Both low resistivity zones and low velocity zones are distributed in the middle-lower crust of the western Lhasa terrane, Tibetan Plateau, China. Some estimates from electrical resistivity data suggest large volume fractions of silicate melts that are difficult to reconcile with seismic velocity data that prefer lower volumes. A second conductive phase, such as saline fluids, that drastically reduces the conductivity but does not significantly affect the seismic velocity because of its low volume may be able to explain these differences. In this study, a 3-D model of the electrical resistivity structure is generated on a profile along longitude 85°E from a latitude of 29°N to 32.5°N. Based on experimental measurement of melts and alkali-rich fluids (e.g., H ₂O-NaCl), we estimate the volume fraction of each phase that is required to explain the conductive anomalies observed in the geophysical model. The model reveals that the maximum bulk conductivity of the mid-lower crust in the south (1.52 S/m) is much higher than the conductivity of the mid-lower crust in the north (0.18 S/m) when taking 31°N as a rough boundary, near Coqen region. We hypothesize that the conductive zones in the south of the Coqen region may result from a silicate melt and alkali-rich fluid (multicomponent) system. In contrast, partial melting alone can explain the conductive zones in the north. The hypothesis can reconcile the predictions from electrical resistivity data and seismic data, and it corresponds well with zircon Hf isotope data. For example, a combination such as the presence of <1% NaCl-bearing aqueous fluids in addition to 5-10% partial melt can reconcile electrical conductivity data and seismic data. We propose that the contributions from partial melt or saline fluids are controlled by the distinct tectonic dynamics in each region. Furthermore, the model compatible with the idea that the Indian lower crust subducted northwards beneath the Lhasa terrane and may not extend far beyond the Indus-Yarlung Zangbo suture (approximately 30-31°N). The widespread distribution and interconnection of crustal conductors at different depths is consistent with the lateral migration of materials. However, both geophysical data sets agree that some anomalies are discontinuous along the profile. Furthermore, the low-angle subducted Indian Plate with no obvious tearing feature and a low volume of melts may have contributed to the absence of long, continuous, N-S-trending normal faults in this region.

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The tectonic dynamics of the Lhasa-Gangdese terrane, southern Tibet, are still unclear and open questions persist regarding the structure, physical properties, and rheology of the lithosphere. A three-dimensional electrical resistivity model was generated beneath the Lhasa terrane, 79°−94°E and 28°–33°N, an area of ∼1,500 by ∼600 km, using data from 537 magnetotelluric measurements. To overcome computational challenges, we present an approach in which multiple high-resolution models from overlapping subregions are combined by means of an error-weighted averaging scheme to construct a continuous electrical resistivity model. Based on the electrical structure, the rheology of the mid-lower crust was investigated by constraining the melt fraction, in order to explore controls on dynamic mechanisms from a whole-system perspective. The models shed light on the vertical and lateral migration of crustal materials, magma transportation, and continental deformation in the Lhasa terrane. The model shows resistive features in the south that may represent the Indian plate, and conductive features above this that may represent the migration of materials beneath the Lhasa terrane, both of which vary substantially along the Indus-Yarlung Zangbo suture. In the mid-lower crust, conductive features (with a likely corresponding decrease of the effective viscosity) may be related to underplating, magma reservoirs, mineralization sources, and vertical motion of buoyant materials. A weakened mid-lower crust has consequences for surface deformation and may have contributed to the formation of (north-south) rift zones. Furthermore, the inhomogeneous distribution of weakened areas in the mid-lower crust has implications for the local lateral migration of materials.@en

