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The article presents information about various papers related to geophysics. Some of the papers are "The Seismic Structure of the Saurashtra Crust in Northwest India and Its Relationship With the Reunion Plume," by G.S.P. Rao and H.C. Tewari, "Maximum Horizontal Stress Orientations in the Cooper Basin, Australia: Implications for Plate-Scale Tectonics and Local Stress Sources," by S. D. Reynolds, S. D. Mildren, R. R. Hulls, J. J. Meyer and T. Flottmann, "A Magnetic Field Model With Daily Variations of the Magnetospheric Field and Its Induced Counterpart in 2001," by V. Lesur, S. Macmillan and A. Thomson.

We present a method for measuring the dispersion of higher mode surface wave phase velocities from a single seismogram using a hierarchical transdimensional Bayesian approach. The 1-D shear velocity profiles down to 800 km depth between sources and seismic stations are regarded as the controlling parameters to tune the phase velocities of fundamental and higher modes. The misfits between synthetics and real waveforms indicate whether the phase velocities are recovered well from the data. We use Monte Carlo Markov chains to approximate the posterior distribution of each model parameters and assess the uncertainties from these probability density functions. These techniques can test models of varying dimensions while being parsimonious, thereby letting the data themselves control the complexity of the solution. Another advantage is that the algorithm can decide how much data noise is needed in order to explain the data without overfitting them. The data noise can be treated as an unknown and different noise levels can be applied to the different time windows considered. The posterior noise distributions can then be used as an indicator of the quality of the waveform fit within each frequency–time window. We considered phase velocities between 50 and 200 s for each mode and performed a reliability analysis to determine which modes and periods are reliably constrained. In this paper, we first present the method and demonstrate its feasibility with synthetic tests, which show that the technique is robust. We then illustrate it with applications to real data. We applied the method to two paths sampling Australia using earthquakes at regional distances and obtained results that agree well with previous studies. The new method can be used in regional and global tomographic studies to obtain phase velocity maps and 3-D models of seismic velocities and anisotropy at depths that are not well resolved by fundamental mode surface waves or body waves. [ABSTRACT FROM AUTHOR]

In the absence of quantitative relationships, a cross-gradient function can be used to correlate unknown physical properties in a joint inversion of geophysical data sets. It introduces a structural correlation between properties. A commonly used, inexact approach adds a weighted cross-gradient term as a penalty to the cost function being minimized during inversion. This weighting factor needs to be tuned to balance the regularization and cross-gradient terms. In this paper we propose nonlinear weighting for the cross-gradient function which addresses the very different magnitudes of the cross-gradient and regularization terms. This approach also couples the weighting factors for the regularization and correlation terms reducing the number of tuning parameters. The approach is investigated for a synthetic case. Results are also shown for the 3-D joint inversion of high-resolution magnetic and gravity anomaly data from Southern Queensland in Australia with over 30 million cells. [ABSTRACT FROM AUTHOR]

Crustal scale seismic images provide information on the geometry of subsurface structure. In this paper we examine shear zones as they provide geometrical constraints on the evolution of the crust and as they provide pathways for the migration of mineral-rich fluids from the lower crust. However, they typically appear in seismic images of the deep crust as laterally continuous bands of discontinuous reflections with individual reflections often having high amplitudes. Geological mapping of exposed shear zones show them to have a complex 3-D structure yet crustal-scale seismic reflection surveys use single or at the most only a few profiles, and therefore only create 2-D images of these structures. The processing and imaging of the multifold common midpoint (CMP) data assumes that the seismic energy comes entirely from within the plane of the section. In this paper, we use full-waveform 3-D synthetic data to consider the effects that 3-D topography on a reflector has on reflection character on a 2-D profile. We base our synthetics on an observed shear zone and test models with both a single layer and anastomizing layers. We show that topography on the reflector out of the plane of the section may cause spurious events both above and below the expected target depth. We derive the basic understanding using a simple isotropic homogeneous model, however, we then demonstrate that this is a robust phenomenon and is endemic on all 2-D sections even if the overburden is not homogeneous. We demonstrate that we obtain similar results with a velocity gradient or, for a more extreme case, with a 2-km-deep basin filled with low-velocity sediment. For crustal scale seismic profiles, in particular, the effect is pervasive as neither stacking nor migration can discriminate against out-of-plane energy and the 2-D stack represents the 3-D seismic response of a broad swath centred on the profile. However, we conclude that using the modelled data it is possible to identify qualitatively where there is significant contamination from out-of-plane topography and show examples from a shear zone in the Archaean Yilgarn Block in Western Australia. [ABSTRACT FROM AUTHOR]

In this paper, we report the 3-D electrical conductivity distribution beneath the Australian continent in the depth range 410–1600 km, which we have imaged by inverting C-response estimates from a regional network of geomagnetic observatories. The inversion scheme is based on a quasi-Newton optimization method while the forward algorithm relies on an integral-equation approach. To properly account for the ocean effect in responses at coastal observatories we included a high-resolution (1° × 1°) fixed thin laterally varying surface conductance layer. As starting model in the inversion we considered a laboratory-based 3-D conductivity model of the region obtained from seismic surface wave data and thermodynamic modelling. This model provides a good fit to observed C-response estimates supporting its choice as initial model. The most striking feature of the obtained 3-D model is a high-conductivity anomaly in the lower part of the mantle transition zone (MTZ; 520–660 km depth) beneath southeastern Australia implying considerable lateral as radial heterogeneity in the conductivity structure. The high-conductivity region appears to be 0.5–1 log units more conductive than previous global and other regionalized 1-D models. Further analysis using laboratory-based conductivity models combined with thermochemical phase equilibrium computations shows that the strong conductivity anomaly implies water contents of around 0.1 wt per cent in the upper part and >0.4 wt per cent in the lower part of the MTZ. This implies a large MTZ water reservoir that likely totals one to three times that which currently resides in the oceans. The amount of water in the lower MTZ appears to exceed the experimentally determined water storage capacity of the main lower MTZ mineral ringwoodite, which, as a result, undergoes dehydration-induced partial melting. Including contributions to conductivity from a thin melt layer (20 km thick) located in the mid-MTZ increases conductivity locally in the melt layer to ∼1 S m−1, that is, about 0.5 log units more conductive than the average surrounding mantle. This provides an adequate explanation for the strong conductivity anomalies observed beneath part of the continent and points to lateral variations in melt in the MTZ. [ABSTRACT FROM AUTHOR]

