Direct geological information in Antarctica is limited to ice free regions along the coast, high mountain ranges or isolated nunataks. Therefore, indirect methods are required to reveal subglacial geology and heterogeneities in crustal properties, which are critical steps towards interpreting geological history. We present a 3D crustal model of density and susceptibility distribution in the Wilkes Subglacial Basin and the Transantarctic Mountains (TAM) based on joint inversion of airborne gravity and magnetic data. The applied “variation of information” technique enforces a coupling between gravity and magnetic sources to give an enhanced inversion result. Our model reveals a large-scale body located in the interior of the Wilkes Subglacial Basin interpreted as a batholithic intrusive structure, as well as a linear dense body at the margin of the Terre Adélie Craton. Density and susceptibility relationships are used to inform the interpretation of petrophysical properties and the reconstruction of the origin of those crustal blocks. The petrophysical relationship indicates that the postulated batholitic intrusion is granitic, but independent from the Granite Harbour Igneous Complex previous described in the TAM area. Emplacement of a large volume of intrusive granites can potentially elevate local geothermal heat flow significantly. Finally, we present a tectonic evolution sketch based on the inversion results, which includes development of a passive continental margin with seaward dipping basalt horizons and magmatic underplating followed by two distinct intrusion events in the Wilkes Subglacial Basin with Pan-African ages (700 - 551 Ma) and Ross ages (550 - 450 Ma).

We present a new approach that allows for the inversion of quantities derived from the observed data using non-diagonal data covariance matrices. For example, we can invert approximations of apparent resistivity and phase instead of magnetotelluric impedance using this methodology. Compared to the direct inversion of these derived quantities, the proposed methodology has two advantages: i) If an inversion algorithm allows for the specification of a full data covariance matrix, users can invert for arbitrary derived quantities by specifying the appropriate covariance matrix instead of having to rely on the inversion code to have implemented this feature. ii) It is fully compatible with the assumptions of least-squares optimization and thus avoids potential issues with bias when inverting quantities that are non-linear functions of the original data, We discuss the theory of this approach and show an example using magnetotelluric data. However, the same method can be applied to other types of geophysical data, for example gravity gradient measurements.

This article is composed of three independent commentaries about the state of ICON principles (Goldman et al., 2021) in the Geomagnetism, Paleomagnetism, and Electromagnetism (GPE) section and discussion on the opportunities and challenges of adopting them. Each commentary focuses on a different topic: Global collaboration, reproducibility, data sharing and infrastructure; Inclusive equitable, and accessible science: Involvement, challenges, and support of early career, BIPOC, women, LGBTQIA+, and/or disabled researchers; Community engagement, citizen science, education, and stakeholder involvement. Data sharing practices and open repository use still varies strongly between GPE communities. Some have a long tradition of data sharing, others are only starting it. Globally, GPE leadership is strongly dominated by white males and diversity may increase through the creation of Science Equality Commissions. Improved global stakeholder involvement can increase research impacts and help fight inequalities. In all investigated topics we see promising beginnings but also recognize obstacles that include a lack of funding, a lack of understanding of diversity, and prioritizing short-term gain over long-term benefit. Nonetheless, we are hopeful that our community will embrace ICON science.

The Southern African Magnetotelluric Experiment (SAMTEX) involved the collection of data at over 700 sites in Archean to Proterozoic southern Africa, spanning features including the Kalahari Craton, Bushveld Complex and voluminous kimberlites. Here, we present the first 3D inversions of the full SAMTEX dataset. In this paper, we focus on assessing the robustness of the 3D models by comparing two different inversion codes, jif3D and ModEM, and two different subsets of the data, one containing all acceptable data and the other containing a smaller selection of undistorted, high-quality data. Results show that the main conductive and resistive features are imaged by all inversions, including deep resistive features in the central Kaapvaal Craton and southern Congo Craton and a lithospheric-scale conductor beneath the Bushveld Complex. Despite this, differences exist between the jif3D and ModEM inverse models that derive mainly from the differences in regularization between the models, with jif3D producing models that are very smooth laterally and with depth, while ModEM produces models with more discrete conductive and resistive features. Analysis of the differences between these two inversions can provide a good indication of the model resolution. More minor differences are apparent between models run with different subsets of data, with the models containing all acceptable data featuring higher wavelength conductivity variations than those run with fewer stations but also demonstrating poorer data fit.

