Rifting and basin inversion in the eastern margin of the Japan Sea

Abstract Extensional 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 an en échelon arrangement, 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 interval of about 50 km trending parallel to the arc.     

Refined Early to Middle Miocene diatom biochronology for the middle- to high-latitude North Pacific

Abstract   Refined numerical ages of the diatom biohorizons of the Early to Middle Miocene (11–18 Ma) period in the Neogene North Pacific are presented based on the direct correlation between biostratigraphy and magnetostratigraphy at Site 887 on the Patton–Murray Seamount in the northeastern Pacific. Sampling intervals of 0.02–0.04 my allowed the determination of the ages of the biohorizons to be more precise than previous studies. The secondary biohorizons established in the northwestern Pacific have been proven to be useful also in the northeastern Pacific, and are linked to magnetostratigraphy directly for the first time. The refined diatom biochronology established in this study will provide a vital basis for the study of the Neogene marine sediments of the middle- to high-latitude North Pacific, which rarely yield calcareous microfossils. Denticulopsis praedimorpha var. prima n. var. is described.