黑濑川地体源于亚洲大陆东缘何处？

The point at issue: The Kurosegawa Terrane is composed of continental fragments transecting Mesozoic terranes of accretionary complex in Southwest Japan (Fig. 1). It is an attenuated tectonic sliver and considered to be allochthonous with respect to the main part of Southwest Japan. The problem of which continental block in the East Asian continental margin is the source of the Kurosegawa Terrane has puzzled Japanese geologists for many years. Firstly, we try to approach this issue based on the analysis of fusulinacean assemblage in accreted terranes composed of subduction complex in the Pacific Rim. Secondly, by applying the result of this analysis we try to locate the source of the continental fragments of the Kurosegawa Terrane. Thirdly, we try to prove its validity with a new paleomagnetic study.     

Kurosegawa Terrane in Southwest Japan: Disrupted Remnants of a Gondwana-Derived Terrane

The Kurosegawa Terrane is an anomalous, disrupted, Paleozoic and Mesozoic lithotectonic assemblage characterized by fragments of continent and continental margins. It is located in Southwest Japan where it lies between two Mesozoic subduction complex terranes. The Kurosegawa Terrane is an exotic and far-travelled geologic entity with respect to its present position. Limestones of the Kurosegawa Terrane formed along a continental margin yield fusulinacean fossils Cancellina, Colania and Lepidolina. Accordingly, the Kurosegawa Terrane was once situated within the Colania-Lepidolina territory in the East Tethys-Panthalassa region at a palaeo-equatorial latitude, possibly close to the eastern margin of the South China and/or Indochina-East Malaya continental blocks. These blocks had rifted from Gondwana by late Devonian. They drifted northwards, passing through the Colania-Lepidolina territory in mid-Permian time, and amalgamated with the proto-Asian continent during the late Triassic. Subsequently, during the Cretaceous, parts of the allochthonous continental blocks and their associated tectonic collage were transpressed, dispersed, and displaced from the southeastern periphery of Asia towards the north. As a result, the Kurosegawa Terrane is formed as a disrupted allochthonous terrane, characterized by a serpentinite melange zone, lying between the adjoining Mesozoic subduction complex terranes.     

Geology and tectonics of Japanese islands: A review – The key to understanding the geology of Asia

The age of the major geological units in Japan ranges from Cambrian to Quaternary. Precambrian basement is, however, expected, as the provenance of by detrital clasts of conglomerate, detrital zircons of metamorphic and sedimentary rocks, and as metamorphic rocks intruded by 500 Ma granites. Although rocks of Paleozoic age are not widely distributed, rocks and formations of late Mesozoic to Cenozoic can be found easily throughout Japan. Rocks of Jurassic age occur mainly in the Jurassic accretionary complexes, which comprise the backbone of the Japanese archipelago. The western part of Japan is composed mainly of Cretaceous to Paleogene felsic volcanic and plutonic rocks and accretionary complexes. The eastern part of the country is covered extensively by Neogene sedimentary and volcanic rocks. During the Quaternary, volcanoes erupted in various parts of Japan, and alluvial plains were formed along the coastlines of the Japanese Islands. These geological units are divided by age and origin: i.e. Paleozoic continental margin; Paleozoic island arc; Paleozoic accretionary complexes; Mesozoic to Paleogene accretionary complexes and Cenozoic island arcs. These are further subdivided into the following tectonic units, e.g. Hida; Oki; Unazuki; Hida Gaien; Higo; Hitachi; Kurosegawa; South Kitakami; Nagato-Renge; Nedamo; Akiyoshi; Ultra-Tamba; Suo; Maizuru; Mino-Tamba; Chichibu; Chizu; Ryoke; Sanbagawa and Shimanto belts.The geological history of Japan commenced with the breakup of the Rodinia super continent, at about 750 Ma. At about 500 Ma, the Paleo-Pacific oceanic plate began to be subducted beneath the continental margin of the South China Block. Since then, Proto-Japan has been located on the convergent margin of East Asia for about 500 Ma. In this tectonic setting, the most significant tectonic events recorded in the geology of Japan are subduction–accretion, paired metamorphism, arc volcanism, back-arc spreading and arc–arc collision. The major accretionary complexes in the Japanese Islands are of Permian, Jurassic and Cretaceous–Paleogene age. These accretionary complexes became altered locally to low-temperature and high-pressure metamorphic, or high-temperature and low-pressure metamorphic rocks. Medium-pressure metamorphic rocks are limited to the Unazuki and Higo belts. Major plutonism occurred in Paleozoic, Mesozoic and Cenozoic time. Early Paleozoic Cambrian igneous activity is recorded as granites in the South Kitakami Belt. Late Paleozoic igneous activity is recognized in the Hida Belt. During Cretaceous to Paleogene time, extensive igneous activity occurred in Japan. The youngest granite in Japan is the Takidani Granite intruded at about 1–2 Ma. During Cenozoic time, the most important geologic events are back-arc opening and arc–arc collision. The major back-arc basins are the Sea of Japan and the Shikoku and Chishima basins. Arc–arc collision occurred between the Honshu and Izu-Bonin arcs, and the Honshu and Chishima arcs.     

