Mesozoic Sedimentology, Basin Analysis And Tectonic Evolution In The North Margin Of The Dabie Orogenic Belt, China

This research focuses on the sedimentology, basin analysis and tectonic evolution of Mesozoic strata in the north margin of the Dabie Orogenic Belt, using an integrated approach of modern sedimentology, continental dynamics, petrology, geochemistry and isotope chronology. The topics of the study include: (1). establishing the time framework of Mesozoic strata in the north margin of the Dabie Orogenic Belt; (2). analysis of lithofacies and sedimentary systems; (3). ascertaining the paleocurrent direction of the Mesozoic basin and reconstructing the paleogeography; (4). deciding the material components and original structure sequences of the basin provenance, and coupling relationship between the sediments of the basin and geologic units of the provenance; (6). establishing the Mesozoic tectonic evolution history of the Dabie Orogenic Belt and discussing the collisional mechanism of the Belt. The results are listed as follows.1.The framework of Mesozoic strata in the north margin of the Dabie Orogenic Belt was reestablished according to the discovery of the tuff beds and volcanic debris in Zhougongshan and Fenghaungtai Formations, along with their isotope ages and the correlations of sedimentary facies and heavy minerals. This result lays a solid foundation for the research.2.Based on the study of petrology facies, facies association and sedimentary sequences, the sedimentary processes and systems are established. They consist of debris and hyper concentration flow deposits, channel deposit of braided and meandering streams, and alluvial fans, floodplains, fan-deltas and lacustrine turbidities. Based on these results, the framework of sedimentary system in the study area is established.3.The Mesozoic paleogeography of the north margin of the Dabie Orogenic Belt was established according to the paleocurrent study. Water system of the sedimentary basin is divided into two classes: one is transverse water system adjacent to the south margin of the basin , which flows from the south to the north, the other is longitudinal water system close to the midst and flows front the west to the east. The provenance of the basin is located in the south, which covers the Dabie Orogenic Belt.4.It is the first time that eclogite conglomerates are discovered in Maotangchan and Fenghuangtai Formations. This suggests that ultra-high pressure metamorphic rocks in the Dabie Orogenic Belt exhumed to the earth's surface in Lower Cretaceous. The geochemistry data indicates that the protoliths of the eclogite gravels formed in a structure setting of volcanic arc.5.Based on the analysis of their source rocks, the Mesozoic detrital rocks in Feixi area are mainly derived from metamorphic rocks of Fuziling and Luzhengguan Groups, and the provenance of Fenghuangtai Formation is a disappeared ultra-high pressure metamorphic rock unit characterized with inland arc eclogites. The Dabie complex suffered denudation in Neogene and constitutes the source rocks of Zhengyangguan Formation. These facts indicate that the Dabie Orogenic Belt was uplifted rapidly before the early Jurassic (205-180Ma), and went through a long period of tectonic calm during the Jurassic. At the late stage of Upper Jurassic (135ma), the provenance areas underwent intense and sustained uplift and denudation. In the period of Lower Cretaceous, the tectonic setting of the provenance areas turned into strike-slip. In the Later Cretaceous (95Ma), the provenanceareas and the margin of the basin uplifted and suffered denudation together.6.The Mesozoic sedimentation in the north margin of the Dabie Orogenic Belt is diachronous. From east to west, the onset of the sedimentation is successively in Lower-Middle Jurassic, Middle-Later Jurassic and Lower Cretaceous. This indicates that the exhumation of the Dabie Orogenic Belt was diachronous, and that its uplift happened from east to west and was mainly controlled by the NE fault system.7.The Dabie Orogenic Belt is of transformation and superposition character. Its new superimpose...