The Seve—Köli Nappe Complex of the Handöl—Storlien—Essandsjøen area, Scandinavian Caledonides

Abstract

The Seve Nappes of the area comprise three tectonic units, the Blåhammarfjället Nappe overlain by the Snasahögarna Nappe and the Täljstensvalen Complex. The Blåhammarfjället Nappe is dominated by schists and amphibolites; they exhibit an inverted metamorphic trend ranging from upper greenschist to middle or upper amphibolite facies. The overlying Snasahögarna Nappe mainly consists of granulite facies K-feldspar-sillimanite gneisses. The Täljstensvalen Complex comprises amphibolites and kyanite (± staurolite) gneisses and schists. It contains imbricated slices from the underlying and possibly also from the overlying units. The Seve Nappes are overlain by Köli units, the lower of these being the Handöl Formation, comprising calcareous mica schists, metavolcanites, metagabbro and ultramafic rocks. This unit is overlain by the more extensive Visjön Formation, dominated by garbenschiefer. The grade of both units is generally upper greenschist to lowermost amphibolite facies. In the Essandsjøen area, the Köli units exhibit a normal metamorphic trend, ranging from lower amphibolite to greenschist facies. The Seve and Köli Nappes thin rapidly westwards but some units reappear in the south-western part of the area. They are overlain by the Trondheim Nappe Complex. Inter- and intra-unit blastomylonites indicate that a composite nappe complex was formed by Caledonian, possibly syn-metamorphic thrusting. The thrust between the Seve and the Köli is a zone of uppermost greenschist or amphibolite facies blasto-mylonites in the east; further west it is marked by greenschist facies phyllonites. This change is ascribed to events post-dating the emplacement of the Köli onto the Seve. It possibly indicates that the formations in the Köli actually are separate tectonic units. In the Köli and schistose parts of the Seve, four Caledonian deformation episodes (D₁-D₄) are recorded. The different tectonic styles in units of the Seve-Köli Nappe Complex reflect rheologic differences but probably also dissimilar early structural histories. A two-stage model for the Caledonian tectonic evolution is suggested. The first stage involved overall shortening and thrusting leading to the development of a composite nappe pile. The second stage in the model was characterized by large horisontal movements probably due to gravitational collapse of the pile. The smooth basement culminations may have been formed in response to the gravitational instability caused by the superposition of dense nappes onto the Baltic Shield.

Key Words:

• Thrust nappes
• greenschist facies
• amphibolite facies
• granulite facies
• retrogression
• mylonite
• imbrication
• pseudotachylite
• phyllonite
• trondhjemite
• garbenschiefer
• pangenesis
• Autochthon
• Lower Allochthon
• Middle Allochthon
• Upper Allochthon
• Seve-Köli Nappe Complex
• Trondheim Nappe Complex
• Offerdal Nappe
• Särv Nappes
• Seve Nappes
• Köli Nappes
• Blåhammarfjället Nappe
• Snasahögarna Nappe
• Täljstensvalen Complex
• Handöl Formation
• Visjön Formation
• Tännforsfältet
• Rørosfeltet
• south central Caledonides
• N6305
• N6324
• E1237
• E1149