Geophysical imaging reveals topographic stress control of bedrock weathering

Abstract

Bedrock fracture systems facilitate weathering, allowing fresh mineral surfaces to interact with corrosive waters and biota from Earth's surface, while simultaneously promoting drainage of chemically equilibrated fluids. We show that topographic perturbations to regional stress fields explain bedrock fracture distributions, as revealed by seismic velocity and electrical resistivity surveys from three landscapes. The base of the fracture-rich zone mirrors surface topography where the ratio of horizontal compressive tectonic stresses to near-surface gravitational stresses is relatively large, and it parallels the surface topography where the ratio is relatively small. Three-dimensional stress calculations predict these results, suggesting that tectonic stresses interact with topography to influence bedrock disaggregation, groundwater flow, chemical weathering, and the depth of the "critical zone" in which many biogeochemical processes occur.

Reviews and syntheses: on the roles trees play in building and plumbing the critical zone

“…2a and c), the relationship among h, topography, and trees may depend on hillslope position (i.e., crest, sideslope, toe). For example, near ridge crests and in valley bottoms, the stress fields vary markedly, affecting the distribution of fractures (Wyrick and Borchers, 1981;St. Clair et al, 2015).…”

“…In landscapes where the ratio of the regional horizontal compressive tectonic stresses to near-surface gravitational stresses is relatively large, these stresses may promote the opening of fractures at great depth under ridges (St. Clair et al, 2015). One might expect that trees in such locations will have a limited role in shaping the CZ architecture because of the prevalence of deep regolith with deep or widely spaced fractures.…”

“…Clair et al, 2015). An increase in the sharpness of a ridge (increased convexity) or an increase in topographic relief and narrow valley spacing can generate stress concentrations sufficient to fracture bedrock along ridge crests and valley bottoms, respectively (Miller and Dunne, 1996;St. Clair et al, 2015).…”

“…However, if fracturing ultimately controls the distribution of roots in unweathered rock (hypothesis 1), the CZ may alternately be shaped from the bottom up. For example, frac-turing under hills has been posited to be controlled by the state of tectonic stress and how it interacts with topographic unloading (St. Clair et al, 2015). Such ideas suggest that distribution of trees and their access to water and the nature of the CZ may be ultimately dictated by bottom-up, tectonic controls.…”

Designing a network of critical zone observatories to explore the living skin of the terrestrial Earth

“…Many papers have been published exemplifying this approach to map out the subsurface architecture (Befus et al, 2011;Holbrook et al, 2014;Orlando et al, 2015;Olyphant et al, 2016). Now, geophysicists travel among CZOs to image the subsurface with a battery of instruments to image the belowground landscape (St. Clair et al, 2015). In another example, after the Boulder Creek CZO began emphasizing slope aspect as a useful natural experiment to examine controls on CZ architecture and function in 2009, similar analyses at other CZOs led to highlighted linkages among aspect, water, biota, regolith structure, and episodic events (West et al, 2014;Ebel et al, 2015;Pelletier et al, 2017).…”