RESEARCH ◥ itational stresses and regional tectonic stresses. REPORTS Theoretical calculations have shown that the presence of topographic features such as ridges and valleys can cause vertical and lateral var- GEOMORPHOLOGY iations in the shallow subsurface stress field (15) and that these stress perturbations can be large enough to alter bedrock fracture patterns Geophysical imaging reveals (14, 16). Some observational evidence suggests that topographic stresses influence near-surface fracture distributions (9, 17, 18)andgroundwater topographic stress control flow (19–21), but it is unknown whether topo- graphic stresses systematically affect the spatial of bedrock weathering distribution of bedrock weathering. In this study, we used geophysical surveys of seismic velocity J. St. Clair,1*† S. Moon,2*†‡ W. S. Holbrook,1 J. T. Perron,2 C. S. Riebe,1 S. J. Martel,3 and electrical resistivity to image the thickness of B. Carr,1 C. Harman,4 K. Singha,5 D. deB. Richter6 the weathered zone in three landscapes with similar topography but different tectonic stress Bedrock fracture systems facilitate weathering, allowing fresh mineral surfaces to interact conditions, and we tested whether the geometry with corrosive waters and biota from Earth’ssurface,whilesimultaneouslypromoting of the weathered zone in each landscape matches drainage of chemically equilibrated fluids. We show that topographic perturbations to the modeled topographic stress field. regional stress fields explain bedrock fracture distributions, as revealed by seismic velocity Acalculationofthestressesbeneathanideal- and electrical resistivity surveys from three landscapes. The base of the fracture-rich ized topographic profile in the presence of re- zone mirrors surface topography where the ratio of horizontal compressive tectonic gional tectonic stress illustrates how topographic stresses to near-surface gravitational stresses is relatively large, and it parallels the stress might influence the geometry of the weather- surface topography where the ratio is relatively small. Three-dimensional stress ed zone (Fig. 1). We calculated the total stress calculations predict these results, suggesting that tectonic stresses interact with field as the sum of the ambient stress due to topography to influence bedrock disaggregation, groundwater flow, chemical weathering, gravity and tectonics and the stress perturba- and the depth of the “critical zone” in which many biogeochemical processes occur. tion due to topography (22). From the stress on October 29, 2015 field, we calculated two scalar quantities that act as proxies for two mechanisms that could eathered bedrock and soil are part of the how rapidly groundwater flowing toward the river influence the abundance of fractures. As a proxy life-sustaining layer at Earth’ssurface channels can drain bedrock pores of chemically for shear fracturing or shear sliding on existing commonly referred to as the “critical equilibrated water (1). Another hypothesis, based fractures, we calculated the failure potential (F) zone.” Its thickness depends on the on reactive transport modeling, predicts that the (23), defined as (smc – slc)/(smc + slc), where s W competition between erosion, which re- propagation of the weathered zone depends on indicates a stress, and the subscripts denote the moves weathered material from the land surface, the balance between mineral reaction kinetics most compressive (mc) and least compressive (lc) and weathering, which breaks down rock mechan- and groundwater residence times (2, 6). These principal stresses, with compression being posi- www.sciencemag.org ically and chemically and thus deepens the inter- mechanisms are not mutually exclusive, and both tive. As a proxy for opening-mode displacement face between weathered and fresh bedrock (1–3). emphasize the importance of fluid flow through on fractures, we used the magnitude of slc. Erosion at the surface can be studied both di- bedrock. This implies that the development of Because we do not have prior knowledge of the rectly by observation (4)andindirectlyusing fracture systems, which provide preferred fluid- orientations or abundances of existing fractures isotopic tracers (5). In contrast, weathering at flow paths in bedrock (7, 8), should help regulate in a given landscape, we used both quantities depth is generally obscured by overlying rock the downward propagation of weathering. to represent the propensity for generating open and soil, making it more difficult to study. In Bedrock fractures may be inherited from past fractures. A larger F indicates that new shear this Report, we combine results from landscape- tectonic or thermal events, but the common ob- fractures are more likely to form and that exist- Downloaded from scale