Significance of crustal-scale shear zones and synkinematic mafic dykes in the Nagssugtoqidian orogen, SW Greenland: a re-examination

Abstract

The published accounts of the structural geology and tectonic evolution of the Early Proterozoic Nagssugtoqidian orogen, SW Greenland, require modification in several fundamental respects. The geometry of the orogen has been attributed to important displacements on major conjugate shear zones and thrust zones. From new field observations, we are unable to confirm that crustal-scale shear zones have played an important role in Nagssugtoqidian tectonics. The ‘dextral’ component of ‘conjugate’ shear zones is demonstrably sinistral and transpressive, strike-slip shear zones are incipient, and a major thrust zone within the orogen cannot be confirmed at its projected inland location. The sinistral strike-slip Nordre Strømfjord shear zone is an order of magnitude smaller than previously thought. It comprises an array of non-linked segments of annealed mylonite and cannot have accommodated large displacements. The dextral ‘Itivdleq shear zone’ is characterised by heterogeneous, sinistral, noncoaxial flow, but compared with deformation elsewhere in the orogen, it is not a large-scale zone of strain localisation. ‘Ikertôq thrust zone’ is not an orogen-scale shear zone, but appears to be part of a large-scale, rheological boundary. Application of an en relais fracture array model to what is classically identified as the Kangaˆmiut mafic dyke swarm suggests that the sinistral vorticity determined throughout much of the Nagssugtoqidian orogen is detectable in the Archean foreland up to 150 km south of the orogenic front.