Geophysical Interpretations

The lack of sub-horizontal reflectors within the core of the central uplift (region C, Fig. 10) is due to the rotation into vertical structures, where a still existing reflective pattern not can be detected by vertical incidence measurements.

The termination of seismic reflectors can neither be related to the transient crater nor to the excavated crater. It is the result of strong upturning and dismembering of lithologies caused by the collapse flow.

The collapse flow is thus the likely reason for the observed patterns of seismic reflectors seen in the sections down to ca. 14 km depth. At this level reflectors appear undisturbed. The seismic surveys do however not extent far enough outside the structure to enable a comparison with conditions in the nearby undisturbed uppermost crust.

Fig. 14. Examples of lineations in granites of the central rise. The lineation is penetrative through the normally massive granite (a and b). The size of samples is ca. 10 cm.

Fig. 14. Examples of lineations in granites of the central rise. The lineation is penetrative through the normally massive granite (a and b). The size of samples is ca. 10 cm.

In addition to the lineation also slightly stained curved surfaces are developed. These lineations have been interpreted in the geological map by Kresten et al. (1991) as shatter cone features.

An alternative interpretation is the formation of slickensides by the collapse flow. The two examples are from the NW sector of the peak ring.

Example of shatter cone developed in Siljan granite from the northern part of the central uplift.

Fig. 15. Fragmentation into large clasts of the sedimentary formations of the ring. Location is at the limestone quarry Kallholen in the NW part of the ring.

The extent at depth of mega-clasts of Paleozoic cover rocks and their orientation is of considerable interest to study. In this case more detailed gravity measurements may be needed. If these clasts mainly contain limestone they would have a generally higher density compared to the surrounding crystalline basement. Where sedimentary rocks occur, MT measurements or vertical electrical sounding (VES) measurements can be used as a depth constraint for the gravity interpretation

Additional MT measurements have the potential to better map the extent of impact induced fracturing at depth and should be extended over the entire structure and a significant part of its surroundings.

The extent of fracturing is of particular interest for geothermal energy prospecting. If the fractured rock volume at depth can be exploited as a heat exchange medium, very large thermal energy resources could be made available (Henkel 2003).

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