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The Fault in our Hill: Characterisation of the Rangitawa Tephra, and insight into tectonic influences at Hillcrest Road

This study aimed to identify deformation and characterise the Rangitawa Tephra in terms of geomechanical and geochemical properties, and relate these properties to the deformation potential/activity. It was hypothesised there is a fault present in the site at 59 Hillcrest Road, and that the Rangitawa Tephra deforms plastically and accommodates deformation thereby reducing the amount of deformation in the geological units above. Field analyses included stratigraphic observations, unit descriptions, structural observations and anomaly identification, sensitivity measurements, and description of soft sediment deformation structures. Laboratory analyses consisted of particle-size, mineralogy, chemistry, and microstructure analyses and geomechanical behaviour identification through Atterberg Limits and Triaxial shear testing. Field analyses showed the presence of soft sediment deformation in the form of offset and deformed bedding at the base of the Rangitawa Tephra unit, and in the Hamilton Ash Formation above, along with a significant offset of approximately 6 metres within the cutting at 59 Hillcrest Road. This is inferred to be a normal fault, with the surrounding deformation seismites resulting from seismic triggers in the area along this fault plane. Laboratory analyses were used to characterise the Rangitawa Tephra at Hillcrest Road based on geomechanical, chemical, and mineralogical properties indicated the Rangitawa is bimodal extra-fine silt and extra-fine sand with an average of medium silt texture, behaving as a low compressibility silt. The clay mineralogy present as halloysite, dehydrated in Atterberg Limits testing, but hydrated otherwise. Triaxial shear testing identified the Rangitawa Tephra at Hillcrest to be of normally consolidated material, exhibiting ductile-plastic characteristics. The properties indicated during laboratory analyses were used to provide explanations for the deformation structures observed in the exposure at 59 Hillcrest Road. Field analyses along with the laboratory characterisation of the Rangitawa Tephra confirm the presence of a fault plane at the exposure. The thesis concludes the Rangitawa Tephra exhibits plastic deformation characteristics, presenting the potential to exhibit cyclic softening. These findings confirm the presence of ductile displacement, and support the soft sediment deformation structures observed at the base of the Rangitawa Tephra, and in the Hamilton Ash formation, are the direct result of paleoseismic influences within the area. It can then be suggested this fault zone may be active (displacement younger than 100 ky.) (GNS, 2017). The ductile plastic characteristics of the Rangitawa Tephra suggests deformation within and above this unit could be present, masked by the atypical displacement features and reduced by the accommodated deformation by the Rangitawa Tephra, suggesting the inferred ages on other faults found within the basin may be much younger than expected.
Type of thesis
Gibbons, V. (2020). The Fault in our Hill: Characterisation of the Rangitawa Tephra, and insight into tectonic influences at Hillcrest Road (Thesis, Master of Science (Research) (MSc(Research))). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/14258
The University of Waikato
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