Structural evidence for large-scale top-to-the-north normal displacement along the Median Tectonic Line in southwest Japan

Abstract   The Median Tectonic Line (MTL) is a first-order tectonic boundary that separates the Sanbagawa and Ryoke Metamorphic Belts. Strike-slip movements on the MTL have been well documented by many workers. New field based structural studies in the Sanbagawa Belt close to the MTL reveal a large number of secondary faults and semibrittle shear bands indicating a top-to-the-north normal sense of displacement. The strikes of these shear zones and their spatial distributions suggest that development of these structures is related to movements on the MTL. These results imply that the MTL has a large-scale normal shear component on a regional scale that can help account for the exhumation of the Sanbagawa Belt. Our proposed history of the MTL can also account for changes in the geometry of folds in the Sanbagawa Belt.     

Seismotectonics of the Median Tectonic Line in southwest Japan: Implications for coupling among major fault systems

The relationship between the slip activity and occurrence of historical earthquakes along the Median Tectonic Line (MTL), together with that of the fault systems extending eastward has been examined. The MTL is divided into three segments, each containing diagnostic active faults. No historical earthquakes have been recorded along the central segment, although the segment has faster Quaternary slip rates compared with the other segments that have generated historical earthquakes. This discrepancy between earthquake generation and slip rate can be explained by a microplate model of southwest Japan. The microplate model also provides spatial and temporal coupling of slip on adjacent fault systems. In the context of this model, slip on adjacent faults reduces the normal stress on the MTL. Historical data and paleoseismic evidence indicate that slip on this segment occurs without significant strong ground motion. We interpret this as indicating anomalously slow seismic slip or aseismic slip. Slip on the central segment of the MTL creates transpressional regions at the eastern and western segments where historical earthquakes were recorded. Alternatively, the earthquakes at the eastern and western segments were triggered and concentrated shear stress at the edge of the segments resulted in postseismic slip along the central segment. The sequence of historical events suggests that the MTL characteristically does not produce great earthquakes. The microplate model also provides a tectonic framework for coupling of events among the MTL, the adjacent fault systems and the Nankai trough.     

西南日本中央剪切构造带晚第四纪以来位错特征与地震

野外考察和室内综合分析表明,日本中央构造线的右旋位错自上新世以来一直延续至今.资料表明,晚更新世以来的平均位错速率达6 .1 m m/a .高滑动速率而近1 ka 来无强破坏性地震发生的事实意味着该断裂现今可能处在蠕滑运动为主的相对稳滑状态.断裂带结构简单、走向稳定、高角度而无横向断裂切割等特点有利于断裂的蠕滑运动.     

Kinematics, structure and relationship to metamorphism of the east-west flow in the Sanbagawa Belt, southwest Japan

Abstract Deformation in the Sanbagawa Belt is characterized by ductile flow in an east-west direction sub-parallel to its length. The east-west flow (D₁) caused large-scale recumbent folding of the metamorphic sequence in central Shikoku, which can explain the inverted thermal structure of this region. Chemical zoning of metamorphic minerals associated with D₁ microstructures also suggest that the east-west flow developed under retrograde conditions. D₁ is therefore related to exhumation rather than subduction processes. A variety of kinematic indicators show that during the east-west flow, deformation was partitioned into structurally continuous domains with opposed senses of shear. This suggests that bulk deformation was not simple shear but included a component of flattening.     

Parentage of low‐grade metasediments in the Sanbagawa belt,eastern Shikoku,Southwest Japan,and its geotectonic implications

This study examines the geology of low‐grade (chlorite zone) metamorphic rocks in the Sanbagawa belt and of a Jurassic accretionary complex in the Northern Chichibu belt, eastern Shikoku, Japan. The bulk chemistries of metasandstones and metapelites in the Sanbagawa belt of eastern Shikoku are examined in order to determine their parentage. The Sanbagawa belt can be divided into northern and southern parts based on lithology and geologic structure. Geochemical data indicate that metasediments in the northern and southern parts are the metamorphic equivalents of the KS‐II (Coniacian–Campanian) and KS‐I (late Albian–early Coniacian) units of the Shimanto belt, respectively. The depositional ages of the parent sediments of low‐grade metamorphic rocks found in the Sanbagawa belt and the Jurassic Northern Chichibu belt, indicate a north‐younging polarity. In contrast, sedimentological evidence indicates younging to the south. These observations suggest that a tectonic event has resulted in a change from a northerly to southerly dip direction for schistosity and bedding in the Sanbagawa and Northern Chichibu belts of eastern Shikoku. The younging polarity observed in the Sanbagawa and Northern Chichibu belts, together with previously reported data on vitrinite reflectance and geological structure, indicate that the Northern Chichibu belt was part of the overburden formerly lying on top of the Sanbagawa low‐grade metamorphic rocks.     

