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

Imaging of groundwater that interacts with surface watercourses is essential for providing detail needed to accurately manage both resources. It is particularly important where one resource is saline or otherwise polluted, where spatial quantification of the interacting resources is critical to water use planning and where losses from surface waterways need to be minimized in order to transport water long distances. Geo-electric arrays or transient electromagnetic devices can be towed along watercourses to image electrical conductivity (EC) at multiple depths within and beneath those watercourses. It has been found that in such environments, EC is typically related primarily to groundwater salinity and secondarily to clay content. Submerged geo-electric arrays can detect detailed canal-bottom variations if correctly designed. Floating arrays pass obstacles easily and are good for surveying constricted rivers and canals. Transient electromagnetic devices detect saline features clearly but have inferior ability to detect fine changes just below beds of watercourses. All require that water depth be measured by sonar or pressure sensors for successful elimination of effects of the water layer on the data. Presentation of the data using a 3D presentation technique where EC is imaged along vertical ribbons drawn along the watercourses is almost essential for handling the data produced because the meandering paths of rivers and canals combined with the shear volume of data typically acquired results in a geo-referencing dilemma that cannot be accommodated using traditional presentation techniques.

An extensive set of EC Imaging case studies, distributed across canals and rivers of the Australian Murray-Darling Basin, has been collected. They reveal the interaction of various rivers and canals with the underlying groundwater resources. At some sites, watercourses cross prior river channel sands that are being recharged and are suitable for use as water storages with low evaporation losses. Geophysical imagery can be used to optimize recharge and design of such underground storages. At some other sites, little connection between aquifers and surface watercourses is evident. Finally, at downstream ends of geological basins, sites where saline groundwater flows into, or is on the verge of flowing into rivers are evident.

Recent research voyages using the Australian Marine National Facility, have discovered one of the world’s most youthful, magmatically-active backarc basins: the Coriolis Troughs. In detail, the localised structural consequences of the general clockwise rotation and extension inferred for the basement block of the southern New Hebrides Arc has been clearly revealed by 30 kHz multi-beam, sonar swath mapping of the Coriolis Troughs, and major submerged volcanoes in the southern part of the Arc. A left-lateral, transtensional faulting regime predominates both within the Troughs and the submarine Gemini-Oscostar (20° 59’S, 170° 17’E) and Volsmar (21° 30’S, 170° 11 ’E) volcanic complexes.

Gravity and GPS surveys were conducted on the Windmill Islands, East Antarctica, during the 2004/05 summer season. The aims of the study were to investigate the subsurface geology of the area and to deduce the changes in gravity and rock surface heights since the first surveys of the area were conducted almost five decades earlier, caused by glacial isostatic readjustment. The area became ice free following the end of the Last Glacial Maximum (LGM) 5,500 to 8,000 years ago and is thought to contain signals as to the state of nearby Law Dome.

G-model Lacoste and Romberg gravity meters were used to make precise gravity measurements, std +/- 0.04 milligal. Two calibrated hand held single frequency Garmin GPS receivers were used to determine position including elevation, the standard deviation of the elevation being 2.7 m. Ninety three gravity stations were established. Additionally, forty three observations from a survey in 1993-94 were re-reduced and merged with the 2004-05 data to allow a complex three-dimensional subsurface model to be constructed.

The Windmill Islands are at the rock/ice interface and thus this work has given an insight into the likely underlying bedrock structures and variability below Law Dome.

This survey has set the foundation for future time-series gravity work in the Windmill Islands.

Mafic igneous rocks in the Georgetown Inlier of north Queensland are found within the Palaeoproterozoic Etheridge Group, and comprise the extrusive Dead Horse Metabasalt and intrusive Cobbold Metadolerite. This study presents new major, trace and rare earth element data for the mafic rocks, and compares them to existing data from similarly aged mafic rocks of other Palaeoproterozoic terranes in eastern Australia. Based on the geochemical and limited geochronological evidence, both suites of mafic rocks are relatively evolved low-K tholeiites with strong Fe-enrichment trends (Fe203(t) 6.81-21.11 wt%), and were emplaced between ca. 1700 and 1650 Ma. Despite pervasive metamorphism and recrystallization, the rocks appear geochemically similar to basic gneisses of the Broken Hill Block and mafic rocks of the Mt Isa Eastern Succession, particularly the Toole Creek Volcanics. This relationship is of regional significance, as it may point to a tectonic relationship between the terranes, and hence help place the Georgetown Inlier within the Proterozoic framework of northern Australia.

Two 2D anisotropie prestack depth migration algorithms that use wavefield extrapolation are evaluated for tilted transversely isotropie media (TTI).

The first method is anisotropie phase-shift-plus-interpolation (A-PSPI) that is based on an analytical solution of frequency-dispersion enabling the use of an arbitrary distribution of velocities and anisotropie parameters. The second method is anisotropie reverse-time (A-RT) that uses the full wave equation.

We evaluate the accuracy and efficiency of there algorithms. Examples are presented that use numerical and physically modelled data along with a structurally complex real data example. Both A-PSPI and A-RT have excellent performance with A-RT maintaining the advantage of using the full wave equation.

Traveltime computations are an integral part of modelling and imaging seismic data by providing efficient kinematic information on the location of propagated energy. The traveltimes may be computed analytically using simplifying assumptions, or may be estimated on a complex geological structure using raytracing or gridded traveltimes.

A basic requirement for the propagation of gridded traveltimes is the estimation of one point on a corner of a square, given the traveltimes on the other three corners. A number of solutions are available to solve for the unknown time and are based on either a plane-wave assumption, a finite difference solution to the Eikonal equation, or an assumption that the wavefront at the square is curved. A solution for a curved wavefront assumption requires estimating the center of curvature, and requires solving a quartic equation. An alternate method is presented to estimate the center of curvature for a curved wavefront that uses an iterative procedure and does not require solving the quartic equation.

This paper uses large sets of whole-rock geochemical data to make comparisons between the komatiitic rocks of Abitibi (AGB) and Norseman-Wiluna (NWGB) greenstone terranes.

The NWGB komatiite suite has a much higher proportion of highly olivine-enriched cumulates than the AGB suite, as indicated by data-density distributions on plots of MgO vs FeO and MgO vs Cr, although average compositions of the komatiite magmas in the two belts were not significantly different. NWGB komatiites appear generally more contaminated, on the basis of various ratios of strongly to moderately incompatible low-mobility trace elements.

