Structural characteristics and geochronology of thrust faulting in the Jiulianshan anticlinorium, the Lower Yangtze region, East China

摘要

The Jiulianshan anticlinorium is located at the southern part of the Mount Maoshan-Jiulianshan in the Lower Yangtze region, where a number of new-found thrust faults were identified. Movement of these thrust faults had resulted in Palaeozoic strata lying unconformably above Cretaceous strata. However, the Lower Yangtze region was mostly under an extensional tectonic regime during the Late Mesozoic, which is inconsistent with existence of the nappe in the Mount Maoshan-Jiulianshan. Therefore, what the structural characteristics are and when these thrust faults developed are important for understanding the evolution of the South China Block. Based on detailed field observations, these new-found thrust faults outcropped in the Liqiao, Yangxian, and Miaopu areas. These faults, SE-dipping deeply, developed a meters-wide fault-and-fracture zone of width in meters. The footwall was composed of the Early Cretaceous pyroclastic rock or tuff, which indicates that these faults developed since the Early Cretaceous. Two undeformed dykes formed in the fault-and-fracture zone of Miaopu area and a footless intrusion in the hanging wall, which give emplacement ages of ca. 132Ma by using zircon U-Pb LA-ICP-MS dating and thus implies that thrust fault motion occurred prior to or around this time. The volcanic tuff and pyroclastic rocks, located in the footwall of the Yangxian and Miaopu thrust fault, are dated at ca. 137 and 134Ma, respectively, indicating that fault motion must have occurred after this time. The Qiaotouyang pluton emplaced in the core of one of the folds in the fold-and-thrust belt at ca. 141Ma, which indicates that the fold-and-thrust belt of Tongling Uplift formed prior to 141Ma. Integrating other evidences, such as magmatic quiescence, large-scale and short-duration volcanic eruptions, and beginning time of metamorphic core complex and rift basin, it is inferred that Early Yanshanian thrust faults were reactivated during the Late Mesozoic, resulting in thrusting w