In mid-1991, the Bureau of Mineral Resources (BMR; now the Australian Geological Survey Organisation, AGSO) commenced a program of deep-seismic acquisition on the southern North West Shelf with the intention of providing a regional framework data set for explorersin this highly prospective segment of Australia's continental margin. This program is part of a wider program that is acquiring a regional grid of deep-seismic data along the length of the North West Shelf. In particular, the program aims to: *Determine the broad regional structural framework of the southern North West Shelf byexamining the boundaries between the major structural elements; *Determine the deep crustal structure of the region; *Assess the control of deep structure on the development of the major hydrocarbon fieldsand plays, and in particular the structural and depositional effects resulting from reactivation of these structures; and *Acquire a set of high-quality seismic tie lines linking the deeper exploration wells throughout the region, to allow regional seismic correlations. To address these aims, a multi-cruise program was devised during which deep-seismic data are being recorded. The first survey, SNOWS-1 (for Southern h_Torth Eest Shelf; AGSO Survey 101) was concentrated in the Barrow and Dampier Sub-basins and inner Exmouth Plateau. 1654 km of good-quality seismic data tied to 20 exploration wells were recordedand processed; these data frequently show basin structure down to a depth of at least 10 s two-way time (TWT). The second survey, SNOWS-2 (AGSO Survey 110), acquired morethan 2800 km of high-quality deep seismic data along 13 lines in the Beagle, Dampier, and Barrow Sub-basins, and over the full width of the Exmouth Plateau in mid-1992. These lineswere tied to 21 exploration wells, of which 3 were also tied during SNOWS-I, and again show reflections down to 12 s TWT. The third survey in the program, SNOWS-3 (AGSO Survey 120), took place in July-August 1993, and acquired a total of 4052.5 km of high-quality deep-seismic data along 14 lines, tiedto 6 exploration wells (of which 2 were also tied on SNOWS-2) and Ocean Drilling Program Site 765 in the offshore Canning Basin, northeast extremity of the Carnarvon Basin, andO^Argo Abyssal Plain. SNOWS-3 ties in with the SNOWS-2 survey to the southwest and withthe Browse Basin deep-seismic survey (AGSO Survey 119, acquired in June, 1993) to the northeast. The seismic data were recorded from a 4800 m streamer, configured with 192 x 25 m active groups. The record length was 16 seconds and the sample interval 2 msec. The seismicsource consisted of tuned airgun arrays with a total volume of 49 litres (3000 cu in). Shots were fired every 50 m at an average ship speed of 5 knots, providing 48-fold coverage.Streamer noise levels were low, with the average noise levels on acceptable channels generally being less than 1.5 rnicrobars. Navigation for the survey was provided by two differential Global Positioning Systems (DGPS), using shore reference stations in Dampier and Broome, adjacent to the survey area.Full differential coverage was achieved for all but a few minutes of the survey. Positionalaccuracy is estimated to be probably better than +I- 10 metres. The offshore Canning Basin is a complex amalgam of a WNW-ESE trending EarlyPalaeozoic intra-cratonic basin overlain by a NE-SW trending Late Palaeozoic to Mesozoicpassive margin basin (the Westralian Super-basin)

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The Eastern Goldfields Province (EGP) is a late Archaean orogenic belt located on the eastern margin of the Yilgarn Craton. The EGP is characterised by a prominent NNW-trending tectonic grain developed largely during regional ~E-W oriented shortening. A simplistic sequence of largely progressive compressional deformation events (D1 to D4+) has been used by previous workers to explain the development of the EGP. Regional `D2? was thought to be responsible for most of the finite strain, and therefore a reliable marker event for correlating deformation histories across the region. A re-examination of the deformation history has shown that `D2? (Kalgoorlie Orogen of Weinberg et al., 2003) was not a single progressive event. Rather, it was episodic and involved several stages of switching of the tectonic mode (compression-extension switching) between ~2665 Ma and ~2655 Ma. The sequence of events during `D2? was an early stage of ~E-W shortening (D2a), followed by extension with basin formation and normal fault movement (D2E), and finally a second stage of ~E-W compression (D2b). We introduce this sequence of events as the Wangkathaa Orogeny, more complex in tectonics and broader in geographical scope than the Kalgoorlie Orogen. Features of the Wangkathaa Orogeny are well illustrated by the geometry and geological relationships that occur in the Kurnalpi Terrane (Welcome Well area) and the Kalgoorlie Terrane (Ora Banda, Kambalda, and the Boorara Domains) of the central and southern EGP. Available geochronology indicates that this sequence of events was likely to have been diachronous across the entire EGP and the eastern Southern Cross Province (of the Yilgarn Craton). Recognition of these separate (episodic) stages of ~E-W oriented compression and extension is important in our understanding the tectonic evolution of the EGP. Switching tectonic mode between compression and extension will change the state of mean-stress in a rock mass (e.g., from net dilation to net constriction and vice versa), and is an effective method in transporting and trapping mineral-rich fluids. The paradox of the different timings established for `D2? across the province are explained by the Wangkathaa Orogeny, especially if it were diachronous, as different workers dated different phases of an episodic orogeny.

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HYC and the McArthur River region are located in northern Australia within the Northern Territory on the eastern edge of the uBatten Trough" adjacent to the Emu Fault Zone (Fig.1). The McArthur Region Geology is presented on Figure 1 and shows that the HYC deposit lies to the east of the NE-plunging Barney Hill anticline and immediately west of the Western Fault Block which is bounded to the east by the Emu Fault Zone containing horsts of Masterton Sandstone. The "Cooley Breccias" lie along the western edge of the Western Fault Block juxtaposed with the HYC sequence with unkown relationships that were the focus of this study. The HYC-Cooley drillholes logged in this study are listed in Table 1 and located on Figure 2. Table 1 also indicates the holes that were quantitatively structurally logged, the orientation method applied and the detailed Plans and Sections that accompany this report.

