Search

The two Large Low Velocity Provinces (LLVPs) in the lowermost Earth mantle are thought to affect large‐scale heat and material transport, governing mantle evolution. LLVPs have been interpreted as thermochemical piles of recycled oceanic crust (ROC) and/or other dense rock types. However, the role of ROC intrinsic viscosity in pile formation and related effects on mantle evolution remain poorly understood. Using mantle convection models, we show that, while ROC intrinsic density controls pile formation, intrinsic viscosity determines whether piles are internally convecting or stratified. Only high‐viscosity, stratified piles can preserve material over several billions of years. Pile stratification is therefore required to reconcile geochemical evidence for the survival of ancient reservoirs. Compositionally layered piles are also consistent with geophysical observations that point to vertical gradients in LLVP properties. As mineral physics constraints point to low‐viscosity ROC, our results suggest that LLVPs may be partly formed by early basal‐magma‐ocean cumulates. Plain Language Summary: Earthquake‐produced vibrations travel long distances inside the Earth, but slow down when they cross two large blobs that lie thousands of kilometers below Africa and the Pacific Ocean. The nature of these anomalies is controversial, but it is thought that the crust, the outermost rocky peel that is continuously formed on the Earth's surface, may penetrate the planet's interior, sink and accumulate to form large piles. We employ a specialized code that is widely used to simulate movements of masses inside rocky planets over billions of years, including the process of crust formation and accumulation. We experiment with how dense and soft/strong (i.e., easy/hard to deform) this sunken crust is, relative to the surroundings. We investigate how these largely unknown properties affect pile formation and find that crustal strength has a marginal role compared to its density. However, only high strength makes piles layered and able to store ancient material, consistent with current observations. As recent laboratory experiments point to a soft crust, our results suggest that the piles observed today mainly contain "primitive" material that settled in the deep Earth during the very early stages of our planet's formation, instead of crust. Key Points: Intrinsic viscosity of deep‐sunken oceanic crust does not affect its segregation and accumulation into pilesHigh‐viscosity piles are stratified and able to store Archean‐age materialsLarge low velocity provinces may not be mainly formed of low‐viscosity oceanic crust [ABSTRACT FROM AUTHOR]

La Palma, Canary Islands, had its largest historical eruption in 2021. From January 2022 to May 2023 there were >2,100 seismic events, primarily at depths ≤20 km, prompting us to update the deformation and modeling study, using interferometric synthetic aperture radar observations and a last generation interpretation tool. We detect the evolution of the remaining magmatic body in the SW portion of the island, with arrival of new magma moving into the oceanic crust out to sea, and a pressurized zone in the central‐eastern area, at regions of structural weakness. The current source characteristics have some similarities to the early stage dynamics prior to the 2021 eruption. Operational and multidisciplinary studies must continue to monitor either their stabilization or growth and destabilization. The ability to identify magma ascent using only deformation data over short time periods allows us to characterize unrest patterns and provide new insights into volcanic processes. Plain Language Summary: Following the 2021 eruption at La Palma, Canary Islands, seismic activity has persisted that may be a precursor to future volcanic activity. Here we perform an updated deformation and modeling study that characterizes the evolution of the remaining magma in the SW area of the island. A fresh injection of magma occurs in the oceanic crust off the coast, along with a pressurized region in the central‐eastern area of the island where crustal weakness exists close to recent volcanic eruptions. The 2022–2023.4 process has similar characteristics to the evolution prior to the 2021 eruption. Ongoing studies should focus on monitoring the increase or decrease in this activity. Improved characterization of changing volcanic activity over short time periods can provide new insights into ongoing volcanic processes. Key Points: We determine the current unrest is similar to that prior to the 2021 eruption warranting attention by hazard response decision makersWe detect a shallow magmatic intrusion in the SW of the island migrating W into the submarine crust of the islandWe also detect a shallow pressurized zone located in the central eastern region of the island [ABSTRACT FROM AUTHOR]

