The thermochemical structure of the lithosphere exerts control on melting mechanisms in the mantle as well as the location of volcanism and ore deposits. Imaging the complex interactions between the lithosphere and asthenospheric mantle requires the joint inversion of multiple data sets and their uncertainties. In particular, the combination of seismic velocity and electrical conductivity with data proxies for bulk composition and elusive minor phases is a crucial step towards fully understanding large-scale lithospheric structure and melting. We apply a novel probabilistic approach for joint inversions of 3D magnetotelluric and seismic data to image the lithosphere beneath southeast Australia. Results show a highly heterogeneous lithospheric structure with deep conductivity anomalies that correlate with the location of Cenozoic volcanism. In regions where the conductivities have been at odds with sub-lithospheric temperatures and seismic velocities, we observe that the joint inversion provides conductivity values consistent with other observations. The results reveal a strong relationship between metasomatized regions in the mantle and i) the limits of geological provinces in the crust, which elucidates the subduction-accretion process in the region; ii) distribution of leucitite and basaltic magmatism; iii) independent geochemical data, and iv) a series of lithospheric steps which constitute areas prone to generating small-scale instabilities in the asthenosphere. This scenario suggests that shear-driven upwelling and edge-driven convection are the dominant melting mechanisms in eastern Australia rather than mantle plume activity, as conventionally conceived. Our study offers an integrated lithospheric model for southeastern Australia and provides insights into the feedback mechanism driving surface processes.
Abstract We present two‐dimensional electrical resistivity models of two 40 km magnetotelluric (MT) profiles across the Frome Embayment to the east of the northern Flinders Ranges, South Australia. The lower crust shows low resistivity of 10 Ω m at around 30 km depth. The middle crust is dominated by resistive (>1000 Ω m) basement rocks underlying the Flinders Ranges. Adjacent to the ranges, conductive lower crust is connected to vertical zones of higher conductivity extending to just below the brittle‐ductile transition at ∼10 km depth. The conductive zones narrow in the brittle upper crust and dip at roughly 45° beneath the surface. Zones of enhanced conductivity coincide with higher strain due to topographic loading and sparse seismicity. We propose that fluids are generated through neotectonic metamorphic devolatilization. Low‐resistivity zones display areas of fluid pathways along either preexisting faults or an effect of crustal compression leading to metamorphic fluid generation. The lower crustal conductors are responding to long‐wavelength flexure‐induced strain, while the upper crustal conductors are responding to short wavelength faulting in the brittle regime. MT is a useful tool for imaging crustal strain in response to far‐field stresses in an intraplate setting and provides important constraints for geodynamic modeling and crustal rheology.
SummaryA magnetotelluric survey, comprising 40 stations, has been completed in the southern Yilgarn Craton. The preferred resistivity cross section through the crust and upper mantle shows the local lithosphere comprises three distinct units separated by steep boundaries. The central unit, interpreted as equivalent to the Southern Cross Domain has a resistive crust overlying a more conductive mantle. The two units on either side comprise a conductive lower crust overlying a resistive mantle. Dipping narrow zones of increased conductivity in the crustal part of the model correlate with known surface structures. The eastern margin of the Southern Cross Domain as inferred from deep crustal and mantle resistivity occurs about 50 km to the west of the Ida Fault, the margin of the domain at the surface. The three fold subdivision of the local lithosphere is consistent with the geologically and geochemically defined terranes and domains in this part of the Yilgarn.Current models for regional mineral exploration targeting emphasize the significance of major geological structures and the edges of cratonic blocks as areas of greatest prospectivity. The South Yilgarn MT dataset demonstrate that such features can be located based on variations in the electrical conductivity of the lower crust and mantle, which can be measured in a cost effective manner using the magnetotelluric method.
