九岭山南缘断裂带运动学研究

KINEMATIC STUDY ON THE SOUTHERN MARGINAL FAULT ZONE OF THE JIULING MOUNTAINS,JIANGXI PROVINCE

江南晚元古代碰撞造山带中段九岭山南缘断裂带的运动学研究揭示出三个不同时期的强烈构造变形作用。第一期从北向南仰冲推覆作用，对应一个晚元古代洋盆关闭，板块碰撞造山过程；第二期从ＳＷ向ＮＥ的左旋走滑动性剪切作用，对应一个早古生代华南地体的斜向拼贴增生活动；第三期从北向南的逆冲－推覆作用，对应一个印支构造期的侧向挤压机制。现代地表上的九岭古岛弧南缘断裂带，是一个以走滑剪切力学作用占优势的片麻状糜棱岩带。宏观韧剪组构和显微构造标志为这个断裂带提供了重要的运动学证据。

The Nanchang-Wanzai-Pingxiang fault zone is located in the central part of the late Proterozoic Jiangnan collision belts and is a large ductile shear zone or mylonitic zone with a width of 2-5km. It extends laterally in the ENE-WSW trend for more than 300km along the southern boundary of the Jiuling Mountains, Jiangxi Province. The paper analysed in detail the kinematic feature for the fault zone and the neighbouring areas on its both sides, finding a plenty of geological evidences of polyphase deformation. Our researches reveal three kinds of the most significant shear deformations with different ages and sizes. They can be summed up from north to south or from older to newer as follows:First, the late Proterozoic thrusting deformation with the sense of shear from north to south.The mineral and stretching lineation and the shearing foliation are widely developed in the mylonitic and metamorphic rocks and strata of the Jiuling volcanic arc to the north of the fault zone, lineations plunging 30°±to the NNW 340° ±direction and mylonitic foliations dipping 30-40° northward suggest a slip shear deformation along the foliation planes. The non-coaxial asymmetric fabrics such as pyrite pressure shadows, 'snowball' structures of robated garnets, boudins in an 'en echelon' pattern and 'book-inclined' structures of phyllite slices indicate clearly a thrusting deformation with the sense of shear from north to south. The analysis of isotopic ages and regional tectonics suggests that the thrusting shear deformation was resulted from the late Proterozoic plate collisional orogeny accompanied by final closure of ancient ocean basins in the Jiangnan.Secondly, the early Paleozoic sinistral strike-slip ductile shear deformation.A strike slip shear zone is limited in the Nanchang-Wanzai-Pingxiang zone and are composed of mylonitic gneisses, mylonitic greenschists and mylonitic granites. This mylonitic zone has a linear cartographic shape with mostly subhorizontal lineations and subvertical foliations. The most intense mylonitization occurs in the center of the shear zone and becomes gradually weaker on both sides. Kinematic analysis indicates a sinistral strike-slip movement from SW to NE. Mesoscopic fabric and microstructures in the zone provide some important kinematic evidence. The strike-slip mechanism is related to the early Paleozoic amalagamation and accretion of the South China terranes in a oblique direction toward the southeast margin of the Yangzi plate.Third, ductile-brittle foreland thrusting nappe from north to south in the Indosinian orogeny.From the late Paleozoic to the early Mesozoic, a large size of ductile-brittle thrusting deformation took commonly place in the interior of the Ping-clang-Leping basin to the south side of the fault zone. A series of klippes, composed of the Proterozoic greenschists and the late Paleozoic limestones, fragmental rocks, etc., distribute intermittently in a E-W direction in the basin and capped onto the late Triassic coal-bearing strata. Only brittle fractures and weak deformed rocks are found on or near the thrusting plane. 'Nail' type slickensides and cleaveges, etc. on the fault plane give a sense of movement from north to south. This deformation is related to the indosinian. When the Qinling Tethys was closed and the Pacific plate subducted toward the South China continent, then, the studied area was strongly compressed and thrusted southward.The fault zone of the southern margin of the Jiuling Mountains is mainly an early Paleozoic sinistral strike-slip ductile shear zone or gneissose mylonitic zone where there are a plenty of polyphase structural information. Thus, kinematic studies of the Nanchang-Wanzai-Ping-clang zone is favourable to further understand the plate tectonic framework and the restoration of the evolutional history for the Jiangnanand the South China regions.