The Cascade Rock Avalanche: Structure and deformation in a catastrophic rock avalanche

Abstract

Catastrophic rock avalanches involving large volumes of bedrock are primary drivers of surface erosion in mountainous landscapes and pose a considerable threat to both human life and infrastructure. These events typically involve > 106 m3 of material and show increased mobility and long runouts compared to relatively small landslide events. Investigations of these deep-seated, catastrophic events have previously been undertaken to explore the relationships between mobility, volume, and physical properties of the rock mass. Several processes are thought to play a significant role in the increased mobility, including mechanical grain fragmentation, acoustic grain fragmentation, frictional melting, and mixing between the rock avalanche and substrate. To further constrain the role of these processes, this thesis will investigate the structural and deformation aspects of a rock avalanche deposit located in South Westland, New Zealand. The Cascade Rock Avalanche (CRA) was first identified as a large volume catastrophic rock avalanche by Barth (2013), having initially been interpreted as glacial moraine in previous studies. The CRA contains three main lithological groups: the Dun Mountain Ultramafic Group, the Greenland Group, and the Brook Street Volcanic Group. The deposit consists of c. 0.75 km3 of material and was likely triggered by a rupture on the Alpine Fault c.660 A.D. Observations from fieldwork presented in this thesis reveal characteristic rock avalanche textures and facies that formed during transportation. These facies include a carapace, quasi-bedding defined by the alignment of blocky material. Typical textures that had formed include fine-grained material in shear bands, jigsaw-style fractured clasts, liquefied material, and survivor clasts. Shear bands within the Greenland Group are subvertical, while those in the Dun Mountain Ultramafic Group are shallowly dipping. The quasi-bedding planes in the Dun Mountain Ultramafic Group are broadly parallel to the shear bands. These results indicate that there may have been structural and lithological controls on the formation of the shear bands and bedding surfaces during transportation of the rock avalanche. Three samples were selected for quantitative grain size and shape analysis to establish whether there were structural and lithological controls on grain fragmentation processes during transportation. Two samples represent shear band material and the other sample represents material from the body of the CRA. Results indicate that shear band material is texturally more mature than body material. Both of the shear band samples have a smaller mean grain size (5.39 µm and 6.13 µm) and higher fractal 3-dimensional (D), between 2.61 and 3.17) than the body material (mean grain size = 7.62 µm; D3D = 2.32 –3.27). The D values are comparable to those found in tectonic fault rocks. However, the D values established for both the shear band and body material are both above and below theoretical models of fragmentation which indicates that multiple fragmentation processes have taken place. Grain shape analysis provides evidence that grains within shear bands are more circular and have fewer grain-boundary asperities than grains within the bulk avalanche material. In conjunction with the field observations, the grain size and shape analyses indicate that grain fragmentation played a significant role in transportation and mobility of the CRA, likely contributing to the long runout of the material.