Both factors which are likely contributors to the much higher Ni sulfide resource endowment of the NWGB. The combination of high degrees of contamination and presence of olivine adcumulates in the NWGB attests to the presence of exceptionally high-intensity, prolonged eruptions, capable of forming long-lived entrenched magma pathways, represented by highly olivine-enriched cumulates, and capable of melting substrates to form orebodies. This is in contrast with more episodic, lower volume eruptions in the AGB. The contrast is interpreted as the result of crustal structure and tectonic setting, rather then the size and intensity of mantle plume sources.

The dramatic changes in the landscape since European settlement have radically altered regional water balances in semi-arid areas, which have not only resulted in land salinisation, but have and will cause salinisation of aquifers over the next 100 or so years in semi-arid areas of the Murray Basin. This process is triggered by two main processes - land clearing and extractions for irrigation.

The small amount of salt in rainfall has been concentrated by native mallee vegetation over thousands of years and is stored in the root zone, with salinities approaching that of sea water. Clearance of this native vegetation for dryland cropping has not only increased recharge, but has initiated the flushing of this salt down to underlying aquifers which often contain good quality groundwater.

Irrigation extractions from shallow permeable aquifers have led to the recirculation and concentration of salt.

These salinisation processes require a new interpretation of what sustainability means for groundwater development.

In the last 100 years about two million (mostly eclogitic) diamonds were mined from Tertiary alluvial deposits at Bingara-Copeton, in the low metamorphic grade Phanerozoic New England Fold Belt, NSW. Even today their hard rock source remains unknown, so we targeted several basanitic igneous bodies to the south west of Bingara, and recovered more than 3000 garnets and five diamonds from samples of modern stream sediment and loam.

Chemical classification of the garnets identified some from diamond eclogite and the Cr-poor megacryst suite indicating a mantle source, and formation near or within the diamond stability field. We defined an additional subgroup of UHP-crustal garnets with inherited continental crustal chemistry and unusual exsolution microstructures. These are similar to garnets from schist/orthogneiss, formed from deeply (>200km) subducted leucocratic continental crust, exhumed in the UHP terrane of Dabie Shan, China. They also compare with rare garnet inclusions in diamonds.

Exsolution microstructures in garnets (world wide) are rare. They indicate a chemical change in UHP garnet due to exhumation of slab material and re-equilibration under lower pressure conditions. Such decompressed garnets from Bingara provide evidence for partial exhumation of deeply subducted diamond eclogite-grade oceanic crust and UHP-metamorphic continental crust to much shallower levels.

This study predicts a buried diamond-bearing UHP terrane in the northern New England district, and a two-stage delivery of subduction diamonds and garnets to the surface; firstly by slab exhumation to relatively high levels, then by entrainment in shallow-sourced basanitic magma. Deep-sourced kimberlites and lamproites therefore are not necessary for diamond delivery here, and are unknown from eastern Australia.

Laser Raman spectroscopy is used to identify mineral inclusions in situ within six Copeton diamonds and to determine the remnant internal pressure on them, namely 31-34 kbar for coesite, 13.6 and 23 kbar for diopsidic omphacite, and 8 kbar for grossular garnet. Using a published linear model of the host-inclusion volume system, each remnant pressure value generates a PT locus of diamond formation. The intersections of these define a range of diamond formation conditions from 250 ΰC, 43 kbar to 950 ΰC, 58 kbar with estimated intersection errors of α 70 ΰC and α 4 kbar. This range encompasses at least two sets of ultrahigh pressure (UHP) formation conditions derived from different protoliths involved with the termination of subduction, namely subducted oceanic slab (lower temperatures: eclogitic diamonds) and subducted continental rocks (higher temperatures: calcsilicate diamonds). Coesite has been found as a compound inclusion with omphacite (lower temperature set) and with grossular (higher temperature set). The measurements indicate that the Copeton stones are UHP diamonds (not cratonic), and their Carboniferous argon age dates on pyroxene inclusions should be interpreted as ages of crystallisation. The results and implications are consistent with a local source for Copeton diamonds, and imply there is a buried Carboniferous UHP terrane within eastern Australia.

Pb-Pb step-leaching (PbSL) is a digestion technique based on sequential acid treatment of a mineral resulting in the selective recovery of radiogenic and common Pb components from the crystal lattice, making single-phase Pb-Pb dating possible. Developed originally by R. Frei and colleagues at Bern, the technique has had mixed success. Initial studies produced reliable dates for silicates and mixed sulphides, however there is some conflict surrounding the validity of the PbSL isochrons, with some authors claiming that isochrons might be the product of initial Pb heterogeneity and/or Pb mixing due to post crystallisation U or Pb introduction.

This study builds on previous observations and tests the reliability of PbSL in directly dating ore minerals such as sulphides (chalcopyrite, pyrite, pyrrhotite/pentlandite) and oxides (magnetite), in an attempt to obtain the timing of mineralisation from a phase that is unambiguously linked to the ore-forming process. This is based on the often surprisingly high U contents (relative to what might be anticipated), yielding rather higher than expected initial U/Pb ratios in several types of low-Pb sulphides. The technique is applied to well-constrained samples from a number of predominately Australian mineral deposits covering a range of mineralization types.

Recent drilling in Voisey’s Bay has uncovered significant wallrock mineralization in what was before thought to be a sub-economic occurrence of sulphides within the Reid Brook Dyke. Although technically disseminated, the sulphides within the dyke are electrically well connected and can act as a single conductor of kilometres in size and with a conductance of thousands of Siemens. Using the standard transmitting frequency of the time, we were unable to discriminate the conductance of the disseminated from the massive sulphides. In 2003 we revisited the area with BHEM transmitting at 1Hz with more success. From the low frequency data, we saw evidence for the first time of conductive features that did not coincide with the dyke orientation.

In 2004 we began to utilize the superposition of two data sets from two separate transmitting loops to adjust the angle of the inducing field and null couple to these in-dyke disseminated sulphides. In doing so, we obtained greater precision on the orientations of the conductors, and were able to discriminate between in-dyke conductivity, and wall rock related conductivity. We are now integrating this method with other structural techniques to further develop the deposit model.

This paper attempts to highlight the fundamental role played by tectonics and deformation in the formation and subsequent modification of many komatiite associated nickel sulphide deposits in the Eastern Goldfields province of the Archaean Yilgarn Craton of Western Australia. The influence of tectonics and deformation is manifest at a variety of scales, ranging from hand specimen to craton scale, and has a real impact on how these deposits are targeted, explored, delineated and mined. The influence of tectonics and deformation is as fundamental to the genesis of these deposits as are the well documented magmatic and volcanic processes generally considered to be crucial to their formation.

