The Precambrian palaeomagnetic record: the case for the Proterozoic Supercontinent

The case for a single supercontinent in Proterozoic times is examined in the context of a modified reconstruction with the Precambrian shields collectively comprising a primary lens-shaped body of crust. The palaeomagnetic data base is now sufficient to provide a definitive test of this model and it is shown that palaeopoles from the major shields conform to a single narrow path from ca. 2600 to 570 Ma. The agreement is especially compelling because the model requires no stringent data selection although it is itself highly rigid with adjustments only necessary at 1100 Ma and then applicable only to peripheral shields. There is close correspondence of the predominant field polarities from the constituent shields giving added confidence that the same polarities are being correlated, and the model supports the view that the global palaeoradius has been approximately constant since 2600 Ma. The pre-2200 Ma palaeopoles from Laurentia define "track 6" which is shown to be precisely applicable to the contemporaneous record from the African, the Australian, and possibly the Indian, Shields. The post-2200 Ma palaeopoles record higher rates of apparent polar wander (a.p.w.) movement although the magnitude of these movements is identified within one shield (the Laurentian) alone, and the data from other shields continue to conform to the unique reconstruction. The a.p.w, path is best constrained by the data from anorogenic igneous units over the intervals 2600-1800 and 1500-1000 Ma with second rank data from sedimentary successions in all of the major shields supporting the path defined by the igneous data.The a.p.w, paths over the intervals 1800-1500 and 1000-850 Ma are represented predominantly by uplift magnetisations in metamorphic terrains of the Fennoscandian and Laurentian Shields, although they are matched by magnetisations in supracrustal successions in Africa, South America and Australia, and in Africa and Siberia, respectively. The weakest part of the analysis covers the interval 800-570 Ma and the solution cannot yet be regarded as unique because it relies on magnetisations from sedimentary rocks; on the model developed here these data conform to a single a.p.w, path which specifically subdivides at the base of the Cambrian as the supercontinent was dismembered. Many Precambrian tectonic lineaments are brought into parallelism on this model and confirm the validity of the palaeomagnetic reconstruction. They include the later greenstones (2900-2200 Ma) formed in a permobile environment, and the straight belts and mobile belts formed in a coherent ensialic environment; there appears to have been a long continuity in the prevailing stress system in the crust linked to evolving small-scale to large- scaleaesthenosphere systems. This analysis confirms that the continental crust has been a highly coherent unit since the beginning of Proterozoic times and restricts the tectonic models applicable to these times. The possibility of multi-continent plate tectonics is no longer an issue: it is not possible that the shields could have been repeatedly separated and returned to the same unique configuration necessary to satisfy the single a.p.w, path. Models of lithosphere evolution require that the continental crust stabilised towards the end of the Archaean retained a much higher relative strength than the oceanic lithosphere until late Proterozoic times.