Research On Relationship Between Tectonic Evolution Of The Tan-Lu Fault Zone And Adjacent Basins

The Tan-Lu fault zone is a large-scale strike-slip fault zone and important geological boundary striking NNE in East China, which extends about 2400 km. More than fifty years studies on the large fault zone suggest that it has experienced polyphase and complicated evolution history since Mesozoic , and is a complex geological body which has strike-slip, extensional and compressive behaviors on different segments during different geological stages. On the one hand the active stage and regime of the fault zone was controlled by the activities of deep structure and the regional tectonic setting, but on the other hand the active stage and regime significantly has modified or affected the evolution of its adjacent structures, and has important effects on the tectonic subsidence, sedimentary filling and petroleum geological conditions in the adjacent Meso-Cenozoic basins. There are many Meso-Cenozoic basins adjacent to the Tan-Lu fault zone, including basins rich in hydrocarbon, such as the Bohai Bay Basin and the Subei Basin, also including basins without oil and gas, such as the Jiaolai Basin and the Southern North China Basin. Therefore, putting the Tan-Lu fault zone and its adjacent basins, which is independent in time, space on each other, under the same tectonic system to research their evolution relationship in the macro level, we can understand the effects of the Tan-Lu fault zone on the formation, evolution as well as petroleum generation, migration and accumulation of its adjacent sedimentary basins, and grasp the law of geological conditions and the petroleum enrichment, which is meaningful for targeting new strata, optimizing favorable exploration blocks in the eastern old oilfields.On the basis of the field geology research and laboratory analysis, the dissertation systematically studied the structural features, the formation temperature and pressure environment, and the formation time of the late sinistral shear belt along the Tan-Lu fault zone. The structure characteristics, such as the microstructures of mylonite, the outcrop-scale S-C structures, the rotation porphyroblasts, indicates the late shear belt formed due to the sinistral activities of the Tan-Lu fault zone. The newformed mineral assemblage, deformation behaviors in mylonite, and muscovite electron probe test results show that the formation temperatures of the late shear belt is 400-500℃, and the formation pressure is low pressure environment; Through two muscovite, 6 biotite ⁴⁰Ar/³⁹Ar dating, this study obtained a representative age (149.8±0.9 Ma) which was the cooling age of the late shear belt due to the sinistral activity of the Tan-Lu fault zone, that is, the second sinistral activity of the Tan-Lu fault zone occurred at the Late Jurassic and before 150Ma. Timing of magmatic activities in North China in the Early Cretaceous shows that the earliest age of magmatic activities is 135Ma (found in the Qingshan Formaition in the Mengyi basin) and 133Ma (found in the Huangshiba Formation along the Tan-Lu fault zone), and the peak time of magmatic eruption is 120-110Ma, which was widely distributed in most rift basins. The above magmatic activities indicate that both the Tan-Lu fault zone and the eastern North China Craton were involved into intensively extensional faulted events in the Early Cretaceous due to a tensile tectonic setting. According to the stratigraphic sequence of Cretaceous in the eastern North China craton, before the large-scale volcanic eruption represented by the Qingshan Formation, the rift basins were filled with a set of continental sediments including the Laiyang Group, the Mengyin Formation and the Zhuxiang Formation, etc. Some rift basins, such as the Hefei and Jiaolai basins, showed the half-graben or graben structures controlled by the Tan-Lu fault zone, and the stratigraphic thickness increases markedly near the boundary normal faults, thus the stratigraphic age of the faulted basins indicates the extensional activities of the Tan-Lu fault zone continued to Paleogene; In addition, the formation time of metamorphic core complex distributed generally in the eastern North China Craton, concentrates in the earliest(145 Ma) to middle (116 Ma) Early Cretaceous, and indicates the extensional activities of the Tan-Lu fault zone occurred at the beginning of the Early Cretaceous. Therefore, the evidence, including magmatic activities in the eastern North China Craton, faulted basin development and the formation of metamorphic core complexes and so on, demonstrates that the extensional activities of the Tan-Lu fault zone started at the beginning of the Early Cretaceous and continued to Paleogene.Combined with previous results and based on the fault slip data from field work along the Tan-Lu fault zone and adjacent areas, we analyzed the regional stress fields, and summarize stress field evolution since Late Jurassic. This study divides the stress field evolution into 6 stages, and focused on 3 stages of tectonic stress field evolution and their relationship with the evolution of the basins. Three stages are the compression in Late Jurassic, the regional extension and rifting in Early Cretaceous, and regional extension with a strike-slip component along the Tan-Lu fault zone during Late Cretaceous to Paleogene.The study established the relationship between activities of the Tan-Lu fault zone and its adjacent basin development since Late Jurassic, through analyzing sedimentary filling, regional tectonics and stress fields. The relation of activities between the fault zone and its adjacent tectonics can be divided into 4 stages: (1) During the Late Jurassic period, the high-speed, oblique subduction of the Pacific ocean plate occurred underneath the eastern Eurasia plate amrgin, which caused the eastern China in a sinistral transpression stress field and hiatus of Late Jurassic deposition. Sinistral