We present a new S-wave velocity tomographic model of the upper mantle beneath the Australian Plate and its boundaries that we call Aus22. It includes azimuthal anisotropy and was constrained by waveforms from 0.9 million vertical-component seismograms, with the densest data sampling in the hemisphere centred on the Australian continent, using all available data covering this hemisphere. Waveform inversion extracted structural information from surface waves, S- and multiple S-waves and constrained S- and P-wave speeds and S-wave azimuthal anisotropy of the crust and upper mantle, down to the 660-km discontinuity. The model was validated by resolution tests and, for particular locations in Australia with notable differences from previous models, independent inter-station measurements of surface-wave phase velocities. Aus22 can be used to constrain the structure and evolution of the Australian Plate and its boundaries in fine detail at the regional scale. Thick, high-velocity (and, by inference, cold) cratonic lithosphere occupies nearly all of western and central Australia but shows substantial lateral heterogeneity. It extends up to the northern edge of the plate, where it collides with island arcs, without subducting. Diamondiferous kimberlites and lamproite deposits are underlain by cratonic lithosphere, except for the most recent diamondiferous lamproites in the King Leopold Orogen. The rugged eastern boundary of the cratonic lithosphere resolved by the model provides a lithospheric definition of the Tasman Line. Just east of the Tasman Line, an area of intermediate-thick lithosphere is observed in the southern part of the continent. The eastern part of Australia is underlain by thin, warm lithosphere, evidenced by low seismic velocities. All the sites of Cenozoic intraplate volcanism in eastern Australia are located on thin lithosphere. A low-velocity anomaly is present in the mantle transition zone (410–660 km depths) beneath the Lord Howe and Tasmanid hotspots, indicative of anomalously high temperature and consistent with a deep mantle upwelling feeding these hotspots and, possibly, also the East Australia hotspot. High seismic velocities at 200–410 km depth below New Guinea indicate the presence of slab fragments, probably linked to the subduction of the Australian Plate. High seismic velocities are observed in the transition zone below northeast Australia and indicate the presence of subducted lithospheric fragments trapped in the transition zone, possibly parts of the former northern continental margin of Australia. [ABSTRACT FROM AUTHOR]

In recent years, there has been a considerable expansion of deployments of portable seismic stations across Australia, which have been analysed by receiver function or autocorrelation methods to extract estimates of Moho depth. An ongoing program of full-crustal reflection profiles has now provided more than 25 000 km of reflection transects that have been interpreted for Moho structure. The Moho data set is further augmented by extensive marine reflection results. These new data sources have been combined with earlier refraction and receiver function results to provide full continental coverage, though some desert areas remain with limited sampling. The dense sampling of the Moho indicates the presence of rapid changes in Moho depth, and so, the Moho surface has been constructed using an approach that allows different weighting and spatial influence depending on the nature of the estimate. The inclusion of Moho results from continental-wide gravity inversion with low weighting helps to resolve the continent-ocean transition and to provide additional control in the least sampled zones. The refined distribution indicates the presence of widespread smaller-scale variations in Moho structure. Strong lateral contrasts in crustal thickness remain, but some have become more subdued with improved sampling of critical areas. The main differences from earlier results lie in previously poorly sampled regions around the Lake Eyre Basin, where additional passive seismic results indicate somewhat thicker crust though still with a strong contrast in crustal thickness to the cratonic zone to the west. [ABSTRACT FROM AUTHOR]

Most studies of sea level budget only indicate the stationary causes for the global mean sea level (GMSL) rise over the course of a time span of interest, providing limited information on temporary changes in the GMSL budget. In this contribution, we present an instantaneous budget of the GMSL for the period of 2003–2015, which offers new insights on the time evolution of the GMSL budget. We use a space-state model to compute the instantaneous rates of GMSL and their contributing sources (barystatic and steric contributions), to investigate the causes of the GMSL instantaneous budget, which accounts for and quantifies low-frequency variations in GMSL rise. Combining the barystatic GMSL instantaneous rates (estimated from satellite gravimetry) along with a particular choice of steric GMSL instantaneous rates (inferred from an Argo product) achieves closure of the GMSL instantaneous budget with a standard deviation of 0.4 mm yr−1. We find that the barystatic GMSL rate is primarily responsible for the striking fluctuations in the GMSL instantaneous rates between 2009 and 2015. For example, over only 20 months (July 2010 to February 2012), the GMSL (barystatic) rate increases from 0.32 (0.71) to 6.12 (4.55) mm yr−1. These strong fluctuations mainly result from hydrology and mountain glacier mass variations rather than mass loss in ice sheet, for instance, Australia slowed ∼1.2 mm yr−1 equivalent contribution to GMSL rise over the year 2010, which is approximately four times the linear rate of Antarctica mass loss. [ABSTRACT FROM AUTHOR]

Gravity gradient tensors (GGTs) are used to investigate the density of subsurface structures in the Earth's crust and can reduce ambiguity during data interpretation. However, costs and research area restrictions often prevent their application during surveys, thereby limiting their utility. To address this limitation, a matrix equation based on the Taylor series expansion that uses the gravity vector and its neighbours was formulated to obtain the GGTs. Higher-order derivatives of the gravity vector were utilized to constrain the calculation, which improved the accuracy of the transformation. Synthetic data were used to demonstrate that the proposed approach improved accuracy when the radial component of the gravity vector was transformed into GGTs. This approach was also applied to gravity data from the Otway Basin in Australia. Compared with the measured GGT, the results obtained using the proposed approach had a relative error of 0.46. [ABSTRACT FROM AUTHOR]

The equivalent source technique is a powerful and widely used method for processing gravity and magnetic data. Nevertheless, its major drawback is the large computational cost in terms of processing time and computer memory. We present two techniques for reducing the computational cost of equivalent source processing: block-averaging source locations and the gradient-boosted equivalent source algorithm. Through block-averaging, we reduce the number of source coefficients that must be estimated while retaining the minimum desired resolution in the final processed data. With the gradient-boosting method, we estimate the sources coefficients in small batches along overlapping windows, allowing us to reduce the computer memory requirements arbitrarily to conform to the constraints of the available hardware. We show that the combination of block-averaging and gradient-boosted equivalent sources is capable of producing accurate interpolations through tests against synthetic data. Moreover, we demonstrate the feasibility of our method by gridding a gravity data set covering Australia with over 1.7 million observations using a modest personal computer. [ABSTRACT FROM AUTHOR]

