Rift processes in the Westralian Superbasin, North West Shelf, Australia: insights from 2D deep reflection seismic interpretation and potential fields modelling

Abstract

Acquisition of long-offset (8–10 km), long-record length (12–18 sec), 2D reflection seismic and ship-borne potential fields data (WestraliaSpan by Ion/GXT and New Dawn by PGS) on the North West Shelf of Australia provide the opportunity to study rift processes in the context of modern models for rifted margins (Manatschal, 2004). Basement and Moho surfaces were interpreted on seismic reflection data. Refraction models from Geoscience Australia constrain Moho depth and initial densities for gravity modelling through standard velocity-density transformation. 2D joint inversion of seismic reflection and gravity data for Moho depth and basement density constrain depth to basement on seismic. 2D gravity and magnetic intensity forward modelling of key seismic lines constrain basement thickness, type and density. Late Permian and Jurassic-Early Cretaceous rift zones were mapped on seismic reflection data and constrained further by inversion and forward modelling of potential fields data. The Westralian Superbasin formed as a marginal basin in Eastern Gondwana during the Late Permian rifting of the Sibumasu terrane. Crustal necking was localised along mechanically-weak Proterozoic suture belts or Early Paleozoic sedimentary basins (such as Paterson and Canning). Mechanically-strong cratons (such as Pilbara and Kimberley) remained intact, resulting in necking and hyper-extension at their edges. Late Permian hyper-extended areas (such as Exmouth Plateau) behaved as mechanically-strong blocks during the Jurassic to Early Cretaceous continental break-up. Late Permian necking zones were reactivated as failed-rift basins and localised the deposition of the Jurassic oil-prone source rocks that have generated much of the oil discovered on the North West Shelf.

Low‐Angle Normal Faults on the NW Shelf of Australia: Implications for Late Paleozoic Rifting

“…Analysis of potential fields data and refraction data suggests that the crust of the Exmouth Plateau in the Northern Carnarvon is thin (<6 km), extremely dense (∼3,000–3,200 kg/m³), and seismically fast (∼7.2–7.6 km/s) compared to typical continental crust (Belgarde et al., 2015). Crustal‐scale syn‐tectonic faults thin the crust within the “necking” domain at the edge of the Pilbara craton and controlled the formation of inner rift basins during later Mesozoic extension (Deng et al., 2021).…”

“… (a) Crust domains of the NW Shelf of Australia (modified from Belgarde et al., 2015 and Reeve at al., 2021). NCB, Northern Carnarvon Basin; OCB, Offshore Canning Basin; BRB, Brose Basin; BOB; Bonaparte Basin.…”

“…2D and 3D seismic data provide clear and detailed information of basin structures in the upper to mid crustal levels (0–20 km). A 2D regional seismic line (Figure 1) across the NW Shelf continental margin was acquired and processed by ION Geophysical in 2014 (Belgarde et al., 2015). It is over 700 km long with a record length of 18 s (TWT), deep enough to interpret Moho and basement architectures beneath the thick sedimentary succession.…”

“…The North West Shelf of Australia is a passive continental margin that extends for over 2,000 km in length and up to 600 km in width (AGSO North West Shelf Study Group, 1994; Belgarde et al., 2015; Direen et al., 2008; Longley et al., 2002; Marshall & Lang, 2013; Reeve et al., 2021). It encompasses four large basins with the Northern Carnarvon Basin in the southwest (Figure 1a).…”

“…It encompasses four large basins with the Northern Carnarvon Basin in the southwest (Figure 1a). The margin developed through widespread continental rifting in the Late Paleozoic that formed the Westralian Superbasin (Yeates et al., 1987) and during which continental crust of the Northern Carnarvon Basin was thinned by two‐thirds to ∼10 km (Figure 1b) (Belgarde et al., 2015; Hill, 1994; Pryer et al., 2014; Stagg & Colwell, 1994). Post‐rift deposition amassed massive “layer‐cake” package of Triassic sediments of about 10–15 km in thickness on the Exmouth Plateau (Longley et al., 2002; Marshall & Lang, 2013; Stagg & Colwell, 1994).…”