MARINE TERRACES AND THEIR TECTONIC DEFORMATION ON THE NORTHEAST COAST OF THE NORTH ISLAND, NEW ZEALAND

The northeastern corner of the North Island, New Zealand, is situated at the southern terminal of the Tonga-Kermadec island arc and trench system, and its east coast runs nearly parallel to the axis of the Kermadec Trench, whereas the trend of its north coast, roughly speaking, is normal to the trench (Fig. 1). Raukumara Range extends approximately parallel to its east coast and terminates close to its north coast, where Matakaoa Range runs from east to west separated from the former by a narrow depression (Fig. 6). The area east of Raukumara Range is hills composed of the Neogene formations and the Taupo Volcanic Zone lies to the west of the range. In this region, marine terraces are developed fairly well on the north coast from East Cape to Cape Runaway and on the southeast coast of the Bay of Plenty as far west as Ohiwa Harbour. Volcanic ash derived from the Taupo Volcanic Zone is extensively distributed in this region and covers the terraces. Its maximum thickness is about 20m in the westernmost part of the region. Volcanic ash beds are grouped into the youngest, the Hamilton and the Tablelands tephra formations (Fig. 2). Fission track ages of the four horizons in the middle and lower members of the Tablelands tephra formation were measured to be 0. 19 to 0. 21m. y. B. P. (Iso et al., to be published). The Ohinewai ash located at the lowest horizon of the Hamilton tephra formation was estimated at about 0.15m, y. B. P.by Vucetich et al. (1978) . A deeply weathered paleosol is developed at the uppermost: horizon of the Hamilton tephra formation and is unconformably overlain by the Rotoehu: ash, which is the lowest horizon of the youngest tephra formation and was dated at 41, 700 14C y. B. P. (Pullar et al., 1973). By tephrochronological and geomorphological means, the marine terraces in this region_ are classified, in descending order, into Matakaoa, Otamaroa, Te Papa and Te Araroa Terraces and can be recognized fairly easily over almost the whole region (Fig. 3 and Fig. 4 A-F). Matakaoa Terrace is conformably covered with the Tablelands tephra formation. This terrace is clearly of marine origin, though small in cereal extent, in the northeasternn part of the region, but chiefly forms extensive uplands of fluvial origin, mantled with thick volcanic ash in the southwestern part. Its height ranges from 130 to 20 m. This terrace is capped by the three tephra formations, and is inferred to have been formed in the pe nultimate interglacial age. Otamaroa Terrace is the most extensive terrace in the region, and its initial surface is. preserved fairly well and is conformably overlain by the upper horizon of the Hamilton. tephra formation. Its height ranges from 300 to 10m. Terrace deposits are mainly com. posed of well-sorted rounded gravel and sand 2 to 5m thick, which at several places fill depressions more than 20m deep in the underlying rocks. This suggests that this terrace was formed under a transgressive condition. Fossil mollusca obtained from a silt bed of this terrace in the vicinity of whangaparaoa indicates marine conditions a little warmer than at present (A. G. Beu, personal communication). From its topographical and sedimen tological features, as well as the ages of tephra overlying it, Otamaroa Terrace iss judged to have been formed in the last interglacial age about 0.12m. y. B. P. Te Papa Terrace is mainly distributed in the northeastern part of the region and is less widely developed than Otamaroa Terrace. Its surface is conformably covered by the Rotoehu ash, and its height ranges from 215 to 10m. Terrace deposits consist chiefly of marine gravel and sand 2 to 3m thick, but at a few places they exceed 10m thick. This. terrace, therefore, is inferred to have been formed during a minor transgression.