Geological modelling based on gravity data from the central part of the Swedish Caledonides

Abstract

Gravimetric measurements have been carried out within a c. 500 km² area of western Jämtland in the Swedish Caledonides. Using different types of regional-residual gravity field separations, various geological models have been tested to fit the measured gravity data. Information on the geology and density of the rocks have reduced the number of possible models which are presented along two profiles. These models confirm the existence of antiforms and synforms running approximately parallel to the mountain range. The main profile crosses the two synforms over which positive anomalies are identified. Both in the western Tännfors Synform and the eastern Åre Synform this is explained by the presence of high density Seve rocks, in the former case occurring beneath a cover of Köli Nappes. The depth to the base of the Seve Nappe units in the Tännfors Synform is calculated at 4.5 km and in the Åre Synform at 3 km. The maximum depth to the interface between Köli and Seve rocks in the Tännfors district is 3.2 km. Gravimetric models of the basement rocks in the windows, and difference in physical properties between the crystalline rocks of the windows and those of the autochthonous basement east of the Caledonian thrust front, imply that the basement exposed in the antiforms is allochthonous. A gravity minimum east of the Åre Synform is interpreted as a southern continuation of low density granites related to the Olden Complex. The inferred thickness of these low density rocks suggests that it may be a part of the autochthonous basement.

Key Words:

• Gravimetric interpretation
• rock densities
• geological models
• Caledonides
• Tännfors Synform
• Åre Synform
• basement windows
• Jämtland
• Sweden