The unconformable distribution of Caledonian and Indosinian strata in Jiangnan-Xuefeng area

Abstract：The tectonic nature of Jiangnan - Xuefeng orogenic belt is a controversial issue which has aroused great attention among geologists in China. The nature of the Caledonian and Indosinian unconformity in Xuefeng region reflects the sphere of influence in the two stages of orogeny. Based on field geological survey, the authors have found that Jing'an - Xupu - Jingzhou fault and Xiushui - Cili - Baojing - Sandu fault are boundaries of different types of unconformity and the limits of influence in Caledonian orogeny. On the southeast side of Jing'an - Xupu - Jingzhou fault there is a strong fold deformation zone with well developed angular unconformity. In the vast area on the west side of Xiushui - Cili - Baojing - Sandu fault there is no obvious fold deformation, but vertical uplift is well developed, correspondingly forming parallel unconformity. The area between the two faults is a weak fold deformation zone with well developed slightly angular unconformity. The distribution of different types of unconformity in Indosinian period, which is bounded by Puqi - Cili - Baojing fault and Xianning - Hefeng - Longshan fault, has certain continuity relationship with the dsitribution of the Caledonian orogeny. In Indosinian movement, the area on the east side of Puqi - Cili - Baojing fault was a strong fold deformation zone, with well developed angular unconformity, whereas parallel unconformity was well developed on the west side of Xianning - Hefeng - Longshan fault. Between the two major faults there is slightly angular unconformity. From the distribution of different types of unconformity in Caledonian and Indosinian orogeny, it can be concluded that the influence range in Indosinian movement advanced more westward than that in Caledonian orogeny, and the Xuefeng uplift area was only on the western edge of the two stages of intracontinental orogeny, rather than acted as the structural center.