Abstract. Ultraviolet radiation is the key factor driving tropospheric photochemistry. It is strongly modulated by clouds and aerosols. A quantitative understanding of the radiation field and its effect on photochemistry is thus only possible with a detailed knowledge of the interaction between clouds and radiation. The overall objective of the project INSPECTRO was the characterization of the three-dimensional actinic radiation field under cloudy conditions. This was achieved during two measurement campaigns in Norfolk (East Anglia, UK) and Lower Bavaria (Germany) combining space-based, aircraft and ground-based measurements as well as simulations with the one-dimensional radiation transfer model UVSPEC and the three-dimensional radiation transfer model MYSTIC. During both campaigns the spectral actinic flux density was measured at several locations at ground level and in the air by up to four different aircraft. This allows the comparison of measured and simulated actinic radiation profiles. In addition satellite data were used to complete the information of the three dimensional input data set for the simulation. A three-dimensional simulation of actinic flux density data under cloudy sky conditions requires a realistic simulation of the cloud field to be used as an input for the 3-D radiation transfer model calculations. Two different approaches were applied, to derive high- and low-resolution data sets, with a grid resolution of about 100 m and 1 km, respectively. The results of the measured and simulated radiation profiles as well as the results of the ground based measurements are presented in terms of photolysis rate profiles for ozone and nitrogen dioxide. During both campaigns all spectroradiometer systems agreed within ±10% if mandatory corrections e.g. stray light correction were applied. Stability changes of the systems were below 5% over the 4 week campaign periods and negligible over a few days. The J(O1D) data of the single monochromator systems can be evaluated for zenith angles less than 70°, which was satisfied by nearly all airborne measurements during both campaigns. The comparison of the airborne measurements with corresponding simulations is presented for the total, downward and upward flux during selected clear sky periods of both campaigns. The compliance between the measured (from three aircraft) and simulated downward and total flux profiles lies in the range of ±15%.
Surface-based monitoring of mass transfer caused by injections and extractions in deep boreholes is crucial to maximize oil, gas and geothermal production. Inductive electromagnetic methods, such as magnetotellurics, are appealing for these applications due to their large penetration depths and sensitivity to changes in fluid conductivity and fracture connectivity. In this work, we propose a 3-D Markov chain Monte Carlo inversion of time-lapse magnetotelluric data to image mass transfer following a saline fluid injection. The inversion estimates the posterior probability density function of the resulting plume, and thereby quantifies model uncertainty. To decrease computation times, we base the parametrization on a reduced Legendre moment decomposition of the plume. A synthetic test shows that our methodology is effective when the electrical resistivity structure prior to the injection is well known. The centre of mass and spread of the plume are well retrieved. We then apply our inversion strategy to an injection experiment in an enhanced geothermal system at Paralana, South Australia, and compare it to a 3-D deterministic time-lapse inversion. The latter retrieves resistivity changes that are more shallow than the actual injection interval, whereas the probabilistic inversion retrieves plumes that are located at the correct depths and oriented in a preferential north–south direction. To explain the time-lapse data, the inversion requires unrealistically large resistivity changes with respect to the base model. We suggest that this is partly explained by unaccounted subsurface heterogeneities in the base model from which time-lapse changes are inferred.
The resistivity structure of the crust is broadly expected to be homogeneous, with highly resistive lower crustal rock overlain by more conductive rock in the upper crust. However, observed data shows that although the upper crust is typically resistive, the lower crust can be much more conductive. The presence of such high electrical conductivity in the lower crust is remarkable and suggests a substantial highly connected material, melt or fluid. Has the low resistivity structure been present since inception, or is it the result of a later overprinting event. The secondary objective to establish how such a low resistivity region can be preserved over such an extended time scale.Data has been collated from magnetotelluric (MT), and geomagnetic depth sounding (GDS) surveys collected over the last thirty years. Three different methods have been used to model thousands of data points. A thin-sheet inversion of thousands of GDS data has been used to place constraints on the regional scale electrical conductance. Inversions of the MT data in both 2D and 3D have provided more detailed models of how the Moho is connected to the upper crust.Strong correlations were observed between major tectonic domains (such as the Gawler Craton) and regions of high resistivity within the crust. The 2D profiles show broad regions of low resistivity at the boundary between the upper and lower crust (10-15 km depth), with low resistivity zones extending for tens of km. Above the boundary, the low resistivity regions transform in to narrow pathways penetrating through the resistive upper crust and the areas with the lowest resistivity were found to have a strong correlation with known major mineral provinces. This leads to the suggestion that crustal low resistivity anomalies are likely a product of fluxes of fluid and possibly melt from the upper mantle and lower crust.
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