Reconstruction of Tectonic Stress Magnitude of the Yanshanian Period in the Pingdingshan Mining Area by Using the Microstructural Estimation Method.

To investigate the tectonic deformation mechanism and quantitative characteristics of the paleo-tectonic stress field in the Pingdingshan mining area, based on the theoretical method of tectonophysics and rock mechanics, the macroscopic, microscopic, and ultra-microscopic tectonic deformation characteristics and mechanisms were studied with the observation data of the tectonic deformation. The evolution periods of the paleo-tectonic stress field were obtained, and the main diagenetic period of tectonite was determined in the mining area. The magnitudes of paleo-tectonic stress were quantitatively calculated in the Late Yanshanian period of the mining area. The results showed that the macro-fractures of the Permian-Carboniferous sandstone formation were mainly tectonic fractures and some fractures were diagenetic fractures. The types of tectonic fractures included the layer-controlled tensile fractures, the cross-cutting shear fractures, and the bedding-parallel shear fractures. The types of microscopic fractures included tensile fractures, shear fractures, compressive-shear fractures, and tenso-shear fractures in the research area. The ultra-microscopic tectonic types of tectonites included the dislocation lines and dislocation walls. And the intracrystalline plastic deformation mechanism of tectonites was dominated by the dislocation glide and dislocation climb. The Pingdingshan mining area mainly got through three periods of tectonic stress field movement, including the Indosinian period, Yanshanian period, and Himalayan period. The direction of the maximum principal stress had experienced a transformation process of NS → NW → NE → NNE. Under a multistage tectonic stress field, the mining area formed a geological structure on a series of NWW-NW direction folds and faults with NNE-NE direction secondary faults under the control of the Likou syncline. The main diagenetic period of tectonites was the Late Yanshanian period in the mining area. Based on the lattice dislocation density measurement method, the ultra-microscopic structure of the secondary quartz enlarged edge on tectonite samples was tested in the diagenetic period. The magnitude of paleo-tectonic stress was obtained in the Late Yanshanian period, and the values were 40.70-62.50 MPa. The result accuracy was verified by the subparticle size method. And the closer the Likou synclinal axis was, the greater the paleo-tectonic stress was. Research results provide the theoretical basis for quantitative analysis of the paleo-tectonic stress field in the Pingdingshan mining area.