The Nordre Str¿mfjord shear zone is exposed for a length of 170 km within the Precambrian Nagssugtoqidian Mobile Belt of West Greenland. Its eastern 100 km consists of a 15--20 km wide zone of syndeformational granulite facies rocks. The movement was sinistral with a displacement of 120 km across the zone. The strain across the shear zone margin increases exponentially but levels off to a near constant value over major sections of the inner zone. This profile suggests a growth model, in which the marginal zones are focii of instantaneous deformation that moves outward as hardening occurs along the inner boundaries and weakening occurs along the outer boundaries. This growth model allows the minimum possible viscosity change associated with the zone to be calculated from the logarithmic strain gradient and from the width across which weakening occurs. When applied to an area within the granulite grade part of the shear zone, this model indicates a decrease in viscosity by a factor 10 to 50, and the functional form of this weakening shows that the parameter undergoing change enters the exponential part of the appropriate constitutive equation. The available evidence seems to exclude shear heating as a viable weakening mechanism but supports chemical weakening. It is hypothesized that 'hydrolytic weakening' was responsible. At higher crustal levels a fundamental change in geometry of the Nordre Str¿mfjord zone occurs. The applicability of the present model is thus restricted to deep crustal levels. In conclusion it is illustrated that the model is of relevance to small scale shear zones as well and may also be representative of mantle deformation.