Observations from different disciplines have shown that our planet is highly heterogeneous at multiple scale lengths. Still, many seismological Earth models tend not to include any small-scale heterogeneity or lateral velocity variations, which can affect measurements and predictions based on these homogeneous models. In this study, we describe the lithospheric small-scale isotropic heterogeneity structure in terms of the intrinsic, diffusion and scattering quality factors, as well as an autocorrelation function, associated with a characteristic scale length (a) and RMS fractional velocity fluctuations (ε). To obtain this characterization, we combined a single-layer and a multilayer energy flux models with a new Bayesian inference algorithm. Our synthetic tests show that this technique can successfully retrieve the input parameter values for 1- or 2-layer models and that our Bayesian algorithm can resolve whether the data can be fitted by a single set of parameters or a range of models is required instead, even for very complex posterior probability distributions. We applied this technique to three seismic arrays in Australia: Alice Springs array (ASAR), Warramunga Array (WRA) and Pilbara Seismic Array (PSAR). Our single-layer model results suggest intrinsic and diffusion attenuation are strongest for ASAR, while scattering and total attenuation are similarly strong for ASAR and WRA. All quality factors take higher values for PSAR than for the other two arrays, implying that the structure beneath this array is less attenuating and heterogeneous than for ASAR or WRA. The multilayer model results show the crust is more heterogeneous than the lithospheric mantle for all arrays. Crustal correlation lengths and RMS velocity fluctuations for these arrays range from ∼0.2 to 1.5 km and ∼2.3 to 3.9 per cent, respectively. Parameter values for the upper mantle are not unique, with combinations of low values of the parameters (a < 2 km and ε < ∼2.5 per cent) being as likely as those with high correlation length and velocity variations (a > 5 km and ε > ∼2.5 per cent, respectively). We attribute the similarities in the attenuation and heterogeneity structure beneath ASAR and WRA to their location on the proterozoic North Australian Craton, as opposed to PSAR, which lies on the archaean West Australian Craton. Differences in the small-scale structure beneath ASAR and WRA can be ascribed to the different tectonic histories of these two regions of the same craton. Overall, our results highlight the suitability of the combination of an energy flux model and a Bayesian inference algorithm for future scattering and small-scale heterogeneity studies, since our approach allows us to obtain and compare the different quality factors, while also giving us detailed information about the trade-offs and uncertainties in the determination of the scattering parameters. [ABSTRACT FROM AUTHOR]

Seismic full waveform inversion (FWI) is a state-of-the-art technique for estimating subsurface physical models from recorded seismic waveform, but its application requires care because of high non-linearity and non-uniqueness. The final outcome of global convergence from conventional FWI using local gradient information relies on an informative starting model. Bayesian inference using Markov chain Monte Carlo (MCMC) sampling is able to remove such dependence, by a direct extensive search of the model space. We use a Bayesian trans-dimensional MCMC seismic FWI method with a parsimonious dipping layer parametrization, to invert for subsurface velocity models from pre-stack seismic shot gathers that contain mainly reflections. For the synthetic study, we use a simple four-layer model and a modified Marmousi model. A recently collected multichannel off-shore seismic reflection data set, from the Lord Howe Rise (LHR) in the east of Australia, is used for the field data test. The trans-dimensional FWI method is able to provide model ensembles for describing posterior distribution, when the dipping-layer model assumption satisfies the observed data. The model assumption requires narrow models, thus only near-offset data to be used. We use model stitching with lateral and depth constraints to create larger 2-D models from many adjacent overlapping submodel inversions. The inverted 2-D velocity model from the Bayesian inference can then be used as a starting model for the gradient-based FWI, from which we are able to obtain high-resolution subsurface velocity models, as demonstrated using the synthetic data. However, lacking far-offset data limits the constraints for the low-wavenumber part of the velocity model, making the inversion highly non-unique. We found it challenging to apply the dipping-layer based Bayesian FWI to the field data. The approximations in the source wavelet and forward modelling physics increase the multimodality of the posterior distribution; the sampled velocity models clearly show the trade-off between interface depth and velocity. Numerical examples using the synthetic and field data indicate that trans-dimensional FWI has the potential for inverting earth models from reflection waveform. However, a sparse model parametrization and far offset constraints are required, especially for field application. [ABSTRACT FROM AUTHOR]

The degree to which the lowermost mantle influences behaviour of the geodynamo has been debated over the past quarter century. Our analysis of a comprehensive set of 17 Cenozoic palaeomagnetic transitional field records obtained from lavas in the Southern Hemisphere provides robust evidence of stable mantle control since the Pliocene. The records come from a region where—given a significantly weakened axial dipole—the magnetic field today would be largely controlled by the non-axial dipole (NAD) flux patch currently emanating from Earth's outer core beneath western Australia. The palaeomagnetic recording sites from west to east include the south Indian Ocean, eastern Australia, New Zealand and French Polynesia. The analysed records contain from 2 to 26 sequential transitional virtual geomagnetic poles (VGPs). 10 of the 17 records supply at least one VGP within a narrow longitudinal band 10°-wide between 60°S and the equator, centred along 102.4°E. That is, transitional data from 59 per cent of the Cenozoic recordings are found to reside in a region that encompasses a mere 2.8 per cent of the VGP transitional area on Earth's surface. A robust Monte Carlo approach applied to this data set, one that takes into account the number of transitional VGPs contained in each record, finds this result highly improbable (p-value  = 0.0006). The present-day pattern of vertical flux at the core–mantle boundary shows an anomalously strong, thin Southern Hemisphere longitudinal band off the west coast of Australia that strikingly coincides with this unusual palaeomagnetic finding. We conclude with a high degree of confidence that this band of flux has remained virtually unmoved for at least the past 3 Myr. Seemingly independent of the behaviour of the axial dipole, our findings indicate that it has dominated the magnetic field over an area of considerable size during attempts by the geodynamo to reverse polarity. [ABSTRACT FROM AUTHOR]

We propose a new framework for incorporating petrophysical and geological information into voxel-based geophysical inversion. By developing the geophysical inverse problem from a probabilistic perspective, we redesign the objective function and the iteration steps as a suite of cyclic optimization problems in which three separate MAP optimization problems are solved using geophysical, petrophysical and geological data, respectively. By quantitatively linking these data into a single framework, we recover a final inverted model that reproduces the observed, or desired, petrophysical and geological features while fitting the geophysical data. To achieve our goal we replace the Gaussian prior, used in the Tikhonov inversion approach, by a Gaussian mixture model. After each geophysical model update, the mixture parameters (means, variances and proportions) are determined by the geophysical model and the expected characteristics of the lithologies through another optimization process using the expectation–maximization algorithm. We then classify the model cells into rock units according to the petrophysical and geological information. These two additional steps over the petrophysical and geological data result in a dynamic update of the reference model and associated weights and guide the inversion towards reproducing the expected petrophysical and geological characteristics. The resulting geophysical objective function does not require extra terms to include the additional petrophysical and geological information; this is an important distinction between our work and previous frameworks that carry out joint geophysical and petrophysical data inversion. We highlight different capabilities of our methodology by inverting magnetotelluric and direct-current resistivity data in 1-D and 2-D, respectively. Finally, we apply our framework to inverting airborne frequency domain data, acquired in Australia, for the detection and characterization of saline contamination of freshwater. [ABSTRACT FROM AUTHOR]

