Deformation-related coalification: Significance for deformation within shallow crust

With a low mechanical strength and high Poisson's ratio, coal can exhibit brittle or ductile deformation when exposed to lower levels of stress, strain rate, pressure, and temperature compared to the adjacent sedimentary rocks (sandstone, mudrock, and carbonate). The physical and optical properties of deformed coals have been extensively examined. However, it is still unclear if within the shallow crust, tectonic stresses can enhance the coalification and how the coal organic structure responds to ductile deformation. Here, dynamic-related coalification was reviewed to identify the significance of ductile deformation within the shallow crust. Compared with the brittle and transition stages, the dynamic-related coalification initiated within the ductile deformation could also exert a more substantial impact, resulting in a distinct evolution such as enhanced structural alignment, increased curvature in aromatic molecules, higher aromaticity, and generation of secondary defects. Although these alterations are similar to those resulting from normal thermometamorphism, their patterns are influenced by the stress mode and tectonic exposure, which was related to the host basins. Either tension or extrusion stresses under a high strain rate (corresponding to normal or high-angle reverse faults) are favorable for brittle deformation, where the dynamic-related coalification was presented as a stress degradation process. However, ductile deformation (or even rheology) was produced from compressive-stress, compressive-shear stress, or pure shear stress accompanying low strain rates (corresponding to the nappe, layer slip, and strike-slip faults). There, the deformation-related coalification was promoted to a greater extent and was present as stress condensation. Thus, the deformation extent of coal and the accompanying deformation-related coalification is of significance for identifying the ductile deformation within the shallow crust. The outcome of this paper can be applied to the geological analysis of coalfield structure in tectonically complex regions.