Geological structure characteristics of the middle segment of the Xuefeng orogen

There exist different view points about the geological structure characteristics of the Xuefeng orogen. The authors studied such aspects of this problem as deformation types, deformation epochs and intensities, geometrical and kinematic properties, dynamic mechanisms and tectonic regimes in Huaihua- Liangyaping area lying in the eastern part of the middle segment of the Xuefeng orogen through tectonic profile and regional geological survey and, as a result, reached the following conclusions: ①There existed main deformation types such as slaty cleavages, crenulation cleavages, folds, thrust faults, large-scale kink zones, normal faults and dextral strike-slip faults. ②There existed notable compressional deformations with NW to NWW compression and NE to NNE-trending folds and thrust faults in both Caledonian movement and Early Mesozoic tectonic movements (Indosinian movement and Early Yanshanian movement). Slaty cleavages and large-scale kink zones were formed in Caledonian movement. ③The middle segment of the Xuefeng orogen can be divided into the east zone and the west zone with Xupu- Jingzhou fault as the boundary, which served as the root and the middle zone of Xuefeng thrust system respectively. There existed higher deformation in the east zone than in the west zone in Caledonian movement, as evidenced by the existence of slaty cleavages in the east zone and the absence of slaty cleavage and the similarity between the Upper Palaeozoic and the underlying strata in attitude in the west zone. Unconformity, tectonic uplift and thrust faults indicate that there existed stronger deformation in the east zone than in the west zone in Early Mesozoic tectonic movement. ④Dip directions of slaty cleavages, fold axial surfaces and thrust faults indicate that there existed back- thrust structures in both Caledonian movement and Early Mesozoic tectonic movements in the east zone of the Xuefeng orogen, but the middle belts of the two period back thrust structures didn't coincide with each other, and the middle belt in Early Mesozoic was about 25km west of Caledonian. ⑤ The east side of Xupu- Jingzhou fault subsided violently in both early Nanhuan and Late Palaeozoic, suggesting that the east zone of the Xuefeng orogen was a structurally weak zone with lower strength of crust, and therefore turned into the root zone of the Xuefeng thrust system.