Abstract:

A series of NNE trending folds were formed by the major tectonic deformation in western Hubei Province and the major structural features of this region had been investigated systematically. However, a comprehensive study on paleotectonic stress field of the foldings has not been underway till now. Previous researchers had only studied on the direction of paleotectonic stress according to the fold features and fault slip directions and, had caused a number of different points of view on the Yanshanian tectonic evolution of Lichuan and Qiyueshan folds. The present paper gives a systematical research on the paleotectonic stress field of Yanshanian period including the stress direction and magnitude. According to the stratigraphic contact relationship in this region, the Jurassic and Triassic strata are in the same tectonic layer, due to their conformity relationships. However, the Cretaceous system is unconformably covered on the Jurassic and Triassic strata, which indicates that the tectonic event was occurred between the Late Jurassic and Early Cretaceous. On the basis of the data measured in the field on the occurrences of the conjugate shear joints and the attitude of the strata on both limbs of the foldings, combining with the bulk analysis on the characteristics of tectonic deformation to determine the compressive zone, it comes to the author that the directions of maximum principal compressive stress axes are generally NWSE trending. The quartz sandstone samples collected from the Jurassic and Triassic strata were tested to investigate and statistically analyze the intracrystalline free dislocation density by transmission electron microscope, then to calculate the differential stress valuesby using the empiric relationship formula. The results of the above research will provide a solid foundation for the quantitative research on paleotectonic stress field in that area.

Key words:  paleotectonic stress field, intracrystalline free dislocation density, differential stress values, Yanshanian period, western Hubei