Kurosegawa Terrane as a Transform Fault Zone in Southwest Japan

The Kurosegawa Terrane intervening in the Jurassic-Early Cretaceous accretionary complexes along the Pacific side of the SW Japanese Islands is a serpentinite mélange zone. It contains various kinds of exotic rocks, for example, granitoids, metamorphic rocks, Siluro-Devonian deposits and is intimately associated with Cretaceous forearc basin deposits. The terrane is regarded as a key to clarify the Mesozoic geotectonic history of the western circum-Pacific orogenic belts. The current model, in which the formation of the Kurosegawa Terrane is attributed to nappe-movement or sinistral strike-slip faulting, can explain neither the mode of occurrence of the Kurosegawa Terrane we observed in eastern Kii Peninsula nor the array of evidence obtained from the Ryoke Terrane southward to the Shimanto Terrane. We suggest a new hypothesis in which the Kurosegawa Terrane was a transform fault zone that originated because of oceanic ridge subduction along the southern margin of the coeval accretionary prism (Butsuzo T.L.) in the late Early Cretaceous. Our model is mainly based on new geological evidence from the Kurosegawa Terrane in eastern Kii Peninsula where the deepest erosion level is exposed due to neotectonic uplift.     

Discovery of Paleozoic rocks at northern margin of Sambagawa terrane,eastern Kyushu,Japan: Petrogenesis,U–Pb geochronology and its tectonic implication

Ordovician diorite-quartz diorite mylonite (Saganoseki quartz diorite) was discovered in Sambagawa metamorphic terrane at the northern margin of Saganoseki Peninsula, Kyushu Island, Japan. The LA-ICP-MS zircon U–Pb geochronology revealed that the intrusion age of Saganoseki quartz diorite was 473.3 ​± ​3.6 ​Ma. These rocks show the volcanic arc affinity based on the trace element composition. On the basis of geochronological and geochemical results, Saganoseki quartz diorite is considered to be a member of the Early Paleozoic igneous rocks of the Kurosegawa tectonic zone. Saganoseki quartz diorite is located just south of the Median Tectonic Line (MTL) and is in close contact with the pelitic and psammitic schist without any brittle shear zone. U–Pb ages of detrital zircon grains from two psammitic schists show the estimated sedimentation age of early Late Cretaceous, indicate that these psammitic schists are the member of Sambagawa metamorphic rocks. Together with these results and the mode of occurrence in the field, we argue that the Early Paleozoic igneous rocks of the Kurosegawa tectonic zone existed as an upper structural unit of the Sambagawa terrane. This relationship is the key to reconstruct the Mesozoic tectonics of Japan as a part of East Asia, and its evolution through time.     

Gondwanaland origin, dispersion, and accretion of East and Southeast Asian continental terranes

