Post-collisional extensional and thrust-nappe structures in northern part of Dabie Mountains

Thrust-nappe structures and extensional structures simultaneonsly occur in the northern part of the Dabie Mountains. The systematic structural study reveals that extensional structures along the Mozitan-Xiaotian fault and thrust-nappe structures that take the Jinzhai-Shucheng fault as their frontal thrust share the same shear sense, and display a transitional relationship from the ductile extensional structure of deep level in the south to the ductile-brittle and brittle thrust-nappe structure of shallow level in the north. The extensional and thrnst-nappe structures in the region are explained to result from post-collisional processes by the continuous subduction of the Yangtze continental block and the extension induced by the uplifting of the core part of the Dabie Mountains, which are components of the exteasional structures produced in the exhumation process of the ultrahigh pressure metamorphic rocks in the Dabie Mountains. Because of the frontal blocking in the process of the north-westward spreading, the extension and detachment of the low-grade metamorphic rocks along the Mozitan-Xiaotian fault was transformed into the northwestward thrustiag, resulting in the thrust-nappe structures. They developed in the period of 200-170 Ma, maybe last till the late Jurassic.     

Continental subduction channel processes: Plate interface interaction during continental collision

The study of subduction-zone processes is a key to development of the plate tectonic theory. Plate interface interaction is a basic mechanism for the mass and energy exchange between Earth＇s surface and interior. By developing the subduction channel model into continental collision orogens, insights are provided into tectonic processes during continental subduction and its products. The continental crust, composed of felsic to mafic rocks, is detached at different depths from subducting continental lithosphere and then migrates into continental subduction channel. Part of the subcontinental lithospheric mantle wedge, composed of perido- tile, is offscrapped from its bottom. The crustal and mantle fragments of different sizes are transported downwards and upwards inside subduction channels by the corner flow, resulting in varying extents of metamorphism, with heterogeneous deformation and local anatexis. All these metamorphic rocks can be viewed as tectonic melanges due to mechanical mixing of crust- and man- lie-derived rocks in the subduction channels, resulting in different types of metamorphic rocks now exposed in the same orogens. The crust-mantle interaction in the continental subduction channel is realized by reaction of the overlying ancient subcontinental lithospheric mantle wedge peridotite with aqueous fluid and hydrous melt derived from partial melting of subducted continental basement granite and cover sediment. The nature of premetamorphic protoliths dictates the type of collisional orogens, the size of ultrahigh-pressure metamorphic terranes and the duration of ultrahigh-pressure metamorphism.     

A late Mesozoic high plateau in eastern China: Evidence from basalt vesicular paleoaltimetry

Eastern China is made of a number of large and small continental fragments throughout late Paleozoic and Mesozoic time,and is still the best natural laboratory for examining kinematic models of continent tectonics.The paleoelevation history of eastern China provides direct insight into the tectonic processes in China.Here we present basalt-vesicularity based estimates of the paleoaltimetry of the early Cretaceous basalts in northern East China,which indicate that the studied basalt units were at an elevation of more than 4 kilometers in late Mesozoic.Two vesicular basaltic rocks have been collected form bottom to top along the lava flow outcrop in eastern Inner Mongolia.After the digital precessing to the sample cross-sections,and taking advantage of the stereological conversion method to acquire the bubble size distributions,paleoelevation estimate was calculated for East China in the early Cretaceous.The results show that the elevation has been near 4700±750 m when the lava flows was cooling,implying that there were highland regions in the northern part of the East China Plateau during the early Cretaceous.Combined with other geological evidence,we conclude that there was a high plateau with elevation near 5000 m above sea level in Eastern China during the late Mesozoic.This ancient high plateau in eastern China was most likely formed by the collision of the north and south China blocks.     