The southern margin of Australia, formed by the separation of Australia and Antarctica in theCretaceous, has long been considered an example of a classic, non-volcanic, passive, rifted margin. In recent times it has been cited as an excellent example of a 'lower plate' margin inthe terminology of detachment models of passive margin formation. As the margin has been relatively sediment-starved since breakup, and particularly in the Tertiary, it represents anexcellent opportunity to test detachment models via reflection seismic data and drilling. In this document, we propose that a total of seven holes be drilled by the Ocean DrillingProgram in the central Great Australian Bight (GAB). Five of these holes are located close tothe previously interpreted continent-ocean boundary (COB), which high-quality seismic datasuggest may actually be a metamorphic core complex. The holes have the following basic aims: 1.Test the applicability of detachment tectonics to passive margin formation by drillingthrough a master detachment to extended lower continental crust; 2.Sample acoustic basement at several sites to characterise the tectonic and magmaticprocesses that have been influential at high rates of continental extension and/or duringearly and slow oceanic spreading. In the event that the interpretation of a metamorphic core complex proves to be incorrect and oceanic crust is encountered, then two additional holes are proposed to: 1.Date the oldest sediments above oceanic crust in the South Australian Abyssal Plain, andhence constrain the currently ill-defined age of onset of spreading between Australia andAntarctica and refme the breakup history of eastern Gondwanaland. 2.Date and characterise the extensional listric faulting that is prominent in possible oceaniccrust on the South Australian Abyssal Plain. As the data on which these sites are proposed is sparse and, in some cases, old, the AustralianGeological Survey Organisation (AGSO) proposes to carry out a deep-seismic program in thecentral GAB that will allow a refmal interpretation and consequent up-grading or modificationof the proposed sites in 1995/96.

The southern margin of Australia has long been considered an example of a classic passive rifted continental margin. In recent times, it has been cited as an example of a 'lower plate' margin in the terminology describing detachment models of passive margin formation.However, despite extensive study by both industry and government since the early 1970s, some fundamental aspects of the structure and geological history of the margin remain speculative. It is proposed here to use the AGS0 research vessel Rig Seismic to acquire 4087 km of deep-seismic data (16 s record length) in the central Great Australian Bight (GAB) and across the continental margin south of Western Australia. The survey has three principal objectives: 1. To enhance understanding of the tectonic evolution / event history of the southern margin in support of DPIE' s Acreage Release Program and thus to encourage successfulpetroleum exploration. 2. To provide the necessary framework data in support of a proposal submitted to the Ocean Drilling Program (ODP) for drilling in the GAB; and 3. To provide data in the GAB that support Australia's claim to a Legal Continental Shelf beyond the 200 n.m. Exclusive Economic Zone (EEZ) as defmed under the 1982 UNConvention on the Law of the Sea (UNCLOS). The specific scientific aims of the work include: - Definition of the deep crustal structure of the region and the mode of margin formationacross a number of key transects. In particular, to image the key detachment surfaces which are believed to have controlled the extensional processes. Interpretation of the deepstructures will be enhanced by seismic refraction data recorded by Lamont-Doherty Geological Observatory in 1976 (Talwani et al., 1979). - Determination of the location and structural setting of the continent-ocean boundary. In places, stratified tilt-blocks that lie oceanwards of the magnetically determined COBsuggest that either the COB identification is incorrect, old oceanic crust has been re-rifted, or the seismic data are imaging an amalgam of oceanic crust and continental slivers. - Determination of the structure and origin of the enigmatic Diamantina Zone, west of the GAB, and its relationship, if any, to the change in seafloor spreading rate at 44 Ma. Is the crust to the north oceanic, or is it highly extended continental?

The presence of dipping reflectors and zones of high reflectivity in oceanic crust are beingincreasingly recognised on modern seismic reflection data. Some workers have related thereflection characteristics of oceanic crust and its associated Moho to spreading rate - slow-spreading crust is proposed to have the richest variety of reflections, but with a poorly imagedMoho, where as fast-spreading crust is suggested to be largely transparent but with a strongreflection Moho. The various reflection characteristics of slow-spreading crust have beenassociated with both tectonic and/or magmatic origins, but whatever their causes, they areclearly important to understanding the accretion processes that occur at spreading ridges.Although imaging of the seismic characteristics and reflector geometries of oceanic crust hasimproved dramatically in recent years, without direct information on the nature of thereflecting zones it will not be possible to uniquely constrain their origins and to test competing models. Recent AGSO deep-seismic data over the Argo Abyssal Plain off northwest Australia provideexcellent imaging of both strong crustal reflections and an associated Moho from the world'soldest (Late Jurassic) preserved oceanic crust. This area represents an ideal location at whichto test the origin of a variety of oceanic crustal reflectors, and thus could make some majoradvancements in our understanding of spreading ridge accretion processes. The proposedsites, in conjunction with the nearby Site 765, will provide a unique insight into the nature,origin, variability and deformation of the oceanic crust in the region. Although the chilling willpush the capabilities of the Joides Resolution to the limit, this is likely to be the case for anysites targeting such objectives within oceanic crust. The main general objectives of the proposal are to: 1.Test models for the formation (tectonic and/or magmatic) of major crustal reflectionfeatures and zones in oceanic crust, and determine their significance to crustal accretionprocesses at spreading ridges; 2.Determine the cause of the reflectivity of the upper part of oceanic basement and itsimplications for the composition and physical properties of the crust; 3. Examine the thermal and mechanical evolution of oceanic crust as it ages and moves awayfrom the spreading ridge, and, in particular, the nature and cause of any late-stagereactivation of primary structures.

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Legacy product - no abstract available