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Implications of Australian seismic and gravity measurements for the structure and composition of the upper mantle

Created 16/10/2025

Updated 16/10/2025

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In a previous paper densities of crustal layers were inferred from seismic refraction surveys in Australia and surrounding marine areas. These indicated substantial variations in crustal mass. As the free-air gravity field does not show anomalies corresponding to these, it is inferred that compensating mass variations must occur in the upper mantle. Sub-crustal mantle densities, inferred from Pn velocities, in general do not provide the required mantle mass distribution; however in some parts of the continent observed increases in seismic velocity at depths of 60 to 100 km suggest density changes which would lead to approximate compensation at about 130 km depth, corresponding to the top of a low-velocity layer suggested by surface wave studies. Marine crustal masses are reasonably close to a common value, but the wide variation of Pn velocities implies a corresponding variation of densities which would counteract this compensation if they persisted in depth. It is suggested that the Pn velocities represent comparatively thin layers, and that deeper density changes occur so that compensation takes place at 80 to 100 km, at the top of the sub-oceanic asthenosphere. The West Australian shield has the highest crustal mass, and also the highest Pn velocity, which implies a further relative increase in mass with depth. If the sub-shield mantle is assumed to consist of refractory peridotite with a relatively low density corresponding to its Pn velocity, the discrepancy in crustal mass with respect to the other areas is reduced with increasing depth, but is still not eliminated at depths less than about 160 km. This suggests that the sub-shield mantle above this depth has enough strength to support the differential pressure associated with the excess crustal mass. This conclusion is in accordance with other evidence, suggesting that sub-shield or sub-continental mantle differs from sub-oceanic mantle to depths of several hundreds of kilometres, and hence that the return flow compensating for plate-tectonic motions cannot take place at depths of 100 to 200 km, as often supposed.

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Field Value
Title Implications of Australian seismic and gravity measurements for the structure and composition of the upper mantle
Language eng
Licence Not Specified
Landing Page https://data.gov.au/data/dataset/3eceee48-a31b-4100-9e1d-0ff8ef2ac228
Contact Point
Geoscience Australia Data
clientservices@ga.gov.au
Reference Period 20/04/2018
Geospatial Coverage
Map data © OpenStreetMap contributors
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Data Portal Geoscience Australia

Data Source

This dataset was originally found on Geoscience Australia "Implications of Australian seismic and gravity measurements for the structure and composition of the upper mantle". Please visit the source to access the original metadata of the dataset:
https://ecat.ga.gov.au/geonetwork/srv/eng/csw/dataset/implications-of-australian-seismic-and-gravity-measurements-for-the-structure-and-composition-o