2.2.2.2 Computer-assisted reconstructions

Wegener pointed out that it was not possible to fit the continents together using their present coastlines, which are influenced by recent sedimentary deposits at the mouths of major rivers as well as the effects of coastal erosion. The large areas of continental shelf must also be taken into account, so Wegener matched the continents at about the edges of the continental shelves, where the con­tinental slopes plunge into the oceanic basins. The match­ing was visual and inexact by modern standards, but more precise methods only became available in the 1960s with the development of powerful computers.

In 1965 E. C. Bullard, J. E. Everett and A. G. Smith used a computer to match the relative positions of the con­tinents bounding the Atlantic ocean (Fig. 1.6). They digi­tized the continental outlines at approximately 50 km intervals for different depth contours on the continental slopes, and selected the fit of the 500 fathom (900 m) depth contour as optimum. The traces of opposite continental margins were matched by an iterative procedure. One trace was rotated relative to the other (about a pole of relative rotation) until the differences between the traces were minimized; the procedure was then repeated with different rotation poles until the best fit was obtained. The optimum fit is not perfect, but has some overlaps and gaps. Nevertheless, the analysis gives an excellent geometric fit of the opposing coastlines of the Atlantic.

Fig. 1.6 Computer-assisted fit of the Atlantic-bordering continents at the 500 fathom (900m) depth (after Bullard et al., 1965).

A few years later A. G. Smith and A. Hallam used the same computer-assisted technique to match the coastlines of the southern continents, also at the 500 fathom depth contour (Fig. 1.7). They obtained an optimum geometric reconstruction of Gondwanaland similar to the visual match suggested by du Toit in 1937; it probably represents the geometry of Gondwanaland that existed in the Late Paleozoic and Early Mesozoic. It is not the only possible good geometric fit, but it also satisfies other geological evidence. At various times in the Jurassic and Cretaceous, extensional plate margins formed within Gondwanaland, causing it to subdivide to form the present "southern continents." The dispersal to their present positions took place largely in the Late Cretaceous and Tertiary.

Fig. 1.7 Computer-assisted fit of continents that formed Gondwanaland (after Smith and Hallam, 1970)

Pangaea existed only in the Late Paleozoic and Early Mesozoic. Geological and geophysical evidence argues in favor of the existence of its northern and southern con­stituents - Laurasia and Gondwanaland - as separate enti­ties in the Early Paleozoic and Precambrian. An important source of data bearing on continental reconstructions in ancient times and the drift of the continents is provided by paleomagnetism, which is the record of the Earth's ancient magnetic field. Paleomagnetism is described in Section 5.6 and summarized below.