Post emplacement tectonism has led to the deformation, and in many cases, substantial modification of these essentially magmatic deposits. This ranges from centimetric scale remobilisation as a consequence of metamorphic recrystallisation, through the localised predominantly mechanical remobilisation of sulphides during brittle-ductile deformation of the rock mass, to partial or complete decoupling of sulphides from their parent ultramafics over distances of up to 800 metres in brittle-ductile shear zones and even nappes. Recognition of such modifying processes and consequent three dimensional architecture is essential for successful exploration, orebody modelling and mining of these deposits.

Furthermore, regional tectonic processes may also exert a considerable fundamental influence on the primary emplacement and localisation of komatiites and associated nickel sulphide mineralisation. Many of the thick komatiite sequences and major nickel sulphide accumulations of the Eastern Goldfields province are proximal to cratonic scale NNW trending shear zones which are generally accepted to represent D2 reverse and strike slip fault systems formed during the closure of ensialic greenstone basins. It is proposed that some of these structures represent inverted early extensional faults and that the ultramafics were extruded and emplaced as local volcanic centres adjacent to these basin bounding faults in rift basins and half grabens. If valid, this has important implications for the targeting of, and exploration for komatiite associated nickel sulphide deposits in the Archaean Yilgarn Craton.

This paper explores these ideas by specifically highlighting key geological features of the Emily Ann, Maggie Hays, Waterloo and Honeymoon Well nickel sulphide deposits together with general observations from other deposits within the large scale regional geological context and four dimensional structural framework of the Eastern Goldfields province as a whole.

GeoScience Victoria (GSV) is introducing a new service delivery model to improve capture, storage and delivery of geoscience information. This major project has prompted significant effort to establish a data model with accompanying vocabularies to govern what data to store in the model, and how to arrange and group that data.

The principles behind the vocabulary are that it must be robust, flexible, expandable and hierarchical. The vocabulary, like the data model itself, must satisfy both internal business requirements and international efforts with respect to information exchange in a global environment. There are three components to the vocabulary: the terms themselves with their definitions/synonyms; arranging the terms into schemes; and the grouping of those schemes to provide meaningful information (handled by the data model). Our solution manages the vocabulary in an open source technology, with terms (and synonyms) arranged in hierarchies.

The process demonstrated that while computers may require rigid terminology from us, in some cases this is a real challenge. As an evolving science, many areas of geology have terms that mean different things to different people, with no single correct meaning.

Common vocabularies are essential to the exchange of geoscience information at a national or international level, and GSV strongly encourages their development.

The White Hills Gravel is a widespread gold-bearing palaeoplacer in western and north-central Victoria. Its age is unconstrained by palaeontology but has been considered as Late Cretaceous to early Tertiary. Using coarse (> 1mm) concentrates of heavy minerals from an occurrence of the WHG near St Arnaud, we have obtained evidence for the age and provenance of the formation. U-Pb SHRIMP dating of two populations of zircons gave ages of 74.5 and 68.2 Ma, indicating a maximum age of latest Cretaceous for the WHG. The zircons are believed to have been sourced from basaltic volcaniclastic deposits since removed by erosion. These also provided coloured sapphires, spinel and possibly the sole diamond recovered from the concentrates. Such an assemblage is characteristic of eastern Australian gemfields associated with Cainozoic basaltic lavas. Other minerals in the concentrates include schorl, rutile, andalusite, anatase and metamorphic corundum. U-Pb LA-ICPMS dating of rutile gave an age of 393 Ma, within the age range of Early Devonian granites in the region, which have intruded Cambro-Ordovician metasediments (St Arnaud Group). Sources within contact metamorphosed pelitic metasediments and associated pegmatites are indicated for the rutile, and probably for schorl, andalusite and anatase. Corundum-bearing rocks are unknown in the region, so these may have a more distal and older source. Investigation of heavy mineral concentrates from other occurrences of the White Hills Gravel may enable regional correlation of sources and provide more evidence for particular depositional models

New insights into the crustal structure and evolution of the Mt Isa Eastern Succession have come from constructing a crustal scale 3D model. A revised chronostratigraphy, detailed aeromagnetic data, multiscale wavelet edges of potential field data, and reprocessing and reinterpretation of seismic data suggest hitherto unsuspected links between upper and lower crustal structures.

Three major approximately NS structures are interpreted to underlie the Eastern Succession, respectively along the edge of the Wonga belt, under the Mitakoodi anticline and at the western margin of the contiguous Soldiers Cap outcrop. Major contrasts in potential field data occur along these structures; they also correspond to major changes in stratigraphy. They are interpreted to have originated as extensional faults during basin formation, which were positively inverted during the Isan orogeny from 1.6 to 1.5 Ga.

Inversion apparently involved both the cover sequences and basement rocks, although net extension is preserved on some parts of the major structures. Reactivation of these structures in the ban orogeny may explain some of the complexity of structure within the Eastern Succession, which has hitherto been interpreted as evidence for multiple overprinting deformation. Furthermore, the localisation of contractional features over extensional structures suggests that the Eastern Succession is parautochthonous. The two largest mineral deposits in the Eastern Succession (Cannington and Ernest Henry) are proximal to the easternmost major crustal structure, suggesting the importance of these structures to mineralization.

In seismic reflection tomography, the velocity model of the subsurface is updated by back-projecting travel-time residuals along ray-paths. The travel-time residuals are picked from the seismic data itself and the methodology used to gather these picks is a fundamental part of any velocity inversion workflow. In particular, the density at which the residuals are represented in the four-dimensional data space (inline, crossline, offset and depth/time) appears to have a significant effect on the precision of the velocity updates that are output from the tomographic inversion. Utilising dense, hyperbolic- (or parabolic-) fitting means that the residuals are finely sampled in the data space but does not necessarily represent their true values with great accuracy. Dense, non-hyperbolic fitting offers a similarly fine sampling but with greater adherence to the true residual value. These two methodologies have been compared and contrasted on a complex synthetic dataset. It can be seen from this comparison that the dense, non-hyperbolic tomography offers greater potential for resolving small-scale velocity heterogeneities in the Earth.

A “holistic” method for simultaneously estimating conductivity and calibration models from 1-D inversion of time-domain airborne electromagnetic (AEM) data is proposed. The work extends the concept of holistic inversion that been successfully applied to frequency-domain AEM data. The entire multi-component airborne dataset and available independent conductivity and interface-depth data are simultaneously inverted. A spline-based conductivity model covering the complete survey area is estimated. Unmonitored elements of the system geometry are included as unknown parameters of the calibration model and are solved for in the inversion.

