川东北新元古代克拉通裂陷的厘定及其深层油气意义

四川盆地安岳震旦-寒武系特大型气田的发现,极大地鼓舞了相关学者对四川盆地深层油气勘探新领域的探索热情,然而,对于川东北地区深层古地理格局的认识尚存在不少争议。本文通过对四川盆地北缘、东北缘新元古代-寒武纪地层剖面的综合调查与区域对比研究,基于关键事件序列的系统厘定,重建了川东北及邻区的新元古代-寒武纪原型盆地地层格架;同时,结合盆内最新实施的深层钻井和地球物理资料解析,基本厘定了川东北克拉通裂陷的演化时限、发育范围、充填序列、区域古地理格局及其演化过程。研究显示川东北克拉通裂陷的演化经历了三个阶段:(1)新元古代板溪期-震旦纪早期为伸展断陷期,与南华裂谷系开启同步,表现为快速充填,区域沉积分异显著;(2)震旦纪陡山沱晚期-寒武纪筇竹寺期为伸展沉降期,沉积厚度小,但烃源岩相对发育;(3)寒武纪沧浪铺期至寒武纪末为挤压扰曲与充填期,发育局限台地-潟湖相含膏岩系。不同于绵阳-长宁克拉通裂陷,川东北克拉通裂陷开启早,其早期为补偿性裂谷充填、中晚期则为欠补偿的碳质泥页岩和碳酸盐岩沉积组合。由此可见,这一新认识或将对川东北深层油气勘探方向预测具有重要意义。

甘陶河群形成时代和构造环境:地质、地球化学和锆石SHRIMP定年

甘陶河群是华北克拉通中部带早前寒武纪基底的重要组成．主要由中基性玄武岩、砂岩、白云岩等组成，本文报道了甘陶河群变质玄武岩的锆石SHRIMPU-Pb年龄和地球化学组成．岩浆锆石的^207Pb／^206pb加权平均年龄为2087±16Ma（MSWD=1.3），其Hf同位素变化范围非常大，εHr（t）值从－7.17到＋0．45．除岩浆锆石外，岩石中还存在大量约2．5Ga捕获锆石,全岩Nd同位素变化范围为＋4．0-0．8．在原始地幔标准化的蛛网图中，所有样品都没有明显的Zr和Hf异常、部分样品也没有Nb和Ta异常．亏损地慢的部分熔融以及随后的地壳混染是这些玄武岩最可能的成因．野外地质特征、大量捕获锆石的存在、同位素和微量元素地球化学都表明甘陶河群是在大陆基底上打开的．结合前人对中部带其他古元古代火山沉积建造的研究，本文指出华北克拉通中部带在古元古代时期为大陆裂谷环境，在太古宙末期中部带地质体和东部陆块已经形成了统一的陆块．

Destruction of the North China Craton

A National Science Foundation of China （NSFC） major research project, Destruction of the North China Craton （NCC）, has been carried out in the past few years by Chinese scientists through an in-depth and systematic observations, experiments and theoretical analyses, with an emphasis on the spatio-temporal distribution of the NCC destruction, the structure of deep earth and shallow geological records of the craton evolution, the mechanism and dynamics of the craton destruction. From this work the foUowing conclusions can be drawn： （1） Significant spatial heterogeneity exists in the NCC lithospheric thickness and crustal structure, which constrains the scope of the NCC destruction. （2） The nature of the Paleozoic, Mesozoic and Cenozoic sub-continental lithospheric mantle （CLM） underneath the NCC is characterized in detail. In terms of water content, the late Mesozoic CLM was rich in water, but Cenozoic CLM was highly water deficient. （3） The correlation between magmatism and surface geological response confirms that the geological and tectonic evolution is governed by cratonic destruction processes. （4） Pacific subduction is the main dynamic factor that triggered the destruction of the NCC, which highlights the role of cratonic destruction in plate tectonics.

