Geologic Evidence For Continental Drift

The continental reconstructions discussed in Sections 3.2.3 and 3.2.4 are based solely on the geometric fit of continental shelf edges. If they represent the true ancient configurations of continents it should be possible to trace continuous geologic features from one continent to another across the fits. The matching of features requires the rifting of the supercontinent across the general trend of geologic features. This does not always occur as the location of the rift is often controlled by the geology of the supercontinent, and takes place along lines of weakness that may run parallel to the geologic grain. However, there remain many geologic features that can be correlated across juxtaposed continental margins, some of which are listed below.

1 Fold belts. The continuity of the Appalachian fold belt of eastern North America with the Caledonian fold belt of northern Europe, illustrated in Fig. 3.5, is a particularly well-studied example (Dewey, 1969). Within the sedimentary deposits associated with fold belts there is often further evidence for continental drift. The grain size, composition, and age distribution of detrital zircon minerals in the sediments can be used to determine the nature and direction of their source. The source of sediments in the Caledonides of northern Europe lies to the west in a location now occupied by the Atlantic, indicating that, in the past, this location must have been occupied by continental crust (Rainbird et al., 2001; Cawood et al., 2003).

2 Age provinces. The correlation of the patterns of ages across the southern Atlantic is shown in Fig. 3.6, which illustrates the matching of both

Palynology Baltic

Figure 3.5 The fit of the continents around the North Atlantic, after Bullard et al. (1965), and the trends of the Appalachian-Caledonian and Variscan (early and late Paleozoic) fold belts (dark and light shading respectively). The two phases of mountain building are superimposed in eastern North America (redraw from Hurley, 1968; the Confirmation of Continental Drift. Copyright © 1968 by Scientific American, Inc. All rights reserved.)

Figure 3.5 The fit of the continents around the North Atlantic, after Bullard et al. (1965), and the trends of the Appalachian-Caledonian and Variscan (early and late Paleozoic) fold belts (dark and light shading respectively). The two phases of mountain building are superimposed in eastern North America (redraw from Hurley, 1968; the Confirmation of Continental Drift. Copyright © 1968 by Scientific American, Inc. All rights reserved.)

Precambrian cratons and rocks of Paleozoic age (Hallam, 1975).

3 Igneous provinces. Distinctive igneous rocks can be traced between continents as shown in Fig. 3.7. This applies both to extrusive and intrusive rocks, such as the belt of Mesozoic dolerite, which extends through southern Africa, Antarctica, and Tasmania, and the approximately linear trend of Precambrian anorthosites (Section 11.4.1) through Africa, Madagascar, and India (Smith & Hallam, 1970).

4 Stratigraphic sections. Distinctive stratigraphic sequences can also be correlated between adjacent continents. Figure 3.8 shows stratigraphic sections of the Gondwana succession, a terrestrial sequence of sediments of late Paleozoic age (Hurley, 1968). Marker beds of tillite and coal, and sediments containing Glossopteris and Gangamopteris flora (Section 3.5) can be correlated through South America, South Africa, Antarctica, India, and Australia.

Gangamopteris Fossil Remains

jllll;} Cratons | 1 Younger mobile belts

Figure 3.6 Correlation of cratons and younger mobile belts across the closed southern Atlantic Ocean (redrawn from Hurley, 1968, the Confirmation of Continental Drift. Copyright © 1968 by Scientific American, Inc. All rights reserved.)

Proto Gondwana
Figure 3.7 Correlation of Permo-Carboniferous glacial deposits, Mesozoic dolerites, and Precambrian anorthosites between the reconstructed continents of Gondwana (after Smith & Hallam, 1970, with permission from Nature 225,139-44. Copyright 1970 Macmillan Publishers Ltd).

5 Metallogenic provinces. Regions containing manganese, iron ore, gold, and tin can be matched across adjacent coastlines on such reconstructions (Evans, 1987).

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