Study On The Growth, Linkage Of Boundary Faults And Fault-Related Folds In Zhu I Depression, Pearl River Mouth Basin

The normal fault of faulting basin, in general, can be classified into four types based on their scales, that is the primary basin-controlling fault, secondary sag-controlling fault, tertiary zone-controlling fault, and forth-level trap-controlling fault. The secondary sag-controlling fault, as a boundary fault, which characteristics and evolution is often restricted the geometry, formation and evolution of a depression. The evolution of a boundary fault is often a process of isolated faults growth and linkage, analysis on the fault growth and linkage is a research hotspot and long-term focus on scientific item of an extensional basin.Fault and fold, as the most two common structural types of a basin, the discussion on the relationship between the two has experienced a long time. An origin relationship is existed between the faulting and folding, and faulting always form a fault related-fold. The early study on the fault-related fold mainly considered that fold is formed in the extrusional or strike-slip tectonic stress settings, but the extensional folds, formed in the extensional setting, is relatively weak. With the deepening research on the structural deformation of an extensional basin, in recent years, the extensional fault-related fold gradually is a study hot topic of scholars at home and abroad. Different viewpoints on the types of extensional folds between different scholars, as diversity origin and patterns of fault-related folds, although the characteristcs of different extensional fold types were divided from the geometry and original mechanism, there is less discussion on the extensional fault-related fold from the perspective of the fault linkage and evolution.In order to resolve the above main problems, we selected Zhu I depression in Pearl River Mouth Basin (PRMB), as the research object, to discuss the characteristic of fault growth and linkage and the main extensional folds patterns which were formed in the different fault linkage modes. PRMB, as a non-typical passive continental margin basin, has the basic properties of passive continental margin basin when the South China Sea was separated into southern and northern mainland ca. 33Ma. The regional tectonic scope of the basin coveres the continental crust and ocean-continent transition belt, and transition to the oceanic crust. Under the complex structural setting of PRMB, there are abundant boundary faults and multiple types of extensional folds in Zhu I depression, which provide a good research object for this study.Based on well controlling high-precision 3D seismic survey, the geometry of border faults and the framework of "Four-order sequences" is established in PRMB, combined with the basic theories and approaches of fault analysis, to analyze the north and south border fault growth and linkage of the sag in Zhu I depression, PRMB, and proposed the different fault linkage modes. The fault related-folds formed in the process of border faults evolution, meanwhile, based on the origin relationship between the fault and folds, using high-precision 3D seismic data to disscuss the fault-related folds formation modes with different fault linkage patterns, from two aspects that fault related fold geometry character description, and couple the response relationship with fault linkage and evolution.This article has been recognized that there are some differences between the western and east ern zones border faults of Zhu I depression, based on the study of border faults geometry and kinematic characteristics of Enping sag, Xijiang sag and Huizhou sag in Zhu I depression. The result show that the eastern belt, Huizhou sag, whose boundary fault is high-angle listric normal fault, long-term inherited faults developed in the period of rift and post-rift, and the strike of these faults is complex and changeable, show an arc shape in the plane. The eastern belt, including Xijiang sag and Enping sag, whose boundary fault is low-angle listric normal fault, only developed in the rift period, and the strike of these faults is NE.Based on the responding relationship between the sequence stratigraphy structure and developmental characteristics and boundary faults, and on the basis theory that the sediment within the rift basins records the interaction between the sedimentary history and fault evolution, we identified the border faults geometry through high-precision 3D seismic data, combined with the drilling data to demarcate the sequence interface accurately, under the framework of third-order sequence, and using the "fourth-order layers" technology, combining with conventional fault research technology, such as the fault activity and fault displacement distance relationship curve to discuss the growth and linkage and evolution of south and north boundary faults in Zhu I depression. Research results show that the sequence migration and evolution of the "fourth-order layer" can directly reflect the growth and the connecting process of boundary faults, through the migration and evolution of sequence rebuilting the the stretching process of boundary faults, and proposed firstly the fault complex linkage and transform model. Fault complex linkage and transformation is that the fault have a complex structure model in the process of fault