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

Middle Devonian (Tabberabberan) deformation in the central parts of the sou thern Lachlan Fold Belt resulted in 50% shortening in the upper crust and a complete cessation of marine sedimentary environments thereafter. Isostati cally support-ed changes in elevation generated during Middle Devonian defo rmation in southeastern Australia are examined through geodynamic modelling . Constraints are placed on the nature of the lower crust of the Lachlan Fo ld Belt tin particular its density) and insights are gained into the Palaeo zoic tectonic evolution of this unusual orogenic belt. The evolution of str ain in the Lachlan Fold Belt is quantified using crustal and (whole) lithos pheric thickening factors fe and fl. Relative changes in the thickness of t he crust and lithospheric mantle generated during orogenesis and associated physical responses in the deforming lithosphere (such as changes in isosta tically 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 charact eristics of this part of the southeastern Australian crust. as well as meta morphic and structural evidence. From the results of the isostatic modellin g 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)), ge ologically reasonable estimates for isostatically supported elevation are c alculated for central Victoria both before and after Middle Devonian deform ation (0 +/- 250 m and between similar to 1700 and 2500 m, respectively). C onversely, if a lower density crustal substrate is modelled, elevations gen erated during Middle Devonian deformation are > 4000 m. which is considered unrealistically high for southeastern Australia. Further constraints can a lso be placed upon the actual style of Middle Devonian structural thickenin g in the Lachlan Fold Belt. It is proposed that the entire crustal section (both upper and lower crust) has been homogeneously thickened by imbricatio n, as opposed to a subduction-style model in which only the overlying sedim entary pile would thicken above a detached lower crust.