Laterally constricted shear zone system - the Nordre Stromfjord steep belt, Nagssugtoqidian Orogen, W Greenland

Many ductile shear zones are interpreted to operate by simple shear flaw but some form under other flow regimes. Lineations and foliations in such shear zones can lie obliquely to those in simple shear zones, which can lead to erroneous tectonic interpretations on the assumption of simple shear flow. This paper describes a gently dipping shear zone system from the N-central segment of the Palaeoproterozoic Nagssugtoqidian orogen of W. Greenland, which operated with a lateral constriction component. This resulted in the development of upright folds with axes parallel to the transport direction where the constriction component is weak. Where it is strong, a linear fabric and even a subvertical foliation normal to the rotation axis of bulk flow developed. This steep foliation is interpreted as the origin of the Nordre Stromfjord steep belt, previously interpreted as a crustal-scale sinistral transcurrent shear zone. Shear zones of this type may occur elsewhere and shear zone fabrics should therefore be carefully analysed before the direction of tectonic transpost can be determined.