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New Perspectives Workshop, September 2014
Presented by AIG Victoria and AusIMM Central Victoria Branch, Romsey Victoria

The Lachlan Orocline of Eastern Australia. Giant folds, the geodynamic processes that can form them, and how these new understandings have potential to revolutionise the resource prospectivity game in Eastern Australia.

Ross Cayley

Geological Survey of Victoria, Melbourne, Australia

Abstract

Since the first applications of modern plate tectonic theory to Eastern Australia, the Lachlan Fold Belt (LFB) has been repeatedly interpreted within the paradigm of an unusually wide and squat, composite orogen, an amalgamation of multiple linear, orogen-parallel Early Paleozoic accretion or rifting events and multiple arc complexes, some developed independently, some simultaneously. Such models are a poor fit to key Ordovician palaeogeographic constraints, and don’t take account of the highly variable mix of accretionary and intra-plate characteristics and microcontinents embedded within the orogen.

Modern structural and stratigraphic mapping, and aeromagnetic, gravity and deep seismic reflection data now constrains a new geodynamic model: a simple unifying solution for the whole LFB from far north Queensland to Tasmania. This model develops the idea that only one, continent- dipping, subduction zone was active in eastern Australia in the Ordovician-Early Devonian. Mega- folding about vertical axes in the Silurian reoriented the orogen into a giant Z-shaped orocline, giving the appearance of ‘multiple’ coeval systems of opposing dip when simple cross-sections are constructed across it. The Lachlan Orocline core is 400-1200 km in amplitude with well-defined limbs enclosing a fault-disrupted and extended core. It’s formation involved continent-scale reorientations of the geology. The orocline developed in a trans-tensional upper-plate setting, chasing a slab that was retreating oceanward in asymmetric southeast-directed roll-back, triggered by a microcontinent collision – Tasmania. The orocline effectively doubled apparent LFB width, disrupting and redistributing any- and all- mineral systems formed within the orogen prior to 400Ma in the process.

The new model explains: patterns in aeromagnetic data; long-standing paleomagnetic data complexity; palaeogeography; provenance and stratigraphic relationships; structural vergence ‘reversals’; the transition to crustal extension and the wave of Silurian/Early Devonian arc- and back- arc magmatism that swept the LFB. It’s a brand new conceptual template for predicting the location of buried mineral systems in all of Australia’s eastern States, from the vast plains of southern Queensland to the highlands of Tasmania (examples will be presented). Eastern Australia is now the global type-locality for a new geodynamic model that rethinks the lower mantle processes that drive continental collisions.

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About the Speaker

Ross Cayley is a senior structural and field geologist at the Geological Survey of Victoria (GSV). He is developing a radical new geodynamic model for the Australian continent for the Cambrian – Devonian, in conjunction with Robert Musgrave (GS NSW).  An overview of this work was incorporated as the global type-locality for a new geodynamic model for continent collision developed with Monash University research collaborators. Published recently in NATURE, this work has fundamental implications for terrane prospectivity analysis across Australia, and globally.

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