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A field study in the coesite province, the deepest unit of the Norwegian Caledonides, gives new constraints on the rheological behaviour of the continental crust during exhumation. Lithological heterogeneities and differential retrogression led to crustal‐scale boudinage during the late‐orogenic intense E–W stretching event in the footwall of the Nordfjord‐Sogn Detachment. The main gneissic lithologies display a modest but widespread syn‐exhumation migmatization. Textural criteria allow estimation of a 30% fusion rate. Partial melting mostly post‐dates eclogitization and is synchronous with ductile stretching and top‐to‐west shearing. Presented observations suggest that the melt reactions and migmatization resulted in a soft rheology. During subduction to ~ 100 km depth and subsequent exhumation, crustal viscosity can be reduced by up to four orders of magnitude. Models are discussed that consider a …