Present‐Day Stress Field Influences Bedrock Fracture Openness Deep Into the Subsurface

Abstract

Fracturing of bedrock promotes water-rock interactions and influences the formation of the life-sustaining layer of soil at Earth's surface. Models predict that present-day stress fields should influence bedrock fracture openness, but testing this prediction has proven difficult because comprehensive fracture data sets are rarely available. We model the three-dimensional present-day stress field beneath the deglaciated, low-relief landscape of Forsmark, Sweden. We account for ambient regional stresses, pore pressure, topography, sediment weight, and seawater loading. We then compare the modeled stresses to a data set of~50,000 fractures reaching depths of 600 m at Forsmark. We show that modeled failure proxies correlate strongly with the fraction of observed open fractures to depths of~500 m. This result implies that the present-day regional stress field, affected by surface conditions and pore pressure, influences fracture openness in bedrock hundreds of meters beneath the surface, thereby preparing the rock for further weathering. Plain Language Summary The "critical zone"-the life-sustaining part of the Earth that extends from the top of the tree canopy to the bottom of permeable bedrock-is essential for ecosystems and agriculture. The opening of bedrock fractures and onset of water-rock interaction are crucial to the formation of the critical zone. Within the bedrock, the intensities of horizontal regional forces and vertical gravitational forces typically increase with depth. These force intensities, or stresses, are modified by surface effects associated with topography, the weight of overlying seawater and sediment, and by groundwater pressure. However, the influence of these surface effects on fractures has been difficult to observe because comprehensive fracture data sets are rare. In this study, we examine whether, and to what depths, bedrock may fracture under the influence of stress associated with surficial conditions. We compare bedrock stress calculations with~50,000 fractures from 18 cores reaching depths of 600 m at Forsmark, Sweden. We find that the present-day stress field influences the opening of fractures to depths of 500 m, contributing to the formation of the critical zone and the preparation of rock for weathering hundreds of meters beneath the surface, much deeper than previously thought.

Seismic Imaging of a Shale Landscape Under Compression Shows Limited Influence of Topography‐Induced Fracturing

“…The three-dimensional topographic stress regime was calculated using the boundary element model Poly3D (Thomas, 1993) following procedures as in St. Clair et al (2015) and Moon et al (2017Moon et al ( , 2020. The subsurface total stress fields were quantified as the sum of (a) ambient stress due to horizontal compression and gravity, and (b) stress perturbation from actual topography.…”

Quantifying Depth‐Dependent Seismic Anisotropy in the Critical Zone Enhanced by Weathering of a Piedmont Schist

“…Clair et al, 2015). Both types of stresses are taken into account by topographic stress models, which can be used to predict the potential of opening-mode or shear fracturing (hereafter referred to as fracture potential) (Moon et al, 2017(Moon et al, , 2020Slim et al, 2015). Spatial variations in fracture potential and bulk weathering may correlate as stresses under strong compression induce fracturing, create pathways for water to infiltrate, and initiate mineral dissolution (St. Clair et al, 2015).…”

“…Clair et al (2015). Then, we calculated proxies of both shear and opening-mode failure potential on fracture planes of varying strikes and dips, following methods detailed by Moon et al (2020). The proxy for shear failure potential is calculated as shear traction (τ) divided by normal traction (σ n ) on the fracture surface.…”

“…This type of CZ architecture, where the boundary between fracture‐rich zones and intact bedrock mirrors surface topography, has been interpreted to result from the combined effects of topographic and strong regional compressive stresses (Moon et al., 2017; Slim et al., 2015; St. Clair et al., 2015). Both types of stresses are taken into account by topographic stress models, which can be used to predict the potential of opening‐mode or shear fracturing (hereafter referred to as fracture potential) (Moon et al., 2017, 2020; Slim et al., 2015). Spatial variations in fracture potential and bulk weathering may correlate as stresses under strong compression induce fracturing, create pathways for water to infiltrate, and initiate mineral dissolution (St. Clair et al., 2015).…”

Warmer, Wetter Climates Accelerate Mechanical Weathering in Field Data, Independent of Stress‐Loading

“…Rock cracking brought on by tectonic, topographic, and environmental stresses arising at Earth's surface (∼0–500 m depth; e.g., Moon et al., 2020) embodies mechanical weathering. Because the vast majority of such stresses are less than rock's critical strength, mechanical weathering is likely governed by subcritical cracking (Eppes & Keanini, 2017; Eppes et al., 2018; Hall, 1999; Martel, 2011; Molnar, 2004; Stock et al., 2012; Walder & Hallet, 1985).…”