Geophysical Investigations of the Siljan Impact Structure A Short Review

Herbert Henkel1 and Sven Aaro2

'Department of Land and Water Resources Engineering, Royal Institute of Technology, S-10044 Stockholm, Sweden ([email protected])

2Geological Survey of Sweden, Division of Geophysics, Box 670, S-75128 Uppsala, Sweden ([email protected])

Abstract. Siljan in southwest Sweden is the largest impact structure in western Europe, with a present topographic diameter of ca. 75 km. Recent age determinations indicate an age of 377 Ma. The bedrock geology of the region has recently been re-mapped by the Geological Survey of Sweden in the scale of 1:50 000. There is now complete coverage with airborne geophysics. New maps of the geophysical data have been prepared for this review. In connection with the Deep Gas Project, further geophysical studies were made and two drill holes were sunk to over 6 km depth in the central uplift of the structure. The Deep Gas Project produced a large number of reports and publications, which are listed in the summary report of Juhlin (1991). Some of the results are compiled and shortly summarized here. Digital elevation data are available with 50 m spatial resolution, and a gray tone map has been prepared with the regional trend removed. A profile of these data shows that the peak ring of the structure is still visible in the morphology.

Fig. 1. Location of the Siljan Impact Structure. Impact structures smaller than 20 km in diameter are shown with square symbols. Modified from Abels et al. (2000).

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