Talc-phengite-albite assemblage in piemontite-quartz schist of the Sanbagawa metamorphic belt, central Shikoku, Japan

Abstract The talc (Tlc) + phengite (Phn) + albite (Ab) assemblage is newly confirmed in MnO^total-rich (1.65 wt% in average) piemontite-quartz schists from the intermediate- and high-grade part of the Sanbagawa belt, central Shikoku, Japan. Talc is in direct contact with Phn, Ab and chlorite (Chl) with sharp boundaries, suggesting that these four phases mutually coexist. Other primary constituents of the Tlc-bearing piemontite-quartz schist are spessartine, braunite, hematite (Ht), crossite/barroisite and dolomite. Phlogopite (Phl) rarely occurs as a later stage mineral developing along the rim of Phn. The studied piemontite-quartz schist has mg# (= Mg/(Mg + Fe²⁺)) ~ 1.0, because of its high oxidation state. Schreinemakers' analysis in the KNMASH system and the mineral assemblage in the Sanbagawa belt propose a possible petrogenetic grid, in which the Tlc–Phn assemblage is stable in a P-T field surrounded by the following reactions: lower-pressure limit by Chl + Phl + quartz (Qtz) = Phn + Tlc + H₂O as proposed by previous workers; higher-pressure limit by glaucophane + Qtz = Tlc + Ab + H₂O; and higher-temperature limit by Tlc + Phn + Ab = Phl + paragonite + Qtz + H₂O. Thermodynamic calculation based on the database of Holland & Powell (1998) , however, suggests that the Tlc–Phn stability field defined by these reactions is unrealistically limited around 580–600 °C at 11.6–12.0 (± 0.7) kbar. Schreinemakers' analysis in the KNMA-Fe³⁺-SH system and the observed mineral assemblage predict that Chl + crossite = Tlc + Ab + Ht + H₂O is a preferable Tlc-forming reaction in the intermediate-grade part of the Sanbagawa belt and that excess Ab + hematite narrows the stability field of the Tlc–Phn assemblage.     

Phengite K-Ar ages of schists from the Sanbagawa southern marginal belt, central Shikoku, southwest Japan: Influence of detrital mica and deformation on age

Abstract K–Ar age determinations were carried out on phengite separates from pelitic schists collected systematically from the Sanbagawa southern marginal belt and the associated area. The petrography and phengite chemistry by electron probe micro-analyzer (EPMA) revealed the existence of detrital white micas in the schist that have an extremely older age (108 Ma) in comparison with the neighboring schists (88 Ma) without any detrital mica. The ages become gradually older from the north ( ca 78 Ma) to the south ( ca 90 Ma) except for some samples that contain detrital micas and/or have been reactivated thermally by intrusives. The age is interpreted as an exhumation-cooling age that has been controlled by the ductile deformation of the host rocks that have never experienced a culmination temperature higher than 350°C which corresponds to the closure temperature of the K–Ar phengite system. The southward aging of the recorded ages in the extensive chlorite zone of the central Shikoku, from the Dozan river area of the north ( ca 65 Ma) to the study area of the south ( ca 85 Ma) through the Asemi river area ( ca 75 Ma), is explained in terms of increasing exhumation/cooling rates of the host rocks from north to south. The phengite K–Ar ages in the pelitic schists from the Kyomizu tectonic zone, which is classically considered as a remarkable thrusting shear zone, have no significant difference in comparison with that of the neighboring schists. This fact suggests that the latest stage of brittle deformation during exhumation/uplift has not significantly affected the ages of phengite in the schists.     

郯庐断裂带南段与大别──胶南造山带构造复合（叠加）的显微构造证据

对那庐断裂带南段构造岩特别是中深层次韧性剪切带中的糜棱岩进行了研究自北向南于部庐断裂带主干断裂沿线选取6条有代表性的初一脆性剪切带进行地质测量,应用高分辨透射电子显微镜对构造岩中变形石英的位错结构进行观察,求得了韧性剪切作用下构造岩石所受的古差异应力和应变速率等一系列应变参数从显微构造角度提出了郊庐断裂带与大别一胶南造山带构造复合（叠加）的证据：构造岩石的一组应变参数是郊庐断裂带的产物,严格受其中的韧性剪切带控制,另一组系大别-胶南造山带与部庐断裂带复合的产物还讨论了都庐断裂带南段中深层次构造岩的变形一变质历史     

Determination of paleo-pressure for a natural gas pool formation based on PVT characteristics of fluid inclusions in reservoir rocks——A case study of Upper-Paleozoic deep basin gas trap of the Ordos Basin

~~Determination of paleo-pressure for a natural gas pool formation based on PVT characteristics of fluid inclusions in reservoir rocks——A case study of Upper-Paleozoic deep basin gas trap of the Ordos Basin~~     

新疆北部构造区带地震活动状态分析

在对新疆北部地区地质构造、构造运动强度、历史强地震活动特征、地壳缩短速率以及局部应力场特征综合研究的基础上,将新疆北部地区划分为阿尔泰地震带、乌鲁木齐、北天山西段、中天山和东天山5个地震构造区带.通过计算上述各构造区带年应变能释放均值、折合震级、不同震级下限的地震年发生率、b值和应变加速释放模型参数m值等参数,对各构造区带中地震活动状态进行了定量分析,进而提取了各构造区带地震活动状态的特征指标,为地震趋势分析和判定提供了定量的依据.