GEOCHRONOLOGY OF UPLIFTED LATE PLEISTOCENE COASTAL TERRACES, CAPE KIDNAPPERS AND RIVERSDALE BEACH, HIKURANGI SUBDUCTION MARGIN, NORTH ISLAND, NEW ZEALAND

Uplifted flights of late Pleistocene coastal terraces occur along multiple segments of the Hikurangi subduction margin, North Island, New Zealand. Terrace preservation at these sites indicates locally moderate rates of tectonic uplift, combined with minimal horizontal change in coastline position over time. Terrace ages at two of these sites (Cape Kidnappers and Riversdale Beach) are constrained in this study by IRSL luminescence on marine and fluvial terrace deposits, tephrochronology, and eustatic sea level curve correlation.

At Cape Kidnappers (southern Hawkes Bay), a single prominent marine terrace is cut across Pleistocene marine sediments of the Kidnapper’s Group. This surface is gently folded in a subtle anticline, with terrace inner edge elevations ranging from ~135-200 m above msl. At least four lower fluvial terraces are inset below the marine terrace within local canyons and gullies, and a prominent Holocene bench wraps around its seaward edge. Between Riversdale Beach and Flat Point (central Wairarapa coast), a flight of four marine terraces are cut into Cretaceous-Neogene forearc basin sediments. These terraces are broadly folded along a margin-parallel syncline, with inner edge elevations ranging from ~15-175 m above msl.

18 sediment cores were extracted from terrace deposits at 11 sites in these two areas and dated via Optically Stimulated Luminescence (IRSL). Using Central Age Models, feldspar grains were examined and dated for age of burial. Tephra horizons within terrace cover beds at Cape Kidnappers provide additional minimum age constraints. Correlation of terrace inner edge elevations with the global sea level curve yields further age constraints and provides a means of calculating local uplift rates. Preliminary results indicate net uplift rates at Cape Kidnappers of 1.1-1.7 m/ky, while at Riversdale, uplift rates range from 0.2-0.8 m/ky. Tectonic uplift at both sites is the product of forearc shortening above the Hikurangi megathrust.