FIELD INVESTIGATION OF UPLIFTED PLEISTOCENE MARINE TERRACES, RIVERSDALE BEACH TO FLAT POINT, SOUTHERN WAIRARAPA COAST, NORTH ISLAND, NEW ZEALAND

The Hikurangi convergent plate margin lies along the east coast of the North Island, New Zealand, where the Pacific plate subducts northwestward beneath the Australian plate at ~45 mm/yr. Between Riversdale Beach and Flat Point, a flight of four well-preserved marine terraces records late Quaternary forearc uplift and folding along the southern Wairarapa coastline. Based on tread elevations and spacing, these terraces are interpreted as late Pleistocene in age. Prior mapping (Ghani, 1978) indicates the presence of a synclinal fold oriented subparallel to the coastline, causing a gentle down-warping of terrace treads in the central part of the field area.

This terrace sequence was visited during fieldwork conducted in 2018 and 2019 as part of the NSF SHIRE Project. Terrace cover beds were described and sampled for optically stimulated luminescence (OSL) age dating at 5 locations. Aerial drone imagery was also recorded. LiDAR topographic data from Land Information New Zealand (LINZ) will be processed in ArcGIS to map the terrace sequence in detail and to generate coast-normal topographic profiles. OSL age results and LiDAR derived terrace inner edge elevations will provide important constraints necessary for evaluating long-term uplift rates and folding along this coastal segment.

The Riversdale to Flat Point marine terraces are the product of late Pleistocene glacio-eustatic sea level fluctuations, coupled with long-term tectonic uplift and coastal emergence. This study will provide new insights on patterns of upper-plate deformation and marine terrace formation along the southern Hikurangi margin.