Hadean zircons provide a potential record of Earth's earliest subduction 4.3 billion years ago. It remains enigmatic how subduction could be initiated so soon after the presumably Moon‐forming giant impact (MGI). Earlier studies found an increase in Earth's core‐mantle boundary (CMB) temperature due to the accumulation of the impactor's core, and our recent work shows Earth's lower mantle remains largely solid, with some of the impactor's mantle potentially surviving as the large low‐shear velocity provinces (LLSVPs). Here, we show that a hot post‐impact CMB drives the initiation of strong mantle plumes that can induce subduction initiation ∼200 Myr after the MGI. 2D and 3D thermomechanical computations show that a high CMB temperature is the primary factor triggering early subduction, with enrichment of heat‐producing elements in LLSVPs as another potential factor. The models link the earliest subduction to the MGI with implications for understanding the diverse tectonic regimes of rocky planets. Plain Language Summary: Plate tectonics remains unique to Earth, but when and how it started is debated. Earth's oldest minerals indicate a clement surface by 4.3 Ga, resembling the modern Earth with its granitic crust and oceans. Granite is most easily explained as originating from subduction. However, the mechanisms for subduction initiation, especially so soon after the Moon‐forming giant impact, remain elusive. Earlier studies indicate that the core‐mantle boundary (CMB) temperature is increased due to accumulation of the impactor's core during the impact. Our recent work further shows that the lower half of Earth's mantle remains mostly solid after this impact and that parts of the impactor's mantle might have survived as the two seismically‐observed large low‐shear velocity provinces (LLSVPs). In this study, we perform whole‐mantle convection models to illustrate that strong mantle plumes can arise, weaken the lithosphere, and eventually initiate subduction ∼200 Myr after the giant impact. Our systematic computations reveal that the hot CMB temperature after the impact is the primary factor determining whether there is early subduction initiation, with enrichment of heat‐producing elements in LLSVPs as another potential contributor. Our model ties the Moon's formation to incipient subduction, providing insights for understanding the diverse tectonic regimes of rocky planets. Key Points: The mantle thermochemical structure left by the Moon‐forming impact triggers strong plumes that may have initiated the first subductionThe strong mantle plumes may rise from a heated core or from large low‐shear velocity provinces enriched in heat‐producing elementsEarly giant impact events may have a profound influence on the diverse tectonic regimes of rocky planets [ABSTRACT FROM AUTHOR]

Oceanic volcanic islands have a complex internal structure due to their geological evolution. La Palma is the second youngest in the Canary Islands. Historical eruptions occurred in the Cumbre Vieja (CV) volcanic system, including the last 2021 Tajogaite eruption. The seismicity before, during, and after the Tajogaite eruption provides valuable information for a better understanding of deep volcanic processes. We applied the receiver function technique to image the crustal and upper mantle structures beneath the CV volcanic complex. We investigated the first 50 km of the lithosphere, identifying various seismic discontinuities. After a thin layer of recent volcanic rocks, we found a layer corresponding to the old oceanic crust. Beneath the Moho, we found a significant decrease in the seismic velocities until about 37 km of depth. This low‐velocity layer corresponds to a zone of partial melting with interconnected magmatic chambers that feed the volcanic activity in CV. Plain Language Summary: Oceanic volcanic islands have a complex internal structure. Our study is important to understand volcanic activity in its geodynamic context better. The last eruption occurred in the Cumbre Vieja volcanic complex on La Palma Island (2021) and had an unexpected magnitude in terms of lava volume, explosivity, and a significant impact on the economy and society of the island. This study used receiver function analysis to image the lithosphere up to a depth of 50 km. This seismological technique exploits the recordings of teleseismic events (i.e., earthquakes located between 30 and 90° from the stations) for this purpose. We used an advanced inversion technique to study the structure of the crust and the upper mantle beneath the volcano. The most relevant finding is a zone of anomalous low seismic velocities extending from 13 to 37 km depth. This zone is likely to host partial melt and magma chambers and extends over a much wider and thicker volume than expected from previous studies. Previous studies also identified two magma chambers: the upper one, located at about 10–15 km, and the lower one, beneath 25 km depth. These findings provide new insights into the current paradigms about the internal active oceanic volcanic islands. Key Points: Receiver function analysis allowed us to image the crustal and upper mantle seismic structure beneath Cumbre Vieja volcano in La PalmaMain velocity discontinuities in the crust show the contacts between the recent volcanic rocks, the old oceanic crust and the upper mantleA low‐velocity zone extending from the Moho up to about 37 km of depth indicates the presence of a broad zone of partial melting [ABSTRACT FROM AUTHOR]