evolution,that is "hard linkage" and "soft linkage" coordinated development coupling with along-strike and lateral linkage pattern. The view of fault complex linkage and transformation proposed, not only points out the boundary faults growth with along-strike, at the same time, also has the vertical direction of lateral connection mode. The lateral linkage means the growth and linkage of transverse faults, which were formed in the relay ramp broken stage, has also a characteristic of complex linkage and transform. The lateral linkage modes reveals the broken relay ramp is may not connected both sides of the normal fault by a new transverse fault,relay ramp break process also experienced a complex evolutionary process, according to the process of the lateral faults growth connection can be divided into three stages, the isolated normal faults, synthetic overlapping, and hard linkage.Based on the discussion of dynamic mechanism of Zhu I depression, and do a research on the boundary faults growth, from the perspective of double restriction fault growth and development regional stress and local stress. Research results showed that the fault linkage besides the along-strike linkage model and lateral model, also has an oblique connection mode. Due to the influence of basement properties and pre-existing structure, lead to the the various difference of the local stress and regional stress direction in different distribution areas, give rise to the southern and northern border faults presents a complex characteristic of fault stike and patterns. When regional stress and the local stress direction is consistent, the growth and development of fault interaction mainly is along-strike model, with along-strike segment and tip growth modes; When regional stress and the local stress direction oblique, oblique linkage mode is the main way of the fault growth, which show multiple segmentation fault overlapped and transformation, causing the transformation of sag-controlling roles.Statistical and comparative analysis the activity intensity of the north and south boundary fault in Zhu 1 depression, the fault under the control of the thickness of strata, and sequence development cycle. Discuss the different fault growth linkage mode and the same connection mode in different evolution stages of the boundary faults influence depression or sag development. The results of the study show that, different fault connection mode result in different depression or sag development. The strongest activity of faults which vertical segmented connection control maximum thickness of strata. Sedimentary sequence develop up to the full during the rift period. Oblique connection weaker, longitudinal-terminal growth fault weakest. It also shows that the multiple section type fault connection reflect the strong activity of the fault during the rift period. It is strongly controlled the development of depression or depression. The single section type faults is relatively weaker on control depression than the segmented fault. In addition, the border fault of the same connection model is also different for the contorl of depression or sag evolution in different stages. According to the evolution of the fault, the same fault connection model is divided into three stages, including the early stage, the middle stage and the mature stage. The early stage faults are isolated normal faults, then the faults begin to interaction and couple in the middle stage, the mature stage faults transfonn from the early small isolated faults into a large fault. Based on the analysis of the fault activity, the thickness of stratum and the evolution of sequences, it shows that the control for the depression or sag are enhanced form the early stage to the mature stage.Based on high precision 3D seismic data to identify the fault related fold type which were formed near the boundary fault, through the statistics and analysis of the geometry characteristics of fault related fold, has been recongnized that the different fault connection modes would have different fault related fold development mode in Zhu I depression. Among them, under the along strike-segment linkage mode, the fault-related folds development model is lateral growth mode, the fold hinge perpendicular to fault strike, formed in the fault connection zone, the lateral displacement gradient fold is the main type of this mode.Within the along strike-tip growth fault linkage mode, the fault-related folds development model is vertical growth mode, fold hinge parallel to the fault strike, drag fold is the most common representative of this style. Within the oblique fault linkage mode, the fault-related folds development model is oblique develop mode, oblique fault conversion fold and oblique fault bend fold were the typical representative. Oblique fault transform fold formed near the connect zone of two or more segmented faults. These fold axis approximate consistent or oblique with fault tend, the former is associated with high-angle border faults, the corresponding boundary faults of the latter one are low-angle normal faults, but the fold hinge of the two are oblique to the fault strike; Oblique fault related folds which have a genetic relationship with the fault surface morphology, formed at the change section of fault dip of oblique fault, the fold hing of this kind folds are oblique with the fault strike.