The variation of temperature in the crust is difficult to quantify due to the sparsity of surface heat flow observations and lack of measurements on the thermal properties of rocks at depth. We examine the degree to which the thermal structure of the crust can be constrained from the Curie depth and surface heat flow data in Southeastern Australia. We cast the inverse problem of heat conduction within a Bayesian framework and derive its adjoint so that we can efficiently find the optimal model that best reproduces the data and prior information on the thermal properties of the crust. Efficiency gains obtained from the adjoint method facilitate a detailed exploration of thermal structure in SE Australia, where we predict high temperatures within Precambrian rocks of 650 °C due to relatively high rates of heat production (0.9–1.4 μW m−3). In contrast, temperatures within dominantly Phanerozoic crust reach only 520 °C at the Moho due to the low rates of heat production in Cambrian mafic volcanics. A combination of the Curie depth and heat flow data is required to constrain the uncertainty of lower crustal temperatures to ±73 °C. We also show that parts of the crust are unconstrained if either data set is omitted from the inversion. [ABSTRACT FROM AUTHOR]

The 280-km-long San Cristobal Trough (SCT), created by the tearing of the Australia plate as it subducts under the Pacific Plate near the Solomon and Vanuatu subduction zones, has hosted strike-slip earthquake sequences in 1993 and 2015. Both sequences, which likely represent a complete seismic cycle, began along the oldest section of the SCT—the portion farthest from the tear that has experienced the most cumulative displacement—and migrated to the younger sections closer to the tear. The SCT's abundant seismicity allows us to study transform boundary development—a process rarely observed along a single fault system—through observations of earthquake rupture properties. Using the spectral ratio method based on empirical Green's functions (EGFs), we calculate the corner frequencies of three M w ∼7 2015 earthquakes and colocated smaller earthquakes. We utilize two different spectral ratio stacking methods and fit both Brune and Boatwright models to the stacked spectral ratios. Regardless of stacking methods and spectral models, we find that the corner frequencies of the 2015 M w ∼7 earthquakes decrease slightly with distance from the tear. Assuming a constant rupture velocity and an omega-square spectral model, this corner frequency decrease may be due to an increase in rupture length with distance from the tear. The spectrum of the 2015 earthquake farthest from the tear also deviates from the omega-square model, which may indicate rupture complexity. Stress drop estimates from the corner frequencies of the 2015 M w ∼7 earthquakes range between 1 and 7 MPa, whereas stress drop estimates of their EGFs range from ∼0.05 to 10 MPa with most values between 0.1 and 1 MPa. Independent evidence from a second moments analysis of the 2015 earthquake sequence also indicates a possible increase in rupture length with distance from the tear, confirming the results from the spectral ratio analysis. We also observe an increase in normalized centroid time-delay values, a first-order proxy for rupture behaviour, with distance from the tear for the 2015 sequence. A similar trend for the 1993 sequence suggests that earthquake rupture varies systematically along the SCT. Since distance from the tear corresponds to cumulative fault displacement, these along-strike rupture variations may be due to a displacement-driven fault maturation process. [ABSTRACT FROM AUTHOR]

We apply virtual deep seismic sounding (VDSS) to data collected from both permanent and temporary seismic stations in Australia with the goal of examining (i) the resilience of the method to the presence of complex lithospheric structure and (ii) the effectiveness of different approaches for estimating bulk crustal properties (namely thickness and Vp). Data from the permanent station WRAB in the Northern Territory is ideal for benchmarking VDSS (large number and favourable distribution of recorded earthquakes), with the results from several approaches agreeing on a thickness of 40–42 km. Application of VDSS to data from the temporary BILBY array, a linear distribution of broadband stations that traverses central Australia, shows that strong Moho reflections can be retrieved with as few as two earthquakes even at the transition between crustal blocks of different character and in the presence of thick sedimentary basins. Crustal thickness varies between 36 and 54 km and compares well with the reflectivity character of nearby deep seismic reflection lines. Furthermore, we find that off-line estimates of crustal thickness, calculated by binning the source regions according to back-azimuth, produce values of crustal thickness that are consistent with the regional geology. Overall, we find that VDSS is a powerful technique for estimating crustal thickness and velocity due to its insensitivity to complex short-wavelength structure and requirement of a small number earthquakes to produce a stable result. However, not all schemes tested for extracting bulk crustal properties appear to be robust and stringent data quality checking is still required during implementation. [ABSTRACT FROM AUTHOR]

Conventional full-waveform inversion (FWI) often fails to retrieve the unknown model parameters from noisy seismic data. A successful FWI implementation usually requires to follow a multistage recovery approach, starting from the retrieval of the lower model wavenumbers (tomography) to those with the higher resolutions (migration). Here, we propose a new method based on the flux-corrected transport (FCT) technique often used in computational fluid dynamics for the removal of instabilities in a shock profile. FCT involves three finite-difference steps: a transport, a diffusion and an antidiffusion process. This third step involves non-linear operators such as maximum and minimum, which are non-differentiable in a classic sense. However, since the seismic source wavelet and the corresponding wavefield are relatively smooth and continuous in nature without any strong ripples like shock waves, we exclude the non-linear step from FCT, which allows us to evaluate the novel FWI gradient efficiently. As a result, we achieve a converging FWI model by gradually reducing the diffusive flux-correction. We demonstrate the versatility of FCT-based FWI on a noisy synthetic data set from the Marmousi II model and a marine field data set from offshore Australia. [ABSTRACT FROM AUTHOR]

