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The thickness of the crust is usually defined seismologically by the depth to the Mohorovicic discontinuity or Moho. The Moho is defined as the boundary below which the seismic p-wave velocity of the rocks increases to values above about 7.8 km/s. These velocities are typical for mantle rocks under the expected temperature and pressure conditions. There are arguments that the Moho may not always be the base of the crust from a petrological point of view. In any case, the transition from crustal to mantle compositions may be gradational, and this is reflected by the common observation of a velocity gradient at the base of the crust.There are significant variations in the depth of the Moho under Australia. In general, within the Archean regions of Western Australia the Moho is relatively shallow with a large velocity contrast at the transition between the crust and the mantle. It is significantly deeper under the Proterozoic North Australian Platform, under Central Australia and Phanerozoic Southeastern Australia. Thicker crust in general is reflected in higher surface elevation, although the relationship between crustal thickness and elevation is not linear. Where the Moho is deep there is a very broad transition from crustal to mantle velocities. Other regions of Australia for which data exist generally have average depths to Moho.Crustal thickness patterns reflect the mechanisms of continental growth and tectonic evolution. The relative thickness of the upper and lower crust are characteristic of various styles of extensional or compressional tectonics and other processes such as underplating. Images across the Australian continental margin clearly demonstrate the role that upper and lower crustal extension have on crustal growth, by sedimentary deposition and magmatic emplacement on and within the attenuated crust.

This 2D deep crust seismic reflection survey is part of the joint project between the Geological Survey of Western Australia and Geoscience Australia and is a base study of the South Perth Basin linked to possible future geo-sequestration in the region. It consists of recording seismic signals down to 8 seconds two-way-time depth to image the rock layers below the earths surface. This geophysical method allows the upper crust to be imaged and assists in providing an understanding of the crustal architecture of the study region. Terrex Seismic, a sub-contractor, undertook the geophysical data acquisition. The data were processed to produce industry standard 2D land seismic reflection data. Raw data for this survey are available on request from clientservices@ga.gov.au

Gold deposits in the Archaean Eastern Goldfields Province in Western Australia were deposited in greenstone supracrustal rocks by fluids migrating up crustal scale fault zones. Regional ENE-WSW D2 shortening of the supracrustal rocks was detached from lower crustal shortening at a regional sub-horizontal detachment surface which transects stratigraphy below the base of the greenstones. Major gold deposits lie close to D3 strike slip faults that extend through the detachment surface and into the middle to lower crust. The detachment originally formed at a depth near the plastic-viscous transition. In orogenic systems the plastic-viscous transition correlates with a low permeability pressure seal separating essentially lithostatic fluid pressures in the upper crust from supralithostatic fluid pressures in the lower crust. This situation arises from collapse in permeability below the plastic-viscous transition because fluid pressures cannot match the mean stress in the rock. If the low permeability pressure seal is subsequently broken by a through-going fault, fluids below the seal would flow into the upper crust. Large, deeply penetrating faults are therefore ideal for focussing fluid flow into the upper crust. Dilatant deformation associated with sliding on faults or the development of shear zones above the seal will lead to tensile failure and fluid-filled extension fractures. In compressional orogens, the extensional fractures would be sub-horizontal, have poor vertical connectivity for fluid movement and could behave as fluids reservoirs. Seismic bright spots at 15-25 km depth in Tibet, Japan and the western United States have been described as examples of present day water or magma concentrations within orogens. The likely drop in rock strength associated with overpressured fluid-rich zones would make this region just above the plastic-viscous transition an ideal depth range to nucleate a regional detachment surface in a deforming crust.

The Broadmere structure occurs in the Batten Trough in the southern McArthure Basin. Analysis of seismic data over the basin provides a basinfill architecture and an insight into the determining fluid pathways throughout the evolving basin. System requirements: The presentation on this CD was created using standard Adobe Acrobat PDF format. You will need version 3.0.1 or later of the Adobe Acrobat Reader, to view the presentation.

