Chapter in Geoscience Australia Record for Northern Yilgarn Seismic Workshop

No abstract available

Deep-seismic reflection data across the Archaean Eastern Goldfields Province, northeastern Yilgarn Craton, Western Australia have provided information on the crustal architecture and on several of its highly mineralised belts. The seismic reflection data contain has images of several prominent crustal scale features, including an eastward thickening of the crust, 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 smaller deposits. Other non crustal-penetrating structures within the area do not appear to be as well endowed metallogenically as the Laverton structure. The seismic reflection data have also imaged a series of low-angle shear zones within and beneath the granite-greenstone terranes. Where the low-angle shear zones intersect the major crustal-penetrating structures, a wedge shaped geometry is formed. This wedge geometry forms a suitable fluid focusing geometry where upward to sub-horizontal moving fluids are focused and then distributed into the nearby complexly deformed greenstones.

As part of Geoscience Australia's Southwest Margin Project, two major marine surveys were undertaken (from October 2008 to February 2009) to investigate the resource potential of deep-water frontier areas on the southwest Australian continental margin. 1. Southwest Australian Margin Regional Marine 2D Seismic Survey (S310) - Areas covered by the seismic survey include the Mentelle Basin, North Perth Basin (Zeewyck and Houtman Sub-basins), Southern Carnarvon Basin and the Wallaby Plateau. Data acquired: 7300 kilometres of 2D seismic (12 second record length, 8 km solid streamer), gravity and magnetic data. In addition to the new seismic reflection data, Geoscience Australia has reprocessed selected open-file industry seismic lines in the offshore Northern Perth Basin (11,700 line km) that provides ties to most wells in the Abrolhos and Houtman sub-basins. 2. Southwest Australian Margin Marine Reconnaissance Survey - The marine reconnaissance survey investigated the geology and marine environments of the offshore North Perth and Southern Carnarvon Basins and the Wallaby Plateau. Data acquired: multibeam swath bathymetry (230,000 km2), gravity and magnetics (25,000 line km), sub-bottom profiler (25,000 line km), geological samples (190 rocks from 53 dredge sites)

Seismic reflection, seismic refraction and portable broadband data collected within Western Australia's Yilgarn Craton, in particular the Eastern Goldfields Province, are providing detailed images of several of its highly mineralized terranes as well as new insights into the crustal architecture of the region. When the results from these seismic techniques are integrated, the results are providing a better understanding of the structure of the crust and lithosphere beneath the Yilgarn Carton, from the surface to depths in excess of 300 km.

New 2D seismic data acquired in the Mentelle Basin by Geoscience Australia in 2008-09 has been used for a seismic facies study of the post-rift succession. The Mentelle Basin is a large deep to ultra deep-water, frontier basin located on Australia's southwestern margin about 200 km southwest of Perth. The study focused on the post-rift sequences deposited following the breakup between Australia and Greater India. Stratigraphic wells DSDP 258 and DSDP 264 provide age and lithological constraints on the upper portion of the post-rift succession down to mid-Albian strata. The depositional environment and lithology of the older sequences are based on analysis of the seismic facies, stratal geometries and comparisons to the age equivalent units in the south Perth Basin. Fourteen seismic facies were identified based on reflection continuity, amplitude and frequency, internal reflection configuration and external geometries. They range from high continuity, high amplitude, parallel sheet facies to low continuity, low amplitude, parallel, subparallel and chaotic sheet, wedge and basin-fill facies. Channel and channel-fill features are common in several facies as well as a mounded facies (probably contourite) and its associated ponded turbidite fill. A progradational sigmoidal to oblique wedge facies occurs at several stratigraphic levels in the section. A chaotic mound facies, probably comprising debrite deposits, has a localised distribution. Seismic facies analysis of the post-rift sequences in the Mentelle Basin has contributed to a better understanding of the depositional history and sedimentation processes in the region, as well as provided additional constraints on regional and local tectonic events.

Between October 2008 and February 2009, Geoscience Australia undertook two major surveys off the coast of Western Australia. Areas of interest included the Mentelle and northern Perth Basins, the Southern Carnarvon Basin, the sourthern Exmouth Sub-basin (Northern arnarvon Basin) and the Wallaby Plateau. These surveys collected a range of data, including 7300 kilometers of industry-standard seismic reflection data and 43000 line kilometers of gravity and magnetic data. In addition to the new data collected, Geoscience Australia has reprocessed 11700 line kilometres of open file 2D seismic data that exists within the survey area. This data is available for purchase as part of the Southwest Margin Data Package.

The new acquisition of multibeam bathymetry data along with potential field, seismic data and sediment and rock samples has provided a large quantity of new data in the Northern Lord Howe Rise. A detailed study of the relationships between the surface and sub-surface features over the Capel and Faust basins suggests that seafloor deformation is linked to the underlying basement architecture. Numerous seafloor and sub-surface geological features have been identified and mapped over the study area. Their nature, distribution and relationships have been analysed to propose their formative mechanisms. Most of these features are related to buried igneous intrusions and fluid flow either located within depocentre megasequences or along basement bounding faults. The co-genetic geological features indicate that fluid flow is mainly driven by igneous activity. The ongoing fluid flows, after each magmatic pulse has re-utilised pre-existing fluid conduits. Major depocentres have been identified over the study area and could be prospective for petroleum exploration. Potential source, reservoir and seal rocks are likely to be present in the capel and Faust basins. Volcanic activity has driven the geology and fluid flow over the study area since at least the Upper Cretaceous and has to be considered when assessing the petroleum prospectivity of the Capel and Faust basins and also elsewhere in the Lord Howe Rise.

A new 460 km long transect combines 6-12 fold explosive and 60 fold vibroseis data, proving that it is possible to re-use old, low fold seismic data by utilizing new processing techniques. We demonstrate comparability in resolution of reprocessed dynamite and new vibroseis data. Reprocessing of the dynamite data using a vertical stacking technique that simulates DMO corrections, followed by detailed velocity analysis, has resulted in an improved seismic section that images dipping structures not detected in the original processing.A new 460 km long transect combines 6-12 fold explosive and 60 fold vibroseis data, proving that it is possible to re-use old, low fold seismic data by utilizing new processing techniques. We demonstrate comparability in resolution of reprocessed dynamite and new vibroseis data. Reprocessing of the dynamite data using a vertical stacking technique that simulates DMO corrections, followed by detailed velocity analysis, has resulted in an improved seismic section that images dipping structures not detected in the original processing. Several deep penetrating shear zones in the Broken Hill block dip mostly to the SE, and link to a shallow thrust belt located further to the west. Because of the large depth of penetration, these shear zones may have conducted fluids from deeper levels, which has important implications for mineral exploration.

Presentations from the SA Seismic and MT workshop 2010.