East and Southeast Asia is a complex assembly of allochthonous continental terranes, island arcs, accretionary complexes and small ocean basins. The boundaries between continental terranes are marked by major fault zones or by sutures recognized by the presence of ophiolites, mélanges and accretionary complexes. Stratigraphical, sedimentological, paleobiogeographical and paleomagnetic data suggest that all of the East and Southeast Asian continental terranes were derived directly or indirectly from the Iran-Himalaya-Australia margin of Gondwanaland. The evolution of the terranes is one of rifting from Gondwanaland, northwards drift and amalgamation/accretion to form present day East Asia. Three continental silvers were rifted from the northeast margin of Gondwanaland in the Silurian-Early Devonian (North China, South China, Indochina/East Malaya, Qamdo-Simao and Tarim terranes), Early-Middle Permian (Sibumasu, Lhasa and Qiangtang terranes) and Late Jurassic (West Burma terrane, Woyla terranes). The northwards drift of these terranes was effected by the opening and closing of three successive Tethys oceans, the Paleo-Tethys, Meso-Tethys and Ceno-Tethys. Terrane assembly took place between the Late Paleozoic and Cenozoic, but the precise timings of amalgamation and accretion are still contentious. Amalgamation of South China and Indochina/East Malaya occurred during the Early Carboniferous along the Song Ma Suture to form “Cathaysialand”. Cathaysialand, together with North China, formed a large continental region within the Paleotethys during the Late Carboniferous and Permian. Paleomagnetic data indicate that this continental region was in equatorial to low northern paleolatitudes which is consistent with the tropical Cathaysian flora developed on these terranes. The Tarim terrane (together with the Kunlun, Qaidam and Ala Shan terranes) accreted to Kazakhstan/Siberia in the Permian. This was followed by the suturing of Sibumasu and Qiangtang to Cathaysialand in the Late Permian-Early Triassic, largely closing the Paleo-Tethys. North and South China were amalgamated in the Late Triassic-Early Jurassic and finally welded to Laurasia around the same time. The Lhasa terrane accreted to the Sibumasu-Qiangtang terrane in the Late Jurassic and the Kurosegawa terrane of Japan, interpreted to be derived from Australian Gondwanaland, accreted to Japanese Eurasia, also in the Late Jurassic. The West Burma and Woyla terranes drifted northwards during the Late Jurassic and Early Cretaceous as the Ceno-Tethys opened and the Meso-Tethys was destroyed by subduction beneath Eurasia and were accreted to proto-Southeast Asia in the Early to Late Cretaceous. The Southwest Borneo and Semitau terranes amalgamated to each other and accreted to Indochina/East Malaya in the Late Cretaceous and the Hainanese terranes probably accreted to South China sometime in the Cretaceous.     

Kurosegawa zone and its bearing on the development of the Japanese Islands

The Kurosegawa zone in southwest Japan is a 600 km long serpentinite mélange in the Chichibu terrains. It runs generally E-W but is slightly oblique to the subparallel arrangement of the Ryoke, Sanbagawa and Chichibu belts of Southwest Japan. A variety of geological units occurs in the Kurosegawa zone:

1. (1) granodiorite, gneiss and amphibolite of ca. 400 Ma,

2. (2) Siluro-Devonian formations,

3. (3) Upper Carboniferous to Jurassic formations,

4. (4) Upper Jurassic to Lower Cretaceous formations,

5. (5) serpentinite and

6. (6) low- to medium-grade metamorphic rocks of various baric types (ages, 220, 320, 360 and 420 Ma by K-Ar).

The most widespread is a high-pressure intermediate group of metamorphic rocks. Serpentinite is emplaced along the faults between and within the constituent units.Rocks of the Kurosegawa zone represent a mature orogenic belt along a continental margin or an island arc. Its original site as constrained by paleomagnetism was near the equatorial area. Here, 400 Ma old paired metamorphism and related magmatism took place. The island arc or microcontinent migrated northward to collide with the Eurasia plate during Late Jurassic, thus consuming the intervening ocean.     

Late Paleozoic tectonic -magmatic evolution history of the northeastern ChinaSCIEI北大核心CSCD

Northeastern China is suited in the eastern part of the Central Asian Orogenic Belt, and it is mainly composed of Erguna Massif, Xing'an Massif, Songnen-Zhangguangcai Range Massif, Jiamusi Massif, and Nadanhada Terrane. The Late Paleozoic magmatism was relatively intense accompanied with multiple stages of amalgamation in several microcontinents, therefore these magmatic products are an important media in recording the Late Paleozoic tectonic evolution history of the northeastern China. According to the petrological, geochronological, and geochemical characteristics of Late Paleozoic igneous rocks in the northeastern China, we found that the Late Paleozoic magmatism was based on Carboniferous -Permian igneous rocks. The Early Carboniferous magmatic products are gabbro, diorite and granite, the Late Carboniferous magmatic products are mainly composed of granitoids with minor gabbro, and the Permian magmatic products are mainly granitoids. Meanwhile, these Late Paleozoic igneous rocks mostly exhibit typical arc characteristics. In addition, the Late Paleozoic igneous rocks in eastern Jilin and Heilongjiang provinces are mainly Permian granitoids with minor gabbro, and these Permian igneous rocks show typical arc characteristics. Combined with petrological, geochronological, geochemical and isotopic characteristics, we suggest that the Late Paleozoic igneous rocks in the Great Xing'an Range and eastern Jilin and Heilongjiang provinces underwent different magmatic evolution history, and the microcontinents in NE China had different crustal growth history.     

