Thrust And Decollement System Of The Xuefeng Intracontinental Tectonic System

This thesis investigates ranges and distribution of disconformity influenced by each tectonic events since Caledonian period on the basis of a comprehensive fieldwork to angular disconformities and structures; Besides, combined field geological surveys with geophysical data interpretation, the whole detachment is analyzed as well as the entire thrust-decollement system in the Xuefeng intracontinental tectonic system. Thirdly, tectonic units are subdivided in the principle of structure style. At last, by a comparison with tectonic events and magmatism in the surroundings of South China, a geodynamic model of thrusting and decollement in the Xuefeng intracontinental tectonic system is proposed. The major conclusions are presented as follows. 1) Progressive deformation in the Xuefeng intracontinental tectonic system refers to double meanings:â‘ Caledonian progressive deformation from one stage to another, andâ‘¡diachronous progressive deformation since late-Triassic. The Caledonian and Indosinian deformations are constrained at the east of the Xiushui-Mayang-Sandu line and at the east of the Cili-Baojing-Sandu Fault, respectively.But the Yanshanian deformation range propagated westward to the Huayingshan Fault from the southeast continental margin. Therefore, it is obvious that the diachronously progressive deformation propagated from east to west since the late Triassic.2) Multi-layer detachment fault system are identified and the involved layers can be divided into the upper group, the middle group and the lower group. The upper group is made up of Upper Permian and Lower Triassic. The middle group consists of Lower Silurian. Lower Cambrian and Lower Sinian constitute the lower group. In the northwest of the Xuefeng uplift belt, the major detachment faults propagated vertically by jumping from the lower group to the middle group and to the upper group from SE to NW.3) In the second stage of Caledonian movement, top-to-west thrusting developed in the study area except top-to-east thrusting east of the Xuefeng uplift. During Indosinian and Yanshannian period, thrusting and uplifting continued to develop westward while the Xuefeng uplift belt formed a large-scale, asymmetrical, fan-shaped pop-up structure. Structures in the Xuefeng uplift and its east part resulted from superimposition between Caledonian structures and Indosinian-Yanshannian structures. To the northwest of the Xuefeng uplift belt, the thrust-decollement style varied with "thrust-fault-bending-fault-propagation-detachment-conjugate kink" from east to west. However, thin-skin thrusts and decollements, which were only produced in the Qianzhong uplift on the southwest of the Xuefeng uplift belt (Guizhou), were the result of Yanshannian movement.4) The Ziyun-Luodian Fault is a southeast boundary fault of the Xuefeng intracontinental tectonic system, which includes various structures on each segement. Furthermore, this fault can be divided into 4 segments, and pop-up structures, ramp-faults, imbricate thrustings and wedge-thrustings are identified. Shallow structure of the Fault was a series of top-to-southwest thrust faults which were decoupled with one large-scale normal fault of the fault at depth, Its shallow thrusting is related to the Yanshannian extrusion of the Precambrian basement from the Xuefeng uplift belt. 5) Combined with timing of major faulting, deformation propagation velocity, as well as timing and geochemical characteristics of magmatism and the evolution of the ancient Pacific Plate, it is deduced that Caledonian, Indosinian and Yanshannian events in south China are all intracontinental events. Caledonian event is probably caused by convergence of two intracontinental blocks, i.e. Cathaycian and Yangtze Blocks. Indosinian event, however, is a result of crust overriding under the far-field compression induced from three continental margins. Mesozoic lithospheric thinning of South China, which relates with changes of subduction direction of the ancient Pacific Plate, may make direct influence on the thrust- decollement system during Yanshannian period.