The "Hayachine Tectonic Belt" borders the Southern Kitakami and Northern Kitakami Massifs, and is characterized by widespread basic to ultrabasic rocks named the Hayachine Complex. In the central part, i. e. Hayachinesan-Oguni district, the Hayachine Complex consists of the Nakadake Serpentinite, Kagura Complex and Koguro Formation, in ascending order. The Nakadake Serpentinite is composed of serpentinite and serpentinized ultrabasic rocks, and the Kagura Complex is a complex of trondhjernite, dolerite, gabbro and ultrabasic rocks. The Koguro Formation is composed mainly of dolerite and basalt and is overlain by thick terrigenous sedimentary rocks of Ordovician? to Early Silurian age. The Hayachine Complex is, therefore, not an allochthonous block, but the basement of the "Tectonic Belt". The eastern boundary fault of the "Tectonic Belt" is a reverse one and steeply dips southwest. The geologic structure in the northeastern margin of the "Tectonic Belt" and in the Northern Kitakami Massif, as a whole, is characterized by overturned folds with axial planes dipping to the southwest. On the contrary, in the southwestern margin of the "Tectonic Belt" and in the northeastern margin of the Southern Kitakami Massif, high-angle reverse faults and asymmetric or overturned folds with southwest vergence are prevalent. The basement rocks, the Hayachine Complex, occur in the " Tectonic Belt", and thus the boundary zone between the Southern and Northern Kitakami Massifs seems to form a broad anticline, which associated with longitudinal reverse faults on both flanks. The Hayachine Complex is infered to have been formed in a rift zone, which originated on the continental margin, in Ordovician or Early Silurian time. During Middle Palaeozoic to Early (Middle?) Mesozoic, the "Tectonic Belt" was a site of passive margin. In the latest Jurassic, the Palaeozoic strata with the Hayachine Complex in the passive margin began to uplift, in connection with the accretion in the Northern Kitakami Massif, and leading to the formation of the said anticline. The "Hayachine Tectonic Belt" is the axial part of this anticline.

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In this paper we describe and discuss the characteristic sedimentary structures observable on three exposures of the Jurassic in the Shizukawa area in South Kitakami. The selected exposures are: southwestern coast of Niranohama, eastern coast of Gongen, and northeastern coast of Shizuhama. These exposures are the best sections of the Niranohama, Hosoura and Arato Formations, constituents of the Shizukawa Jurassic. All the sequences established in these sections are exclusively composed of clastic sediments of various grain size. Sedimentary structures are well developed and suggest the environments ranging from breaking zone to base-of-slope. The Niranohama coast is the type section of the Niranohama Formation. The succession here is divided into the lower part composed of alternation of arkose sandstone and bituminous shale, characterized by wave ripples and convolute bedding, and the upper part of of sandstone-dominant alternation, characterized by wave ripples, including hummocky cross stratifications. This represents the environment from breaking zone to storm-dominated shore-face to upper offshore. The Gongen coast represents the lower and middle parts of the Hosoura Formation. The sequence observable here is composed of strongly bioturbated black mudstone intercalated with thin storm-generated sheet sandstone layers. Associated sedimentary structures suggest the base-of-slope to basin floor environment. The Shizuhama section covers a fraction of the Arato Formation. The lithology is essentially similar to that of the previous Shizuhama section. The whole sequence observable in this section is, however, more strongly affected by submarine sliding which has left a number of slump scars in the sequence. Upper slope environment is suggested for this section. The above observations lead us to the conclusion that the Shizukawa Group is the stratigraphical sequence representing a transgressive cycle beginning with high-energy breaking zone environment to low energy offshore environment. The Hashiura Group is another transgressive cycle succeeding the former, in which the Arato Formation represents the slope stage of sedimentation.

