Toyama Trough Research Articles

日本列島周辺の海底地質 その４ 日本海

Topographies of the Japan Sea can be divided roughly into three parts; the continental borderland lying from Hokkaido to San'in Region, the Yamato Ridge and the Korea Plateau of the continental relic, and the deep-sea basin in the northern Japan Sea. The deepest part of the Japan Sea is about 3700m deep located near the eastern margin of the Japan Basin. The age of the Japan Sea is estimated to be younger than 30Ma. The recent topographic features of the Japan Sea have been built mainly by the large-scale deformation and reorganization of topography since late Pliocene. The continental borderland, east of the Toyama Trough, in the northeastern area has very complicated topographies with folding and faulting. In contrast to it, the continental borderland of southwestern area has been comparatively calm in structural activity since late Pliocene. Erosional terraces in late Pliocene to early Pleistocene are reserved around the summit of the Yamato and Oki Ridges and on the basement covered by Quaternary strata forming a marginal terrace. The eastern margin of the Japan Sea is structurally the most active zone, where the lithospheric convergence occurs by the strong compression. The main belt of convergence in the eastern margin of the Japan Sea can be traced by the distribution of epicenters and aftershock areas of great earthquakes of M7 class along the Okusiri Ridge, the Mogami Trough, and the valley of the Sinano River.

富山トラフの変動地形と地質構造

Hydrographic Department, Japan Maritime Safety Agency conducted multichannel seismic reflection and refraction survey and Sea Beam bathymetric survey in the Toyama Trough extending northward in the southeastern margin of the Japan Sea. The purpose of the survey is to reveal the geological structure and tectonic landform of the Toyama Trough and to examine the hypothesis that the Toyama Trough is subduction zone.Seismic profiles across the Toyama Trough do not support the presence of distinct subduction zone along the trough because the basement of the trough is not traceable beneath the Sado Ridge. This suggests that the Toyama Trough is different in geological feature from typical subduction zone such as the Japan Trench or the Nankai Trough, where subducting oceanic basement is clearly traceable beneath the overriding plate in seismic profiles. Further, seismic profiles show that the sedimentary layers thicken to the east in some places and to the west in the other places. These features indicate that convergent plate boundary is not present, or only extremely incipient plate boundary is present along eastern margin of the Toyama Trough. However, the tectonic landform and geological structure characterized by the presence of active reverse faults indicate that Toyama Trough has been subjected to compressional stress.

口絵富山トラフ周辺の地形・地質構造

口絵富山トラフ周辺の地形・地質構造

日本海東縁地域の地震活動とメカニズムおよびテクトニクス

Extensive studies on seismic activity and focal mechanism of major to moderate-size earthquakes that occurred along the eastern margin of the Japan Sea and the northern part of the Fossa Magna regions are reviewed and discussed in relation to regional tectonics. The validity of a prevailing hypothesis is also tested from various observations to see if the Northeast Honshu arc is part of the North American plate.Seismicity along the Japan Sea coastal regions extends southwestwards across the Toyama trough in one way, and also in other way to the northern Fossa Magna through a zone near the Shinano river. It has been noticed that the seismicity show temporarily successive properties similar to migration phenomena. Five major earthquakes along the Japan Sea regions have been interpreted as having thrust-type mechanisms, and the depth distribution of aftershocks of two larger events and the dip of submarine active faults suggest an eastward dipping fault plane.The maximum compressive stress derived from the focal mechanisms of more moderatesize earthquakes indicates E-W to ESE-WNW orientations, changing gradually from the northern to southwestern part of the coastal regions. In inland regions west of the northern Fossa Magna, the general trend appears oriented again in the ESE-WNW direction, which is found to be well consistent with the direction of principal compressive strains derived from geodetic triangulation surveys over the last 80 years.All the above evidence suggests that the regions under consideration may be a tectonically active, convergent zone, and might be regarded as a zone of a possible plate boundary. There is no direct evidence, however, suggesting an eastward incipient subduction of the Japan Sea lithospheric plate, from observations of submarine topography and upper crustal structure beneath the regions.Numerical calculations show that the suggested hypothesis could partly account for the observed directions of compressive stress in Northeast Honshu but is apparently inconsistent with those in Southwest Honshu. The results suggest that the Northeast Honshu arc may better be regarded as a “micro-plate” which receives strong compressive stress not only from the westward movement of the Pacific plate but also from the west side. One of possible sources of the driving stress from the west side might be an eastward movement of the “Amurian plate” which is one of micro-plates detached from the Eurasian plate. Another possible source could be bilateral extension of the central Japan Sea region or of the regions covering Northeast China and the Korean peninsula.

Provenance Variation in Turbidite Sediments, Sea of Japan

ABSTRACT Provenance variations in turbidite deposits from DSDP Sites 299 and 301 in the Sea of Japan have been determined on the basis of petrographic and electron microprobe analyses of quartz and feldspar in the 34-74µ size interval. Site 299 receives most or all of its sediment in this size interval from central Japan via the Toyama Trough. Site 301 is located in the middle of the Sea of Japan and receives detritus from the main Asian continent as well as from the volcanic island arc. Reflecting different types of source rock, the sediments from the island arc (Site 299) contain less quartz, more feldspar, more plagioclase An >= 50, less alkali feldspar Or85-100 and a higher mean An content of the unaltered plagioclase grains (40.2% vs. 29.2%) than Site 301. At Site 299 the quartz/ eldspar n < 1.54 ratio increases with depth below the sea floor. This may represent either increasing separation from the Asian continent or increasing volcanism and albitization on the island arc. There are no differences in the detritus in the 34-74 µ size interval in sand versus mud beds at site 299.