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

The underbelly of the Lachlan Orocline: origins and distribution of the middle crust in the Tasmanides

Bob Musgrave

Geological Survey of New South Wales, Maitland NSW, Australia 


Aeromagnetic data over eastern Australia reveal a pattern of domains defined by systematic regional highs and lows, emphasised by low-pass filtering, over which are superimposed shorter (<20 km) wavelength anomalies related to mappable geology and its inferred subsurface continuation. Long-baseline levelling by Geoscience Australia has clarified the definition of these magnetic domains, and confirmed that they are not an artefact of grid merging. Geothermal and teleseismic data indicate that neither variation in Curie depth nor upper mantle magnetisation can produce the long- wavelength pattern. Hence, domain-wide variations in magnetisation at the middle to lower crustal level are presumably the cause of these long-wavelength features. Although reversed polarity remanence could contribute to deeply sourced negative magnetic anomalies, the correspondence of magnetic low domains with the Proterozoic Curnamona Craton and the Ordovician Macquarie Arc, and of a high domain with the western Lachlan Orogen floored by Cambrian ocean crust, suggests that the control may be simply stark contrasts in lower to middle crustal susceptibility. Moho thickness determined by the AusMoho model mimics the pattern of susceptibility domains, suggesting a relationship between tectonic history and mid to lower crustal composition. Implicit in this analysis is a division of the domains into continental (or continent-like) and oceanic basement, with implications for the tectonic evolution of the Tasmanides and the distribution of mineral systems in eastern Australia. The mid to lower crust below the Macquarie Arc appears to be continent-like, and the Thomson Orogen is a compound feature, comprising both attenuated continental/arc crust and oceanic crust. Seismic evidence supports both interpretations.

The distribution of crustal type fits well with the model of accretion of Vandieland into one end of the Tasmanide subduction system in the Ordovician, and subsequent congestion of the subduction system and oroclinal migration of the Macquarie Arc to chase subduction roll-back southwards. Continent-like middle crust below the Macquarie Arc may in fact indicate a highly mature oceanic-subduction setting, similar to that reported for the Izu-Bonin arc in the western Pacific. Geochemically, the Macquarie Arc resembles the western, rear-arc component of the Izu-Bonin system, which shares with the Macquarie Arc the elements of back-arc extension that have led to the recent reassessment by Quinn et al, who have suggested that the “Macquarie Arc” is in fact a back-arc, extensional feature. A purely back-arc rift system is unlikely to have produced the magnetic and velocity profile seen below the Macquarie Arc, but an Izu-like rear-arc may be a compromise that satisfies all observations, which begs the question: Did the frontal arc of the Macquarie Arc system follow the retreating trench?

About the Speaker

Bob Musgrave is the Research Geophysicist in the Geological Survey of NSW. Bob graduated with his PhD in geology and geophysics from the University of Sydney in 1987. His PhD, and postdoctoral research at Victoria University of Wellington, the Australian National University, and the University of Tasmania, applied palaeomagnetism to the tectonics of the southwest Pacific. Bob joined the Ocean Drilling Program in Texas as a Staff Scientist from 1991 through 93. Since that time he has continued his interest in ocean drilling and applications of rock magnetic techniques to marine geology issues (particularly gas hydrates), and he recently took part in his fifth ODP/IODP expedition. Bob returned to Australia to lecture in geophysics at La Trobe University, where he was based until the Earth Sciences department there was shut down in 2003. Bob joined GSNSW in 2005, which had then recently moved to Maitland, as a senior geophysicist. Bob has continued operating his palaeomagnetic/rock magnetic laboratory (the PALM lab), which he transferred to the University of Newcastle. Bob’s continued interest in applications of geophysical research to a range of topics, from application of rock magnetism to magnetic anomaly characterisation, potential field inversion, geological and tectonic interpretation of the mid and lower crust, and the tectonic history of the palaeo-Pacific margin from the Neoproterozoic to the present day, saw him talk the NSW bureaucracy into creating a new position of “Research Geophysicist” within the Geological Survey. The free range this affords him, and the “publish or perish” pressure that he is now exposed to, drive him to creative collaboration with colleagues in other surveys, industry, and academe.

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