Conventional 1-D inversion methods invert each airborne sample in isolation from other samples. However, by simultaneously considering all of the available information together in a holistic inversion formulation, we are able to exploit the inter-component and spatial coherency characteristics of the airborne data. The formulation ensures that the conductivity and calibration models are optimal with respect to the airborne data and prior information.

There is a well documented relationship between Au mineralized quartz and CO2 rich fluid inclusions within that quartz, indicating the CO2 rich nature of the parent hydrothermal fluids. This relationship has been suggested to be a function of CO2 buffering the Ph of the geothermal fluids in a range which enhances gold solubility. It is also known that the acoustic decrepitation method can identify quartz formed from fluids which had high CO2 contents, as CO2 rich fluid inclusions decrepitate upon heating at temperatures well below that of aqueous inclusions trapped under the same pressure and temperature conditions. CO2 contents at least as low as 5 mole % are readily detectable using the acoustic decrepitation method.

The decrepitation method has been applied to a number of gold deposits in Victoria and shows that CO2 is a common but not ubiquitous constituent of the fluid systems which deposited the quartz and gold. Although the precise relationship between CO2 and gold is not clear, the method provides a means to “fingerprint” quartz veins as an exploration guide. Samples from the Meguma terrane in Nova Scotia also show widespread but variable levels of CO2 rich fluids in the gold deposits.

Knowledge of the CO2 contents of quartz systems is a valuable exploration tool and the acoustic decrepitation method is the easiest way to acquire this information for exploration purposes as it is an automated and quick instrumental procedure.

Recent mapping by the Geological Survey of New South Wales in the Cobar district has led to an improved understanding of the Ordovician and Siluro-Devonian rocks. Mapping to date has been undertaken in the Sussex area, centred approximately 50km northeast of Cobar. The Ordovician Girilambone Group turbidite package has been subdivided, on the basis of lithological associations and conodont assemblages in cherts, into a separate Early Ordovician package and a Middle to Late Ordovician package. The depositional relationship between these two packages is not clear but an unconformity cannot be ruled out. The mapping has also confirmed the existence of slivers of Siluro-Devonian Cobar Supergroup rocks within the Sussex area. These occur both unconformably upon and in fault contact with the Girilambone Group. The lithological associations within the slivers are identical, suggesting that the sequence was once a more extensive sheet across the area. The existence of Cobar Supergroup rocks in the Sussex area increases its prospectivity for Cobar-style Au-Cu-Pb-Zn-Ag deposits. Reconnaissance inspections of areas north of the Sussex area suggest that rocks previously mapped as Ordovician may in fact be Cobar Supergroup equivalents, in turn increasing that area’s prospectivity.

A new workflow is used to harness the synergy of borehole data and surface seismic in seismic data processing. It fully utilizes the borehole information in every step of seismic data processing. In this workflow, a model is first constructed from the borehole data and surface seismic data. At each step of data processing, processing parameters are optimized to ensure that the results are consistent with the model.

The advantage of calibrating the seismic processing parameters via the use of borehole data is the significant reduction of many uncertainties for seismic interpretation and to increases the reliability of seismic data for quantitative interpretation. Two case studies in China clearly demonstrate the advantages of this new workflow.

The Jacaré Nickel Laterite deposit lies in the western part of the Carajas region, north-central Brazil. Originally identified in the 1970’s and then abandoned, the property was acquired by Anglo American Brazil in 2000 and has passed through several progressive stages of exploration.

An initial investigation indicated that nickeliferous laterites were present over the northern part of the 18kms long, 2.5kms wide ridge at Jacaré, the result of deep tropical weathering of a bedrock mafic-ultramafic intrusive complex. Subsequent exploration has shown that the deposit contains three types of nickeliferous material comprising a siliceous laterite, a ferruginous laterite and a saprolite. Surface mapping, drill road mapping and sampling, pitting, drilling, and a reconnaissance TEM survey have all been employed to progress a geological evaluation and understanding of the deposit, outline the position and inter-relationships of the three main types of mineralization and identify additional resources.

Nickeliferous saprolite in the northern and central parts of the deposit would be amenable to pyrometallurgical treatment, while the ferruginous laterites of the southern block could potentially feed a hydrometallurgical plant.

The deposit at Jacaré is seen as an important component of the Anglo American nickel portfolio in Brazil, which includes the existing mine at Codemin and the Barro Alto project. Exploration continues at the Jacaré site, which is 100% controlled by Anglo American Brazil.

GeoScience Victoria (GSV) has recently completed detailed mapping and geophysical modelling of a large portion of the eastern Melbourne Zone in Central Victoria. This region encompasses the Walhalla and Woods Point goldfields, as well as subsidiary goldfields such as Enochs Point and Tallangallook.

The work has recognised two north-trending structural subzones. The western thick-skinned subzone abuts the eastern thin-skinned subzone along a high-angle fault system. This fault system links into a basal detachment system that may underlie both subzones and separates the Melbourne Zone from its structural basement: Cambrian igneous rocks that are part of the Selwyn Block. This new structural understanding helps explain the location of the main goldfields, which lie adjacent to the transition between the subzones.

The Pilbara and Yilgarn Cratons in Western Australia record the Earth’s early history from ca. 3.7 Ga to ca. 2.5 Ga. This paper highlights recent scientific work, and advances in understanding the similarity and differences in granitic rocks, greenstone stratigraphy, structural styles, and tectonic settings of the major granite–greenstone terranes within the cratons, and then discusses the crustal evolution trends from the Paleo- to Neoarchaean. We conclude that in the Pilbara and Yilgarn Cratons, the Paleoarchaean areas are characterized by vertical tectonics, whereas the Meso- to Neoarchaean areas are dominated by horizontal tectonics — plate tectonic processes may have started in the Mesoarchaean.

We propose an approximate two-dimensional inversion procedure for transient electromagnetic data. The method is a two-stage procedure, where data are first inverted with ID multiple-layer models. The ID model section is then considered as data for the next inversion stage that produces the 2D model section. For moving platform data there is translational invariance and the second part of the inversion becomes a deconvolution. The convolution kernels are computed by perturbing one model element in an otherwise homogeneous 2D section and calculating full nonlinear responses. These responses are then inverted with ID models to produce a ID model section. This section is the convolution kernel for the deconvolution.

