Another look at reflection seismic data acquired over the Siljan Ring impact structure, central Sweden

Several high quality reflection seismic profiles were acquired over the Siljan Ring impact structure in the mid- to late-1980s using an explosive source and 80 to 120 channels. Furthermore, two deep boreholes, Gravberg-1 and Stenberg-1, were drilled to more than 6 km depth in the area in search for abiogenic gas. Boreholes to these depths in Precambrian rock are rare and provide some ground truth for interpretation of the seismic data. Results from the seismic data have been presented in a number of publications where we have demonstrated that data have (1) helped to improve our understanding of the impact cratering process (Juhlin and Pedersen, 1987, 1993), (2) allowed us to identify the source of some of the seismic reflectivity in Precambrian basement (Juhlin, 1990; Papasikas and Juhlin, 1997) and (3) provided preliminary mapping of the Moho in the area with near vertical incidence seismic data (Juhojuntti and Juhlin, 1998). Even though these studies were initiated more than 40 years ago, the data can still provide new insight into crustal structure and the potential to image it in detail. Reprocessing of the data in a consistent manner with modern software and careful merging of the various profiles allows different perspectives of the data to emerge compared to earlier results. In particular, a strong band of reflectivity at about 5 s TWT (c. 15 km) is present throughout most of the area. Reflections from this band appear to be of a different nature from the shallower more distinct reflections that are known to originate from dolerites that have intruded into the surrounding granitic rock. Whether or not all the shallow reflections originate from dolerites is still an open question. I will present the latest results from the reprocessing, an expanded interpretation of the crustal structure and some suggestions for future research.

Juhlin C., 1990, Interpretation of reflections in the Siljan Ring area based on results from the Gravberg-1 borehole, Tectonophysics, 173, 345-360.

Juhlin C. and Pedersen L. B., 1987, Reflection seismic investigations of the Siljan impact structure, Sweden, J. Geophys. Res., 92, 14113-14122.

Juhlin C. and Pedersen L.B., 1993, Further constraints on the formation of the Siljan impact crater from seismic reflection studies, Geologiska Föreningens i Stockholm Förhandlingar 115, 151-158.

Juhojuntti N. and Juhlin C., 1998, Seismic lower crustal reflectivity and signal penetration in the Siljan Ring area, central Sweden, Tectonophysics, 288, 17-30.

Papasikas N. and Juhlin C., 1997, Interpretation of reflections from the central part of the Siljan Ring Impact structure based on results from the Stenberg-1 borehole, Tectonophysics, 269, 237-245.