Change from thrusting to syncollisional extension at a mid-crustal level: an example from the Palaeoproterozoic Nagssugtoqidian Orogen (West Greenland)

The Palaeoproterozoic Nagssugtoqidian Orogen in West Greenland represents a mid- to deep-crustal section through a collisional orogen with a complex intrusive, tectonic, and metamorphic history. In the northeastern central part of the orogen, in the Ussuit area, Palaeoproterozoic intrusive and supracrustal rocks are sandwiched between slices of Archaean rocks forming a stack of lithotectonic units. Juxtaposition of these units occurred during west- to northwest-vergent thrusting along ductile shear zones (= D1) associated with a foliation formed at upper amphibolite facies conditions. D1 structures were folded (= FA) and then reactivated and locally omitted by localized east- to east-northeast-vergent extensional ductile shear zones (= D2) at near-peak metamorphic conditions. Shallowly east-plunging, transport-parallel upright folds (= FB) affect but are also truncated by D2 extensional shear zones, suggesting an interplay between FB folding and D2 shearing, compatible with a scenario of overall shortening perpendicular to fold axial surface during simultaneous extension parallel to fold axes. Consequently, the structures preserved in the Ussuit area document a deformation history which results from a change from thrusting to syncollisional extension occurring in mid- to lower crustal levels at peak metamorphic conditions.