Geochronology and Sr–Nd–Hf isotopic composition of the granites, enclaves, and dikes in the Karamay area, NW China: Insights into late Carboniferous crustal growth of West Junggar

New whole-rock major and trace elements, and zircon U–Pb and Hf–Nd isotope compositions are reported for the Karamay dikes, enclaves, and host granites in the West Junggar, NW China. Zircon U–Pb dating of the Karamay pluton yields an age of 300.7 ± 2.3 Ma for the enclave and 300.0 ± 2.6 Ma for the host granite, which was intruded by dike with an age of 298 Ma. The host granites exhibit relatively low SiO2 contents and A/CNK and Ga/Al ratios, low initial 87Sr/86Sr ratios (0.703421–0.703526) and positive εHf(t) (5.5–14.1) and εNd(t) (7.3–8.1) values with a young model age, suggesting that they are I-type granites and were mainly derived from a juvenile lower crustal source. The enclaves and dikes belong to an andesitic calc-alkaline series and have high MgO concentrations at low silica content and positive εHf(t) (7.6–13.2, 14.2–14.9) and εNd(t) (6.8–8.3, ∼6.9) values. They are enriched in LILEs (Rb, Ba and U) and LREE and depleted in HFSEs (Nb and Ta) with insignificant negative Eu anomalies, indicating that the melts were derived from an enriched lithospheric mantle modified by subducted oceanic crust-derived melts and minor fluids, followed by fractional crystallization. The Karamay host granites and enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and lithospheric mantle-derived magmas, and were intruded by the unmixed dikes subsequently. The upwelling mantle through a slab window in an island arc environment might have triggered partial melting of the lithospheric mantle and its subsequent interaction with the granitic magma, further suggesting that the ridge subduction played an important role in the crustal growth of West Junggar

Large-scale thrusting at the northern Junggar Basin since Cretaceous and its implications for the rejuvenation of the Central Asian Orogenic Belt

The Wulungu Depression is the northernmost first-order tectonic unit in the Junggar Basin. It can be divided into three sub-units: the Hongyan step-fault zone, the Suosuoquan sag and the Wulungu south slope. The Cenozoic strata in the basin are intact and Mesozoic–Cenozoic deformation can be observed in the Wulungu step-fault zone, so this is an ideal place to study the Mesozoic–Cenozoic deformation. By integration of fault-related folding theories, regional geology and drilling data, the strata of the Cretaceous–Paleogene systems are divided into small layers which are selected as the subjects of this research. The combination of the developing unconformity with existing growth strata makes it conceivable that faults on the step-fault zone have experienced different degrees of reactivation of movement since the Cretaceous. Evolutionary analyses of the small layers using 2D-Move software showed certain differences in the reactivation of different segments of the Wulungu Depression such as the timing of reactivation of thrusting, for which the reactivity time of the eastern segment was late compared with those of the western and middle segments. In addition the resurrection strength was similarly slightly different, with the shortening rate being higher in the western segment than in the other segments. Moreover, the thrust fault mechanism is basement-involved combined with triangle shear fold, for which a forward evolution model was proposed

Kinematic evolution of fold-and-thrust belts in the Yubei-Tangbei area: Implications for tectonic events in the southern Tarim Basin

The Yubei-Tangbei area in the southern Tarim Basin is one of the best-preserved Early Paleozoic northeast-southwest trending fold-and-thrust belts within this basin. This area is crucial for the exploration of primary hydrocarbon reservoirs in northwestern China. In this study, we constructed the structural geometric morphology of the Yubei-Tangbei area using geophysical logs, drilling, and recent two- and three-dimensional (2-D and 3-D) seismic data. The Early Paleozoic fault-propagation folds, the Tangnan triangle zone, fault-detachment folds, and trishear fault-propagation folds developed with the detachment of the Middle Cambrian gypsum–salt layer. According to a detailed chronostratigraphic framework, the growth strata in the Upper Ordovician–Lower Silurian layer formed by onlapping the back limb of the asymmetric fault-propagation folds, which therefore defines the timing of deformations. The changes in kink band hinges and amplitudes in the Permian–Carboniferous and Cenozoic folding strata suggest that the evolution of the fold-and-thrust belts followed a sequential evolution process rather than a simultaneous one. Above the pre-existing Precambrian basement structure, the Yubei-Tangbei fold-and-thrust belts can be divided into four tectonic evolution stages: Late Cambrian, Late Ordovician to Early Carboniferous, Carboniferous to Permian, and Cenozoic. The northwestern-verging Cherchen Fault is part of the piedmont fold-and-thrust system of the southern Tarim foreland basin. We interpreted its strata as a breakthrough trishear fault-propagation fold that developed in three phases: Mid–Late Ordovician, Silurian to Middle Devonian, and Triassic to present. These tectonic events are responses of the Altyn-Tagh and Kunlun collisional orogenic belts and the Indian-Eurasian collision. The inherited deformation and structural modification in the southern Tarim Basin may be an indicator of the growth and evolution of peripheral orogens