Abstract The two Large Low Velocity Provinces (LLVPs) in the lowermost Earth mantle are thought to affect large‐scale heat and material transport, governing mantle evolution. LLVPs have been interpreted as thermochemical piles of recycled oceanic crust (ROC) and/or other dense rock types. However, the role of ROC intrinsic viscosity in pile formation and related effects on mantle evolution remain poorly understood. Using mantle convection models, we show that, while ROC intrinsic density controls pile formation, intrinsic viscosity determines whether piles are internally convecting or stratified. Only high‐viscosity, stratified piles can preserve material over several billions of years. Pile stratification is therefore required to reconcile geochemical evidence for the survival of ancient reservoirs. Compositionally layered piles are also consistent with geophysical observations that point to vertical gradients in LLVP properties. As mineral physics constraints point to low‐viscosity ROC, our results suggest that LLVPs may be partly formed by early basal‐magma‐ocean cumulates.

Seismic surveys along subduction zones have identified anomalously high ratio of P‐ to S‐wave velocity (VP/VS) in the subducting oceanic crust that are possibly due to the presence of pore water. Such interpretations postulate that the pore structure is homogeneous at the scale of the seismic wavelength. Here we present the first statistical evidence of a heterogeneous pore structure in oceanic crust at scales larger than laboratory samples. The spatial correlation of measured bulk density profiles of the crustal section of the Samail ophiolite suggests that the pore structure is heterogeneous at scales smaller than ∼1 m. Wave‐induced fluid flow cannot follow the loading during the seismic wave propagation at this estimated heterogeneity, which implies that fluid‐filled microscopic pores and cracks have a limited impact on the observed high VP/VS anomalies in the subducting oceanic crust. Large‐scale cracks may therefore play an important role in shaping these anomalies. Plain Language Summary: Seismic studies along subduction zones have identified unusually high ratios of P‐ to S‐wave velocity (VP/VS) in the subducting oceanic crust, which indicates the presence of water‐filled cracks and pores. The close link between pore water and local seismic activity highlights the importance of quantitatively interpreting these seismic anomalies in terms of pore characteristics. Previous interpretations have assumed that the microscopic pore structure is quite homogeneous, even at macroscopic scales as large as the seismic wavelength. However, our analysis of a bulk density profile of the crustal section of the Samail ophiolite, Oman, which is a fossilized oceanic plate preserved on land, indicates that the pore structure is more heterogeneous than previously assumed. This means that the fluid flow within the unit volume that represents the macroscopic physical properties of the rock cannot follow the wave‐induced loading during seismic wave propagations. This results in a relatively small impact of water on the seismic velocity, as inferred from theoretical models that predict the effective elastic properties of rock containing fluid‐filled cracks. Therefore, microscopic cracks may not have a large impact on the high VP/VS values of subducting oceanic crust, whereas large‐scale cracks may play a more significant role. Key Points: The bulk density of the crustal section of the Samail ophiolite is more spatially heterogeneous than previously assumedThe effect of fluid‐saturated microcracks on low‐frequency seismic velocities is modeled as an unrelaxed condition for this heterogeneityThe high VP/VS anomaly in the subducting oceanic crust can be explained by both microcracks and large‐scale cracks [ABSTRACT FROM AUTHOR]

