Geology of the Nordhallen—Duved—Greningen area in Jämtland, central Swedish Caledonides

Abstract

A lithotectonic sequence has been studied in the Nordhallen—Duved—Greningen area and found to consist of a Precambrian basement overlain by four distinct thrust-nappe units—the Kallsjon, Quartzite—Mylonite, Seve and Köli Nappes. This nappe sequence is folded by the Mullfjället Antiform, the western limb of which composes much of the investigated area. An important tectonic feature, recognized during the investigation, is the lenticular geometry of these nappe units, as well as individual formations within the highest tectonic unit—the Köli Nappe. The stratigraphy within the Köli Nappe of the Tännforsen area is described in detail and correlated with the classic Köli stratigraphy in Västerbotten. Study of the minor structures within the Köli Nappe reveals a polydeformational sequence (F₁, F₂, F₃). Isoclinal to tight folding occurred prior to (F₁) and during (F₂) the development of the main foliation (S₂). The third-phase structures (F₃) deformed the main foliation and are open, asymmetric kink folds and box folds. The peak of the metamorphism of the Köli rocks, represented by garnet and biotite porphyroblastesis, was established prior to the formation of S₂.