Mesozoic–Cenozoic sedimentary rock records and applications for provenance of sediments and affiliation of the Simao Terrane,SW China

ABSTRACT

As the north part of Simao Terrane, Lanping Basin is located between the Sanjiang Tethys Orogen (STO) and Yangtze Block, also the junction zone between the Gondwanaland and Cathaysian old land (Pan Huaxia mainland), which includes Yangtze and Cathaysian Blocks. The aim of this study is to decipher the provenance of the sedimentary units in the Lanping Basin and affiliation of Simao Terrane by the U-Pb ages, Hf isotope of detrital zircons and whole-rock geochemistry. The whole-rock geochemistry and the mineral composition indicate that most of the Triassic–Paleocene sedimentary rocks are derived from the upper crust and exhibit recycled orogen features. The detrital zircon U-Pb ages from the North Simao Terrane are consistent with the magmatic events during Early Neoproterozoic and Permian in the Western Yangtze Block. And the detrital zircons ages from North Simao Terrane show same distribution features as the Permian–Triassic magmatic rocks, which are distributed in the Simao Terrane and along major sutures. These comparisons suggest that the clastic sediments in Lanping Basin (North Simao Terrane) are derived from Early Neoproterozoic and Permian magmatic rocks from Western Yangtze Block, Permian–Triassic magmatic rocks from Simao Terrane, along Jinshajiang, Garz-Litang and Ailaoshan Sutures. The comparison of the detrital zircon age distributions shows that Simao Terrane and Yangtze Block exhibited similarity tectonic setting in the evolution history, especially during Paleoproterozoic–Late Paleozoic. This suggests that the Simao Terrane is part of Cathaysian old land, although Simao Terrane was separated from Yangtze Block for short period during Early Paleozoic. Besides, the Hf mapping, stratigraphic succession, paleogeography and paleomagnetism in SW China support that Simao Terrane has a Cathaysian old land-affinity, rather than one involving Gondwanaland.     

西藏安多地区早古生代及中生代造山记录:来自安多微陆块-南羌塘锆石U-Pb年代学及Hf同位素研究

安多地区位于青藏高原腹地,为拉萨地体、羌塘地体及安多微陆块的结合部位,是研究拉萨地体、羌塘地体起源以及特提斯造山过程的关键位置。我们对采自安多地区的前中生代基底岩石及侏罗系沉积岩样品进行了岩石学、锆石U-Pb年代学及Hf同位素研究。研究结果表明:安多花岗片麻岩中锆石同时记录了510～505Ma岩浆年龄以及187Ma变质年龄;187Ma的变质锆石与510～505Ma的岩浆锆石具有相似的Hf同位素模式年龄(1.7～1.5Ga),表明寒武纪花岗岩主要来源于古老地壳重熔。碎屑锆石年代学分析结果揭示了安多微陆块石英岩具有498～484Ma、800～1000Ma和1800～1950Ma的年龄峰值,与南羌塘地体及特提斯喜马拉雅碎屑锆石年龄分布特征相似,表明其在早古生代时位于冈瓦纳大陆北部印度陆块边缘。南羌塘坳陷东南部中侏罗世砂岩及钙质砂岩碎屑锆石年代学分析结果显示其具有182～171Ma、450～600Ma、800～1000Ma、1800～1950Ma及2400～2600Ma的年龄峰值,这种年龄分布特征与安多微陆块及南羌塘地体相似,而与拉萨地体不同,说明南羌塘坳陷东南部下-中侏罗统物源主要来自安多微陆块及南羌塘地体,在早-中侏罗世时安多微陆块与南羌塘地体已经发生了碰撞造山。     

Provenance comparisons between the Nambucca Block,Eastern Australia and the Torlesse Composite Terrane,New Zealand: connections and implications from detrital zircon age patterns