Kinematic classification, structural modeling and prospective fields of the foreland thrust belts in Midwest China

According to the distribution and transfer directions of thrust displacements under which the size, shape and pattern of thrust belt are controlled, the foreland thrust belts are classified into four types in middle and western China: (1) Type I, all the fault displacements in the thrust belt transfer from the mountain front to the basin along detachment. This type is the most popular in central and western China and can be divided into three subtypes: Kuche type, southwestern Sichuan type and Jiuquan type. (2) Type II, represented by southern Junggar Basin, in which there are fault displacements transferring to both basin and orogenic belt directions. (3) Type III, represented by Fusha thrust belt in southwestern Tarim Basin, in which all the fault displacements transfer toward the mountain front as deep structural wedges propagating toward the basin direction. (4) Type IV, represented by southwestern and northern Qaidam Basin, in which the transfer of fault displacements is restrained by the size, shape and boundary of the basin, and the stable foreland part and typical wedge sedimentary structure not produced. On basis of the above four types and the modeling of complicated structures, four new exploration areas are identified: the deep imbricate structural wedges in southern Junggar Basin, late Paleozoic passive continental margin sequence under the reverse Cambrian nappe in northwestern Sichuan, under-coal structures in middle and eastern Kuche, and footwall covered structures in northwestern Junggar Basin. Key words: foreland thrust belt, complex structural analysis, kinematic classification, structural modeling, restoration strain, Midwest Chin

Kinematics of syn-tectonic unconformities and implications for the tectonic evolution of the Hala'alat Mountains at the northwestern margin of the Junggar Basin, Central Asian Orogenic Belt

AbstractThe Hala'alat Mountains are located at the transition between the West Junggar and the Junggar Basin. In this area, rocks are Carboniferous, with younger strata above them that have been identified through well data and high-resolution 3D seismic profiles. Among these strata, seven unconformities are observed and distributed at the bases of: the Permian Jiamuhe Formation, the Permian Fengcheng Formation, the Triassic Baikouquan Formation, the Jurassic Badaowan Formation, the Jurassic Xishanyao Formation, the Cretaceous Tugulu Group and the Paleogene. On the basis of balanced sections, these unconformities are determined to have been formed by erosion of uplifts or rotated fault blocks primarily during the Mesozoic and Cenozoic. In conjunction with the currently understood tectonic background of the surrounding areas, the following conclusions are proposed: the unconformities at the bases of the Permian Jiamuhe and Fengcheng formations are most likely related to the subduction and closure of the Junggar Ocean during the late Carboniferous–early Permian; the unconformities at the bases of the Triassic Baikouquan and Jurassic Badaowan formations are closely related to the late Permian–Triassic Durbut sinistral slip fault; the unconformities at the bases of the middle Jurassic Xishanyao Formation and Cretaceous Tugulu Group may be related to reactivation of the Durbut dextral slip fault in the late Jurassic–early Cretaceous, and the unconformity that gives rise to the widely observed absence of the upper Cretaceous in the northern Junggar Basin may be closely related to large scale uplift. All of these geological phenomena indicate that the West Junggar was not calm in the Mesozoic and Cenozoic and that it experienced at least four periods of tectonic movement

Upper Palaeozoic gas accumulations of the Yimeng Uplift, Ordos Basin

This paper analyzes the Upper Palaeozoic gas accumulations and conditions of the study area by using the definition and method of petroleum system, and summarizes the natural gas accumulation pattern. The Upper Palaeozoic natural gas in the Yimeng Uplift is mainly from source rocks of Taiyuan and Shanxi Formations in the southern part and Wushenqi area. Braided channel sandbodies of alluvial fan, river channel sandbodies and distributary channel sandbodies of delta plain in the Shanxi and Xiashihezi Formations are the most favorable reservoirs. The thick mudstone layers of the Shangshihezi and Shiqianfeng Formations which cover the whole region constitute the regional seal rock, and there are also many local seal rocks. Oil source, reservoir and seal rocks form a good relationship in time and space and they form two types of assemblage which are the self-generating and self-preserving assemblage in the southern part of the Yimeng Uplift and the lower-generating and upper-preserving assemblage in the southern and northern parts. Natural gas migrated to the north through the migration pathways composed by advantageous sandbody, unconformity, faults and cracks. Structural traps and the distal accumulation pattern dominate in the northern part, while the southern part is characterized by lithologic traps and the proximal accumulation pattern. Key words: Yimeng Uplift, Upper Palaeozoic, geologic accumulation element, migration, trap, accumulation patter

