The Superposed Structural Style Of Longshan Dome In Xiangzhong Basin And The Process Of Sandbox Modeling

The Xiangzhong Basin in Hunan Province underwent polyphase deformation and formed a series of tectonic domes.Although previous study focused on the field investigation and geological research,the structural nature and forming process of tectonic dome still a matter of debate.These different opinions barren hinder the understanding for geologic evolution of the region and adjacent area.Therefore,in this study,we chosen the Longshan dome in Xiangzhong Basin as the research objective,and conducted detailed filed geological investigation,structural analysis and sandbox modelling.The results suggest that Longshan dome is a result of deformation,including overprint of two phase or more phases of deformations.The structural pattern of the dome was controlled by the fault-accommodation folds.Based on the development and distribution of angular unconformities and the detachment fault of basement,we revealed two weak layers which is the potential detachment on the region.Attitude of strata and foliation on the region,combined with fold pattern with foliations indicate that the Longshan dome is as result of NE-trending folds superposed on the EW-trending folds.Based on the involvement of strata in deformation and geochronology of deformation and magmatic rocks,we suggest that the Longshan dome is a consequence of superposition of Caledonian and Yanshanian deformations.Furthermore,interpreted with measurement of foliation and lineation on the region,we reconstructed the stress field of the two phases of deformations and structural evolution.We designed the sandbox modelling for two types of interfere folds.According to similarity principal,to observe the different characteristics of deformation in weak and strong layers,we set different thickness,number and materials for competent and noncompetent layers in the models.The structural analysis and sandbox modeling suggest that the two-phased superposed models support that the Longshan dome formed by interfere of Caledonian EW-trending upright fold and NNE-trending folds.There are two key point during the folding process: the axial surface and hinge of early folds were folded and migrated during folding and were cut by the high-angle normal faults;the weak layers and detachment result in variable deformation in different layers and different interfere pattern in different depth.Thus,competent and noncompetent layers result in different structural styles and types of interfere folds,which is the key factor for the formation of superposed dome Based on structural analysis of outcrop-scale deformations,deformation behavior of different layers in the sandbox models and mechanism of interfere folds,we proposed a new mechanism for the process of superposition.