Page 81 - Global Tectonics

68 CHAPTER 3

Triassic - Jurassic

Present

Cambrian
L. Permian

Pennsylvanian
Devonian

Pennsylvanian
Cambrian
Permian
Devonian
Jurassic

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 of paleomagnetic and paleoclimatic data, is very com-

segments describe diverging APW paths until the parable to that of Creer (1965). All APW paths for
hairpin at time 8 signals a change in direction of Gondwana have the south pole during Carboniferous
motion to one of convergence. After suturing at times in the vicinity of southeast Africa, as did
time 12 the two segments follow a common polar Wegener (Fig. 1.3), and the Ordovician pole position
track. in northwest Africa, where there is evidence for a
The southern continents, plus India, are thought minor glaciation in the Saharan region at this time
to have formed a single continent, Gondwana, from (Eyles, 1993).
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 Gond-
wana (Torsvik & Van der Voo, 2002). The track of 3.6.6 Paleogeographic
the path relative to South America can be compared reconstructions based on
with the very early path given by Creer (1965) (Fig.
3.13b). The seemingly greater detail of the path paleomagnetism
shown in Fig. 3.16 may however be unwarranted.
There is considerable disagreement over the details of Reconstructions of the relative positions of the main
the APW path for Gondwana, presumably because of continental areas at various times in the past 200 Ma
the paucity of sufficient reliable data (Smith, 1999; are best achieved using the very detailed information

McElhinny & McFadden, 2000). Interestingly the path on the evolution of the present ocean basins pro-
favored by Smith (1999), based on a detailed analysis vided by the linear oceanic magnetic anomalies