Geomechanics of polygonal fault systems: a review

Layer-bound systems of polygonal faults are found in sequences of very fine-grained sediments that have typically undergone passive subsidence and burial. In the absence of tectonic extension, the heave of the faults must be complemented by horizontal compaction of the sediments. Density inversion, syneresis and low coefficients of friction on fault planes have all been proposed as causal mechanisms for the development of polygonal fault systems, but most sequences that contain polygonal faults are not underlain by sediments of lower density and there is a lack of evidence to support the idea that syneresis is responsible. Laboratory measurements of clay properties and a recent field test based on well data strongly suggest that low coefficients of residual friction in fine-grained sediments are key to the growth of faults that eventually develop into polygonal systems. However, coefficients of residual friction apply to faults only after initial slip has taken place, so some other mechanism must be responsible for the initial nucleation of the faults. Various speculative suggestions have been made, but there is no evidence that nucleation of those faults that evolve into polygonal systems differs fundamentally from the processes involved in the nucleation of other faults in soft sediments.