A description and interpretation of the Proterozoic Kobbefjord Fault Zone, Godthåb district, West Greenland

Abstract

The Kobbefjord Fault Zone (KFZ) is recognized as the largest of a group of early Proterozoic dextral transcurrent fault zones that cut Archaean gneisses in the vicinity of Godthåb. Detailed fjeld study of the KFZ reveals that it corresponds to a zone of deformation and alteration - 100 m wide, which completely obliterate pre-existing structures in country rock gneisses. Rocks within the KFZ are dominantly quartz-rich laminated mylonites characterized by regionally consistent NE-SW trending subvertical foliations and subhorizontallineations. Such mylonites were produced by intense ductile deformation, although local pseudotachylites and cataclasites occur which indicate minor amounts of brittIe deformation as well. Alteration in the KFZ involved pervasive pink staining and/or bleaching, and development of hydrated mineral assemblages (Chl + Ep + Musc), indicating an important role for fluid flux during the faulting process. Movement on the KFZ was probably initiated in the time period 2000-1800 Ma, as it displaces mafic dikes which appear correlative to dikes to the north of the Godthåb area which yield ages in this range. Deformation in the KFZ most plausibly represents internal deformation of a stable continental block subjected to compression on at least two sides. Data for fabric elements in the KFZ and for associated conjugate shears indicate that the principal compressive stress (o1) for this faulting episode was oriented W-NW, but precise relationships to deformation events on the margin of this block have yet to be deduced.