comparable to the early conditions shown here. (d) As an approximation this diagram indicates the horizontal stress that can be generated as the result of gravity-glide, to which must be added the influence of the asthenospheric pressure (Pa).

and c in Figure 6.28a) in the direction of plate motion, approximately normal to the embryonic spreading-ridge. Hence, although the initiation of the 135 Ma phase of break-up of Gondwana was initiated by an impact deep within that super-continent, the opening of the S Atlantic progressed from the ESE to the WNW.

By this same mechanism, the portion of S America cut off by the new spreading-ridge would be driven to the SE, where it eventually made contact with Antarctica, to become the Antarctic Peninsula. However, such a collision between the two continental units requires one to assume that there was either a subduction zone between them, or else they made contact along a major strike-slip fault. This southerly spreading-ridge has been represented as a straight line. However, its development may well have been influenced by preexisting weakness in Gondwana.

As may be inferred from the track shown in Figure 6.28b, this development and widening of the new spreading-ridge must have occurred within 1.1 Ma (from 135.1 to 134 Ma) so that from 134 to 133 Ma, the

Figure 6.28 (a) Showing how the S Atlantic, although initiated to the north by the Paraña impact, 'opened' from the south. (b) Detail of track from 138-133 Ma, showing the sharp change in direction and rate of movement of the developing S American Plate: the initial amazing rate of movement from 135.1 to 134 Ma and the rapid slowing and change of direction from 134 to 133 Ma.

energy of the impact 'ran out of stream'. Thereafter the pace of spreading slowed significantly, with a significant change in the direction of movement of the S American plate.

A comparable major fracture must, almost certainly, have been initiated, running NE to NNE from the Paraña impact site. However, the track of this fracture is more clearly defined by the junction of the coastlines between S America and Africa (Figure 6.29).

This north-easterly major fracture, which would also be struggling to become an embryonic spreading-ridge, did not have the energy to penetrate far into the massive N African area of Gondwana. A simplified diagram of the mechanistic situation is indicated by the insert diagram in Figure 6.29. This represents a (horizontal) cantilever, which, along its 3000 km length, is acted upon by about 8 kb of magmatic pressure, which, of course, generates a tremendous force on the S American Plate. The bending of the (horizontal) cantilever generates tensile stresses along a vertical plane at right angles to the 3000 km long (NE-SW) fracture. These tensile stresses eventually become sufficiently large to cause vertical failure of the lithosphere, more or less at right angles to the embryonic spreading-ridge. Once this vertical fracture develops it becomes infilled with melt from below. (Indeed, it is probable that the St Helena hotspot developed at this time.) S America was then able to detach itself along this and other similarly formed fractures by strike-slip movement.

This conceptual model is supported by simple mechanical analysis and experimental experience. Hence, we suggest that the proposed model provides a viable explanation for the parting of S America from the rest of Gondwana and the opening of the S Atlantic— which the plume theory fails to provide.

Figure 6.29 Opening of the northern part of the S Atlantic and the mode of splitting from Africa as the result of vertical fractures induced by the spreading-ridge pressure on the 'cantilever' represented by the inset diagram.

Figure 6.29 Opening of the northern part of the S Atlantic and the mode of splitting from Africa as the result of vertical fractures induced by the spreading-ridge pressure on the 'cantilever' represented by the inset diagram.

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