Study on the developmental characteristics and mechanism of shale FPZs

•The evolution and anisotropy of fracture process zones with closed symmetric fractures in shale are investigated qualitatively and quantitatively.•The progressive failure process of fractured shale is revealed based on crack tip opening displacement.•The anisotropy of initiation fracture toughness and unstable fracture toughness of Mode I fracture of shale are studied.•The specific tensile softening constitutive forms of shale in different fracture orientations are determined.