The Research On Mesozoic Tectono-Sedimentary Relationship Between Dabie Orogen And Hefei Basin

The introduction of system theory into earth science resulted in research of basin-range coupling. To be a bridge connecting orogenic belt and peripheral basins, mass transfer was employed as a key in the research between orogen tectonic evolution and basin sedimentary filling. This, therefore, became a focus in the study of earth dynamics. Provenance analysis can depict the history of basin filling, furthermore it can provide a series of clues for the understanding of evolution in source areas. For these reasons, it has become an important approach in basin-range coupling.Dabieshan Orogen has become a hotspot in geological research both in China and abroad for its large scale outcrop of HP/UHP metamorphic rocks. A new challenge in this region lies in the integration of tectonic evolution of Dabieshan Orogen with sedimentary filling of Hefei Basin.In this paper, integrated sedimentological, geochemical, petrological and mineralogical studies are carried out on Mesozoic sedimentary rocks of Hefei basin. Major conclusions are as follows:1. Tectonic units of Dabieshan Orogen and their rock compositionsFrom north to south, Dabieshan Orogen can be divided into four belts, namely, North Huaiyang retroarc flysch belt, North Dabie arc complex belt, South Dabie collision complex belt and Susong metamorphic complex belt. The faults dividing these units are Xinyang-Shucheng Fault, Mozitan-Xiaotian Fault, Shuihou-Yingshan Falut (shear zone), Mamiao-Taihu Fault (shear zone) and Xiangfan-Guangji Fault respectively. Located in the middle of Dabieshan Orogen, the N-S trending Shangma Fault divides the orogen into two parts, with Dabie Block (include North Dabie arc complex belt and South Dabie collision complex belt) in east and Hongan Block in west. Each unit has its characteristic rocks.2. Basement characteristics, fault features and tectonic units of Hefei BasinVertically, the basement of Hefei Basin is recognized to have double-layer structure, and is divided into Archaean-Paleoproterozoic high-grade metamorphic rocks and Neoproterozoic-Late Palaeozoic low grade metomorhpic/original marine structure layer。Horizontally, Feizhong Fault and Liuan Fault act as boundaries and divide the basement of Hefei Basin into three parts, i.e., North China platform basement, continental margin transitional basement and North Huaiyang basement. Hefei Basin is controlled by four boundary faults and can be divided into four secondary tectonic units since Mesozoic, and each unit has its own subunits. Unique tectonic setting endow abundant structure styles to Hefei Basin, such as extensional structure, compressive structure and strike-slip structure.3. Mesozoic sedimentary facies assemblages, strata distribution and sedimentary cyclicityLiuan Fault divides Hefei Basin into two deposition systems. The north system (i. e., main body of basin) is composed of Jurassic-Cretaceous deposition. Fanghushan Formation, Yuandongshan Formation and Zhougongshan Formation belong to late Lower Jurassic-early Middle Jurassic deposition, late Middle Jurassic-Upper Jurassic deposition and early Lower Cretaceous deposition, respectively. The south system is mainly composed of Jurassic-Cretaceous clastic rocks interbeded by volcanic rocks and pyroclastic rocks, including Sanjianpu Formation, Fenghuangtai Formation/Maotanchang Formation, and Heishidu Formation. Sanjianpu Formation span late Middle Jurassic-Lower Jurassic; Fenghuangtai Formation/Maotanchang Formation is the result of earlist Cretaceous deposition-volcanic eruption cyclicity, while Heishidu Formation formed in late Early Cretaceous. The strata between different systems can be well correlated. The dominant facies in Hefei Basin in Mesozoic include alluvial fan, fluvial and lacustrine facies. The evolution of facies and sedimentary cyclicity with time shows synchrony and continuity between the north and the south system.4. Mesozoic paleoenvironment variance in Hefei BasinIn late Early Jurassic-early Middle Jurassic, the preponderant paleocurrent directions indicate the feature of longitudinal drainage system from west to east in Hefei Basin. Surprisingly, paleocurrent directions from north to south also exist in this region. In late Middle Jurassic-Late Jurassic, the paleocurrent is attributed to lateral drainage system and