Cataclastic Processes in Small Scale Faults at Loch Laird Recreational Reserve, Benmore, New Zealand

Abstract

The microstructural deformation processes that occur during earthquakes and how these are represented in the fault rocks are not well understood. As cataclastic fault rock is formed, there is an evolution of particle size distribution and formation of different particle shapes. Three faults were investigated in the Loch Laird Recreational Reserve, Benmore, Waitaki, which have accommodated varying amounts of slip (0.64 - 6.8m). Using high-resolution imaging capabilities on a Scanning Electron Microscope, we produced particle maps that distinguish grains according to their size and shape. These maps were constructed for the main cataclasite components: quartz and feldspar. Particle size distribution (PSD) data indicate that there is no relationship between fractal dimension (D) and amount of slip on the fault plane. The fractal dimension is consistent across all faults regardless of total slip. Therefore, it is proposed that the fractal dimension within cataclasite is established at displacements smaller than those accommodated by these faults. Particle size analysis show that, as particle size decreases, angularity decreases but circularity and convexity increase. From this study alone, it is not possible to infer what effect this change in particle shape has on the frictional strength of the fault. Observations from thin sections and consistency of PSD with theoretical predictions indicate that constrained comminution is the deformation mechanism that initially reduces particle size. Abrasion is thought to be the dominant particle size reduction for relatively smaller particles, inferred by examination of the circularity and convexity shape data.