TY - JOUR A1 - Clarke, Brian A. A1 - Burbank, Douglas W. T1 - Bedrock fracturing, threshold hillslopes, and limits to the magnitude of bedrock landslides N2 - Bedrock fracturing and rock strength are widely believed to influence landscape morphology and erosional resistance. Yet, understanding of the quantitative relationship between rock-mass strength and landscape evolution remains limited. Here we present a new application of seismic refraction surveys that uses variations in seismic velocity to interpret differences in bedrock fracture density with depth. We use a comparative study of Fiordland and the western Southern Alps of New Zealand to examine how differences in rock type and bedrock fracturing influence landscape morphology and landslide response to rock uplift. In both regions, slopes appear invariant with differential rock-uplift rates and slope distributions reveal modal hillslope angles of similar to 32 degrees. The majority of landslides initiate on slopes steeper than the modal hillslope angle, however, landslide magnitude-frequency distributions reveal order-of-magnitude differences between the regions, with Fiordland experiencing considerably smaller and less frequent landsliding events. Landslide-driven denudation rates of similar to 9 mm/yr in the western Southern Alps and between similar to 0.1 and 0.3 mm/yr in Fiordland approximate estimates of long-term rock-uplift rates for each region. The invariance of hillslope angles, near-normal slope distributions, predominance of landslide initiation on slopes steeper than modal values, and the apparent balance between rates of uplift and landslide-driven erosion suggest that hillslopes in both regions are at threshold angles. Their similar modal slopes further suggest that both ranges are characterized by equivalent rock-mass strength, despite striking differences in lithology. Additionally, our seismic analysis reveals nearly identical surface p-wave velocities. The unexpected equivalence of both modal slopes and surface velocities between these lithologically distinct ranges is attributed to contrasting degrees of surface fracturing that have differentially affected the intact rock properties, such that they now yield equivalent surface velocities and hillslope-scale strengths. Given that surface fractures help regulate threshold angles by modulating hillslope strength; we propose that shallow seismic velocities may provide a quantitative proxy for rock-mass strength. We define two contrasting fracture and landsliding environments. In Fiordland, dense geomorphic fracturing that is focused within the shallow subsurface appears to limit the depth and magnitude of most bedrock landslides. Conversely, in the western Southern Alps, tectonic forces produce pervasive fracturing with depth that results in larger, and deeper landslides. Our data suggest that bedrock fracturing at the Earth's surface modulates threshold hillslope angles, whereas the depth of bedrock fracturing influences the magnitude and frequency of landslide response to tectonic rock uplift. Y1 - 2010 UR - http://www.sciencedirect.com/science/journal/0012821X U6 - https://doi.org/10.1016/j.epsl.2010.07.011 SN - 0012-821X ER - TY - JOUR A1 - Clarke, Brian A. A1 - Burbank, Douglas W. T1 - Quantifying bedrock-fracture patterns within the shallow subsurface Implications for rock mass strength, bedrock landslides, and erodibility JF - Journal of geophysical research : Earth surface N2 - The role of bedrock fractures and rock mass strength is often considered a primary influence on the efficiency of surface processes and the morphology of landscapes. Quantifying bedrock characteristics at hillslope scales, however, has proven difficult. Here, we present a new field-based method for quantifying the depth and apparent density of bedrock fractures within the shallow subsurface based on seismic refraction surveys. We examine variations in subsurface fracture patterns in both Fiordland and the Southern Alps of New Zealand to better constrain the influence of bedrock properties in governing rates and patterns of landslides, as well as the morphology of threshold landscapes. We argue that intense tectonic deformation produces uniform bedrock fracturing with depth, whereas geomorphic processes produce strong fracture gradients focused within the shallow subsurface. Additionally, we argue that hillslope strength and stability are functions of both the intact rock strength and the density of bedrock fractures, such that for a given intact rock strength, a threshold fracture-density exists that delineates between stable and unstable rock masses. In the Southern Alps, tectonic forces have pervasively fractured intrinsically weak rock to the verge of instability, such that the entire rock mass is susceptible to failure and landslides can potentially extend to great depths. Conversely, in Fiordland, tectonic fracturing of the strong intact rock has produced fracture densities less than the regional stability threshold. Therefore, bedrock failure in Fiordland generally occurs only after geomorphic fracturing has further reduced the rock mass strength. This dependence on geomorphic fracturing limits the depths of bedrock landslides to within this geomorphically weakened zone. Y1 - 2011 U6 - https://doi.org/10.1029/2011JF001987 SN - 2169-9003 SN - 2169-9011 VL - 116 IS - 20 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Acosta, Veronica Torres A1 - Schildgen, Taylor F. A1 - Clarke, Brian A. A1 - Scherler, Dirk A1 - Bookhagen, Bodo A1 - Wittmann, Hella A1 - von Blanckenburg, Friedhelm A1 - Strecker, Manfred T1 - Effect of vegetation cover on millennial-scale landscape denudation rates in East Africa JF - Lithosphere N2 - The mechanisms by which climate and vegetation affect erosion rates over various time scales lie at the heart of understanding landscape response to climate change. Plot-scale field experiments show that increased vegetation cover slows erosion, implying that faster erosion should occur under low to moderate vegetation cover. However, demonstrating this concept over long time scales and across landscapes has proven to be difficult, especially in settings complicated by tectonic forcing and variable slopes. We investigate this problem by measuring cosmogenic Be-10-derived catchment-mean denudation rates across a range of climate zones and hillslope gradients in the Kenya Rift, and by comparing our results with those published from the Rwenzori Mountains of Uganda. We find that denudation rates from sparsely vegetated parts of the Kenya Rift are up to 0.13 mm/yr, while those from humid and more densely vegetated parts of the Kenya Rift flanks and the Rwenzori Mountains reach a maximum of 0.08 mm/yr, despite higher median hillslope gradients. While differences in lithology and recent land-use changes likely affect the denudation rates and vegetation cover values in some of our studied catchments, hillslope gradient and vegetation cover appear to explain most of the variation in denudation rates across the study area. Our results support the idea that changing vegetation cover can contribute to complex erosional responses to climate or land-use change and that vegetation cover can play an important role in determining the steady-state slopes of mountain belts through its stabilizing effects on the land surface. Y1 - 2015 U6 - https://doi.org/10.1130/L402.1 SN - 1941-8264 SN - 1947-4253 VL - 7 IS - 4 SP - 408 EP - 420 PB - American Institute of Physics CY - Boulder ER -