Three-dimensional geometrical and kinematic characteristics of boundary faults in Minfeng subsag Dongying Sag, Bohai Bay Basin

Under the constraint of high-precision 3D seismic data and well data, through 2D and 3D analysis, the geometrical and kinematic characteristics of the boundary faults of Minfeng subsag, Jiyang depression, Bohai Bay Basin were analyzed. According to the strike, dip and dip-angle, the boundary fault of the study area is divided into three segments, i.e. west, center and east. It is thought that the fault surface is a curved surface consisting of several inclined surfaces linked by axial plane (i.e. transversal axial plane and vertical axial plane), and a three-dimensional geometry model of the boundary fault was built. The Minfeng boundary fault surface is divided into 11 subzones by 6 transversal axial planes and 4 vertical axial planes. The kinematic characteristics of the Minfeng boundary fault were analyzed on the basis of the normal fault-bend folds theory, and the three balanced cross-sections in west, central and east segments are compared. It is concluded that the Minfeng boundary fault mainly experienced rotary motion in its early stage of development, and moved in the inclined shear fault-bend fold pattern in the late stage. There developed listric fault in the central segement, seat type normal faults in east and west segments of the Minfeng boundary fault. The activity intensity of west and central faults was affected by Shengbei fault, the rotation degree of Minfeng boundary fault weakens from west to east. The reverse degree of the Minfeng boundary fault in creased from west to east at the end of the Paleogene Dongying Formation deposition. Key words: Dongying Sag, Minfeng subsag, boundary fault, 3D, geometrical features, kinematics, fault-bend fold, balanced cross- sectio

Distribution of Carboniferous source rocks and petroleum systems in the Junggar Basin

With pre-Cambrian continental nuclei and Early Paleozoic accretionary folded belts as the collaged basement, the Junggar Basin underwent an evolution of intra-cratonic basin after the transitional period of Carboniferous to Permian. The Carboniferous Period was a critical phase for the cratonization of Junggar Terrane, with two continental passive margins developed on both the southern and northern margins, and with rifts and upswells in the basin interior. The rifts were full-filled with volcanic clastic rocks up to 2 000 to 5 000 m in thickness. Influenced by the marine transgression, the regional extended source-rocks of the Lower Carboniferous Dishuiquan Formation(C1d) and the Upper Carboniferous Batamayneishan Formation(C2b) occurred within or around the basin, with the former one as oil and gas source rocks, while the latter one as effective gas source rocks. Volcanic rocks in the Batamayneishan Formation are the main reservoirs. Two petroleum systems were formed: C1d-C (.) and C2b-C2b (!). The tectonic events in the Late Jurassic and Neogene gave rise to the formation, modification, and finalizing of the Carboniferous prospects. The difference between the evolutionary process of each structural unit resulted in the differentiation of the petroleum systems horizontally. A series of oil and gas fields have been discovered in Carboniferous, such as the Wucaiwan, Shixi, Kelameili, and Chepaizi oil and gas fields as well as the oil-bearing Block-2, 4, and 6 in the Karamay Oilfield. The exploration has proven that the Carboniferous has great potential for oil and gas discovery. 摘 要: 准噶尔盆地是在前寒武纪陆核和早古生代增生褶皱带的拼合基底之上经历石炭纪—二叠纪过渡阶段后形成的克拉通内坳陷盆地，石炭纪是准噶尔及邻区克拉通化过渡发展的关键时期，在准噶尔地块南、北缘发育了被动大陆边缘，地块内部断陷与凸起相间，断陷内沉积了厚度达2 000～5 000 m的火山碎屑岩系。受海侵范围控制，盆地内部及周缘发育区域展布的石炭系下统滴水泉组和上统巴塔玛依内山组两套有效烃源岩，前者可能是油、气烃源岩，后者是有效的气源岩，上石炭统巴塔玛依内山组火山岩是主要的储集体，由此形成了C1d—C(.)和C2b—C2b(!)两个含油气系统。中侏罗世晚期—晚侏罗世和新近纪以来的构造活动导致了石炭系圈闭的形成、改造与最终定型。不同构造单元后期演化历史的差异导致石炭系含油气系统在平面上产生分异。迄今在石炭系已发现了五彩湾、石西、克拉美丽、克拉玛依二/四/六井区、车排子等多个油气田，油气勘探实践表明准噶尔盆地石炭系具有较好的勘探前景。图8表3参41 Key words: Carboniferous, hydrocarbon source rock, volcanic rocks, petroleum system, Junggar Basi