Co-evolution of Rock Strength, Erosion, and Steep Topography

Abstract

Rock-mass strength is recognized as an important control in geomorphology that influences the morphology of landscapes and modulates feedbacks between surface processes, tectonics, and climate. Despite this general recognition, many of the factors controlling rock-mass strength in the near-surface are not widely understood, largely due to challenges in quantifying strength over the appropriate spatial scales. Although the strength of small rock samples can be readily measured with laboratory techniques, fractures and weathering gradients at progressively larger spatial scales dramatically reduce the strength of rock masses compared to their intact (unfractured) counterparts. Due to a lack of approaches that incorporate discontinuities into strength estimates, the contribution of rock-mass properties to geomorphic processes and topographic relief remains unquantified. In this dissertation, I address gaps in our understanding of the role of rock mass strength in geomorphology by testing new methods for quantifying scale-dependent rock mass strength, using these new tools to quantify gradients in strength across environmental variables, and assessing the contribution of rock strength to topographic form and erosion. Research activities focus on an inverted sedimentary basin within the Western Transverse Ranges of southern California, USA, where differences in fault activity have exposed sedimentary rocks with variable burial histories. With Chapters 2 and 3, I test new approaches to quantifying outcrop- and hillslope-scale near-surface rock-mass strength using slope-stability models and field methods. With these new techniques, I resolve order-of-magnitude differences in strength that appear to be related to diagenetic changes associated with the maximum burial depth of young clastic sedimentary rocks. I resolve smaller differences in strength (300 kPa – 1.5 MPa) that are positively correlated with mean erosion rates, which I hypothesize reflects decreased weathering extents with increasing erosion rates for mountain ranges experiencing faster fault slip rates.

Assessing the contribution of rock-mass properties to the evolution of landscapes requires recognizing patterns of long-term fault activity and erosion. In Chapter 4, I resolve differences in the timing of reverse fault localization, initiation, and propagation, as well as long-term erosion rates, using low-temperature thermochronometry. Inverse thermal modelling of Miocene to Pleistocene apatite (U-Th-Sm)/He cooling ages and partially reset zircon (U-Th)/He cooling ages on eleven vertical transects reveal that deformation localization was likely driven by contrasts in the rheology and strength of the lithosphere, rather than by restraining bend tectonism.

With estimates of near-surface rock-mass strength and long-term erosion rates mapped across the landscape in Chapters 2-4, I evaluate the contribution of rock-mass strength to topography in Chapter 5. Where erosion rates and climate are spatially uniform, I find a non-linear relationship between topographic metrics and rock-mass shear strength, implying that rock-mass strength sets the topographic structure of the mountain range. Chapter 6 builds on this observation, and I find that post-wildfire erosion as quantified from repeat airborne-LiDAR surveys is positively correlated with local slope and the strength of the underlying bedrock. Rather than setting the erodibility of earth materials directly, I hypothesize that rock-mass strength controls event-driven erosion by setting the steepness of the overlying landscape. Collectively, interpretations put forward in this dissertation highlight the complex interplay between tectonics, erosion, topography, and the mechanical evolution of rock to transportable, erodible materials.