1991
Contact relationships between the Askvoll group and the basement gneisses of the Western Gneiss Region (WGR), Sunnfjord, Western Norway
71
1
pp. 15-27

The Askvoll group, comprising a thick mylonitic sequence. is situated between the Caledonian nappes and the underlying Western Gneiss Region (WGR) and defines the Kvamshesten Detachment Zone on the south side of the Devonian Kvamshesten basin. The tectonostratigraphy of the Askvoll group is subdivided into threc units: the upper Gjervika Unit comprises quartzofeldspathic mylonites and marbles: the middle Vikaneset Unit consists of metasediments. greenschists. ftaser gabbros. plagiogranites and amphibolite bands; and the lower Kumle Unit comprises metapsammite and garnet mica schist with lenses of garnet amphibolites. The WGR consists of migmatitic and granitic gneisses which are interfolded with amphibolites. metagabbros and metapsammites. The peak metamorphic assemblages preserved in the detachment zone are characteristic of amphibolite facies metamorphism and are observed in both the basement and the Askvoll group. Eclogites have been observed in the gneisses further to the east showing high-grade metamorphism of the basement. Progressive retrogression to low greenschist-facies metamorphism accompanied penetrative shear deformation and mylonitization of the Askvoll group. The contact between the WGR and the Askvoll group is characterized by a change in lithology and structural style along a broad zone. in part associated with the Kumle Unit that truncates the isoclinally folded rocks of the basement. The Gjervika Unit and the Vikaneset Unit are interpreted to be an allochthonous cover sequence to the underlying basement, whereas the Kumle Unit could be part of either the supracrustals in the basement or the lowermost part of the Askvoll group. The deformation and retrograde metamorphism, which are assigned to the orogenic collapse, occurred prior to the deposition of the Middle Devonian basins (380 Ma) and have overprinted and obliterated older tectonic fabrics.

0029-196X