The Structural Style And Structural Physical Simulation Study Of Laizishan Dome In Guizhou Nanpanjiang Basin

Nanpanjiang basin in Guizhou province came into being many dome structures after experienced the Indosinian-Yanshanian orogeny, and the domes controlled the distribution of low temperature deposits, Laizishan dome which controls Lannigou super large Au deposit is a perfect example. Detailed field geological survey indicate that the domes are the results of superimposed folds, the structural types under the controls of layered deformation and superposition.According to tectonic unconformities owing to the Guangxi movement during late Silurian to early Devonian, the Indosinian movement during Mid Triassic to early Jurassic and the Yanshanian movement during Mid Jurassic to early Cretaceous and structural characteristics, we divided the basin cover into three main structural layers including Caledonian、Hercynian and Indosinian – Yanshanian layer. Based on the structural layers, we ascertained three main weak layers including carbonaceous black shale of the Niutitang formation(?1n), argillaceous rocks and calcareous mudstone of the Yujiang formation(D1y), coal seam of the Longtan Formation(P2l) and argillaceous rocks and siltstone of the Luolou formation(T1l) through mechanical parameters of formation rock, these weak layers is also like to be main detachment layers, according to the shear relationship of axial cleavages inferred that the NNE trend fold superposed the NW trend fold.Based on the attitude of strata, the NW and NE structural deformation sub-region in which the geometry of the fold is stable are ascertained in the method of π circle projection. Combined cross sections of sub-region folds and projection analysis we know that the Laizishan dome is formed by the NW and NNE trend vertical and horizontal folds. We screened out the axial cleavages matched with sub-region folds by measuring and projection analyzing of regional cleavages. Combined with the strata involved into superposed deformation and magmatic rocks chronology data, we deduced the Laizishan dome was the result of Yanshanian deformation, the early fold was the result of lateral compression by Xuefengshan during early Yanshanian and the later fold was the result of thrust-nappe in southwest of Guizhou.Based on the above geological analysis, the author established two kinds of sandbox model about superposition, there are three sets of weak layer in each model. Observing the deformation behavior of strong and weak layers in the process of deformation through changing the material of weak layer, the time and speed of formations, the purposes are to confirm the main factors that affect the superimposition styles and types and compare the superimposition phenomenon produced by the conjunction of structures and compounding of structures.Finally, through the structural analysis and sandbox modelling, we draw that Laizishan dome is the result of the late Yanshanian NNE trend buckling fold superposed on the early Yanshanian NW trend buckling fold, the superposed type belongs to the migration fold superimposition of compound action. Sandbox modelling proves that the bending and migrating of axial plane and hinge of the early fold produce the superimposition phenomenon. The layered deformation action of weak layers makes the differences of superposed interference patterns in different depth, and the difference of rock strength and the intensity of deformation are the main factors that affect the superimposition styles and types.