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The NNW-trending Onikobe-Yuzawa Mylonite Zone extends for 45 km from Onikobe Caldera, Miyagi Prefecture to Yuzawa, Akita Prefecture, northeast Honshu, Japan. The mylonite was derived from Cretaceous tonalite and granodiorite. The field relationship between the mylonite and the foliated tonalite is gradational. The mylonite is intruded by the Paleogene massive tonalite. The K-Ar ages of the mylonite range from 75.6 Ma to 88.5Ma. These facts indicate that the mylonitization proceeded during Late Cretaceous. Dynamically recrystallized quartz aggregates in mylonite are distinguished into P-and S-types. They were partly changed into blastomylonite through the intrusion of the younger tonalite. The micro-structure of the mylonites shows left-lateral shear deformation. The pre-Tertiary metamorphic and granitic rocks on the western side of the mylonite zone are comparable to those in the Abukuma Belt. The pre-Tertiary basement rocks on the east of the mylonite zone are composed of highly to weekly metamorphosed volcanic and sedimantary rocks probably of Paleozoic ages, and the Cretaceous granitic rocks, which are similar to those of Kitakami Belt. Hence, the Onikobe-Yuzawa Mylonite Zone is probably a northern extension of the Hatagawa Fracture Zone, which is a boundarv between the Abukuma and Kitakami Belts.

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A total of 217 oriented samples were collected from the Paleozoic rocks at 63 sites on various localities in the South Kitakami Mountains for paleomagnetic measurement. The experimental results suggest that most of the remanent magnetization of the Paleozoic rocks in this area probably owes to the progressive overprinting by a high blocking temperature component. This magnetic overprinting is believed to be related to the plutonism which occurred in Early Cretaceous age. Our former interpretation of Devonian and Carboniferous pole positions and paleolatitudes of the Kitakami Mountains is retracted accordingly.

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The Tamadare Amphibolite Complex in the western Hitachi district, which is mainly composed of amphibolite group derived from gabbroic-quartzdioritic plutonics with a small amount of biotite schist and siliceous schist, is regarded as a basement complex lying under the Hitachi metamorphic rocks fomed by the Abean erogenic movements. The amphibolite group can be devided into three facies: (1) gneissose facies, (2) schistose facies, and (3) boundary facies. The structural revolutionary trend of basement complex, field occurrence of the abovementioned facies and textural changes of hornblende in the light of petrofabric investigation suggest that two stages of metamorphism should have been superimposed in the Tamadare Amphibolite Complex. One of the metamorphism (pre-Abean event probably) which made up gneissose facies characterized by coarse-grained relict minerals and relict textures of plutonics origin, together with marked microfolding structure. The other is the tectonic event (possibly Abean orogenic movement) which converted the gneissose facies into schistose facies characterized by fine-grained secondary minerals, together with preferred orientation of hornblende and strikingly schistose texture. The first metamorphism (formation of gneissose facies) is presumably correspondent to the metamorphism which concerned with the formation of the Nishidohira metamorphic complex, whilst the second one (formation of schistose facies) is identical with the tectonic event which formed the Hitachi metamorphic rocks.