The composition of the lower crust is a key factor in understanding tectonic activity and deformation within the Earth. In particular, the presence or absence of melt or fluids has strong control on tectonic evolution. Multi-physics inversion results from the western United States demonstrate that tectonic inheritance plays a much stronger role in determining the location of melt in the lower crust than previously thought. Even in a currently active area such as the Yellowstone Hotspot, fluid dominated structures and fluid free regions are juxtaposed. This is incompatible with the commonly used model of recent tectonic activity as a main controlling factor for the presence of fluids or melt. These results have global implications for how geophysical models are interpreted and how they can be related to geodynamic simulations.

Integration of multiple geophysical methods in combined data analysis is a key practice to reduce model uncertainties and enhance geological interpretations. Electrical resistivity models resulting from inversion of marine magnetotelluric (MT) data, often lack depth resolution of lithological boundaries, and distinct information for shallow model parts. This is due to the nature of the physics i.e. diffusive method, model regularization during inversion, and survey setup i.e. large station spacing and missing high frequency data. Thus, integrating data or models to constrain layer thicknesses or structural boundaries is an effective approach to derive better constrained, more detailed resistivity models. We investigate the different impacts of three cross-gradient coupled constraints on 3D MT inversion of data from the Namibian passive continental margin. The three constraints are a) coupling with a fixed structural density model; b) coupling with satellite gravity data; c) coupling with a fixed gradient velocity model. Here we show that coupling with a fixed model (a and c) improves the resistivity model most. Shallow conductors imaging sediment cover are confined to a thinner layer in the resulting resistivity models compared to the MT-only model. Additionally these constraints help to suppress vertical smearing of a conductive anomaly attributed to a fracture zone, and clearly show that the seismically imaged Moho is not accompanied by a change in electrical resistivity. All of these observations aid interpretation of an Earth model indicating involvement of a plume impact in continental break-up during the early Cretaceous.

Inverse methods form the basis of many investigations of the structure of the lithosphere-asthenosphere system as they provide the basis for physics-based subsurface imaging from surface and/or near-surface measurements. Steady increases in computational capabilities and methodological improvements have resulted in increasingly detailed three-dimensional models of the Earth based on inverse methods. While these models can show an impressive array of features, it may be difficult for non-specialists to assess which aspects can be considered reliable and which are tenuous, or are artefacts of the mathematical formulation or data collection. In this paper we address the fundamental issues of feature reliability due to limited resolution and model sensitivity to data noise for researchers who do not work with intimately with inverse methods. We include and introductory overview of the mathematical formulation of inversion methods and define commonly used terms and concepts. We then present two case studies based on data from USArray in the western United States. The first case study utilizes magnetotelluric array data to construct a three-dimensional model of electrical resistivity to a depth of approximately 300 km. We use this example to demonstrate fundamental issues regarding data fit, data coverage, and model parameterization. The second case study discusses how we can incorporate petrological and mineral physics information directly into the inversion approach to create models that are compatible with constraints on the temperature and composition of the lithosphere. We will discuss the implications for practical use of these models in interpretations and provide guidelines on how to evaluate such models.

The tectonic history of Southern Africa includes Archean formation of cratons, multiple episodes of subduction and rifting and some of the world’s most significant magmatic events. These processes left behind a compositional trail that can be observed in xenoliths and measured by geophysical methods. The abundance of kimberlites in southern Africa makes it an ideal place to test and calibrate mantle geophysical interpretations that can then be applied to less well-constrained regions. Magnetotellurics (MT) is a particularly useful tool for understanding tectonic history because electrical conductivity is sensitive to temperature, bulk composition, accessory minerals and rock fabric. We produced three-dimensional MT models of the southern African mantle taken from the SAMTEX MT dataset, mapped the properties of $\sim36000$ garnet xenocrysts from Group I kimberlites, and compared the results. We found that depleted regions of the mantle are uniformly associated with high electrical resistivities. The conductivity of fertile regions is more complex and depends on the specific tectonic and metasomatic history of the region, including the compositions of metasomatic fluids or melts and the emplacement of metasomatic minerals. The mantle beneath the $\sim 2.05$ Ga Bushveld Complex is highly conductive, probably caused by magmas flowing along a lithospheric weakness zone and precipitating interconnected, conductive accessory minerals such as graphite and sulfides. Kimberlites tend to be emplaced near the edges of the cratons where the mantle below 100 km depth is not highly resistive. Kimberlites avoid strong mantle conductors, suggesting a systematic relationship between their emplacement and mantle composition.