Cratonization and the Ancient North China Continent:A summary and review

Cratonization is a key geological process to form stable continental masses with a considerable scale.The Precambrian global cratonization and formation of supercratons in the world is an unrepeated event in the history of the Earth's formation and evolution.Mainly based on study of early Precambrian geology in Eastern Hebei Region and combining other Archean regions in the North China Craton (NCC),the author proposes a two-stage cratonization model of the NCC.The first stage took place at the end of Neoarchean of ～2.5 Ga (boundary time between Archean and Proterozoic),when several micro-blocks were amalgamated together with amphibolite-granulite facies metamorphism and intrusion of crustal-melting granites to form the present-scale NCC.The second cratonization event is cratonic reworking,corresponding to rifting-subduction-collision at 2.3-1.97 Ga and subsequent extension-uplifting related to upwelling mantle at 1.97-1.82 Ga,which could be linked to,respectively,assembly and breaking up of the Columbia Supercontinent.Three main Paleoproterozoic mobile belts in the NCC record that small remnant Neoarchean ocean basins and continental rift basins within the craton were opened and finally closed,and metamorphosed to greenschist-amphibolite facies at ～2.0-1.97 Ga.After that,high-grade granulite facies (HT-HP and HT-UHT) metamorphism with abnormally high heat occurred at 1.97-1.82 Ga.A metamorphism-migmatization event that includes lower crust of the NCC uplifting as a whole,intrusion of mafic dyke swarms,continental rifting and anorogenic magmatic action took place in 1.82-1.65 Ga,marking that the second cratonization of the NCC was finally accomplished and started to evolve to a period of stable continent (platform).

Decratonic gold deposits

The North China Craton(NCC) hosts numerous gold deposits and is known as the most gold-productive region of China. The gold deposits were mostly formed within a few million years in the Early Cretaceous(130–120 Ma), coeval with widespread occurrences of bimodal magmatism, rift basins and metamorphic core complexes that marked the peak of lithospheric thinning and destruction of the NCC. Stable isotope data and geological evidence indicate that ore-forming fluids and other components were largely exsolved from cooling magma and/or derived from mantle degassing during the period of lithospheric extension. Gold mineralization in the NCC contrasts strikingly with that of other cratons where gold ore-forming fluids were sourced mostly from metamorphic devolatization in compressional or transpressional regimes. In this paper, we present a summary and discussion on time-space distribution and ore genesis of gold deposits in the NCC in the context of the timing, spatial variation, and decratonic processes. Compared with orogenic gold deposits in other cratonic blocks, the Early Cretaceous gold deposits in the NCC are quite distinct in that they were deposited from magma-derived fluids under extensional settings and associated closely with destruction of cratonic lithosphere. We argue that Early Cretaceous gold deposits in the NCC cannot be classified as orogenic gold deposits as previously suggested, rather, they are a new type of gold deposits, termed as "decratonic gold deposits" in this study. The westward subduction of the paleo-West Pacific plate(the Izanagi plate) beneath the eastern China continent gave rise to an optimal tectonic setting for large-scale gold mineralization in the Early Cretaceous. Dehydration of the subducted and stagnant slab in the mantle transition zone led to continuous hydration and considerable metasomatism of the mantle wedge beneath the NCC. As a consequence, the refractory mantle became oxidized and highly enriched in large ion lithophile elements and chalcophile elements(e.g., Cu, Au, Ag and Te). Partial melting of such a mantle would have produced voluminous hydrous, Au- and S-bearing basaltic magma, which, together with crust-derived melts induced by underplating of basaltic magma, served as an important source for ore-forming fluids. It is suggested that the Eocene Carlin-type gold deposits in Nevada, occurring geologically in the deformed western margin of the North America Craton, are comparable with the Early Cretaceous gold deposits of the NCC because they share similar tectonic settings and auriferous fluids. The NCC gold deposits are characterized by gold-bearing quartz veins in the Archean amphibolite facies rocks, whereas the Nevada gold deposits are featured by fine-grained sulfide dissemination in Paleozoic marine sedimentary rocks. Their main differences in gold mineralization are the different host rocks, ore-controlling structures, and ore-forming depth. The similar tectonic setting and ore-forming fluid source, however, indicate that the Carlin-type gold deposits in Nevada are actually analogous to decratonic gold deposits in the NCC. Gold deposits in both the NCC and Nevada were formed in a relatively short time interval(<10 Myr) and become progressively younger toward the subduction zone. Younging of gold mineralization toward subduction zone might have been attributed to retreat of subduction zone and rollback of subducted slab. According to the ages of gold deposits on inland and marginal zones, the retreat rates of the Izanagi plate in the western Pacific in the Early Cretaceous and the Farallon plate of the eastern Pacific in the Eocene are estimated at 8.8 cm/yr and 3.3 cm/yr, respectively.

