Musgrave, Dr Robert1
1Geological Survey Of New South Wales, Maitland, Australia
Ordovician to early Silurian calc-alkaline volcanic-, associated intrusive-, and shallow-marine sedimentary rocks comprise the Macquarie Arc, the principal arc system in the Lachlan Orogen. Although Pb-isotope ratios and positive εNd suggest intraoceanic origins, there are competing hypotheses regarding the arc’s tectonic setting. The range of proposed tectonic models include:
- simple intraoceanic models, in which subduction initiated in the Ordovician over a west-dipping subduction zone, an east-dipping zone that later reversed polarity, or a south-dipping zone within a back-arc that later rotated anticlockwise;
- two-stage intraoceanic models, in which the Ordovician arc developed on an earlier intraoceanic Cambrian arc;
- the Lachlan Orocline model, which suggests the Macquarie Arc arose close to the Cambrian convergent margin of Gondwana;
- or in a very different viewpoint, the Macquarie ‘Arc’ may have been an extensional system in a back-arc, far behind the frontal arc located outboard of the New England Orogen.
Seismic reflection and refraction surveys across the Macquarie Arc, with corresponding potential-field models, were previously interpreted to indicate a MORB-like substrate, consistent with proposed tectonic models of type 1 and (arguably) 4. However, petrophysical parameters yielded by these potential-field models are inconsistent with this interpretation. Long-wavelength, deeply sourced signals from a variety of geophysical techniques can clarify the composition of the basement, and these differing datasets show remarkable agreement. Long-wavelength aeromagnetic features qualitatively indicate a basement of low magnetic susceptibility. Quantitative modelling limits this to values typical of intermediate intrusive rocks characteristic of either mature arc or continental basement. Seismic velocity from the AuSREM model maps out a belt of low p-wave velocity, consistent with an intermediate chemistry, in the mid- to lower crust, which corresponds closely to the areas of magnetic susceptibility lows. Long-wavelength, long period MT signals recorded by the AusLAMP array map out conductivity highs that continue from mid (10 km) to lower (40 km) crustal depth and broadly correspond to the Macquarie Arc.
Petrophysical interpretation of an intermediate composition in the middle and lower crust below the Macquarie Arc is incompatible with a MORB-like composition and requires the basement of the Macquarie Arc to be either a pre-existing arc, a continental fragment, or a very thick arc in which little of any original MORB remains in situ. Tectonic models of types 1 and 4 are therefore ruled out. Recently reported magmatic Cambrian zircons from Macquarie Arc rocks are consistent with models of either type 2 or 3, but Pb isotope links between the Macquarie Arc and the forearc of the Cambrian Mount Wright Arc on the Gondwana margin suggest support for a version of model type 3, the Lachlan Orocline model. The Eocene to Recent forearc to rear-arc system of the Izu-Bonin Arc, south of Japan, shares geochemical, geophysical and scale similarities with the Macquarie Arc. This analogy suggests a genetic linkage between the Macquarie Arc, the Cambrian boninite–tholeiite–calc-alkaline belts in Victoria and offshore NSW, and the Cambrian Ponto Group on the margin of the Delamerian orogen in northwest New South Wales.
Robert (Bob) Musgrave: Research Geophysicist with the Geological Survey of New South Wales. Multidisciplinary geophysicist and tectonic interpreter, with more than 30 years’ experience in palaeomagnetism, tectonics, magnetic and gravity interpretation and modelling, and marine geophysics. Bob built and runs the PALM palaeomagnetic/petrophysics laboratory at the University of Newcastle.