Research Review: North Australian Craton Margin Geometry and Relationship to Deep Crustal Structures

This is a review of an interesting extended abstract presented at the AEGC Conference held in Brisbane in 2023.

Geometry of the margins of the North Australian Craton and correlations with upper crustal structures Authors: Fatemeh Amirpoorsaeed, Peter Betts, Anindita Samsu, Robin Armit, Alexander Cruden.

Craton margins:

• Craton margins are dipping discontinuities, wedge-shaped crustal and lithospheric geometries where younger, weaker rocks collide with stronger, older craton rocks.
• They can be found in locations with high topography, higher metamorphic grade, and boundaries or transitions in lithospheric geometry found in geophysical data.
• There are two types of craton margins, inward dipping and outward dipping.
• Craton margins can be structurally complex because they may record multiple episodes of extension and inversion that can reactivate pre-existing structures.
• The structures associated with craton margins act as pathways for fluid migration and circulation in the crust, contributing to mineral endowment.

Inward Dipping Geometry – Mount Isa Inlier

• The Mount Isa Inlier underlies the eastern margin of the NAC.
• This major lithospheric boundary, between the Mount Isa Block and the burried Numil Seismic Block, coincides with a zone of intense faulting.
• There are three major major crustal-scale structures within the Mt Isa portion of the NAC, the Gidyea Suture Zone, Pilgrim Fault Zone, and the Western Edge Fault.
• These structures all dip toward the craton’s interior.
• Listric faults sole into the regional decollements, which in turn connect with the suture or major fault.
• The Mount Isa Inlier records localized deformation over a distance of ~300 km parallel to the margin.
• The significant rheological contrast between the Mount Isa Block and the adjacent Numil Block, which has a noticeably different seismic signature, results in asymmetrical deformation localized on the major faults at the craton margin at the beginning of extension.
• Asymmetrical graben structures bounded by steeply-dipping normal faults occur in the hanging wall of the Cloncurry Fault.
• The estimated bulk extensional strain in narrow rifts is typically <30%.
• Shortening on the eastern margin of the NAC gave rise to a complex history of inversion.

Outward Dipping Geometry – Arunta Inlier

• The Arunta Inlier forms the southernmost outcropping part of the transition from the NAC.
• The crust thickens to the south, from about ~36km at the adjacent Tanami and Davenport Provinces to ~55km at the southern Arunta Inlier.
• The Arunta Inlier contains the east-west oriented Willowra and Warumpi Sutures.
• These sutures are antithetic to one another.
• The Arunta Inlier margin is an outward dipping structure dominated by crustal penetrating structures.
• The deformation style is symmetrical with a wide deformed zone in the strong mobile belt south of the craton.
• The structures on the north side of the Willowra Suture are crustal penetrating structures, a typical signature of thick-skinned inversion.
• Seismic reflection and gravity data, as well as seismic modeling, suggest that the crust of the Southern Arunta Inlier, along the Warrumpi Suture, has been thrust beneath the overriding lower crust and uppermost mantle of the Central Arunta.

Conclusion

• The NAC margins correspond to zones of intense faulting in the upper crust along major lithospheric boundaries.
• The eastern margin of the NAC dips toward the craton interior while the southern margin dips southward away from the craton interior.
• In the east, there are three major fault zones: the Gidyea Suture Zone, the Pilgrim Fault Zone, and the Western Edge Fault, which all dip towards the craton interior.
• On the southern margin of the Arunta Inlier, there are two major sutures: the Willowra Suture and the Warrumpi Suture.
• The faults in the footwall of the Willowra Suture are more widely spaced and penetrate through the crust to the Moho.

Critical Comment and Scope for Further Work

Below are comments on potenital areas to expand this work:

• Limited Discussion of Underlying Causes: While the paper thoroughly describes the different geometries of the eastern and southern margins (inward vs. outward dipping), it does not go into great detail about the potential causes of these differences. Further discussion on the geodynamic processes responsible for these contrasting geometries would enhance the understanding of the NAC’s evolution.  In addition, 3D modelling of the major structures across the inlier using all of the available deep seismic data could be very revealing and might show an explicit relationship to mineralization particulalry within the craton to the west of the Gidyea Suture.
• Influence on Mineralization: The introduction mentions that craton margin structures can act as pathways for fluid migration, contributing to mineral endowment. However, the paper doesn’t explore this aspect in the context of the NAC margins. Investigating the potential relationship between the observed geometries and mineralization patterns could be a valuable addition.
• Broader Implications: The conclusion emphasizes the correlation between structural architecture and craton margin geometry but doesn’t elaborate on the broader implications of these findings. Discussing how these insights could be applied to other craton margins or contribute to a better understanding of craton formation and evolution would add value to the paper.
• Additional Data and Modeling: The paper primarily relies on gravity, magnetic, and seismic reflection data. Incorporating other geophysical datasets, such as the growing volume of broadband magnetotelluric data, could provide a more comprehensive understanding of the crustal structure and dynamics. Additionally, more advanced 3D modeling techniques could further refine the interpretations and improve the visualization of the subsurface structures.

Original Abstract

The margins of the North Australian Craton (NAC) are major lithospheric boundaries that correspond to zones of intense faulting in the upper crust. The sutures defining the edges of the eastern and southern margin of the NAC have different geometries. The eastern margin (Mount Isa Inlier) is characterized by a west-dipping geometry that dips toward the interior of the craton (inward), whereas the southern margin (Arunta Inlier) dips to the south away from the craton interior (outward). The causes of these different geometries are not well-understood, nor is their influence on later phases of deformation. We have used gravity, magnetic and seismic reflection data to investigate the influence of the NAC margin geometry on the distribution and connectivity of structures in the upper crust. These geophysical data were used to interpret major crustal boundaries, including major sutures and shallow structures. Three major structures occur on the eastern margin of Mount Isa Inlier: the Gidyea Suture Zone, the Pilgrim Fault Zone, and the Western Edge Fault. Each of these major structures dips towards the interior of the craton. Our interpretation reveals the presence of shallow listric structures that sole out into regional decollements, which are antithetic to the major crust-penetrating faults. We observe two major sutures along the southern margin of the Arunta Inlier: the Willowra Suture and the Warrumpi Suture. The Willowra Suture represents the southernmost edge of the NAC. Seismic reflection data reveals that the structural style is very different to that of Mount Isa. Faults in the footwall of Willowra Suture are more widely spaced and penetrate through the crust to the Moho. There is no obvious link between the geometry of this margin and upper crustal structures. By comparing these findings from the Mount Isa and Arunta Inliers we can establish relationships between the structural architecture and the geometry of the craton margin.

Geometry of the margins of the North Australian Craton and correlations with upper crustal structures Authors: Fatemeh Amirpoorsaeed, Peter Betts, Anindita Samsu, Robin Armit, Alexander Cruden.