Late Mesoarchean crust growth event： evidence from the ca. 2.8 Ga granodioritic gneisses of the Xiaoqinling area, southern North China Craton

Recently, more attention has been paid to Precambrian magma events and crustal growth of the North China Craton(NCC), accompanying with controversy in activity stages and dynamic mechanism. In this study, we report the Archean(2802 ± 13 Ma) granodioritic gneisses in the Taihua Complex from the Xiaoqinling area, located in the southern margin of the NCC. The zircon Lu–Hf isotope analysis of the rocks showed ^(176)Hf/^(177)Hf ratios of0.280977–0.281228, corresponding to ε_(Hf)(t) values ranging from-3.5 to +6.6, and two–stage Hf model ages varying from 2836 to 3409 Ma. It was confirmed that late Mesoarchean(2.9–2.8 Ga) juvenile crust made contribution to the source material of these ca. 2.8 Ga granodioritic gneisses in the Xiaoqinling area. The whole rock geochemical data indicate that the granodioritic gneisses are high-K calc-alkaline series, probably generated at relatively high pressure and temperature, and formed under the continental arc setting. Statistically, we conclude that the magmatic activities during 2.9–2.7 Ga may represent the most intense crustal growth events in the NCC and these Archean rocks at different locations in southern NCC underwent a similar crustal evolution history.

Regional differences in crustal structure of the North China Craton from receiver functions

Moho depth and crustal average Poisson＇s ratio for 823 stations are obtained by H-n： stacking of receiver functions. These, to- gether with topography and receiver function amplitude information, were used to study the crustal structure beneath the North China Craton （NCC）. The results suggest that modified and preserved crust coexist beneath the craton with generally Airy-type isostatic equilibrium. The equilibrium is relatively low in the eastern NCC and some local areas in the central and western NCC, which correlates well with regional geology and tectonic features. Major differences in the crust were observed beneath the eastern, central, and western NCC, with average Moho depths of 33, 37, and 42 km and average Poisson＇s ratios of 0.268, 0.267 and 0.264, respectively. Abnormal Moho depths and Poisson＇s ratios are mainly present in the rift zones, the northern and southern edges of the central NCC, and tectonic boundaries. The crust beneath Ordos retains the characteristics of typical craton. Poisson＇s ratio increases roughly linearly as Moho depth decreases in all three parts of the NCC with different slopes. Receiver function amplitudes are relatively large in the northern edge of the eastern and central NCC, and small in and near the rifts. The Yanshan Mountains and southern part of the Shanxi rift show small-scale variations in the receiver-function ampli- tudes. These observations suggest that overall modification and thinning in the crust occurred in the eastern NCC, and local crustal modification occurred in the central and westem NCC. Different crustal structures in the eastern, central, and western NCC suggest different modification processes and mechanisms. The overall destruction of the crustal structure in the eastern NCC is probably due to the westward subduction of the Pacific Plate during the Meso-Cenozoic time; the local modifications of the crust in the central and western NCC may be due to repeated reactivations at zones with a heterogeneous structure by successive thermal-tectonic events during the long-term evolution of the NCC.

Late Mesozoic magmatism and tectonic evolution in the Southern margin of the North China Craton

Late Mesozoic granitic magmatism(158–112 Ma) are widespread in the southern margin of the North China Craton(NCC), contemporary with many world-class Mo-Au-Ag-Pb-Zn polymetallic deposits. There are abrupt changes in the elements and isotopic compositions of these granites at about 127 Ma. The early stage(158–128 Ma) granites show slightly or no negative Eu anomalies, large ion lithophile elements enriched and heavy REE depleted(such as Y and Yb), belonging to typical I-type granite. The late stage(126–112 Ma) granites are characterized by A-type and/or highly fractionated I-type granite, with higher contents of SiO2, K2 O, Y, Yb and Rb/Sr ratio and lower contents of Sr, δEu value and Sr/Y ratio than that of the early-stage granites.Moreover, the whole rock Nd and Hf isotopic compositions of the granites younger than 127 Ma show more depleted than those of the older one. The two stages of Late Mesozoic granites were derived from a source region of the ancient basement of the southern margin of the NCC incorporated the mantle material. The late stage(126–112 Ma) granites contain more fractions of mantle material with depleted isotopic composition than the early ones. The granites record evidence for a strong crust-mantle interaction. They formed in an intracontinental extensional setting which was related to lithospheric thinning and asthenospheric upwelling in this region, which was possibly caused by westward subduction of the Paleo-Pacific plate. 127 Ma is an critical period of the transformation of the tectonic regime.

