Lithospheric Structure and Melting Processes in Southeast Australia: New Constraints From Joint Probabilistic Inversions of 3D Magnetotelluric and Seismic Data.

The thermochemical structure of the lithosphere controls melting mechanisms in the mantle, as well as the location of volcanism and ore deposits. Obtaining reliable images of the lithosphere structure, and its complex interactions with the asthenosphere, requires the joint inversion of multiple data sets and their associated uncertainties. In particular, the combination of seismic velocity and electrical conductivity, along with proxies for bulk composition and elusive minor phases, represents a crucial step toward fully understanding large‐scale lithospheric structure and melting processes. We apply a novel probabilistic approach for joint inversions of 3D magnetotelluric and seismic data to image the lithosphere beneath southeast Australia. The results show a highly heterogeneous lithosphere 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 (a) boundaries of geological provinces, elucidating the subduction‐accretion process in the region; (b) distribution of leucitite and basaltic magmatism; (c) independent geochemical data, and (d) 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 primary mechanisms for melting in eastern Australia, contrary to the conventional notion of mantle plume activity. Our study presents an integrated lithospheric model for southeastern Australia and provides valuable insight into the mechanisms driving surface geological processes. Plain Language Summary: The lithosphere is the Earth's outermost rigid layer and the focus of important geological processes, such as earthquakes, volcanism, and mineralization. The location of these processes often coincides with deep discontinuities in the lithosphere. Imaging the structure of the lithosphere using geophysical techniques is crucial to fully understand the nature of these processes. The most reliable images of the lithosphere are obtained by joint analysis of two or more geophysical data sets. In particular, the combination of magnetotellurics (an electromagnetic technique) and seismic data holds great potential because of their complementary sensitivity to the Earth's properties. Furthermore, our understanding of the variability of the lithospheric structure is enhanced by combining this joint analysis with a probabilistic approach. This is because probabilistic methods provide a large number of models that can explain the data. Given the substantial data coverage in southeast Australia, we use a new probabilistic approach for the joint analysis of magnetotelluric and seismic data to image the lithospheric structure beneath this region. Our results show a complex lithosphere in line with the location of volcanism and the tectonic history of the region. The lithospheric composition derived from the models provides significant insights into melt production in the area. Key Points: We apply a novel approach for joint probabilistic inversions of 3D magnetotelluric and seismic dataWe use the new method to image the lithosphere‐asthenosphere system beneath southeastern of AustraliaThe imaged lithospheric structure provides insights on metasomatism and melt production in the region [ABSTRACT FROM AUTHOR]