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The stability field of hydrous phases carried by subducting slabs regulates water transport along the subduction pathway. The ultimate vertical distribution of this water at different depths in Earth’s mantle is governed by the thermal state of the slab. A warm slab is considered to lose water mainly in the upper mantle. However, whether a warm slab can carry water into the lower mantle (LM) is uncertain because of the scarcity of geophysical observations. We report an electrical conductivity model of the mantle transition zone (MTZ) and uppermost LM beneath North America to confirm the water-transporting ability of a warm slab. A high-conductivity anomaly was identified beneath the western United States. The LM portion of this anomaly is interpreted as a hydrous region containing rehydrated stishovite, whereas the lower MTZ portion of the anomaly is ascribed to the presence of water (~0.8 wt%) released by the hydrous stishovite. We speculate that warm slabs, such as the Farallon slab, can cause the breakdown of large amounts of dense hydrous magnesium silicates, releasing water mainly into the upper mantle. This water rehydrates stishovite when percolating through the slab, enabling the slab to continue to transport water into the LM. The identified high-conductivity anomaly, together with previously recognized electrical features in the upper mantle, demonstrates the existence of a trans-MTZ water transport system associated with the warm subduction of the Farallon slab.

The magma plumbing systems of volcanoes in subduction and divergent tectonic settings are relatively well known, whereas those of intraplate volcanoes remain elusive; robust geophysical information on the magma pathways and storage zones is lacking. We inverted magnetotelluric data to image the magma plumbing system of an intraplate monogenetic volcanic field located above the stagnant Pacific slab in northeast China. We identified a complex, vertically aligned, low-resistivity anomaly system extending from the asthenosphere to the surface consisting of reservoirs with finger- to lens-like geometries. We show that magma forms as CO2-rich melts in a 150-km-deep asthenospheric plume crossing the whole lithosphere as hydrated melt, inducing underplating at 50 km depth, evolving in crustal reservoirs, and erupting along dikes. Intraplate volcanoes are characterized by low degrees of melting and low magma supply rates. Their plumbing systems have a geometry not so different from that of volcanoes in subduction settings.

Songyuan is the most earthquake prone area in northeast China. Since 2006, earthquakes have occurred in the area in the form of swarms, with a maximum magnitude of M L5.8. There is much controversy about the cause of the Songyuan earthquakes. We attempted to determine the cause using a three-dimensional electrical conductivity structure inverted from a regional network of magnetotelluric data in the Songyuan area. The L-BFGS inversion method was applied, with a full-impedance tensor data set used as the inversion input. Combined with an evaluation of the earthquake locations, the resistivity model revealed a northeast-oriented hidden fault running through the Songyuan earthquake area (SEA), which was speculated to be the preexisting Fuyu-Zhaodong Fault (FZF). Our resistivity model also found an apparent lithospheric low-resistivity anomaly beneath the earthquake area, which breached the high-resistivity lithospheric mantle and stalled at the base of the crust. A petrophysical analysis showed that this lower crustal low-resistivity anomaly was most likely attributed to hydrated partial melting, which could release water into the lower crust during later magma emplacements. While weakening the strength of the FZF, these ascending fluids also increased the pore pressure in the fault, further reducing the shear strength of the fault. Shear stress action (a fault strike component of the east-west regional compress), together with possible near-surface disturbances, may drive the fault to slip and trigger the earthquakes in Songyuan. It is possible that the continuous replenishment of fluids from the deeper mantle forces the Songyuan earthquakes into the form of swarms. We infer that the Songyuan earthquakes could be attributed to a combination of preexisting faults, regional stress, and deep fluids associated with plate subduction, and near surface disturbances might induce the earthquakes in advance. The Songyuan earthquakes are inherently induced earthquakes, fed by deep fluids.