Destruction of North China Craton:Insights from temporal and spatial evolution of the proto-basins and magmatism

Temporal and spatial evolution of proto-basins and magmatism in the North China Craton might provide information of its destruction.Overall,the destruction of the North China Craton is a heterogeneous process of bottom upward and from margin toward interior,related to multiple interactions between the craton and its surrounding plates.The interior of craton would be initially destructed during the Early-Middle Jurassic.Subduction of the Paleo-Asian Ocean Plate may have destructed the northern margin of the North China Craton.Collision of the Yangtze Plate with the North China Craton is significant for the magmatic and tectonic activities during the Late Triassic-Middle Jurassic.Subductions of the Izanagi and Paleo-Pacific plates lead to the ultimate destruction of the North China Craton.Temporal and spatial evolution of the proto-basins and related magmatism in the North China Craton indicate that the dominant mechanism of the cratonic destruction may be thermal mechanical-chemical erosion.

The Dayingzi detachment fault system in Liaodong Peninsula and its regional tectonic significance

Large scale lithosphere thinning is an important characteristic of the destruction of the North China Craton （NCC） during the late Mesozoic. A series of extensional structures were developed under extensional setting, among which is the Dayingzi detachment fault system （DFS）. The DFS is constituted by three parts, volcano-sedimentary basins at the hanging wall, the Dayingzi-Huanghuadian detachment fault zone, and Paleoproterozoic metamorphic rock series and Mesozoic plutons at the footwall. In the section across the detachment fault zone, there is a sequence of tectonites including fault gouge, microbreccia, cataclastic-mylonites, mylonites, and gneissic biotite monzonite granite. Microstructural characteristics of tectonites and electron backscatter diffraction （EBSD） patterns of quartz indicate that the rocks from the footwall experienced a process from upper greenschist facies to lower greenschist facies. SHRIMP and LA-ICP MS U-Pb dating of zircons from the volcanic rocks in the basins, the tectonic evolution of the DFS is summarized as follows： 1） regional extension started at 135.0±1.2 Ma ago, when the detachment fault cut through the middle crust. Faulting induced the upwelling of magma and eruption of volcanic rocks and deformed a series of medium-acid volcanic rocks; 2） after 135.0±1.2 Ma, a large scale detachment faulting was active cross-cutting the mid-upper crust. The western margin of Jurassic and Triassic granite was ductilly and brittly sheared; besides, the Cretaceous volcanoedimentary rocks were tilted when the master fault approached the surface; 3） at around 127±1 Ma, the detachment fault stopped its activity and was intruded by the unsheared Cretaceous granite near Chaoyang. Comparison with the Liaonan metamorphic core complex （MCC） and other extensional structures in Liaodong Peninsula led to a general trend of including three zones in the Peninsula： MCC zone, detachment fault systems （DFS） zone, and half graben zone. MCC commonly cuts through the mid-lower crust, DFS through the mid-upper crust, and half graben through the upper crust. Therefore, development of the extensional structures in Liaodong Peninsula indicates that they are the results of crustal extension and thinning at different crustal levels. They may provide a deep insight into the dynamic mechanism, history of destruction and lithosphere thinning of the North China Craton （NCC）. Liaodong Peninsula, detachment fault system, Cretaceous extension, lithosphere thinning, North China Craton

Origin of Baotoudong syenites in North China Craton:Petrological, mineralogical and geochemical evidence

Baotoudong syenite pluton is located to the east of Baotou City, Inner Mongolia, the westernmost part of the Trias- sic alkaline magmatic belt along the northern margin of the North China Craton （NCC）. Zircon U-Pb age, petrological, miner- alogical and geochemical data of the pluton were obtained in this paper, to constrain its origin and mantle source characteris- tics. The pluton is composed of nepheline-clinopyroxene syenite and alkali-feldspar syenite, with zircon U-Pb age of 214.7±1.1 Ma. Diopside （cores）-aegirine-augite （rims）, biotite, orthoclase and nepheline are the major minerals. The Bao- toudong syenites have high contents of rare earth elements （REE）, and are characterized by enrichment in light rare earth ele- ments （LREE） and large ion lithophile elements （LILE; e.g., Rb, Ba, Sr）, depletion in heavy rare earth elements （HREE） and high field strength elements （HFSE）. They show enriched Sr-Nd isotopic compositions with initial 87Sr/86Sr ranging from 0.7061 to 0.7067 and eNd（t） values from -9.0 to -11.2. Mineralogy, petrology and geochemical studies show that the parental magma of the syenites is SiO2-undersaturated potassic-ultrapotassic, and is characterized by high contents of CaO, Fe2O3, K2O, Na2O and fluid compositions （H2O）, and by high temperature and high oxygen fugacity. The syenites were originated from a phlogopite-rich, enriched lithospheric mantle source in garnet-stable area （〉80 km）. The occurrence of the Baotoudong sye- nites, together with many other ultrapotassic, alkaline complexes of similar ages on the northern margin of the NCC in Late Triassic implies that the lithospheric mantle beneath the northern margin of the NCC was previously metasomatized by melts/fluids from the subducted, altered paleo-Mongolian oceanic crust, and the northern margin of the craton has entered into an extensively extensional regime as a destructive continental margin in Late Triassic.

