Strong slope‐aspect control of regolith thickness by bedrock foliation

Abstract

The porous near‐surface layer of the Earth's crust – the critical zone – constitutes a vital reservoir of water for ecosystems, provides baseflow to streams, guides recharge to deep aquifers, filters contaminants from groundwater, and regulates the long‐term evolution of landscapes. Recent work suggests that the controls on regolith thickness include climate, tectonics, lithology, and vegetation. However, the relative paucity of observations of regolith structure and properties at landscape scales means that theoretical models of critical zone structure are incompletely tested. Here we present seismic refraction and electrical resistivity surveys that thoroughly characterize subsurface structure in a small catchment in the Santa Catalina Mountains, Arizona, USA, where slope‐aspect effects on regolith structure are expected based on differences in vegetation. Our results show a stark contrast in physical properties and inferred regolith thickness on opposing slopes, but in the opposite sense of that expected from environmental models and observed vegetation patterns. Although vegetation (as expressed by normalized difference vegetation index [NDVI]) is denser on the north‐facing slope, regolith on the south‐facing slope is four times thicker (as indicated by lower seismic velocities and resistivities). This contrast cannot be explained by variations in topographic stress or conventional hillslope morphology models. Instead, regolith thickness appears to be controlled by metamorphic foliation: regolith is thicker where foliation dips into the topography, and thinner where foliation is nearly parallel to the surface. We hypothesize that, in this catchment, hydraulic conductivity and infiltration capacity control weathering: infiltration is hindered and regolith is thin where foliation is parallel to the surface topography, whereas water infiltrates deeper and regolith is thicker where foliation intersects topography at a substantial angle. These results suggest that bedrock foliation, and perhaps by extension sedimentary layering, can control regolith thickness and must be accounted for in models of critical zone development. © 2020 John Wiley & Sons, Ltd.

What Do P-Wave Velocities Tell Us About the Critical Zone?

“…Although high-frequency sonic velocity logs in the CZ remain rare, our data show that P-wave seismic velocity is scale-dependent. Given the interdisciplinary nature of CZ science, it is likely that the 2D profiles of velocity will be used to inform or test conceptual models of CZ evolution (West et al, 2019;Leone et al, 2020). It is important to realize that the velocities in seismic refraction tomography represent large-scale averages over the scale of their dominant wavelength.…”

“…During a SSR survey, the travel times between a seismic source to an array of geophones are measured and inverted to generate a 2D tomogram of seismic velocity (Woodward, 1992;Sheehan et al, 2005;Gance et al, 2012;Zelt et al, 2013). This type of SSR surveying is a robust method that has been used on crustal scales (Mooney and Brocher, 1987;Korenaga et al, 2000;Hayes et al, 2013), hillslope scales (Whiteley and Eccleston, 2006;McClymont et al, 2012;Thayer et al, 2018;Leone et al, 2020), and over outcrops (Holbrook et al, 2014;Flinchum et al, 2018b). SSR sources utilize a low-frequency (10s of Hz) source that produces relatively large wavelengths (10s−100s of meters).…”

“…CZ structure may depend strongly on strong contrasts in hydrogeologic properties such as porosity or hydraulic conductivity (Dralle et al, 2018;Rempe and Dietrich, 2018;Hahm et al, 2019). In some locations, lithologic fabric or foliation (Novitsky et al, 2018;Leone et al, 2020), composition (Hahm et al, 2014;Buss et al, 2017) or geochemical reaction rates (Dixon and Thorn, 2005;Gabet and Mudd, 2009;Hilley et al, 2010;Braun et al, 2016) may control CZ structure and evolution. Testing any of these process-based hypotheses about CZ evolution, or developing new hypotheses, requires quantifying CZ architecture over large areas.…”

Profiles of Near‐Surface Rock Mass Strength Across Gradients in Burial, Erosion, and Time

“…A. Marshall & Roering, 2014). Fracture generation and weathering are sensitive to a number of environmental factors, including climate (Gabet et al., 2010; Gallen et al., 2015; Goudie, 2016), tectonics (DiBiase et al., 2018; Kirkpatrick et al., 2021; Molnar et al., 2007; Neely et al., 2019), burial depth of sedimentary rocks (Townsend, Gallen, & Clark, 2020), and geomorphic position on the landscape (Gabet et al., 2015; Leone et al., 2020; Medwedeff et al., 2019; Slim et al., 2015). Quantifying the relationships between these environmental variables and fracturing, weathering, and the resultant rock mass strength at appropriate spatial scales will further our ability to accurately model the evolution of active mountainous regions with respect to strength.…”

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

“…Nonetheless, understanding how rock fabric is modified during weathering is important, particularly for hydrological purposes. For example, foliation, joints, and fracture networks have been shown to correlate with the directional hydraulic conductivity of their host media (Gerke & Vangenuchten, 1993;Neuman, 2005;Zimmerman & Bodvarsson, 1996), affect the infiltration rate of meteoric water (Leone et al, 2020), and act as the most prevalent sources of permeability in crystalline rocks (Nutter & Otton, 1969). Prior research has been primarily geared towards helping predict groundwater movement in unweathered bedrock.…”