6533b834fe1ef96bd129d4fe

RESEARCH PRODUCT

Tectonic evolution in the Archaean and Proterozoic

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description

Kre 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 volurnlnous production of tonalite-trondhjemite-granodlorite magmas is still uncertain, and both piate ma@ and intrapiate scenarios arc possible. Deformanon atylea in greenstones and adjacent high-grade orthognersses 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 cxtl-aordinary hhlding to erogenic 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 laland a16 accretion, ophiolite obducdon, foreland thrust and fold belts and exotic terranes. One of the best documented example for this tectonic environment is the Arabian-Nubian shield. .4lthough it is doubtful whether modern plate tectonic settings can serve as analogues for all Precambrian tectonic styles, it i\ clear that global crustal evolution since the early Archaean was governed by the motion of plates which repeatedly came togethel TV fotm supercontinents and then dlspersetl again. We shall not understand the detailed mechanism of Precambrian crustal itLton>cs before we learn horn LO irltejpret recent large-scale crustal deformation and its causes.