Temperature, strain rates, and rheology: the key parameters controling strength variations in the Australian lithosphere

The Australian plate is composed of tectonic features showing progression of the age from dominantly Phanerozoic in the east, Proterozoic in the centre, and Archean in the west. These tectonic structures have been investigated in the last three decades using a variety of geophysical methods, but it is still a matter of debates of how temperature and strength are distributed within the lithosphere. We construct a thermal crustal model assuming steady state variations and using surface heat flow data, provided by regional and global database, and heat generation values, calculated from existing empirical relations with seismic velocity variations, which are provided by AusREM seismic tomography model. The lowest crustal temperatures are observed in the eastern part of the WAC and the Officer basin, while Central and South Australia are regions with anomalously elevated heat flow values and temperatures caused by high heat production in the crustal rocks. On the other hand, the mantle temperatures, estimated in a previous study, applying a joint interpretation of the seismic tomography and gravity data, show that the Precambrian West and North Australian Craton (WAC and NAC) are characterized by thick and relatively cold lithosphere that has depleted composition (Mg# > 90). The depletion is stronger in the older WAC than the younger NAC. Substantially hotter and less dense lithosphere is seen fringing the eastern and southeastern margin of the continent. Both crustal and mantle thermal models are used as input for the lithospheric strength calculation. Another input parameter is the crustal rheology, which has been determined based on the seismic velocity distribution, assuming that low (high) velocities reflect more sialic (mafic) compositions and thus weaker (stiffer) rheologies. Furthermore, we use strain rate values obtained from a global mantle flow model constrained by seismic and gravity data. The combination of the values of the different parameters produce a large variability of the rigidity of the plate within the cratonic areas, reflecting the long tectonic history of the Australian plate. The sharp lateral strength variations are coincident with intraplate earthquakes location. The strength variations in the crust and upper mantle is also not uniformly distributed: In the Archean WAC most of the strength is concentrated in the mantle, while the Proterozoic Officer basin shows the largest values of the crustal strength. On the other hand, the younger eastern terranes are uniformly weak, due to the high temperatures.