Redefined crustal structure of the Buchan Rift, northeast Victoria: evidence from potential field modelling of newly acquired land-based gravity data

ABSTRACT

The Buchan Rift, in northeastern Victoria, is a north–south-trending basin, which formed in response to east–west crustal extension in the Early Devonian. The rift is filled mostly with Lower Devonian volcanic and volcaniclastic rock of the Snowy River Volcanics. Although the structure and geometry of the Buchan Rift and its major bounding faults are well mapped at the surface, a discrepancy exists between the surface distribution of the thickest rift fill and its expected potential field response. To investigate this variation, two new detailed land-based gravity surveys, which span the rift and surrounding basement rocks in an east–west orientation, have been acquired and integrated with pre-existing government data. Qualitative interpretation of the observed magnetic data suggests the highly magnetic rocks of the Snowy River Volcanics have a wider extent at depth than can be mapped at the surface. Forward modelling of both land-based gravity data and aeromagnetic data supports this interpretation. With the Snowy River Volcanics largely confined within the Buchan Rift, resolved geometries also allow for the interpretation of rift boundaries that are wider at depth. These geometries are unusual. Unlike typical basin inversions that involve reactivation of rift-dipping faults, the bounding faults of the Buchan Rift dip away from the rift axis and thus appear unrelated to the preceding rifting episode. Limited inversion of previous extensional rift faults to deform the rift-fill sequences (e.g. Buchan Synclinorium) appears to have been followed by the initiation of new reverse faults in outboard positions, possibly because the relatively strong igneous rift fill began to act as a rigid basement ramp during continued E–W crustal shortening in the Middle Devonian Tabberabberan Orogeny. Overthrusting of the rift margins by older sediments and granite intrusions of the adjacent Tabberabbera and Kuark zones narrowed the exposed rift width at surface. This scenario may help explain the steep-sided geometries and geophysical expressions of other rift basins in the Tasmanides and elsewhere, particularly where relatively mechanically strong basin fill is known or suspected.

KEYWORDS:

• Buchan Rift
• rift inversion
• forward modelling
• gravity
• magnetics
• Tabberabberan
• Snowy River Volcanics

Acknowledgements

The authors would like to acknowledge the Geological Survey of Victoria for funding this research. Ken Sherry and Suzanne Haydon are thanked for help with acquisition and processing. Thank you also to Abe Jones, Dani Majer-Kielbaska and Masha Pastuhov for their assistance in the field. John and Nicky from Wombargo are thanked for all their help and hospitality during each of the field seasons. Constructive reviews by Peter Betts and Mark Lackie improved the manuscript, including by expanding its scope to include a more general discussion of the mechanics of overthrust rift margins, and are greatly appreciated. Publication is with permission of the Director of the Geological Survey of Victoria.

Disclosure statement

No potential conflict of interest was reported by the authors.