The outcrop pattern of Caledonian allochthons in northern Norway is controlled by two late fold sets: cross folds (F3), which have axes at a high angle to the structural trend of the orogen, and back folds (F4), which have axes parallel to the structural trend but verging in the opposite direction from that of nappe emplacement. Interference of cross folds and back folds produces Ramsay type I (dome and basin) and type II (boomerang or canoe) interference patterns on all scales. The Ofoten area contains the closure of a regionally extensive, northeast-trending back fold, the Ofoten synform. Within its core are klippen of the structurally highest nappes preserved in this portion of the Caledonides. At Tysfjord, 15 km south of Ofoten, some of the deepest structural levels within the orogen are exposed in the core of the Tysfjord cross-fold culmination.
The Norwegian Caledonides contain two strike-parallel belts of gneiss domes underlain by Precambrian basement rocks flanking an elongate structural basin to which the Ofoten synform in part corresponds. In Ofoten- Tysfjord, these domes and basins reflect a large-scale cross-fold-back-fold interference pattern. Structural characteristics of the cross folds and back folds indicate that they formed in response to layer-parallel shortening, implying that gneiss domes in these areas reflect interference of folds formed in crustal compression rather than diapirism. Because this dome-and-basin pattern is present along virtually the entire length of the Norwegian Caledonides, we suggest that other gneiss domes in Norway may have a similar origin.
The orientation of the cross folds in Ofoten-Tysfjord suggests a component of sinistral shear affecting the orogen after the Early Silurian and before the Late Devonian. Paleomagnetic and structural data from other parts of the Caledonides suggest large-scale mid-Paleozoic (post-collisional) lateral shear, commonly with a sinistral sense. Although Paleozoic strike-slip faults have not been identified in the Caledonides at this latitude, we suggest that the cross folds may be an alternative expression of large-scale sinistral shear.