Apparent polar wander curves

Paleomagnetic data can be displayed in two ways. One way is to image what is believed to be the true situation, that is, plot the continent in a succession of positions according to the ages of the sampling sites (Fig. 3.13a). This form of display requires the assumption of the paleolongitudes of the sites. The other way is to regard the continent as remaining at a fixed position and plot the apparent positions of the poles for various times to provide an apparent polar wander (APW) path (Fig. 3.13b). As discussed above, this representation does not reflect real events, but it overcomes the lack of control of paleolongitude and facilitates the display of information from different regions on the same diagram.

The observation that the apparent position of the pole differed for rocks of different ages from the same continent demonstrated that continents had moved over the surface of the Earth. Moreover, the fact that APW paths were different for different continents demonstrated unequivocally that relative movements of the continents had taken place, that is, continental drift had occurred. Paleomagnetic studies thus confirmed and provided the first quantitative measurements of continental drift. Figure 3.14a illustrates the APW paths for North America and Europe from the Ordovician to the Jurassic. Figure 3.14b shows the result of rotating Europe and its APW path, according to the rotation parameters of Bullard et al. (1965), to close up the Atlantic Ocean. The APW paths for Europe and North America then correspond very closely from the time the continents were brought together at the end of the Caledonian orogeny, approximately 400 Ma ago, until the opening of the Atlantic.

APW paths can be used to interpret motions, collisions, and disruptions of continents (Piper, 1987), and are especially useful for pre-Mesozoic continents whose movements cannot be traced by the pattern of magnetic lineations in their surrounding ocean basins (Section 4.1.6). Figure 3.15 represents the full Wilson cycle (Section 7.9) of the opening and closure of an ocean basin between two continents. Before rifting, the two segments A and B of the initial continent have similar APW paths. They are unlikely to be identical as it is improbable that the initial rift

Figure 3.13 Two methods of displaying paleomagnetic data: (a) assuming fixed magnetic poles and applying latitudinal shifts to the continent; (b) assuming a fixed continent and plotting a polar wander path. Subsequent work has modified the detail of the movements shown. Note that the south pole has been plotted (redrawn from Creer, 1965, with permission from the Royal Society of London).

Figure 3.13 Two methods of displaying paleomagnetic data: (a) assuming fixed magnetic poles and applying latitudinal shifts to the continent; (b) assuming a fixed continent and plotting a polar wander path. Subsequent work has modified the detail of the movements shown. Note that the south pole has been plotted (redrawn from Creer, 1965, with permission from the Royal Society of London).

and final suture would coincide. After rifting the two segments describe diverging APW paths until the hairpin at time 8 signals a change in direction of motion to one of convergence. After suturing at time 12 the two segments follow a common polar track.

The southern continents, plus India, are thought to have formed a single continent, Gondwana, from late Pre-Cambrian to mid-Jurassic time. During this period, of approximately 400 Ma, they should have the same polar wander path when reassembled. Figure 3.16 illustrates a modern polar wander path for Gondwana (Torsvik & Van der Voo, 2002). The track of the path relative to South America can be compared with the very early path given by Creer (1965) (Fig. 3.13b). The seemingly greater detail of the path shown in Fig. 3.16 may however be unwarranted. There is considerable disagreement over the details of the APW path for Gondwana, presumably because of the paucity of sufficient reliable data (Smith, 1999; McElhinny & McFadden, 2000). Interestingly the path favored by Smith (1999), based on a detailed analysis of paleomagnetic and paleoclimatic data, is very comparable to that of Creer (1965). All APW paths for Gondwana have the south pole during Carboniferous times in the vicinity of southeast Africa, as did Wegener (Fig. 1.3), and the Ordovician pole position in northwest Africa, where there is evidence for a minor glaciation in the Saharan region at this time (Eyles, 1993).

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Responses

  • quartilla
    What is aparent polar wonder (APW)?
    6 months ago
  • teresa
    What is aparent polar wondering?
    5 months ago

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