Observations of the incipient and penultimate stages of Holocene marine terrace development

Flights of Holocene marine terraces are useful for reconstructing past earthquakes, but coastal erosion can remove terraces from the landscape, potentially leading to incorrect estimates of earthquake magnitude and frequency. Relatively little effort has been afforded to studying terrace erosion processes, and this paper presents the first field evidence that we are aware of documenting terrace erosion rates. Two case studies from New Zealand provide a unique opportunity to observe the beginning and end phases of terrace development. We present downwear and backwear erosion measurements, showing that both sets of processes are important. Micro-erosion meter measurements from Kaikoura Peninsula, South Island, confirm that downwear processes are modifying new marine terraces that were created when the peninsula was uplifted about 1 m during the 2016 earthquake. Erosion rates were high immediately following uplift as the relatively barren intertidal rock shore platform rapidly transformed into an incipient marine terrace with cover deposits. However, the Kaikoura earthquake uplifted shore platforms only a small distance above the upper tidal limit and ongoing downwear and backwear erosion may begin to remove parts of this terrace in future decades. We explored this prospect with a case study at Mahia Peninsula, North Island, where 100-300 years have elapsed since the last terrace-forming earthquake. Historical photographs were used to document about 80 years of backwear erosion. Terrace erosion rates have been nearly constant through this period, and extrapolation implies that the terrace will be removed in places by 2030. The erosion data in this paper provide new insights into how terraces can be removed from the landscape, but there are many complicating factors. To help understand these factors we present a new conceptual model of marine terrace creation and destruction for soft-rock coasts.

Automated summary

Holocene marine terraces are important for studying past earthquakes, but erosion can remove them from the landscape, leading to incorrect estimates of earthquake magnitude and frequency. This paper presents the first field evidence of terrace erosion rates and highlights the importance of both downwear and backwear erosion processes. Case studies in New Zealand demonstrate how new terraces can be modified by downwear processes following uplift during an earthquake, and how ongoing erosion can remove parts of older terraces. The study also proposes a new conceptual model for terrace creation and destruction on soft-rock coasts.