Characteristics Of Compounding-Combine Fold Superposition And Dynamics Of The Xuefengshan Tectonic System

There are lots of complex geological structures in the Xuefengshan tectonic system, This is one good natural laboratory to study intra-continental orogeny, especially for superposition of folds. Based on field observations and pre-existing data, This thesis carefully analyzes each tectonic events that happened in the Xuefengshan tectonic system. The Xuefengshan tectonic system in this thesis is subdivided into several areas based on different types of superposition of folds. Then this thesis selects to describe some typical superposition styles of folds for detailed structural analysis, for establishing fold superposition models and deducing a process of formation and evolution of fold superposition, and discussing characteristics of dynamics of the Xuefengshan tectonic system. Finally, four major conclusions have been made as follows.1. Progressive deformation—one major characteristic of the Xuefengshan tectonic system. Based on field observations, there is not the Caledonian unconformity at the west side of the line of Xiushui- Chili- Baojing- Sandou, it is known that the western boundary of Caledonian tectonic movement is the line of Xiushui- Chili- Baojing- Sandou. The Indosinian unconformities are strictly constrained at the east of the line of Hefeng-Laifeng-Sandou. Tthe Yanshanian unconformities, however, are controlled by the Huaying fault, just being at the east side of this fault. Therefore, it is obvious that the diachronously progressive deformation propagated from east to west.2. Structural diversity—another characteristics of the Xuefengshan tectonic system. There are different striking tectonic traces in the Xuefengshan area. NE-trending, EW-trending and SN-trending structures are majority, and NNE-trending, NEE-trending, NW-trending structures are secondary. The major reason to so many tectonic lines is the limitations and transformations of boundary faults.3. There are abundant superposition styles of folds, such as single-arc, chain-arcs, triangle-arcs, coaxial-superposition, crossing-superposition and limitation-superposition. The main structures east of the Xuefengshan are Dome-and-Basin fold superposition, while Rhomb-Arc west of the Xuefengshan.4. Dynamic evolution of the Xuefengshan tectonic system is also proposed here. In Caledonian, the Yunkai and Guidian blocks moved towards the Yangtze block from south to north, following the northwestward collision of the Cathaysian block to the Yangtze Block. Indosinian fold superposition, however, is a result of extrusion of the Qinling-Dabie Micro-continent and intracontinental collission of the Yangtze Block with the Cathaysian Block. While Yanshannian events are related to changes of subduction directions of the ancient Pacific Plate. For example, during Early Yanshannian it has a NW-directed subduction, while it has W-directed subduction in late Yanshannian.