Proterozoic palaeomagne tism and single continent plate tectonics

Summary. Proterozoic (2700–570 Ma) palaeomagnetic poles from the major shields conform to a single apparent polar wander path on a contiguous reconstruction which brings into alignment, both with each other and with the long axis of the primary continental crust, tectonic and magmatic features ranging from late Archaean (≤ 2900 Ma) to late Proterozoic (1100 Ma) age. This discovery implies that the continental lithosphere behaved on a megatectonic scale as a single coherent plate during this era, subject to internal deformation but not breaking up in a brittle fashion until late Precambrian times. Peripheral parts of the Supercontinent were redistributed over a short interval at ca. 1100 Ma but the continental crust was not dismembered and widely dispersed until 570–530 Ma. This latter episode correlates with the Proterozoic-Phanerozoic transition and defines the onset of multi-continent Plate Tectonics. The Proterozoic apw record comprises nine large loops with periods ranging from 270 to 110Ma and averaging 200Ma. Between 2150 and 1000 Ma the loops had a similar signature and their resultant effect was to return the continental lithosphere repeatedly to a stable position with its centre at the geographic pole, probably by the Goldreich-Toomre mechanism. This periodicity correlates with theoretical and experimental studies of mantle convection predicting avalanche-type behaviour due to build up of instability in the surface boundary layer with cycles of 50–200 Ma. The consolidation of the continental lithosphere defines several phases in the history of mantle convection: (1) between 3500 and 2800 Ma the sialic scum resulting from differentiation of a thickened and subducting lithosphere was accumulated into the primary body by a large scale mantle convection system; (2) this operated simultaneously with a subcontinental-lithosphere small-scale (50–100 km) convection system showing a transition from bimodal to cylindrical roll mode represented by the later greenstone belts (2900–2200 Ma) and straight belts (ca. 2800–2400 Ma); (3) a transition to larger-scale episodic subcontinental convection is represented by the Proterozoic mobile belts (2200–1100 Ma); (4) a modification of the major suboceanic lithosphere convection is suggested by reorganization of the peripheral shields at 1100 Ma, and (5) a subdivision of this major system resulted in continental dispersal during Lower Cambrian times.