Marine terrace genesis on an active tectonic coast: rates and processes of rock coast erosion prior to uplift during the 2016 Kaikōura earthquake inferred from cosmogenic 10Be concentrations.

Marine terraces (paleo shore platforms that have been raised above sea level) are common landforms on rock coasts around the globe. Ongoing uplift (relative sea level fall) can result in sequences of marine terraces that get younger towards the modern coast. Where their age can be determined, these marine terraces are often used to infer past rates of tectonic uplift and/or relative sea level history. Marine terrace formation can also result from coseismic uplift during large earthquakes and thus these terraces may provide an archive of past high magnitude earthquake events. Cosmogenic radionuclide geochronology has previously been used to constrain the age of raised marine terraces. More recently, cosmogenic isotopes have been used to investigate rates of cliff retreat and shore platform erosion on active rock coasts, revealing that rock coasts may harbour appreciable concentrations of these isotopes prior to uplift to form marine terraces. This inherited signal could be a key source of uncertainty and potential bias in the marine terrace record. The recent M_w 7.8 Kaikoura earthquake on 14^th November 2016 presents a globally unique opportunity to assess the extent of this uncertainty. Here, we interpret a transect of cosmogenic radionuclide ¹⁰Be concentrations across a section of rock coast raised by 6.5 m during the 2016 earthquake. We invert ¹⁰Be concentrations for the most likely rates and processes of rock coast evolution in order to ask a) how much of the ¹⁰Be inventory in raised marine terraces has been inherited at the time of uplift and terrace formation and b) whether the coast has formed entirely during the Holocene or could be inherited from the past? The results will have implications for the interpretation of paleoseismicity. Uplift rates from dated marine terraces will inform assessment of coastal vulnerability in the face of sea level rise, which could be countered by land upheaval.