Structural Pattern And Regional Evolution Of The Developmental Area Of The Weng'an Dome In The Eastern Yangtze Block

The Weng'an dome is located at the combine zone between the Hubei-Chongqing-Hu'nan-Guizhou tectonic belt and the east-west striking Qianzhong uplift,which is close to the eastern margin of the Yangtze Block,and it is essentially a structural superimposed dome.As a representative of the dome group,its tectonic-thermal evolution is of great significance for understanding of the tectonic evolution of the Xuefeng Intraplate Orogenic Belt,and even for the formation and evolution of tectonic patterns in South China.Through the comprehensive analysis based on geological mapping and previous data results of this area,5 different types of unconformity surfaces are identified from the bottom upward within the Weng'an dome and its peripheral areas.They are Nh/Pt3 unconformity surface,D2/S2 unconformity surface,T3/T2 parallel unconformity surface,K2/underneath strata unconformity surface and Kz/underneath strata unconformity surface.Corresponding to the unconformities and the key parallel unconformity,five major structural layers,which are distinguished in the Weng'an Dome and its peripheral areas,are the Pt3 structural layer(?),Nh-S2 structural layer(?),D2-J2 structural layer(?),K2 structural layer(?),and Kz structural layer(?)and several sub-structural layers.Tectonic episodes of the D1,D2,D1,D3 and D4 are divided from early to late.The D1 deformation is represented by NNW-striking and near-upright spaced cleavage,which only occurred within the Pt3 structural layer.The detrital zircon age results dated the D1 at 772 to 691 Ma.The D2 deformation is represented by east-west striking fold,the dextral strike-slip and thrust fault,and east-west striking and near-upright spaced cleavage.The D2 occurred in the Nh-S2 and its underlying structural layers is dated at 426 to 415 Ma by detrital zircon U-Pb age results.The D1 period,which is represented by the parallel unconformity between the upper and lower Triassic and is dated at 246-233 Ma,has special significance for regional geological comparation.The D3 period is represented by NE-SW-srtiking fold,strike slip-thrust fault,cleavage and associated secondary structural assemblages.These of the D3 deformation occurred within the D2-J2 and its underlying structural layer to modify and superimpose the D1,D2 and D1 deformations.The D3 is dated at 168-150 Ma by detrital zircon U-Pb age results.The D4 occurred within the K2 and its underlying structural layer is characterized by high-angle normal faults and small rift basins.Taken together,the Weng'an dome and its adjacent areas experienced D1,D2,D3 and D4 tectonic episodes.The D1 reveals the Late Neo-Proterozoic orogenic and post-orogenic extensional tectonics preserved in the eastern margin of the Yangtze Block indicate Rodinian orogenic belt.The D2 represented by the EW-striking fault-fold belts reveals the strong Caledonian tectonic movement within the Yangtze Block.The D3 NE-striking fold-fault belts re-deformed the earlier deformations and built the basic tectonic framework represents the Yanshanian movement in the eastern margin of the Yangtze Block.The D4 period is characterized by regional uplift and formation of local fault basins occurred in the Cenozoic.It i s worth noting that a weak Indosinian movement represented by the D1 parallel uncomformity in this area,although it was strongly occurred in the Yangtze Block.We concluded that the representative Weng'an dome in the Yangtze Block is a typical superimposed dome,which experienced multi-stage structural superimposition and reconstruction.