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 i ntrusive, tectonic, and metamorphic history. In the northeastern central pa rt of the orogen, in the Ussuit area, Palaeoproterozoic intrusive and supra crustal rocks are sandwiched between slices of Archaean rocks forming a sta ck of lithotectonic units. Juxtaposition of these units occurred during wes t- to northwest-vergent thrusting along ductile shear zones (= D1) associat ed with a foliation formed at upper amphibolite facies conditions. D1 struc tures were folded (= F-A) and then reactivated and locally omitted by local ized east- to east northeast-vergent extensional ductile shear zones (= D2) at near-peak metamorphic conditions. Shallowly east-plunging, transport-pa rallel upright folds (= F-B) affect but are also truncated by D2 extensiona l shear zones, suggesting an interplay between F-B folding and D2 shearing, compatible with a scenario of overall shortening perpendicular to fold axi al surface during simultaneous extension parallel to fold axes. Consequentl y, the structures preserved in the Ussuit area document a deformation histo ry which results from a change from thrusting to syncollisional extension o ccurring in mid- to lower crustal levels at peak metamorphic conditions.