Geometry and origin of a polygonal fault system

A fault array in South Australia, interpreted from a 3D onshore seismic sur vey, shows fault traces on the lowermost mapped horizon of a shale-dominate d sequence which outline polygonal cells averaging 1.4 km in diameter. The cell boundaries coincide approximately with the downward terminations and n ear convergence of conjugate pairs of normal faults. The pattern becomes le ss spatially ordered on higher horizons where faults still show a near-isot ropic strike distribution. Maximum throws, c. 80 m, occur c. 400 m above th e downward terminations of the faults. The faults have a systematic geometr ic relationship with folds, with anticlines in the mutual hanging walls of fault pairs and broader footwall synclines that define the shallow dish for ms of the polygons. Polygon boundaries coincide with anticlinal ridges on t he 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 d ensity inversion, the data do not exclude the possibility of lateral extens ion.