ISOSTATIC CONSTRAINTS ON THE CENTRAL VICTORIANLOWER CRUST: IMPLICATIONS FOR THE TECTONIC EVOLUTION OF THE LACHLAN FOLDBELT
G.J. O'HALLORAN*, AND P.F REY**
*Esso Australia Ltd, 12 Riverside Quay, Melbourne, Vic. 3001, Australia
**Dept. of Earth Sciences, Monash University, Clayton, VIC 3168, Australia
In: Australian Journal of Earth Sciences (1999), v. 46, pp. 633-639.
ABSTRACT
Middle Devonian (Tabberabberan) deformation in the central parts ofthe southern Lachlan Fold Belt resulted in > 50% shortening in the uppercrust (Gray 1988), and a complete cessation of marine sedimentary environmentsthereafter (Cas 1983). Isosatically supported changes in elevation generatedduring Middle Devonian deformation in southeastern Australia are examinedthrough geodynamic modelling. Constraints are placed on the nature of thelower crust of the Lachlan Fold Belt (in particular its density) and insightsare gained into the Palaeozoic tectonic evolution of this unusual orogenicbelt. The evolution os strain in the Lachlan Fold Belt is quantifiedusing crustal and (whole) lithospheric thickening factors fc andfl. Relative changes in the thickness of the crust and lithospheric mantlegenerated during orogenesis and associated physical responses in the deforminglihtophere (such as changes in isostatically supported elevation) are portrayedin fc-fl space. A model for the strain history (i.e. the changingthickness of crust with time) of central Victoria has been proposed bytaking into account known present-day characteristics of this part of thesoutheastern Australia crust, as well as metamorphic and structural evidence. From the results of the isostatic modelling it is suggested that the LachlanFold Belt is largely floored by dense (?oceanic) crust. Assuminga relaticely dense lower-crust (>2.9 g.cm-3), geologically reasonable estimatesfor isostatically supported elevation are calculated for central Victoriaboth before and after Middle Devonian deformation (0±250 m and between~1700 and 2500 m, respectively). Conversely, if a lower density crustalsubstrate is modelled, elevations generated during Middle Devonian deformationare >4000m, which is considered unrealistically high for southeastern Australia. Further constraints can also be placed upon the actual style of MiddleDevonian structural thickening in the Lachlan Fold Belt. It is proposedthat the entire crustal section (both upper and lower crust) has been homogeneouslythickened by imbrication, as opposed to a subduction-style model in whichonly the overlying sedimentary pile would thicken above a detached lowercrust.
ACKNOWLEDGMENTS
GOH acknowledges receipt of an APA scholarship and Monash University PostgraduatePublication Award. The manuscript benefited from a constructive reviewand helpful comments by Dave Gray and an anonymous reviewer.
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