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.