| Based on geological structural interpretation of latest 2D and 3D seismic profiles, combined with drilling data, log measurement, bore specimen and gravity and magnetic data, geometry and kinematics characteristic of main faults in Qiongdongnan Basin (QDNB) have been studied by using theoretical analysis of geologic analysis such as plate tectonic theory, basin dynamic analysis, analytic structural geology et al. Structural styles and fault growth models were summed up. At the same time, balanced cross sections were used to renew the evolution of faults system in QDNB; numerical modeling was used to analyze the deformation characteristic of faults; developmental mechanism of faults in QDNB had been researched based on regional geological setting analyse. Basin architecture and depositional system tract depends upon a complex interaction with the evolution of fault propagation. Furthermore,3D seismic data was secondary treated using coherence technique and was secondary structure elucidated. The basic characteristic of intrastratal faults in QDNB and their formation mechanism were also discussed.There were three groups fault system in QDNB, including NE-trend faults, NW- trend faults and nearly EW-trend faults. NE-trend faults were the dominant fault system, EW-trend faults were the secondary and NW-trending faults were least. Beside No.1 fault, which was strike-slip fault and the boundary fault of Yinggehai basin, the other NW-trend faults were adjusted faults, but also with a certain strike-slip weight. The main faults which control the development of basin or sag were NE-trend faults, such as No.2 fault, No.6 fault, No.11 fault and eastern part of No.5 fault. EW-trend faults, which were dominate formed during the deposition period of the Lingshui formation, were subsidiary faults generated by the regional SN-trend stretching. But No.3fault system and western part of No.5fault system were also EW-trend faults due to the impact of the No.1 strike-slip fault zone. No.6 fault and No.11 fault tendency was NW, and the other main faults tendency was SE. All the fault dips, except No.11 fault, were greater than 30°as high-angle normal faults. The main structure of fault system was simple single faults; and fault terrace structures were mainly located in terminal portion of faults such as No.5 fault, No.11 fault, No.6 fault, and No2. fault.There were lots of similarities of main faults activity characteristic during the geological history of QDNB. Many small faults with small displacements and low displacement rates were active during Eocene period. This characteristic was testable in basin record as numerous, isolated, bounded by small faults. During Early-Oligocene period, the faults were active with larger displacements and higher displacement rates suddenly. In Late-Oligocene period, the regional extensional stress field changed form NW-tension into SN-tension, but all the main faults were continually active with larger displacements and higher displacement rates; but vast of small faults were inactive lead to the number of faults sharp reduction in the performance of fault plane map; at the same time, a lot of EW-trend faults were beginning active. Starting from the Early Miocene, the main faults were nearly inactive, the basin entered into the period of thermal subsidence stage. The spatial and temporal migration of main active faulting was clear revealed. The faults activity rates were significantly higher in central depression zone than the faults in northern depression zone; fault system in eastern was mainly in its central and southern, and fault system in western had little faults activity difference. The center of single fault activity (such as No.6 fault and No.11 fault et al.) migrated from east to west during Early Oligocene to Late Oligocene, and the center of the whole fault system in QDNB also migrated from east to west during Late Oligocene to Early Miocene.A large number of intrastratal faults have been identified in the early-middle Miocene Meishan Formation and Huangliu Formation shallow-bathyal mudstone in QDNB. Those faults were all extensional and planar with most dips above 45°and less than 1000m long, but the faults strike in the different regions vary widely. In the northeast part of the studying area, the intrastratal faults were composed of EW-trending densely faulted belt and SN-trending linear faults, the EW-trending densely faulted belt were resulted from gravity-driven mechanical, while the SN-trending linear faults were resulted from volumetric mechanical and also were influenced by gravity-driven. In the central part of the studying area, the intrastratal faults were composed of radial faults and other faults which perpendicular to the former, the radial faults were resulted from hydraulic fracture mechanical, while other faults were resulted from volumetric mechanical and also were influenced by radial faults. In the southwest part, the intrastratal faults were polygonal faults, which were resulted from volumetric mechanical. The polygonal faults were perpendicular to the tectonic faults in tectonic faults-affected area.Structural styles and transfer zones were systematic summarized and compartmentalized in QDNB. The structural styles, base on different formation mechanism of Structural styles, were divided into extension structural, diapiric and the polygonal faults three types. Fault and fold were basic research of structure; fault, according to the fault occurrence, can be divided into platy normal faults and the listric normal fault; and fold, according to their geometry characteristics, can be divided into longitudinal fold, drape fold and transverse fold. The diapiric structure was divided into mud diapiric and volcanic diapiric, according to the different diapiric material. Three types of transfer zones, includes synthetic, convergent and divergent, were dug out in QDNB. And the synthetic transfer zone can be further divided into part overlapping, overlapping cover and transform fault. The development patterns of fault system in QDNB were summarized by two types——simple propagation and propagation amalgamation. Finite element numerical simulation was applied for modeling the process of fault formation and evolution:the maximum displacement of a fault was proportional to the trace length's power exponent. No.6 fault and No.11 fault accord with the single fault evolution pattern:the middle part of the fault was characteristic as vertical growth and lateral extension was developed at the breakpoint. No.5 fault and No.2 fault were attributed to muti-faults evolution pattern:high stress was stimulated at the convergence point of the faults that vertical expansion was developed to connect the faults. The maximum displacement transported to the convergence point at the same time. The shearing stress in the Western QDNB had important influence on fault growth. The maximum displacement transferred to the direction of shearing stress. The maximum stress was present to breakpoint away from the breakpoint. The stress concentration around the strike-slip fault was not apparent, and the faults were pinnate ranged. It was indicated that Honghe strike-slip faults had important influence on fault activity in Western QDNB.In particular, the processes of fault propagation, growth, linkage and death were major tectonic controls on QDNB architecture and the three-dimensional evolution of QDNB. Depositional system, especially the distribution and its evolution of sand body morphology, was controlled by both basin architecture and the evolution of fault system. On the other hand, the region of maximum depositional thickness of rifted basin reflects on the region of maximum displacement rate of fault, and the migration of center of subsidence reflects on the migration of fault action centre. |
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