Debate is ongoing as to which tectonic model is most consistent with the known geology of southeast Australia, formerly part of the eastern margin of Gondwana. In particular, numerous tectonic models have been proposed to explain the enigmatic geological relationship between Tasmania and the mainland, which is separated by Bass Strait. One of the primary reasons for the lack of certainty is the limited exposure of basement rocks, which are masked by the sea and thick Mesozoic–Cenozoic sedimentary and volcanic cover sequences. We use ambient noise tomography recorded across Bass Strait to generate a new shear wave velocity model in order to investigate crustal structure. Fundamental mode Rayleigh wave phase velocity dispersion data extracted from long-term cross-correlation of ambient noise data are inverted using a transdimensional, hierarchical, Bayesian inversion scheme to produce phase velocity maps in the period range 2–30 s. Subsequent inversion for depth-dependent shear wave velocity structure across a dense grid of points allows a composite 3-D shear wave velocity model to be produced. Benefits of the transdimensional scheme include a data-driven parametrization that allows the number and distribution of velocity unknowns to vary, and the data noise to also be treated as an unknown in the inversion. The new shear wave velocity model clearly reveals the primary sedimentary basins in Bass Strait as slow shear velocity zones which extend down to 14 km in depth. These failed rift basins, which formed during the early stages of Australia–Antarctica break-up, appear to be overlying thinned crust, where high velocities of 3.8–4.0 km s−1 occur at depths greater than 20 km. Along the northern margin of Bass Strait, our new model is consistent with major tectonic boundaries mapped at the surface. In particular, we identify an east dipping velocity transition zone in the vicinity of the Moyston Fault, a major tectonic boundary between the Lachlan and Delamerian orogens, which are part of the Phanerozoic accretionary terrane that makes up eastern Australia. A pronounced lineament of high shear wave velocities (∼3.7–3.8 km s−1) in the lower crust of our new model may represent the signature of relict intrusive magmatism from failed rifting in the early stages of Australia–Antarctica break-up along a crustal scale discontinuity in the Selwyn Block microcontinent which joins Tasmania and Victoria. [ABSTRACT FROM AUTHOR]

Different mechanisms have been proposed as explanations for seismic anisotropy at the base of the mantle, including crystallographic preferred orientation of various minerals (bridgmanite, post-perovskite and ferropericlase) and shape preferred orientation of elastically distinct materials such as partial melt. Investigations of the mechanism for D" anisotropy usually yield ambiguous results, as seismic observations rarely (if ever) uniquely constrain a mechanism or orientation and usually rely on significant assumptions to infer flow patterns in the deep mantle. Observations of shear wave splitting and polarities of SdS and PdP reflections off the D" discontinuity are among our best tools for probing D" anisotropy; however, currently available data sets cannot constrain one unique scenario among those suggested by the mineral physics literature. In this work, we determine via a forward modelling approach what combinations of body wave phases (e.g. SKS, SKKS and ScS) are required to uniquely constrain a mechanism for D" anisotropy. We test nine models based on single-crystal and polycrystalline elastic tensors provided by mineral physics studies. Our modelling predicts fast shear wave splitting directions for SKS, SKKS and ScS phases, as well as polarities of P- and S- wave reflections off the D" interface, for a range of propagation directions, via solution of the Christoffel equation. We run tests using randomly selected synthetic data sets based on a given starting model, controlling the total number of measurements, the azimuthal distribution, and the type of seismic phases. For each synthetic data set, we search over all possible elastic tensors and orientations to determine which are consistent with the synthetic data. Overall, we find it difficult to uniquely constrain the mechanism for anisotropy with a typical number of seismic anisotropy measurements (based on currently available studies) with only one measurement technique (SKS, SKKS, ScS or reflection polarities). However, data sets that include SKS, SKKS and ScS measurements or a combination of shear wave splitting and reflection polarity measurements increase the probability of uniquely constraining the starting model and its orientation. Based on these findings, we identify specific regions (i.e. North America, northwestern Pacific and Australia) of the lowermost mantle with sufficient ray path coverage for a combination of measurement techniques. [ABSTRACT FROM AUTHOR]

A method for subsurface recognition of blind geological bodies is presented using combined surface constraints and 3-D structural modelling that incorporates constraints from detailed mapping, and potential-field inversion modelling. This method is applied to the Mount Painter Province and demonstrates that addition of low density material is required to reconcile the gravity signature of the region. This method may be an effective way to construct 3-D models in regions of excellent structural control, and can be used to assess the validity of surface structures with 3-D architecture. Combined geological and potential-field constrained inversion modelling of the Mount Painter Province was conducted to assess the validity of the geological models of the region. Magnetic susceptibility constrained stochastic property inversions indicates that the northeast to southwest structural trend of the relatively magnetic meta-sedimentary rocks of the Radium Creek Group in the Mount Painter Inlier is reconcilable with the similar, northeast to southwest trending positive magnetic anomalies in the region. Radium Creek Group packages are the major contributor of the total magnetic response of the region. However field mapping and the results of initial density constrained stochastic property inversion modelling do not correlate with a large residual negative gravity anomaly central to the region. Further density constrained inversion modelling indicates that an additional large body of relatively low density material is needed within the model space to account for this negative density anomaly. Through sensitivity analysis of multiple geometrical and varied potential-field property inversions, the best-fitting model records a reduction in gravity rms misfit from 21.9 to 1.69 mGal, representing a reduction from 56 to 4.5 per cent in respect to the total dynamic range of 37.5 mGal of the residual anomaly. This best-fitting model incorporates a volumetrically significant source body of interpreted felsic, low density material (1012 m3) impinging on the central-west of the Mount Painter Inlier and overlying Neoproterozoic sequences, and the emplacement of more mafic affinities in the northeast and east. The spatial association and circular geometry of these granitoid bodies suggests an affinity with the Palaeozoic ∼460–440 Ma British Empire Granite that outcrops in the Mount Painter Inlier. The intrusion of this additional material in the Palaeozoic could either be the product of; or contributed to, an increased local geotherm and heat flow in the region during the Palaeozoic. [ABSTRACT FROM PUBLISHER]

The mantle component of the Australian Seismological Reference Model (AuSREM) has been constructed from Australian-specific sources, primarily exploiting the wealth of seismic sources at regional distances around Australia recorded at portable and permanent stations on the continent. AuSREM is designed to bring together the existing information on Australia, from both body wave and surface wave studies and provide a synthesis in the form of a 3-D model that can provide the basis for future refinement. The model is grid based with a 0.5° sampling in latitude and longitude, and is designed to be fully interpolable, so that properties can be extracted at any point.For the upper mantle the primary source of information comes from seismic surface wave tomography, supplemented by analysis of body wave arrivals and regional tomography which provide useful constraints on the relation between P- and S-wave speeds in the mantle lithosphere. A representative model has been developed to capture the features of mantle structure drawing on a range of studies. The mantle structure is represented by grid values at 25 km intervals in depth from 75 to 300 km. Shallower structure is linked to the AuSREM crust through the recent Moho depth model of Kennett et al., which exploits all available sources of seismological information. Below 300 km depth and in the surrounding area AuSREM is linked to the S40RTS model of Ritsema et al. [ABSTRACT FROM AUTHOR]

