Tectonic incorporation of the upper part of oceanic crust to overriding plate of a convergent margin: An example from the Cretaceous–early Tertiary Mugi Mélange, the Shimanto Belt, Japan

A tectonic mélange exposed on land is examined to reveal relationships between mélange formation, underplating, and deformation mechanisms, focusing on the deformation of basaltic rocks. The studied Mugi Mélange of the Shimanto Belt is composed of a shale matrix surrounding various blocks of sandstone, pelagic sediments, and basalts. The mélange was formed during Late Cretaceous to early Tertiary times in a subduction zone under P–T conditions of 150–200 °C and 6–7 km depth as estimated from vitrinite reflectance and quartz veins fluid inclusions. The mélange represents a range of deformation mechanisms; pressure solution with micro-scale cataclasis in the shale matrix, brittle tension cracking in the blocks, and ubiquitous strong cataclasis in the basal portion of basaltic layers. The cataclastic deformation in the basalts suggests a breakage of a topographic high in the seismogenic depth.     

Early Tertiary fluid flow and pressure–temperature conditions in the Shimanto accretionary complex of south-west Japan: constraints from fluid inclusions

The pressure and temperature history of the Tertiary Shimanto belt of south-west Japan has been elucidated by analysing fluids trapped in quartz crystals which grew syn-kinematically along late-stage brittle faults. The samples come from three areas that span the Paleogene exposures on the Muroto Peninsula of Shikoku Island. Applying microthermometric and laser Raman microsampling techniques to coeval water-rich and carbonic fluid inclusions, we have constrained the pressure and temperature conditions that accompanied a widespread and kinematically distinct phase of deformation. The results suggest elevated geothermal gradients during late-stage deformation, conditions that are in disaccord with previous plate reconstructions that have depicted old, thermally mature Pacific crust subducting beneath Eurasia during the early to middle Tertiary. These conditions can most easily be accounted for by including an additional plate boundary in the western Pacific during Paleogene time. Plate reconstructions that include the Kula plate in this region are therefore consistent with our findings. In addition, our results provide clues to the conditions that likely accompany seismogenic deformation at active convergent plate boundaries.     

Small-scale mélange fabric between serpentinite block and matrix: Magnetic evidence from the Mitsuishi ultramafic rock body, Hokkaido, Japan

The Mitsuishi ultramafic rock body in Hokkaido, Japan, consists mainly of serpentinized peridotites that originated from a depleted mantle. This study aims to show new evidence of small-scale mélange fabric of serpentinite matrix in the rock body. Each serpentinite block in the serpentine matrix shows large and stable intensities of natural remanent magnetization (NRM). However, the directions of serpentinite blocks' NRM in the matrix are randomly scattered. A Curie temperature (Tc) of 580 °C corresponding to pure magnetite was also observed. Additionally, there is no evidence of heating over 580 °C after serpentinization. The blocks in the matrix must have obtained crystallization remanent magnetization (CRM) during serpentinization. The directions of the blocks' characteristic remanent magnetization (ChRM) are also scattered. It shows that serpentinite blocks were magnetized prior to uplifting. The results of the study indicate that the magnetic carrier of the serpentinite blocks in the matrix is mainly composed of magnetite, and it can keep original magnetization before uplifting. The results also imply that the scattering directions of NRM indicate the presence of small-scale mélange fabric of serpentinite matrix.     

西南日本黒濑川带的解剖：晚二叠世和早侏罗世增生混杂岩的构造-地层学与浅变质作用

The Kurosegawa belt forms a relatively narrow terrane that can be traced continuously throughout southwest and central Japan. The major constituent continental fragments of the Kurosegawa belt include Early Paleozoic granitic rocks, high-grade metamor-phic rocks, Carboniferous metamorphic rocks (epi-dote-amphibolite facies), Triassic-Early Jurassic metamorphic rocks (pumpellyite-actinolite facies), serpentinites of unknown age, Silurian–Devonian volcanoclastic rocks intercalated with limestones, and Permian–Jurassic shallow marine sediments (e.g., Ichikawa et al., 1956; Maruyama et al., 1984; Faure, 1985; Yoshikura et al., 1990; Aitchison et al., 1991, 1996; Hada et al., 1992, 2001; Isozaki et al., 1992). These diverse rock suites are highly disrupted, form-ing lenticular bodies within the Late Permian accre-tionary complex (AC) which collectively are covered by younger (Cretaceous) marine to brackish water sediments (e.g., Aitchison et al., 1991; Isozaki et al., 1992). We characterize the tectono-stratigraphic ar-chitecture and low-grade metamorphism of the accre-tionary complex preserved in the Kurosegawa belt of the Kitagawa district in eastern Shikoku, Southwest Japan, in order to understand its internal structure, tectono-metamorphic evolution, and assessments of displacement of continental fragments within the complex.     

