Systematic spacing and topological variations in layer-bound fault systems

Polygonal fault systems, sometimes termed layer-bound faults, are extraordinary features of many fine-grained sedimentary successions and have been described in a significant number of 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 characteristics of 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. This systematic spacing of fault pairs indicates, (a) the presence of a mechanically weaker layer at the base of the fault system and (b) that the horizontal shortening required by the volume loss due to compaction can be accommodated without reaching saturation with respect to fault intensity. Furthermore, topological analysis indicates that areas with different patterns also have different intersection and branch characteristics, and these differences suggest that the growth of layer-bound faults is not explained by a single model. The findings have implications for the genesis and growth of layer-bound fault systems and the potential for cross-stratal fluid flow.