A Kinematic Model And Dynamic Cause Of Quaternary Tectonic Movement Of Southeastern Coastal Basins In Fujian Province

Present-day tectonic activities are obvious and intensive in the eastern coast area of Fujian province, mainland of China. Many researchers have studied Quaternary tectonics of coast areas of Fujan province. These studies involve activity of faults, earthquakes and the relationship between them; sediments, tectonics and earthquake activity in coastal basins; and relationship of coastal sediment to crustal vertical movement or to sea level change.The tectonics of coastal area of southeastern Fujian is controlled by two groups of faults: northeast and northwest strike faults. The northeast strike faults are the Zhenghe-Dapu fault zone, Changle-Zhao'an fault zone and Seashore fault zone. The northwest strike faults are the Hanjiang fault zone, Jiulongjiang fault zone, Yong'an-Jinjiang fault zone, Xinghuawan fault zone and Minjiang fault zone. The Changle-Zhao'an fault zone is an important great deep fault zone, it was active mainly before mid Pleistocene and has been weakened since late Pleistocene. The Changle-Zhao'an fault zone has some present earthquakes, but its intensity is weaker than the Seashore fault zone either in space-density or in time-frequency. The Seashore fault zone is a great deep marine fault zone and may be the west-boundary fault of the Taiwan strait which has been subsided greatly since Cenozoic. This fault zone is the most important present earthquake active zone. The northwest strike fault zones are a group of newer and more active sinistral tensile fault zones. Their depths may be less than the northeast trending fault zones, but nearly has cut all the northeast strike fault zones. This fault groups are distributed regularly (equal strike direction and distance), it's intensity of earthquake activity increases southwards, and the strong earthquakes took place at the cross site of the northeast strike fault zones and northwest strike fault zones generally.Geologically, Yanshan granite, Jurassic and Cretaceous terrestrial-pyroclastic tephra sedimentary formation are dominant in southeastern coastal areas of Fujian province. Quaternary sediments are distributed in eastern coastal basins and plains. There are magmatic and granitization dynamometamorphic rocks along the Changle-Zhao'an fault zone. Basalt younger than Neogene appears in southern part of the Changle-Zhao'an fault zone in Zhangpu and Longhai city. The volcano was active in late Pleistocene.In the Fujian coast area, the Cenozoic sediment is shallow, mainly is Quaternary. Pleistocene is divided into lower Pleistocene Tianbao formation, middle Pleistocene Tong'an formation and upper Pleistocene Longhai and Dongshan formation. Holocene is Changle formation mainly. In this area the Quaternary sedimentation is controlled by the Changle fault zone and the northwest strike fault group, the sediment is mainly located in the Fuzhou basin, Changle plain, Putian plain, Quanzhou basin, Tong'an plain, Zhangzhou basin and Longhai plain. These basins are located at the cross sites of the Changle fault zone and the northwest strike fault mainly.There is a lot of research achievements about Quaternary basins and tectonics in the Fujian coastal area. But there is less profound research about formation and tectonic development of basins, evolvement of big regional Quaternary tectonics and the dynamical model of this area. For deeper studying the faults and their activity in this area, recently the projects of active faults detection in big cities and earthquake possibility assessment have been carried out successively in Fuzhou, Quanzhou, Zhangzhou and Xiamen cities recently. Using the information of these data and correlative results of other studies in the Fujian coastal area, Taiwan strait and other regions, combining the author's exploration of faults in the cities, this thesis analyzes synthetically tectonics and its movement of the Quaternary basins in the southeastern coastal area of Fujian province and has acquired more understanding of Quaternary tectonic movement and its dynamic cause of this region.The progresses of the research are as follows:1. Studying methodBy studying symbolic strata of ancient sea level, analyzing their geochron and elevation, comparing strata of ingression of basins, this thesis has determined the basins' tectonic subsidence.Summarizing a lot of study achievements of sediment, tectonics and space-time relation of tectonic activity in the area, comprehending the international progress of this study field, according to present data and the character of the basins' sediment, the thesis proposes the depth-time (D-T) method which analyzes basins using drilling data. Applying this method we can efficiently and visually assess activity of faults of basins and analyze the tectonic activity of basins at different locations and times in Quaternary.2. Analysis of basin structureThe historic earthquakes and the most current small earthquakes are located