Geomechanics of polygonal fault systems: a review

Abstract

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.

Displacement field in contraction‐driven faults

“…Shin et al [2008] also show that shear displacement can localize under these stress conditions when the sediment exhibits post peak strain softening, and listed multiple soil conditions that exhibit this behavior. Low residual friction on polygonal fault planes is one possible mechanism to explain the displacement accumulation, as identified by Goulty [2002Goulty [ , 2008 and Goulty and Swarbrick [2005]. However, low residual friction does not explain the genesis of faults or the localization of shear into planes, and this model must also explain the magnitude of the observed displacement fields associated with polygonal faults for the range of physical properties that capture the diversity of sediment types in which polygonal faults are hosted.…”

Seismic Expression of Polygonal Faults and Its Impact on Fluid Flow Migration for Gas Hydrates Formation in Deep Water of the South China Sea

“…The model of low coefficient of friction raised by Goulty [8] may support the function of conduits of polygonal faults as well as conventional faults. It was demonstrated that low coefficients of residual friction in fine-grained sediments might be key to the formation of the polygonal system, and there was no evidence that nucleation of those faults that evolve into polygonal systems differs fundamentally from the processes involved in the nucleation of conventional faults in soft sediments [8].…”

“…It was demonstrated that low coefficients of residual friction in fine-grained sediments might be key to the formation of the polygonal system, and there was no evidence that nucleation of those faults that evolve into polygonal systems differs fundamentally from the processes involved in the nucleation of conventional faults in soft sediments [8]. Previous studies show that polygonal faults often initiate at shallow burial depth [2,7,29].…”

“…With increasing buried depth, compacted dewatering continuously occurs in layers underlying polygonal faults, and expelled fluids migrate up through polygonal faults and rock porosities to arrive at the seabed. However, with relation to different formation models of polygonal faults, the transporting mechanisms of fluid flows through polygonal faults are controversial [6,8]. The preferred way of fluid flows may be episode eruptions provided by Cartwright [6].…”

“…Up to now, more than 50 basins have found the existence of polygonal faults. Some geologists presented different formation mechanisms, including density inversion [3,4], gravity sliding or collapse [5], episodic hydrofracturing [6], "volumetric contraction" [7], and low coefficients of friction [8].…”