LANDSCAPE FEATURES INFLUENCE BEDROCK FRACTURE OPENNESS IN THE DEEP SUBSURFACE (Invited Presentation)

Fracturing of bedrock promotes water-rock interactions and the formation of the life-sustaining layer of soil at Earth’s surface. Theoretical models predict that topography and loads from sediment and water should influence bedrock fracturing in three dimensions by perturbing gravitational and tectonic stress fields. Testing model predictions of the depth and distribution of topographically-affected fractures has proven difficult because comprehensive fracture datasets in three-dimensions for the subsurface are rare. Thus, it is not clear how strongly fractures respond to landscape features or how deep the effects extend. Here, we compare models of three-dimensional stress fields that include effects of topography, sediment weight, ocean loading, and pore water pressure with a dataset of ~50,000 fractures documented from 18 cores reaching depths of 600 m in the Precambrian crystalline rocks at Forsmark, Sweden. Stress proxies for the formation or reactivation of fractures correlate strongly with the fraction of observed fractures that are open (have a visible aperture in cores) at depths down to 500 m, despite the low topographic relief in Forsmark (~ 40 m). This result implies that present-day stress fields, perturbed by landscape features, can induce or reactivate fractures, potentially beginning to form the critical zone as deep as hundreds of meters beneath the land surface.