Caledonian nappe emplacement: an example from Uløya, Northern Norway

Abstract

On Uløya, an island located east of the Lyngen peninsula in northern Troms, Norway, four Caledonian nappes are exposed. These are from bottom to top: part of the Kalak Nappe Complex (KNC), the Vaddas, the Kåfjord and the Nordmannvik nappe. In the present study the geology of these nappes is described, macroscopic and microscopic deformation structures are analyzed, and phase equilibrium modeling is presented to define Caledonian deformation and emplacement of the nappes. A penetrative foliation dipping at an angle of 20-40° to the SW displays a pervasive stretching lineation plunging NW or SE. Nappe emplacement took place by thrusting to the SE as indicated by the dominant sense of shear observed in the field and in thin section. Thrusting was accompanied by strong shearing documented by the penetrative foliation and various mylonites. Two Caledonian fold generations were identified, a first generation of isoclinal to tight folds and a second generation of closed to open folds, which refold the first generation folds. Both generations have fold axes parallel to the Caledonian stretching lineation trending NW-SE, indicating a typical Caledonian orientation. Metamorphic conditions estimated in the KNC, the Vaddas, the Kåfjord and the Nordmannvik nappe are in accordance with metamorphic conditions of a subduction and collision environment as reconstructed for the Caledonian orogeny. Parts of Baltica were subducted beneath Laurentia followed by collision of the two continents. The nappes show increasing metamorphic conditions (pressure and temperature) with the exception of the Vaddas nappe, which shows much higher pressures than the overlying Kåfjord and Nordmannvik nappes. A different subduction mechanism or out-of-sequence-thrusting may have been active to transport the Vaddas nappe to greater depth than the overlying nappes. Gneisses of the KNC and the Nordmannvik nappe itself show migmatization which in the Nordmannvik nappe was overprinted by subsequent Caledonian solid-state shearing. P-T estimates of 8.8-9.8 kbar and 760-775°C from Gasser et al. (2015) for Neoproterozoic migmatization in meta-sedimentary rocks of the KNC are very similar to conditions of migmatization estimated for the Nordmannvik nappe in the present work (~8.5-10 kbar and ~750-780°C), possibly indicating a common previous tectonometamorphic history of the KNC and the Nordmannvik nappe. Caledonian solid state shearing occurred at higher pressures and lower temperatures than migmatization.