Although carbonates are the primary form of carbon subducted into the mantle, their fate during recycling is debated. Here we report the first coupled high‐precision Zn and Mo isotope data for Cenozoic intraplate basalts from western China. The exceptionally high δ66Zn values (+0.39 to +0.50‰) of these lavas require involvement of recycled carbonates in the mantle source. Variable δ98Mo compositions (−0.39 to +0.27‰) are positively correlated with Mo/Ce, best interpreted as mixing between isotopically light Mo from dehydrated oceanic crust and heavy Mo from recycled carbonates, which is also supported by positive coupling between δ66Zn and δ98Mo. Modeling reveals that involvement of ≤5% carbonate‐bearing oceanic crust fully resolves the observed δ66Zn–δ98Mo mantle heterogeneity probed by intracontinental basalts. Our study demonstrates that combined δ66Zn–δ98Mo data sets for mantle‐derived magmas can track recycled surficial carbonates in Earth's interior, providing a powerful geochemical tool for deep carbon science. Plain Language Summary: Carbon is an element of life and studying its geological cycle is crucial for understanding Earth's evolution including formation of a life‐supporting atmosphere. Here we report the first combined high‐precision Zn and Mo isotope data for Cenozoic intraplate lavas from western China, showing that the basalts record ≤5% carbonate‐bearing oceanic crust components in their mantle source. Our results provide new evidence for surficial carbonates being delivered into the deep upper mantle, which adds to the debate about the deepest extent of the terrestrial carbon cycle. Key Points: First combined zinc (Zn) and molybdenum (Mo) isotope data for mantle‐derived magmas to track the fate of subducted carbonatesZn−Mo isotopic compositions of Cenozoic Tarim basalts suggest surficial carbonates being delivered into the deep upper mantleWe highlight the utility of combined Zn−Mo isotope data as a powerful tool in deep carbon science [ABSTRACT FROM AUTHOR]

The paradigm of plate tectonics holds that ocean plates are rigid during drift and only experience tectonic deformation at subduction zones, but new findings from the Pacific challenge this idea. Geological and geophysical evidence from the Ontong Java, Shatsky, Hess, and Manihiki oceanic plateaux indicates that extensional deformation during plate drift is a widespread phenomenon across the Pacific plate. These anomalously thick oceanic plateaux are weaker regions of the ocean lithosphere and more prone to tectonic deformation. Numerical geodynamic models demonstrate that a slab pull force from distant subduction plate boundaries can be effectively transmitted to oceanic plateaux through strong ocean lithosphere and cause substantial extension during plate drift. Our findings reveal that a wide expanse of the Pacific has experienced syn‐drift plate tectonics linked to pull from the western Pacific subduction factory. Plain Language Summary: New findings from the Pacific Ocean challenge the conventional understanding of plate tectonics. It was previously believed that oceanic plates remained rigid during plate drift and only experienced deformation at subduction zones. However, geological and geophysical evidence from the Ontong Java, Shatsky, Hess, and Manihiki oceanic plateaux suggests that extensional deformation is a common occurrence during plate drift. These plateaux, which are weaker regions of the ocean lithosphere, are more susceptible to tectonic deformation. Through numerical geodynamic models, we have demonstrated that the slab pull force exerted by distant subduction plate boundaries can effectively cause substantial extension in these oceanic plateaux. This study reveals a significant presence of intra ocean plate deformation associated with the Western Pacific subduction factory. Key Points: Seismic and petrologic data indicate that oceanic plateaux on the Pacific Plate are undergoing extensional deformation during plate driftNumerical modeling finds a slab pull force may be causing the extension as the force is transmitted far away from the subduction zoneOceanic plates can experience substantial tectonic deformation during their drift to subduction [ABSTRACT FROM AUTHOR]

