Kinematic analysis of sinistral cataclastic shear zones along the northern margin of the Mino Belt, central Japan

In this paper, cataclastic shear zones along the northern margin of the Mino Belt, central Japan are described, and the significance of the shearing in the tectonic evolution of SW Japan is examined. The Mino Belt in SW Japan is composed of accretionary complexes of Jurassic to Early Cretaceous age. Field investigation revealed that remarkable cataclastic shear zones trending east to northeast run along the northern margin of the Mino Belt. Closely spaced cleavage is developed in these shear zones. Lineation on the cleavage plunges at shallow to moderate angles. Deformation structures (e.g. composite planar fabric and asymmetric structure of clasts) in the sheared rocks clearly indicate a sinistral sense of shear. The shearing ceased by latest Cretaceous time, because the sheared rocks are overlain by unsheared Upper Cretaceous volcanic rocks. The sinistral shearing may be closely related to Cretaceous sinistral movement along the eastern margin of Asia. Sinistral shearing along the northern margin of the Mino Belt can be considered as a key for re-examining the tectonic development of SW Japan.     

黑龙江省那丹哈达与日本美浓地区古地磁结果对比及意义

在那丹哈达地区采集了中三叠世红色硅质岩样品，古地磁分析测试结果表明，该岩样的磁性特征与日本西南美浓地区同时代硅质岩具有相似性，反映了二者是同源同构造环境下的产物。依据沉积地层组合和古地磁数据，那丹哈达和日本美浓地区原为同一地体，而且三叠纪以来均存在大规模北向位移。同时，古地磁数据还表明了日本海是约在８０Ｍａ，相当于晚白垩世开始扩张的，致使那丹哈达与日本美浓两地体相互分离     

Provenance of Jurassic accretionary complex: Mino terrane, inner zone of south-west Japan – implications for palaeogeography of eastern Asia

Provenance and tectonic history of the Jurassic accretionary complex, Mino terrane, located in the Inner Zone of south‐west Japan, were studied using sandstone framework composition and mudrock geochemistry. Modal analysis of sandstones shows that the tectonic setting of the source area for the studied Mino terrane clastic rocks was uplifted basement, largely dominated by high‐grade metamorphic terrain composed of quartz and feldspar, especially plagioclase. The textural and mineralogical immaturity, extent of alkali and alkaline earth element leaching, low chemical index of alteration values and depleted rare earth element (REE) contents suggest rapid uplift and erosion within the source terrain and a relatively weak weathering intensity. Factor analysis revealed that grain‐size effects governed compositional heterogeneity in the studied sediments. Provenance of the sediments is interpreted as being plagioclase‐enriched felsic basement rock, such as granodiorite, within a continental margin and evolved arc tectonic setting rather than active volcanic arc. Lack of a contribution from active volcanic arcs may have resulted from the cessation of volcanism during the reorganization of the subducting plate system and/or erosion of arc volcanics and exposure of basement. Considering the previous studies on palaeogeography and palaeocurrent reconstruction, the north‐eastern part of the Yeongnam massif in the Korean Peninsula is interpreted as the most probable source area for the studied turbidites. The results of mixing calculation for Mino terrane sediments suggest that Precambrian leucocratic granite and the basement rock of the Cretaceous Gyeongsang Basin shed large amounts of sediments to the Mino trench, whilst Precambrian granitic gneiss and the Triassic pluton supplied lesser amounts. The results of this study reveal that, although active subduction–accretion processes were occurring, the Mino trench was bordered by continental basement rocks. This knowledge contributes to enhanced understanding of the Jurassic palaeogeography of the east Asia continental margin.     

地层特征对比研究在地体解析中的意义

不同地区地层特征的对比研究,是地体解析的重要方法之一。在对中国那丹哈达地区和日本美浓地区出露的地层、岩石等进行对比研究后认为,在老第三纪日本海尚未形成之前它们是连在一起的统一的地体。三叠纪时它们在赤道附近生成,侏罗纪—白垩纪时随板块运动增生于亚洲东部大陆边缘,白垩纪—老第三纪时左行剪切北移,新第三纪时因日本海的扩张而分裂移动到现今的位置。     

Chrome spinel in normal MORB‐type greenstones from the Paleozoic–Mesozoic Mino terrane,East Takayama area,central Japan: Crystallization course with a U‐turn

Phenocrystic chrome spinel crystallized in normal MORB‐type greenstones in the East Takayama area. Associated phenocryst minerals show a crystallization sequence that was olivine first, followed by plagioclase, and finally clinopyroxene. Chrome spinel ranges from 0.54 to 0.77 in Mg/(Mg+Fe²⁺) and 0.21 to 0.53 in Cr/(Cr+Al); the Fe³⁺ content varies from 0.07 to 0.22 p.f.u. (O = 4). Significant compositional differences of spinel were observed among the phenocryst mineral assemblages. Chrome spinel in the olivine–spinel assemblage shows a wide range in Cr/(Cr+Al), and is depleted in Fe²⁺ and Fe³⁺. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage is Fe²⁺‐ and Fe³⁺‐rich at relatively high Cr/(Cr+Al) ratios. Basalt with the olivine–plagioclase–spinel assemblage contains both aluminous spinel and Fe²⁺‐ and Fe³⁺‐rich spinel. The assumed olivine–spinel equilibrium suggests that chrome spinel in the olivine–spinel assemblage changed in composition from Cr‐ and Fe²⁺‐rich to Al‐ and Mg‐rich with the progress of fractional crystallization. Chrome spinel in the olivine–plagioclase–clinopyroxene–spinel assemblage, on the other hand, exhibits the reversed variations in Mg/(Mg+Fe²⁺) and in Cr/(Cr+Al) ratios that decrease and increase with the fractional crystallization, respectively. The entire crystallization course of chrome spinel, projected onto the Mg/(Mg+Fe²⁺)–Cr/(Cr+Al) diagram, exhibits a U‐turn, and appears to be set on a double‐lane route. The U‐turn point lies in the compositional field of chrome spinel in the olivine–plagioclase–spinel assemblage, and may be explained by plagioclase fractionation that began during the formation of the olivine–plagioclase–spinel assemblage.     

从板块构造到地体

从板块构造的发生、发展到地体概念的提出，论述了地体解析和拼贴构造，并根据中国东部的地体研究，特别是中国东北那丹哈达地体和日本美浓地体的对比研究，论述了亚洲东部中生代构造发展史，认为在侏罗纪时，亚洲大陆边缘的地体拼贴活动已经开始，形成了拼贴沉积的复合地体；白垩纪时由于大陆边缘的侧向挤压和离散作用，这些地体产生左行运动和变形；早第三纪晚期由于日本海的扩张，形成了弧形的日本列岛。