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The Hayachine Tectonic Belt (HTB) is a boundary zone between the North Kitakami Terrane (NKT) and the South Kitakami Terrane (SKT). It is characterized by intense tectonic deformation and slicing by numerous faults, along which serpentinite bodies were emplaced. The following eight geologic units constitute the HTB and its vicinity. The predominant lithology and contained fossils of these units are as follows: Unit A mudstone, bedded chert (with Triassic conodonts) and chert-laminite. Unit B mudstone, sandstone and bedded chert (with Jurassic radiolarians). Sometimes show olistostromal occurrences. Unit C basalt, basaltic volcaniclastics, massive chert and muddy/siliceous laminite. Unit D foliated serpentinite with tectonic blocks-slices of amphibolite, basalt, chert and pelitic schistose rocks. Unit E ultramafic mass occasionally containing gabbroic layers. Unit F basalt, dolerite and gabbroic rocks with minor amount of quartzdioritic rocks. Unit G volcaniclastic sandstone, mudstone (with Silurian brachiopods), limestone and acid tuff. Unit H quartzofeldspathic sandstone, mudstone and conglomerates with abundant Middle Silurian fossils. Unit A apparently is equivalent to the NKT, Units F-H are assigned to the SKT. The HTB is represented by Units B-E. The NKT and SKT are discontinuously sutured by HTB. Three different fault systems are recognized within the HTB: (1) north-dipping thrust faults, (2) WNW-trending high-angle faults, and (3) NNW-trending high-angie faults, in formative order. The thrust faults are recognized only beneath the Hayachine Ultramafic Mass, which is the largest one in the HTB. The WNW-trending faults dominate the other two, and are characteristic tectonic components of the HTB. These are associated with foliated serpentinite bodies often accompanying tectonic block-slices. The NNW-trending faults comprise both the western and eastern sides of the HTB, showing strike-slip dislocation. Formation of the NNW-trending faults occurred during the Early Cretaceous. The lithologic assemblage of geologic units of the HTB is very similar to that of the Motai Group, which has been considered generally as pre-Devonian or pre-Silurian metamorphic basement in the Motai-Matsugadaira Belt (MMB). These two belts formed a continuous geologic body prior to sinistral dislocation by such NNW-trending faults as the Hizume-Kesen'numa Fault. The geotectonic view of the HTB is presented hypothetically as follows. At first, an accretionary complex including olistostromal facies with Triassic-Jurassic chert blocks was formed at convergent margin. This convergent process possibly persisted until the end of Jurassic. The SKT collided with the margin to form an allochthonous terrane. Formation of the nappe structure might have been associated with the collision event. Subsequently, WNW-trending high-angle faults formed, with possible transcurrent dislocation and intense deformation. This dislocation rearranged the accretionary complex to form the HTB as the boundary tectonic belt between the NKT and SKT. Finally, an Early Cretaceous NNW-trending strike-slip fault system further dissected the HTB. Present distribution of the HTB, SKT, NKT and MMB occurred prior to the Early Cretaceous (110-120 Ma) granitic rocks that were commonly intruded into these terranes.

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As the biostratigraphic works play an important role in the geological considerations on the Cretaceous formations, recent works on the Cretaceous strata and the contained fossils in Northeast Japan are reviewed. Marine sediments in the Kitakami and the Abukuma areas frequently involve index species of ammonites and other groups. Therefore, the geological age of the formations in these areas can be determined by using indices in terms of the global scale. The result of the correlation on available evidence is shown in a chart. The ammonite bearing sequences from the Berriasian to Valanginian (e. g. Isokusa, Nagasaki, and Koyamada Formations) should be restudied for the precise correlation. As to the strata from Hauterivian to Barremian (e. g. Omoto and Yokonuma Formatins, Ofunato Group), further discovery of indices is expected. These sequences are important for the correlation with non-marine sequences not only in Kwanto area and Inner Zone of Southwest Japan but also in Korea and China in terms of the international scale, because marine and non-marine sediments laterally change each other in the Kitakami area. Occurrence of various non-marine molluscs are especially noted. The Upper Aptian to Lower Albian is well represented by the Miyako Group which exhibits successively the ammonite zones. They are safely correlated with the standard zones in southern England and France. Restoration of the Miyako Group on paleogeography and paleoecology is possible. The necessity of paleomagnetic analysis should be emphasized in order to restore the Cretaceous circumstances precisely. The Cenomanian to Turonian sediments are not known in Northeast Japan. The coast line in those times was withdrawn eastward. The Coniacian to Santonian Futaba Group is clearly denned by inoceramid zones. This group predominates in Japan on abundant occurrences of reptiles. The Kuji Group and its correspondents consist mainly of Santonian sediments which include texanitine ammonites frequently from shallow shelf facies. The occurrence of texanitines from the Kitakami should be compared with those from Hokkaido and Kyushu. Further study is needed for the zonal subdivision of the Santonian by species of the Texanitinae.

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