Coastal dune ridge systems as chronological markers of palaeoseismic activity: a 650-yr record from southwest New Zealand

A study of two shore- parallel dune ridge sequences in southwest New Zealand shows that tectonic activity has been the primary controlling influence in their formation since at least AD 1450. The timing of dune- building episodes at the mouths of the Haast and Okuru Rivers was determined using the ages of colonizing trees. Episodic dune formation was indicated by clear discontinuities in tree ages, with distinct cohorts having colonized successive, newly formed, dune ridge-swale units. At both sites, four dune ridge- swale units have formed since AD 1450, with each unit closely postdating an Alpine fault rupture ( c. AD 1460, c. AD 1615, AD 1717, AD 1826). Colonizing cohorts of trees started growing within 20 - 46 years after an earthquake at both sites, and all known major regional earthquakes have resulted in a dunebuilding episode. No other dunes are present at either site. Progradational coastal dune systems have potential as a tool for palaeoseismic studies. In regions with high background levels of sediment delivery to limit erosion/ burial of dunes and with little coseismic subsidence, dune systems may preserve a spatially discrete record of major earthquake- induced sedimentation events over the Holocene. Earthquakes are a key driver of palaeoenvironmental change and coastal plain development in this tectonically active region.