Detrital zircon U–Pb LAM-ICPMS age patterns for sandstones from the mid-Permian –Triassic part (Rakaia Terrane) of the accretionary wedge forming the Torlesse Composite Terrane in Otago, New Zealand, and from the early Permian Nambucca Block of the New England Orogen, eastern Australia, constrain the development of the early Gondwana margin. In Otago, the Triassic Torlesse samples have a major (64%), younger group of Permian–Early Triassic age components at ca 280, 255 and 240 Ma, and a minor (30%) older age group with a Precambrian–early Paleozoic range (ca 1000, 600 and 500 Ma). In Permian sandstones nearby, the younger, Late Permian age components are diminished (30%) with respect to the older Precambrian–early Paleozoic age group, which now also contains major (50%) and unusual Carboniferous age components at ca 350–330 Ma. Sandstones from the Nambucca Block, an early Permian extensional basin in the southern New England Orogen, follow the Torlesse pattern: the youngest. Early Permian age components are minor (<20%) and the overall age patterns are dominated (40%) by Carboniferous age components (ca 350–320 Ma). These latter zircons are inherited from either the adjacent Devonian–Carboniferous accretionary wedge (e.g. Texas-Woolomin and Coffs Harbour Blocks) or the forearc basin (Tamworth Belt) farther to the west, in which volcaniclastic-dominated sandstone units have very similar pre-Permian (principally Carboniferous) age components. This gradual variation in age patterns from Devonian–late Carboniferous time in Australia to Late Permian–mid-Cretaceous time in New Zealand suggests an evolutionary model for the Eastern Gondwanaland plate margin and the repositioning of its subduction zone. (1) A Devonian to Carboniferous accretionary wedge in the New England Orogen developing at a (present-day) Queensland position until late in the Carboniferous. (2) Early Permian outboard repositioning of the primary, magmatic arc allowing formation of extensional basins throughout the New England Orogen. (3) Early to mid-Permian translocation of the accretionary wedge and more inboard active-margin elements, southwards to their present position. This was accompanied by oroclinal bending which allowed the initiation of a new, late Permian to Early Triassic accretionary wedge (eventually the Torlesse Composite Terrane of New Zealand) in an offshore Queensland position. (4) Jurassic–Cretaceous development of this accretionary wedge offshore, in northern Zealandia, with southwards translation of the various constituent terranes of the Torlesse Composite Terrane to their present New Zealand position.     

Variation of Cl and SO3 Contents of Microphenocrystic Apatite in Intermediate to Silicic Igneous Rocks of Cenozoic Japanese Island Arcs: Implications for Porphyry Cu Metallogenesis in the Western Pacific Island Arcs

Abstract. Determinations of SO₃ and Cl contents of igneous accessory apatite were carried out on Late Cenozoic intermediate to silicic intrusive and volcanic rocks in the Japanese island arcs of the western Pacific rim including the southwestern Kuril arc (eastern Hokkaido), Northeast Japan arc (southwestern Hokkaido through northeastern Honshu to central Honshu), Izu‐Bonin arc, Kyushu‐Palau ridge, Southwest Japan arc (northern Kyushu) and northern Ryukyu arc (southern Kyushu). These were compared to those from the Western Luzon arc, Philippines, to better understand the metallogenesis of porphyry Cu deposits in the western Pacific island arcs. In addition, SO₃ and Cl contents of accessory apatite in the Cretaceous magnetite‐series granitic rocks in the Kitakami belt (northeastern Honshu) and the Miocene ilmenite‐series granitic rocks in the Outer Zone of Southwest Japan (southern Kyushu) were also examined. Microphenocrystic apatites in shallow intrusions associated with porphyry Cu deposits in the Western Luzon arc contain >0.1 wt% S as SO₃. Such high SO₃ contents of microphenocrystic apatite are a common characteristic of hydrous mag‐matism in the Western Luzon arc, from 15 Ma old tonalitic plutonic rocks of the Luzon Central Cordillera to present‐day volcanism at Mount Pinatubo. The accessory apatite in intrusive rocks associated with porphyry Cu deposits, especially those at the Santo Tomas II deposit, show significantly high Cl contents (>2 wt%). The SO₃ contents of microphenocrystic apatite in most of the hydrous silicic rocks along the volcanic front, in andesites related to native sulfur deposits, and in Miocene and younger shallow granitic intrusions in northeastern Honshu, are generally <0.1 wt%. On the other hand, the SO₃ contents of apatite in such rocks from eastern Hokkaido, southwestern Hokkaido, Izu, northern Kyushu and southern Kyushu are similar to those from the Western Luzon arc. The SO₃ contents of accessory apatite in the Cretaceous magnetite‐series granitic rocks in the Kitakami belt are variable, whereas those of the Miocene ilmenite‐series granitic rocks in southern Kyushu are extremely low. The Cl contents of accessory apatite in some rocks of the Northeast Japan arc, Izu‐Bonin arc and Southwest Japan arc are significantly high. In terms of the Cl and SO₃ contents of microphenocrystic apatite, Cenozoic Japanese arc magmatism show similarities with arc magmatism associated elsewhere with porphyry Cu mineralization, except for the most of northeastern Honshu of the Northeast Japan arc. Apatite commonly occurs as inclusions in other phenocrystic phases. Thus the variation in SO₃ contents of apatite is a feature of early stage magmatic differentiation. The SO₃ contents of microphenocrystic apatite are considered to reflect the redox state of the magma source region or fluids encountered during magma generation.     

