ニュージーランドの変動地形に関する最近の研究

抄録

This paper discusses recent studies since 1975 on the tectonic geomorphology of New Zealand, acountry which is well-known for work on this subject. The regional character of the tectonic landforms is controlled mainly by the tectonic setting of New Zealand at the boundary between the Australian (Indian) Plate to the west and the Pacific Plate to the east, with opposed subduction zones at the northeast and southwest margins and a complex zone of shear deformation linking the two. The Taupo Volcanic Zone in North Island is a volcanic arc and marginal basin characterised by volcanism and crustal extension.
Active faults and deformed marine and fluvial terraces are prominent tectonic landforms. Detailed research on the Alpine Fault are being supplemented by intensive works on active reverse faults. In South Island this includes trenching studies. The characteristics of the deformed topography and trench logs across the Dunstan Fault and Pisa Fault in the Otago area demonstrate the close similarity between reverse faulting here and many active reverse faults in Japan. The deformation of fluvial terraces and the height of postglacial transgressive deposits and marine terraces indicate a very high uplift rate in the Southern Alpes east of the Alpine Fault which is ascribed to vertical drag along the Alpine Fault due to the plate convergence.
Marine terrace studies, especially in North Island and some parts of South Island have made much progress. In North Island, the last interglacial terrace reaches a maximum height of 300m and the postglacial terrace a maximum of 27m. The deformation pattern shows regional differences. In North Island, landward tilting away from the Hikurangi Trench is observed on the eastern coast. Folding with short wavelength and small-scale block faulting dominate the central to southeast coast and the south coast. Downtilting towards the sea continued in the Taranaki area during the middle to late Quaternary. Holocene marine terraces can be used as reference surface for the reconstruction of paleoseismicity. Fourteen tectonic regions are identified on the east coast of North Island on the basis of the number of subdivided Holocene marine terraces and their radiocarbon ages. Some of offshore seismogenic faults are inferred from submarine landforms and records of obtained seismic profiling. In South Island, discussion has focused on the recognition of very high terrace remnants and on age estimation of terraces which have no datable materials and key beds for correlation.
Tectonic landforms have been used as references for compiling a Quaternary uplift map. The maximum uplift rate of 17mm/yr is located on the east side of the Alpine Fault, South Island. The highest uplift rate in North Island is 4mm/yr north of Gisborne and Turakirae Head near Wellington and was derived from Holocene marine terrace data. It is, however, still difficult to establish a temporal sequence for uplift rates because of insufficient age control.