geophysical imagingandstress-fieldmodel- servation that fracture abundance declines with ing fractures oriented obliquely to the smc and ing to evaluate hypotheses about the relationship depth (9) suggests that near-surface processes slc directions are more likely to dilate as a result of subsurface weathering to surface topography. are capable of generating new fractures or reacti- of sliding on the rough fracture surfaces (24). A One hypothesis is that weathering at depth vating existing ones. Examples of potential near- smaller slc indicates that fractures oriented near- is regulated by the hydraulic conductivity and surface fracturing mechanisms include wedging ly perpendicular to the slc direction will be more porosity of bedrock and the incision rate of river due to growth of ice crystals (10), salt crystals (11), likely to open (20, 25). Given that open fractures channels into bedrock, which together determine or roots (12); volume expansion due to weather- permit water to flow more quickly through bed- ing reactions of certain minerals (13); and stress rock and enhance the rate of chemical weather- 1Department of Geology and Geophysics and Wyoming perturbations associated with surface topography ing, we expect that zones of larger F or smaller Center for Environmental Hydrology and Geophysics, (14). Some of these mechanisms require specific slc correspond to zones of more weathered University of Wyoming, Laramie, WY 82071, USA. conditions that only occur in certain geographic bedrock. 2 Department of Earth, Atmospheric and Planetary Sciences, regions, rock types, or parts of the subsurface. In a scenario with a horizontal land surface Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 3Department of Geology and Geophysics, For example, ice weathering requires repeated and a stress field determined by both gravity University of Hawaii, Honolulu, HI 96822, USA. 4Department freeze-thaw cycles and is therefore limited to and a low, uniform ambient horizontal compres- of Geography and Environmental Engineering, The Johns depths of a few meters, and volume expansion sion, F declines and s increases with depth 5 lc Hopkins University, Baltimore, MD 21218, USA. Hydrologic due to mineral weathering is most effective in beneath the land surface (Fig. 1, A and D). If to- Science and Engineering Program, Colorado School of Mines, Golden, CO 80401, USA. 6Nicholas School of the rocks containing abundant biotite or hornblende. pography is added and the ambient horizontal Environment, Duke University, Durham, NC 27708, USA. In contrast, surface topography always perturbs compression remains low, contours of F and slc *Corresponding author. E-mail: [email protected] (J.S.C.); the bedrock stress field. Here we focus on this generally parallel the surface (Fig. 1, B and E). [email protected] (S.M.) †These authors contributed equally to potentially widespread control on bedrock fractures. This surface-parallel pattern occurs because s this work. ‡Present address: Department of Earth, Planetary, and lc Space Science, University of California–Los Angeles, Los Angeles, Bedrock stress fields can be evaluated by con- is determined primarily by the overburden in CA 90095, USA. sidering the effects of local topography on grav- this scenario, and therefore it increases with 534 30 OCTOBER 2015 • VOL 350 ISSUE 6260 sciencemag.org SCIENCE RESEARCH | REPORTS depth similarly beneath ridges and valleys (fig. lowest F or highest slc at which fractures open To test our conceptual model, we investigated S1A). However, if the ambient horizontal stress enough to accelerate chemical weathering. After three field sites in the United States with similar becomes strongly compressive—so that smc at crossing this boundary, rocks will only experi- topography but different ambient tectonic stress shallow depths parallels the topographic sur- ence higher F and lower slc and should become regimes. Gordon Gulch (Fig. 2B), in the Boulder face, and slc becomes small relative to smc (fig. even more weathered. Creek Critical Zone Observatory, Colorado (3), is S1, E and F)—then contours of F and slc resem- A dimensionless ratio of tectonic stresses to underlain by Precambrian gneiss and has weak ble mirror images of the land surface, plunging gravitational stresses that accounts for topographic tectonic compression (st =1MPa;s* =0.2). below ridges and rising beneath valleys (Fig. 1, perturbations captures the difference between Calhoun Critical Zone Observatory, South Caro- CandF).Thismirror-imagepatternoccurs the surface-parallel scenario (Fig. 1, B, E, and lina (Fig. 2C) (26), is underlain by Neoproterozoic because slc becomes much smaller beneath ridges H) and the surface-mirroring scenario (Fig. 1, or Cambrian granite gneiss and has stronger tec- than