西藏拉萨地块过铝质花岗岩中继承锆石的物源区示踪及其古地理意义

富含继承锆石的过铝质花岗岩一般来源于富铝质岩石(如变泥质岩)的部分熔融,因而分析这些继承锆石的U-Pb年龄可以像分析沉积岩碎屑锆石的U-Pb年龄一样,提供过铝质花岗岩源区物质中碎屑沉积物物源区的丰富信息。本文报道了中部拉萨地块早侏罗世过铝质花岗岩的全岩地球化学和锆石U-Pb年代学数据,结合拉萨地块已有二叠纪和晚三叠世过铝质花岗岩的继承锆石年代学数据,总结了目前已有的拉萨地块过铝质花岗岩的继承锆石U-Pb年龄特征(共199个谐和测点)。这些过铝质花岗岩属强过铝质S型花岗岩,其中的继承锆石定义了1250~1100Ma(峰值1181±14Ma)和550~450Ma(峰值494±7Ma)2个最突出的年龄群,分别可比于拉萨地块古生代沉积岩的碎屑锆石年龄峰值(约1170Ma)和寒武纪火山岩的侵位时代,明显不同于西羌塘、安多和特提斯喜马拉雅新元古代-古生代沉积岩中的碎屑锆石年龄频谱。拉萨地块过铝质花岗岩中约1181Ma的继承锆石,可能与拉萨地块古生代沉积岩中的同期碎屑锆石一样,都来自澳大利亚南西部Albany-Fraser造山带和东南极Wilkes等地,而约494的继承锆石,既可能来自澳大利亚西部,也可能来自拉萨地块本地。本文提供了拉萨地块与澳大利亚大陆北缘具有古地理联系的过铝质花岗岩继承锆石U-Pb年龄证据。拉萨地块的研究实践表明,采用过铝质花岗岩继承锆石和古生代沉积岩碎屑锆石相结合的锆石U-Pb年代学方法,可为重建冈瓦纳大陆北缘其它微陆块的古地理和构造岩浆演化提供重要约束。     

Zircon Dating and Geological Implications of Granitic Gneiss in the Metamorphic Zone of Gaoligong Mountains in Western Yunnan, China

The general classification of intermediate-acid intrusive rocks in the metamorphic zone of Gaoligong Mountains as one of the metamorphic terranes of Proterozoic Gaoligong Mountains is problematic regarding the intrusion stage and age, as well as the subsequent metamorphism and deformation. In this study, we investigated granitic gneiss in the metamorphic zone of Gaoligong Mountains based on the 1:50,000 regional geological survey of Qushi Street (2011-2013) and SHRIMP U-Pb zircon geochronology. Results showed that the SHRIMP U-Pb zircon dating of granitic gneiss ranged from 163.5±5.7 Ma to 74.0±2.0 Ma. Thus, the granitic gneiss was grouped into orthometamorphic rocks (metamorphic intrusions). The dating data of granite rocks associated with intense metamorphism and deformation were divided into three groups, 163.5±5.7 to 162.3±3.1 Ma, 132.2-101.0 Ma and 99.4±3.5-74.0±2.0 Ma, which respectively represented three independent geologic events including an important magma intrusion with superimposed metamorphic effects in the late Middle Jurassic, regional dynamic metamorphism and superimposed reformation of fluid action in the early Cretaceous, and dynamic metamorphism dominated by ductile shear and metamorphism starting from the late Cretaceous.     

Spatial variations in damage zone width along strike-slip faults: An example from active faults in southwest Japan

Field investigations reveal spatial variations in fault zone width along strike-slip active faults of the Arima–Takatsuki Tectonic Line (ATTL) and the Rokko–Awaji Fault Zone (RAFZ) of southwest Japan, which together form a left-stepping geometric pattern. The fault zones are composed of damage zones dominated by fractured host rocks, non-foliated and foliated cataclasites, and a fault core zone that consists of cataclastic rocks including fault gouge and fault breccia. The fault damage zones of the ATTL are characterized by subsidiary faults and fractures that are asymmetrically developed on each side of the main fault. The width of the damage zone varies along faults developed within granitic rocks of the ATTL and RAFZ, from ∼50 to ∼1000 m. In contrast, the width of the damage zone within rhyolitic tuff on the northwestern side of the ATTL varies from ∼30 to ∼100 m. The fault core zone is generally concentrated in a narrow zone of ∼0.5–∼5 m in width, consisting mainly of pulverized cataclastic rocks that lack the primary cohesion of the host rocks, including a narrow zone of fault gouge (<0.5 m) and fault-breccia zones either side of the fault. The present results indicate that spatial variations in the width of damage zone and the asymmetric distribution of damage zones across the studied strike-slip faults are mainly caused by local concentrations in compressive stress within an overstep area between left-stepping strike-slip faults of the ATTL and RAFZ. The findings demonstrate that fault zone structures and the spatial distribution in the width of damage zone are strongly affected by the geometric patterns of strike-slip faults.     

