Skip Navigation | ANU Home | Search ANU | Directories
The Australian National University
Research School of Earth Sciences
Printer Friendly Version of this Document
RSES SITE SEARCH
Untitled Document

Seismic Investigations of Lithospheric Transitions between the Northern and Southern Australian Cratons (BILBY)

Sara H. Pozgay and Brian L.N. Kennett

Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia

 

Figure 1. BILBY station map.  Red squares indicated proposed station locations; black circles are prior broadband instrumentation; orange stars are major landmarks.  Topography scale (in meters) shown at bottom.

 

We aim to determine the nature of the transition at lithospheric depths between the northern and southern Australian cratons.  What are the controlling factors of the regions with anomalously slow velocities beneath the central Australian intercratonic suture zones?  Do these intercratonic transitions propagate with depth and, if so, in what manner?  To answer these questions, 25 broadband seismic stations were deployed in August-September 2008 and will remain operational for approximately one year.

Much of the Australian continent is an amalgamation of several smaller cratons and multiple orogenic events.  The transitions between any two cratonic regions, however, do not necessarily reflect the same processes.  Beneath the Capricorn Orogen (connecting the Yilgarn and Pilbara cratons), seismically fast wavespeeds (relative to a reference earth velocity model) and low attenuation are relatively continuous from 75 to >300 km depth.  These large-scale observations suggest that a simple thermal origin is a likely explanation for the seismic signature of the orogen.  However, much of central Australia is different; the central intercratonic belt regions exhibit low attenuation as for the case of the Capricorn Orogen, but slow wavespeeds persist at ~75 km depth.  Only 25 km deeper, wavespeeds are fast and become relatively continuous at ~125 km depth.  These observations suggest that the physical mechanism responsible for the observed seismic signature is more complex than simple thermal variations would predict. 

What is the nature of this slow wavespeed region around 75 km depth beneath these intercratonic suture belts?  Do the transitions between them extend through the whole lithosphere, as suggested by prior studies of the central Australian (Arunta Block) region?  Several orogenic events should have made a lasting imprint through the whole lithosphere (e.g. the Alice Springs Orogeny 400-300 Ma), however these effects have yet to be imaged at the appropriate resolution to determine the nature of different lithospheric sections.  Prior seismic deployments in the region were largely designed to image either crustal fault systems or deep lithosphere and upper mantle structures and, therefore, can provide only first-order constraints on the intercratonic suture belts.

We deploy an ambitious seismograph station configuration to image these lithospheric transitions between the northern and southern Australian cratonic regions and across parts of the intercratonic suture belts.  The experiment configuration is designed specifically to connect the Gawler Craton in the south to components of the North Australian Craton - specifically, through the Musgrave Block and to the northern side of the Arunta Block, and to connect the Arunta Block with the Mt. Isa Block.  A combination of rigorous analysis tools, such as velocity and attenuation tomography coupled with receiver functions, will help to provide a comprehensive understanding of the amalgamation of continental cratons and the associated intercratonic transitions.  In addition, a significant amount of information will be added to the present understanding of intercratonic suture belts and further constraints on the Australian lithospheric structure and overall continental amalgamation will be realised.

Figure 2. Station near Alice Springs.