Abstract
Downwearing rates obtained from tectonically active coasts provide evidence of rapid rock breakdown following coseismic uplift. These measurements help solve puzzles about ‘missing' marine terraces and have implications for accurate reconstruction of past sea levels, earthquakes and rock coast evolution. However, very few detailed erosion datasets exist for uplifted shore platforms, making it uncertain if erosion records from one coast can be extrapolated to other regions with similar tectonic, geologic, or geomorphic characteristics. Here we present new downwearing rates from an inter-tidal mudstone shore platform at Kahutara Point, Māhia Peninsula, New Zealand that was uplifted likely by ∼3 m within the last 300 years. Downwearing rates were measured over 49 months/4.05 years using the Micro-Erosion Meter (MEM) and Structure-from-Motion photogrammetry: the mean annual downwearing rate was 1.08 mm yr⁻¹, while the total erosion at individual MEM stations ranged from 0.44 to 14.37 mm (equivalent to mean downwearing rates of 0.11 to 3.54 mm yr⁻¹). Orthophotographs of the eroded rock surfaces indicate the combined role of marine processes (waves and tides), sub-aerial weathering processes, salt weathering and biological activity in the erosion of the mudstone platform surface. The downwearing rates from Kahutara Point, Māhia Peninsula are statistically similar to previously published downwearing rates (1.23 mm yr⁻¹) from mudstone platforms at Kaikōura Peninsula, New Zealand, that were obtained prior to the 2016 Kaikōura earthquake, but are significantly different from perturbed post-uplift downwearing rates (2.25 mm yr⁻¹) obtained from the same MEM stations following coseismic uplift of ∼1 m. This work provides valuable data to support comparison of the development of shore platforms and marine terraces at Kaikōura and Māhia at different stages of their tectonic evolution.