Bedrock fracturing, threshold hillslopes, and limits to the magnitude of bedrock landslides

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.

New insights into the mechanics of fluvial bedrock erosion through flume experiments and theory

“…Plucking involves the removal of large chunks of rock along intersection joint planes, and it may dominate erosion rates in welljointed rock types (e.g., columnar basalt) and during large floods that are capable of entraining and transporting large blocks (e.g., Malde, 1968;Baker, 1973;Lamb and Fonstad, 2010;Lang et al, 2013;Lamb et al, 2014). Bedrock can be jointed caused by primary (e.g., cooling joints in basalt) or secondary causes (e.g., tectonic-and topographic-induced fractures; Miller and Dunne, 1996;Martel, 2006;Clarke and Burbank, 2010). Fluid flow and sediment-transport processes can also contribute to fracturing rock or loosening of preexisting joints through chemical and physical weathering, hydraulic wedging of sediment into crack openings (Hancock et al, 1998), crack development from saltating clasts, and hydraulic pressure fluctuations (Hancock et al, 1998;Whipple et al, 2000).…”

Significance of rockfall magnitude and carbonate dissolution for rock slope erosion and geomorphic work on Alpine limestone cliffs (Reintal, German Alps)

“…However, the qualitative and quantitative significance of different magnitudes for rock wall retreat and talus slope evolution has been poorly addressed. This is important since high-and lowmagnitude rockfalls and rock slope failure respond to different promoting and triggering conditions (Clarke and Burbank, 2010) and leave different deposits such as talus slopes and rock avalanche deposits (Fort et al, 2009). The magnitude distribution will also influence the geomorphology of headwalls and talus slopes (Densmore et al, 1997), coupling and decoupling in mountain catchments , the overall performance of sediment flux in mountain systems (Korup, 2005;Lin et al, 2008) and the imposed hazard potential (Evans, 2002;Krautblatter and Moser, 2009).…”

Digital Elevation Model Differencing and Error Estimation from Multiple Sources: A Case Study from the Meiyuan Shan Landslide in Taiwan

“…As a result, a profiling method that stack elevation values on parallel, equally-spaced tangents has been developed, i.e., the swath profile method. Applications of the swath profile method can often be found in the analysis of large-scale regional terrain issues, such as tectonic structures or landslides [44][45][46].…”