Heterogeneity in the thermal state of the lower crust and upper mantle beneath eastern Australia

Abstract

Xenolith-derived paleogeotherms for the lower crust and upper mantle beneath the eastern margin of the Australian craton (EMAC) and beneath the Phanerozoic Tasman Fold Belt (south-eastern Australia ? SEA) indicate distinct thermal regimes. The SEA geotherm passes through 800°C at 7 kbar and 1200°C at 25 kbar. The strong curvature of the constructed geotherm from 10 to 30 kbar indicates significant advective heat transfer in the lower crust and upper mantle. The EMAC geotherm lies some 150 to 200°C below the SEA curve and stretches from 600°C at 7 kbar to 1000°C at 21 kbar. This is well above the steady state conductive geotherm calculated from (low) surface heat flow for cratonic areas. The lower crustal suite xenoliths from EMAC and SEA are dominated by mafic lithologies. The EMAC mafic xenoliths represent igneous intrusions into the lower crust and uppermost mantle metamorphosed to granulites and eclogites. The large lateral variations in temperature at the base of the crust (depending on tectonic environment) are significant to the relative stability of eclogite and granulite mineral assemblages. The restriction of lower crustal eclogite suites to cratons or craton margins, and their apparent absence from younger terranes, is a consequence of this temperature difference. The definition of distinct thermal regimes in different tectonic settings provides a basis for examining the response of magnetic properties and Vp-depth profiles with changes in temperature. The seismic Moho and the crust mantle boundary do not coincide in areas that have high geothermal gradients. Differences in the seismic characteristics of cratonic areas and areas with high geothermal gradients can be modelled as the effect of temperature on similar lithological columns and do not necessarily imply large petrographic differences. A lower crust beneath cratonic areas dominated by eclogite facies assemblages may be misinterpreted seismically as mantle.