Congratulations to Kirk Townsend who successfully defended his thesis on Wednesday July 21st, 2021.

Advisor: Marin Clark

Abstract:

Rock-mass strength is widely expected to influence the morphology and topographic evolution of mountain ranges. However, many of the factors controlling rock strength in the near-surface environment are not widely understood because rock-mass strength is difficult to quantify at spatial scales relevant to geomorphology. Consequently, the contribution of scale-dependent rock-mass strength to geomorphic processes and topographic relief remains largely uncharacterized.
In this dissertation, I test new approaches to quantifying rock strength averaged over outcrop- and hillslope-scales, which incorporate the strength-limiting effects of fractures and weathering within rock masses. I apply these techniques to the Western Transverse Ranges of southern California, USA, where I resolve order-of-magnitude differences in near-surface strength related to geologic and environmental controls. With new quantitative estimates of rock-mass strength mapped across the landscape, I assess the interdependencies of rock strength, topographic form, and erosion driven by active faulting over long timescales, and wildfire over short timescales. Key questions include:

  • Does active tectonism reduce rock strength by adding fractures, or increase strength by reducing weathering?
  • How sensitive is steep topography to the strength of the underlying bedrock?
  • How does rock strength influence the erodibility of landscapes?

Collectively, interpretations put forward in this dissertation highlight the complex interplay between active tectonics, erosion, topography, and the mechanical evolution of rock to transportable, erodible materials.