Strength variations of the Australian continent: Effects of temperature, strain rate, and rheological changes

Abstract

The Australian continent is composed of several geologic provinces, showing a general age progression from Archean in the west to Phanerozoic in the east. The lithospheric heterogeneity and complex tectonic history of this region make it a key area for studying the thermal and rheological structure of the geological provinces and testing the influence of different conditions, such as temperature, rheology, and strain rate on the plate strength. In a previous study, temperature and compositional variations of the Australian upper mantle have been determined based on a joint interpretation of the seismic tomography and gravity data. In this study, we further implement a thermal model of the crust, based on available surface heat flow data from regional and global database. The crustal and upper mantle thermal models show different anomalies distribution, indicating a significant variation of the thermal conditions with depth. The new thermal models are used to estimate strength and effective elastic thickness ( Te ) distribution in the lithosphere. For this aim, we assigned the rheology of the crust based on the seismic velocities provided by the AuSREM model and used the strain rate values obtained from a global mantle flow model, constrained by seismic and gravity data. The maximal strength and Te are found in the West Australian Craton, on account of the low temperatures in the lithospheric mantle. We found that locations of the intraplate earthquakes attend to sharp changes in the lithospheric strength. Comparison of the results with those obtained for uniform rheology and strain rate, indicate that in the Officer basin the variations of the crustal rheology enhance the effect of temperature changes, while in the Yilgarn craton they reduce it. On the other hand, the lower values of the strain rate in the cratons than in the Phanerozoic regions influence the strength/ Te in the opposite way with respect to temperatures. Integrated strength (Pa m) and strength (Pa) variations along three cross-sections. Horizontal and vertical axes of the cross-sections display the distance and depth (both in km), respectively. (a) Integrated strength of the lithosphere. The three black lines show the location of the cross-sections; (b) Integrated strength of the crust. White circles show the earthquakes location, occurred from 1990 to present-day, with moment magnitudes (Mw) >2.5, taken from the USGS seismic catalogue ( https://earthquake.usgs.gov/earthquakes/search/ ); (c) Integrated strength of the mantle lithosphere; (d) Percentage of crustal strength; (e) Strength variations along three cross-sections (A-A′, B-B′, and C-C′, respectively). • A crustal themal and rheological model of the Australian continent is set up. • Crustal thermal anomalies correlate with surface heat flow variations. • Upper mantle thermal anomalies correlate with age of the tectonic features. • The strength distribution is strongly influenced by the temperature variations.