The Characteristic And Evolution Of Gliding Detachment Structure In Jiujiang-Ruichang Area

Jiujiang-Ruichang area is in the northern of Jiangnan Uplift, which is between Qinling-Dabie orogenic belt and Jiangnan Uplift. It belongs to the Jiangnan-Xuefeng intra-continental tectonic system, and it assumes an arc-shaped structural belt in the mass. Jiujiang-Ruichang area developed multi-level, multi-period and multi-type gliding detachment. Basement detachment fault, the unconformity between Neoproterozoic and Sinian and Silurian are the territorial detachments, which control over the detachment structures. Deformation is really different in different levels. Four detachment systems (presinian basement, Cambrain-Ordovician, Silurian and Devonian-Triassic) can be concluded.Different types detachment structures developed in different periods and the background of geotectonica. (1) Extrusional detachment:the appearance character of fractures is imbricate thrust fault; the appearance character of folds is Jura-type detachment folds and fult-related folding. Involved basement thick-skinned structure change to cap detachment thin-skinned structure. (2) Extensional detachment:in the name of Lushan metamorphic core complex, the footwall of basement detachment fault is ductility-plasticity formation zone. The formation charactures are complicated superposed folds and flowage fold; the hanging wall is brittleness formation zone, compose structural slices, monocline and graben basin. (3) Gravitational detachment:The structural pattern are major double-deck slip-fold type and back detachment slip sheet type. Extention belt of trailing edge show detachment fold whose axial surfaces dip to SE and normal slip fault sharp dip to NW. Part of normal slip fault underlay with the detachment fault system; The detachment fault's character of middle transition belt show bed-parallel simple shear deformation. Extrusion belt of leading edge show imbricate thrust fault and Jura-type detachment folds.Based on the mesoscopic and microscopic investigation,40Ar/39Ar geochronology studies on the basement detachment fault in Aikou and trailing edge extention belt of gravitational detachment system in Shilipu, zircon LA-ICP-MS U-Pb geochronology studies on the Mesozoic pluton (Yujing granitic intrusion, Haihui granitic intrusion, Donggushan granitic intrusion), ESR geochronology studies on the a quartz veins in northern Jiangnan uplift were carried out, which constrain the evolution history of gliding detachment structure in Jiujiang-Ruichang area: Extrusional detachment during Late-Jurassic to Early Cretaceous, Extensional detachment in original Early Cretaceous(140Ma), Gravitational detachment in the lateness of Early Cretaceous(113Ma) and late Himalayan(43.2-3.4Ma).Discussion on the geological problems of Jiujiang-Ruichang area is with structural analysis, results of geochronology study, stable isotope, homogenization temperature of fluid inclusion, analysis of regional geologic information. (1) Relation between extensional detachment and magmatism:Lushan MCC was the result of different scale extensional detachment and magmatism during the Mesozoic. In the shallow crust, its specific formation process as follows: Extensional detachment was earlier than magmatic emplacement. Decompression of the basement detachment fault leaded the uplift and emplacement of deep magmatic. Then magmatic emplacement leaded the Lushan massif uplifting, which provided the motivity for the gravitational detachment of the cover. (2) Cenozoic geotectonic evolution of Jiujiang-Ruichang area:the evolution history can be classified into two phases. The former is extend and faulted depressions stage (61.5～75Ma). Normal faults and fault basins formed during this time. The latter is squeezing uplift stage (43.2～3.4Ma). Its main feature was gravitational detachment structure. The Indosinian and Yanshanian structures had been intensely reconstructed during Himalayan. Entire eastern China experienced distinct tectonic movement, including southern North China, mid-lower Yangtze and northern Jiangnan uplift. (3) Deformational environment of cap detachment structure:the detachment of cover rocks is shallow (3～5km) thin-skinned structure.