SIGNIFICANCE OF CRUSTAL-SCALE SHEAR ZONES AND SYNKINEMATIC MAFIC DYKES IN THE NAGSSUGTOQIDIAN OROGEN, SW GREENLAND - A REEXAMINATION

The published accounts of the structural geology and tectonic evolutio n of the Early Proterozoic Nagssugtoqidian orogen, SW Greenland,requir e modification in several fundamental respects. The geometry of the or ogen has been attributed to important displacements on major conjugate shear zones and thrust zones. From new field observations, we are una ble to confirm that crustal-scale shear zones have played an important role in Nagssugtoqidian tectonics. The 'dextral' component of 'conjug ate' shear zones is demonstrably sinistral and transpressive, strike-s lip shear zones are incipient, and a major thrust zone within the orog en cannot be confirmed at its projected inland location. The sinistral strike-slip Nordre Stromfjord shear zone is an order of magnitude sma ller than previously thought. It comprises an array of non-linked segm ents of annealed mylonite and cannot have accommodated large displacem ents. The dextral 'Itivdleq shear zone' is characterised by heterogene ous, sinistral, noncoaxial flow, but compared with deformation elsewhe re in the orogen, it is not a large-scale zone of strain localisation. 'Ikertoq thrust zone' is not an orogen-scale shear zone, but appears to be part of a large-scale, rheological boundary. Application of an P n relais fracture array model to what is classically identified as the Kangamiut mafic dyke swarm suggests that the sinistral vorticity dete rmined throughout much of the Nagssugtoqidian orogen is detectable in the Archean foreland up to 150 km south of the orogenic front. (C) 199 7 Elsevier Science Ltd.