A horizontal tectonic regime in the Archaean of Greenland and its implications for early crustal thickening

The Archaean complex of Greenland consists of a layered sequence of gneisses whose history can be traced back from 2,600 m.y. to before 3,750 m.y. ago. Quartzo-feldspathic gneisses that are considered to be derived from intrusive granitic rocks make up more than 80% of the complex. They were emplaced during at least two separate periods of plutonism. The remainder of the complex consists of supracrustal rocks, mainly amphibolites and semipelitic gneisses with smaller amounts of quartzite and calcarous rocks, and metamorphosed basic igneous rocks dominated by leucogabbros and anorthosites. Lithologic units of different ages and provenances were intercalated by nappe-like folding and thrusting and were intruded by syntectonic sheets of calc-alkaline granitic rocks. The resulting sequence was repeatedly folded and deformed under high grade metamorphic conditions and the layering was further emphasised. This combination nappe-like folding, thrusting and granite injection is considered to have resulted in a considerable amount of crustal thickening. Granulite facies metamorphism ensued at depth as the base of the thickened sialic mass dried out. In this way a stable continental mass was formed with a refractory base of high grade rocks depleted in radioactive elements. It is suggested that the driving force responsible for the folding, thrusting and generation and intrusion of calc-alkaline magmas was some form of sub-horizontal movement within the mantle and between the mantle and thin crust. This dominantly horizontal tectonic regime is contrasted with the dominantly vertical tectonic regime described from the greenstone belt—granite terrain of southern Africa.