bedrock fracture spacing controls on hillslope and channel erosion in steep landscapes

Open Access
- Author:
- Neely, Alexander
- Graduate Program:
- Geosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 07, 2020
- Committee Members:
- Roman Alexander Dibiase, Dissertation Advisor/Co-Advisor
Roman Alexander Dibiase, Committee Chair/Co-Chair
Donald Myron Fisher, Committee Member
Elizabeth Ann Hajek, Committee Member
Xiaofeng Liu, Outside Member
Mark E Patzkowsky, Program Head/Chair - Keywords:
- steep
bedrock fracture
erosion
mountain
river
hillslope
sediment
grain size - Abstract:
- Steep topography is commonly associated with rapid rates of tectonic uplift, erosion, and a higher risk of geologic hazards; however, steep topography can also be supported in tectonically inactive regions by bedrock that resist erosion. Distinguishing between steep, tectonically active, landscapes and steep tectonically-dormant landscapes underlain by resistant bedrock, requires and understanding of how rock material properties affect the relationship between topography and erosion. Bedrock fractures reduce rock strength by orders of magnitude relative to intact rock samples and control the size of detached sediment that must be transported; therefore, bedrock fractures serve as a measureable rock strength property that can be coupled to erosional process. Here, I compare topography and erosion rates between two mountain ranges that have similar topographic and climatic drivers of erosion, but large differences in bedrock fracture density. I use new structure-from-motion photogrammetry remote-sensing techniques to measure fracture spacing at high resolution and over large spatial scales, and I show how measurements of bedrock fracture density can be incorporated into conceptual models that describe the erosion of hillslopes, debris-flow dominated channels, and fluvial rivers. I highlight three findings: (1) for a given hillslope morphology, higher bedrock fracture density increases soil cover, soil transport, and hillslope erosion; (2) higher bedrock fracture density reduces the grain size of sediment supplied to rivers; and (3) reduced sediment grain size resulting from denser bedrock fracture spacing increases the mobility of sediment, which leads to more frequent sediment transport and river incision. While tectonic processes generate topographic relief through rock uplift, tectonic fracturing also lowers topographic relief by reducing sediment grain size and bedrock strength. This feedback may lead to a dichotomy where high-relief topography either reflects young, tectonically-active landscapes with densely fractured bedrock, or tectonically-inactive landscapes underlain by sparsely-fractured crystalline bedrock exhumed as the core of ancient mountain ranges.