Pangea

The reconstruction of the North and South Atlantic Oceans by Bullard et al. (1965), when combined with that of Smith and Hallam (1970) for Gondwana, created the first formal reconstruction of the supercontinent of Pangea, which existed just prior to the opening of the North Atlantic Ocean at 175 Ma (Table 5.2). Therefore, to investigate the existence of Pangea before 175 Ma, a comparison of the reference APWPs for Laurussia (Table 7.1) and Gondwana (Table 7.3) is appropriate. Such a comparison is made in Fig. 7.9 for the poles between 180 and 320 Ma after rotating North America to northwest Africa about an Euler pole at 61.3N, 343.2E through an angle of +79.5° (counterclockwise), following Lottes and Rowley (1990).

The reference poles for Laurussia and Gondwana are not significantly different at 180 and 200 Ma, just prior to the opening of the Atlantic Ocean. However, for earlier times the Gondwana poles all plot systematically eastwards of those from Laurussia. Although the overall lengths of the two paths between 180 and 320 Ma are almost identical, the angular changes in pole positions for successive 20-Myr intervals are quite different. It must first be determined whether the difference arises from a data problem or from a reconstruction problem. The difficulty of assigning poles from the Gondwana continents into the 20-Myr time groupings has already been noted in §7.2.3. Restricting the Gondwana poles to West Gondwana (Africa and South America) substantially reduces the eastward trend of the Gondwana path, as shown in Fig. 7.9. Thus, it appears that the poles from the individual Gondwana continents do not, when reconstructed, match each other as well as they might. The larger circles of 95% confidence for the mean Gondwana poles compared with those from Laurussia reflect this fact.

Two factors may be combining to create the problem illustrated in Fig. 7.9. The first is age assignment for each pole and the second is a reconstruction problem. A change in age assignments would move the positions of the mean poles north or south and therefore account for differences in angular change between pole positions; the easterly trend of the Gondwana path would remain. However, it does appear as though the poles have not been reconstructed tightly enough, so perhaps the reconstruction of East Gondwana with West Gondwana is not tight enough. For example, Gondwana reconstructions do not fully account for the extension that occurs on continental margins prior to sea-floor spreading (e.g., see Tikku and Cande, 1999).

The factors described above have led to many discussions regarding the reconstruction of Pangea between Late Carboniferous and Early Jurassic, as has been well summarized by Smith and Livermore (1991). Taken at their face value

Fig. 7.9. Comparison of the reference poles for Laurussia (Table 7.2, solid circles) and Gondwana (Table 7.3, open circles) in the co-ordinates of northwest Africa after rotation of North America to northwest Africa. Ages are shown in millions of years (italics for Gondwana poles). The APWP with data restricted to West Gondwana (open squares) is shown as a dashed line. Circles of 95% confidence are shown for each of the mean poles (dashed for West Gondwana only).

Fig. 7.9. Comparison of the reference poles for Laurussia (Table 7.2, solid circles) and Gondwana (Table 7.3, open circles) in the co-ordinates of northwest Africa after rotation of North America to northwest Africa. Ages are shown in millions of years (italics for Gondwana poles). The APWP with data restricted to West Gondwana (open squares) is shown as a dashed line. Circles of 95% confidence are shown for each of the mean poles (dashed for West Gondwana only).

the difference in pole paths requires that Gondwana be rotated with respect to Laurussia in some way so as to bring the two paths into coincidence. Morel and Irving (1981) referred to the conventional reconstruction of Pangea just prior to the opening of the Atlantic Ocean as Pangea A1 (Fig. 7.10a). Van der Voo and French (1974) proposed a reconstruction that brought the older parts of the curves into coincidence (Fig. 7.10b), named by Morel and Irving (1981) as Pangea A2. This involves rotating Laurussia about an Euler pole located in the Sahara at 19.3N, 0.7W through an angle of about +20° (counterclockwise) relative to its position in Pangea A1. Here the cratonic edge of South America is fitted closely to the Gulf coast of North America by tightly closing the Gulf of Mexico. Although parts of Mexico would then overlap with South America, this is a geologically plausible scheme considering the existence of displaced terranes in this part of Mexico.

In order to bring the poles for the Early Permian (280 Ma) into coincidence, Irving (1977) and Morel and Irving (1981) proposed Pangea B (Fig. 7.10c). Here Gondwana is rotated clockwise so that the northwest coast of South America is placed against the Atlantic margin of North America and Africa lies to the south of eastern Europe and southwestern Asia. Hallam (1983) generalized this approach by naming the paleomagnetic reconstructions for 240, 280, and

Fig. 7.10. Various reconstructions of Pangea, (a) Pangea Al, the conventional reconstruction just prior to the opening of the Atlantic Ocean (Bullard et al., 1965). (b) Pangea A2, with the cratonic edge of South America fitted closely to the Gulf coast of North America (Van der Voo and French, 1974). (c) Pangea B, with the northwest coast of South America fitted to the Atlantic coast of North America (Morel and Irving, 1981). (d) Pangea C, with the northwest coast of South America fitted against southeast Europe (Smith et al., 1981). After Smith and Livermore (1991), with permission from Elsevier Science.

320 Ma proposed by Smith et al. (1981) as Pangea C (Fig. 7.10d). Alternative paleomagnetic reconstructions made by fitting linear segments of the Laurussia and Gondwana paths have been termed Pangea D by Smith and Livermore (1991). They are similar to Pangea A2 and require rotation of Laurussia about an Euler pole located in central Africa at 6S, 19E through an angle of about +30° (counterclockwise) relative to its position in Pangea Al.

