Paleomagnetism of the North American Precambrian: a look at the data base |
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| Authors: | Jean L Roy |
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| Institution: | Geomagnetic Laboratory, Division of Geomagnetism, Earth Physics Branch, Ottawa Canada |
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| Abstract: | More than half of the approximately 650 paleopoles reported from all Precambrian cratons have been obtained from North America. The observations have spurred a proliferation of apparent polar wander paths to support different hypotheses for the tectonic evolution of the continent. Many of the reasons for this development can be traced to the inadequacy of the data base with its average of only 1.6 poles 10 Ma?1 (as compared to 7 poles 10?1 Ma for the last 300 Ma). Even when it is assumed that a ·primary’ pole has been derived from each rock unit sampled, the total number of ·primary’ poles (excluding the 900–1200 Ma interval) yields an average of only 0.4 pole 10 Ma?1. An inspection of the data base shows a non-systematic sampling of a non-continuous (fragmented) paleomagnetic record with gaps as large as tens and even hundreds of Ma. More importantly, a search through the literature reveals that very few poles have an age known to better than 50 Ma and for many poles the age bracket could be as large as a few hundreds of Ma. This is partly owing to the fact that radiometric and paleomagnetic sites rarely originate from the same localities. It is shown that this limited data base with all its deficiencies and uncertainties cannot be used to support certain hypotheses, or for the drawing of a meaningful apparent polar path for the whole of the Precambrian. It appears that, because of the enormous time scale, Precambrian paleomagnetism should rather be directed towards the construction of segments of polar paths such as those in the 900–1200 Ma interval. Comparison of well dated contemporaneous segments from different parts (or structural provinces) could be used most effectively to delineate relative movements, if any, and reconstruct the mosaic of the past continent for certain time intervals. One of the recent developments of Precambrian paleomagnetism has been to show that the apparent pole did not remain constrained to the equatorial region, but made several excursions into high latitudes. |
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