Investigating fault scarp degradation in jointed basalt in southwestern Iceland
Cassandra Brigham, University of Washington
Juliet Crider, University of Washington
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Fault scarps hold information key to illuminating a region's recent tectonic history if their morphological evolution can be deciphered. Scarps within fractured bedrock are very common throughout the world, but their relative or absolute morphological ages have proven difficult to determine. This project focuses on constraining the characteristics of young fault scarps (with ages ranging from 3 to 9 ka) in jointed basalt, as a first step to establishing morphological ages for these types of scarps.
Fieldwork was conducted in the summer of 2017 in southwest Iceland, where the on-land portion of the Reykjanes Ridge spreading center is manifested by surface-breaking normal faults, fractures and fissures that cut late Quaternary-to-recent volcanic rocks. Structure-from-motion photogrammetry was used along selected fault segments in order to create a precise surface model of portions of four scarps.
An initial conceptual model was that scarps start with a dominant vertical free face that retreats through time as material topples onto an increasingly large talus slope, until the slope reaches the top of the scarp. However, topographic profiles of the studied scarps reveal along-strike heterogeneity in scarp morphology, with similar form types found at the different fault sites. This suggests that age is not a primary control on the morphology of young fault scarps in jointed basalt; joint characteristics and original fault structure may be more likely to influence scarp morphology in the early stages of scarp degradation. This study also recorded geomorphic characteristics, such as the overall height of the scarp, the original scarp structure, the spacing of the bedrock joints, the height of the free face relative to the talus slope, the size of the talus material, and the amount of vegetation and lichen cover, that will be used in future studies to constrain the drivers of early scarp degradation.
The results of this project will inform a broader study that seeks to clarify the links between fault scarp morphology, degradation mechanisms and retreat rates, an important step in determining a quantitative model of fault scarp evolution in jointed rock. Such a model would permit the rapid dating and characterization of fault activity in Quaternary-to-recent faulting in regions of extension around the Earth.