This paper presents new interpretations of the distribution of magmatic and pre-rift rock packages in the Exmouth-Gascoyne margin, based on the integrated interpretation of two deep crustal transects with existing seismic reflection, refraction, gravity and magnetic data. Interpretations are constrained by data from sparse ODP and petroleum drilling, and dredging. There is evidence for significant accumulation of magmatic rocks and their clastic derivatives infilling extensional fault-controlled basins developed in a broad volcanic margin transition (VMT) zone between the outer Exmouth Plateau and true oceanic crust. These rocks have distinctive seismic facies in the form of Seaward Dipping Reflector Sequences (SDRS), and are dense and magnetised. Most significantly, these packages give rise to potential field anomalies that have previously been interpreted as due to seafloor spreading. Recognition of these packages in a VMT zone has implications for the recognition of the inboard edge of unequivocal oceanic crust, the Oceanic Volcanic Margin Boundary (OVMB). Notably, in the volcanic margin transition zone off the Exmouth Plateau, the main locus of igneous activity is spatially offset from a previously recognised high velocity zone, suggesting that these two phenomena may not be temporally related. Seismically imaged differences in total thinning and partitioning of thinning between upper and lower crust provide support for models of depth dependent thinning previously proposed for this margin.

A recent passive seismic survey to investigate the variations in crustal structure across the Yilgarn craton has shown significant contrasts in seismic models between neighbouring terranes/superterranes. The Eastern Goldfields showed a unique variability in crustal structure in agreement with a recent reinterpretation of terrane boundaries within the Yilgarn craton. We further investigate the Eastern Goldfields region using a 3-way approach which combines conventional passive seismic analysis with innovative seismic noise-correlation methods and constraints from active source data. The conventional passive seismic analysis enables the receiver function S-velocity structure, and hence composition, of the lower crust to be constrained. The noise-correlation analysis allows seismic model in the 5-15 km depth range to be determined and provides medium resolution coverage across regions not previously explored using active seismic methods. Where active source data have been acquired, shallow structure and deeper seismic velocity determinations are added, providing an unprecedented combination of seismic constraints on the structure of this complex and economically important region. We find that, although some individual terrane boundaries within the new Eastern Goldfields reinterpretation are open to question, the concept of the multi-terrane amalgamation is substantially justified by the exceptional variability of the lower crustal structure. Upper crustal structure is often characterised by seismic discontinuities which may represent detachment surfaces or layered structure that varies between terranes over a sub-100 km length scale. The accretionary history of the superterrane and associated regional tectonic setting of numerous formations of economic significance would now appear to be beyond question.

Most crustal-scale seismic reflection surveys use single profiles, and are an attempt to create two-dimensional images of three-dimensional structures. CMP data are stacked and migrated assuming that the seismic energy comes from within the plane of the section. However, three-dimensional topography on an interface results in out-of-plane reflected energy coming into the plane of the section, and energy from the plane of the section being lost from the plane of the section. Interfaces with low to moderate relief image as a zone of reflections in which the top of the zone reproduces the shape of the interface within the plane of the reflector fairly accurately, and energy lower in the zone of reflections is from out of the plane. However, if the relief on the interface is significant, reflections from shallower levels of the interface out of the plane of the section can arrive before those from deeper parts of the interface in the plane of the section. This makes interpreting both the position of the interface in the plane of the section and the amount of relief on the interface difficult. Three-dimensional topography on the interface out of the plane of the section generates more diffractions than does two dimensional topography. The amount and nature of diffracted energy in stacked data is a qualitative indicator of structure in and out of the plane of the section. For a synthetic three-dimensional interface with relief ranging over a number of wavelengths, reflection amplitudes up to twice the primary reflection strength were observed; destructive interference produced very weak reflections elsewhere. The absolute amplitude of the strongest reflections is therefore a poor indicator of impedance contrast for reflectors with significant three-dimensional topography.

Depths to magnetic basement point data were generated in three areas (North Queensland; Gawler-Curnamona; Arunta-Georgina-Amadeus-Musgrave) for the Regional Geodynamic Project, part of the Onshore Energy Security Program. The point depths are interpretations of the depth to magnetic basement (in metres) which is generally the top of crystalline basement. However, due to geological complexity, including variability of magnetic properties of some geological units, the point depths may not coincide with the top of crystalline basement. The point depths should be viewed in conjunction with the map legends of previously released Depth to Magnetic Basement Maps of the three areas. The map legends provide an explanation of which geological units the depth estimates are mapping.

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