SUMMARY A meaningful interpretation of seismic measurements requires a rigorous quantification of the uncertainty. In an inverse problem, the data noise determines how accurately observations should be fit, and ultimately the level of detail contained in the recovered model. A common problem in seismic tomography is the difficulty in quantifying data uncertainties, and thus the required level of data fit. Traditionally, the complexity of the solution model (defined by both the number of basis functions and the regularization) is defined arbitrarily by the user prior to inversion with only limited use of data errors. In the context of multiscale problems, dealing with multiple data sets that are characterized by different noise variances and that span the Earth at different scales is a major challenge. Practitioners are usually required to arbitrarily weigh the contribution of each data type into the final solution. Furthermore, the basis functions are usually spatially uniform across the velocity field and regularization procedures are global, which prevents the solution model from accounting for the uneven spatial distribution of information. In this work we propose to address these issues with a Hierarchical Bayesian inversion. The new algorithm represents an extension of the transdimensional tomography to account for uncertainties in data noise. This approach has the advantage of treating the level of noise in each data set, as well as the number of model parameters, as unknowns in the inversion. It provides a parsimonious solution that fully represents the degree of knowledge one has about seismic structure (i.e. constraints, resolution and trade-offs). Rather than being forced to make decisions on parametrization, level of data fit and weights between data types in advance, as is often the case in an optimization framework, these choices are relaxed and instead constrained by the data themselves. The new methodology is presented in a synthetic example where both the data density and the underlying structure contain multiple length scales. Three ambient noise data sets that span the Australian continent at different scales are then simultaneously inverted to infer a multiscale tomographic image of Rayleigh wave group velocity for the Australian continent. The procedure turns out to be particularly useful when dealing with multiple data types with different unknown levels of noise as the algorithm is able to naturally adjust the fit to the different data sets and provide a velocity map with a spatial resolution adapted to the spatially variable information present in the data. [ABSTRACT FROM AUTHOR]

SUMMARY We introduce a new method of rapid interpretation of magnetic vector and tensor field data, based on ideas of potential field migration which extends the general principles of seismic and electromagnetic migration to potential fields. 2-D potential field migration represents a direct integral transformation of the observed magnetic fields into a subsurface susceptibility distribution, which can be used for interpretation or as an a priori model for subsequent regularized inversion. Potential field migration is very stable with respect to noise in the observed data because the transform is reduced to the downward continuation of a well-behaved analytical function. We present case studies for imaging of SQUID-based magnetic tensor data acquired over a magnetite skarn at Tallawang, Australia. The results obtained from magnetic tensor field migration agree very well with both Euler deconvolution and the known geology. [ABSTRACT FROM AUTHOR]

SUMMARY The GOCE mission, a part of ESA's Living Planet Programme, aims at improved gravity field modelling at high spatial resolution. On-board GOCE a gradiometer, in combination with a scientific on-board GPS receiver, measures the earth's gravity field with unprecedented accuracy. These measurements have been used to compute GOCE gravity field solutions and combined GOCE/GRACE solutions. The main difference between the solutions is how they incorporate the required a priori information, which consists either of existing gravity field models or Kaula's rule for the signal variances of the gravity field. We assessed four series of models by comparing the gravity gradients they predict with the measured GOCE gradients. The analysis of the gravity gradients fits may reveal differences between the different solutions that can be attributed to the solution strategy, assuming that the measurement errors are homogeneous. We compared the GOCE gradients with existing state-of-the-art global gravity field models and conclude that the gradient errors are indeed globally homogeneous, with the exception of the cross-track gradient especially south of Australia. Furthermore, we find that the use of existing global gravity field models as a priori information should be avoided because this may increase the gradient residuals in spatial and spectral domain. Finally, we may conclude that the GOCE and GRACE data are compatible and complementary. [ABSTRACT FROM AUTHOR]

SUMMARY Since 2004 more than 7000 km of full-crustal reflection profiles have been collected across Australia to give a total of more than 11 000 km, providing valuable new constraints on crustal structure. A further set of hitherto unexploited results comes from 150 receiver functions distributed across the continent, mostly from portable receiver sites. These new data sets provide a dramatic increase in data coverage compared with previous studies, and reveal the complex structure of the Australian continent in considerable detail. A new comprehensive model for Moho depth across Australia and its immediate environment is developed by utilizing multiple sources of information. On-shore and off-shore refraction experiments are supplemented by receiver functions from a large number of portable stations and the recently augmented set of permanent stations, and Moho picks from the full suite of reflection transects. The composite data set provides a much denser sampler of most of the continent than before, though coverage remains low in the remote areas of the Simpson and Great Sandy deserts. The various data sets provide multiple estimates of the depth to Moho in many regions and the consistency between the different techniques is high. In a number of instances, differences in estimates of Moho depth can be associated with the aspects of the structure highlighted by the particular methods. The new results allow considerable refinement of the patterns of Moho depth across the continent. Some of the thinnest crust lies beneath the Archean cratons in the Pilbara and the southern part of the Yilgarn. Thick crust is encountered beneath parts of the Proterozoic in Central Australia, and beneath the Palaeozoic Lachlan fold belt in southeastern Australia. The refined data indicate a number of zones of sharp contrast in depth to Moho, notably in the southern part of Central Australia. [ABSTRACT FROM AUTHOR]

A lithospheric tomography experiment was conducted in the Adelaide Fold Belt of South Australia, to elucidate the 3-D velocity structure down to depths of several hundred kilometres in a region of major geological and tectonic significance. The data comprised arrival times for compressional waves from 148 teleseismic events recorded on a temporary array of 40 digital seismographs spanning an area of 200 × 300 km. A new irregular shortest path ray tracer and non-linear inversion scheme were used to map the crust- and upper-mantle structure. Synthetic 3-D imaging tests were first performed to validate the procedure and establish resolution limits for the particular earthquake distribution and recording geometry. Wave speed anomalies of up to 4 per cent relative to the IASP91 layered earth model were found. The boundary between a prominent velocity high and a prominent velocity low in the shallow (crustal) depth slices seems to coincide with the synclinal axis of Adelaidean and Early Palaeozoic sediments. Other velocity features at shallow depths correlate with gravity highs and lows, and compare favourably with structure revealed from explosion seismic profiles recorded in the area. At depths greater than 60 km, the tomographic images reveal isolated velocity highs enclosed by regions of lower velocity within the fold belt. These features, of dimensions tens of kilometres, could indicate the existence of fragments of old mantle broken off from the thick subcontinental lithosphere beneath the Gawler Craton and scattered in the Rodinian break-up rift zone. [ABSTRACT FROM AUTHOR]

