Gently dipping fracture zones in Paleoproterozoic metagranite, Sweden: Evidence from reflection seismic and cored borehole data and implications for the disposal of nuclear waste

Abstract

The Svecokarelian orogen in the coastal area north of Stockholm, central Sweden, is characterized by a complex deformation belt. In this belt, bedrock domains strike WNW-NW, dip steeply, and are affected by high ductile strain. These domains anastomose around tectonic lenses that are up to a few tens of kilometers in length and are a few kilometers wide. The bedrock within the lenses is folded and, in general, affected by lower ductile strain. Site investigations for the disposal of highly radioactive nuclear waste are presently in progress within one of these lenses. Approximately 16 km of high-resolution (10 m source and receiver spacing, 100 channels) seismic data were acquired in 2002 along five separate profiles (2 to 5 km in length) within this tens. In the southeast, reflection orientations are well determined where profiles cross one another. Here, there is a nearly one to one correlation of distinct reflections on the seismic sections with hydraulically conductive fracture zones in cored boreholes, indicating that these fracture zones control the reflectivity. However, smaller lenses of amphibolite within the fracture zones may enhance the reflectivity. The reflections dip gently to the SSE and SE, and the zones in the boreholes are dominated by fractures that show the same dip direction or are subhorizontal. A working hypothesis for the gently dipping fracture zones suggests that these structures formed as minor thrust faults during the later stages of the Svecokarelian orogeny. Mineral fillings and coatings along fractures in these zones indicate later reactivation. The current stress field, a high intensity of fractures in the zones at a high angle to CF3, high in situ stresses in the bedrock, and rapid glacial unloading with the development of high differential stresses may explain the large volume of groundwater in these zones. The reflection seismic data, in combination with the verifying work presented in the cored borehole data, have steered the subsequent focus in geoscientific studies to the northwestern part of the candidate area. Copyright 2006 by the American Geophysical Union.