It is generally agreed that Pangea Al existed just prior to the opening of the Atlantic Ocean at 175 Ma. The Pangea B and C configurations must therefore have evolved to the Al configuration, presumably during the (Permo-) Triassic. The transition between Pangea B and Al requires a dextral strike slip (referred to as the "Tethys Twist" - see Van der Voo, 1993) of 3500 km with respect to Laurussia at rates of up to 100 km Myr"1. The transition from Pangea C to Al would require an even larger displacement (see Fig. 7.10). There is no plausible geological evidence that would account for a megashear of such magnitude. However, some recent analyses of paleomagnetic data by Muttoni et al. (1996) and Torcq et al. (1997) still favor Pangea B. Pangea B and C are discounted here as implausible, and both Smith and Livermore (1991) and Van der Voo (1993) agree that the solution that is both geologically plausible and paleomagnetically acceptable is that of Pangea A2. Final resolution of this problem requires the restudy of many of the Permian and Triassic poles of Gondwana. In addition, the paleomagnetic data strongly suggest that the reconstruction of Gondwana needs careful reassessment.

7.2.5 Paleogeography: 300 Ma to the present

Pangea formed in the Late Carboniferous (-300 Ma) when the Rheic Ocean between Gondwana and Laurussia closed. At that time the south pole lay in Antarctica (Fig. 7.8) and most of Gondwana was covered by an ice sheet. Asia as a continent did not exist and Siberia, Kazakhstan, and North and South China lay in equatorial latitudes. Because of the controversy surrounding the evolution of Pangea during the Permo-Carboniferous (§7.2.4), a discussion of global paleogeography will be restricted to Late Permian and younger times at this point. An overview of Earth history since 1000 Ma is given in §7.4.3, which includes a representation of the paleogeographic information here in a cladistic-style diagram (see Fig. 7.24).

Paleomagnetic data for the major blocks on the surface of the Earth can be combined with the information derived from marine magnetic anomalies summarized in §5.3.1 (see Fig. 5.19) to produce paleogeographic maps through time. Such a set of maps has been compiled by Scotese (1997) and is shown in Fig. 7.11 for times since the Late Permian (255 Ma). The maps include topographic and sea level information and are set in their correct paleolatitude according to paleomagnetic data. Paleogeographic maps for the Paleozoic based on combined paleomagnetic and geological information can be found in Scotese and McKerrow (1990) and are updated by Scotese (1997). A much more complete set of maps than those presented here can be viewed on the internet site http://www. scotese. com.

In the Late Permian (255 Ma), Pangea dominated global paleogeography (Fig. 7.1 la). North and South China and Indochina were located in low latitudes, whereas Gondwana occupied high latitudes. Turkey, Iran, and the terranes of Tibet were the components of the Cimmerian continent postulated by Sengor et al. (1984). As a protocontinent located in Gondwana, Cimmeria was situated just north of India and Arabia, whereas the Paleotethys Ocean lay to the north. At 255 Ma Cimmeria had just separated from Gondwana resulting in the initial opening of the Tethys Ocean. Siberia and Kazakhstan were situated in high latitudes but lay close to Europe.

By the Middle Triassic (237 Ma) Siberia and Kazakhstan had amalgamated with Europe (Fig. 7.11b) to form the Ural Mountains. The Tethys Ocean was well developed and the map shows Cimmeria occupying a position between Gondwana and Eurasia. However, it should be noted that the latest paleomagnetic data from Iran (Besse et al., 1998) suggest that Iran had already merged with Eurasia at that time (see §7.3.5, Fig. 7.16). South China and Indochina lay in equatorial latitudes and North China had by then moved into intermediate northerly latitudes. In Early Jurassic time (195 Ma) the Tethys Ocean was fully developed (Fig. 7.11c). North and South China had not yet amalgamated but lay close to the southern margin of Eurasia. The Amurian Seaway separated North China from Siberia.

Pangea started to break up about 175 Ma when Africa separated from North America. The breakup of Gondwana commenced at about 160 Ma when Antarctica (together with Greater India) separated from Africa (see Table 5.2). At about 130 Ma, North America separated from Iberia and South America commenced its separation from Africa. Note that East Gondwana (Greater India, Madagascar, Antarctica, and Australia) first drifted southwards on separation from Africa. Then Greater India and Madagascar separated from Antarctica at about 130 Ma and drifted northwards. By Mid-Cretaceous time (94 Ma), the major blocks that make up present-day Eurasia (with the exception of India) had amalgamated and the development of narrow North and South Atlantic Oceans had occurred (Fig. 7.1 Id).

By the latest Cretaceous (69 Ma), Greater India had separated from Madagascar and was drifting rapidly northwards in the central Indian Ocean (Fig. 7.1 le). The Tethys Ocean had by then virtually disappeared. Although the separation between Australia and Antarctica commenced at about 95 Ma (see Table 5.2), at 69 Ma they still lay close to one another because the rate of separation was initially very slow. The Labrador Sea between Greenland and North America opened at about 85 Ma and was still only a narrow seaway at 69 Ma. The opening of the South Atlantic continued and South America and Africa were well separated at this time (Fig. 7.1 le). The North Atlantic between Europe and Greenland did not open until about 60 Ma (see Table 5.2).

By the Eocene (50 Ma) Greater India was about to collide with Asia, resulting in the formation of the Himalayas and underthrusting of the Indian plate. The separation between Antarctica and Australia was well under way (Fig. 7.1 If) and the Indian Ocean was well developed. The North Atlantic Ocean between Europe and Greenland had started to open. Meanwhile, opening of the Labrador Sea continued.

Fig. 7.11(a-c). Global paleogeographic maps for (a) Late Permian (255 Ma), (b) Middle Triassic (237 Ma), and (c) Early Jurassic (195 Ma). Oceanic trenches are shown in red. From Scotese (1997), see also http://www.scotese.com.

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