No abstract

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~32°. 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~9 mm/yr in the western Southern Alps and between~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.

Although variations in insolation and emergent feedbacks among soil moisture, vegetation, and soil cohesion are commonly invoked to explain topographic asymmetry that depends on aspect, few studies have directly quantifi ed the effi ciency of regolith transport along hillslopes of opposing aspect. We utilize meteoric 10 Be concentrations in regolith (n = 74) to determine mass fl ux along equatorial-facing and polar-facing hillslopes in three forested upland watersheds in and adjacent to the Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania (USA). In combination with regolith depth measurements and high-resolution topography, these fl uxes allow us to evaluate transport rate laws and the effi ciency of regolith creep. Concentrations of meteoric 10 Be in regolith along six separate transects imply that regolith fl ux is similar along all hillslopes, despite differences in topographic gradient and regolith thickness. Comparison of fl ux with regolith depth and topographic gradient reveals that transport depends on regolith depth, and that regolith creep is twice as effi cient along low-gradient, south-facing slopes with thin regolith as compared to steep, north-facing slopes mantled with thicker regolith. We suggest that the observed topographic asymmetry in these watersheds has evolved over geologic time as a result of differences in the frequency of freezethaw events between hillslopes of opposing aspect.

[1] 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.Citation: Clarke, B. A., and D. W. Burbank (2011), Quantifying bedrock-fracture patterns within the shallow subsurface: Implications for rock mass strength, bedrock landslides, and erodibility,