Gravity and magnetic modelling of crustal structure in central Victoria: what lies under the Melbourne Zone?

Three gravity and magnetic profiles through the central and eastern part of the Melbourne Zone, Lachlan Fold Belt, have been modelled, with tight control from surface geology to investigate the deep crust. The largely non-magnetic upper crust (folded Ordovician to Lower Devonian strata of the Melbourne Zone intruded by Devonian granite plutons) has been modelled using recently acquired density measurements, providing a good match with the gravity profiles. The top of the basement is modelled at 10-15km depth in the centre of the zone, and 8-10km depth in the south of the zone, reflecting a thinning of the folded cover sequence toward the south, consistent with outcrops of basement Cambrian rocks on the south coast of Victoria. Long-wavelength magnetic anomalies in the Melbourne Zone are modelled as highly magnetic tabular bodies lying in the basement below the folded strata of the Melbourne Zone. The only rock type that matches the density and magnetic susceptibility is serpentinised ultramafic rock. Thus, a large ophiolitic body is thought to exist in the upper part of the basement to the Melbourne Zone, at depths of 15-25km in the central part of the zone, and 10-15km in the south of the zone. This unit is correlated with the Cambrian mafic-ultramafic complex of Tasmania, agreeing with previous correlations of the Selwyn Block with Tasmania.