Mark Ireland, Chris K Morley, Richard Davies

Polygonal fault systems are extraordinary features of many fine grained sedimentary succession and have been described from a significant number of deepwater sedimentary basins over the last two decades. Their formation represents an important mechanism by which fine grained sediments compact often resulting in a variety of complex patterns for which several controlling factors have been proposed. Here three-dimensional seismic data from the North West Shelf of Australia are used to interpret previously undescribed characteristic of a layer bound fault systems where systematic horst and graben structures are the dominant structural style. Conjugate fault pairs which form the horsts and grabens frequently have a systematic spacing with graben bounding faults exhibiting a spacing of half that of the horst bounding faults. It is interpreted that this systematic spacing of fault pairs indicates the presence of a ductile layer at the base of the fault system. Furthermore, using topological analysis areas with different patterns and contrasting fault interactions and intersections, are used to show that the growth of layer bound faults may not be explained by a single model of growth. The regular spacing and style of faults described indicate that the growth of layer bound fault systems are strongly influenced by both layer thickness and the ductility of underlying sediments. The findings have implications for the genesis and growth of layer bound fault systems.