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Electrical anisotropy is increasingly recognized as an important aspect of the resistivity models required to explain magnetotelluric (MT) observations. However, a limited number of practical MT inversion algorithms that can consider anisotropy have been published to date. To address this problem, we have developed a three‐dimensional (3‐D) MT inversion algorithm that recovers a 3‐D resistivity model that considers arbitrary electrical anisotropy. The inversion uses the same inversion algorithm as the widely used ModEM inversion algorithm, and a novel forward modeling algorithm to consider the anisotropic Earth. The algorithm was tested on both synthetic and field MT data. Inversions considered both a completely general anisotropy tensor with six components and approximations with less parameters. Synthetic inversions show that the two horizontal components of resistivity and the anisotropy strike can be well recovered, while the vertical component of resistivity is poorly resolved, primarily because current flow in MTs is dominantly horizontal. The synthetic examples confirm the limitation of the axial anisotropic inversion technique when applied to MT data produced by a resistivity model with arbitrary anisotropy. The synthetic inversions also showed that inversion of data from an isotropic model will not result in an artificially anisotropic model. Compared to the isotropic inversion model of the real MT data, the anisotropic model clearly shows some features that are consistent with the mapped geology. As expected, the results showed that a given data set can be fit by a range of models, with an inherent trade‐off from 3‐D heterogeneity to 3‐D anisotropy. This uncertainty can be reduced with the use of prior information in the inversion. Plain Language Summary: The resistivity model of real Earth generally contains anisotropic features. Field magnetotelluric (MT) data may be significantly influenced by these features. To interpret such data, inversion algorithms which can consider electrical anisotropy are highly demanded. However, a limited number of practical MT inversion algorithms that can consider anisotropy have been published to date. To address this problem, we have developed a three‐dimensional (3‐D) MT inversion algorithm that recovers a 3‐D resistivity model that considers arbitrary electrical anisotropy. The algorithm was tested to be effective on both synthetic and field MT data. Compared to the isotropic inversion models of the tested MT data, the resolved anisotropic models are more reliable. Key Points: A three‐dimensional non‐linear conjugate gradient inversion algorithm is implemented for the inversion of magnetotelluric (MT) data to produce an electrical resistivity model with arbitrary anisotropyInversion examples of synthetic and real MT data validate the usefulness of the algorithmReal MT data inversion obtains an anisotropic model which is more consistent with the mapped geology than the corresponding isotropic model [ABSTRACT FROM AUTHOR]

Magnetotelluric (MT) and wide‐angle seismic reflection/refraction surveys play a fundamental role in understanding the crustal rheology and lithospheric structure of the Earth. In recent years, the integration of the two methods in order to improve the robustness of the inversion has started to gain attention. We present a new approach for joint 3‐D inversion of MT and wide‐angle seismic reflection/refraction data to accurately determine crustal structures and Moho depth. Based on H‐κ stacking of teleseismic receiver functions, we estimate an initial reference Moho. This is used as input for the subsequent MT/seismic joint inversion, where the Moho interface is updated and crustal structures are added to the model. During the joint inversion process, structural similarity is facilitated through the cross‐gradient constraint. Synthetic model tests show an improvement of the inversion results over separate inversions. In particular, the tests based on two geologically realistic models demonstrate that the crustal structure and even the trade‐off between velocity and Moho interface can be sufficiently resolved by combined MT and seismic data sets when using the estimates from analysis of receiver functions. These results show that the new method can provide useful constraints on crustal structures including their geophysical properties and discontinuities. Key Points: A new coupled algorithm is developed for 3‐D imaging of magnetotelluric and wide‐angle seismic reflection/refraction dataTwo strategies are presented to mitigate the problem of accurately determining crustal structures and Moho interface simultaneouslyThe inclusion of teleseismic receiver functions is proved to be effective in the new algorithm [ABSTRACT FROM AUTHOR]

The eastern terminations of the Himalayan orogeny, named Namche Barwa, are considered a vital natural laboratory in the Tibetan plateau for geodynamics due to its distinctive geological and geomorphological characteristics. Magnetotelluric (MT) data measured at 83 sites around the Namche Barwa are imaged by three-dimensional (3-D) inversion to better reveal the crustal structure of the eastern Himalaya. The results show a complex and heterogeneous electrical structure beneath the Namche Barwa. The electrical conductors distributed in the middle and lower crust around the Namche Barwa provide additional evidence for the 'crustal flow' model if they are considered as some parts of the flow in a relatively large-scale region. The near-surface resistivity model beneath the inner part of Namche Barwa conforms with the locations of hot spring and fluid inclusions, the brittle-ductile transition, and the 300°C-400°C isotherm from previous hydrothermal studies. Relatively resistive upper crust (>800 Ωm) is underlain by a more conductive middle to lower crust (<80 Ωm). The electrical characteristics of the thermal structure at shallow depth indicate an accumulation of hydrous melting, a localized conductive steep dipping zone for decompression melting consistent with the 'tectonic aneurysm' model for explaining the exhumation mechanism of metamorphic rocks at Namche Barwa. The results also imply that both surface processes and local tectonic responses play a vital role in the evolution of Namche Barwa. An alternative hypothesis that the primary sustained heat source accounts for the local thermal-rheological structure beneath Namche Barwa is also discussed. [ABSTRACT FROM AUTHOR]