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ON, 273.6E

also showed no significant TPW for the Tertiary and that the cumulative amount of TPW since the Early Cretaceous is also insignificant.

The calculations made by McElhinny (1973b) are repeated here and are summarized in Table 7.5. The summary data for the Eocene and Paleocene for the Pacific plate have been used as given in Table 5.3. There are no reliable data for the early Tertiary of South America, so North and South America are considered as a single plate using the data in Table 6.5. Originally, Le Pichon (1968) regarded North and South America as a single plate, and analysis of Pacific Ocean magnetic anomalies suggests that negligible movement occurred between North and South America during the Tertiary (see Fig. 5.25). For Eurasia, the data from Europe (see Table 6.6) have been combined with those from South China (see Table 6.9). Data for Africa are given in Table 6.14. The break between the India and Australia plates is only a recent one (Wiens et al., 1985). Therefore, they are considered to be a single plate throughout the Tertiary and the data from Tables 6.14 and 6.16 have been combined. The position of the o

Fig. 7.26. Estimates of the vectors nVP for the six major plates covering the Earth's surface during the past 50 Myr as given in Table 7.5. The situation corresponds with that illustrated in Fig. 7.25a.

Antarctic plate is found by subtracting the relative motion between Antarctica and Australia (from sea-floor spreading) from the Australian data.

The magnitude and poles of rotation of the six vectors nVP calculated in Table 7.5 are illustrated in Fig. 7.26. The resultant vector Vm has pole of rotation at ON, 126E and magnitude 2.5° corresponding to motion of the whole lithosphere of 2.5° along the 36° meridian. Within the error limits this is not discernible from zero and confirms the previous result of McElhinny (1973b).

For earlier times there has been much speculation regarding the possibility that TPW can been seen in the paleomagnetic APWPs for the major continents. Van der Voo (1994) observed that the APWPs for the Late Ordovician-Late Devonian interval for Laurentia, Baltica and Gondwana have nearly identical looping shapes (see §7.2) that can be brought into superposition. Also the less well-known paths for Siberia (Fig. 6.7) and South China (see Fig. 6.9a) reveal similar lengths. Kirschvink et al. (1997) suggested that Vendian to Cambrian data for all the major continents show anomalously fast rotations and latitudinal drift that might be attributed to TPW, although this is disputed by Torsvik et al. (1998). Mound et al. (1999) suggested that TPW involving movement of the rotation pole with respect to the Earth by 90° in a matter of 10 Myr would produce dramatic changes in sea-level of up to 200 m. Therefore a possible test for such TPW could lie in accurate estimates of sea-level changes for the time interval involved.

During the Permo-Triassic (295-205 Ma) the APWP for Pangea is about 35° in length (§7.2.4). This means that Pangea rotated through an angle of 35° with respect to the rotation axis about an Euler pole located on the equator. Marcano et al. (1999) argued that the rest of the world (mainly composed of the Panthalassic Ocean) rotated about the same Euler pole in the same sense as Pangea, so that TPW occurred during the Permo-Triassic at a rate of about 0.4° Myr"1. On the basis of the above observations, Evans (1998) speculates that the geoid, and hence TPW, may be a legacy of supercontinental breakup, which can persist even after the next supercontinent has begun to form.

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