Basement-cover relationships on northern Vanna, Troms, Norway.

Remapping of mylonitized Precambrian tonalitic basement gneisses on north em Vanna has shown them to be allochthonous (Skipsfjord Nappe) relative to the metatonalites further south. The allochthonous rocks are separated from the tonalites on northern Vanna by a major high-angle brittle normal fault, possibly of Mesozoic age. The Skipsfjord Nappe is composed of three major lithotectonic units; a lower and upper mylonite-gneiss sheet separated by a metasedimentary sequence (Kvalkjeften group). The latter probably has a tectonically disturbed depositional contact against the lower mylonite-gneiss sheet. The lower third of the Kvalkjeften group is composed of metapsammite and calcareous metapelite (Geitdalen fm.), whereas the upper part is dominated by metapelites with some minor quartzite towards the top (Brattfjell fm.). Both the allochthonous mylonite gneisses and the Kvalkjeften group are transected by mafic sheets, now transformed to biotite-chlorite schists. A correlation with the dike intruded portions of the Kalak Nappe Complex is suggested. The mylonitized upper portion of the basement, the allochthonous mylonite gneisses and the psammitic lithologies within the Kvalkjeften group all have a well-developed mylonitic foliation with a NW-SE trending stretching lineation. The basement rocks north of Vannareid are transected by an anastomosing network of steeply dipping shear zones. The shear rones strike NNE and dip steeply WNW. They are interpreted to be due to crustal shortening at a late stage in the Caledonian orogeny.