Australia: The Land Where Time Began

A biography of the Australian continent 

Lachlan Orogen - Crustal Complexity revealed from Telesceismic Receiver Functions

According to the authors¹ debate is continuing about the tectonic evolution of southeast Australia, especially the nature and causes of its accretion to the core of the continent to the west that is of Precambrian age. Useful clues with regard to this issue can be provided by seismic imaging of the crust. When the elastic properties of the interior of the Earth are being mapped seismic tomography imaging, a powerful tool, is often used, though in most cases it does not allow the depth of discontinuities such as the Moho (Mohorovicic) to be well constrained. For this study the authors¹ used an alternative imaging technique, called receiver function RF, for seismic stations near Canberra, in the Lachlan Orogen. They investigated the shear-wave-velocity profile in the crust and upper mantle and variations in the depth of the Moho beneath the Lachlan Orogen, and the nature of the transition between the mantle and the crust. They used a number of styles of RF analyses - to obtain the best compressional shear velocity (V_p/V_s) ratio and thickness of the crust they used HK stacking, to investigate the potential dip of the layers of crust and anisotropy they used nonlinear inversion for the shear-wave-velocity structure and inversion of the observed variations of RFs with back-azimuth. The authors¹ suggest the thick crust, up to 48 km, and the nature of the crust-mantle transition in the Lachlan Orogen, that is mostly intermediate, could be the result of the presence of underplating at the crust base, and possibly to the existing thick piles of mafic rocks from the Ordovician that are present in the mid and lower crust. It is suggested by results from numerical modelling of RFs at 3 stations, CAN, CNB and YNG, that variations that were observed with back-azimuth could be related to a complex structure that is situated beneath these stations, and there is a likelihood of both a Moho that is dipping and anisotropy of the crust. The authors¹ say that crustal thickening to the west, beneath the CAN station, that was revealed by their analysis, could have resulted from slab convergence. They also suggest the crustal thickening may be related to the broad Macquarie Volcanic Arc, that is rooted in the Moho. In the eastern Lachlan Orogen a strong N-S structural trend and to the preferred crystallographic orientation of minerals that are seismically anisotropic in the lower and middle crust, that are related to the convergence of the Palaeopacific plate, may lead to the crustal anisotropy.

Sources & Further reading

1. Fontaine, F. R., H. Tkalcic, and B. L. N. Kennett. "Crustal Complexity in the Lachlan Orogen Revealed from Teleseismic Receiver Functions." Australian Journal of Earth Sciences 60, no. 3 (2013/04/01 2013): 413-30.