| Kongnan region in Huanghua depression is one of the areas that earliest deposited Cenozoic sediments in Bohai Bay basin. This region develops complete Palaeogene and Neogene filling sequences, and different tectonic deformation were investigated in the past decades, such as extension, strike-skip and inversion structures. These structures recorded the whole Cenozoic tectonic evolution of the central region of Bohai Bay basin. Based on the earlier released2D seismic and drilling data, previous studies provided us comprehensive understanding of sedimentary system, sequence stratigraphy, structure and their evolution. However, structure and tectonics in Kongnan region and the relationship between strike-slip and extension structures in Lanliao fault zone at central area of Bohai Bay basin should be detail studied.The whole research was guided by the theory of the basin dynamic, structure analyses and tectostratigraphy. Base on drilling, logging data and newly released high-resolution3D seismic data which cover the whole Kongnan region, we synthetically studied the Cenozoic deforming framework and evolution of the basin combined with the method of low-temperature thermochronology. Meanwhile, the quantitative analysis was used to determined tectonic evolution of the main fault systems and reconstructed the ancient structure frameworks at different geological periods of the Huanghua depression. Finally, based on the geological background of Bohai Bay basin, the evolution and deformation at the different periods were analyzed and reconstructed. And the tectonic dynamics and the deforming mechanism of the Kongnan region and central areas of Bohai Bay basin, was discussed in the background of surrounding plates and deep earth.This paper is comprised of six chapters:The first chapter mainly discussed the purposes and significance of this thesis, summarized the recent international frontier researches and futher study of extensional basin with strike-slip faulting. Based on the previous study on tectonics in Kongnan region, we utilized some new theories and methods about sedimentary basin dynamic analysis and numerical modelling, structure analysis and low-temperature thermochronology. By means of interdisciplinary study, tectonic evolution of Cenozoic stratigraphy in Kongnan region has been studied in detail. And we demonstrated the feasibility and significance of this research. According to our workflow, we present the research contents, methods and the technique in our thesis. In general,in order to reconstruct the tectonic deformation patterns of Cenozoic stratigraphy of Kongnan region, we use the method of structural analysis, tectonic reconstruction and basin dynamic. Cenozoic tectonic evolution of Kongnan region has been studied in our thesis. Then using the method of low-temperature thermochronological analysis, great tectonic framework changes occurred has been confirmed. In all, synthesizing the research results, we analyzed the tectonic evolution of Cenozoic stratigraphy in the central area of Bohai Bay basin and its dynamic background.The second chapter introduced the basic geology of Kongnan region, Huanghua depression, including geography, tectonism, basin basement, deep structure of lithosphere and Proterozoic-Cenozoic fillings. Kongnan region is located to the south of Huanghua depression, and it is a NNE trumpet-shaped trending basin. Kongnan region is in the southern of Huanghua depression. To the north of Qikou region, it is Shenqiongzhuang-kongdian-yangshanmu buried hill. To the south, it is Linqing depression and Wuqiao sag. The basin boundary faults are Chengxi fault to the east, and Cangdong fault to the west respectively.. Our study area is on the intersection of Taihang basement and Luxi basement. Its lithosphere has obvious stratification, and the mantle upwelling towad the lithosphere strongly. This basin had been experienced4stages of tectonic movements since Proterozoic, and the basin trap4suits of sediments overlying the basement. They are Ek, Es, Ed, Ng, Nm, Qp, which are our study sequence in this study.The third chapter recognized the main Cenozoic geological surfaces and tracked the main structure sequence interfaces of the Huanghua depression, based on the lithology of well logs, the changes of well log curves, and the reflection structure and their termination styles of seismic events, especially the boundary of Upper Ekl-Ek1Ⅲ which has a great tectonic significance.. Then, by studying of the high-resolution3D seismic profiles, we recognized and tracked the Upper Ekl-EklⅢ, and summarized the identification of this surface. EklⅢ Winly presents onlap or downlapon seismic profiles. As the western and eastern part of underlayer was truncated by EklⅢ, Lower Ekl presents reduction from central thick part to the sides; while, above of the EklⅢ, Upper Ekl appears as onlap to EklⅢ, and it shows a standard fault depression lack basin with wedge-shaped strata. What is more, in the wedge-shaped strata near the edge of the fault, seismic reflection was characterized by