General Monthly Meeting
Lachlan to New England:
Evolution from an Extensional to
Contractional Accretionary Orogen
Professor William J. Collins, Department of Geology, School of Geosciences, University of Newcastle.
Due to unforseen circumstances this talk was cancelled.
Date: Wednesday, 6th March, 2002
Time: 6:00 for 6:30 pm
Venue: Macquarie University
The New England Fold Belt (NEFB) of coastal eastern Australia is commonly regarded as an archetypal subduction/accretion complex that is readily explicable in a modern plate tectonic framework. However, the inland, older Lachlan Fold Belt (LFB) is anomalous and enigmatic, with a plethora of hypotheses still failing to provide an adequate explanation for its existence.
The contrasting character of the two fold belts can be reconciled by considering the behaviour of the subducting plate. In the LFB example, the vast belt of granites and repeated development of sedimentary basins (or troughs) indicates a history of protracted crustal extension throughout its development. This migration of magmatism occurs in response to ongoing rollback of the subducting slab, interrupted by short crustal thickening events, similar to what has happened in the SW Pacific over the last 100 million years. In contrast, the narrow belt of granites and lack of widespread basin in the NEFB suggest a near-stationary subduction zone, similar to that seen in the present- day Andes.
These different modes of subduction essentially reflect the density of the subducting slab: if old, cold and dense, it sinks faster than it translates horizontally and the slab "rolls back" toward the ocean, creating troughs and potentially voluminous granites in the upper plate. This is the LFB situation. On the other hand, if young and warm, the slab is buoyant and more likely to translate horizontally rather than sink. Accordingly, the slab is forced beneath the overriding continental margin, coupling with it and generating orogenic contraction. As a result, narrow belts of granites are localised near the continental margin. This situation exemplifies the NEFB. Therefore, the contrasting character of the two fold belts can be reconciled by a subduction model that invokes changing slab density with time.