Features And Causes Of Indosinian Structures In The Xuefengshan Tectonic System

The eastern South China Continental Block can be subdivided into the Yangtze Block and the Cathaycian Block. The Xuefengshan Introcantinental Tectonic System is one important part of the Yangtze Block. By outdoor investigating in the Xuefengshan area, we study evolutionary process of Indosinian intracontinental structures in South China, which is very important for understanding the structural evolution of the Early Mesozoic in South China. We surveys the Xuefengshan Precambrian basement and its neighboring, analyzing paleogeography and granite distribution, focusing on Indosinain distribution and features of the angular unconformity, folds and faults. Mainly by structural analysis, with a combination of geophysics, petrology and paleomagnetism, this paper tries to reveal features of Indosinian evolution, structures and geodynamics. At last, the following conclusions are summerized:(1) The distribution of the Indosinian unconformities in the Xuefengshan Tectonic System reveals four kinds of contact interfaces, including high-angle unconformity, low-angle unconformity, disconformity and conformity and becoming younger and younger to the west. Then the intensity of tectonism becoming weaker and weaker to the west. The distortion area is wider in earlier times, and retreats to the east at latest.(2) Boundary effect of intracontintal:the Indosinian folds are subdivided into two directions of folds in terms of axial traces, northeast and meridional striking folds, which were between the north and south trending thrusts. One of the most important reason is a control of different block borders under the same stress field, not paleo-Pacific ocean, or Qinling-Dabie continent.(3) Differential rotation leading to two-stage intracontinental deformation. The first-genenration NEE-striking folds are rotated from the earlier NNE-striking folds or NE-striking due to late block rotation. While, the second-generation NNE-striking folds are also under the same stress field for the first-generation ones. There is an overall rotation of the South China, rather than a differential rotation between the Yangtze and Cathaysian blocks after Indosinian orogeny, but there is a differential rotation between the two blocks in this time.(4) The southern and northern structural lines are perpendicular to each other. The reason for perpendicular tectonic lines is that the first-genenration NEE-striking folds are rotated from the earlier NNE-striking folds due to late block rotating. The second-genenration NNE-striking folds superimpose the NEE-striking folds. The intracontinental shortening between the Yangtze and Cathaysian Blocks was earlier than the collision of the South China Block with the Qinling-Dabie Micro-continent, and the Yangtze Block and the Qinling-Dabie Micro-continent had rotated in early Indosinian orogeny, these resulted in a difference between those earlier structural lines formed in early Triassic, the perpendicular relation to these structural lines was at last remained although they underwent small rotation between the Yangtze and Cathaysian Blocks.