Abstract
Shore platforms are erosional landforms on rock coasts shaped by wave erosion, tidal variations, weathering processes, sea level dynamics and tectonism. Studies have focused mostly on elucidating the relative role of waves and weathering as erosive processes and largely neglected the interplay between tectonics and erosion. Models depicting shore platform evolution are based on the assumption that platforms develop under stable or fluctuating sea levels in tectonically stable regions. Yet, many rock coasts evolve with the addition of vertical land movements.
This thesis investigated the rates and patterns of erosion at two sites affected by tectonic uplift at different times and shaped by marine and weathering processes. Erosion measurements were carried out on shore platforms at Kaikōura and Kahutara Point Māhia Peninsulas, New Zealand. These study sites provide unique opportunities to examine the developmental stages and erosional processes shaping inter-tidal shore platforms and marine terraces following coseismic uplift. Cross-scale erosion measurements were collected over five years using the Micro-erosion meter (MEM) at fifty-one sites and repeat photographs subjected to Structure-from-Motion (SfM) photogrammetry at forty-three sites. A MEM site covered ~0.01 m2 (microscale) and SfM covered 1 m2 (mesoscale) in order to assess sub-millimetre to metre scale erosion rates, erosion products and examine processes contributing to shore platform development.
Over one year, the mean annual erosion rate was 2.244 mm/yr from the MEM and 2.608 mm/yr using SfM microscale point clouds (PCs). Statistically, erosion rates from the MEM and SfM PCs are equivalent at the microscale, and either approach can measure shore platform erosion. The mean erosion rate calculated from the twenty-four mesoscale SfM PCs across the larger platform area (1 m2) was 10.299 mm/yr and statistically different from the MEM erosion rates. These differences in erosion rates demonstrated that a significant amount of platform erosion was undetected by the MEM.
Five years of MEM data complemented with SfM orthomosaics revealed that erosion rates were faster after the 2016 uplift of shore platforms at Kaikōura Peninsula. Platform erosion rates increased by 104%, from a pre-uplift rate of 1.100 mm/yr to a post-uplift rate of 2.247 mm/yr. The MEM data revealed that the mudstone lithologies respond faster to uplift events and previously reported seasonal trends in erosion rates have disappeared post-uplift. Results showed that erosion rates on the Kaikōura Peninsula were controlled by distance from the seaward edge, elevation and tidal characteristics, while lithological differences and microtopography determined the size of erosion products. Erosion rates measured from the inter-tidal mudstone shore platforms at the Kahutara Point, Māhia Peninsula, uplifted between 100 to 300 years ago, ranged from 0.069 to 5.820 mm/yr, with a mean annual erosion rate of 1.937 mm/yr. There was no spatial pattern in erosion rates cross-shore or between elevation groups at Kahutara Point, suggesting erosional processes are of equal efficiency because the entire platform is eroding uniformly in the inter-tidal zone.
The shore platform downwearing rates presented in this study represent two of three quantitative rates from tectonically active coasts and the first MEM lowering rates from inter-tidal mudstone platforms on an open coast on the North Island of New Zealand. The results demonstrate how tectonism can fundamentally alter how waves, tides, and weathering processes influence rock shore erosion rates. This thesis presents a modified rock erosion model that captures the morphological and microtopographic factors influencing shore platform development.