The tectonic setting of the giant Olympic Dam iron oxide-copper-gold ore deposit in South Australia has been uncertain to date. Given the economic significance of the Olympic Dam deposit and its influence in defining the mineral deposit class, resolving its tectonic setting is important. To help address this, two orthogonal deep seismic reflection profiles, centred on the ore deposit, were recorded to 18 s TWT. An approximately north-south line, 193 km long, was oriented as near to a regional dip direction ?defined by potential-field data? as land access would allow. It imaged units of the Archaean?Proterozoic Gawler Craton and a possible allochthonous Proterozoic terrane to the north. A shorter east-west cross-line (57 km long) provided information on any out-of-plane structures imaged on the longer profile in the region of the ore deposit. The seismic data show that the Olympic Dam deposit lies between two distinctly different pieces of crust. To the north of the deposit and the craton, the upper crust beneath the cover sequences shows south-dipping reflectors, interpreted as shear zones that cut through an upper crust of moderate sub-horizontal reflections and a lower crust of higher-amplitude reflections. To the south, the upper crust and lower crust have signatures indicating thrusting towards the craton, in a pattern indicative of a fold and thrust belt of crustal scale. However, in the south, the upper crustal deformation is decoupled from lower crustal deformation at a 3-4km thick band of sub-horizontal reflectors centred at about 12km depth (4s TWT). This zone is mostly un-deformed, except where, in places, it is thrust and duplicated. The middle to lower crust in the region between the two distinctly different crustal types in the north and south contains few reflections, and appears anomalous but homogenised compared with the crust to the north and south, and is interpreted as a likely source region for the Burgoyne batholith which hosts and is approximately coeval with the Olympic Dam ore deposit. Four hypotheses are tested for the lower crustal heat source that generated the granite melt. The intrusion of mafic sills into the lower crust is not favoured because no evidence of widespread seismically reflective underplate can be seen in the data. Heating by a plume is not favoured because the region contains no plume-related geological evidence, and the signature in the seismic data would probably be similar to that of underplating. Extension is not favoured, because the data show no evidence of crustal extension accompanied by surface subsidence and sediment deposition; rather they can be interpreted to show crustal shortening at the time. Heating by radioactive decay is worthy of further consideration because of the high concentrations of heat producing elements in the granite.

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The 2003 Gawler Craton Seismic Survey was conducted with the principal aim of imaging the crustal architecture of the eastern Gawler Craton. The seismic data were collected from two nearly orthogonal traverses, totalling 250 line-km, centred on the Olympic Dam IOCG deposit. They show the Archaean-Proterozoic crystalline basement is dominated by thrusts and duplexes, the upper and lower crust characterised by thrust-complexes, and the middle crust characterised by a layer, up to five kilometres thick, shortened by duplexing. In the lower crust, transcrustal shear zones appear to transect the Moho. The comparatively homogeneous deep crust of the northern part of Traverse 1 is dominated by sub-horizontal reflections rather than dipping reflections and may be a separate terrane. The Moho is at about 40-42 km depth. We propose that the thrust-complexes of the upper crust, although initiated during previous orogenies, were active at 1.59 Ga, the time of Fe oxide Cu-Au mineralisation, and provided the first-order fluid pathways of the minerals-system. No mafic plutons were interpreted.

Steeply dipping reflectors have been recognised on stack data for the Lachlan Fold Belt (up to 60º) and the Tanami Province (up to 70º), demonstrating that significant dip filtering does not occur during acquisition with regional parameters. DMO correction is essential during processing to correctly image such features. These events appear to be real reflections not reflected refractions or diffractions, as proved by analysis of amplitude, frequency and moveout on shot records. In the Tanami case, the reflection is related to a small circular feature in the potential field image, possibly the expression of a granite stock. Migration proved problematical, as most migration algorithms could not simultaneously migrate the steep dips and remain artefact free in other parts of the section, suggesting that interpretation must be done on multiple sections in such cases.

The 2005 Tanami Seismic Survey consisted of 720 km of deep crustal seismic reflection data acquired along 4 lines. The processing was aimed at obtaining a high quality image of the crust from the Moho to the surface, with particular emphasis on imaging shallow features and steeply dipping reflectors. The key processing steps applied here included refraction and automatic statics, spectral equalisation, detailed velocity analysis pre and post DMO, and omega-x migration. Near surface features were better imaged using a floating datum technique for refraction statics application, with only the residual CDP static applied before NMO correction, and the mean CDP static after migration. Offset regularization of CDP gathers pre DMO employed trace interpolation based on dip coherency which had the dual advantage of improving signal to noise, and normalizing amplitudes in low-fold areas, thus reducing migration smiles.

The Curnamona Province Deep Seismic Reflection Traverse was completed in 2004 after the survey was postponed due to heavy rain in mid 2003. The survey involved the collection of approximately 200 km of seismic data along a single east-west orientated seismic traverse that extended from the 1996 Broken Hill seismic traverse, 96AGS-B1A, westwards towards the Flinders Ranges.

Data and results from the 2003 Gawler Craton seismic survey were released at Gawler Craton: State of Play 2004 and Seismic Workshop, held in Adelaide last August. The survey consisted of two nearly orthogonal lines centred on the giant Olympic Dam Fe oxide Cu-Au deposit; 02GA-OD1, a north-south line 193 km long and 02GA-OD2, an east-west cross line 57 km long.

The research project, M346, was divided into two phases. The initial phase involved collection of 70 km of deep seismic reflection data along two sub-parallel traverses (01AGSNY2 and 01AGSNY4) to the south of the regional traverse (Figure 1). The second phase of the project concentrated on the processing of the seismic data, followed by interpretation of these seismic reflection data jointly by the researchers and industry participants. This research project was designed to provide information on the uppermost crustal structure within specific mineralised regions within the Laverton Tectonic Zone of the Eastern Goldfields and image the internal structural and stratigraphic relationships within this zone.

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