The geological characteristics of oceanic-type UHP metamorphic belts and their tectonic implications: Case studies from Southwest Tianshan and North Qaidam in NW China

The geological characteristics of ultrahigh-pressure （UHP） metamorphic belts formed by deep subduction of oceanic crust are summarized in this paper. Oceanic-type UHP metamorphic belt is characterized by its protolithlc assemblage of typical oceanic crust, the peak metamorphic temperature 〈600℃, P-T path undergoing blueschist facies during prograde and retrograde metamorphic evolution, reepectively, with low geothermal gradient of cold subduction. The further study of oceanic-type UHP metamorphic belt is very significant for constructing metamorphic reaction series of cold subduction zone, for understanding how aqueous fluids were transported into deep mantle and for classifying the types of UHP metamorphism in cold subduction zone. The uplift and exhumation mechanism of oceanic UHP metamorphic rocks is one of the most challenging problems in the study of UHP metamorphism, which is very important for understanding the geodynamic mechanism of solid Earth. As a traveler eubducted into the mantle depth end then uplifted to the surface, oceanic-type UHP metamorphic belts witness the bulk process from the subduction to exhumation and is an ideal target to study the geochemical behavior end cycling of elements in subduction zones. The tectonic evolution of one convergent orogenic belt can be usually divided into two stages of oceanic subduction and followed continental subduction and collision, and the two best-established examples of orogenic belts are Alpa and Himalaya. Therefore, the study of oceanic-type UHP metamorphic belt is the frontier of the current plate tectonic theory. As two case studies, the current status and existing problems of oceanic-type UHP metamorphic belts in Southwest Tianshan and North Qaidam, NW China, are reviewed in this paper.     

On the timing and duration of the destruction of the North China Craton

The timing and duration of the destruction of the North China Craton, which is pivotal to understanding the destruction mechanism and its geodynamic controlling factors, still remain controversial. On the basis of the principles of magma genesis and evolution, first we outline magmatic expressions that can be related to cratonic destruction, then use magmatic and basin evolution trends to constrain the timescale of the lithospheric thinning in North China. The main conclusions include： （1） the thinning of the lithosphere beneath the North China Craton might have started, at least locally, since late Carboniferous-late Triassic, attained its climax during the late Jurassic-early Cretaceous, and continued till the end of late Cretaceous-early Cenozoic. The destruction of the North China Craton was a relatively slow, rather than a dramatic process. （2） The weakened lithospheric zones along the margins and interiors of the craton played an important role in cratonic destruction, partly accounting for the heterogeneous pattern of cratonic destruction. （3） The tectonic factors that controlled the destruction of the North China Craton may be multiple. The late Carboniferous southward subduction of the Paleo-Asian plate and the late Triassic collision between North China and South China may have re-activated the craton by influencing the thermal and integral structure of the craton. The Pacific subduction underneath the eastern Asian continent played a determinant role in the cratonic destruction, governing the distribution patterns of post-Mesozoic basins and major tectonic configuration, temporal change of magmatism and formation of the North-South gravity lineament.     

Late Mesozoic interaction between crust and mantle in coastal area of Zhejiang-Fujian

This paper summarizes the lithogeochemical features of composite lavas, dikes and plutons composed of early Cretaceous associated basic-acidic igneous rocks from the coastal area of Zhejiang-Fujian. On the basis of the studies on Nd, Sr, Pb isotopic signatures and geochemical characteristics of large ion lithophile elements (L1LE) , light rare earth elements (LREE) and high field strong elements ( HFSE) of these contemporary basic-acidic igneous rocks, it is considered that the source enrichment caused by the subduction of Pacific plate and contamination of crustal rocks and melts during ascending process may lead to the formation of basaltic magmas with a high LILE concentration. These late Mesozoic composite basic-acidic igneous rocks are the products of the interactions between crust and mantle in active continental margin.     

A perspective view on ultrahigh-pressure metamorphism and continental collision in the Dabie-Sulu orogenic belt

The study of continental deep-subduction has been one of the forefront and core subjects to advance the plate tectonics theory in the twenty-first century. The Dabie-Sulu orogenic belt in China crops out the largest lithotectonic unit containing ultrahigh-pressure metamorphic rocks in the world. Much of our understanding of the world＇s most enigmatic processes in continental deep-subduction zones has been deduced from various records in the Dabie-Sulu rocks. By taking these rocks as the natural laboratory, earth scientists have made seminal contributions to understanding of ultrahigh-pressure metamorphism and continental collision. This paper outlines twelve aspects of outstanding progress, including spatial distribution of the UHP metamorphic rocks, timing of the UHP metamorphism, timescale of the UHP metamorphism, the protolith nature of deeply subducted continental crust, subduction erosion and crustal detachment during continental collision, the possible depths of continental subduction, fluid activity in the continental deep-subduction zone, partial melting during continental collision, element mobility in continental deep-subduction zone, recycling of subducted continental crust, geodynamic mechanism of postcollisional magmatism, and lithospheric architecture of collision orogen. Some intriguing questions and directions are also proposed for future studies.     

