GEMOC ARC National Key Centre

Yvette H. Poudjom Djomani1, Suzanne Y. O'Reilly1, William L. Griffin1.2
1. GEMOC Macquarie, 2. CSIRO Exploration and Mining

Precambrian western Australia is separated from Phanerozoic eastern Australia by the Tasman Line and this line marks the boundary between a tectonically and thermally relatively young active domain to the east and a cold, thick old lithosphere to the west. The nature of this boundary has been further refined using mantle-derived xenolith and mineral data on a transect that traces the change from Phanerozoic eastern NSW across the Tasman Line to the Eyre Peninsular in Proterozoic South Australia. The paleogeotherm decreases westward from greater than 50mW/m2 to around 40mW/m2 in South Australia and it is evident that the lithosphere thickens from the eastern to the western sections. This information on the thermal state of the lithosphere is important for the interpretation of the gravity and topography dataset for Australia using the method of determining elastic plate thickness, a measure of the strength of the lithosphere. Gravity and topography data of the whole continent has now been assembled. The gravity data are provided by AGSO while the topographic data are supplied by AUSLIG.

We present preliminary results of our gravity modelling in the Mount Isa domain in Northeastern Australia. Gravity data from the Mt Isa area show large central positive Bouguer anomalies which correspond to the core of Mt Isa Inlier. These large positive anomalies are interpreted as due to the presence of high density material in the crust, related to the occurrence of magmatic bodies in the lower crust. Enhancement of the gravity data shows that the anomalies are oriented NNW to NS in the north, and change to a NE-SW direction in the South. These trends correspond to the two major directions of faulting that affected the area. The central positive anomalies are surrounded by large negative anomalies on either side of the Inlier, and reflect a difference in crustal structure and densities in the area.

We then use the gravity and topographic grids to estimate the strength of the lithosphere, or its effective elastic thickness (EET or Te) as a function of the wavelength. The results of Te analysis show lateral variations of the lithospheric strength within the area, with Te between 34 and more than 60 km. Moreover, to the west of the central part, Te is an average of 34 km, then slightly higher (40 km) in the south east, and finaly the central part shows larger Te values (60-64 km). These results show that the lithosphere is stronger in the central part of the Mt Isa Inlier. The lithospheric domains recognised using gravity modelling techniques correspond to those defined using a combined interpretation of gravity, magnetic and geological data. Work is currently in progress to define lithospheric domains and structural differences between western and eastern Australia. An analogous study in northern Siberia (Poudjom Djomani et al., 1999) showed that such modelling could delineate lithospheric domains corresponding to major tectonic terranes mapped at the surface.

Acknowledgements. We thank AGSO and AUSLIG for providing the gravity and the relief data of Australia, respectively.

References
Poudjom Djomani, Y.H., Griffin, W.L., Natapov, L.M., Erincheck, Y., O'Reilly, S.Y. and Morgan, P. (1999). Mapping lithosphere-scale structures on the Siberian platform. Flexural rigidity and lithospheric mantle composition: relationships to major tectonic features. Internal report for Western Mining Corporation, December 1999.