Steep extrusion of late Archaean granulites in the Northern Marginal Zone, Zimbabwe: evidence for secular change in orogenic style

Abstract

The Northern Marginal Zone (NMZ) of the Limpopo Belt in southern Zimbabwe is an essentially plutonic terrain, now consisting of granulite-gneisses, that was assembled by extensive magmatic under- and intraplating between ca. 2.7 and 2.6 Ga during late Archaean crustal shortening. In this study, we present the first evidence for regional-scale normal shear in the NMZ. Normal shear has been identified along an up to 500m wide, steeply dipping belt of mylonites, the Mtilikwe shear zone. Geochronologic and mineralogical-textural data indicate that normal shear occurred coeval with crustal shortening that was accommodated along gently-dipping thrust zones. The bulk strain pattern consisted of horizontal shortening, vertical stretching, and sub-vertical extrusion of a crustal segment. Horizontal shortening persisted throughout the orogenic evolution in the NMZ and no features typical of gravitational collapse in modern orogens are seen. This could imply that there was never major overthickening of the crust by thrusting, which may be a characteristic feature of Archaean orogeny.