ASEG Extended Abstracts - ASEG2006 - 18th Geophysical Conference, 2006

Dating of shallow faults is crucial to understanding of the mechanism of earthquake generation and future seismic risk. However, radiometrie dating of low temperature authigenic minerals in shallow fault zones has been a major challenge because of the contamination of fault rocks by older mineral phases and the overprinting of the syn-tectonic isotopie signature by post-tectonic re-crystallisation events. We present a technique for a reliable dating of fault movements using a combined application of low temperature geochronology, mineralogy and isotope-trace element geochemistry. Such combined studies further allow constraining the origin and migration of seismically mobilised fluids along active fault zones. We investigated illitic clay minerals from fault gouges and late Quaternary carbonate deposits in co-seismic fissures along currently active fault zones in Turkey. Our results indicate that the North Anatolian Fault Zone has been active since the Late Paleocene – Early Eocene that followed immediately the continental collision related to the closure of the Neotethys Ocean. Fault movement is considered to have driven deeply sourced fluids, with metamorphic fluids being supplied by the compression along the Neotethyan orogenic suture zone. Precise dating of the late Quaternary carbonate deposits by U-series geochronology provide important constraints on the late Quaternary fissure generation related to the neotectonic processes and active faulting. Isotopie and trace element data indicate that the fissure carbonate deposits precipitated from deeply hydrothermal fluids. Mobilisation of deep fluids and their surface effusion is attributed to tectonic processes such as seismic pumping. K-Ar and U-series dating of clay and carbonate minerals respectively in seismically active areas is very promising for further studies for dating of major earthquake events and their recurrent intervals, with significant implications for the Australian active fault systems.

Both multiple, reflections and multiple diffractions are present in seismic data acquired in the Makassar Straits, offshore Indonesia. The application of 3-D SRME shows significant improvements over 2-D SRME results in various areas. In addition, several other processing methods were utilized to specifically attenuate any remaining multiple diffractions.

The Tasman Fold Belt System in eastern Australia provides a record of the Palaeozoic geological history and growth of the Australian continent along the proto-Pacific margin of Gondwana inboard of an extensive and long-lived subduction system. The Hodgkinson and Broken River provinces represent prominent geological elements of this system and together form the northern Tasman Fold Belt System. Integration of newly-obtained data with a time-space evaluation of the geological evolution of the Hodgkinson and Broken River provinces, as well as other terranee in the northern Tasman Fold Belt System, allows for the development of a geodynamic model for the Palaeozoic evolution of the northern Tasman Fold Belt System. Our model indicates that three cycles of extension – contraction occurred during the Palaeozoic evolution of the northern Tasman Fold Belt System. Episodes of extension were controlled by roll-back of the subduction system along the Australian margin, whereas episodes of contraction resulted from accretion following the arrival of positively buoyant segments (i.e., micro-continental blocks / oceanic plateaux) at the subducting trench. Our composite interpretative model on the geodynamic evolution of the northern Tasman Fold Belt System integrates the timing of the development of mineral deposits throughout this part of the system and provides a significant advancement in the understanding of Palaeozoic geodynamics along the margin of Gondwana in northeast Australia.

Results of coupled mechanical and fluid flow modelling provide insights into some of the factors that controlled gold mineralisation and deformation in the Hodgkinson Province in northeastern Queensland, Australia. These results aid in resolving why many north-south to northwest-southeast trending, second-order fault zones throughout the Hodgkinson province are spatially associated with gold deposits. The simplified, regional-scale model geometry used in this study permitted variation of boundary conditions, material properties and stress regimes. This included variation of lithological properties, the absence of fault zones in some models, and the application of plane strain or transpression. The model outcomes illustrate the importance of a listric fault geometry for focusing deformation and fluid flow in zones of relatively high permeability and / or low rock strength. Also, the orientation of fault zones with respect to the dominant far-field compressive stress regime is shown to be a key factor controlling deformation and fluid flow. In addition, fault bends and terminations are shown to influence significantly the distribution of deformation and fluid flow in our models. Our results provide an improved understanding of first-order factors that may have controlled localisation of deformation and fluid flow in the regional geological architecture of the Hodgkinson Province. Focused fluid flow arising from localised dilation and permeability increase plays an important role in the formation of ore deposits, hence the outputs of our modelling may be of significant value for future exploration in the Hodgkinson Province, and analogous regions elsewhere. Moreover, our study illustrates the usefulness of numerical modelling as a tool for testing multiple scenarios, leading to improved conceptual understanding of geological systems.