Within its limitations, the approximate 2D inversion performs well. Theoretical modeling shows that it delivers model sections that are a definite improvement over the ID model sections. A field example shows ....

The Neoproterozoic oceans that closed as Gondwana amalgamated left a cryptic, yet recognisable, record in the resulting orogens. These consist of Neoproterozoic oceanic or ocean-margin protoliths, relatively high-pressure metamorphic belts separating distinct crustal domains, and Neoproterozoic provenance fronts.

The southern Indian Palghat-Cauvery shear zone system, is interpreted as the remains of the Neoproterozoic Mozambique Ocean, because: a) it separates Archaean and Palaeoproterozoic granulites from the extensive Neoproterozoic-Cambrian Southern Granulite Domain; b) it contains mafic gneisses that may have formed within a Neoproterozoic Ocean; c) it preserves distinctive high pressure, ultra-high temperature granulites that were metamorphosed in the Cambrian Malagasy Orogeny, and; d) it isolates southern Indian Proterozoic metasediments that are unlikely to be sourced from north of the shear system.

The Sepon sedimentary-rock hosted gold deposits (SHGD) in Laos contain micro-disseminated gold associated with sulphide minerals. New insights into the nature of gold occurrence in the Sepon SHGD through Laser Ablation Inductively Couple Plasma Mass Spectrometer (LA-ICP-MS) investigations have detected gold mostly occurring in overgrowth rims on disseminated pyrite (main-gold stage) containing: (1) high gold grades along the pyrite rims, ranging from >2 to 187 ppm Au with associated high concentrations of Ag, As, Cu, Sb, Tl, Zn, and; (2) and gold poor cores with inclusions of Co, Ni and Pb and very low levels of As, Cu, Sb and Tl.

Sulphur isotope values from the Sepon SHGD main-gold stage pyrite are similar to the light 634S values reported for the late gold-ore stage pyrite occurring in the Nevada Carlin-like gold deposits.

Previously unrecognised ultramafic units in the Kalgoorlie-Kanowna area have recently been discovered using airborne hyperspectral sensing. Some of these units are up to 1 km in length and 200 m in width and were identified through airborne mineral mapping of talc. These areas lack significant magnetic expression, have little or no outcrop or subcrop and comprise a surface dominated by talc-bearing soil pediment. Exploration drill spoil in these areas confirms the presence of weathered talc schists underlying the soils. These results suggest that even in well mapped areas of deeply weathered greenstone belts, airborne hyperspectral sensing can complement traditional geophysical data by mapping non-magnetic, talc-bearing ultramafic rocks, which may be potential hosts to massive nickel and base metal mineralisation.

This paper summarises key results from the recently completed MERIWA M370 project which demonstrated that new generation airborne hyperspectral technologies can deliver seamless accurate maps of mineral abundances and mineral physicochemistries for the 1:100 000 scale Kalgoorlie-Kanowna special mapsheet. Twenty-three mineral products were produced, most of which have significance for regolith and alteration mapping as well as improving our understanding/modelling of fundamental geologic processes. Key spectral-mineral products delivered for the M370 project include: kaolin disorder for mapping transported versus in situ regolith materials; gypsum for salt lakes; talc for mapping non-magnetic ultramafic rocks; white mica chemistry for Archaean hydrothermal alteration; and pyrophyllite, which previously had not been recognised. The mineral maps of white mica chemistry reveal “big footprint” hydrothermal cells that have important implications for Au exploration in the region.

Although salinity is widely regarded as a significant geohazard within Australia, there is no nationally consistent approach to salinity risk management. Salinity risk assessment, prediction or management, is limited by the variety of meanings of “risk” in its popular usage. A salinity risk framework based on the Australian/New Zealand Standard on Risk Management (AS/NZS 4360:2004) is currently being developed in the Corangamite region of south west Victoria, one of the priority regions in the National Action Plan for salinity and water quality. This paper outlines a process for salinity risk assessment that considers salinity in the broader context, both secondary salinity as a threat to assets, and primary salinity which constitutes the regions most valuable environmental assets.

In the most recent Federal and State salinity frameworks, the emphasis has been placed on the protection of community and catchment assets. In this context, the adoption of a national risk management framework based on the principles of AS/NZS 4360:2004 is both logical and timely. The standard provides a systematic, disciplined and rigorous approach to salinity risk management. It provides logical and defendable processes and practices for the assessment of salinity risk. It can inform the development of strategies and decision making to protect all classes of assets which are threatened by changes to salinity processes, even those where the salinity itself is the asset (e.g. a saline wetland or estuary).

The North Fiji backarc basin (NFB) began opening ~12 Ma in response to subduction reversal along the Vanuatu section of the Proto-Vitiaz arc. The triangular shape of the basin reflects ongoing (since ~5 Ma) southward propagation of the backarc spreading centre in response to westward roll-back of the Vanuatu trench and eastward roll-back of the Tonga trench. During this time (5-3 Ma), a short-lived intraoceanic arc, the Hunter Ridge, formed along the southeast margin of the NFB. This arc magmatism occurred in response to a short period of NNW-directed subduction of the South Fiji basin crust under the NFB. The southern tip of the North Fiji backarc basin (22 °S, 174 °E) is a volcanically active submarine triple junction between the backarc basin spreading centre, Vanuatu trench and Hunter Fracture zone. This area has been mapped and sampled during the SS 10/2004 cruise of R/V “Southern Surveyor”. Volcanic rocks erupted within 100 km cover the entire spectrum of subduction-related magmas from backarc basin basalts to boninites, arc tholeiites, calc-alkaline basalts, high-Mg andesites and adakites. Subduction-related lavas in the area are dominated by olivine-clinopyroxene-phyric basalts with high-Mg phenocrysts (Mg# to 94). We will present detailed geology, geochemistry and mineralogy (including melt inclusion compositions) of all volcanic rock from this region and discuss their origin.

Estimates of seismic hazard are strongly influenced by the location and recurrence of large earthquake events. Understanding such events is inherently difficult because recurrence intervals for large events in a given intra-plate region are typically much greater than the period over which earthquakes have been instrumentally recorded. Consequently, there is often very little data with which to robustly estimate seismic hazard. One way of enlarging the dataset is to study pre-historic large events using palaeoseismological methods, and there are many probable palaeo-fault scarps in the southwest of Western Australia. Studying these, along with historic fault scarps, places constraints on the geological setting of large events.

