Discussion

The main contributions to this investigation involved rationalising the gravimetric field characteristics of lithospheric-scale substructures of South Australia. This section presents a brief discussion of significant elements of the interpretation of the datasets that were used to construct the three-dimensional models of the South Australian lithosphere in the context of two- and three-dimensional potential field modelling.

Geophysical boundaries of the Gawler Craton

Profile modelling of crustal level sources in the gravity data indicates that the geometry of the Archaean Gawler Craton at its margins varies significantly in the third dimension. The western margin is a markedly steep, west-dipping suture defined by an intense negative anomaly low. The structure of this boundary is curvilinear and marks an abrupt transition into Archaean and Palaeoproterozoic rocks in the west. Modelling of the northern cratonic margins indicate an analogous sub-vertical, north-dipping suture. The eastern margin, referred to by Betts (1999) as the Kimban Suture Zone, is in contrast a relatively shallow, east-dipping suture. Crustal thicknesses defining the basal contact between the cratonic margins and the lower crust are however comparable in depth.

Although unsubstantiated, division of the Gawler Craton into large elements to essentially characterise the different gravity signatures of the Archaean complexes has agreed somewhat with the measured gravity data. The modelled western cratonic element differentiates the Mulgathing Complex predominantly in the central and northern regions of the Gawler Craton from the Sleaford Complex found largely in the south and east of the Gawler Craton. The contacts of each cratonic element may represent possible ancient suture zones to protoliths of the Mulgathing and Sleaford complexes.

Origin of various anomalies

The source of the relatively moderate to high, polygonal shaped long wavelength, regional gravity anomaly in the central Gawler Craton has been ascribed to a mantle derived mafic underplate intruding the lower crust (Blissett et al., 1993). Modelling of this anomaly demonstrates the likelihood and existence of such a body. This high-density sheet-like mass is interpreted as the remnants of a possible mafic underplate.

Modelling of various regional-scale anomalies in the east and northern regions of the state adjacent to the eastern margin of the Gawler craton indicate the presence of relatively shallow to mid-crustal level dense bodies of unknown origins. The modelling of displaced crustal blocks beneath and further north of the Stuart Shelf cannot however, be directly correlated (in three-dimensions) from the calculated anomaly response. These bodies exhibit a much higher signature than indicated by the measured dataset and the nature of the source appears to emanate from relatively deeper crustal levels than modelled in the profiles. It is possible that these bodies represent shallow crustal-level plutons. The existence of such bodies however, remains speculative.

Petrophysical Characteristics

The forward modelling technique of calculating a gravity response from a geological model requires; (i) defining geometrical dimensions; and (ii) assigning homogeneous rock properties. The latter is an inherent assumption in both the two- and three-dimensional geophysical modelling undertaken in this investigation. The density values used in both forms of the modelling represent a first-order approximation on the three-dimensional distribution of anomalous cryutal blocks. Although the densities are not definitive of any one particular rock type, they do however provide a primary constraint on the crustal composition and also aid in defining contacts between rocks of similar characteristics.

Matching of the gravity signatures across the profiles reveal rocks of the Palaeo- and Mesoproterozoic exhibit an overlap of density values. Nonetheless, modelling of the Archaean crust indicate average values (2921 kg/m³) typically greater than rocks of the Palaeoproterozoic (2782 kg/ m³). Likewise density values of the Palaeoproterozoic are in general, greater on average than rocks of the Mesoproterozoic (2646 kg/ m³). The central Gawler Craton province of the Gawler Range Volcanics display very similar ranges of values between the Gawler Range Volcanics, Hiltaba Suite Granitoids and associated sediments. In addition, Neoproterozoic rocks of the Adelaidean Fold Belt, Stuart Shelf and the Officer Basin reflect comparable average modelled density values (2652 kg/ m³) to those of the Kararan Orogen.

Level of modelling

The upper crustal-level models show that geometrical sources of the high frequency domain modelled against the gravimetric profiles do not, in general, extend to depths greater than approximately 10 km. The profile modelling of crustal structures at depth cannot however, be viewed in isolation without an understanding of the level of information associated with the gravimetric dataset. The measured dataset provides information on the three-dimensional distribution of anomalous masses in the Earth’s crust (Telford et al., 1995). Modelling of this field requires interpretation and or knowledge of the structures as it is progresses with depth. The image processing technique of depth-slicing (Berger & Reudavey, 1996), where a separation filter such as a matched filter is used to effectively isolate a cumulative ensemble of sources at a given depth, can facilitate better resolution of modelling and interpretation. Unfortunately, the matched filtering technique currently available (and employed in this investigation) does not provide a clean separation of frequencies for different depths (pers. comm. P.McInerney) and is at best a ‘pseudo depth slicing’. For this reason, the two-dimensional profile modelling was not conducted on the matched filtered image because certain frequencies of the data, which was meant to have been removed, were reintroduced into the filtered dataset. Modelling and interpretation of the deeper crustal-level components was restricted to matching the broader and longer wavelength components of the profile. There are clear and obvious inaccuracies associated with this procedure, such as the unknown level at which the modelling is undertaken.

Conclusions

This study presents an example of the use of regional-scale geological and geophysical datasets for modelling geometrical and petrophysical properties in an area largely concealed beneath sedimentary cover. Two- and three-dimensional forward gravimetric modelling of lithospheric-scale structures of South Australian reveals geometrical and petrophysical differences in the crustal architecture between the Archaean Gawler Craton and interpreted margins of Palaeo- to Meso-proterozoic accretionary complexes immediately adjoining and surrounding the cratonic nucleus.

Profile modelling of the shallow crustal-level structures inherent in the high frequency domain of the South Australian gravimetric dataset indicate supracrustal sequences of the Kimban and Kararan Orogen do not extend to depths greater than ~10 km. The modelled geometry, size and crustal levels of these accretionary terranes reveal a marked change of crustal composition throughout particular rock types. This lateral change of rock property is reflected in discrete modelled blocks exhibiting predominantly sub-vertical contacts likely related to normal faulting.

Profile modelling of the deeper crustal-level structures of the gravimetric dataset indicates differences in the modelled geometry and crustal level response of the Archaean nucleus when compared to the modelled interpretation of the Gawler Craton and supracrustal sequences developed in the upper crustal-level models. This is apparent where each of the profiles is modelled to essentially match the relatively longer wavelength components of the gravimetric dataset.

The modelled profiles were subsequently integrated into a two three-dimensional geometrical models, which provided the framework for rock property modelling. This model demonstrates that the interpreted geophysical boundaries of the Archaean nucleus vary in geometry. The eastern cratonic margins dips shallowly to the east while the western and northern cratonic margins sub-vertically dip to the west and north respectively.

The generation of synthetic gravimetric anomaly responses from the three-dimensional models provided a test for comparison of the two- and three-dimensional interpretations against the measured dataset. The modelled anomaly response of the shallow crustal-level structures demonstrates a poor level of correlation with the measured dataset. Although the modelled profiles show a good match of the data, the transition to a three-dimensional model interpretation does not support a direct correlation of the most of the interpreted structures. The modelled gravimetric response of the deeper crustal-level sources demonstrates a moderate level of correlation with the match-filtered dataset. Despite the apparent discrepancies between the model response and target data, the generated field gives some confidence to the interpretation of the lithospheric-scale geometry.

Acknowledgements

The authors wish to thank PIRSA for the support of this study and for permission to publish. This contribution is released with the permission of the Director of the Australian Crustal Research Centre.

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