SIMS U-Pb dating of rutile within eclogitic xenoliths in the Early Cretaceous adakitic rocks of the Xuzhou-Huaibei area, China: Constraints on the timing of crustal thickening of the eastern North China Craton

This paper reports the results of secondary ion mass spectrometry （SIMS） U-Pb dating of rutile within eclogitic xenoliths from the Early Cretaceous adakitic rocks in the Xuzhou-Huaibei area of China and discusses the geological significance of these new dates. The dating of rutile from pargasite-bearing eclogite and garnet clinopyroxenite （retrograde eclogite） yielded ages of 209 ± 25 and 132 ± 7 Ma, respectively. The former is consistent with the timing of the first stage of exhumation of the Dabie-Sulu orogenic belt and provides evidence of crustal thickening within the southeastern margin of the North China Cra- ton during the early Mesozoic, related to subduction and collision between the Yangtze and North China cratons. The latter age is similar to the timing of emplacement of the hosting intrusion and indicates that the rutile U-Pb system was triggered after uplifting by the intrusion of the hosting magmas. It implies that garnet clinopyroxenite was previously at the depth where the temperature was above the futile U-Pb closure temperature.

Timing of uplift and evolution of the Lüliang Mountains,North China Craton

This study analyses evidence for reformed basin development and basin-mountain coupling associated with devel- opment of the Ordos Basin and the Laliang Mountains, China. Gaining an improved understanding of the timing and nature of uplift and evolution of the Ltiliang Mountains is important for the reconstruction of the eastern sedimentary boundary of the Ordos Basin （a major petroliferous basin） as well as for providing insight into the evolution and breakup of the North China Craton （NCC）. Based on systematic sampling for fission track analysis, it is suggested that the main phase of uplift of the Laliang Mountains occurred since later part of the Early Cretaceous. Three evolutionary stages of uplift and development are identified： slow initial uplift （120-65 Ma）, accelerated uplift （65-23 Ma）, and intensive uplift （23 Ma to present）, with the ma- jority of the uplift activity having occurred during the Cenozoic. The history of uplift is non-equilibrium and exhibits complex- ity in temporal and spatial aspects. The middle and northern parts of the Ltiliang Mountains were uplifted earlier than the southern part. The most intensive episode of uplift activity commenced in the Miocene and was associated with a genetic cou- pling relationship with the eastern neighboring Cenozoic Shanxi Grabens. The uplifting and evolutionary processes of the Ltiliang Mountains area since later part of the Early Cretaceous share a unified regional geodynamic setting, which was ac- companied by uplift of the Mesozoic Ordos Basin and development of the neighboring Cenozoic Shanxi Grabens. Collectively, this regional orogenic activity is related principally to the far-field effects of both the compression sourced from the south- western Tibet Plateau and westward subduction of the Pacific Plate in Cenozoic.

Three-dimensional crustal velocity structure model of the middle-eastern north China Craton (HBCrust1.0)

Lithosphere thinning and destruction in the middle-eastern North China Craton(NCC), a region susceptible to strong earthquakes, is one of the research hotspots in solid earth science. All 42 seismic wide-angle reflection/refraction profiles have been completed in the middle-eastern NCC. We collect all the 2-D profiling results and perform gridding of the velocity and interface depth data, building a 3-D crustal velocity structure model for the middle-eastern NCC, named HBCrust1.0, by using the Kriging interpolation method. Our result shows that the first-arrival times calculated by HBCust1.0 fit well with the observations. The result demonstrates that the upper crust is the main seismogenic layer, and the brittle-ductile transition occurs at depths near interface C(the interface between upper and lower crust). The depth of interface Moho varies beneath the source area of the Tangshan earthquake, and a low-velocity structure is found to extend from the source area to the lower crust. Based on these observations, it can be inferred that stress accumulation responsible for the Tangshan earthquake may have been closely related to the migration and deformation of the mantle materials. Comparisons of the average velocities of the whole crust, the upper and the lower crust show that the average velocity of the lower crust under the central part of the North China Basin(NCB) in the east of the craton is obviously higher than the regional average. This high-velocity probably results from long-term underplating of the mantle magma.

