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"Symbols and method of averaging are as given in Table 6.5.

"Symbols and method of averaging are as given in Table 6.5.

extensional rift system from the Benue Trough into central and northern Africa as shown in Fig. 6.19a.

Lottes and Rowley (1990) compiled rotation parameters that allow for the differential rotation between northwest Africa, southern Africa and northeast Africa. Data from the main Arabia craton have been included with data from Africa after allowing for the opening of the Red Sea by rotation of Arabia to northeast Africa about an Euler pole at 31.3N, 25.5E through an angle -8.5° (clockwise; see §5.1.1 for Euler rotation conventions). Therefore, in analyzing paleomagnetic data from Africa, pole positions are rotated to the co-ordinates of West Africa for times before 130 Ma. For times between 130 and 100 Ma the paleomagnetic poles have been partially rotated, and for times after 100 Ma they have not been rotated. The rotation parameters are as follows.

(i) Southern Africa to West Africa: rotation about an Euler pole at 9.3N, 5.7E through an angle -7.8° (clockwise).

(ii) Northeast Africa to West Africa: rotation about an Euler pole at 19.2N, 352.6E through an angle -6.3° (clockwise).

(iii) Arabia to West Africa: rotation about an Euler pole at 26.2N, 11,2E through an angle of -14.2° (clockwise).

Mean pole positions for Africa are given in Table 6.11 and the corresponding APWP is illustrated in Fig. 6.20a. The earlier paleomagnetic studies in Africa were mainly carried out in southern Africa with reviews of these data being given by Gough et al. (1964), McElhinny et al. (1968), and Brock (1981). Significant data were later acquired from northern Africa, mainly by workers

Fig. 6.20. Phanerozoic APWPs with each pole position and its circle of 95% confidence labeled with the mean age in millions of years.

(a) Africa, using the data listed in Table 6.11.

(b) Madagascar, using the data listed in Table 6.12.

Fig. 6.20. Phanerozoic APWPs with each pole position and its circle of 95% confidence labeled with the mean age in millions of years.

(a) Africa, using the data listed in Table 6.11.

(b) Madagascar, using the data listed in Table 6.12.

from French and German laboratories. Currently, 90 pole positions representing 807 sites are summarized for Africa in Table 6.11.

Reviews of data from Africa over the past two decades have generally been made in the context of the reconstruction of Gondwana (Morel and Irving, 1978; Kent and Van der Voo, 1990; Bachtadse and Briden, 1990, 1991; Schmidt et al„ 1990; Li et al., 1990; Van der Voo, 1993). This is because no single Gondwana continent by itself has unambiguous data through the Paleozoic to define each APWP with certainty. Of particular interest is the section of the path from Late Ordovician through Early Carboniferous times. Part of the problem has been due to the fact that most of the results in this time interval have been derived from the Tasman Fold Belt in eastern Australia, and it has been argued that these could be derived from displaced terranes and are not therefore related to the cratonic part of Australia. This problem will be discussed more fully in relation to the Australian APWP.

Bachtadse and Briden (1991) revised their previous view (Bachtadse and Briden, 1990) of the Gondwana Paleozoic APWP on the basis of new data from Devonian ring complexes in the Bayuda Desert, Sudan. Two possible primary components of magnetization were distinguished and it was argued (quite reasonably) that the high-temperature component was primary, and that the intermediate-temperature component was secondary. This scenario relegated all eastern Australian poles to the displaced terrane category and produced a simple APWP. However, new data from central Australia now clearly define the trend of the Gondwana APWP during the time interval Early Devonian to Early Carboniferous (Chen et al., 1995). They are compatible with those from eastern Australia and the pole path is in the opposite sense to that proposed by Bachtadse and Briden (1991). It turns out that, if the interpretation of the high- and intermediate-temperature components is reversed (i.e., the intermediate-temperature component is primary and the high-temperature component is secondary), the data then become compatible with the Australian data. An alternative is that the age of the ring complexes is incorrect. Therefore this apparently important result has not been included in the analysis of the APWP here. It appears that the basic structure of the paths proposed by Bachtadse and Briden (1990), Kent and Van der Voo (1990), Schmidt et al. (1990), and Li et al. (1990) is probably correct. The Gondwana path for the Phanerozic is discussed more fully in §7.2.3.

The discussion of the Late Ordovician to Early Carboniferous poles for Africa and Gondwana is further complicated by the Late Carboniferous and Early Permian poles from Africa because the pole from the Late Carboniferous Dwyka varves of central Africa (McElhinny and Opdyke, 1968) is often taken to be a key pole in discussion of the Gondwana APWP. However, this pole differs by 30.7° from five well-grouped Late Carboniferous poles from northern Africa. Two of the sites in the Dwyka varves (those with the largest applied structural correction) are from the K1 beds of the Galula coalfield in Tanzania. These are located adjacent to the Tertiary Rukwa rift where the tectonic activity associated with the rifting relates to the structural corrections applied. Similarly, the pole from the Early Permian K3 beds located in the same section of the Galula coalfield differs by 41° from the pole determined from the K3 beds in the Songwe-Kiwira and Ketewaka-Mchuchuma coalfields to the east (Opdyke, 1964). Recent data from these same beds to the east (Nyblade et al., 1993) confirm the previous result of Opdyke (1964). There are 8 Early Permian poles from northern Africa and these agree well with those from the eastern localities of the K3 beds in Tanzania. However, the K3 Galula pole differs by 50° from the mean of all the 10 remaining Early Permian poles. This pole is clearly an outlier from the main group. Therefore, the results derived from the Galula coalfield adjacent to the Rukwa rift have not been included in the analysis.

There appears to be a rapid polar shift of about 25° between the Early and the Late Permian, but lack of good data in this time interval means that the details cannot be determined. During the Mesozoic the mean pole positions appear to circulate around a position at about 65N, 250E although the means listed in Table 6.12 suggest there is some fine structure involved. During the Triassic the mean pole position is located near 60N, then during the Early/Middle Jurassic it is more northerly (at 74N) than its subsequent position during the Late Jurassic

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