The Deep Structure of the Australian Continent
from Surface-Wave Tomography

Frederik J Simons, Alet Zielhuis and Rob D. van der Hilst

Department of Earth, Atmospheric and Planetary Sciences
Massachusetts Institute of Technology (MIT)
Cambridge MA 02139, USA

Abstract

We present a new model of three-dimensional variations of shear wave speed in the Australian upper mantle, obtained from the dispersion of fundamental and higher-mode surface waves. We used Rayleigh wave data from the portable arrays of the SKIPPY project and from permanent stations (from AGSO, IRIS and GEOSCOPE), amounting to about 1600 source-receiver combinations. AGSO data have not been used before and provide better data coverage of the Archean cratons in western Australia. Compared to previous studies we also improved the vertical parameterization and the weighting scheme that accounts for variations in data quality and we reduced the influence of epicenter mislocation on velocity structure. The dense sampling by seismic waves provides for unprecedented resolution of continental structure, but the wave speed beneath westernmost Australia is not well constrained owing to insufficient station coverage. Global compilations of geological and seismological data (using regionalizations based on tectonic behavior or crustal age) suggest a correlation between crustal age and the thickness and composition of the continental lithosphere. However, the age and the tectonic history of crustal elements vary on wavelengths much smaller than have been resolved with global seismological studies. Using our detailed regional upper mantle model we investigate how the seismic signature of tectonic units changes with increasing depth. At large wavelengths, and to a depth of about 200 km, the inferred velocity anomalies corroborate the global pattern and display a progression of wave speed with crustal age: slow wave propagation prevails beneath the Paleozoic fold belts in eastern Australia and wave speeds increase westward across the Proterozoic and reach a maximum in the Archean cratons. The high wave speeds that we associate with Precambrian shields extend beyond the so-called Tasman Line, which marks the eastern limit of Proterozoic outcrop. This suggests that parts of the Paleozoic fold belts are underlain by Proterozoic lithosphere. We also infer that the North Australia craton extends off-shore into southwestern Papua New Guinea and beneath the Indian Ocean. For depths in excess of 200 km a regionalization with smaller units reveals a more complex pattern. Some tectonic subregions of Proterozoic age are marked by pronounced velocity highs to depths exceeding 300 km, but others do not and, surprisingly, the Archean units do not seem to be marked by such a thick high wave speed structure either. The Precambrian cratons that lack a thick high wave speed ``keel'' are located near passive margins, suggesting that convective processes associated with continental break-up may have destroyed a once present tectosphere. Our study suggests that deep lithospheric structure can vary as much within domains of similar crustal age as between units of different ages, which hampers attempts to find a unifying relationship between seismological units and crustal age domains.

Figures

1. Figure 01 Multiple scales of geological variability in Australia
   
2. Figure 02 Locations of the SKIPPY, IRIS, GEOSCOPE, and AGSO stations
   
3. Figure 03 Body and surface-wave phases: the use of group-velocity windows
   
4. Figure 04 Sensitivity Fréchet kernels for surface wave propagation
   
5. Figure 05 Waveform fitting by Partitioned Waveform Inversion
   
6. Figure 06 Path coverage of the tomographic study
   
7. Figure 07 Results of resolution experiment in linear tomographic inversions
   
8. Figure 08 Current surface wave speed model: mapviews
   
9. Figure 09 Current surface wave speed model: profiles
   
10. Figure 10 Local Earth models with two-station method
    
11. Figure 11 Comparison of present model with global models: spatial and spectral domains
    
12. Figure 12 Four-part regionalized representation of the shear wave speed
    
13. Figure 13 Age-dependent wave speed variations with depth.
    
14. Figure 14 Detailed regionalized representation of the shear wave speed
    
15. Figure 15 Definition and names of the coarse regionalization (not included in paper)
    
16. Figure 16 Definition and names of the fine regionalization (not included in paper)
    

Frederik Simons