>> A B S T R A C T

>> 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 wide-spread 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 transition
>> from a small internal strain of the crust to viscous flow during
>exhumation.