Normal fault networks and their spatial relationships in Plio-Quaternary sedimentary series: A case study in the Zanjan Depression, NW Iran

Abstract

Well-exposed normal fault networks in the Plio-Quaternary sedimentary series of the Zanjan Depression, Iran, were studied in order to analyse the spatial relationships and evolution of fault and damage zone structures in low-burial (~200 m) conditions. Despite the poorly-consolidated, geologically young nature of sediment, fault networks show several characteristics more typical of faulting in lithified strata. Key controls on the evolution of fault networks are found to be fault interactions, segment linkage, and the mechanical stratigraphy resulting in dip refraction, fault segmentation, the development of fault bends and steps, vertical fault overlap zone breaching, and bed-parallel slip. The data show a decrease of fracture frequency away from the fault cores. The obtained data show that damage zones width increases with fault displacement in a non-linear manner. Furthermore, damage zones appear more deformed and wider in the hanging-wall than in the footwall. A plot of damage zone width versus total fault throw reveals a positive but non-linear correlation, and the best-fit function corresponds better to a power-law relationship. The relationship between displacement and length shows a sub-vertical growth path with large displacement gradients typical of faulting in partially-lithified strata, due to shale ductility accommodating fault tip-related strains. A model for the evolution of the normal fault system is proposed based on spatial distribution of deformation, particularly fault segmentation, segment linkage, and damage zone development, as influenced by initial fault spacing, mechanical stratigraphy and the low burial conditions.