Measuring erosion on uplifting coasts: Shore platform downwearing rates and pattern on inter-tidal rocks at Māhia Peninsula, New Zealand

Abstract

Recent erosion rates from tectonically active coasts provide evidence of rapid rock breakdown following coseismic uplift. These rates are needed to solve puzzles about 'missing' marine terraces that prevent accurate reconstruction of past sea levels, earthquakes and rock coast evolution. Yet little scientific effort has been put into investigating the impact of tectonism on shore platform development. It is also presently unclear if erosion records from one coast can be extrapolated to other regions with similar tectonic, geologic, or geomorphic characteristics. We present shore platform downwearing rates and processes measured using the traversing micro-erosion meter (TMEM) and Structure-from-Motion photogrammetry from an inter-tidal shore platform uplifted by 3.1 m between 100 to 300 years ago. The site is a mudstone platform at Kahutara Point, Māhia Peninsula, North Island, New Zealand. Over 1.43 years, the mean annual downwearing rate was 1.94 mm/yr, while the total erosion at individual TMEM stations ranged from 0.29 to 8.31 mm (equivalent to mean erosion rates of 0.07 to 5.82 mm/yr). We found a lack of any spatial pattern in erosion rates cross-shore, suggesting the equal efficiency of waves and weathering processes. Orthophotographs of the eroded rock surfaces support the combined role of marine processes (waves and tides), sub-aerial weathering processes, salt weathering and biological activity in the erosion of the mudstone platforms at Kahutara Point. The mean erosion rate of 1.94 mm/yr from the Māhia Peninsula is similar to the mean post-uplift rate of 2.25 mm/yr reported for the Kaikōura Peninsula, New Zealand, where platforms were uplifted by ~1 m in 2016. This result suggests a comparable pattern of erosion response at both sites following co-seismic uplift and provides the first field evidence to support the comparison of the Kaikōura and Māhia shore platforms, thus helping to inform marine terrace development and destruction.