Recent research has shown that shallow tremors tend to occur in spatial association with heterogeneous conditions, which are formed by seamounts or ridges on the subducted oceanic crust. In the southern Kuril Trench, shallow tremors have been observed on the western side, although no apparent heterogeneity in the subducted oceanic crust has been identified. To investigate structures spatially associated with these shallow tremors, we processed reflection seismic data acquired along seven profiles. We examined structures ranging from the seafloor to the top of the oceanic crust, within a depth of up to 20 km. Our results reveal that undulations observed at the top of the subducted oceanic crust are related to the distribution and number of shallow tremors. The undulations would play a crucial role in the distribution of shallow tremors. Plain Language Summary: Shallow tremors, often occurring near subducted seamounts and ridges underground, are small earthquakes that take place at shallower depths of a plate boundary compared to megathrust earthquakes. While the mechanisms behind these two types of earthquakes are not yet fully comprehended, there is an observed correlation, particularly in terms of their locations. At the southern Kuril Trench, shallow tremors have only been detected on the western side, where there aren't any subducted seamounts or ridges. Seismic surveys carried out in the southern Kuril Trench have provided detailed images of the subsurface structures. These images indicate that shallow tremors are more likely to happen in areas where the subducted oceanic crust has pronounced undulations. It's worth noting that only these undulations on the oceanic crust—and not other significant structures—have been linked to the occurrence of shallow tremors. Key Points: Seismic reflection surveys in the Kuril Trench image undulations in the subducted oceanic crust, as well as strong overlying reflectorsAmplitudes and wavelengths of undulations correlate with shallow tremor occurrencesUndulations and the distribution of strong reflections on the plate boundaries shown in previous studies also generally correspond [ABSTRACT FROM AUTHOR]

Marine multichannel and wide‐angle seismic data constrain the distribution of seamounts, sediment cover sequence and crustal structure along a 460 km margin‐parallel transect of the Hikurangi Plateau. Seismic reflection data reveals five seamount up‐to 4.5 km high and 35–75 km wide, with heterogeneous internal velocity structure. Sediment cover decreases south‐to‐north from ∼4.5 km to ∼1–2 km. The Hikurangi Plateau crust (VP 5.5–7.5 km/s) is 11 ± 1 km thick in the south, but thins by 3–4 km further north (∼7–8 km). Gravity models constructed along two seismic lines show the reduction in crustal thickness persists further east, coinciding with a bathymetric scarp. Gravity data suggest the transition in crustal thickness may reflect spatial variability in deformation and lithospheric extension associated with plateau breakup. Variability in the thickness of subducting crust may contribute to differences in megathrust geometry, upper‐plate stress state and high‐rates of contraction and uplift along the southern Hikurangi margin. Plain Language Summary: The thickness of crust arriving at subduction zones exerts a major influence on the configuration, structure, and distribution of stresses at plate boundaries. A region of thickened oceanic crust, the Hikurangi Plateau, is subducting beneath the North Island of New Zealand. To understand the structure of this Plateau, we analyze seismic data along a 460 km long transect, which reveals five large seamounts, and a 3–4 km reduction crustal thickness between the southern and northern areas of the Plateau. Gravity data show this difference is persistent across the plateau and reveal structures suggesting the crust may have thinned when the Hikurangi plateau separated from a second plateau (Manihiki), which is thought to have formed at the same time. We propose that the subduction of thicker crustal along the southern Hikurangi margin may contribute to the shallow angle of the subducting plate, and high rates of uplift and deformation in the overthrusting plate. Key Points: Crustal thickness of the Hikurangi Plateau reduces by ∼30% between the southern (10–11 km) and northern (7–8 km) PlateauCoincident contrasts in plateau fabric may attribute the thickness contrast to rifting from the Manihiki PlateauThickness contrasts may impact the geometry of the subducting plate and rates of upper‐plate compression and uplift along the Hikurangi margin [ABSTRACT FROM AUTHOR]

Abstract Seismic surveys along subduction zones have identified anomalously high ratio of P‐ to S‐wave velocity (VP/VS) in the subducting oceanic crust that are possibly due to the presence of pore water. Such interpretations postulate that the pore structure is homogeneous at the scale of the seismic wavelength. Here we present the first statistical evidence of a heterogeneous pore structure in oceanic crust at scales larger than laboratory samples. The spatial correlation of measured bulk density profiles of the crustal section of the Samail ophiolite suggests that the pore structure is heterogeneous at scales smaller than ∼1 m. Wave‐induced fluid flow cannot follow the loading during the seismic wave propagation at this estimated heterogeneity, which implies that fluid‐filled microscopic pores and cracks have a limited impact on the observed high VP/VS anomalies in the subducting oceanic crust. Large‐scale cracks may therefore play an important role in shaping these anomalies.

