Stress field influences on bedrock fracture openness in the deep subsurface at Forsmark, Sweden

Abstract:

Fracturing of bedrock promotes water-rock interactions and influences the formation of the life-sustaining layer of soil at Earth’s surface. Mechanically-based models predict that present-day stress fields should influence bedrock fracture openness, but testing this prediction has proven difficult because comprehensive fracture datasets are rarely available. We model the three-dimensional present-day stress field beneath the deglaciated, low-relief landscape of Forsmark, Sweden, accounting for ambient regional stresses, pore pressure, topography, sediment weight, and seawater loading. We then compare the modeled stresses to a dataset of ~50,000 fractures reaching depths of 600 m at Forsmark. We show that bedrock failure proxies correlate strongly with the fraction of observed open fractures at depths to ~500 m. In addition, we find that the strength of the correlation between observed fractures and the failure proxies varies with fracture aperture, the presence of secondary minerals, and position with respect to localized deformation zones. These results imply that the present-day regional stress field, affected by surface conditions and pore pressure, influences the generation or reactivation of fractures, contributing to the initial breakdown of bedrock hundreds of meters beneath the surface.