ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00139.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractBased on the spatial distribution of the focal areas from previous large earthquakes, a significant seismic gap was found off the coast of the Akita Prefecture at the eastern margin of the Sea of Japan. We forecast that the 75 km segment of plate boundary may be ruptured by a large thrust earthquake with a magnitude of 7.5 in the near future. The time of occurrence is uncertain, but the recurrence interval of large earthquakes in this region suggests that it will be no later than the end of next century. The time sequence of large earthquakes in the eastern margin of the Sea of Japan shows a systematic shortening of the earthquake interval for the past 250 years. This feature is simulated successfully by a simple mechanical model that represents a plate boundary by segmented blocks, each of which is composed of a spring friction slider system. We further propose to revise the conventionally accepted plate boundary between the Eurasian and Okhotsk plates in the eastern margin of the Sea of Japan; the observed data of seismic activity and recent crustal deformation indicate that the southern part of the plate boundary traverses the land region from Niigata to Otari along the Shinanogawa seismic belt. The fault geometries of previous large earthquakes show that the mode of plate convergence in the eastern margin of the Sea of Japan is a collision type rather than a simple eastward subduction as was postulated in early studies.

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00140.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractExtensional basin formation and subsequent basin inversion in the southern area of the eastern margin of the Japan Sea were studied on the basis of the interpretation of seismic profiles (total length approximately 15 000 km) and the fossil analyses of 77 sea‐bottom samples. Rift (Early to Early Middle Miocene), post‐rift (Middle to Late Miocene), pre‐inversion (Late Miocene to Pliocene) and inversion stages (Pliocene to Quaternary) were differentiated by the extension and contraction of the crust. Many small‐scale rifts were formed in the Sado Ridge and the Mogami Trough during the rift stage, simultaneous with back‐are spreading of the Japan Sea. Most of the rifts were east‐ or southeast‐facing, rotational half‐grabens bounded by west‐dipping normal faults at their eastern boundaries. The syn‐rift sequence can be divided into lower and upper units by an erosional surface. The sequences are presumed to be composed mainly of fining‐upward sediments. The trend of most rifts is north‐northeast with the remainder being of east‐northeast‐bias. The north‐northeast trending rifts are distributed widely in the Sado Ridge and Mogami Trough and do not show anen échelonarrangement, suggesting that they were formed mainly by pure extension nearly perpendicular to the arc. The east‐northeast trending rifts are presumed to have been developed by a north‐northwest extension in the late rift stage, which may have accompanied a right‐lateral movement in the eastern margin of the Japan Sea. During the post‐rift stage, the rifts and adjacent horsts subsided and became covered by the post‐rift sequence, characterized by parallel and continuous reflections. This suggested no significant tectonic movements in this period. In the pre‐inversion stage many of the rifts subsided again, presumably because of down‐warping due to weak compressional stress. The normal faults reactivated as reverse faults during the inversion stage due to an increase in compressional stress. Many of the rifts have been uplifted and transformed into east‐vergent asymmetric anticlines. The basin inversion is greatest in the Sado Ridges and in the Dewa Bank Chain, while it is least developed in the Mogami Trough and in the western slope of the Sado Ridge, in which some normal faults have not been reactivated. The increase and decrease of the inversion corresponds to the peak and trough of undulation at an interv

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00141.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractThe Nankai Trough, off southwest Japan, is one of the best sites for the study of geomorphic characteristics of a clastic accretionary prism. A recent multibeam survey over the central and eastern parts of the Nankai accretionary prism has revealed a large variation of the topography along the trough axis. Analysis of the bathymetric data suggests the existence of prism deformational features of different scales, such as depressions, embayment structures and cusps. These structures are the results of slope instability caused by basement relief of subducted oceanic plate. Unstable slopes recover by new accretion and development of a low angle thrust. Small‐scale deformation due to the subduction of a small isolated seamount is then adjusted to the regional trend. By contrast, a 30 km indentation of the wedge observed in the eastern part of the Nankai Trough, the Tenryu Cusp, has seemed to retain its geometry. The subducted Philippine Sea plate has deformed greatly near the eastern end of the Nankai Trough, because of the collision between the Izu‐Ogasawara (Bonin) arc and central Japan. Therefore, the indentation may be the result of the continuous subduction of a basement high, such as the Zenisu Ridge, which has been formed under north‐south compression due to the arc‐arc co

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00142.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractFossil worm tubes were collected from the Hayama Group, Miura Peninsula, Japan, together with abundant fossils ofCalyptogena‐Acharaxclams. The fossil worm tubes were well preserved and coated with milky white amorphous silica. Most of the tubes were 1‐3 mm in diameter, and up to 10 cm in length. Worm tubes were found in siltstone and limestone, and formed network‐like assemblages. Elemental mapping on the tube cross‐sections revealed the localization of sulfur, zinc and iron at the worm tubes, which suggests that sulfur‐related metabolism and deposition occurred in association with the worm tubes. High resolution analysis revealed the localization of zinc‐sulfur (sphalerite, ZnS) on the tubes, while iron‐sulfur (pyrite, FeS2) was localized at the center of the tubes. The spatially separate sphaleritization and pyritization imply that epiphytic and endosymbiotic microorganisms perform different sulfur metabolisms, such as sulfate‐reduction and su

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00143.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

AbstractApatite and zircon fission‐track (FT) analyses of the Shimanto accretionary complex and its vicinities, southwest Japan, unraveled the episodic material migration of the deep interiors of the accretionary complex. Apatite data with 100°C closure temperature (Te) generally indicate ~10 Ma cooling throughout the Shimanto complex. In contrast, zircon data with 260°C Teexhibit a wide range of apparent ages as a consequence of paleotemperature increase to the zircon partial annealing zone. In the Muroto and Kyushu regions, maximum temperatures tend to have been higher in the northern, older part of the complex, with indistinguishable temperature differences between coherent and melange units adjacent to each other. It thus suggests, along with vitrinite reflectance data, that older accretionary units occurring to the north sustain greater maximum burial during the accretion‐burial‐exhumation process. Zircon data suggest two cooling episodes: ~70 Ma cooling at widespread localities in the Cretaceous Shimanto Belt and Sambagawa Belt, and ~15 Ma cooling in the central Kii Peninsula. The former is consistent with40Ar/39Ar cooling ages from the Sambagawa Belt, whereas the latter slightly predates the widespread 10 Ma apatite cooling ages. These data imply that the extensive material migration and exhumation took place in and around the Shimanto complex in Late Cretaceous as well as in Middle Miocene. Considering tectonic factors to control evolution of accretionary complexes, the episodic migration is best explained by accelerated accretion of sediments due to increased sediment influx at the ancient Shimanto trench, probably derived from massive volcano‐plutonic complexes contemporaneously placed inland. Available geo‐ and thermochronologic data suggest that extensive magmatism triggered regional exhumation twice in the past 100 Ma, shedding new light on the cordilleran orogeny and paired metamorphi

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00144.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

ISSN:1038-4871    

DOI:10.1111/j.1440-1738.1995.tb00145.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY