AAPG Middle East Region Geoscience Technology Workshop, Rift Basin Evolution and Exploration

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The Value of Deep Seismic Data: An example from the North Western Australia that Exposes Ancient, In-Place Oceanic Lithosphere

Abstract

Deep reflection seismic data from the North West Shelf of Australia demonstrate the likely presence of in-place Permo-Carboniferous oceanic crust beneath parts of the North Carnarvon Basin and require a new model for the tectonic development of the area. It is generally considered that the oldest in-place oceanic crust and lithosphere on the planet is of Jurassic age in the western Pacific Ocean, with potentially older examples in the Eastern Mediterranean (Müller et al 2008; Granot 2016). Here, through integration of deep reflection seismic data, wide angle seismic study results, gravity and isostatic modelling together with other regional geological data, we demonstrate the likely presence of deeply buried, Paleozoic oceanic crust. The new consistent model fits all available datasets, highlights inconsistencies in existing models for the area and proposes an in-place oceanic crust which is at least 75 Million years older than the current oldest recognized oceanic crust buried by, up to 20km of sedimentary section. The geological history of the Australian North West Shelf has been dominated by two tectonic events; one of Permo-Carboniferous age which created the major depocentres of the present-day margin, and a younger Jurassic event which continued until the eventual development of Lower Cretaceous oceanic spreading in the South Indian Ocean. Previous models for the area have often focused on the importance of this later Jurassic event and have considered the underlying crust to be stretched continental material leading to eventual Cretaceous break-up. This focus is understandable given its intimate relationship to the present day Indian Ocean, its extensive and highly prospective structuration and the fact the event is observable and mappable on available existing industry seismic data. Here, we demonstrate that the earlier Permo-Carboniferous event was in fact the dominant event in the history of the basin and lead to the development of a small oceanic basin which has been filled by the significant sediment influx through Permian and Triassic times including the thick Mungaroo delta. We show that the later Jurassic event was, at a crustal scale, quite localized and minor in comparison. These results require a re-evaluation of the plate models for the break-up of Gondwanaland and will also have implications for the expected heat-flow history of the basin which may impact models for hydrocarbon generation in the area. References Granot, 2016, Paleozoic oceanic crust preserved beneath the eastern Mediterranean, Nature Geoscience, 9, p. 701-705. Müller, R.D., Sdrolias, M., Gaina, C. & Roest, W.R., 2008, Age, spreading rates and spreading asymmetry of the world’s ocean crust. Geochemistry. Geophysics