Helicopter-borne time-domain electromagnetic (EM) systems combine the advantages of portability with large transmitter moments. In order to demonstrate the potential of helicopter TEM for shallow bathymetric mapping, a HoistEM survey was conducted near the entrance of Port Jackson (Sydney Harbour) in March 2002. ID inversion of rescaled HoistEM yielded accurate (± lm) water depth predictions to depths of about 55m. The seawater depths inferred from HoistEM were in good agreement with the depths obtained from accurate single-beam and multi-beam sonar soundings.

It was necessary to rescale the HoistEM data in order to account for improper calibration. Re-scaling was possible because a considerable amount of “ground truth” was available. Synthetic TEM decays were computed for numerous models representing shallow water, sediment and basement at a number of locations within the survey area. Measured and calculated data were compared at each delay time, and were found to be linearly related. A rescaling factor was then applied to each decay time, irrespective of water depth.

Given the successful bathymetric mapping demonstration, the potential of airborne EM for seafloor characterisation has been investigated. Accordingly, the sediment thickness inferred from ID inversion of HoistEM has been compared with estimates based on marine seismic studies.

Generally, the sediment thickness, and hence depth to resistive bedrock, inferred from HoistEM was in reasonable agreement with depths estimated from marine seismic data when the seawater was less than about 20 m deep and the sediment was less than about 40 m thick.

Large faults expressed at the ground surface often have clear effects on drainage networks. Smaller scale tectonic activity, or that which occurs beneath sedimentary cover, can have more subtle effects that arise from changes to down-valley slope. These include an increase meander sinuosity or the development of anabranching.

Fowlers Creek arises in the northern Barrier Range (NSW) and its terminal floodout flows across the flat surface of the Bancannia Trough. In the floodout’s proximal and medial reaches the channel is sinuous but generally without active meander development. Active meandering occurs in only in two reaches, and is associated with higher slopes and repeated avulsions, the most recent being during the 20^thC. It is suggested that small-scale uplift may have occurred, increasing the down-valley slope and therefore promoting meander development and avulsion.

More closely spaced topographic data is necessary to support this hypothesis, however Fowlers Creek is one of nine creeks flowing east from the Barrier Range which show planform irregularities several kilometres from the rangefront. This supports the possibility of a concealed tectonic feature, and suggests that it is large-scale and linear.

Major structures (e.g. mines, waste dumps) may be built in sparsely monitored remote locations where earthquake hazard estimation can be difficult. In such places, maps of fluvial geomorphology can be useful indicators of neotectonic activity.

As seismic technology becomes more mature, there is a growing need for seismic reservoir characterization techniques for: the accurate estimate of physical properties of the rock, improved resolution, reliable DHI, estimation of pore pressure for safer drilling, identification of the fractures for the planning of the development wells etc. Theoretically, tools to achieve these reservoir attributes do exist and have been applied with good success. This paper will present a collection of these methods with real data examples. Also, the preconditioning of the seismic data, one of the most important factors for the success of these methods, will be discussed.

The essentials of the unconformity-related U (± precious and base metals, REE...) association are illustrated by the Mesoproterozoic systems of the Athabasca Basin (Canada) and the Alligator Rivers (Australia) regions, sites of major production and high grade resources of U. Significant elements of such systems include

• i)   a regional redox interface corresponding broadly to the unconformity separating an oxidised sedimentary pile from C and/or Fe²⁺ rich reductants in or near the basement;
• ii)   sources of oxidised (high SO4^2-/H2S, evaporite- interacted, moderate to high salinity) fluids in/above the sedimentary basin succession. Such fluids may transport numerous components and their progressive interaction with reductants may give rise to characteristic geochemical and mineralogical patterns at deposit and broader scales;
• iii)   linked fault systems operative during sedimentary basin inversion or subsequent compression;
• iv)   the mechanical contrasts of the basement with the sedimentary cover package;
• v)   and probably, uranium-enriched rocks in the basement and/or in cover sediments and volcanics.

Other well endowed uranium systems, for example the Proterozoic Westmoreland district (Queensland and the Northern Territory), exhibit the key elements of the unconformity-related association, but the main deposits are localised around redox interfaces the faults away from the basal unconformity. A clear picture of the Westmoreland regional mineralised system is provided by integrated airborne geophysical and geological information, providing a model for systems elsewhere.