motion took place along the Tan-Lu fault zone in the Late Jurassic; (2) During the Early Cretaceous, the activities of the Tan-Lu fault zone changed into extension, leading to devilment of rift basins along the Tan-Lu fault zone and its adjacent areas in eastern China. According to the isopach maps of the residual strata, normal faulting along the Tan-Lu fault zone resulted in the sedimentary centers obviously around the Tan-Lu fault zone when the Lower Cretaceous strata were deposited while the sedimentary centers of the Upper Cretaceous strata migrated to the east-west trending, normal faults flanking the Tan-Lu fault zone; (3) Bohai Bay Basin was the intense rifted area in Paleogene, followed by the Subei Basin. The near north-south extension event, combined with the right-lateral slip pull-apart between the Tan-Lu fault zone and the frontal fault of the Taihang Mountains, induced the intensive extension of the Bohai Bay Basin. Under the control by both the near norh-south extension and the right strike-slip pull-apart, the sedimentary centers in the Bohai Bay Basin in Paleogene, migrated from south to north and from west to east regularly. In the end of Paleogene, the sedimentary center migrated to the central part of the Bohai sea where the Tan-Lu fault zone passed through; (4) The extensional activities ended in the study area in the Early Neogene due to the change of regional stress field from extension to compression. As a response to the change of stress field, the inversion widely happened in the faulted basins with termination for most of the rift basins due to uplifting and survival for other basins into depression-type basins (such as the Bohai Bay Basin, Subei Basin) and filled with draping sediments. According to the isopach maps of the Neogene, the Bohai sea segment of the Tan-Lu fault zone was the center of the depression deposition.Focusing on the structural analysis in the Bohai Bay Basin in Cenozoic, this study analyzed Paleogene extensional activities, and compared tectonic subsidence in different basins or sags adjacent to the Tan-Lu fault zone. The results show that the main extension direction is north-south in different sags of the Bohai Bay region during Cenozoic, and extensional activities occurred mainly in the deposition period from the Kongdian Formation to Member 3 of the Shahejie Formation. From the Early Paleogene to the Late Paleogene, the largest extensional quantity, extensional rate, and extensional velocity migrate from both north and south ends in the study area to the Central Bohai. Moreover, the farther from the Tan-Lu fault zone, the smaller subsidence quantity of the depression is, that is, the depressions (such as the Changwei, Jiyang, Bozhong, Liaodong depression) within and near the Tan-Lu fault zone have a larger subsidence quantity, and the depressions far away from the Tan - Lu fault zone (the Linqing, Jizhong depression, etc.) have a smaller subsidence quantity. The above phenomenon shows that the Tan-Lu fault zone is a strong rifting zone during the extensional activities, and controls the development of the strong rifting region.On the basis of researches on the tectonic activities in the eastern North China Craton since Early Cretaceous, combined with the results from previous studies on deep structure, the tectonic subsidence characteristics of the basins adjacent to the Tan-Lu fault zone and its relationship with deep structures were discussed in this study. The main tectonic subsidence centers of the basins in the study area focused on the southern and northern areas of the eastern North China Craton in Early Cretaceous, and in Paleogene the subsidence centers moved to the Bohai Bay basin area within the eastern North China Craton, thus the subsidence centers migrated from the margin to the internal of the eastern North China Craton with the changes in geological ages since Early Cretaceous. The research results shows that tectonic subsidence centers of the basins were mainly controlled by the structures and temperatures of lithosphere while the development characteristics of the rift basins adjacent to the Tan-Lu fault zone since Early Cretaceous were controlled by the four geological factors, namely, thermal state and strength of lithosphere, lithosphere structure, regional extension stress direction and dynamic mechanism.The study systematically analyzed the organic geochemical characteristics and the spatial distribution characteristics of Paleogene source rocks in the basins adjacent to the Tan-Lu fault zone. There are five hydrocarbon source rocks for Eogene, including the Kongdian Formation, Member1, 3 and Member 4 of the Shahejie Formation, and the Dongying Formation, Of which source rocks for the Kongdian Formation and Member 3 of the Shahejie Formation are the major source rocks in the study area, and the geologic ages of the source rocks progressively become younger from south to north. The migration of tectonic subsidence centers controls the source rock development and thicknesses, and the sedimentary fades in the basins obviously controlled organic abundance and type of the source rocks, thus the extending of the Tan-Lu fault zone and tectonic subsidence center migration of the adjacent basins have great effect on quality of source rocks. The extensional activities along the Tan-Lu fault zone in Cretaceous make sedimentary centers in the adjacent basins migrated to the nearby of the Tan-Lu fault zone., such as deep lake -semi-deep lacustrine sedimentary environment mainly appearing near the Tan-Lu fault zone where is suitable for high-quality source rock development. Therefore, the Huming and zhanhua sags in the Jiyang Depression, and the center of the Laiyang sag in the Jiaolai Basin are the favorable exploration areas of Mesozoic exploration. The Weibei, Dongying and Huimin sags in the south of the Bohai bay basin are the favorable exploration areas for the Kongdian Formation and Member 4 of the Shahejie Formation.