Benambran Orogeny in the Eastern Lachlan Orogen, Australia

Abstract

The Benambran Orogeny reflects the accretion of the intra-oceanic Macquarie Arc to the Gondwana Plate. The ultimate cause was the northwards strike-slip transport of the allochthonous Bega Terrane along the eastern margin of Gondwana, into a forearc position outboard of the Macquarie Arc. Ensuing oblique compression in the outboard part of the Gondwana Plate drove the Macquarie Arc into and under its backarc Wagga Basin, represented by the Girilambone – Wagga Terrane, and led to a combination of thrusting and major strike-slip faulting within, inboard and outboard of the arc over 10 million years. In this time frame, the Benambran Orogeny in the Eastern Subprovince of the Lachlan Orogen in New South Wales consists of two phases of exhumation – deformation, at ca 443 Ma (late Bolindian to early Llandovery) and ca 430 Ma (late Llandovery), separated by a relaxation/extensional event. Both phases involved deformation and exhumation of the Macquarie Arc and coeval quartz-rich turbidites and black shales of the Adaminaby Superterrane. Closure of the former backarc basin was facilitated by limited east-dipping subduction that generated the short-lived Fifield arc. The second phase of the Benambran Orogeny also involved deformation and exhumation of overlying Llandovery strata (e.g. the Yalmy Group) and syn- to post-tectonic emplacement of granitoids. In both phases, deformation of Adaminaby Superterrane rocks was more intense than deformation of arc rocks.

Key words:

• arc accretion
• Benambran Orogeny
• Lachlan Orogen
• Ordovician
• plate tectonics
• Tasmanides

Acknowledgements

We thank the rest of the SPIRT team at CODES and at the Geological Survey of New South Wales and the company sponsors for discussions and support. We thank Robin Offler for his unpublished illite data. Ian Percival and Jeff Vassallo provided early comments on the manuscript. Gordon Packham and Fons VandenBerg gave us very helpful, critical journal reviews, and we thank Gordon Packham in particular for his detailed comments on relevant aspects of the geology of New South Wales. Editorial comments from Tony Crawford also helped improve the clarity of the paper. RAG publishes with the permission of the Deputy Director-General of the New South Wales Department of Primary Industries—Mineral Resources.