Comments on: ‘A weighted least-squares fit of the Australian apparent polar wander path for the last 100 Myr’ By R. J. Musgrave

Abstract

With the recent publication of Musgrave’s paper (Musgrave 1989), two Late Mesozoic-Cainozoic pole paths that differ substantially in age calibration have become available for Australia. The earlier path (path 1) is age calibrated almost entirely by remanence directions of sedimentary sequences (Idnurm 1985); the new path (path 2) is calibrated by the remanence directions of volcanics. Path 2 has the special merit that again it puts to use data which fell out of favour in the late 1970s and early 1980s because of serious discrepancies with the Indian data base (e.g., Embleton 1981; Embleton & McElhinny 1982). The two approaches to Australia’s Late MesozoicCainozoic APWP differ fundamentally in the treatment of scatter in pole data. The approach adopted for path 1 attempts to eliminate abnormally large scatters at the source-believed to be the incomplete averaging out of geomagnetic secular variations (for discussion, see Idnurm 1985tby utilizing the time-averaging characteristics of sedimentary sequences. The approach for path 2 relies heavily on statistics to eliminate the scatter. The differences between the paths are of concern especially when they are used for age dating of weathered profiles (the age estimates may differ by up to about 15Ma in the Tertiary, corresponding to an angular difference of So), and Musgrave has attempted to test the paths by comparing their respective age estimates on such units with independent geological constraints. The results are in favour of path 2, but he appears to have made some errors in these comparisons-errors that invalidate the finding. More seriously, the method devised for age calibration of path 2 seems unsound. The first error occurs when the palaeomagnetic age estimates for a New Caledonian laterite are compared with stratigraphic evidence that its age is Early Miocene or older. Musgrave finds that this stratigraphic age limit is consistent with path 2, but not consistent with path 1, and concludes that the test has provided ‘a clear discrimination in favour of the new APWP’. This is not correct. Fig. 1 shows path 1 with the New Caledonian pole AA. Since the 95 per cent confidence circle for AA encloses the 26 Ma calibration pole, path 1 is consistent with an Early Miocene age for the laterite. (As a separate issue, all three New Caledonian laterite poles in Musgrave’s paper have values of A,, that are excessively high for such material-see, for example, his table 1-suggesting unresolved complexities in their remanence.)