crossed Dabieshan Orogen. The paleocurrent of earlist Cretaceous shared the same features as Late Jurassic. In late Early Cretaceous, the paleocurrent of Hefei Basin was emanative.The sandstone particle distribution of Hefei basin in Mesozoic reflects fluvial features. Particle analysis indicates that the fluvial was proximal from late Early Jurassic to early Middle Jurassic; Between late Middle Jurassic and Upper Jurassic, the fluvial in this region transferred from proximal to distal fluvial. The onflow from Dabieshan Orogen to Hefei Basin and distal fluvial in fault basins were formed in Early Cretaceous.5. Mesozoic provenance evolution of Hefei BasinThe trend of gravel components distribution was tracked both in the north and south deposition system of Hefei Basin. In the north system, gravel components show a complex origin in Jurassic. North Huaiyang belt mainly contributed sediments to the basin. Dabie complex provided less sediment to the main body of Hefei Basin. An unknown source area existed in this period and is presumed to be North China platform basement, or North China Plate. There is no deposition in Lower Jurassic, the gravel deposition of Middle to Upper Jurassic mainly derived from North Huaiyang belt and North Huaiyang type basement. Dabie complex provided a small proportion of sediments to the south system. In Cretaceous, Dabie complex provided most of the sediments to the south system. Furthermore, complex rocks spread to the north system.Source rock compositions determined by heavy mineral assembles in the north system of Hefei Basin include sedimentary rock and low-grade metamorphic rock. This indicates that North China Plate was the main source area. Middle to Upper Jurassic heavy mineral assembles has the characteristics of basic magmatic rocks and acid-medium magmatic rocks. The source rocks in Cretaceous are acid-medium magmatites。The evolution trend of source rocks in vertical geological section reflected by Mesozoic heavy mineral assembles in the south system shows that the source rocks are consistent with those in North Huaiyang belt in Jurassic. The contribution of Dabie complex became more and more significant from base to top of the section. In Cretaceous, almost all of sediments were derived from Dabie complex.6. Tectonic evolution of source areas of Hefei Basin in MesozoicThe Lower Jurassic sandstone detrital mode of Hefei Basin indicates that, while its provenance mainly belongs to magmatic arc setting, orogen and continental block setting also appear to be possible settings. This reflects the complexity of source areas. Middle to Late Jurassic tectonic settings of source areas are continental block and collision orogen, exhibiting a clear mixed sources. It is evident that the source areas are recycled orogen and collision orogen in Cretaceous, the simplex clast derived from Dabieshan Orogen affected the filling of Hefei Basin in this period. The presence of magametic provenance and island provenance reflects the extensional setting. The provenance determination from geochemical data confirms the source areas' difference between the north and the south system in Jurassic. The north system is mainly affected by North China plate. In contrast, the south system is mainly affected by Dabieshan Orogen. In Cretaceous, active margin setting is distinguishable. Arc provenance appeared both in the north and south system, which is in accordance with the setting of North Huaiyang belt.7. Tectonic evolution and reconstruction of Dabieshan OrogenTectonic evolution of Dabieshan Orogen is complicated. It is difficult to rebuild the tectonic evolution merely based on Orogen itself. While a series of clues from the sedimentary filling of Hefei Basin are very useful for a better understanding of the tectonic evolution and structure reconstruction of Dabieshan Orogen. According to the filling features, paleocurrent variance, provenance analysis and tectonic setting determination of source areas, the tectonic evolution history and tectonic units can be reconstructed. Though orogency occurred in Late Triassic, the collision orogency in Dabieshan region was less active from Early Jurassic to early Middle Jurassic. Main collision orogency lasted from late Middle Jurassic to Late Jurassic. While large scale orogency occurred in Cretaceous, and belong to thermal doming orogency under regional extensional background.