chaotic reflections and poor continuity. Those features indicate that the upper and lower strata of the EklⅢ have different structure settings. Therefore, this surface has an attibute of tectonic change. In the study, we also tracked other structural sequence boundaries such as Ek3, Es3and Ng, as well as the Pre-Cenozoic sequence boundaries such as C,O and Mz in Kongnan region..After confirmed the main sequence boundaries in Kongnan region, three negative structure units (Cangdong sag, Changzhuang sag, Nanpi sag) and four positive structural units (Kongdian buried hill tectonic belt, Cangshi nose-like structure belt, Xiaoji-Duanliubo fault structure belt and Dengmingsi-Wumaying fault structure belt) were divided. On the basis of the complicated characteristics of the positive structural units in Kongnan region, according to differences in the structure and formation mechanism of this second-class structural unit, the positive structural units were further divided. Kongdian buried hill tectonic belt was further divided into Kongdong, Konglei, Kongxi and Zilaitun-Junmazhan fault structural belts. Dengmingsi-Wumaying fault structural belt was further subdivided into Wumaying, Wumaying-Dengmingsi and Dengmingsi fault structure belt. Xiaoji-Duanliubo fault structure belt was further classified to Xiaoji and Duanliubo fault structural belts. Cangshi nose-like structure belt was further divided to Shenvsi, Dianzi and Yesanbo nose-like structure belts. In addition, this dissertation described basin structure styles and tectonic-stratigraphic basin framework of different tectonic units, and demonstrated that the main cause of the basin structural change in Kongnan region is the active difference between Cangdong and Xuxi faults.The forth chapter analyzed the structure in Kongnan region from the geometry. Then, it described levels, dominate strikes and their geometrical morphology of faults in detail., and recognized the prototype structures, those are extensional structural style, strike-slip structural style, buried hill structural style and igneous rock structural style. After that, we analyzed the activity of fault from the kinematics. According to the statistics of the original gap between each fault from three levels, we confirmed intense active stage of the basin-controlling fault started from Upper Ek1, and the growth of the Kongdong fault was caused by the differential settlement of the basements which controlled by Cangdong and Xuxi faults. Most of the third level faults activity starts from Es3, and during Esl activity stage, fault activity had been constantly enhanced from the center to north and south. Finally, by back stripping analysis technique, combing the fault distribution and its residual strata distribution of each period, paleostructure framework was reconstructed. It shows that the tectonic evolution in Cenozoic of Kongnan region could be divide into depression, rift and strike-slip stretch episodes. During depression episode, there is one subsidence center in the basin, and the deposition has no strong orient, which is a result of the whole depression subsiding. During rifting episode, parallel arrangement of the buried hill uplift and eastern-western depression shows basin was controlled by the NW-SE extensional structure. In strike-slip stretching episode, uplift-depression units and faults showed left-step oblique distribution, which means the tectonic movement was controlled by NNW-SSE extensional stress at this episode, and the whole basin represented an attribute of oblique stretch. According to the calculated of the stretching estimates from the typical structure section, basin stretching rate was minimum in depression episode, and it increased during rifting episode, while decreased in strike-slip episode.The fifth chapter discussed low temperature thermochronology analysis on Kongnan district, multi-sediment provenance of the basin was revealed by the apatite fission track analysis of Kongnan region. And apatite fission track length distribution shows that most samples within the basin experienced a complex thermal event superposition. Age decomposition of apatite showed that the spatial distribution of apatite fission has mixed old and new stratigraphic age, which indicating that the basin showed little gap with the peripheral area and was a lake depression basin in the subsegmental period of Ek2and low Ekl, the denudation rate of the source area was small and the debris were transported for a long time. This paper considered that there did not occur to a significant uplift process in Kongdian burial hill and it maintained a relatively stable state in the top of the Kongdian burial hill during Cenozoic according to the differences in different regions of Kongdian burial hill presented by apatite fission track age. The bulge morphology of Kongdian buried hill we observed today was, actually, the result of the descent of two wings of Kongdian buried hill which caused relative structural uplift, the same structural form also generated with the deformation process