Isostatic constraints on the central Victorian lower crust: Implications for the tectonic evolution of the Lachlan Fold Belt

Abstract

Middle Devonian (Tabberabberan) deformation in the central parts of the southern Lachlan Fold Belt resulted in 50% shortening in the upper crust and a complete cessation of marine sedimentary environments thereafter. Isostatically supported changes in elevation generated during Middle Devonian deformation in southeastern Australia are examined through geodynamic modelling. Constraints are placed on the nature of the lower crust of the Lachlan Fold Belt (in particular its density) and insights are gained into the Palaeozoic tectonic evolution of this unusual orogenic belt. The evolution of strain in the Lachlan Fold Belt is quantified using crustal and (whole) lithospheric thickening factors fc and fl. Relative changes in the thickness of the crust and lithospheric mantle generated during orogenesis and associated physical responses in the deforming lithosphere (such as changes in isostatically supported elevation) are portrayed in fc‐fl space. A model for the strain history (i.e. the changing thickness of crust with time) of central Victoria has been proposed by taking into account known present‐day characteristics of this part of the southeastern Australian crust, as well as metamorphic and structural evidence. From the results of the isostatic modelling it is suggested that the Lachlan Fold Belt is largely floored by dense (?oceanic) crust. Assuming a relatively dense lower crust (>2.9 g.cm–3), geologically reasonable estimates for isostatically supported elevation are calculated for central Victoria both before and after Middle Devonian deformation (0 ±250 m and between ∼1700 and 2500 m, respectively). Conversely, if a lower density crustal substrate is modelled, elevations generated during Middle Devonian deformation are > 4000 m, which is considered unrealistically high for southeastern Australia. Further constraints can also be placed upon the actual style of Middle Devonian structural thickening in the Lachlan Fold Belt. It is proposed that the entire crustal section (both upper and lower crust) has been homogeneously thickened by imbrication, as opposed to a subduction‐style model in which only the overlying sedimentary pile would thicken above a detached lower crust.