INTERACTION OF STRONG COMPRESSIVE STRESSES WITH TOPOGRAPHY: IMPLICATIONS FOR BEDROCK FRACTURES IN FORSMARK, SWEDEN (Invited Presentation)

Topographic perturbation of ambient stress fields can produce vertically and laterally varying subsurface stresses, which can influence the abundance, openness, and orientations of bedrock fractures at depth. The stress state on bedrock fractures can affect the permeability of bedrock, produce non-uniform groundwater flow paths, and affect the rate and distribution of surface processes. In this study, we examine the connections among topographic stress, bedrock fractures, and hydraulic conductivity in Forsmark, Sweden. This area provides an excellent opportunity to study these connections because it has strong horizontal compression (~ 20 MPa) and there are extensive geophysical and hydrogeological datasets available as a result of ongoing investigations of Forsmark as a potential nuclear waste repository site. Previous studies suggested that the strong ambient compressive stresses are probably from regional tectonic stresses or spatial and temporal changes of ice loads. We calculate topographic perturbations of subsurface ambient stress fields in the area surrounding Forsmark using a three-dimensional boundary element model. We use high-resolution topographic maps of the land surface and bedrock surface as zero-traction boundary conditions and apply ambient stress conditions inferred from more than 100 in-situ stress measurements from overcoring and hydro-fracturing. We consider the effects of ice and sediment loads and pore water pressure. Our preliminary results show that the topographic perturbation of the stress fields can extend to 200 m below the surface despite low topographic relief (e.g., ~20 m), which is due in part to the strong, anisotropic ambient compressive stress. The calculated stress distribution is consistent with the observed distribution of hydraulically conductive, sub-horizontal fractures from 23 deep boreholes and has implications for groundwater flow and glacial erosion in the region.