Influence of pre-existing rift faults during the development of an overlying fault network, northern Browse Basin, NW Australia

Many rifts and passive margins form fault networks of different generations through time and it is important to understand how they influence and interact with one another. Using 3-D seismic reflection data from the northern Browse Basin on the Australian NW Shelf, we investigate the interaction between pre-existing Palaeozoic-Mesozoic rift faults and an overlying Neogene normal fault network. We focus on characterising the geometry, organization, throw and strain of the two fault networks in order to constrain the influence of the older rift faults on the development of the younger Neogene faults. The two generations of fault network were mapped in detail and analysed at two stratigraphic horizons relevant to their timing. The older rift faults comprise NE- to ENE-trending conjugate faults that form a series of horsts and grabens with evidence of Palaeozoic and Mesozoic syn-sedimentary fill. Most rift faults do not continue into overlying Paleogene and Neogene post-rift sediments, however, some propagate all the way through the stratigraphic succession. ENE-trending Neogene faults often form conjugate sets and en echelon arrays that cluster directly above the earlier rift faults, suggesting reactivation of the rift faults and interaction and linkage between the two fault networks. Analysis of the spatial distribution of faulting and throw within each cluster of Neogene faults indicates areas of distributed deformation, with numerous faults and lower throws, as well as areas of localized deformation, with fewer faults and greater throws. These correlate with throw distributions along traces of the earlier rift faults where highs and lows in fault throw coincide with localized and distributed areas of Neogene faulting, respectively. Furthermore, 3-D reconstruction of fault planes shows that the Neogene fault segments with the greatest throws are often rotated splays connected at depth to an earlier rift fault plane. Overall there is direct interaction between the two generations of faults, which are both soft and hard linked in places. In both cases the earlier rift faults reactivate and control the spatial organization, displacement distribution and thus strain localization within the later Neogene fault network illustrating the importance of pre-existing structures on fault network development.