The Southwest Seismic Zone is a belt of intra-plate seismic activity within the southwest of Western Australia. The local geological environment comprises deeply weathered Archaean granitoids and supracrustals. Outcrop is extremely sparse. The geological setting of large seismic events is best determined using high-resolution aeromagnetic data. These data allow bedrock geology to be mapped, and in particular identify structures that have been reactivated. This is the first step in establishing whether particular areas are more prone to large events than others.

The 1968 Meckering earthquake (Ms 6.8) resulted in an extensive and complex pattern of surface faulting, comprising faults with a range of orientations with reverse and dextral lateral offsets. Interpretation of new aeromagnetic data show that the surface faulting correlates closely with linear magnetic anomalies, interpreted as dykes/faults and lithological contacts/faults. The geometry of the surface faulting can be explained in terms of the reactivation of northeasterly and northwesterly trending surfaces in a stress regime with an east-west oriented maximum principal stress. Space problems created where these two trends converge led to the creation/reactivation of a linking north-south trending thrust fault which accommodated the greatest vertical displacements recorded for the 1968 event.

A new style of PGE-Au mineralization has recently been identified in Sudbury Ontario where high grades of PGE-Au are associated with low levels of copper-nickel sulphides (low-sulphide high-PGE-Au mineralization). Low-sulphide high-PGE-Au mineralization has become an increasingly higher profile exploration target in the footwall of the Sudbury Basin and is being explored, evaluated and in some instances mined by a number of companies. The Lonmin-lnco Sudbury PGM Joint Venture was established in January 2005 to explore for low-sulphide high-PGE-Au mineralization on six Inco properties.

Geophysical exploration for these targets has focused primarily on using induced polarization methods and, to a lesser degree, electromagnetic methods.

Fully integrating all available geologic and geophysical information is essential for directing drilling of target areas. Results from three properties are presented (Denison, Levack North and Wisner) where massive and disseminated sulphide, and low sulphide high PGE-Au mineralization have been intersected during the 2005 exploration program. These properties continue to have significant exploration opportunities for the discovery of low-sulphide high-PGE-Au mineralization and resources.

Groundwater geochemistry assists in determining the character of deep geological or geophysical targets from a limited number of boreholes, refining location, size and orientation of host target zones, or directly demonstrating presence of alteration or mineralisation. By themselves, uranium analyses can be very misleading in applying groundwater to U exploration. Detailed analyses of multi-element data is required to properly interpret water data, sorting false from significant indicators. Examples from a range of Australian areas reinforce these comments and show that, sometimes, the U indicator is a mineral assemblage, not a U concentration. Ready availability of modelling software and chemical databases allow exploration implications from groundwater data to be easily accomplished.

The Neoproterozoic (~815 Ma) Aries micaceous kimberlite intrudes the central Kimberley Basin, northern Western Australia, and contains a suite of 27 serpentinised ultramafic xenoliths, including spinel-bearing, and rare, metasomatised, phlogopite-biotite and rutile-bearing types, along with minor granite xenoliths. This suite of xenoliths provides new data on the composition and age of the central Kimberley Craton. Proton probe trace-element analysis of pyrope and chromian spinel grains derived from heavy mineral concentrates from the kimberlite has been used to define a Proterozoic geotherm for the central Kimberley Craton, of ~35-40 mW/m². Lherzolitic chromian pyrope, that is highly depleted in Zr and Y, and Cr-rich magnesiochromite xenocrysts (class 1), probably were derived from reduced, depleted garnet peridotite mantle at ~150 km depth. We interpret the highly depleted chromian pyrope xenocrysts to have formed by exsolution from high temperature orthopyroxene in former harzburgite. Sampling of shallower levels of the lithospheric mantle by magmas in the north, and north extension lobes of the Aries kimberlite entrained high-Fe chromite xenocrysts (class 2), and aluminous spinel-bearing xenoliths, where both spinel compositions are anomalously Fe-rich for spinels from mantle xenoliths. This Fe-enrichment may have resulted from Fe-Mg exchange with olivine during slow cooling of the peridotite host rocks. Episodes of Fe-Ti-rich metasomatism in the spinel-facies Kimberley mantle are the likely source of Ti-phlogopite-biotite + rutile and Ti-enriched aluminous spinel ± ilmenite associations in several ultramafic xenoliths. U-Pb SHRIMP ²⁰⁷Pb/²⁰⁶Pb zircon ages for one granite (1851 ± 10 Ma) and two serpentinised ultramafic xenoliths (1845 ± 30 Ma; 1861 ±31 Ma) indicate that the granitic basement and lower crust beneath the central Kimberley Basin are Palaeoproterozoic in age.

Hot rocks occur at depth over significant areas of Queensland offering a number of targets for geothermal energy extraction. On the published maps showing temperatures at 5 kilometre depths, the area beneath the Cooper and Eromanga Basins in southwest Queensland shows as the largest area of hot rocks. Much of this area is underlain by low grade meta-sedimentary rocks. The meta-sedimentary rocks are intruded by granites of different ages. In the central Eromanga Basin, the granites are of Middle Ordovician age. They are s-type granites and are not high heat producing. In the southwest, the granites are of mid Silurian age. They are also s-types and not high heat producing. None the less, they have high residual heat. The Late Carboniferous, high heat producing granite at Habernero in South Australia does cross the border into Queensland, but has no known equivalents in Queensland. High heat producing granites are common in the Mount Isa area and extend under the Eromanga Basin. High heat producing granites are present beneath the Carpentaria Basin. Several heat anomalies of unknown cause occur in western central Queensland. There are limited heat flow measurements in north Queensland. Although there are many high heat producing granites, there are no apparent insulating blanket rocks. The youngest basalt flows in north Queensland are 10,000 years old and new flows could yet occur-there may be heat associated with magma chambers. There are hot springs and warm springs in the area, but they are poorly understood. Exploration for geothermal energy will begin in Queensland in 2006 with the grant of exploration permits.