Paleozoic evolution of continental crust in the Beaujolais-Lyonnais area,northeastern part of the Massif Central,France

In the Beaujolais-Lyonnais area of the northeastern Massif Central accretion of continental and possibly oceanic crustal fragments occurred between Cambrian (?) and early Carboniferous time. Three distinct lithotectonic units (terranes?) have been recognized. The southern (Lyonnais-) Unit consists of medium- to high-grade metamorphics and includes eclogites; it formed in the early Paleozoic. The Brévenne-Unit to the north contains low- to medium-grade metamorphic mafic and felsic volcanics and subordinate sedimentary rocks which possibly originated during the early Paleozoic until Devonian time, in a sialic back-arc environment or along an active continental margin. The Beaujolais-Unit is represented by volcanics on the south and predominantly shallow marine clastics and carbonates on the north. It developed in a late Devonian or early Carboniferous ensialic marginal basin. The peak of metamorphism in the Lyonnais-unit (HP/HT) was reached in Silurian time. Subsequent NW-SE to E-W oriented convergence produced mylonitic foliation, structural imbrication of the Lyonnais basement rocks with the Brévenne-Unit and SE-vergent folds accompanied by low- to medium-grade metamorphism. Late Visean to Namurian N-S to NW-SE directed N-vergent thrusting produced tectonic imbrication of the metamorphic northern Brévenne-Unit with the nonmetamorphic Beaujolais-Unit. In the southern Brévenne-Unit and in the Lyonnais-Unit updoming along right-lateral high-angle normal faults was followed by emplacement of voluminous granitic plutons of crustal origin. Late Carboniferous to early Permian crustal thinning in the Beaujolais-Lyonnais area was associated with N-S trending left-lateral strike-slip faults and E-W to NE-SW trending right-lateral strike-slip faults. Basins that developed along these faults contain continental red beds.     

Initial 87Sr/86Sr ratios of plutonic rocks from Japan

Initial ⁸⁷Sr/⁸⁶Sr rations were determined for more than 80 plutonic rocks in Japan. The ⁸⁷Sr/⁸⁶Sr ratios of gabbroic and granitic rocks show no significant difference in plutonic terranes where both rocks occur closely associated, implying a genetic relationship between them (e.g., Green Tuff belt) or reequilibration at deep level (e.g., Ryoke belt). Wherever granitic rocks occur independently from gabbroic rocks, the granites have higher ratios than the gabbros.Initial ⁸⁷Sr/⁸⁶Sr ratios of the granitic rocks are low (<0.706) in Northeast Japan but high (<0.706) in Southwest Japan, the boundary being the Tanakura Tectonic Line. Within Southwest Japan, the ratios are low along the Japan Sea side of the southernmost area. This regional variation is generally correlated with thickness of the continental crust as deduced from the Bouguer anomaly.Initial ⁸⁷Sr/⁸⁶Sr ratios of the granitic rocks vary from 0.7037 to 0.7124. The low group (<0.706) is considered to consist of essentially mantle-derived magmas contaminated by crustal material in lesser but varying degree, because of its geological setting and initial ⁸⁷Sr/⁸⁶Sr values. The high group may have been formed by contamination of a deep-seated magmas by crustal material or by generation of the main part of the magmas within the continental crust. The ratios of individual belts reflect their own history depending upon age and Rb/Sr ratio of the crustal material.Initial ⁸⁷Sr/⁸⁶Sr ratios of granitic rocks are generally low for the magnetite-series but high for the ilmenite-series. Thus, a negative correlation is observed between initial ratios and ³⁴S for most Cretaceous-Paleogene granites. However, Neogene ilmenite-series granites are low in both initial ⁸⁷Sr/⁸⁶Sr and ³⁴S indicating interaction of the granitic magma with young sedimentary rocks enriched in ³²S.     

