The deformation of the Ashigara region situated between the Izu Peninsula and the Fossa Magna has been ascribed to the regional horizontal compressional tectonic regime. However, contrary to this prevailing view, new field evidence shows that the deformation comes from local stress field in relation to diapiric intrusions and resultant uplift of the Ashigara block. The structural development of the region during the Early and Middle Pleistocene is summarized as follows: First stage-Ashigara monocline was formed in accordance with the subsidence of southwestern part of the Ashigara Basin. Second stage-Formation of small-scale folds in the southwestern flank of the Ashigara monocline and a box-shaped anticline on its axial part. These structures were formed by the plutonic emplacement which uplifted the block and produced strong local compressional force. Third stage-The active rise of the eastern part of the Ashigara basin steeply tilted the thick Shiozawa Formation.
Diatom biostratigraphy and chronostratigraphy of the Yabuta Formation in the Himi-Nadaura area in central Japan is reinvestigated. Ages of three widespread volcanic ash layers in the Yabuta Formation and the base of the No. 3 Globorotalia inflata bed of planktonic foraminiferal biostratigraphy are determined based on the revised chronostratigraphy. The FO (first occurrence) of Neodenticula koizumii is found near the base of the Yabuta Formation, 70m below the previously recognized horizon of the FO of N. koizumii. N. koizumii is rare and sporadic from the base of the Yabuta Formation to the horizon above the TT2 volcanic ash bed, where it abruptly increases in abundance and occurs consistently upward. The RI (rapid increase) of N. koizumii is estimated at 3.0-3.1 Ma from previously reported diatom biostratigraphy for deep sea cores around Japan. Correlation between the standard geomagnetic polarity time scale and the magneto-stratigraphy of the Yabuta Formation is revised with the refined diatom biostratigraphiy. The Gauss/Gilbert boundary is correlated to near the base of the Yabuta Formation. The reversed magnetized horizon in the lower part of the Yabuta Formation is correlated to the Mammoth Subchronozone Ages of the widespread volcanic ash beds and the base of the No. 3 G. inflata bed are calculated with the sediment accumulation curve of the Yabuta Formation based on the above mentioned magneto-biostratigraphy ; YT3 volcanic ash bed about 3.5 Ma; MT2 volcanic ash bed : 2.8-2.9 Ma ; UN volcanic ash bed : 2.6-2.7 Ma; Base of the No. 3 G. inflata bed about 3.1 Ma.
Pyroclastic rock samples collected from the eastern margin of the Ou Backbone Range have a direction of remanent magnetization suggesting vertical-axis rotation. High-temperature component directions of five sites of the Lower Miocene Mizuwake Formation were determined by analyzing results of stepwise thermal demagnetization. A high-temperature component direction of one site at an outcrop is characterized by an easterly direction of reversed polarity, while those of four sites in the type area of this formation, located a few kilometers apart from that outcrop, are marked by a southwesterly direction of the same polarity. By comparing these directions with an Early Miocene reference direction, it is inferred that more than 100° clockwise rotation took place in the type area since 22 Ma. All the sites studied are within a fault zone running along the eastern margin of the Ou Backbone Range, which leads to an inference that the rotation occurred in relation to faulting along the margin.
Ophiolite studies seem be now reaching to a turning point. The genesis of ophiolites, classically regarded as ancient oceanic crust and upper mantle, was seriously debated during 1970s since Miyashiro's proposal on the arc origin for the Troodos ophiolite. Since then, many researchers considered that most of ophiolites were generated at supra-subduction zones. Recently two significant findings for the genesis of ophiolites have been carried out. The first is the discovery of MORBs with arc signatures from the Chile Ridge. The second is that MORBs from different oceans have unique features, implying that the source mantles are heterogeneous and have different histories from ocean to ocean. On the other hand, the genesis of the Oman ophiolite, which is the largest and best exposed ophiolite, is still a matter of serious controversy in spite of numerous publications and its well known geological and structural features. There are still many controversial subjects genesis (supra-subduction zone suite or fast-spreading ridge origin), geometry of layering in the layered gabbro (dipping to ridge axis or opposite) and its origin (magmatic or due to deformation), behavior of upwelling mantle (passive or active), emplacement processes and so on. The correlation between volcanic rocks and plutonic rocks is also still confused. We review previous studies on the Oman ophiolite and introduce recent new findings by Japanese geologist team on the Oman ophiolite. Then we discuss on the perspective of ophiolite studies.
Manganese nodules containing well-preserved radiolarian fossils were discovered for the first time from the Kuzumaki-Kamaishi Belt. They are in the mudstone of chert-clastic sequences and considered to be formed in situ from the mode of occurrences. The radiolarian fauna comprises more than 120 species, including Archicapsa pachyderma, Stichocapsa tegiminis and Trillus elkhornensis. The presence of S. tegiminis and the absence of its descendant Tricolocapsa plicarum indicate early Middle Jurassic age of these nodules, suggesting that a Middle Jurassic accretionary complex exists in the Kuzumaki-Kamaishi Belt.
Calcareous nannofossil biostratigraphy was studied for the Neogene marine Shimajiri Group distributed in Kume-jima Island, Ryukyu Islands, southwestern Japan. Out of 21 samples examined in this study, 14 samples yielded calcareous nannofossils. As was reported in a previous study, Zones CN9 and CN12 of Okada and Bukry (1980) were recognized in this study. In addition, Subzone CN11b (early Pliocene) was newly identified in the lower parts of the sequence where the ages were previously assigned to the late Miocene (upper part of Zone CN9 or older). This result shows the difficulty of establishing the stratigraphy of the subjected group based mainly upon lithotratigraphy, particularly in the lower part. Additional biostratigraphic data are needed to establish the reliable stratigraphy of the marine Shimajiri Group in Kume-jima Island.