Water content in the early Cretaceous lithospheric mantle beneath the south-central Taihang Mountains: implications for the destruction of the North China Craton

Based on studies of the water content of the early Cretaceous Feixian high-magnesium basalts in the eastern part of the North China Craton （NCC）, it has been suggested that the early Cretaceous lithospheric mantle of the eastern NCC was highly hydrous （〉1,000 ppm, HeO wt.） and that this high water content had significantly reduced the vis- cosity of the lithospheric mantle and provided a prerequisite for the destruction of the NCC. The eastern part of the NCC had undergone multistage subduction of oceanic plates from the south, north, and east sides since the early Paleozoic, and these events may have caused the strong hydration of the NCC lithospheric mantle. To determine which subduction had contributed most to this hydration, we measured the water contents of the peridotite xenoliths hosted by the early Cretaceous high-magnesium diorites of Fushan in the south- central part of the Taihang Mountains. Our results demon- strate that the water content of the early Cretaceous litho- spheric mantle beneath the south part of the Taihang Mountains was ~ 40 ppm and significantly lower than that of the contemporary lithospheric mantle beneath the eastern part of the NCC. Thus, the hydration of the early Cretaceous lithospheric mantle of the eastern part of the NCC can be ascribed to the subduction of the Pacific plate from the west side. Thus, the main dynamic factor in the destruction of the NCC was likely the subduction of the Pacific plate.     

Evolution of the Xiaotian-Mozitan fault and its implications for exhumation of Dabie HP-UHP rocks

The Xiaotian-Mozitan fault （XMF） located north of the Dabie orogenic belt separates the North Dabie complex to the south from the Beihuaiyang low-grade metamorphic rocks to the north. It comprises several NW-striking ductile shear zones and brittle faults. The brittle faults obviously overprinted on the ductile shear zones and promoted the development of the volcanic basins in early Cretaceous to the north, which suggests that the brittle faults were normal faults formed in early Cretaceous during doming of the Dabie orogenic belt. The ductile shear zone superposed on the north Dabie gray gneiss, and it is an important channel where the Dabie HP-UHP rocks exhumed. For obtaining new structural constraint on exhumation of the HP-UHP rocks, we present here experimental results on the microstructure, quartz C-axis fabrics and the microprobe analyses of phengite. The ductile shear zone was determined to be formed at a temperature of 600-650 ℃ and pressure of 1.1 GPa by the mineral deformation, microprobe analyses and geobarometry of Si-in-phengite of the mylonite, the results suggest that the mylonite now exposed on the surface experienced an upper amphibolite-facies metamorphism in the lower crust. The mineral stretching lineation varies from horizontal in the east segment to sub-dip in the west. Shear sense indicators from outcrop and thin sections of orientated specimen and quartz C-axis fabrics suggest that the XMF is a sinistral normal fault. The kinematics analysis of the ductile shear zone indicates that the exhumation of Dabie HP-UHP rocks is the results of a SE-directed extrusion and an anticlockwise rotation around its eastern pivot simultaneously.     

Late Mesozoic-Cenozoic décollement structure and its deep geological background in western Shandong,China

