Tectonic evolution in the Archaean and Proterozoic

Abstract

Evidence for continental motion since at least 3.3 Ga and at minimum average velocities comparable to those of today leaves little doubt that Precambrian tectonic styles are essentially comparable to those of the Phanerozoic plate-tectonic regime and were governed by deformation along and within continental plates. The detailed mechanisms for these motions, however, remain a matter of considerable dispute. For the earliest Archaean, from > 4.2 Ga to perhaps 3.9 Ga, plate tectonics appear unlikely because of thin and “soft” lithosphere: high heat flow probably favoured global hotspot activity with crustal growth of Iceland type through vertical magmatic accretion. The tectonic environment for the generation of about 3.9-2.5 Ga granite-gneiss-greenstone terrains with voluminous production of tonalite-trondhjemite-granodiorite magmas is still uncertain, and both plate margin and intraplate scenarios are possible. Deformation styles in greenstones and adjacent high-grade orthogneisses with intercalated shallow-water supracrustal assemblages suggest extensive horizontal shortening and crustal interstacking. This probably resulted from collisional and/or rotational motion and produced significant intracrustal melting as early as 3.9 Ga. By the end of the Archaean, and locally much earlier, crustal thicknesses reached 35–50 km or more, and the extraordinary high global crust-production rate between about 2.8 and 2.5 Ga may be ascribed to both subduction-related lateral accretion and to extensive magmatic underplating, which finally led to large, stable cratons. The apparent worldwide gap in major tectonic and magmatic activity during the early Proterozoic between about 2.5 and 2.2 Ga is an artifact due to lack of precise age data; both intracontinental as well as arc-type magmatic rocks were produced, though only locally. The voluminuous and worldwide crust-formation event at about 2.1-1.8 Ga provides the first evidence of modern-style plate tectonics, many magmatic rocks, such as in the Canadian Trans-Hudson orogen and in the Svecofennian of southwest Finland, have arc-type geochemical signatures, and both obducted ophiolites and blueschist assemblages have recently been discovered. In addition, the Proterozoic is characterized by long, linear mobile belts whose origin and evolution are still under debate. Many of these belts have tectonic features comparable to Phanerozoic collisional orogens, but in Australia and parts of southern Africa some Proterozoic belts appear to have formed through crustal stretching without lithospheric separation, and subsequent shortening leading to orogenic deformation may have been due to delamination and crustal subduction. Towards the end of the Proterozoic, evidence for modern-style plate tectonics becomes abundant through the recognition of island arc accretion, ophiolite obduction, foreland thrust and fold belts and exotic terranes. One of the best documented example for this tectonic environment is the Arabian-Nubian shield. Although it is doubtful whether modern plate tectonic settings can serve as analogues for all Precambrian tectonic styles, it is clear that global crustal evolution since the early Archaean was governed by the motion of plates which repeatedly came together to form supercontinents and then dispersed again. We shall not understand the detailed mechanism of Precambrian crustal tectonics before we learn how to interpret recent large-scale crustal deformation and its causes.