Bottom-hole temperatures (BHT) from oil exploration provide useful constraints on the subsurface thermal regime, but they need to be corrected to obtain the equilibrium temperature. In this work, we introduce several BHT correction methods and compare them using a large Australian data set of more than 650 groups of multiple BHT measurements in about 300 oil exploration boreholes. Existing and suggested corrections are classified within a coherent framework, in which methods are divided into: line/cylinder source; instantaneous/continuous heat extraction; one/two component(s). Comparisons with reservoir test temperatures show that most of the corrections lead to reliable estimates of the formation equilibrium temperature within ±10 °C, but too few data exist to perform an intercomparison of the models based on this criterion. As expected, the Horner method diverges from its parent models for small elapsed times (or equivalently large radii). The mathematical expression of line source models suffers from an unphysical delay time that also restrains their domain of applicability. The model that takes into account the difference of thermal properties between circulating mud and surrounding rocks—that is the two-component model—is delicate to use because of its high complexity. For these reasons, our preferred correction methods are the cylindrical source models. We show that mud circulation time below 10 hr has a negligible effect. The cylindrical source models rely on one parameter depending on the thermal diffusivity and the borehole radius, which are poorly constrained, but the induced uncertainty on the extrapolations remains reasonably low. [ABSTRACT FROM AUTHOR]

The pole position for the late Mesoproterozoic Alcurra (formerly Kulgera) Dyke Swarm (ADS), Musgrave Block, central Australia, has been revised by adding new data from two more dykes. A palaeomagnetic fold test is positive at the 99 percent confidence level and the pole position is latitude = 2.8° S, longitude = 80.4°E . New 40Ar/39Ar determinations on single biotite grains from country-rock gneiss 20 cm from the contact with an Alcurra dyke yield ages from 1068 ± 2 Ma to 1085 ± 2 Ma (2σ). In addition, 40Ar/39Ar determinations on two samples of aggregates of very fine-grained biotite from the mesostasis of a Stuart Dyke (SD), Arunta Inlier, Northern Territory, yield ages of 1059 ± 2 and 1066 ± 3 Ma (2σ). These new ages are in agreement with those of the ∼1070 Ma Warakurna large igneous province, west-central Australia, indicating that both the ADS and SD belong to that suite. Palaeomagnetic poles determined by others for the ADS and SD are indistinguishable from each other but cannot be presently combined as a single pole because the published information on the SD is incomplete. An analysis of the palaeogeography implied by the dispersed palaeomagnetic poles from members of the extended Warakurna suite shows that the present relationships between the major Precambrian cratons (West, North and South Australian), which constitute Australia west of the Tasman Line, are inconsistent with the inferred relationship at ∼1070 Ma. Nor is it apparent that internal deformations within the separate cratons are the cause of the dispersion. Our findings imply that these major Australian Cratons must have finally assembled sometime after ∼1070 Ma. [ABSTRACT FROM AUTHOR]

Numerical models that include pre-existing faults, elasto–visco–plastic rheology, thermal structure, realistic plate boundary conditions, and surface erosion are built to investigate alternative hypotheses of mantle flow geometry beneath central South Island, New Zealand. Bouguer gravity anomalies, teleseismic traveltime delays and seismic velocity structures are used to constrain models. Model predictions are compared with topography, uplift, magnetotelluric resistivity structure, isostatic gravity anomalies (IGAs), and seismicity. We find that the wedge model, in which the mantle lithosphere of the Australia plate is wedged between crust and mantle lithosphere of the Pacific plate, gives reasonable predictions of topography, uplift rate, IGAs, and stress and strain state in the lithosphere. Coupled erosional-tectonic models suggest that tectonic compression primarily determines the asymmetric surface relief, with erosion influencing its detail and having small effects on shallow stresses. Both 2-D and 3-D models suggest that the neotectonics of central South Island is dominated by weak faults. 3-D model results show that the Alpine fault and mantle shear zone accommodate most convergent and strike-slip plate motion, confirming non-partitioned plate motion in this region. Adding the regional strike-slip component has little effect on the convergent velocity field, and only changes the stress field by rotating the horizontal principal stresses out of the plane of the cross-section. [ABSTRACT FROM AUTHOR]

Whether electromagnetic precursors exist is a contentious issue that should be considered in the context of the focal depth and epicentral distance at which earthquakes nucleate, and with due regard to externally driven changes in the electromagnetic fields recorded at the Earth's surface. If high-frequency electromagnetic signals are generated by earthquakes, then these signals may not be detectable because of attenuation occurring between the focal depth of the earthquake and the Earth's surface. This is particularly true in seismotectonic regions (e.g. subduction zones) where saline fluids (including seawater), raised heat flow and partial melts often lead to enhanced conductivities. A technique for examining temporal variations in magnetotelluric (MT) spectra is presented, which: (i) considers both non-inductive and inductive parts of the MT transfer function; (ii) allows electromagnetic skin depths and adjustment distances to be calculated; (iii) takes account of error propagation in the transfer functions and; and (iv) acknowledges possible non-earthquake sources of observed variations in transfer functions and their errors. Data processing tends to eliminate non-stationary electromagnetic disturbances of the source field. Hence the method is more suited to detecting hypothesized changes in subsurface conductivity than short-lived electromagnetic bursts. The technique is applied to MT data from seismotectonic regions in New Zealand (Mt. Ruapehu), the Andes (western Cordillera) and the Mediterranean (Montecristo island) and to a culturally quiet, control site situated on a stable craton in central Australia. We demonstrate that the MT method can resolve apparent conductivity changes of the order of a few percent over 48 hr time intervals and give thresholds for other interval lengths. Comparison of the results obtained from seismotectonic regions with those from central Australia, and consideration of external source field effects, highlight how temporal variations of electromagnetic fields might be speciously interpreted as earthquake precursors. [ABSTRACT FROM AUTHOR]

The Exmouth and Cuvier margins of NW Australia and the adjacent Gascoyne Abyssal Plain (GAP) and the Cuvier Abyssal Plain (CAP) formed when Greater India rifted and separated from Australia during the Late Jurassic and Early Cretaceous. Using new and existing data, we re-examine the magnetic lineations of the GAP and CAP in order to better determine the timing of final breakup, the early seafloor-spreading history and the origin of several bathymetric features. The time of final continental breakup is similar along the middle Exmouth (at M10N or M11) and Cuvier (at M10N) margins, and cannot be the cause of their different (wide versus narrow) rift architecture. In contrast, in the intervening southern Exmouth margin, spreading did not occur until M7 to M4 time. The initial seafloor spreading of the GAP (the area older than M6 in the middle and older than M4 in the south) is associated with excess magmatism. In the CAP, Sonja Ridge was a spreading axis that propagated northward from M6 to M4 time at the expense of Sonne Ridge. Both ridges became extinct during M4 to M3 time when another spreading centre propagated south near the Greater India ocean–continent boundary. This eliminated the 300-km-long-offset Cape Range Fracture Zone and left formerly Indian lithosphere on the Australian plate. An extinct ridge and pseudofault indicate M4 to M0 southward propagation within oceanic crust of the GAP as part of the same early spreading reorganization that simplified the spreading geometry after the rifted margins were fully separated. [ABSTRACT FROM AUTHOR]