The Thomson Fold Belt (TFB) covers a large area of Queensland, mostly under cover where it forms the basement to the Eromanga Basin and to basins beneath the Eromanga Basin. The basement was considered to be metamorphosed quartzose turbidites of Ordovician age intruded by Silurian and Devonian granites. They were thought to be of similar age to rocks in the Lachlan Fold Belt, although a structural discordance had been recognised between the two fold belts. In 2004 some SHRIMP U-Pb dating was carried out on two granites, a volcanic rock at the base of the Devonian Adavale Basin, and a volcanic rock from beneath the Carboniferous Triassic Galilee Basin. The granites provided Middle Ordovician and mid Silurian ages older than expected. The volcanic rocks from the Adavale Basin produced an Early Ordovician age and those from beneath the Galilee Basin a Middle to Late Devonian age. Additional dating in 2005 focussed on volcanic rocks. The presence of Early Ordovician volcanic rocks was confirmed. An early Devonian volcanic sequence was identified in the Adavale Basin. Middle Cambrian volcanic rocks were identified in the far west. The presence of Early Ordovician volcanic rocks overlying the meta-sediments in the eastern TFB meant that the meta-sediments were deposited and metamorphosed prior to this. The meta-sediments are now tentatively correlated with late Neoproterozoic to Middle Cambrian rocks in the Anakie Mier and the Charters Towers Province which were deformed in the Late Cambrian. Rocks of the Cambrian to Late Ordovician Warburton Basin were probably metamorphosed in the Silurian. Further studies are required to unravel the geological history of the TFB.

In 2005, the State of Victoria passed the Geothermal Energy Resources Act, which provides a new framework for the large-scale commercial exploration and extraction of geothermal energy in the State at depths greater than 1 000 m and at temperatures greater than 70°C. Small-scale projects, such as balneology, ground heat pumps and aquaculture, are excluded, and these will continue to be regulated by existing planning, environmental and water legislation.

The first stage of a regional geothermal assessment is the collection and integration of all relevant geological, geophysical and geochemical data from previous studies on the water, minerals and petroleum resources in the study area.

Previous studies have attempted to map the geothermal prospectivity in Victoria, though the reliability of the data collected, and the sources of the data used in these studies, have often been questionable. Consequently, GeoScience Victoria has attempted to identify and collate all open-file geothermal data collected to date in Victoria. In addition, where possible, all of the published data have been traced to their original sources and quality-assessed. The resulting temperature database is considered robust and has been used to construct a preliminary ‘depth to 150°C map for the Gippsland and Otway basins.

The Wildwood deposit is located within the highly prospective Stawell corridor along strike from the Magdala gold deposit in western Victoria. The deposit is covered by Tertiary Murray Basin sediments that at their shallowest are up to 40m thick. The Wildwood deposit displays similarities with the Stawell gold deposit in terms of geology, structure, hydrothermal alteration and mineralisation. Two ore zones have been defined at Wildwood and the resource of these is currently being assessed.

Many low-lying areas of the Mount William Creek valley are affected by dryland salinity due to shallow watertables across the floodplain. The valley contains a sequence of alluvium consisting of a basal gravel and sand unit of Tertiary age, overlain by Quaternary silts and clays. This sequence lies unconformably on a variety of Palaeozoic basement rocks, which outcrop as rolling hills and mountains towards the catchment boundaries. Potentiometric surface configurations and bore hydrograph data show that the basal Tertiary sequence and the overlying Quaternary sequence do not act as a single unconfined aquifer. Rather, they behave as two aquifers, with the Quaternary silts and clays having some confining capacity over the Tertiary sequence. Hydraulic head differences demonstrate that there is potential for downward leakage between these two units at the floodplain margins and upper catchment throughout the year, and that potential for upward leakage exists in the lower catchment following the winter peak in recharge. The area where upward leakage from the Tertiary sequence can be invoked as a salinity-causing mechanism is spatially and temporally restricted, but it does correspond to some large areas of land salinisation. The high groundwater pressures causing potential upward leakage can easily be attributed to head gained in the adjacent Grampians ranges. However, geological restrictions to lateral groundwater flow through the system may also contribute to the high groundwater pressures in the Tertiary sequence.

Cessation of magmatism in the Banda Arc north of Timor about 3 million years ago has been followed by a period rapid continuing uplift, most dramatically evidenced by the presence of Quaternary coral at ~700 m above sea level on the island of Atauro. In order to understand how these processes constrain slab dynamics during the early stage of continental–arc collision (Timor – Banda Arc) we have mapped the island of Atauro in detail. The basic features of Atauro geology are described, and it is demonstrated how they may relate to slab processes deep beneath.

New seismic reflection data and dredged rock samples confirm that the submarine Kenn Plateau is a thinned continental fragment. It is twice the size of Tasmania and lies 400 km east of central Queensland beyond Cato Trough. East and northeast trending faults separate thinly sedimented basement highs from sedimentary basins with >2 km of Cretaceous and younger strata. In the Late Cretaceous the future plateau fitted against Australia south of Marion Plateau, and consisted of Late Triassic and younger basins unconformably overlying the New England Fold Belt. After Lower Cretaceous rift volcanism, the Kenn Plateau itself formed by Late Cretaceous extension and breakup. Paleocene drifting moved it northeast and rotated it 30° anticlockwise, leaving behind Tasman Basin oceanic basalts. Siliciclastic sediments poured into the basins from the Australian mainland and local highs. After a regional Paleocene-Eocene boundary unconformity, siliciclastic sedimentation resumed nearshore. In deep water, deposition of radiolarian chalks ended at the regional Eocene-Oligocene boundary unconformity, and pure biogenic carbonates accumulated in warming surface waters. From the Middle Eocene, calcarenite formed on the shelves of the subsiding plateau. Some seismic profiles show Middle to Late Eocene compression, probably related to New Caledonian obduction to the east. Hotspots formed parts of two volcanic chains as the plateau moved northward: in the west, Upper Eocene and younger volcanics of the Tasmantid chain; and in the east Upper Oligocene and younger volcanics of the Lord Howe chain. The subsiding volcanoes were fringed by reefs, some of which have persisted until now; others have subsided to form guyots. The plateau has subsided 2000 m or more since its breakup and is now subject solely to pelagic carbonate sedimentation.

