Not all folds and thrusts in the Yangtze foreland thrust belt are related to the Dabie Orogen: Insights from Mesozoic deformation south of the Yangtze River

摘要

The Yangtze foreland fold-and-thrust belt is located between the Mesozoic Dabie collisional orogen and the Mesozoic Jiangnan intraplate orogen in east-central China. Three regional deformation events are recognized in the Dabie Orogen north of the Yangtze River. The earliest regional deformation (D1) corresponded to a southeastward compression that occurred during the subduction of the South China Plate below the North China Plate. Top-to-the-southeast shearing coeval with Late Permian-Early Triassic blueschist facies metamorphism, was possibly associated with the development of regional gneissosity in the Dabie complexes and flat-ramp-style thrusts within the Neoproterozoic- Palaeozoic sedimentary cover, north of the Yangtze River. The main ductile deformation (D2) is also characterized by southeast-verging overturned and recumbent folds, coeval with the early stage of exhumation of the high-pressure rocks. A top-to-the-SW thrusting and WNW- striking open folds belong to a separate, later deformation phase, D3. Previous ~(40)Ar-~(39)Ar mica dates ranging between 245 and 205 Ma suggest Late Permian-Early Jurassic ages for the D1 to D3 events. The foreland folds and thrusts related to these deformations in the Dabie Orogen flank the eastern and western 'limbs' of the Dabie Orogen but are absent in the front of it. In contrast to the deformation north of the Yangtze River, the deformation of Neoproterozoic-Palaeozoic sedimentary cover in the Jiangnan Orogen south of the Yangtze River is generally characterized by north-verging folding and top-to-the-north thrusting from Triassic to Jurassic times. Based on the differences of deformation and structural polarity between the south and north areas of the Yangtze River, we propose that the Yangtze foreland fold-and-thrust belt in the direct front of the Dabie Orogen is unrelated to the Dabie Orogen but resulted from the Jiangnan Orogen.