A modern structural regime in the Paleoarchean (∼3.64 Ga); Isua Greenstone Belt, southern West Greenland

Abstract

The footwall gneisses beneath the western part of the Paleoarchean (∼3.8 Ga) Isua Greenstone Belt, southern West Greenland, are interpreted here in terms of a ∼3.64 Ga stack of mylonitic crystalline thrust-nappes, the oldest example known on Earth. In present coordinates, the kinematic history of the thrust-nappe stack is couched in terms of initial longitudinal (strike-parallel) thrusting towards the southwest, followed by transverse thrusting to the northwest, and subsequent extensional collapse of the thickened crust toward the southeast. Diorite and tonalite that form the western margin of granitoids, structurally overlying the western part of the Isua Greenstone Belt and its footwall, contain ∼3.5 Ga mafic dykes, some of which are deformed and/or truncated at fault contacts within the granitoids. Accordingly, a component of the deformation structurally above the Isua Greenstone Belt occurred after ∼3.5 Ga, significantly later than the formation of the underlying mylonitic nappes, probably during the Neoarchean. The structural regime of mylonitic thrust-nappe stacking is very similar to that in modern mountain belts. It would appear that the deformational behaviour, rheological constitution and overall strength of Paleoarchean and modern continental crust were similar.