Tectonostratigraphy and structure of the Kjukkelen area, Børgefjellet, north-central Norway

Abstract

The Kjukkelen area in the Børgefjellet National Park occupies a key position concerning the tectonostratigraphical relationship between the Køli Nappes north and south of the Børgefjellet Window (BW). The thickness of the Køli Nappes between the rock units of the BW and the overlying Helgeland Nappe Complex (HNC) is locally reduced to 75 m in the Kjukkelen area. Several Køli Nappes (Stikke, Gelvenåkko and Leipikvattnet N.) consisting of Røyrvik Group lithologies wedge out along the southwestern margin of the BW. Only the Gjersvik Nappe consisting of Limingen Group lithologies enters the narrow passage between the BW and the HNC. North of the passage, Køli lithologies belong to the Røyrvik Group. It is uncertain, which nappe unit they can be correlated with. Rocks belonging to the Limingen Group reappear further north in the Jofjället Nappe which attenuates southward beneath the HNC. Investigations in the Kjukkelen area show that the Jofjället Nappe appears at the tectonostratigraphic level of the Gjersvik Nappe and thus might be its lateral continuation. The low-grade metamorphic phyllites and schists of the Køli Nappes are strongly deformed by isoclinal folding and transposition, whereas the deformation of the more competent gneisses and intrusions of the BW and the HNC as well as the quartzites of the Susendalen Nappe is dominated by imbrication. The geometry of the deformational structures suggests that thrust movement at the base of the HNC continued after the emplacement of the lower nappes. At least two generations of structures (D₂ and D₃) are thought to be generated by HNC thrusting.

Key Words:

• Tectonostratigraphy
• Køli Nappes
• Helgeland Nappe Complex
• Børgefjellet Window
• Norway