Satoshi Yamakita

Gently north-dipping Median Tectonic Line (MTL) revealed by recent seismic reflection studies, southwest Japan

Abstract The Median Tectonic Line (MTL), with a length of more 1000 km, is the most significant fault in Japan. It juxtaposes the high- P/T Sambagawa metamorphic rocks against the low- P/T metamorphic rocks of the Ryoke belt. The MTL was probably formed in the Cretaceous with many subsequent reactivations. The western segment of the MTL is still active with an almost pure right-lateral sense of motion. Although a great amount of geological information on the MTL has been accumulated, information about the subsurface, especially the deep-seated structure of the MTL, is still insufficient. It has been generally assumed that the MTL is vertical or steeply dipping at depth because of its straight surface trace and its recent lateral motion. Recently, new geophysical data have suggested that the MTL dips gently northward at depth. We have acquired a complementary set of geophysical profiles (seismic reflection and refraction, gravity and MT) across the MTL in east Shikoku. Our results confirm that the MTL dips northward at about 30 to 40 degrees from the surface to about 5 km depth, where it becomes listric. This fault geometry more reasonably explains the reactivation history of the MTL: the motion has occurred on a listric-type fault in so-called oblique or lateral ramp manner.

PERMIAN RADIOLARIAN FAUNAS FROM CHERT IN THE KHABAROVSK COMPLEX, FAR EAST RUSSIA, AND THE AGE OF EACH LITHOLOGIC UNIT OF THE KHABAROVSK COMPLEX

Abstract The Khabarovsk Complex, a Jurassic accretionary complex distributed in and around the Khabarovsk city area, Far East Russia, comprises mélange and schist facies. From the review of previous studies including Russian papers, the lithology and age of the constituent rocks of the mélange facies can be summarized as follows: Upper Paleozoic basic volcanic rocks, mainly pillow lava, and altered gabbro, Upper Carboniferous to Upper Permian fusulinoidean-bearing limestone associated with tuff, Lower and Middle Jurassic siliceous mudstone, Upper Jurassic tuffaceous mudstone, uppermost Jurassic carbonate concretions embedded in mudstone, and age-unknown sandstone. Newly found sequences of limestone-chert and of basalt-chert in the mélange facies crop out along the Amur River in the Khabarovsk city area. A chert sample of the limestone-chert sequence contains Albaillella aff. asymmetrica and Pseudoalbaillella aff. lomentaria, and a chert sample of the basalt-chert sequence includes Follicucullus monacanthus, Follicucullus porrectus, and Pseudoalbaillella cf. yanaharaensis. The radiolarian assemblages from the limestone-chert and basalt-chert sequences have a maximum age of middle Early Permian and late Middle Permian, respectively, overlapping the time of deposition of the fusulinoidean-bearing limestone. The co-occurrence of chert and limestone indicates that the fusulinoidean-bearing limestone was formed on a basaltic topographic high in a pelagic ocean whereas the radiolarians accumulated in a deeper part. Limestone debris occasionally flowed into the depositional site of the radiolarian chert. Although the Khabarovsk Complex was simply considered as a northern extension of the Mino–Tamba Belt of the Inner Zone of southwest Japan, we propose a new correlation based on the lithologic associations. The mélange facies of the Khabarovsk Complex is correlative with one of the Kasugano, Funabuseyama, Nabi, and Yabuhara Formations in the Mino–Tamba Belt, whereas the schist facies is correlative with the Hikami Formation of the Ultra–Tamba Belt.

A natural assemblage of Ellisonia sp. cf. E. triassica Müller (Vertebrata: Conodonta) from the uppermost Permian in the Suzuka Mountains, central Japan

Abstract A natural assemblage of Ellisonia sp. cf. E. triassica Müller was recovered from the uppermost Permian close to the Permian-Triassic boundary in the Nabejiriyama section in the Mt. Ryozen area, Suzuka Mountains, central Japan. This natural assemblage and collections of similar ellisonids from the Taho Formation of Ehime Prefecture, southwestern Japan, demonstrate that the Ellisonia apparatus is composed of eight kinds of elements: breviform digyrate M, symmetrical S0, extensiform digyrate S1 and S2, bipennate S3 and S4, and angulate P1 and P2 elements. The apparatus structure of Ellisonia is comparable to the standard 15-element plan of Carboniferous ozarkodinids and prioniodinids. The apparatus structure of the present natural assemblage is important in discussing the phylogenetic affinity of the genus Ellisonia.

Late Triassic Phaeodarian Radiolaria from the Northern Chichibu Belt, Shikoku, Japan

Abstract. A new genus (Triassiphaeodina gen. nov.) and two new species (Medusetta japonica sp. nov. and Triassiphaeodina niyodoensis sp. nov.) of Late Triassic (Rhaetian) phaeodarian Radiolaria are described from a phosphatic nodule found in mélange rocks of the Northern Chichibu Belt, Shikoku, Japan. The Rhaetian age of the nodule and of the phaeodarian new taxa is based on co-occurring Polycystina Radiolaria, including Bipedis acrostylus Bragin, Livarella densiporata Kozur and Mostler, Fontinella primitiva Carter, and Ferresium sp. A of Carter (1993). This finding shows that phaeodarian Radiolaria were already represented in Late Triassic oceans, with morphologies similar to those known in the Late Cretaceous and Cenozoic, from which they have previously been reported. The new taxa described herein represent the oldest known phaeodarian fossils.

A probable shark dorsal fin spine fragment from the Early Triassic of the Arrow Rocks sequence, Whangaroa, northern New Zealand

The ornament on a small external cast in pink chert shows considerable similarity with that of various Middle Palaeozoic and Triassic fish genera. It comes from the Permian–Triassic Oruatemanu Formation of Arrow Rocks, Whangaroa area, eastern Northland. Conodont faunas from a few metres above and below the sample allow correlation with the Neospathodus pakistanensis zone of the Early Triassic, which is assigned to the late Dienerian (late Induan), with adjacent conodont zone faunas in their correct stratigraphic association. The cast is assumed to be that of a small fragment of fin spine, most likely from the junction area of the crown and root on the right-hand side of a dorsal fin spine, possibly anterior, of a marine ctenacanthoid shark, a basal shark order not previously recorded from New Zealand.

Research progress in the Chichibu Composite Belt-Reports from western Shikoku

秩父累帯の主要部は, ジュラ紀の付加体からなる. 付加体に含まれるチャートや石灰岩などの海洋性物質について, その堆積年代や含まれる化石の古生物地理学上の特徴を明らかにすることにより,秩父累帯の形成モデルに制約を与えることができる. また, 地質構造の特徴は, 付加体形成から現在にいたるまでのプロセスを反映している, 秩父累帯を構成する主要なユニットについて, 露頭のようすや山塊の表情を紹介する. 斗賀野ユニットと三宝山ユニットは南部秩父帯に, 柏木ユニット, 住居附ユニットおよび沢谷ユニットは北部秩父帯にそれぞれ属する.