COASTAL DUNES IN SOUTH WESTLAND, NEW ZEALAND, PROVIDE A RECORD OF PALEOSEISMIC ACTIVITY ON THE ALPINE FAULT

A study of shore-parallel dune ridge sequences in South Westland, New Zealand, shows that tectonic activity has been the primary controlling influence in their formation since at least AD 1200. The timing of dune building episodes at the mouths of five 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.

Regional episodes of coastal progradation and dune formation have quickly followed all of the five known Alpine fault earthquakes since AD 1200 (c. AD 1826, 1717, 1615, 1440, 1230). Colonizing cohorts of trees started growing within 10-46 years after an earthquake, and all known major regional earthquakes have resulted in a dune building episode. This reflects rapid transport of large post-seismic sediment pulses from mountain catchments to the coast and accumulation of this material as a dune ridge. This study provides the first demonstration of this link. The dune sequences also provide evidence of one other previously unrecognized regional aggradation event at around AD 1500, just c. 50 yr after a large Alpine fault earthquake; it is unclear what caused this dune building episode, but an earthquake origin is possible.

Coastal dunes have great potential for paleoseismic application because the spatial separation of earthquake-induced sediment pulses on a prograding coast allows clear identification of successive events including events closely spaced in time. While the system described in this study could be unique to South Westland, coastal dune systems may have potential to improve paleoseismic understanding over the Holocene for other plate boundary faults near coastal areas.