Crustal architecture of central Victoria: results from the 2006 deep crustal reflection seismic survey

Abstract

A ∼400 km long deep crustal reflection seismic survey was acquired in central Victoria, Australia, in 2006. It has provided information on crustal architecture across the western Lachlan Orogen and has greatly added to the understanding of the tectonic evolution. The east-dipping Moyston Fault is confirmed as the suture between the Delamerian and western Lachlan Orogens, and is shown to extend down to the Moho. The Avoca Fault, the boundary between the Stawell and Bendigo Zones, is a west-dipping listric reverse fault that intersects the Moyston Fault at a depth of about 22 km, forming a V-shaped geometry. Both the Stawell and Bendigo Zones can be divided broadly into a lower crustal region of interlayered and imbricated metavolcanic and metasedimentary rocks and an upper crustal region of tightly folded metasedimentary rocks. The Stawell Zone was probably part of a Cambrian accretionary system along the eastern Gondwanaland margin, and mafic rocks may have been partly consumed by Cambrian subduction. Much of the Early Cambrian oceanic crust beneath the Bendigo Zone was not subducted, and is preserved as a crustal-scale imbricate thrust stack. The seismic data have shown that a thin-skinned structural model appears to be valid for much of the Melbourne Zone, whereas the Stawell and Bendigo Zones have a thick-skinned structural style. Internal faults in the Stawell and Bendigo Zones are mostly west-dipping listric faults, which extend from the surface to near the base of the crust. The Heathcote Fault Zone, the boundary between the Bendigo and Melbourne Zones, extends to at least 20 km, and possibly to the Moho. A striking feature in the seismic data is the markedly different seismic character of the mid to lower crust of the Melbourne Zone. The deep seismic reflection data for the Melbourne Zone have revealed a multilayered crustal structure that supports the Selwyn Block model.

Key words:

• backarc
• Bendigo Zone
• Governor Fault
• Heathcote Fault
• Lachlan Orogen
• Melbourne Zone
• Moyston Fault
• orogenic gold
• seismic reflection
• Selwyn Block
• Stawell Zone
• Tabberabbera Zone
• tectonics
• Victoria.

Acknowledgements

In 2006, the Predictive Mineral Discovery Cooperative Research Centre (pmd*CRC) managed a project that principally involved the collection of seismic reflection data in central Victoria by the National Research Facility for Earth Sounding (ANSIR) on behalf of Geoscience Australia, the Victorian Government (Department of Primary Industries and Department of Innovation, Industry and Regional Development, through Geoscience Victoria), Ballarat Goldfields NL (now part of Lihir Gold Ltd), Gold Fields Australasia Ltd, Perseverance Corporation Ltd and Leviathan Resources Ltd (now merged with Perseverance Corporation Ltd, which in turn has been acquired by Northgate Minerals Ltd). We thank staff from these organisations that supported the project. We thank Tim Barton and Hugh Tassell for their contribution to the planning and acquisition of the seismic data and Graham Begg and John Dewey for helpful discussions about mantle lithosphere behaviour. We also thank Silvio Messimo, Malcolm Nicoll, Leonie Jones and Ken Sherry for producing the figures, and David Champion, Natalie Kositcin, Chris Fergusson and Brian Kennett for their constructive reviews which greatly improved the manuscript. Publication is with permission of the General Managers of Geoscience Australia and GeoScience Victoria.