Late Proterozoic paleomagnetism and tectonic models: a critical appraisal

Abstract

Interpretations of Proterozoic orogenic belts in terms of plate tectonic processes have been widely divergent. Published models range between the extremes of no relative motions between continental nuclei (implying ensialic orogenic processes) on the one hand, to large-scale relative motions with oceans opening and closing (resulting in continent-continent collisions) on the other hand. Paleomagnetic data can, in theory, contribute significantly to this debate; however, as shown in this paper, several tectonic interpretations on the basis of paleomagnetic data have been premature. A critical continent in many of the previous models is Africa. In order to test hypotheses, for instance, for the late Proterozoic-Cambrian Pan African orogeny, a compilation of paleopoles has been made for Africa, with age ranges falling fully or partially within the interval of 1150 to 500 Ma. A quantitative comparison of the quality of this African dataset with the Phanerozoic poles for North American and Europe shows that the late Proterozoic paleopoles of Africa generally have very low reliability. It appears that the data from other Gondwana continents are equally unreliable and even less abundant. This means that currently the dataset of Gondwanaland cannot be used with confidence for the testing of tectonic models such as the Precambrian supercontinent, at least for the time after 1150 Ma. Well-dated late Proterozoic paleopoles from the three cratonic nuclei within Africa (Congo, Kalahari, West Africa) define relatively short apparent polar wander path segments, but each with different age ranges. This implies that they cannot be compared with each other to test relative motions between the cratonic nuclei and that a choice between ensialic and ensimatic models for the Pan African orogenic belts cannot yet be based on paleomagnetic data. While this does not imply that the tectonic models (e.g. those of Piper and McWilliams) are wrong, it does mean that substantial paleomagnetic support for them will have to wait more and higher-quality paleopole determinations with better dating precision.