The ultramafic horizons of the Wiluna domain (Agnew-Wihina Greenstone Belt, Western Australia) form two main tabular and sheet-like fractionated units, which generally comprise a 5-10m-thick basal pyroxenite layer, a 50-100m-thick meso-to-orthocumulate-textured peridotite horizon, which is overlain by a core of wehrlite, ranging 10-50m in thickness, grading upwards into oikocrystic pyroxenites (30-50m-thick) and upper gabbroic margins (30-50m-thick). Primary textures are exceptionally well preserved: pegmatoidal and dendritic (harrisitic) olivine domains are common in all lithologies, whereas spinifex-textured horizons and flowtop breccias are absent. The fractionation sequence and the geochemical profile of the two ultramafic units are almost indistinguishable, suggesting that the belts may have been structurally duplicated. The ultramafic units display a steady trend towards increasing Fe content up stratigraphy, above a narrow basal zone showing a reversed trend. Calculated Ni contents of cumulus olivines indicate that the magma remained sulfur-undersaturated up to about the middle of the unit, where fractionation of a small proportion of sulfide (R>1000) occurred and depleted the upper fractionated levels in Ni. The presence of various anomalous PGE-enriched horizons (∑Pt-Pd= 200-300 ppb) in the fractionated pyroxenite layer directly overlying the basal ortho-mesocumulate-textured unit indicates the occurrence of localised weak reef-style PGE mineralisation. Reef-style PGE mineralisation at Wiluna is more similar in stratigraphic setting, style and composition to PGE-rich disseminated Fe-Cu sulfide mineralisation zones within thick differentiated intrusions (e.g. Platinova reef in the Skaergaard intrusion) rather than to other zones in komatiite-hosted systems.

The ~ 1333 Ma Voisey’s Bay Intrusion represents one of the earliest magmatic events recorded in the development of the Nain Plutonic Suite. The intrusion was likely emplaced at a depth of ~ 14 km.

Sulphide saturation occurred due to the assimilation of bulk Churchill gneiss in a series of sub-chambers below the current level of erosion. The current economic ore zones formed by the accumulation of magmatic sulphides in physical traps from a pulsed series of sulphide-laden magmas derived from the sub-chambers.

The initial melt had an MgO content of > 8 wt % and a geochemical and isotopic composition similar to some Proterozoic flood basalts.

An array of northeast-trending shear zones in the northwest Gawler Craton of South Australia has heen implicated in widely disparate Proterozoic continental reconstructions. Quantitative constraints on timing of movement along these shear zones are sparse, but potentially provide tests for contrasting reconstructions. New ⁴⁰Ar/³⁹Ar data provide improved age constraints for movement along these shear zones and are interpreted to indicate that the Karari, Tallacootra and Coorabie shear zones were active at ~ 1440 Ma. This timing is ~ 100 Ma younger than suggested in several published reconstructions and is also significantly older than Grenvillian-age tectonism in adjacent provinces to the west and north. The shear zones of the northwest Gawler Craton are interpreted to have formed in response to sinistral transpression, and dissect the region into several geological domains of contrasting metamorphic grade. Despite distinct differences in metamorphic grade the various domains exhibit evidence of a shared event-history prior to deformation at ~ 1440 Ma, albeit at different crustal levels. Movement along the shear zones of the northwest Gawler Craton at ~ 1440 Ma therefore appears to have dissected and reorganised a previously coherent crustal block rather than representing a suture zone between distinct terranes.

This paper will present a synopsis of the results from a three year AMIRA project (P588) entitled ”Epithermal Gold-silver Deposits: Geological, Geochemical and Isotopic Vectors to Target Major Deposits”. Research was undertaken at the Ladolam, (Papua New Guinea), Gosowong, and Mt Muro (Indonesia), Cerro Vanguardia (Argentina) and Twin Hills and Bimurra (Australia) deposits. The sizes and styles of the deposits varied greatly from a low-grade to barren small vein-stockwork system at Bimurra and Twin Hills to high grade veins systems of different scales such as Gosowong, Mt Murro and Cerro Vanguardia to a large breccia-hosted low grade, disseminated system at Ladolam.

At each of the deposits alteration mineralogy and zonation were determined using a combination of surface mapping, drill hole logging, petrology, SWIR (PIMA), XRD and potassium feldspar staining. Electron microprobe analyses of specific minerals, whole-rock and trace element whole rock analyses and stable isotope geochemistry were utilised to characterise the geochemical signature of the hydrothermal alteration.

Results of this research have 1) substantially increase our knowledge of the mineralogical and whole-rock geochemical characteristics of hydrothermal alteration associated with low- and intermediate sulfidation epithermal deposits, 2) developed a range of vectors that point towards ore, both on district and deposit scales, which can be used when exploring in epithermal environments and 3) proposed a set of criteria for distinguishing potential ore grade systems from barren or low grade epithermal systems.

The Western Australian Wheatbelt has been essentially geologically stable since the late Permian although the Archaean basement sustained some movement during the break-up of Gondwanaland and the northward drift of Australia from Antarctica. During the Early Cretaceous, Eocene and more recently, the Wheatbelt region’s weathered mantle has been eroded by rivers. The palaeo-valleys have been infilled with terrestrial and marine sediments, and been subject to ongoing deep weathering. During the Pliocene and Quaternary the region experienced alternating arid and pluvial climates. These cyclic episodes shaped the regolith and impacted on vegetation species and catchment water balances and also promoted the accumulation of massive volumes of salt. In more recent times, this stored salt has interacted with vegetation, soils, surface waterbodies and groundwater systems and left a distinctive and pervasive legacy in the landscape.

Salinisation was manifest in the Wheatbelt of WA from as long ago as 2.8 Ma, concentrating in valley floors as arid and pluvial cycles prevailed and whilst the continent migrated northwards. Today, as agricultural development has altered the water balance on 20 M ha of cleared farmland, salinity is again on the move, further degrading the 300,000 ha of variably saline land that existed before the arrival of Europeans, and spreading across an additional 1.1 M ha of formerly non-saline land. Unchecked by reduced rainfall or human induced changes to the water balance, salinity may expand even further, potentially affecting 1.7-3.4 M ha of the Wheatbelt’s agricultural land and its unique and indigenous resources.

This paper reviews the palaeogeography and palaeoclimates of the region and its hydrogeology and examines the nature of its susceptibility to salinisation. It poses questions about the relationship between palaeo-salinity and contemporary salinity, seeking geomorphic evidence to determine whether salinity is likely to expand beyond extant palaeo-salinity markers. Finally, it considers the likely timeframes involved in the recovery of systems from primary salinity and whether clearing-induced salinity will follow patterns similar to those observed from past saline episodes in the region.

Low-temperature thermochronology on apatites from the Gulf Extensional Province in northern Baja California reveals fission-track ages >27 Ma in the Sierra San Pedro Martir and the Sierra Juarez. This indicates that exposed crustal depths are less than about 2km below the pre-extension land surface. Younger ages are found in the strongly rotated hanging wall blocks in the Sierra San Felipe towards the Gulf coast indicating deeper crustal levels. The more sensitive apatite (U-Th)/He system, however, reveals much younger ages along the foot of the main escarpment and in the San Felipe blocks constraining the time of onset of rapid extension to between 7 and 14 Ma.