Geophysical lineaments: Clues to the role of basement architecture in tectono-depositional processes
Elliott, C. I.; Northwest Shelf Study Group,
published: Jan 1, 1999
Open Access (paper can be downloaded for free)
The Northwest Shelf, Western Australia is an example of a passive rift margin and a Palaeozoic/Mesozoic depocentre adjacent to Archaean and Proterozoic terrains. The identification of large-scale discontinuities (lineaments) from regional potential field data suggests that pre-existing basement elements have had a strong influence on the depositional and tectonic history of the rift margin. The lineaments are typically hundreds of kilometres in lateral extent and are generally interpreted as zones or "corridors" often exceeding 100 km in width. In several cases, the gravity corridors can be correlated with those that have been described in earlier publications by Elliott, and can be reconciled with well documented crustal structures (eg. G5 corridor with the Halls Creek Mobile Zone). Several previously unpublished regional lineaments were also identified and importantly, these are clearly seen to transect geological terrain boundaries. Through the integration of numerous image-processed versions of Bouger, Free-Air and Geosat gravity, and shipborne magnetic data sets of the onshore-offshore Northwest Shelf region, a pattern of 20 regional gravity and 10 regional magnetic lineaments has been identified. This has been interpreted to represent, in the most part, an ancient and reactivated sequence of crustal weakness zones. Correlations between the lineament framework and deep-seismic reveal that, at least in the cases investigated, the lineaments reflect zones of deep crustal faulting. The lineaments also coincide with depocentre hinge zones, changes in orientation of depocentre axis, and concentrations of aligned tectonic elements. An important observation is the onshore-offshore continuity of many of the corridors which show a tectonic influence in both environments. This study has demonstrated that regional lineaments can be traced, with apparent lateral continuity, from onshore Proterozoic terrains through the offshore Westralian sedimentary sequence of the Northwest Shelf. The implications of these observations are that pre-existing structural weaknesses have, through reactivation, influenced the tectono-stratigraphic history of the offshore depositional sequence. The importance of identifying and understanding the timing relationships and structural style of these regional lineaments is of paramount importance to the petroleum industry in the sedimentary basins, and to the minerals industry in the older basement terrains.