Geometry and origin of a polygonal fault system.



J. J. Walsh, J. Watterson1, A. Nicol2, P. A. R. Nell, & P. Bretan3.
1 - Present address: Marine Biology Laboratory, Port Erin, Isle of Man, IM9 1LD.
2 - Present address: Institute of Geological & Nuclear Sciences, Gracefield Road, P.O. Box 30368, Lower Hutt, New Zealand.
3 - Present address: Badley Earth Sciences Ltd, North Beck House, Spilsby, Licolnshire PE23 5NB.

Abstract - A fault array in South Australia, interpreted from a 3-D onshore seismic survey, shows fault traces on the lowermost mapped horizon of a shale dominated sequence which outline polygonal cells averaging 1.4km 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. 70m, occur c. 400m above the downward terminations of the faults. The faults have a systematic geometric relationship with folds, with anticlines in the mutual hangingwalls of fault pairs and broader footwall synclines which define the shallow dish forms of the polygons. Polygon boundaries coincide with anticlinal ridges on the interface between the faulted sequence and an underlying 35m 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.


Journal of the Geological Society London, 157, 151-161, 2000.