Abstract
North Island of New Zealand is a plate boundary zone controlled by oblique subduction of Pacific Plate beneath Australian Plate from Hikurangi Trough. Palmerston North area is the boundary between the uplifting northern Tararua Range of axial tectonic belt and depressioning Manawatu plain of eastern edge of Wanganui sedimentary basin. The depressional center of the basin is offshore from the mouth of Manawatu River. We have researched marine and fluvial terraces of the eastern Palmerston North area to clarify its geomorphic history and Quaternary tectonic movement.The results obtained can be summarized as follows:
1) Based on terrace topography, deposits, paleosoil, and tephra bed, terrace surfaces are subdivided into Tokomaru, Forest Hill, Milson, and Ashhurst surfacesin descending order following Fair (1968). Tokomaru terrace is the highest marine terrace in this area and correlated with MIS 5e transgressive stage. The age of this surface is 0.13 Ma.
2) Forest Hill, Milson, and Ashhurst terraces are fluvial in origin. According to our stratigraphic data and Palmer and Pillans (1996), Forest Hill terrace is correlated with Porewa stage dated around 70-80 ka, Milson terrace is correlated with Rota stage dated around 30-50 ka, and Ashhurst terrace is correlated with Ohakea III dated about 18 ka.
3) In Linton Quarry at the southern tip of study area, three marine deposits and Linton tephra bed are observed. Linton tephra at the basal part of the deposits of the Quarry should be correlated with Rangitawa tephra based on stratigraphic evidence and the main chemical components of volcanic glass analyzed using EDS-XMA. Linton tephra originated from Whakamaru ignimbrite in Taupo Volcanic Zone and its age is 0.35 Ma. Two marine beds overlain by Tokomaru bed are correlated with MIS 7 and 9 transgressive stages respectively, but they have no terrace surfaces. This area has been controlled by uplifting since elevation of Tokomaru terrace.
4) Altitudes of shoreline of Tokomaru terrace were measured using the Tremble GPS system toclarify deformation of shoreline at 0.13 Ma. Average uplift rates ranged from 0.8 mm to 1.1 mm per year. Shoreline of MIS 5e was tilted at 0. 003 gradient to the southwest direction. This is caused by cumulation of tilting to depocenter of Wanganui sedimentary basin.
5) Projection of terrace surfaces along Turitea River means that the higher terraces are, the steeper their gradients. In addition, Tokomaru terrace was crossed by younger Forest Hill terrace. This is caused by cumulation of basin building movement of the Wanganui basin. Assuming that Tokomaru surface had been flat, and Forest Hill and Ashhurst terraces had the same gradient as the present riverr bed, average rates of tilting to the northwest direction were accelerated after formation of Forest Hill terrace.