The prospectivity of the unconformity-related association is not restricted to the Mesoproterozoic. Examples of systems which display the essentials of the association include the late Palaeozoic-aged Ben Lomond and Maureen U-Mo deposits (Queensland) and others in Central Asia, plus Palaeoproterozoic U, precious and base metal deposits in Canada and elsewhere.

More effective exploration and regional targeting models will result from the application of the model outlined above.

Three dimensional (3D) magnetic forward modelling of Banded Iron Formation (BE?) as high susceptibility tabular-style bodies was carried out at the Musselwhite Mine, Ontario in 2004. BIF is associated with mineralisation at the mine and the study both created and validated geologically plausible models. The models are based on drill sections, and are built from a central axis of nodes with geometrical and susceptibility attributes describing each node. The model building process reveals information about the geometrical nature of the BIF units. The results highlight the importance of understanding the interaction of the earth’s magnetic field with the geometry of these highly magnetically susceptible bodies (up to 2.5SI) when interpreting the observed magnetic anomalies. The forward modelling algorithm includes a calculation for the effect of demagnetization, and at Musselwhite this is found to be subtle in nature.

Microbialites are important constituents of reefrock in modern coral reefs, and they have proven to be reliable proxies for some aspects of seawater geochemistry. We used radiocarbon dating to evaluate their potential use as environmental archives by establishing their growth rates and temporal relationship with associated coral-algal communities. At Heron Reef, southern Great Barrier Reef, microbialites occupy ~7 % of framework volume, having formed in low-light settings in framework cavities within two metres below the reef flat. Microbialite growth generally was initiated within a few 100’s of years of the death of the surface-dwelling coral or clam substrates upon which they grew after those substrates had been incorporated into the framework by successive framework growth. Microbialites accreted at a mean rate of 2.9 mm/100 yrs and grew for only a few 100’s of years, presumably while conditions were optimal. Hence, microbialites represent ecological factors in reef growth, operating over similar time frames to associated coral-algal communities, but offset in time by as much as hundreds of years, compared to immediately adjacent skeletal framework. Therefore, microbialites can serve as proxies for marine geochemistry over averaged century-scale time frames, but they lack the temporal resolution afforded by, for example, scleractinian corals.

The Arte River Gabbroic Complex is located in eastern Victoria within the Kuark Metamorphic Belt. It is spatially associated with a number of I-type granites, the A-type Murrungowar adamellite and the Scrubby Flat Gabbroic intrusion located further to the north. These are hornblende orthopyroxene gabbros, with two samples containing titanaugite.

Geochemical analyses reveal two distinct magma types in the Arte Gabbroic complex an alkali component, evident in the pyroxene mineralogy, and a tholeiitic component. The complex is associated with a small tonalite/diorite lens with complex but sharp boundaries between the two. Sr-Nd isotope studies reveal two evolutionary arrays, one between this lens and the alkali component and the other between the thoeleiitic and the Scrubby Flat Gabbro located to the north-west of the main Arte River gabbro intrusion.

The Geological Society of Australia Geological Heritage Standing Committee promotes the understanding and conservation of the geological heritage of Australia through identifying and documenting features or sites. This paper outlines the methodology, protocols and procedures for assigning or reviewing geological significance developed by the GSA (Victoria) Geological Heritage subcommittee.

Unstable ground conditions caused by unexpected collapse of existing buried infrastructure or incidents during tunnelling represent major geotechnical risks.

Specially developed crosshole, SUBS and SEWREEL seismic imaging technologies that operate from boreholes and underground openings can be applied to define the subsurface extent of the unstable ground and to monitor the effectiveness of remedial works. These technologies use below ground hydrophone arrays and surface seismic sources.

A case study of a shallow sewer collapse that followed a rainfall event in a beach suburb of Sydney, demonstrates the application of these imaging technologies, showing their ability to define the resulting unstable ground hazard and to assist with geotechnical risk reduction.

As part of the exploration program near the Syama mine in Mali, a 3D resistivity and IP survey was designed for detailed lithological mapping of a partially drilled deposit, about 30 km along strike from the main orebody. Tight line spacing and offsets between transmitting and receiving lines ensured 3D coverage. The survey design had to accommodate simultaneous drilling activity on nearby lines. The combination of resistivity and chargeability measurements map distinct basalt, chert, and graphite lithologies. After 3D inversion, constrained by the available drilling information, the data show structures and folds that will influence resource models.

