Northwest Shelf

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Introduction

The North West Shelf of Australia is a major hydrocarbon province and a prime example of a complex, polyphased rift margin. The main goal of the NWS stream is to develop numerical models that help explain the complex geological history of the NWS in a plate-tectonic and geodynamic framework. A series of projects focusing on key aspects of geodynamics, tectonics and surface processes are detailed below:

 

The formation of marginal basins and continental ribbons

To understand the thermal and dynamics parameters controlling the formation of marginal basins and the detachment of continental ribbons, we have run a number of 2D Underworld experiments. The size of these experiments is large enough to include the entire asthenosphere (down to 660 km), which ensures a self-consistent isostasy, and long enough to allow slab-pull to drive tectonics. This is achieved by forcing the subduction of an oceanic lithosphere until the pull from the subducted slab is strong enough to drive subduction. This occurs for a slab length of about 250 to 300 km. The grid resolution of 2 km (sometimes better) allows resolving first order faults in the upper crust.

A trial and error approach was used to select from laboratory-derived data the thermal and mechanical properties of crust and mantle materials. The aim here was to select plausible values to consistently deliver the following experimental outcomes:

1) The subducting slab must be able to reach the base of the asthenosphere without detachment.

2) The slab terminal velocity should not exceed 10 cm/yr.

3) Deviatoric stresses in the lithosphere must not exceed a maximum value in the range of 200 to 300 MPa for the lithospheric mantle and 100 to 150 MPa for the crust.

4) Subduction should be driven by slab pull alone for a subducted slab no longer than 250-300 km.

5) Subduction zone should be strongly asymmetric.

6) The continental crust must be able to rift under the action of the slab-pull force alone.

7) The continental crust must have a Moho temperature between 500 and 600ºC, and the base of the lithosphere (defined by the isotherm 1300ºC) must be between 100 and 150 km depth.

Part of this work is summarized into two Honours thesis, which examine the formation of continental ribbon in the context of passive margins (Nikita Golkin, 2015) and that of active margins (Bayley Payten 2016).

Understanding the inheritance of Palaeozoic fault patterns on influencing Mesozoic basin architecture

Palaeozoic structures along the North-West margin of Australia have long been recognised as fundamental events responsible for the formation of the offshore basins that comprise this prolific hydrocarbon producing region. However, the tectonic setting in which this rifting occurred remains unclear. New seismic interpretation and mapping of the geometry and tectono-stratigraphy of Permian and Carboniferous structures of the inboard Barrow, Dampier and Beagle sub-basins of the Northern Carnarvon Basin has shown:

1) Two distinct orientations of structures that provide evidence for a poly-phase rift history of the North-West margin during the Palaeozoic.

2) NNE trending faults of the Candace Terrace (Barrow sub-basin) were initiated in the Carboniferous or Devonian but were underfilled, resulting in erosion of the fault block crest and filling of the remnant rift-related topography by conformable sequences of later Permian and Triassic sediments.

3) NE-SW oriented faults of the Mermaid Nose (Dampier sub-basin) experienced a distinct phase of Permian activity and are unconformably overlain by Triassic sediments. In the Beagle sub-basin, both orientations of structures are present.

Throughout the marginal fault systems of the Northern Carnarvon Basin, complex fault geometries and significant deformation of hanging wall strata are associated with reactivation of the eroded fault block crests and overprinting of faults during subsequent Mesozoic extension. This has implications for understanding the geodynamic evolution of a poly-phase extensional continental margin.

More details about this work is can be found in I’Anson et al., (2019) Marine and Petroleum Geology.

Understanding the evolution of source-to-sink systems in the NWS from a continental-scale perspective

The Paleozoic and Mesozoic sedimentary sequences of Western Australia host vast resources of hydrocarbons. Despite their economic importance, the origin of their sedimentary sources is strongly debated. More rigorous understanding of the sediment provenance is important for the hydrocarbon industry as it allows better characterisation of reservoir quality and potential correlation between known hydrocarbon reservoirs and reservoirs yet to be discovered. Comparison of U-Pb zircon chronology and Hf isotopic data (149 samples and 6240 zircon grains) from Triassic and Paleozoic sedimentary units from the Northern Carnarvon, Canning and Officer basins with data from crystalline rocks data indicate:

1) detritus in the Officer, Canning and Northern Carnarvon basins were fed by two main sediment transport pathways: a south to north system with headwaters in the mountains of East Antarctica and a subsidiary network starting in central Australia.

2) Depth thickness scaling relationships, which provide an estimate of drainage area, independent support the notion that this was a supercontinental-scale system.

3) Supercontinental regimes allow sediment dispersal systems to be long-lived, as they provide both an abundant sediment supply, due to erosion of large-scale, collision-related internal mountain systems, and a stable, large-scale configuration that lasts until breakup.

More details about this work is can be found in Morón et al., (2019) Geology.

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