Water in peridotite xenoliths from South China

The water contents of nominally anhydrous clinopyroxene （cpx）, orthopyroxene （opx） and olivine （ol） in peridotite xenoliths hosted by Cenozoic basalts from Yingfengling, Zhangchouchun, Fujitian and Lindi, South China, were measured by Micro- FTIR. All cpx and opx grains contained a certain amount of water, which was indicated by the presence of hydroxyls in the crystal structure. The water contents （H20, ppm） of cpx and opx from peridotite xenoliths of the study areas were 293-981, 183-752, 73-586 and 51-423 ppm, and 82-471, 74-571, 53-170 and 9-135 ppm, respectively. No prominent OH absorption bands were detected for any ol grains, indicating that the water contents were below the detection limit （approximately 2 ppm）. The entire rock contents recalculated according to mineral volume proportions were 49-163, 48-168, 21-111 and 8-40 ppm, respectively. Combined with previously reported data describing the water contents of peridotite xenoliths worldwide, the results presented here suggest that water distribution in the continental lithospheric mantle is spatially heterogeneous at a global scale. The lithospheric mantle of South China is much richer in water than that of the North China Craton, and is close to that typical of off-craton localities, such as the Basin and Range Province, and Massif Central.

Crustal and upper mantle structure and deep tectonic genesis of large earthquakes in North China

From the 1960 s to 1970 s, North China has been hit by a series of large earthquakes. During the past half century,geophysicists have carried out numerous surveys of the crustal and upper mantle structure, and associated studies in North China.They have made significant progress on several key issues in the geosciences, such as the crustal and upper mantle structure and the seismogenic environment of strong earthquakes. Deep seismic profiling results indicate a complex tectonic setting in the strong earthquake areas of North China, where a listric normal fault and a low-angle detachment in the upper crust coexist with a high-angle deep fault passing through the lower crust to the Moho beneath the hypocenter. Seismic tomography images reveal that most of the large earthquakes occurred in the transition between the high-and low-velocity zones, and the Tangshan earthquake area is characterized by a low-velocity anomaly in the middle-lower crust. Comprehensive analysis of geophysical data identified that the deep seismogenic environment in the North China extensional tectonic region is generally characterized by a low-velocity anomalous belt beneath the hypocenter, inconsistency of the deep and shallow structures in the crust, a steep crustalal-scale fault,relative lower velocities in the uppermost mantle, and local Moho uplift, etc. This indicates that the lithospheric structure of North China has strong heterogeneities. Geologically, the North China region had been a stable craton named the North China Craton or in brief the NCC, containing crustal rocks as old as ～3.8 Ga. The present-day strong seismic activity and the lower velocity of the lower crust in the NCC are much different from typical stable cratons around the world. These findings provide significant evidence for the destruction of the NCC. Although deep seismic profiling and seismic tomography have greatly enhanced knowledge about the deep-seated structure and seismogenic environment, some fundamental issues still remain and require further work.

Determining the key conditions for the formation of metamorphic core complexes by geodynamic modeling and insights into the destruction of North China Craton

Metamorphic core complex(MCC) is characterized by the exhumation of lower crust over a large-scale detachment fault, providing natural records for tectonic extension. MCCs are widely identified in the North China Craton(NCC), which have been intensively studied on their structural and geological characteristics. Yet, the condition for the formation of MCCs and their link with NCC destruction are still in debate. In this study, we perform numerical simulations to investigate MCC formation under extension, with a focus on the effect of crustal rheologies. Results indicate that three end-member modes of deformation may occur: the metamorphic core complex mode, the detachment fault-uplifting mode and the pure shear mode. Weaker lower crust and stronger upper crust may promote the formation of MCC. In contrast, stronger lower crust(>1.3×1021 Pa s) may prohibit the exhumation of lower crust(detachment fault-uplifting mode), while weaker upper crust(<7.8×1021 Pa s) may fail to develop detachment faults(pure shear mode). Given that cratons typically have a strong crust, we suggest that the lower crust of NCC was weakened prior to extension, which promoted the formation of MCC in a later stage under the back-arc extension.

