Thermal regimes, anatexis, and orogenesis: Relations in the western Lachlan Fold Belt, southeastern Australia

Abstract

Crustal evolution of the Lachlan Fold Belt of southeastern Australia has involved the mobilisation of significant melt volumes now preserved as granites, suggesting that large parts of the lower crust were at T > 800°C for part of the thermal evolution. This anomalous thermal activity has been variously related to continental extension, magmatic underplating and to thinning of the mantle lithosphere. Overriding considerations reflected in the surface geology must include the tectonic setting, the thermal regime and the state of stress in the crust at the time of anatexis. One-dimensional geothermal modelling suggests that no one mechanism can generate the required thermal regime necessary to produce the volume of granite observed in the crust. The observed geological relationships do not match those predicted for lithospheric thinning due to continental extension. Magmatic underplating can cause melting but must involve extensive source zones and allow continual feeding of the magma to higher crustal levels. The crustal thickening/lithospheric thinning mechanism involves upwelling of hot asthenospheric mantle to the base of the crust but the melt products are confined to a very thin layer at the crust-mantle boundary. Mechanisms for melt extraction and emplacement to higher crustal levels must be explained. In the western Lachlan Fold Belt the presence of a major mid-crustal detachment fault, ductile shear zones in the lower crust and marked internal shortening of the upper and possibly lower crustal blocks suggests that shear-heating may also provide a significant component to the heat budget of the evolving crust. Melting will not occur however, unless thermal equilibration of both the upper and lower plates takes place prior to shear-heating. Such equilibration will involve time scales in the order of 30–50 Ma, unless extra heat is added into the lower crust (e.g. advective input). Inferred flexural rigidity of the lithosphere, related to “basinal” subsidence during the period of orogenesis in central Victoria, suggests that thermal effects related to granitoid intrusion must be largely post-tectonic and that felsic intrusive activity is a consequence of the crustal thickening rather than the cause. Thermal regimes during orogenesis cannot be restricted to a single mechanism. Upwelling of hot asthenosphere causing significantly higher heat input at the base of the crust, coupled with shear-heating and internal strain-heating of the crust during the crustal thickening stage as well as thermal effects related to magma-intrusion into the lower crust result in a complex thermal history during collisional orogenesis.