Their data can be combined after rotation of Europe to Laurentia for times older than 175 Ma. The resulting APWP in North American co-ordinates is then a reference path for Laurussia with which other paleomagnetic data can be compared. The data for the two continents are not uniformly distributed in either case, but, in determining a reference path for the combined data set, they complement each other. For pre-425 Ma, only the data from North Britain have been combined with the data for Laurentia to produce the composite APWP.
The mean poles for this reference path for Laurussia are listed at 20-Myr intervals in Table 7.1. The path has been calculated using 409 groups of data from 4811 sites and represents a large body of paleomagnetic data, more than twice that for the Gondwana continents (§7.2.3).
Asia is a composite continent made up of accreted terranes that merged with Europe in the late Paleozoic/early Mesozoic. The situation is therefore the inverse of that discussed in §7.2.1, where two currently separated continents were once joined together. Figure 7.5 compares the APWPs of Siberia and Europe. For pre-425-Ma times Europe as such did not exist, so the comparison for pre-425 Ma has to be either with North Britain or with Baltica (see Fig. 6.14). Because Siberia subsequently collided with Baltica, it is appropriate to use the APWP for Baltica for that time. The paths are widely separated during the early Paleozoic and merge during the Early Triassic as indicated by the close proximity of the mean pole positions at 235 Ma. However, it should be noted that the APWP for Siberia is not well constrained during the late Mesozoic and Cenozoic, as it is for Europe. Therefore, it is convenient to define a reference APWP for Eurasia by combining the data for Europe and Siberia for Mesozoic
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