パプアニューギニア北東部のサンゴ礁段丘の総合研究 パプアニューギニア，ヒュンオン半島のテクトニックセッティング，地震活動および変動地形 サンゴ礁段丘研究の背景として

Abstract

This paper summarises the tectonic setting and seismicity of the Huon Peninsula, based on recently published papers. Some characteristic aspects of tectonic geomorphology are also briefly reviewed.The Huon Peninsula on the northeastern Papua New Guinea, located on ca. 6° S, is a suitable place for Quaternary sea level study, because coral reef terraces as an indicator of former sea level position are well preserved as separate terraces on land, due to strong uplift on the collision zone between the Australian and Pacific plate. The plate boundary is traced on land as Ramu-Malkahm Fault Zone. The formation of the Huon-Finisterre Range is a result of the collision of these two plates during the Quaternary. Fault plane solution for the major shallow earthquakes indicates that many of them are by reverse faults, strike of which is perpendicular to the fault zone. Thus, it can be said that the north-south oriented compression has been dominated at this fault zone.A flight of coral terraces, up to ca. 1, 000 m in altitude, fringes the northeastern coast of this tectonically active Huon Peninsula. U-series dates have been obtained from the terraces younger than Terrace VII, correlated with isotope stage 5e of the last interglacial maximum. Terrace height shows the northwestward downtilting, with some irregularity, from the maximum uplift rate of 3.5 m/ka. The uplift rate since the isotope stage 5e agrees with that for the Holocene, implying that the same pattern of tectonic uplift with the uniform rate has continued during the late Quaternary.Rapid uplift of this area is a result of repeated coseismic uplift which has an meter-scale uplift and ka-scale interval. Evidence of the coseismic uplift is obtained from both Holocene terraces and late Pleistocene terraces. Some of large landslides disrupting coral terraces are also originated coseismically. Coseismic process is an important tectonic factor for the formation and emergence and also for the destruction of coral terraces. These topics, related to paleoseismicity, are discussed in detail elsewhere in another papers in this special issue.Some landforms, related to active tectonics, such as active faults, uplifted alluvial deltas, and stream system, are also briefly discussed in the paper.