Palaeomagnetic results from the Theresa Creek Volcanics and the Middle Devonian (∼380 Ma) Retreat Batholith, located in the southern Drummond Basin, central Queensland, Australia, reveal up to three components of magnetization. Component 1 is characteristically of reversed polarity with a mean palaeopole at, possibly reflecting a remagnetization event associated with the Middle Carboniferous Alice Springs Orogeny, is found in both units. A shallow to very shallow roughly north–south direction, Component 2, was isolated in seven sites, predominantly within the Retreat Batholith,. The corresponding palaeomagnetic pole lies at. This pole is consistent with the Middle Devonian segment of Australia's apparent polar wander path (APWP) and is thought to reflect a primary magnetization related to the emplacement and cooling of the batholith. Component 3 is found exclusively within the Theresa Creek Volcanics and is characterized by east–west directions with moderate to steep inclinations,. A consistent set of site level virtual geomagnetic pole (VGPs) yield a palaeopole at. The presumed Middle Devonian age for the volcanics is at odds with this direction, with respect to the Australian Early to Middle Palaeozoic APWP, suggesting the characteristic direction found in the volcanics reflects an Ordovician or possibly Early Silurian magnetization age. A pre-Middle Devonian magnetization age for the Theresa Creek Volcanics is supported by a positive contact test between the volcanics and the hornfelsing Retreat Batholith as both units are characterized by stable magnetizations whose palaeopoles are dissimilar from each other and younger segments of Australia's APWP. [ABSTRACT FROM AUTHOR]

Magnetic anomaly and fracture zone data on the Southeast Indian Ridge (SEIR) are analysed in order to constrain the kinematic history of the Macquarie Plate, the region of the Australian Plate roughly east of 145°E and south of 52°S. Finite rotations for Australia–Antarctic motion are determined for nine chrons (2Ay, 3Ay, 5o, 6o, 8o, 10o, 12o, 13o and 17o) using data limited to the region between 88°E and 139°E. These rotations are used to generate synthetic flowlines which are compared with the observed trends of the easternmost fracture zones on the SEIR. An analysis of the synthetic flowlines shows that the Macquarie Plate region has behaved as an independent rigid plate for roughly the last 6 Myr. Finite rotations for Macquarie–Antarctic motion are determined for chrons 2Ay and 3Ay. These rotations are summed with Australia–Antarctic rotations to determine Macquarie–Australia rotations. We find that the best-fit Macquarie–Australia rotation poles lie within the zone of diffuse intraplate seismicity in the South Tasman Sea separating the Macquarie Plate from the main part of the Australian Plate. Motion of the Macquarie Plate relative to the Pacific Plate for chrons 2Ay and 3Ay is determined by summing Macquarie–Antarctic and Antarctic–Pacific rotations. The Pacific–Macquarie rotations predict a smaller rate of convergence perpendicular to the Hjort Trench than the Pacific–Australia rotations. The onset of the deformation of the South Tasman Sea and the development of the Macquarie Plate appears to have been triggered by the subduction of young, buoyant oceanic crust near the Hjort Trench and coincided with a clockwise change in Pacific–Australia motion around 6 Ma. The revised Pacific–Australia rotations also have implications for the tectonics of the Alpine Fault Zone of New Zealand. We find that changes in relative displacement along the Alpine Fault have been small over the last 20 Myr. The average rate of convergence over the last 6 Myr is about 40 per cent smaller than in previous models. [ABSTRACT FROM AUTHOR]

Provides reliable, precisely defines and well-dated Early Permian paleomagnetic poles for Australia from the Mount Leyshon Intrusive complex and the Tuckers Igneous Complex. Association of both complexes with prominent negative magnetic anomalies; Substantial movement through the Mesozoic; Consistency of Early Permian poles with Pangaea B-Type reconstructions of Gondwana and Laurussia.

Examines the determination of the pattern of azimuthal anisotropy for SV waves in the Australian upper mantle. Use of two-stage tomographic procedure for Rayleigh waves; Analysis on the waveform inversion using vertical-component seismograms; Differences between the lateral variations of the SV velocity and azimuthal anisotropy.

Magnetic field fluctuations have been recorded by an array of portable three-component magnetometers at 60 sites across the Eyre Peninsula in South Australia between December 1993 and March 1995. An additional 54 magnetometer data records, collected prior to 1989 and described by Milligan (1989) and Milligan, White & Chamalaun (1989), were included in the analysis. A major conductive feature in the crust, first noted by White & Milligan (1984) as the Eyre Peninsula Anomaly (EPA), is re-examined to assess its continuity to the north of the original arrays and to investigate its relationship with major tectonic features. Magnetic-field time-series were converted to induction arrows in the frequency domain. These induction arrows were initially inverted using the minimum-structure 2-D Occam approach to estimate the electrical conductance of the crust. Following this, thin-sheet forward modelling was used to examine the relationship between the conductance and the dominant tectonic features. The principal results of the modelling are that a narrow conductive feature extends inland from the coast about 160 km before terminating, and the conductance is in the range 3000 to 10 000 S, which decreases inland. A strong correlation exists between the electrical conductance of the Eyre Peninsula and Bouguer gravity anomalies, and in particular the EPA is coincident with a significant Bouguer gravity gradient. There is also good agreement between the locations of the foci of earthquakes of magnitude greater than 4.0 and the EPA. We believe that the anomaly is associated with a geological fracture in the Precambrian upper crust as a result of crustal extension prior to the rifting of Australia from Antarctica in the Jurassic (160 Ma). [ABSTRACT FROM AUTHOR]

SUMMARY New palaeomagnetic data from metamorphic and mafic intrusive rocks of the Albany–Fraser orogen place Australia at high latitudes at ca 1.2 Ga, whereas existing data show that Laurentia occupied low latitudes at this time. No previous reconstructions linking Australia and Laurentia at ca 1.2 Ga are supported by our new data. The two continents moved independently during the early Grenville orogeny, although it is possible that late Mesoproterozoic convergence resulted in an AUSMEX configuration by 1070 Ma. [ABSTRACT FROM AUTHOR]