The Kuril basin was formed by back-arc spreading in Oligo-Miocene time which is deduced from bottom depth, heat flow and geological data in and around the basin. The b sin has a shape of fan which diminishes northeastward and closes just southwestern end of Kamchatka Peninsula. The fan shape strongly suggests that the basin was formed by the rotation of backarc plate (the Okhotsk Plate) around a relative rotation pole located between the Kuril Basin and Kamchatka Peninsula. Geology around the Sea of Okhotsk indicates that the clockwise rotation of the Okhotsk Plate and the fan spreading of the Kuril Basin concurrently took place in Oligo-Miocene time.
The Japan Trench and the northeast Japan arc form a single arc-trench system. The topographic features and the geological history of this arc-trench system are briefly summarized. Distribution of the morphotectonic elements across the system east to west is as follows: An outer swell on the deep sea floor, a group of horsts and grabens bound by normal faults caused by the bending of the Pacific Plate, a deep, flat trench axis covered by thick sediments of both pelagic and hemipelagic nature, a trench inner slope partly exposing Cretaceous mudstone, a gradually uplifting and ponding bench, deep sea terraces covered by thick terrigenous sediments, a non-volcanic outer arc (Kitakami Massif and Abukuma plateau), a volcanic chain (volcanic front), an intra-arc depression, and a marginal sea (Japan Sea). These elements are distributed parallel along the arc. Geophysical parameters are also traceable along the arc. Every geological and geophysical feature around the Japan Trench and the Tohoku Arc show this polarity from the trench axis westward. In the Paleogene, around DSDP Sites 438 and 439 an ancient landmass is indicated by samples and seismic profilers where short-term calc-alkaline volcanic activity took place, boulders of the volcanic rocks then being transported to the site. The Japan Trench forearc area began to subside during Miocene times and diatomaceous hemipelagic mud and the volcanic ashes accumulated in the forearc basin of the Japan Trench area. Since Pliocene time, the Japan Trench area and the Tohoku Arc have gradually been uplifting together with the intrusion of the Tertiary granites. At about 5 Ma a drastic change of the depositional environment in this region resulted from the change of the mode of subduction of the Pacific Plate into the Japan Trench from Mariana-type since 15 Ma, to Chilean-type subduction from 5 Ma to the present.
Tectonic elements of submarine landforms and geologic structures are studied with thereference to the neotectonics concerning to the convergence of the Philippine Sea (PHS) plate with Honshu on the Eurasian (EUR). The northern tip of Izu Arc is morphologicallycharacterized by upwarping with an axis in the direction of 323°, the Niijima-shinkuroseUplift zone, which is oblique to the axis of Izu-Ogasawara volcanic arc. Intersectionand capture of consequent canyons on the northeastern slope of Izu Arc are well observed. This indicates the downbending of PHS associated with subduction along SagamiTrough. The sense of convergence between PHS and the overriding plates might havechanged from northward to northwestward (323°) at about 0.5Ma B. P. This can be evidencedby the following features on the accretionary wedge along Sagami Trough:(1) the graben-like structure in a trend of 325° was filled with the undeformed middle Pleistocenestrata on the accretionary wedge;(2) segmentation of the mechanical plate boundarydue to a forearc tectonics related to APC plate and to the strike-slip motion along the outerridge. However, no significant feature necessary to infer such a change in direction ofrelative motion between EUR and PHS plates is found along the Suruga Trough.
The Ogasawara Arc consists of four ridges; they are the Izu, Shichito (volcanic front), Ogasawara and Shinkurose Ridges. Geology and geomorphology of the arc are reviewed b sed on dredged submarine rocks and subaerial geology of islands on the arc. On the basis of submarine topographic features the Shichito Ridge is divided into three parts (segments 1, 2 and 3). Topographic gaps are found between segments 1 and 2, and 3. The Shinkurose Ridge appears to be displaced at the gaps. The gaps are situated between Smith Rock (Sumisujima) and Torishima Is., and between Sofugan and Nishinoshima Is. The latter is correlated with the Sofugan Tectonic Line (YUASA 1983). Volcanic rock bulk chemistry of the volcanic front, and hypocenter distribution pattern are different between northern and southern parts of the Ogasawara Arc separated by the Sofugan Tectonic Line. Distribution of back-arc depressions and frontal arcs are also different between them. In 1983, BANDY and HILDE proposed three right-lateral transverse faults on the Ogasawara Arc based on the displacement of outer ridge structure, and the existence of gaps on free-air gravity anomaly map. The right-lateral displacement proposed by them is apparently different from left-lateral displacement of the Sofugan Tectonic Line and dislocations within the Shichito Ridge. The Sofugan Tectonic Line may not be the only transverse fault on the Ogasawara Arc. There seem to be other faults between segments 1 and 2 of the Shichito Ridge and elsewhere. The gap, however, affecting the geological and geophysical phenomena such as rock chemistry, and hypocenter distribution is only the Sofugan Tectonic Line.