Aqueous fluids are essential to extract slab‐trapped carbon at forearc to subarc depths and interpret the decarbonation efficiency of global subduction zones. A large amount of carbon survives beyond subarc depths. The behavior of such carbon, however, remains unclear owing to a lack of experimental studies. Here, we investigate the decarbonation behavior of carbonated oceanic crust containing different amounts of H2O at pressures from subarc depths to the top of the mantle transition zone. We find that calcium‐rich carbonatitic liquids can form at temperatures of ∼950–1,150°C at depths below ∼150–300 km, corresponding to warm/cold thermal regimes in most subduction zones. Therefore, hydrous carbonatitic liquids should be pervasive in subduction zones, while extensive dehydration reactions and fluid activities are critical for creating carbonatitic liquids and preventing most of the surviving carbon from being subducted into the deep Earth. Plain Language Summary: The deep recycling of CO2 and H2O in subduction zones is key for understanding the habitability and climate change of Earth. Here, we use a high‐pressure and high‐temperature instrument, the multi‐anvil press, to investigate the fate of subducted carbon in oceanic crust. We find that H2O can depress the decarbonation temperature of subducted oceanic crust by hundreds of °C. Most of the surviving carbon that descends to depths below ∼150 km may be released into the upper mantle by hydrous carbonatitic liquids, rather than entering the lower mantle. The fate of such carbon has not been well constrained, but it is important to understand this process because CO2 released into the convecting upper mantle is key to constraining the origin of the deep carbon reservoir. Key Points: Effect of H2O on the decarbonation behavior of subducted oceanic crust was investigatedHydrous carbonatitic liquids can form at ∼950–1,150°C at depths below ∼150 kmFluid activity is critical for removing most of the surviving carbon beyond subarc depths [ABSTRACT FROM AUTHOR]

This study constructs a model to represent the strain–stress field surrounding a splay fault and demonstrates the accumulation processes of both strain and stress around the splay fault. We consider the case of multiple fault dislocations, construct the model as a function of the effective viscosity in the media, and investigate the influence of the effective viscosity on the strain and stress accumulation patterns. The results show that the strain and stress tend to accumulate in the splay fault and the subducting oceanic crust, and the rate of accumulation varies with the effective viscosity. The accumulation and relaxation of strain and stress are simultaneous, and the slower the effective viscosity, the slower the accumulation rate. We discuss the relationship between the splay faults, fluid, and intraslab earthquakes. Finally, the possibility that effective viscosity may contribute to the mode of occurrence of intraslab earthquakes at the Hikurangi subduction margin is discussed. Plain Language Summary: We construct a simple model to explain the evolution of strain and stress over time in a subduction margin. The model constructed in this study is the case where a fault branches from a plate boundary. We investigate the influence of the effective viscosity of its constituents on the temporal evolution of strain and stress. The results show that the rate of strain and stress accumulation varies with the effective viscosity. Our result may suggests the influence of the effective viscosity on the source distribution of the microearthquakes observed at the Hikurangi subduction margin. Key Points: Temporal evolution of strain and stress caused by the dislocation of tri‐materials with different effective viscosities is calculatedStrain and stress accumulation rate depends on the effective viscosityStrain and stress accumulated at the multiple fault edge strongly influence the stress perturbations in the subducting oceanic crust [ABSTRACT FROM AUTHOR]

