Geometry and origin of a polygonal fault system

Abstract

A fault array in South Australia, interpreted from a 3D onshore seismic survey, shows fault traces on the lowermost mapped horizon of a shale‐dominated sequence which outline polygonal cells averaging 1.4 km in diameter. The cell boundaries coincide approximately with the downward terminations and near convergence of conjugate pairs of normal faults. The pattern becomes less spatially ordered on higher horizons where faults still show a near‐isotropic strike distribution. Maximum throws, c. 80 m, occur c. 400 m above the downward terminations of the faults. The faults have a systematic geometric relationship with folds, with anticlines in the mutual hanging walls of fault pairs and broader footwall synclines that define the shallow dish forms of the polygons. Polygon boundaries coincide with anticlinal ridges on the interface between the faulted sequence and an underlying 35 m thick low velocity, low density, overpressured layer. Although the pattern of ridges defining the polygon boundaries is strikingly similar to experimental spoke and hub patterns formed at the boundaries between viscous materials with density inversion, the data do not exclude the possibility of lateral extension.

Investigating the stratigraphy and palaeoenvironments for a suite of newly discovered mid-Cretaceous vertebrate fossil-localities in the Winton Formation, Queensland, Australia

“…10 residual features are interpreted to be associated with tectonic subsidence related to plate motions and underlying mantle conditions, such as thermal relaxation (Gallagher and Lambeck, 1989;Gurnis et al, 1998;Watterson et al, 2000;Veevers, 2006;Heine et al, 2008). However, it has generally assumed that the flat-lying nature of most stratigraphic units in the Eromanga Basin underwent minimal tectonic deformation during or following deposition and that most of the units are relatively conformable.…”

Displacement field in contraction‐driven faults

“…In typical blind normal faults and polygonal faults, the displacement patterns are commonly symmetric with depth. Notable exceptions are when a weak layer is present near the base or where a shallow propagating fault approaches the free surface (sediment-water interface); in these cases, a significant increase in local displacement gradients is generally observed [e.g., Watterson et al, 2000;Nicol et al, 2003;Stuevold et al, 2003].…”

“…For comparison, we plot the maximum throw d versus fault height H in Figure 2 for a collection of 629 faults in polygonal systems worldwide (Note: height H is measured in the vertical direction). These data were selected from polygonal fault arrays where the vertical displacement variation was generally symmetric about a mid-point (see Figure 1b), and where there was little evidence of anomalously weak layers near the base because they distort displacement distribution on individual faults [Watterson et al, 2000]. Similarly, the aspect ratio of faults is known to impact scaling relationships for faults [Nicol et al, 1996], hence, these data come from faults with aspect ratios strike lengthto-height between 0.8 and 2.6.…”

“…Polygonal faults also represent a major challenge for soil mechanics: they are an expression of shear failure, hence the stress path cannot follow the classical 1D k 0 -stress condition that would prevail under zero-lateral strain conditions. Polygonal faults occur widely in passive continental margin basins where hydrate resources are contained [e.g., Berndt et al, 2003], and occur pervasively in sediments that constitute the major sealing sequences for petroleum accumulations [e.g., Watterson et al, 2000;Stuevold et al, 2003;Cartwright et al, 2007]. Therefore, the understanding of polygonal fault systems can be of crucial importance.…”

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

“…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.…”