The metahyaloclastitic matrix of a unique metavolcanic block reveals subduction in the Somozas Mélange (Cabo Ortegal Complex,NW Iberia): tectonic implications for the assembly of Pangea

The allochthonous Cabo Ortegal Complex (NW Iberian Massif) contains a ~500 m thick serpentinite‐matrix mélange located in the lowest structural position, the Somozas Mélange. The mélange occurs at the leading edge of a thick nappe pile constituted by a variety of terranes transported to the East (present‐day coordinates; NW Iberian allochthonous complexes), with continental and oceanic affinities, and represents a Variscan suture. Among other types of metaigneous (calcalkaline suite dated at 527–499 Ma) and metasedimentary blocks, it contains close‐packed pillow‐lavas and broken pillow‐breccias with a metahyaloclastitic matrix formed by muscovite–paragonite–margarite–garnet–chlorite–kyanite–hematite–epidote–quartz–rutile. Pseudosection modelling in the MnCNTKFMASHO system indicates metamorphic peak conditions of ~17.5–18 kbar and ~550 °C followed by near‐isothermal decompression. This P–T evolution indicates subduction/accretion of an arc‐derived section of peri‐Gondwanan transitional crust. Subduction below the Variscan orogenic wedge evolved to continental collision with important dextral component. Closure of the remaining oceanic peri‐Gondwanan domain and associated release of fluid led to hydration of the overlying mantle wedge and the formation of a low‐viscosity subduction channel, where return flow formed the mélange. The submarine metavolcanic rocks were deformed and detached from the subducting transitional crust and eventually incorporated into the subduction channel, where they experienced fast exhumation. Due to the cryptic nature of the high‐P metamorphism preserved in its tectonic blocks, the significance of the Somozas Mélange had remained elusive, but it is made clear here for the first time as an important tectonic boundary within the Variscan Orogen formed during the late stages of the continental convergence leading to the assembly of Pangea.     

Late Caledonian Ductile Thrusting Deformation in the Central East Kunlun Belt, Qinghai, China and Its Significance: Evidence from Geochronology

A high-angle ductile thrusting deformation with top-to-the-north movement penetratively developed in the Proterozoic-Early Paleozoic metamorphic rocks along the Central East Kunlun belt. The deformed rocks suffered epidote-amphibolite facies metamorphism. On the basis of our previous study, we present more data in this paper to further support that the ducdle thrust deformation occurred in the later Caledonian and more detailed information about the deformation. A zircon U-Pb concordant age of 446±2.2 Ma of a deformed granodiorite in the ductile thrust zone was obtained and can be interpreted as the lower limit of the deformation. A syntectonically crystallized and also strongly deformed hornblende Ar/ Ar dating gives an Ar/Ar plateau age of 426.5±3.8 Ma, which represents the deformation age. A strongly orientated muscovite gives an Ar/Ar plateau age of 408±1.6Ma, representing the cooling age after the peak temperature, constraining the upper limit of the ductile thrust deformation. This ductile thru     

钦-杭结合带南段新元古代俯冲作用:来自粤西贵子混杂岩变基性岩年代学和地球化学的证据

新元古代以来,扬子地块和华夏地块拼贴形成钦-杭结合带。粤西贵子混杂岩为揭示钦-杭结合带南段地质演化提供了一个重要的窗口。贵子混杂岩构造岩块为强烈变形-变质的变基性岩、深海硅质岩,基质为石英岩、云母片岩以及具鲍马序列富锰质硅泥质岩等。对混杂岩中变基性岩年代学和元素地球化学分析显示,锆石U-Pb年龄为948±11Ma,除少部分属低钾(拉斑)外,其余均为钙碱性系列,CIPW标准矿物成分属石英拉斑玄武岩-橄榄拉斑玄武岩组合。变基性岩具有高TiO_2(平均1.85%)、P_2O_5(平均0.21%)的特点;无Eu异常或轻微负异常,配分曲线右倾(球粒陨石标准化(La/Yb)N=2.65~3.89),大离子亲石元素(Rb、Ba、Th、U)富集,高场强元素(Nb、Zr、Hf)轻微亏损,表现为洋壳消减作用下岛弧岩浆岩特征。高的Nb(7.56×10~(-6)~16.11×10~(-6),均大于7×10~(-6))、Nb/U(18.39~25.65)、(Nb/La)N(0.76~0.94,原始地幔标准化)显示为岛弧环境富Nb玄武岩。本研究认为,云开地块出露的这套构造混杂岩,是新元古代古华南洋俯冲在钦-杭结合带南段的记录,可为该带的南东边界提供线索。     

深层碎屑岩自生矿物成因机理及其对储层物性的影响——以渤海歧南断阶带侏罗系为例

自生矿物特征和成因机理对深层碎屑岩储层物性具有重要影响。以渤海海域歧南断阶带侏罗系为例,通过岩心、薄片、扫描电镜、电子探针、同位素、包裹体、X衍射分析等技术手段,对研究区侏罗系深层碎屑岩储层自生矿物的类型、特征、成因机理及对优质储层发育的控制作用进行研究。结果表明：研究区侏罗系碎屑岩属于中孔-低渗储层,非均质性强;主要自生矿物类型为硅质矿物、碳酸盐矿物、黏土矿物3类;早期形成的硅质石英衬垫和碳酸盐胶结物控制了孔隙的发育和演化,沿颗粒表面分布的早期硅质石英衬垫有效抑制了后期石英加大生长,并增强了岩石的抗压能力,有利于原生孔隙保存;早期碳酸盐胶结物增强了储层抗压实能力,并为后期储层遭受溶蚀形成溶蚀孔提供了物质基础,有利于高孔隙储层形成;黏土矿物控制了储层渗透率差异,储层渗透率与自生高岭石体积分数呈正相关性,较高渗透率储层分布于侏罗系中段高岭石富集带内。研究区侏罗系碎屑岩发育原生孔隙体积分数高、微晶石英衬垫发育的Ⅰ类有利储层和粒间溶蚀孔隙较发育、具显著表生成岩作用特点的Ⅱ类有利储层,二者孔隙演化存在明显差异。Ⅰ类有利储层主要受早—中成岩期微晶石英衬垫抗压实作用控制,浅层和深层均可发育高孔隙储层;Ⅱ类有利储层主要受表生期风化淋滤作用控制,可在风化壳附近形成优质储层,整体上Ⅰ类储层物性优于Ⅱ类。