Controls on fracture openness and reactivation in Forsmark, Sweden

Abstract

In crystalline bedrock, the open fraction of the fracture network constitutes the main pathways for fluids. Many observations point out that the state of stress influences the open fraction, likely indicating recent reactivation. But how this occurs is still unresolved. We analyse the conditions for fracture reactivation from fracture data collected in the uppermost 1 km of bedrock in Forsmark, Sweden. The open fraction is mainly correlated to the stress acting normally on the fracture but even away from critical failure, leading us to analyse the potential fluid pressure required for reactivation, Pc. We observe that 100% of the fractures are open when Pc is hydrostatic, and the ratio decreases exponentially to a plateau of ~ 17% when Pc is lithostatic and above. Exceptions are the oldest fractures, having a low open fraction independent of Pc. We suggest that these results reflect past pressure build-ups, potentially related to recent glaciations, and developing only if the preexisting open fraction is large enough.