太平洋西縁地域の最終間氷期の海成段丘

とくに酸素同位体ステージ5eの段丘の認定, 変動様式, 変位の累積性, および関連する諸問題

抄録

This paper aims to review the data of marine and coral terraces of the last interglacial maximum (isotope stage 5e) and Holocene ones on the western Pacific rim, especially from New Zealand and Huon Peninsula, Papua New Guinea. Informations on Taiwan, two islands of New Hebrides and Australia are also briefly mentioned based on published data. Main subjects discussed in this paper are :
1) methods of terrace identification as isotope stage 5e, 2) uplift rate and pattern since stage 5e and its significance, 3) the formation of terraces younger than stage 5e as a function of uplift rate, 4) the formation and subdivision of Holocene terraces with special reference to coseismic uplift, 5) relationship of uplift pattern and rate between the last interglacial terrace and Holocene one, 6) accumulated nature of deformation pattern through time.
In New Zealand, stage 5e terrace is identified at many sites from tephrocronological method and fission track age of major tephras, racemization ratio, and U-series age, TL dates of loess, in addition to morpho-stratigraphic observation of terraces and terrace deposits, such as terrace continuity, presence of thick transgressive deposits burying preexisted valley, and relationship between marine terraces and glacial deposits. However, radiometric age data are still rather limited and age of some important key tephras is still problematical to establish the detailed terrace chronology with accuracy of 1 ×10⁵ year time scale. Detailed deformation patterns have been established at several areas. Uplift rate is usually high at the area, close to subduction zones ; for example, 2.5 m/ka at northeastern coast of North Island. Accelarated uplift toward present is established at several areas, especially at Mahia Peninsula. Holocene terraces are well studied in the east and south coast of North Island and coseismic origin for several regressive terraces are now confirmed. Fifteen subregions are identified on the basis of number of Holocene terraces and radiocarbon dates of regressive coseismic terraces.
Coral terraces of Huon Peninsula provide the key information for Quaternary sea level changes. However, age data for stage 5e is not fully sufficient and two peaks within stage 5e is indicated. The high uplift rate reaching 3.5 m/ka make possible to form many terraces, corresponding to each relative high sea level position with nearly 1 × 10⁴ years interval. Repeated coseismic uplift during the last 50 ka is identified from profiles of late Pleistocene and Holocene terraces. Large eartrhquakes associated with meter-scale uplifts have been repeated at Huon Peninsula during the late Quaternary with recurrence interval of 1, 000-1, 300 years. Coseismic uplift is certainly important tectonic process in this tectonically active coast, although the location of seismogenetic faults responsible for coseismic uplift is not able to establish at this stage. In contrast to New Zealand coast, uplift has proceeded at the same rate since stage 5e.
Very high Holocene uplift rate in eastern Taiwan, up to 10 m/ka indicates the rapid uplift on the collision zone. No comparable data are available from late Pleistocene terraces, however. Coral terraces at Santo and Malekula Islands, close to New Hebrides Trench, show the high uplift rate, which has been accelarated between ca. 40 ka and Holocene. However, U-series ages were not obtained from high terrace which is assumed to be stage 5e. On the north Malekula, terrace uplift pattern is concordant with the 1965 coseismic uplift pattern. Average recurrence interval of large earthquakes is estimated to be 340 years. This interval is discordant with four or five steps within the Holocene coral terrace, and requires further interpretation. Australian coast provides data from stable continent. Uplift rate is usually small, with the highest rate of 0.2 m/ka at Tasmania.