Factors Influencing Morphology of Four Jointed Basalt Scarps in Southwest Iceland
Heinze Fry, Ayla
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This study focuses on detailed description and comparison of morphology and traits of fault scarps in jointed basaltic bedrock in Iceland. I predict that intact rock quality has a greater impact on scarp morphology than internal structure of stacked lava flows. I use geomorphic Rock Mass Rating (RMR) as a proxy for rock quality. I assess talus slopes in conjunction with internal structure of stacked lava flows in intact rock faces to assess how the internal structure impacts scarp evolution and morphology. This investigation is conducted in conjunction with research by Cassandra Brigham, University of Washington PhD student, to investigate the style and tempo of fault scarp growth and degradation in jointed bedrock. We studied four scarps in two areas of Iceland, Thingvellir and the Reykjanes Peninsula. The areas both lie in the West Volcanic Zone (WVZ) of southwestern Iceland. We chose the scarps, formed within faulted intermediate and mafic basalt flows of different post-glacial ages, for their good exposure in a tectonically activity area. I used engineering geology methods, such as discontinuity surveys, scanline surveys for block frequency, and RMR in my analysis. I also used detailed notes from site location to subdomain scale. Orthophoto analysis contributed greatly to the analysis of the rock. The four scarps studied have highly variable along-strike scarp morphology over short distances, used to define domains. For example, walls of near-vertical intact rock faces spanning the height of the scarp are juxtaposed with extensive talus and grassy slopes spanning the height of the scarp, with other morphologies consisting of intact rock face, talus and grassy slope in varying patterns. Despite the highly variable scarp morphology, rock quality, measured by RMR values, are relatively consistent. I also used qualitative observations of stacked lava flow structure and talus piles to assess why and how morphology of the scarps evolve. Flow structure strongly defines discontinuity distribution and properties within the rock, which provide sources of weakness that contribute to potential scarp failure. Where horizontally bound by through-going discontinuities, zones of fine joint-bounded blocks recess under coarse blocks in many locations. These zones may be a source of instability in the rock face, potentially contributing to the toppling of coarser blocks above them. Repeated upward fining sequences due to stacking of lava flows provide multiple locations where fine blocks can recess beneath coarse blocks. Although the impact of stacked lava flow structure on bedrock scarp morphology is qualitatively analyzed, it appears to play at least as significant a role in the morphology of the scarps where observed, if not more than RMR values. Based on quantitative comparisons of different RMR to morphology types within a domain and qualitative analysis of structure of stacked lava flows compared to overall scarp morphology, neither factors individually or likely together completely explain the varied morphology of the scarps. These are preliminary findings based on only four scarps. Many analyses are also limited by the accuracy of the orthophoto, which projects points as if the scarp were perfectly vertical, which is not the case in all locations. For this reason, this study best captures general patterns, not exact values of block and talus sizes, areas, or scarp heights.