Platinum-group element geochemistry of Cenozoic basalts from the North China Craton: Implications for mantle heterogeneity

Forty-two Cenozoic （mostly Miocene） basalt samples from Jining, Chifeng, Fansi, Xiyang, and Zuoquan areas of the North China Craton （the NCC basalts hereafter） were analyzed for platinum-group elements （PGE, including Os, It, Ru, Rh, Pt, and Pd）. Most of them are alkaline basalts and tholeiites and all of them display little crustal contamination. The total PGE contents of the NCC basalts vary from 0.1 to 0.9 ppb, much lower than those of the primitive mantle values of 23.5 ppb. Primitive man- tie-normalized PGE patterns of these basalts define positive slopes and Pd/Ir ratios vary from 1.2 to 25. In terms of both PGE contents and Pd/Ir ratios, they are quite similar to the mid-ocean ridge basalts. There are no obvious negative correlations be- tween PGE vs. MgO, Ni, and Cu in the NCC basalts, indicating that fractional crystallization of olivine, pyroxene, and/or sul- fides during magmatic process cannot be the controlling factor for the observed PGE variation. The observed Pd/Ir variations of the NCC basalts require involvement of non-chondritic heterogeneous mantle sources. Based on Sr-Nd-Pb-Hf isotopic sys- tematics and incompatible-element signatures, a mixing of partial melts from both asthenospheric peridotites and enclosed mantle eclogites at the top of asthenosphere was proposed for the origin of these NCC basalts. The lenses of eclogites are de- rived from upwelling of recycled continental crust during the westward subduction of the Pacific plate from the -600 km dis- continuity zone. The PGE geochemistry of these basalts provides independent evidence to support this conclusion and the ob- served Pd/Ir variations may reflect variations in proportions of tapped peridotitic and eclogitic melts.

Mantle dynamics of the reactivating North China Craton:Constraints from the topographies of the 410-km and 660-km discontinuities

The topographies of the 410-km and 660-km discontinuities have important implications for mantle dynamics.Here we present high-resolution seismic imaging of the 410-km and 660-km discontinuities beneath the North China Craton (NCC) employing the receiver function method.Depth anomalies (deeper or shallower than the global average depths) at both discontinuities were detected by introducing a three-dimensional regional velocity model.The depressions of the 410-km discontinuity are mostly located in the eastern NCC.A local elevation of the 660-km discontinuity appeared in the northwest of the NCC and a significant depression of the 660-km discontinuity is located in the southeast of the NCC.Two dynamic mantle regimes are speculated to explain the formation of the anomalous depth zones in the NCC.One possibility is a complex mantle upwelling linked to edge-derived convection between the stagnant slab and the thick cratonic root.The other potential dynamic regime is slab stagnating,sinking,and induced upwelling at the neighboring slab front.These regimes hint that the mantle flow was possibly dominated by dynamic interactions among the subducting slab,cratonic root,and ambient mantle beneath the NCC.

Thermal convection thinning of the North China Craton: Numerical simulation

We used twodimensional numerical simulations to investigate smallscale convection in the upper mantlelithosphere system with depth and temperaturedependent viscosity. Our aim was to examine the mechanism of craton thinning by thermal con vection. The model domain is 700 km deep and 700 km wide with a resolution of 71x71 nodes and 160000 markers. The ve locity boundary conditions are freeslip along all the boundaries. A thermal insulation condition was applied at the two side walls, with constant temperatures for the top and bottom boundaries. We assumed an initial temperature of 273 K at the upper boundary and 1673 K at the lower boundary, and 1573 K at the bottom of the lithosphere （200 km depth） for the thick, cold, and stable North China Craton （NCC）. We calculated the thermal evolution in the upper mantle when the temperature at its bottom is raised because of lower mantle convection or plumes. The temperature at the bottom of the upper mantle was set at 1773, 1873, 1973, and 2073 K for different models to study the temperature effect on the lithospheric thinning processes. Our endmember calculations show that with the bottom boundary raising the lithosphere can be thinned from a depth of 200 km to a depth of between 100 and 126.25 km. The thinning rates are at mm/y order of magnitude, and the thinning timescale is about 10 Ma.