Early Precambrian plate tectonics

Abstract

In recent years, the application of uniformitarian plate tectonic models to the early Precambrian, particularly the Archaean, has become widespread; however, there are those who believe that the great debate on this question that commenced in the seventies has never been satisfactorily resolved. Significant differences exist in the chemistry of calc-alkaline 'arc' magmas in the Palaeoproterozoic and Archaean, compared with the Phanerozoic, held by some to indicate differences in the nature of the subduction process. The prevalence of komatiite, and the absence of blueschists and ophiolites, are aspects of Archaean geology which can be related directly or indirectly to the higher heat flow generally accepted as characterizing the early Procambrian. Less often acknowledged is the difference in style of continental tectonics in the Archaean prior to 2.6 Ga compared with Proterozoic and younger times, which can be illustrated. for example, by comparing the Palaeoproterozoic orogenic belt system of Laurentia/Baltica with the Archaean terrain of the Superior Province. Using a modified pre-Grenville fit of Laurentia and Baltica, tectonic information from the Palaeoproterozoic belts of Baltica and northeast Laurentia is reviewed. Between 2.4 and 2.0 Ga, rifting and dyke emplacement took place in older cratons leading to the creation of oceanic and intracontinental basins. Then in the period 2.0 - 1.8 Ga, subduction at the active margins of the older cratons, leading to the creation of magmatic arcs, was followed by collision of the cratons, closure of intra-cratonic basins, and accretion of arc terranes. Finally, between 1.8 and 1.5 Ga, a new active margin discordant to the previous ones was formed, with significant changes in convergence direction within the amalgamated continental assembly. The consistency in apparent convergence directions across the various belts of the North Atlantic region in the period 1.9-1.8 Ga suggests a common movement pattern for the cratons in the Palaeoproterozoic period within this region, from which it may be inferred that the cratons behaved in a plate-like manner. In contrast, few, if any, pieces of pre-2.6 Ga continental crust can convincingly be shown to have been large enough or stable enough to be considered as analogues of typical Phanerozoic plates. The consequences of higher Archaean heat flow on oceanic lithosphere strength, and the possibiliy that plumes may have been much more prevalent than now (perhaps replacing ridges as the Main source of new oceanic lithosphere). both raise the question whether plates (in the sense that they are understood to operate in the Phanerozoic) existed in the Archacan, Prior to 2.5 Ga. the surface tectonic pattern may have been dominated by worm, weak, and continuously distorting ocean lithosphere flowing from plume-head regions to downflow regions, where continental crust aggregated but did not form large, strong, plate-like areas. The start of plate tectonics as it is known today may have been prompted by the creation of an end-Archaean supercontinent surrounded by a long subduction zone.