A geometrical and kinematic analysis of the Early-Mesozoic Yueyang-Chibi fold-thrust belt in southern Jiangnan orogen

Abstract：The Early Mesozoic (Late Indosinian-Early Yanshanian) Yueyang-Chibi fold-thrust belt is located in the border area between the Jangnan orogen and the middle Yangtze foreland basin. Detailed geological mapping was conducted to reveal the structural geometry and kinematics of the belt. From south to north, the Yueyang-Chibi fold-thrust belt is composed of Yueyang-Linxiang basement detachment-thrust belt, Taohuaquan-Xiaojiawan cover rock detachment fold belt and Chibi-Jiayu fault-fold-basin. The Yueyang-Linxiang basement detachment-thrust belt is composed of Guozhen syncline, Guanshan anticline, Linxiang overturned syncline and Nieshi anticline in northward direction, making up trough-like folds. Most axial planes of the folds dip south. The unconformity surface between Nanhuan cover rock and Lengjiaxi Group and the detachment fault along the unconformity surface have been folded. In Taohuaquan-Xiaojiawan cover rock detachment fold belt, there occur mainly overturned folds composed of Nanhuan-Silurian and Late Carboniferous-Middle Triassic strata, whose axial planes dip south, with small crosswise sizes. The Chibi-Jiayu fault-fold-basin has a southern boundary fault, namely south-dipping Puqi fault (Jiangnan fault). In the fault-fold-basin there occur Late Triassic-Middle Jurassic sediments, which, together with cover layers, are involved into folds and faults. The faults and the axial planes of the folds dip south in the south of the fault-fold-basin, and the axial planes are vertical in the north. The EW-trending lineament changes to NEE～NE from southwest to northeast of the study area. These structural zonation and deformation characteristics indicate a northward movement direction and the properties of a foreland fold-thrust belt. According to the theory of fault-related folding and on the basis of near-surface geological characteristics, the geotectonic section structure and the deformation dynamic mechanism of the the Yueyang-Chibi fold-thrust belt are considered to be as follows: ① Several detachment faults and south-dipping thrust faults from the south and the lower part to the north and the upper part make up a step-shaped thrust-fault system, which controls the slip-thrust of tectonic blocks, structural framework and deformation zones. ② The Guozhen syncline is a basement-detachment fold; the Guanshan anticline has characteristics of a double detachment fold and a fault-propagation fold; the Nieshi anticline is a fault-bending fold; the Linxiang syncline is a passive fold controlled by anticlines on both sides, with detachment faults along the unconformity on the two wings of the syncline. ③ The trough-like folds of the Yueyang-Linxiang basement detachment-thrust belt are mainly controlled by fold-basement detachment and horizontal compression, with its formation mechanism similar to that of the pinch and swell fold. The authors hold that there exists no long distance thrust nappe in northeastern Hunan and southeastern Hubei.