南部フォッサマグナに関連する地形とその成立過程

抄録

Landforms in and around the South Fossa Magna, which are shown in Figs. 2 and 3, and the history of their growth during the late Cenozoic are described. The pattern of the tectonic landforms and the history of their growth are explained by a tectonic model shown in Fig. 5.
The South Fossa Magna consists mainly of strongly folded and thrust Neogene and Quaternary strata along the subduction and collision zone between the overriding Eurasian plate (or the Eurasian and North American plates) and the subducting Philippine Sea plate. The collision occurred on the north of the Izu Peninsula during the past 1-0.5 Ma. The characteristic landform in the folded zone is flights of elongated domes or ridges, called outer ridges.
There are three areas with different topography and tectonic history in central Honshu to the north of the plate boundary; they are, from west to east, 1) the Akaishi Mts-Tokai lowland and offshore area, 2) the Kanto Mts-Misaka Mts-Tanzawa Mts area, and 3) the Kanto Plain-Miura-Boso Hills and their offshore area. To the south of the plate boundary, there are three different belts in the northeastern part of the Philippine Sea (PHS) plate; they are, from west to east, i) the Shikoku Basin, ii) the inner volcanic arc (Izu Inner Bar), and iii) the outer non-volcanic arc (Izu Outer Bar) of the Izu-Bonin arc.
The PHS plate moved to the north after the spreading of the Shikoku Basin (30-15 Ma BP), and different physical property and somewhat different speed of movement of the three belts gave different tectonic features to the three areas in central Honshu as is shown in Fig. 5 (A). Then the PHS plate collided with Honshu on the northern side of the Izu Peninsula, and changed the direction of movement to the northwest in 1-0.5 Ma BP. Quaternary uplift of the Akaishi, Tenshu, Misaka, and Tanzawa Mts thus started, while the processes of making the outer ridges ceased under a rather extensional stress field in the Miura-Boso Hills and their offshore area of southern Kanto as shown in Fig. 5 (B). Collision between the northeast Japan arc (on the North American plate) and the Southwest Japan arc (on the Eurasian plate) may have occurred along the Fossa Magna in the Quaternary Period.