Data from seismic reflection profiles,drilling,stratigraphy,structural deformation studies and physical rock properties reveal the existence of drcollement structures in both shallow and deep levels in western Shandong,China.The most outstanding shallow décollement structures occur along the regional unconformity surface between the Cambrian and Archean,and the disconformity surface between the Carboniferous and Ordovician.The drcollement structure surface manifests as a fault zone with cataclastic rocks and asymmetrical folds.Some of the cataclastic rocks underwent dynamic metamorphism and hydrothermal alteration,including silicification,marbleization and specularite mineralization.Above the decollement structure,the bottom of the Cambrian might be missing or overprinted because of decollement.The striations,asymmetrical folds and boudinage structures indicate the direction of the main décollement tothe NNW and NNE.A deep level decollement structure occurs at a depth of 12-22 km and up to 30 km distance to the south.The early Cretaceous and Eocene are two main periods of activity,with the Cretaceous decollement probably initiated by mantle upwelling derived from subduction and collision of the Yangtze Plate with the North China Plate along the Tancheng-Lujiang Fault in the late Triassicearly Jurassic.This circumstance implies a multidirection of subduction and collision of these two plates in the early late Mesozoic.(C) 2009 National Natural Science Foundation of China and Chinese Academy of Sciences.Published by Elsevier Limited and Science in China Press.All rights reserved.     

大港探区中生代原型盆地恢复及成盆模式

对大港探区中生代残留盆地进行原型盆地恢复．原型盆地可分为：早中侏罗世坳陷型盆地,晚侏罗至早白垩世断坳共存盆地及晚白垩世热坳陷盆地．中生代原型盆地分布方向为ＮＥ－ＮＮＥ向,分布范围有限,且沉积厚度较小,这与中生代中国东部受伊泽奈崎板块斜向俯冲和华北与扬子板块碰撞所致的挤压环境有关     

福建平潭岛火山岩及其板块碰撞构造

提出在福建东南沿海地区晚白垩世除了产出有拉张地球动力环境下火山（石帽山群火山岩）和深成（魁岐碱性花岗岩类）产物外，在长乐－南澳断裂带以东的平潭岛还产出有挤压地球动力环境下火山产物．其化学成分和岩系演化趋势显著不同于闽东南南园组和石帽山群火山岩，为独立的钙碱性角闪石英安岩系．其成因可能与库拉板块沿台湾中央山脉东侧向台湾海峡陆块的俯冲有关．闽东南地区早白垩世挤压环境火山产物（南园组火山岩）和晚自垩世拉张环境火山－深层岩系则可能与燕山早期台湾海峡陆块向闽东南大陆边缘的碰撞俯冲，而在晚白垩世停止俯冲，同时在台湾中央山脉东侧发生的俯冲作用所造成的“弧后扩张”有关．     

25 years of continental deep subduction

This year marks the 25th anniversary of the discovery of coesite in metamorphic rocks of supracrustal origin. This initiated a revolution of the plate tectonics theory due to intensive studies of ultrahigh pressure metamorphism and continental deep subduction. The occurrence of coesite was first reported in 1984 by two French scientists, C. Chopin and D.C. Smith,     

吉林省四平市石岭镇榆树林村—放牛沟村剖面登娄库组构造演化特征

实测剖面地处吉林省四平市梨树县石岭镇境内,位于距四平市区约30 km的303国道两侧。通过对该剖面登娄库组野外露头的精细测量和细致研究,对其构造样式和动力学机制有了比较清晰的认识。共识别出39条各类断层和多种类型的褶皱,其中断层以小型逆断层为主,少数为走滑断层,走向均为NNE向和SWW向,这与依兰-伊通断裂带走向大致相同。通过对剖面内各断层、断层组合及褶皱形态的综合研究并结合区域构造背景,得出该剖面登娄库组构造运动主要受太平洋板块向欧亚板块运动方向变化的影响,先后经历了早白垩世末期至晚白垩世早期(100Ma左右)左行走滑正断层运动和早第三纪右行走滑逆断层运动两个期次的构造运动。     

湖北谷城观音坪地区多重滑脱-推覆及构造变形

湖北谷城县观音坪地区推覆构造为武当山巨型推覆构造的一部分。该区推覆构造结构复杂,顺层滑脱及韧性逆掩有机地组合在一起构成了多重滑脱-推覆构造。该区地壳经历了多期次、多体制、多层次的复杂变形,但无认是外来岩块还是原地岩系,其构造格局和样式均以同推覆期构造为主导。     

