Tectonic position and evolution of the Balsfjord Series in the North Norwegian Caledonides

The Balsfjord Series in Troms and Finnmark, N-Norway is part of a thrust-related nappe stack emplaced during the Ordovician–Silurian Caledonian orogeny. It overlies the Lyngen Magmatic Complex and Reisa Nappe Complex in the E and is overlain by the Nakkedal and Tromsø nappes in the W. Past research on the geological history of the Balsfjord Series was only undertaken locally and the tectonic meaning of this unit is still poorly understood. This is especially evident considering its role as a low–medium grade unit situated between two high grade complexes with diachronous evolution.

The structural evolution of the Balsfjord Series is characterized by three sets of deformation structures. In low-grade areas, the original bedding S0 was affected by boudinage with generally WSW-ENE-oriented stretching axes. In higher-grade regions, the S0 was folded by tight–isoclinal F1 folds showing flat axial surfaces parallel to the surrounding penetrative metamorphic foliation S1. The FA1 fold axes are parallel with the stretching lineation Ls1, and both show considerable rotation from a NW–SE orientation in the NW towards E–W and ENE–WSW in the SE of the area. The F1 folding was syn-metamorphic as it folded the bedding and simultaneously developed the peak metamorphic assemblage in the S1 fabric. A second deformation phase locally folds the metamorphic fabric S1 and Ls1. It is represented by open–tight F2 folds with flat–moderately dipping fold axial surfaces in higher-grade areas, or by development of deformation bands in low-grade rocks. The latest set of structures is represented by steep F3 folds and associated axial planar cleavage S3. The F3 folding and cleavage development becomes increasingly accentuated closer to the contact of the Balsfjord Series with the Lyngen Gabbro.

Mineral assemblages and P-T estimates show that the Balsfjord Series features an inverse metamorphic gradient with conditions increasing from the SE into higher tectonostratigraphic levels towards the W and NW. Thermodynamic modelling revealed maximum P-T conditions of ~450°C and 6.5 kbar in the garnet-zone of the unit, increasing up to 600 °C and 8 kbar in the staurolite-bearing uppermost levels. U–Pb dating of monazite associated with the peak mineral assemblage yielded ages between ca. 425–435 Ma, coeval with localized deformation of the basement rocks.

Our observations together with published data from the surrounding units suggest a tectonic scenario, which involves two suture/thrust zones. The uppermost Tromsø and Nakkedal nappes reached their metamorphic peak at ca. 450 Ma. Their exhumation to upper crustal levels likely occurred soon after that and there these units remained tectonically dormant. At ~440 Ma, the Nordmannvik Nappe of the Reisa Nappe Complex reached its peak metamorphism as a part of the eastern subduction channel. Final exhumation of the Nordmannvik Nappe and closure of the eastern suture took place at ~430 Ma. This was accompanied/followed by underthrusting of the Balsfjord+Lyngen nappe assembly in the west under the Tromsø+Nakkedal+nappe assembly  causing the deeper burial and peak metamorphism of the Balsfjord Series at around the same time.