The Mesoproterozoic rifts are developed in the Ordos Basin located in the western margin of the North China Plate.Based on the latest 3D seismic data and previous research results,this study intends to discuss the zonal differential deformation characteristics and genetic mechanism of the Mesoproterozoic rifts in the Ordos Basin.NE-trending rifts are developed in the Mesoproterozoic in the south-central Ordos Basin,the main part of which are located near the western margin of the North China Plate.NNW-trending rifts are developed in the north of the basin,while NW-NNW rifts in the Mesoproterozoic in Hangjinqi area.The genetic mechanism of the Mesoproterozoic rifts is related to regional extensional stress field,plate boundary conditions and internal preexisting structures.The main extensional stress direction strikes NWW-SSE(120°)in the western margin of the North China Plate,based on the forward rift trend of the northern Mesoproterozoic.In Hangjinqi area,the reactivation of the existing NWtrending Wulansu fault and NW-NW-trending Daolao fault,results in dextral shear stress field.The boundary between the western margin of the North China Plate and its adjacent plates forms a nearly NS-trending preexisting basement tectonic belt,which intersects with the NWW-SSE(120°)extensional stress at an acute angle of 60°.Therefore,the western margin of the North China Plate is formed by oblique normal faults under oblique extension.Due to the long time span of Columbia Supercontinent breakup(1.8e1.6 Ga),the oblique rift in the south-central Ordos Basin is formed under the continuous oblique extension at the western margin of the North China Plate.
Meng LiXiangbin YanWei ZhangYuanling GuoChaoying LiuLingling FanShuang Yang
In situ zircon U-Pb geochronological and Lu-Hf isotope studies of detrital zircons from Late Mesoproterozoic to Early Neoproterozoic sedimentary units on the southwestern margin of the Yangtze Block have important implications for the tectonic evolution of the Yangtze Block.The Huili Group contains zircons whose ages are mainly Late Archean to Mesoproterozoic(2650-2450,2100-1800,and 1350-1150 Ma).The Dengxiangying Group has one major age population of 1900-1600 Ma,and two subordinate age populations of 1350-1100 and 2300-2000 Ma.Yanbian Group sedimentary rocks have a zircon age population mainly in the range of 970-850 Ma,contemporaneous with the ages of widespread arc-related magmatism in the western Yangtze Block.Combining these results with previous work,the Huili and Dengxiangying groups were most likely deposited during ca.1160 to 1000 Ma in an intra-continental rift basin setting,while the Yanbian Group accumulated during>920 to 782 Ma in a back-arc basin setting at the southwestern margin of the Yangtze Block.In addition,all these results further suggest a tectonic transition from a continental rift basin to a convergent environment at the southwestern margin of the Yangtze Block at 1000-970 Ma.
华北克拉通保存有与Columbia超大陆裂解相关的中元古代岩浆记录。本文综述了华北克拉通中元古代A型花岗岩的时空分布、岩石学和地球化学等特征,讨论了各期次A型花岗岩的成因及构造背景,论述了其对华北克拉通中元古代裂解事件的指示意义。华北克拉通主要发育4期A型花岗岩:1)~1.78 Ga A型花岗岩主要分布于南缘熊耳裂陷槽和晋陕地区;2)~1.70 Ga A型花岗岩主要分布于北缘渣尔泰—白云鄂博和燕辽裂陷槽;3)~1.60 Ga A型花岗岩主要分布于南缘熊耳裂陷槽;4)~1.32 Ga A型花岗岩主要分布于北缘渣尔泰—白云鄂博和燕辽裂陷槽。这4期A型花岗岩均来源于地壳物质的部分熔融,可能与同期伸展作用导致岩浆底侵提供热源有关。整体具有造山后伸展到陆内裂谷环境的演化序列,且存在明显的时空差异。华北克拉通中元古代A型花岗岩记录了4期主要的裂解事件,为华北克拉通参与了Columbia超大陆裂解提供了依据。
The black shale of the Mesoproterozoic Cuizhuang Formation in the Changcheng System in Yongji city,North China Craton,is a potential source rock.Understanding the organic matter enrichment mechanism is crucial for evaluating source rock resources and understanding oil and gas accumulation mechanisms.In this study,we evaluated the sedimentary paleoenvironment and organic matter enrichment mechanisms of shale using thin section observations,mineral composition analysis,organic geochemistry,and elemental geochemistry.We found significant differences in the sedimentary paleoenvironment and organic matter enrichment mechanisms between the lower Cuizhuang Formation and the Beidajian Formation shale.The Cuizhuang Formation was deposited in a late-stage,restricted basin environment during the rift phase,and elemental and geochemical indicators showed that the Cuizhuang Formation was in a suboxic-anoxic water environment,that was influenced by a warm and humid paleoclimate and submarine hydrothermal activities,which promoted the accumulation of organic matter.However,the enrichment of organic matter in the Cuizhuang Formation was mainly controlled by redox conditions.The formation of suboxic-anoxic water environments may be closely related to the warm and humid paleoclimate and submarine hydrothermal activities.Warm conditions promote continental weathering and increase marine productivity,thereby consuming oxygen in the bottom water.Moreover,acidic hydrothermal activity also helps to establish an anoxic environment.Our results reveal the effects controlling various coupled mechanisms dominated by redox conditions,which may explain the development of source rocks in the Cuizhuang Formation.
The Meso-neoproterozoic Bayan Obo rift is located along the northern margin of the North China Craton,and was associated with the break-up of the Columbia supercontinent.During rift evolution,syn-sedimentary deformation occurred due to tectonic activity and earthquakes.Seismic events are recorded in the Jianshan Formation of the Bayan Obo Group,Inner Mongolia,as soft sediment deformation structures in the central Bayan Obo rift.Druse calcite crystals and collapse breccias in the Jianshan Formation may provide information on the rift evolution.The druse calcite crystals are idiomorphic-columnar in shape and associated with graphite,pyrite,and quartz.δ^(13)C values of the graphite are−20‰,indicative of biogenic deoxygenation and formation in water.The druse calcite crystals are inorganic in origin and formed in water at a temperature of 55℃,based on calciteδ^(13)C andδ^(18)O data.The calcite grew in paleo-caves containing fault breccias,with heat derived from faulting.As such,the druse calcite crystals are important evidence for seismic events.The collapse breccias(i.e.,fault breccias)and other indicators of slip show that displacement occurred from NE to SW,which is different from the paleocurrent direction in the Jianshan Formation.The thickness of the collapse breccia is∼200 m,which represents the height of the fault scarp.The strike of the fault scarp was NE-SW,based on the distribution of the collapse breccia.The Bayan Obo and Yanliao rifts experienced rapid NW-SE extension,and developed similar deformation structures at ca.1.6 Ga related to break-up of the Columbia supercontinent.