汤加-克马德克俯冲带的地质构造与地震火山特征

2022年1月15日西南太平洋的洪阿哈阿帕伊岛海底火山发生了爆炸式的剧烈喷发，吸引了全球的关注。洪阿哈阿帕伊岛海底火山位于汤加-克马德克俯冲带，综合前期研究结果，对汤加-克马德克俯冲带的地质构造特征、地震和火山分布进行初步分析，发现：（1）从汤加-克马德克俯冲带弧前向海方向直到俯冲的太平洋板块，构造上主要表现为大规模正断层。（2）路易斯维尔海山链的俯冲将汤加-克马德克俯冲带分为北部的汤加俯冲带和南部的克马德克俯冲带，沿汤加俯冲带板块汇聚率为67～84 mm/a，沿克马德克俯冲带板块汇聚率为41～58 mm/a，板块俯冲速度的差异造成汤加俯冲带和克马德克俯冲带目前俯冲深度的不同。（3）在路易斯维尔海山链以北，太平洋板块上覆沉积物厚度不足0.4 km，而在南侧达到1 km左右，由于俯冲板块上覆沉积物厚度的差异而造成北部的汤加俯冲带和南部的克马德克俯冲带孕育地震能力的差异。这些认识对研究该俯冲带的火山喷发机制、大地震成因机理及其灾害风险具有重要意义。     

Geological record of fluid flow and seismogenesis along an erosive subducting plate boundary

Tectonic erosion of the overriding plate by the downgoing slab is believed to occur at half the Earth's subduction zones. In situ investigation of the geological processes at active erosive margins is extremely difficult owing to the deep marine environment and the net loss of forearc crust to deeper levels in the subduction zone. Until now, a fossil erosive subduction channel-the shear zone marking the plate boundary-has not been recognized in the field, so that seismic observations have provided the only information on plate boundary processes at erosive margins. Here we show that a fossil erosive margin is preserved in the Northern Apennines of Italy. It formed during the Tertiary transition from oceanic subduction to continental collision, and was preserved by the late deactivation and fossilization of the plate boundary. The outcropping erosive subduction channel is approximately 500 m thick. It is representative of the first 5 km of depth, with its deeper portions reaching approximately 150 degrees C. The fossil zone records several surprises. Two décollements were simultaneously active at the top and base of the subduction channel. Both deeper basal erosion and near-surface frontal erosion occurred. At shallow depths extension was a key deformation component within this erosive convergent plate boundary, and slip occurred without an observable fluid pressure cycle. At depths greater than about 3 km a fluid cycle is clearly shown by the development of veins and the alternation of fast (co-seismic) and slow (inter-seismic) slip. In the deepest portions of the outcropping subduction channel, extension is finally overprinted by compressional structures. In modern subduction zones the onset of seismic activity is believed to occur at approximately 150 degrees C, but in the fossil channel the onset occurred at cooler palaeo-temperatures.     

四川盆地南部中下寒武统石膏岩的发现与油气勘探

预测四川盆地覆盖区的中下寒武统优质膏质岩盖层的分布,为油气勘探提供依据。通过石膏露头细测、野外剖面实测与钻录井分析的方法,发现四川盆地南部的雷波县沙坪子西王庙组(-C2x)、火草坪龙王庙组(-C1l)2个中下寒武统石膏露头点,膏质岩层出露6层,厚度21.5m。描述了甘洛县黑马中寒武统大鼻山组(-C2d)石膏露头共5层,厚度18.5m。同时,窝深1井也揭示中下寒武统膏质岩层,在抓抓岩与乐山范店实测剖面上发现了可与石膏露头、钻井类比的膏溶角砾岩。研究表明,雷波-甘洛石膏露头区发育内缓坡相、蒸发岩台地(或塞卜哈)、局限台地相潮间坪亚相与潟湖亚相的石膏岩与膏溶角砾岩沉积,它们与泥晶白云岩或红色泥岩共生;蒸发岩台地堆积紫红色泥质岩与透镜状、结核状、角砾状构造的灰色泥晶含石膏质白云岩;局限台地相发育潮间坪亚相韵律条带、波状构造的石膏质白云岩、含白云质石膏岩与膏溶角砾岩,以及盐湖或潟湖亚相块状纯石膏岩与韵律条带构造的含白云质石膏岩。这些膏质岩的确认与相分析表明四川盆地南部发育一条长60～70km膏质岩带,它对油气勘探具有重要的意义。