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Geophysical imaging reveals topographic stress control of bedrock weathering

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Science  30 Oct 2015:
Vol. 350, Issue 6260, pp. 534-538
DOI: 10.1126/science.aab2210
  • Fig. 1 Hypothesized influence of topography and tectonic stress on weathering zone geometry.

    (A to C) Cross-sectional plots of failure potential (Φ) computed with a numerical stress model (22). Arrows indicate the magnitude of ambient horizontal compression. (A) shows a horizontal surface under weak compression, (B) shows ridges and valleys under weak compression, and (C) shows ridges and valleys under strong compression. The magnitude of the horizontal compression at the mean elevation of the land surface is 0.25ρga in (A) and (B) and 2ρga in (C), where ρ is rock density, g is gravitational acceleration, and a is the horizontal distance from the valley bottom to the ridge top in (B) and (C). This corresponds to σ* values of 0.25 in (B) and 2 in (C) (Eq. 1). Topography is horizontally periodic. (D to F) The same scenarios as in (A) to (C), but showing the magnitude of the least compressive principal stress, σlc. (G to I) Conceptual diagrams of bedrock fracture abundance in the three scenarios.

  • Fig. 2 Maps of study sites.

    (A) Locations of the three U.S. sites. Colors indicate elevation, with greens being lowest and browns being highest. (B) Gordon Gulch, Colorado. (C) Calhoun, South Carolina. (D) Pond Branch, Maryland. Geophysical survey lines are numbered. White lines mark the transects shown in Fig. 3. Elevation data are from the U.S. National Elevation Dataset and the National Center for Airborne Laser Mapping.

  • Fig. 3 Comparisons of topographic stress and seismic velocity.

    Modeled failure potential (A to C), modeled magnitude of the least compressive stress (D to F), and measured P-wave velocity (G to I) for Gordon Gulch [(A), (D), and (G)], Calhoun [(B), (E), and (H)], and Pond Branch [(C), (F), and (I)]. Transect locations are marked in white in Fig. 2. The topographic surface was smoothed for stress calculations (17). Vertical exaggerations are 3.6× at Gordon Gulch, 3.7× at Calhoun, and 2.3× at Pond Branch.

  • Fig. 4 Electrical resistivity and seismic velocity at the Calhoun site.

    The transect is the same as in Fig. 3, B, E, and H. Resistivity (A) and P-wave velocity (B) increase with depth, probably reflecting the closing of fractures. Both images show the surface-mirroring pattern predicted by the stress model (Fig. 3, B and E). Vertical exaggeration is 3×.

Supplementary Materials

  • Geophysical imaging reveals topographic stress control of bedrock weathering

    J. St. Clair, S. Moon, W. S. Holbrook, J. T. Perron, C. S. Riebe, S. J. Martel, B. Carr, C. Harman, K. Singha, D. deB. Richter

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
    • Figs. S1 to S10
    • Tables S1 to S3
    • Full Reference List

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