Tectonic evolution of the Ordovician Macquarie Arc, central New South Wales: arguments for subduction polarity and anticlockwise rotation

Abstract

The Ordovician Macquarie Arc is most widely exposed in the Lachlan Fold Belt of central New South Wales. Complex relationships between the arc and the Ordovician turbidite mega-fan are partly explained by anticlockwise rotation of the arc during the Ordovician. Thus, initially two lobes of the mega-fan formed to the north and south of the east–west-trending arc, using present-day coordinates. The arc consists of the western Goonumbla–Trangie Volcanic Belt, replacing the inappropriate term Junee–Narromine Volcanic Belt, and an eastern composite of the Molong, Rockley–Gulgong and Kiandra Volcanic Belts. These two major segments of the arc are separated by Ordovician quartz turbidites of the Kirribilli Formation, and it is probable that the arc has been duplicated by a sinistral strike-slip fault. Eastonian paleogeographic reconstruction of the eastern segment of the arc highlights a prominent limestone platform in the western Molong Volcanic Belt that grades eastwards into a realm of mainly deep-marine sedimentation and volcanic activity. By analogy with Guam in the western Pacific Ocean, the limestone platform is equated to a frontal arc ridge. This implies that the associated subduction zone was along the western side of the arc and not to the east, as in previous reconstructions. A wide zone of deformed Ordovician quartz turbidites, making up the Girilambone and Wagga–Omeo Zones west of the Macquarie Arc, is interpreted as a subduction complex that formed rapidly in the Late Ordovician. Flipping of the subduction zone was a relatively long event, inferred to have occurred during the latest Ordovician to early Silurian Benambran Orogeny. This was driven by collision of the subduction complex with northern continuations of the Stawell and Bendigo Zones, with a new west-dipping subduction zone forming to the east.

Key Words:

• Lachlan Fold Belt
• Macquarie Arc
• Ordovician
• polarity reversal
• subduction

Acknowledgements

Support has been provided by the University of Wollongong (GeoQuEST research centre). I thank many geologists over the past several years for discussions including Evan Leitch, Lawrence Sherwin, John Miller, David Gray, Martin Scott, Gary Colquhoun, Roger Cameron, Carol Simpson, Sol Buckman and Steve Trigg. Constructive and incisive reviews by David Gray and Fons VandenBerg improved the final manuscript.