along two groups of deep fault zones that indicate the fault zones are active. The deep action of these faults makes decline of surface and is important cause of formation of basins in the Fujian eastern coast area. According the geomorphology of the basins, the movement of historical sea level and the strata of ingression in this area, the thesis has affirmed tectonic attribute of these basins and illustrated the phasic development of these basins: from early fault subsidence to later Pleistocene down-warped sedimentation.According to the drilling data and geochronometry data of active fault detection project in all cities of this area, the paper analyzes and compares depth-time relation of each drilling data (D-T analysis), judges fault activity and sedimentation behaviors in the basins and determines the development trend of basin tectonics.(1) Faults in basinsIn the northwest trending basins (Fuzhou, Quanzhou and Zhangzhou basins) in the mainland, all faults are steep sloped and dipping to centers of basins. The northwest strike faults, which are more active, are located at the deepest portions of basins and at southwest or centers of basins.In the Fuzhou basin the Minhou-Nanyu fault is most possible to produce medium-major earthquakes and is relatively unsafe, Wuhushan fault is secondly. Other faults in the basin are generally safe. The two relatively unsafe faults lie at southwest of the basin.In the Quanzhou basin the Sijiao-Jinjiang fault was active in late Pleistocene. According to the sediment D-T analysis the thesis considers the Zimaoshan-Wushishan fault was active in late Pleistocene. The Sijiao-Jinjiang fault is located at the center of the Quanzhou basin. The Zimaoshan-Wushishan fault lies at southwest of the basin. Other faults were active in early Quaternary or earlier.In the Zhangzhou basin the Gutang-Dameixi fault and north-Jiulongjiang fault were active in late Pleistocene, are relatively unsafe faults. The North-Jiulongjiang fault is northwestern strike in southwest of Zhangzhou urban subarea. The Daishanyan-Hangkeng fault in southwest of the basin is more active than the Zhukeng fault in the Hangkeng-Tianbao subarea of the Zhangzhou basin.On the Xiamen island, the Yuandanggang fault in the northeastern strike fault basin was active in the early stages of late Pleistocene, other faults were active earlier.(2) Sediment in basinsThe sediment in these basins is newer than Pleistocene generally. The sediment D-T analysis of the northwest trending basins in the mainland reveals that the sedimentary center of basins has moved from northeast to southwest.The erosion maybe took place in recent hundreds of years in northeastern part of the Fuzhou basin; the place where the sedimentary speed is higher had moved southwestward during the time 40ka to 8ka. The sedimentary center of the Fuzhou basin has moved from northeast to southwest.In northeastern part of the Quanzhou basin the sediment is earliest (about 80 ka). In 30ka the sediment occurred in middle part of the basin which became the sedimentary center of the basin then. In recent thousands years the sediment in southwestern part of the basin has became speedy, and has be most speedy. The sedimentary center of the basin has moved from northeast to southwest since formation of the basin.In the Zhangzhou basin, the sediment was earlier in northeastern part of Hangkeng-Tianbao subarea, but later its sedimentary speed was lower than in southwestern part of the subarea. Thus the sedimentary center of the basin was in northeastern part early, then moved southwards, had arrived at southwest of the subarea. In the Zhangzhou urban subarea the sediment has had the trend of moving southwards likewise.The sediment of the Yuandanggang basin on the Xiamen Island has had the trend of moving southwards too.According to synthetic analysis of faults and sediments of these basins, the conclusions are as follows: The three northwestern trending basins in the mainland are northwestern strike fault basins; the more active northwest strike faults are located at the center or southwest of basins (i.e. the deepest place); the sedimentary centers of basins have moved from northeast to southwest. The basins are tilting toward southwest.3. The model of regional tectonic movementThe model of Quaternary tectonic movement of southeastern coastal areas of Fujian province is established according toâ‘ basins' property of tilting to southwest shown by the faults and sediment in mainland basins of this area,â‘¡the serial events of sediment in these basins andâ‘¢regional crustal structure.The drilling-exploration and geochronology studies of these basins in Fujian province and other collected data in Chaoshan plain and Zhujiang delta of Guangdong province show the following features.The initial Quaternary sediment in the inland area is older than in the coastal areain the Xiamen island older than in Longhai plain and in Longhai plain older than in Zhangzhou basin. In the mainland area, the initial Quaternary sediment in the