Features And Formation Mechanism Of The Eastern-Sichuan-Western-Hunan-Hubei Fold-thrust Belt In South China

The Eastern Sichuan–Xuefeng fold-thrust belt(CXFTB)is located in the middle and upper Yangtze block of South China.It has received significant attention due to its distinctive deformation style.Exploration surveys have been conducted to reveal the structural geometry of this region and the findings indicate that the strucutral evolution is closely related to the formation of natural gas reserves.However,while surface geological studies and data from surveys for oil and gas have revealed the structural geometric characteristics of this area.However,there have some deformation characteristics such as different fold patterns,structural vergence reversal,and creation of horsetail-shaped folds belts,and none of these very unique geological deformations have been systematically analyzed by previous.Understanding of complex deformation patterns and evolution in this study area remains incomplete.There are multiple mainstream deformation models;e.g,progressive expansion deformation and genetic evolution are commonly applied by geologists.Although the modes of progressive deformation and progressive attenuation deformation have drawn considerable attention to the formation and controlling factors of the ejective and trough-like folds.However,the interior of the fold-thrust belt in the Eastern Sichuan and Western Hubei-Hubei domain has complex deformation characteristics and the nature of its formation is unclear.Simultaneously,the new apatite-fission-tracking data demonstrates that the uplift and erosion of the Dayong and Qiyueshan faults are almost synchronous with the deformation during the propagation process.Therefore,the above explanations of the formation patterns in the study area proposed in previous studies are still insignificant.Hence,the deformation evolution and the formation mechanism in this area should be further explored.Studies adopting 2D analogue modelling have suggested the importance of ductile basal décollements of Cambrian gypsolith,Sinian pelite and Proterozoic schist in the deformation evolution of the CXFTB.However,due largely to the lack of 3D analogue modelling experiments,the deformation evolution of the CXFTB is still far from being fully understood.Therefore,based on the previous studies,the fault system was interpreted via remote sensing,the geological profile was evaluated,and the structural patterns in the study area were systematically analyzed.In this study,we adopt 3D physical models to demonstrate the deformation evolution of the CXFTB.Based on three series total and seventeen groups of different physical models,the following findings were made: 1)The deformation propagation of the fold-thrust belts in eastern Sichuan-Western Hunan and Hubei in Mesozoic is mainly controlled by boundary faults and the rheological structure of the strata.The structural deformation began at the Dayong-boundary fault located at the northern front between the Xuefeng uplift and Qiyueshan-boundary fault in the eastern Sichuan block;it then extended to the northwest along these two boundary faults and terminated at the Huayingshan fault.2)The distribution of the planar structural patterns in the trough-like and ejective folds of the fold-thrust belt was affected by the structural geometries of the Dayong-and Qiyueshan-boundary faults.3)The formation of the horsetail-shaped fold belt in this area is closely related to the shapes of Huayingshan fault and Qiyueshan boundary fault in the Eastern Sichuan;The ratio of brittle / ductile strata and thickness difference of the brittle layers have important controls on the wavelength and pattern of the folds;the appropriate base friction is an important condition for the formation of the horsetail-shaped folds and the difference of the basal rheological strength between the Sichuan Basin,the CXFTB may also have an important effect on the formation of the horsetail-shaped fold belt.We found that the structural deformation in the Western Hunan-Hubei domain was controlled by a ductile layer in the lower crust while the deformation characteristics in the Eastern Sichuan domain were controlled by the crustal rheological structure under the Sichuan block.4)Moreover,the analysis of the strain field in each experimental stage revealed that the natural gas reservoirs are mainly distributed in the weak-strain regions including the gentle-fold belt in the Dazhou and Changning regions in the southeast region of the Sichuan Basin and the synclines in the Western Hunan-Hubei domain;these are promising prospects for natural gas reservoirs.