Use of ancient wave-ravinement surfaces to determine palaeogeography and vertical crustal movements around New Zealand

Wave-ravinement (shallow marine erosion) surfaces are formed during landwards migration of the shoreline due to rising relative sea level. They can be preserved if subsequently buried by sediment, and are easily identifiable on seismic reflection data. We present two examples from New Zealand's north-western offshore frontier region where these surfaces, despite a diachronous nature, represent sequence stratigraphic and geomorphic markers that are used to constrain regional palaeogeography and tectonic history. Thermal subsidence that followed the break-up of Gondwana led to the landwards migration of shorelines across the then-emergent Challenger Plateau and formation of a prominent Late Cretaceous-Eocene wave-ravinement surface. In the Reinga Basin, subsidence of uplifted land areas that had previously emerged during Cenozoic initiation of Tonga-Kermadec subduction was similarly accompanied by formation of transgressive wave-ravinement surfaces. Wave-ravinement surfaces serve as useful proxies for palaeo-sea level and, as the above two examples show, they are powerful tools for characterising regional deformation, uplift and subsidence histories and tectonic influence on relative base level fluctuations (changes in shoreline position). The study of these and other remarkable palaeo-sea-level markers around New Zealand and in the circum-Pacific region provides a different perspective on constraining vertical crustal movements associated with major tectonic events.