of buried hill in Ek2formation. We accounted that the tectonic evolution of Kongnan district was likely to be impacted by the movement of Pacific plate on the basis of dividing the apatite fission track ages into six groups and comparative analyzing them with the major syntaxes events of the Pacific plate and the Eurasian plate.The sixth chapter considered that the tectonic movement in Cenozoic of central tectonic belt of Bohai Bay which including the faulted depression whose main part are Huanghua Depression, Linqing Depression, Dongpu depression and the deep strike-slip structural belt, Huanghua-Texas-Dongming was obviously influenced by the deep strike-slip structure and the tectonic evolution has the characteristics of integrity and regularity after we took the Kongnan district as an entry point and combined with the analysis of basin structure and tectonic evolution of the peripheral depression. During the period of Ek-Es4, the precipitating actions of the middle Bohai Bay were mainly happened in the central tectonic zone whereas the tectonic activity in north-central Huanghua Depression and Dongpu depression were weak. The tectonic movement of north-central Huanghua Depression and Dongpu depression which located in the north and south ends of the central tectonic zone of Bohai Bay was significantly enhanced during the period of Es3-Es2. Strongly rifting makes the the subsidence center migrated from the middle part of structural belt to the north and south ends. During Esl-Ed period, the mobilization of the two largest main stretch faults of central structure in the Bohai Bay, Cangdong fault and Lanliao fault, were sharply weakened, the subsidence center of Northern District in Huanghua Depression eastward to the east of Qikou coastal zone while the subsidence center of Dongpu depression was not under the control of Lanliao fracture and migrated westward and mainly concentrated in the narrow strip of land between the placanticline which diagonally stretched and the Yellow River fracture. The entire central Bohai Gulf tectonic belt fracture activity as well as the migration of the subsidence center showed the reverse symmetrical movement characteristics. From the analysis of periphery regional tectonic and regional plate tectonic reorganization event we can see that the dextral strike-slip motion of Tanlu strike-slip faults and Lanliao strike-slip fault and the pull-apart motion between them were the control factors of the tectonic activity in the Central Bohai Gulf tectonic belt in Cenozoic. Especially in the Eocene, the change in the underthrust direction of the Pacific plate to the Eurasian plate prompted the dextral activities of Tanlu fracture (the Lower Liaohe section) extended to the south and the Lanliao fracture which passed the middle of the Bohai Bay Basin was activated under the drift of underthrust direction, the central structure belt of Bohai Bay constructed with the overall performance of the characteristics of oblique extensional basin, resulting in the concentration to both ends of the strike-slip fault stress field, making the Cangdong Lanliao fracture intense activity and creating the migration of the subsidence center. The acceleration subduction of Pacific westward Eurasia in Oligocene, leading the intensity of dextral activity of Tan Lu fault and Lanliao fault, the forward extensional effect of central structure belt of Bohai Bay Basin weakened dramatically but the strike-slip component affect enhanced sharply under the regional stress conditions.The innovations of this thesis are listed as follows:1. This thesis presnted Ek1Ⅲ is a Cenozoic initial rift boundary in Kongnan region. Together with Es3, these two sequences were caused by great tectonic revolution boundaries in Paleogene stratum filling. These boundaries divided syn-rift stage of tectonic evolution into depression, rift and strike-slip stretch episodes2. Structure style in rift episode has been described in detail, and we also quantitatively analyzed the activity of fault. After that, we reconstructed a tectonic framework figure during this episode, and summarized the migrated subsidence centers from center of the basin to south and north gradually. And we calculated the activity of fault at that time. What is more, we concluded that large-scale right-lateral strike-slip extension of the basement fault is the basin development mechanism in this episode.3. Apatite fission track dating shows that samples from the both wings of buried hill documented better information of the tectonic movement in Cenozoic, and samples from the top of the buried hill only tell the information of sediment provenance. It demonstrates that since sediment on the top of the buried hill, the absolute location had never been changed. That is, the top layer of the buried hill has been relatively stable since Cenozoic, and never been uplifted. We proposed that uplifting at the central of the Kongdian Buried hill resulted from subsidence of two wings of the buried hill. |
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