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To address some of the questions associated with possible changes in geological crustal architecture across the Lachlan Transverse Zone in the Orange-Bathurst region of NSW, the central part of the northeastern Lachlan Orogen was the target for wide-angle seismic profiling during 1997 as part of an Australian Geodynamic Cooperative Research Centre project. A 364 km profile designed to investigate seismic P-wave velocity features in the crust was located north-south across the LTZ with outcrop volcanics of the Ordovician Macquarie Arc in the north and outcrop of Ordovician turbidites and Early Devonian S-type granites in the south.

A wide-angle reflection seismic survey coincident with a regional transect through Northeastern Yilgarn Craton focused on the Leonora-Laverton Tectonic Zone, Western Australia, was carried out to supplement deep seismic reflection studies. The major objectives were: to collect high-density refraction information for offsets of up to 60 km; to carry out a comparative study of near-vertical and wide-angle seismic images of the crust in the study area; to obtain velocity information for the upper crust. The survey deployed 120 short period recorders with a 500 m spacing. Acquisition parameters used for the wide-angle reflection experiment were selected so that it would to fit into the schedule and technology of the conventional reflection survey. The same vibrations were recorded in both surveys simultaneously. The major challenge in processing the wide-angle data was to manage the huge volume of information. The processing sequence included sorting into receiver and source gathers, cross-correlation with reference sweeps and stacking original seismic traces to form single source point traces, producing seismograms from individual traces and finally creating seismic record sections from separate seismograms. High amplitude seismic signal from vibroseis sources was recorded at least up to 50 km offsets in the first arrivals, and later arrivals were observed down to 12 s next to sources. A preliminary upper crustal model developed from the wide-angle data shows that the thickness of a high velocity layer, corresponding to the greenstone rocks, is 4.0-4.5 km. The boundary separating this layer from a low velocity layer below it is possibly a compositional boundary between greenstones and underlying felsic gneisses. There is no evidence for high velocity material below this boundary. Assuming the Moho belongs to deepest reflections modelled, total crustal thickness in the region can be speculatively estimated in the range 32-37 km.

Legacy product - no abstract available

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Deep-seismic reflection data across the Eastern Goldfields Province, northeastern Yilgarn Craton, Western Australia have provided information on the region?s crustal architecture and on several of its highly mineralised regions. The seismic reflection data has imaged several prominent crustal scaled features, including an eastward thickening of the crust across the northeastern Yilgarn Craton, the subdivision of the crust into three broad layers, the presence of a prominent east dip to the majority of the reflections and the interpretation of three east-dipping crustal-penetrating shear zones. These east-dipping shear zones are major structures that subdivide the region into four terranes. Major orogenic gold deposits in the Eastern Goldfields Province are spatially associated with these major structures. The Laverton Tectonic Zone, for example, is a highly mineralised corridor that contains several world-class gold deposits plus many other smaller deposits. Other non crustal-penetrating structures within the area do not appear to be as well endowed as the Laverton structure. In all the major structures a wedge-geometry is formed at the intersection of the east-dipping shear and a low-angle shear zone within its hanging wall. This wedge-geometry forms a suitable trap where the upward and/or sub-horizontal moving fluids were focused into the wedge's apex and then distributed and deposited into the surrounding complexly deformed greenstones.

Crustal-scale seismic surveys mostly collect data along single profiles, and the data processing has an underlying assumption that the data have imaged 2D structure striking at right angles to the seismic profile. However even small amounts of out-of-plane topography on a reflector can result in reflections that do not map the reflector shape accurately. Out of plane energy will migrate within the plane of the section to an apparent depth (represented as two-way-time TWT) that is greater than the depth of the reflection point out of the plane of the section. It will fall within the plane of the section at depths less than, equal to or greater than the intersection of the reflector with the plane of the section, depending on the amount of topographic relief on the reflector out of the plane of the section, and the offset of the topographic relief from the plane of the section. Reflectors that are a single surface can therefore be manifested in the seismic section as a band of several reflections, rather than a single reflection. The top of the band of reflections may not represent the position of the reflector in the plane of the section. More complex reflectors that have a finite thickness because they are made up of several to many anastomosing reflectors caused by altered and anisotropic rock embedded in protolith, will appear as laterally short reflections within a laterally continuous reflection band. Examples of such reflectors would be shear zones, the Moho in some places, and rock with compositional layering. With increasing topographic relief, the top of the reflection band for both single- and multi-layer reflectors will be a poor indicator of the top of the reflector in the Earth. The bottom of the reflection band will always be a poor indicator of the bottom of the reflector. Because out-of-plane energy can arrive at TWTs that are different from those of the reflector in the plane of the section, out-of-plane energy has the potential to interfere constructively or destructively with the in-plane energy. In synthetic data calculated for a simple model assuming one layer and topographic relief of 250m over wavelengths of 4-5 km, similar to that imaged in a real sub-horizontal detachment, amplitudes ranged up to 2.6 times the expected amplitude for the layer. A model with anastomosing layers built to resemble a thick shear zone rather than a discrete fault surface allowed tuning between layers. The effects of out-of-plane energy when combined with the effects of tuning caused amplitudes up to 3.1 times those expected. Larger amplitudes could be achieved if model was contrived. The results indicate that care must be taken when calculating impedance contrasts using real data. The highest amplitude reflections are likely to yield overestimates the true impedance contrast.

Legacy product - no abstract available

Legacy product - no abstract available