SUMMARY Constraining the distribution of water in different regions of the mantle remains one of the significant challenges to comprehend the global deep water cycle. Geomagnetic depth soundings can provide such constraint through the electrical conductivity structure. Hence, this study aims to propose a regularization technique that can estimate previously unavailable C-response. In the method, the objective function comprised an L1-norm measured data prediction error and a spectral smoothness constraint term. We used the data error of C-response to weight the predicted error. The L-BFGS method was introduced to determine the minimum point of the objective function, and the regularization parameter decreased adaptively during inversion. Thus, the geomagnetic data processed yielded high-quality C-responses in 31 stations in Eastern China. In addition, we obtained 1-D electrical conductivity profiles in the mantle transition zone (MTZ) beneath Eastern China from C-responses using the L-BFGS method. Compared with the global 1-D model, the conductivity–depth profiles revealed that the MTZ beneath Eastern China is more conductive in the east but more resistive in the west. The conversion of these conductivities to water content based on the mineral physics suggested that the MTZ beneath Eastern China is characterized by a high water concentration, approximately 0.2 and 1 wt per cent in the upper and lower MTZ, respectively. Owing to the inclusion of more stations, the water-rich region could be constrained roughly to the east of the North–South Gravity Lineament (NSGL). Further considering seismic images in the same area, this water content distribution pattern suggested that the front of the stagnant Pacific Plate in the lower MTZ might have reached the NSGL. However, the dehydration reactions in the stagnant slab were more active in the eastern part. Perhaps, some of these fluids migrated into the upper MTZ and could be the source of the trapped water found in the xenoliths from the deep upper mantle beneath Eastern China.

The three-dimensional (3-D) electrical structure of the upper-mantle was used to examine the deep origins of and relationship among the Cenozoic volcanoes located in Northeast China (NEC). High-quality, long-period magnetotelluric (LMT) full-impedance tensor data were collected in NEC and subjected to 3-D Gauss-Newton inversion in order to construct a resistivity model. The resulting model reveals the presence of multiple localized low-resistivity anomalies (LRAs) within the high resistivity lithosphere beneath NEC. These LRAs partially coincide with Cenozoic volcanoes on the surface. Three LRAs that form a larger, annular LRA were observed in the deep upper mantle beneath the Songliao Basin, whereas vein-like LRAs were found in the asthenosphere that connect the lithosphere and deep upper mantle. Petrophysical analyses suggest that the LRAs may have been caused by fluid-induced melting. Based on our electrical model, we propose that, following dehydration of the subducted Western Pacific slab into the mantle transition zone (MTZ) beneath NEC, the released water migrated upward and caused partial melting at the top of the MTZ beneath the Songliao Basin. Under the effect of buoyancy, the melted mantle formed a thermal upwelling that caused melting of asthenosphere before diapiring at the base of the dry lithosphere. The magma then penetrated structural boundaries (such as thinner, weaker, or activated suture zones) and finally reached the Earth’s surface. This melting and upwelling of hot mantle materials may have resulted in large-scale volcanism in the region throughout the Cenozoic, including the eruption of Changbai Mountain and Halaha Volcanoes. Our results suggest that the Cenozoic NEC volcanoes may all share a similar mode of genesis, and probably originated from the annular LRA in the deep upper mantle.

We describe a three‐dimensional electrical exploration system, and its use in carrying out a three‐dimensional survey on a polymetallic deposit located in China's Gansu Province. A time‐domain‐induced polarization method, along with a controlled‐source audiomagnetotelluric method, was used to survey the same station points for comparison. Then, a three‐dimensional inversion based on the incomplete Gauss–Newton method (for time‐domain‐induced polarization) and a nonlinear conjugate gradient (for controlled‐source audiomagnetotelluric) method were developed and applied in order to model the data sets. Four subsurface targets were mapped by the three‐dimensional electrical exploration method. A strong correlation was found between the zones of the revealed ore‐bearing anomalies and the geological setting. Using the results of the three‐dimensional electrical exploration, we revealed the spatial distribution of the ore‐bearing stratum, as well as the relationship between the ore bodies and the stratum. The results potentially provided evidence for future interpretations of deep geological structures, along with evaluations of mineral resources.

Details on the 3-D magnetotelluric inversion, sensitivity test of resistivity anomalies and melt fraction estimation, and Figures S1–S11.