Structural characteristics and tectonisms around the microcontinent in the outer margin of the Paleozoic-Mesozoic geosyncline of Japan

Consideration of the paleogeography and large and small structures in the outer part (the Sakawa Fold Belt) of the Paleozoic—Mesozoic geosyncline of Japan suggests that the main part of the Japanese Islands has grown up not from an arc—trench system but from a marginal sea basin—microcontinent system: the Chichibu Geosyncline and the Kurosegawa — Ofunato Island Arc, Minor structures are superposed in a complicated way and they are analyzed in terms of the concepts of tectonic level and multiple deformation.Stratigraphic evidence shows that an early deformation was pre-middle Triassic in the Chichibu Terrain but northwards in the Sambagawa terrain it may have continued until early Cretaceous in relation to minor scale subduction within the marginal basin. A late Cretaceous phase of deformation produced the greatest crustal shortening in the microcontinent area of the Kurosegawa Tectonic Zone and was nearly contemporaneous with the intrusion of granitic rocks in the Ryoke Zone Younger secondary eugeosynclines were developed by progressive encroachment on the arc—trench gap south of the Kurosegawa zone from late Permian times onwards.     

Origin of dispersed Permian–Triassic fore-arc basin terranes in New Zealand: Insights from zircon petrochronology

Permian–Triassic fore-arc basin terranes are exposed in New Zealand, but their original positions and tectonic configurations along the eastern Gondwanan margin are not fully understood. To better constrain late Paleozoic and Mesozoic reconstructions, we investigated the provenance of Permian–Triassic marine sandstone units from the Dun Mountain-Maitai Terrane (Maitai Group) and the Kaka Point Structural Belt (Willsher Group). The recognition of abundant volcanic lithic fragments in the sandstone samples, combined with the pattern of detrital zircon ages (unimodal to bimodal 280–240 Ma age distribution), demonstrate that the upper Permian to Middle Triassic volcaniclastic successions were derived from a proximal arc source. The detrital zircon age spectra match magmatic pulses in the adjacent Tuhua Intrusives (Median Batholith), a conclusion similar to that recently proposed for the Brook Street Terrane (Grampian Formation) and Murihiku Terrane (Murihiku Supergroup). Trace-element data from the dated zircon grains provide further evidence for a Median Batholith source and cross-terrane provenance links. The data indicate that 275–230 Ma zircon grains from the Maitai Group, Willsher Group, and Murihiku Supergroup were derived from a common magmatic source, and that the late Permian Longwood Suite (261–252 Ma) in the Median Batholith was a source region for these terranes. Based on the cross-terrane provenance links, we suggest that the Brook Street and Murihiku terranes were deposited in the proximal part of a fore-arc basin, whereas the Dun Mountain-Maitai Terrane represents the distal part of the same basin. Sedimentation in the Maitai Group ceased during the Middle Triassic (∼238 Ma), likely in response to a period of orogenesis at 235–230 Ma (Gondwanide Orogeny) that is widely recognized throughout the southwest Pacific.     

The mesozoic evolution of the mino terrane, central Japan: A geologic and paleomagnetic synthesis

The Mino tectono-stratigraphic terrane, central Japan, underlain by Permian to Jurassic sedimentary and volcanic rocks of various origins, was formed through accretion processes associated with the Mesozoic sea-floor spreading. This conclusion has been reached mainly from the following reasoning:

1. (1) the entire boundary of this terrane is defined by tectonic belts with high-pressure metamorphic rocks and serpentinized ultramafic rocks,

2. (2) the chemistry and petrology of the Permian greenstones demonstrate their affinity with abyssal tholeiitic and alkalic basalts,

3. (3) the widespread, but chaotic, occurrence of Permian greenstones, Triassic cherts, and Jurassic siliceous shales in the younger Jurassic clastic rocks of this terrane suggests extensive post-depositional mixing of strata,

4. (4) the sedimentology of the Jurassic sandstones strongly suggests that they are turbidity-current deposits supplied from cratonic lands,

5. (5) the South-Pacific type fossil assemblage in the Mino terrane shows strong contrast with the North-Pacific type fossil assemblage of the adjacent terranes,

6. (6) the paleomagnetism of the Permian and Jurassic greenstones, the Triassic cherts, and the Jurassic siliceous shales implies long-distance northward drift in Cretaceous time of these rocks from their original low latitudinal regions.

Along with this northward migration, the Mino terrane was accreted with extensive internal deformation to northeast Asia including the present Hida terrane. Recent accumulation of paleomagnetic and paleontologic data in the Pacific peripheral regions appears to support the existence of many allochthonous terranes which migrated from the equatorial regions. The Mino terane may be regarded as one example of these Circum-Pacific allochthons.