Some of the largest and most significant Miocene porphyry copper systems in China are within the Gangdese metallogenic belt on the southern Tibetan Plateau. It has been recognized that the crustal architecture and rheology, derived from regional tectonic events, has direct implications for the evolution and transport of fluids and magmas, and thus for the metallogenesis and prospectivity. Using data from a magnetetolluric array, which intersects the Lhasa–Mozugongka district of the Gangdese metal belt, a 3-D electrical resistivity model was generated, with the goal of investigating the tectonic and rheological controls on the magmatic mineral system. The lithospheric temperature distribution was estimated by applying the Arrhenius equation to conductivity profiles generated from 1-D Monte-Carlo models of long-period magnetotelluric data. The conductivity of partial melts in the lower and middle crust (30–60 km depth) was estimated for local conditions by applying the experimentally-derived equation of X. Guo et al. (2018). Subsequently, we estimated the melt fraction required to explain the observed bulk resistivity in each part of the study area. Variations in the effective viscosity of the lower and middle crust were constrained by the electrical resistivity model by applying the empirical relation of Liu and Hasterock (2016). Beneath the Miocene Cu–Mo deposits in the Lhasa terrane, conductive features in the lower and middle crust are attributed to partial melt fractions of more than 5% and viscosity reductions of 1–2 orders of magnitude. These conductive features may represent the signatures of (ore-controlling) melt/fluid migration channels and deeper melt/fluid source regions in the form of extensive crustal reservoirs of partial melt. Based on the interpretations of the geophysical model, and other available geological and geochemical evidence, a model of the metallogenic dynamics of the Miocene porphyry Cu–Mo deposits is proposed. Overall, the study highlights the applicability of electromagnetic geophysical methods to reliably link resistivity structures to melt/fluid transport channels and sources within a mineral system and supports the hypothesis that crustal rheology exerts a major control on the distribution of ore deposits.@en

The Xainza-Dinggye rift, an approximately north-south trending Cenozoic fault zone across the Lhasa Terrane, is an ideal location to investigate extensional mechanisms in the upper crust and lithospheric deformation caused by the subducting Indian Plate beneath the central-southern Tibetan Plateau. The 3-D electrical resistivity structure was obtained by modeling magnetotelluric data from an array across the rift zone. Using the temperature distribution throughout the crust combined with the pressure and water content, we compute the pure melt conductivity. This enables estimation of the partial melt fraction of large-area conductive zones imaged throughout the crust, and thus allows their rheology and strength to be evaluated. The heterogeneous distribution of high melt fraction areas in the crust has implications for the local continuous migration of fluids in the east-west direction. The electrical structure also reveals a dipping resistive zone beneath the Tethys-Himalaya terrane that represents the subducted Indian Plate. Significantly, its depth is observed to vary substantially from east (deeper) to west (shallower), which may indicate tearing of the plate. We suggest that the cause of extension and the formation of the crustal rift zone is related to tearing of the plate directly below this location, and the subsequent partial melting of the mid-lower crust. Furthermore, the ascent of deep hydrothermal fluids, and heating of the shallow subsurface, likely led to the formation of the congruent Xainza-Dinggye Thermal Belt. Additionally, the emplacement of numerous adjacent magmatic-hydrothermal ore deposits is also likely related to partial melting and hydrothermal fluid migration in the crust.@en

Decentralized multi-agent control has broad applications, ranging from multi-robot cooperation to distributed sensor networks. In decentralized multi-agent control, systems are complex with unknown or highly uncertain dynamics, where traditional model-based control methods can hardly be applied. Compared with model-based control in control theory, deep reinforcement learning (DRL) is promising to learn the controller/policy from data without the knowing system dynamics. However, to directly apply DRL to decentralized multi-agent control is challenging, as interactions among agents make the learning environment non-stationary. More importantly, the existing multi-agent reinforcement learning (MARL) algorithms cannot ensure the closed-loop stability of a multi-agent system from a control-theoretic perspective, so the learned control polices are highly possible to generate abnormal or dangerous behaviors in real applications. Hence, without stability guarantee, the application of the existing MARL algorithms to real multi-agent systems is of great concern, e.g., UAVs, robots, and power systems, etc. In this paper, we aim to propose a new MARL algorithm for decentralized multi-agent control with a stability guarantee. The new MARL algorithm, termed as a multi-agent soft-actor critic (MASAC), is proposed under the well-known framework of “centralized-training-with-decentralized-execution”. The closed-loop stability is guaranteed by the introduction of a stability constraint during the policy improvement in our MASAC algorithm. The stability constraint is designed based on Lyapunov’s method in control theory. To demonstrate the effectiveness, we present a multi-agent navigation example to show the efficiency of the proposed MASAC algorithm.

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