3D Numerical Modelling of Heat Flow Across Heterogeneous Geology in Southeast Australia

Abstract

Our knowledge of the thermal regime of the crust diminishes rapidly with depth. Seismic and magnetotelluric datasets are often plagued by inaccuracy that stems from poor knowledge of the composition of the crust and fluid alteration artefacts. Surface heat flow data, however, offers a reliable framework to constrain the thermal budget of the crust. The collection of 175 heat flow calculations in Victoria offers the highest resolution dataset available in Australia. A 1D interpolation of these data points reveals the heterogeneous nature of heat flow within Victoria. This, however, ignores important geological boundaries that may contribute to heat refraction. To address this, we have imported a 3D geological model of Victoria into Underworld, a finite element Lagrangian particle-in-cell program, to solve heat flow. We include depth-dependent thermal properties of the crust in our Underworld solutions, and forward model this against surface heat flow data. Our results show that incorporating geological structure substantially reduces the uncertainty for areas without direct heat flow measurements. Furthermore, there exists a zone of high heat flow between Bendigo and Ballarat, which correlates with region of low teleseismic velocity and high electrical conductivity that intrudes well into the mid crust. This region also coincides with a high spatial distribution of volcano eruption points that were active within the last 5 million years. Extrapolations of geotherms to depth within this anomalous zone highlights impossibly high temperatures at the base of the crust, in excess of 900C, which suggests a steady-state approach to crustal heat flow breaks down. This indicates that transient heat flow, arising from magma migration, dominates this part of the crust.