2.5D electromagnetic (EM) modelling computes the response due to a 3D source from an arbitrary 2D geoelectrical model which has an infinite strike length. As such, airborne EM (AEM) data can be inverted using 2.5D modelling provided that the geoelectrical cross-section is relatively constant along a strike length that exceeds the AEM system footprint. The program ArjunAir is introduced for modelling and inversion based on a 2D finite-element method that enables the accurate simulation of 3D source excitation for full domain models inclusive of topography, non-conforming boundaries and very high resistivity contrasts. Inversion is based on an iterative Gauss-Newton method that is solved using the damped eigenparameter algorithm. Examples are presented for synthetic frequency and time-domain AEM surveys for which inversion runtimes are in the order of hours.

The Stawell goldfield lies within a reworked portion of the Delamerian Fold Belt in western Victoria and has a structural complexity and a gold-lode style that is markedly different from similar ca 440 Ma aged deposits in the Lachlan Fold Belt such as Bendigo or Ballarat. Comparisons with these other turbidite-hosted gold deposits in Victoria suggest four major differences: (1) presence of a tholeiitic basaltic pile; (2) ductile deformation (DM) over at least 60 million years prior to gold mineralisation; (3) a preferential and localised reactivation of pre-existing structures within a progressively rotating stress field and (4) highly evolved hydrothermal alteration. Of these differences a key element has been the nature of the basaltic pile, which may have promoted alteration of onlapping sedimentary rocks, thereby creating a basis from which a high sulphur turbidite-hosted orogenic gold deposit was formed.

This study concentrates on the petrogenesis of carbonates in the Flinders Ranges with the view that they will give insight as to the nature of carbonate geochemistry with implications on the ocean geochemistry during the Neoproterozoic. The search for the reason for the paucity of dolomite in modern times compared to the Precambrian was one of the driving forces for Precambrian carbonate research to this day. By using petrology, cathodeluminescense, trace element analyses and stable isotope analyses, we hope to add to the growing field of evidence for a different ocean chemistry during the Precambrian. We suggest that dolomite is a common early diagenetic marine phase during the late Proterozoic.

It has been found that there is a debris flow unit in the shales adjacent to the Balcanoona Formation (previously described as a submarine escarpment) near Oodnaminta Hut. At this stage we have identified several prominent fabrics in the partially dolomitised limestone blocks in this debris flow. Most of these fabrics involve some kind of biocementation by microbial structures. There are up to 4 geopetal generations found in these limestone blocks. With the 1^st and 2^nd generations being the most common. Preliminary studies show that there has been significant neomorphic replacement of aragonite to calcite and mimetic replacement of calcite by dolomite.

The Palaeoproterozoic Pine Creek Orogen, northern Australia, forms part of the North Australia Craton, and is a highly prospective terrain, with abundant mineral occurrences including gold and uranium. Geologically, it comprises sedimentary sequences deposited on rifted Archaean basement that were deformed, metamorphosed, and intruded by granitoids and mafic bodies during the ca. 1880-1850 Ma Nimbuwah Event, one of the constituent events of the Barramundi Orogeny.

The stratigraphy of the central Pine Creek Orogen is well-characterised due to its extensive exposure, preservation, and low grade greenschist facies metamorphism. It contains units suitable for isotopie dating, permitting determination of absolute ages within a well-controlled stratigraphic framework. We present new zircon U-Pb SHRIMP results for volcanics and volcanogenic strata from various intervals within the pre-orogenic stratigraphy. These data: (a) indicate the presence of volcanic activity in the Pine Creek Orogen at ca. 2025 Ma; (b) provide revised constraints on the tectono-thermal evolution of the Pine Creek Orogen, specifically a revision of the Nimbuwah Event by some 20 Ma; (c) suggest that volcanism was widespread across the Pine Creek Orogen at ca. 1860 Ma; and (d) permit assessment of relative deposition rates of and time intervals between deposition of the Palaeoproterozoic stratigraphic sequences.

The Daly River is the largest perennial river in the northern part of the Northern Territory. Its catchment has good soils and is earmarked for the future agricultural development. Recognising the nearly pristine character of the catchment, the NT Government placed a Moratorium on major developments, pending completion of an Integrated Regional Land Use Plan. The aim of such plan, based on analysis of hydrological, biological, environmental and social (including Indigenous people) studies, is to develop a balanced holistic catchment management approach. Despite the extensive research, significant knowledge gaps still exist. Most of the multidisciplinary studies of the catchment completed till now were conducted without sufficient attention paid to the interconnection between economic, environmental, social and cultural aspects of the river catchment management. As a result no integrated catchment management plan has as yet been proposed. Further research is needed to fill the existing knowledge gaps.