Metamorphism and timing of thrusting in the Tømmerås Window, central Scandinavian Caledonides

Abstract

The core of the Tømmerås Window is made up of Precambrian rhyolites with dykes and sheets of basic rocks. The Precambrian basement rocks are covered by a thin horizon of metasediments, the Bjørndalen Formation, which corresponds to the lower part of the Jämtland Supergroup in the eastern thrust front. The Bjørndalen Formation is in turn covered by a nappe composed of the same lithologies as the core of the window. The rocks of the Bjørndalen Formation were unmetamorphoscd prior to the Caledonian orogeny. Together with available isotopic mineral cooling ages and ages from fossiliferous strata, metamorphic data are used to determine the physical conditions and time constraints for the Scandian overthrusting in the Tømmerås area. It is concluded that the Scandian thrusting occured later than 435 Ma in the Tømmerås area. The thickness of the overthrusted nappe stack was about 30 km. Erosion down to a thickness of about 8 km at 415 Ma gives an erosion rate of 0.5–2.5 mm yr^-1. Such rapid erosion is inferred to have been accomplished by tectonic activity.

Key Words:

• Scandinavian Caledortides
• Tømmerås Window
• overthrusting
• Jämtland Supergroup
• metamorphism
• erosion rates