Zhangzhou basin is oldest, it becomes newer gradually respectively in two directions to northeast and to southwest _ it is the oldest in Xiamen-Zhangzhou basin and it is the newest in the Fuzhou basin and in the Zhujiang delta. Other sedimentary events of mainland basins in Fujian coastal areas are newer gradually northeastward.Using the result of deep seismic prospectings in this region, the model of regional crustal structure is established. The model shows the following features. The crust in this region is relatively thin. There is a low velocity layer in the bottom of the upper crust. The undersurface of the low velocity layer was upheaved notably around the Seashore faults zone and Taiwan strait, was upheaved around Jiulongjiang faults zone too, was upheaved partially around Quanzhou city. The depth image of superface of upper mantle is like this, but the upheaval is more apparent around the Jiulongjiang fault zone and Yong'an-Jinjiang fault zone.According to the study of tectonic movement trend of these basins, comparing serial sediment in the basins and analyzing crust structure of this region, the model of regional tectonic movement is established:In early Quaternary or pre-Quaternary, a northeastern tension made the upper mantle at southwest to Zhangzhou upheaved, then the tension expanded northeastward, the two groups of faults developed, then the Zhangzhou basin, Quanzhou basin and Fuzhou basin had formed successively; the tension expanded southwestward, then the Chaoshan plain and Zhujiang delta had formed successively too. The tension developed continually and had shaped the Quaternary structural pattern of southeastern coastal area of Fujian province.4. Regional tectonic dynamics analysisAnalyzing the manner of Quaternary movement of the adjacent West Pacific Plate and Philippine Plate carefully (mainly tectonic movement of the west edge of the West Philippine Plate), studying the structure and tectonic movement of the adjacent marine area, Taiwan area and Philippine sea area, discussing the influence of the South Chinese Sea Plate on structure of this region, the paper suggests a reasonable dynamic explanation of tectonic movement of this region.The analysis of evolution of the Pacific basin shows that subduction of the Kula Plate towards NNW , subduction of two oceanic ridges of Pacific Plate under southeastern edge of the Euro-Asia Plate step by step, change of movement direction of Pacific Plate and subduction of Philippine Plate westward _ these important events have effects on formation and evolution of structure of the Fujian-Taiwan area.The Philippine Plate has moved northwestward. Its north edge has subducted under the Ryukyu trench. Its west edge has contacted the Euro-Asia Plate in the Taiwan collision by left-lateral strike slip faults and has contacted South China Sea Plate by the Manila trench. In the Manila trench the South China Sea Plate has subducted under the Philippine Plate. The cooperating action of these two sub-plate has made left-lateral tension around the Bashi strait certainly and has made Bashi tectonic system.Under northwestern compression of Philippine Plate the Fujian eastern costal and seashore areas show uplifted and tension movement. Movement of the Philippine Plate has made left-lateral tension on the Bashi tectonic system. The Bashi tectonic system extends to southwest of Zhangzhou city, is a trigger site of tectonic movement of coastal and circumlittoral areas of Fujian province. The tension stretches from the Zhangzhou-Xiamen area northeastward and southwestward and makes sediment and vertical movement of all basins in the southeastern coast of Fujian province. The movement of the Philippine Plate is rhythmic, so the left-lateral tension made by the movement is multi-episode, the tectonic movement of basins in the southeastern coast area of Fujian province is multi-episode too5. The explanation by multi-layer structure of lithosphereThe causes of crustal extension and fracture are analyzed and discussed by the multi-layer lithosphere tectonic theory. The Quaternary structural movement of this area is explained by the theory.Upper layer of lithosphere (upper crust) is brittle, or mid-brittle. The brittle fracture is the deformation style of rock in upper lithosphere mainly, where fault ruptures and earthquakes happen. The lower layer of lithosphere (lower crust and upper mantle) is ductile, mid-ductile layer or which thickness is bigger than upper layer. The lower layer of lithosphere is the layer which bears the stress and through which the stress translates. Deformation of the lower layer is plastic creep mainly. When lithosphere is subjected to the tension along the Bashi structure, tensional deformation shown by lithosphere deformation spreads from forced end to another end, and the continuous tension formed groups of normal faults in this region. This is a possible cause of deep structure for ordinal development of Quaternary basins in the study area from the Jiulongjiang fault zone in two directions.