K‐rich adakite‐like rocks (KARs) in post‐collisional settings, such as in Tibet, have been widely linked with the melting of pre‐existing thickened crust. Here, we investigate geochemical data of the Late Eocene (38–34 Ma) volcanic rocks including KARs from the southern Qiangtang terrane (SQT) of central Tibet. The data reveal that: (a) the volcanic rocks define a fractionation trend from high‐K alkaline basalt to high‐K calc‐alkaline rhyolite, with a continuous compositional range and (b) they are characterized by a narrow range of depleted Sr–Nd isotopic compositions relative to the pre‐Eocene SQT crust. We contend that the KARs in the SQT resulted from fractional crystallization of hydrous, alkaline melts derived from the lithospheric mantle where fractionation was dominated by amphibole and plagioclase. Partial melting of the lithospheric mantle beneath the SQT was possibly triggered by thermal perturbations owing to the north‐directed subduction of the Indian continental lithosphere beneath southern Tibet. Plain Language Summary: Adakites were originally defined as silica‐rich magmas produced by melting of basalts in subducting oceanic crust. They are Na‐rich with low K2O and K2O/Na2O, and are characterized by high Sr/Y and La/Yb and low Y and Yb. In post‐collisional settings, such as in Tibet, silica‐rich magmatic rocks with high Sr/Y and La/Yb have been identified, but they are not sodic as true adakites, and are rich in K2O, forming K‐rich adakite‐like rocks (KARs). They have been linked with the melting of pre‐existing crustal rocks at high pressures where garnet is present. We investigate geochemical data of the Late Eocene (38–34 Ma) volcanic rocks that include KARs from the southern Qiangtang terrane (SQT) of central Tibet. We conclude that these KARs were not products of crustal melting at high pressures, but resulted from extreme fractional crystallization of lithospheric mantle‐derived, hydrous, primitive alkaline melts. Given that extreme fractional crystallization commonly occurs in a compressive regime, we argue that the lithospheric mantle underneath the SQT underwent melting during a compressive regime possibly due to thermal perturbations, driven by north‐directed subduction of the Indian continental lithosphere beneath southern Tibet. Key Points: Late Eocene K‐rich adakite‐like rocks (KARs) in the S. Qiangtang terrane resulted from fractional crystallization of a primitive alkaline meltFractionation was dominated by amphibole and plagioclaseKARs from the S. Qiantang terrane have a different origin from those of the Lhasa terrane [ABSTRACT FROM AUTHOR]

Hydrothermal activity on mid‐ocean ridges plays an important role in shaping marine chemistry, yet the variability of hydrothermal venting and its forcing mechanism remain elusive. Here, we analyzed a sediment core obtained near the tectonic‐controlled Tianxiu vent field, Carlsberg Ridge, to reconstruct the hydrothermal venting history. The core documented two significant hydrothermal events (H1 and H2) in the past 30 ka. H1 occurred at 24.1–24.5 ka during the Last Glacial Maximum (LGM), while H2 occurred at 10.5–11.6 ka during the deglacial period. Compared to H2, H1 was relatively weak, and it occurred concurrently with a tectonic event. We suggest that H2 was caused by the increased melt production associated with the decompression melting of the upper mantle during sea‐level fall, which is consistent with previously published records, whereas H1 was likely triggered by an intense tectonic event associated with depressurization during the LGM, which was previously unrecognized. Plain Language Summary: Hydrothermal systems are controlled not only by magmatism but also by tectonism. Previous studies have shown that there is a relationship between the variability of hydrothermal activity and sea‐level changes. Increased melt production during glacial periods has been invoked to explain the enhanced hydrothermal activities during glacial terminations. However, little attention has been paid to the relationship between hydrothermal activity and tectonic events during the glacial–interglacial cycle. Based on the analysis of a sediment core collected near the tectonic‐controlled Tianxiu vent field, Carlsberg Ridge, we reconstructed the hydrothermal venting history and found that enhanced hydrothermal activities not only occurred at 10.5–11.6 ka during the last glacial termination but also occurred concurrently with an intense tectonic event at 24.1–24.5 ka during the Last Glacial Maximum. We infer that this earlier hydrothermal event was induced by the improved permeability of the oceanic crust due to depressurization during the glacial sea‐level fall. This is the first time that the hydrothermal venting history of a tectonic‐controlled hydrothermal system in the Indian Ocean is characterized, and the coupling between hydrothermal and tectonic events associated with sea‐level changes is revealed. Key Points: A 30 ka history of hydrothermal and tectonic activity was reconstructed for the Tianxiu vent fieldThe hydrothermal event at 24.1–24.5 ka was triggered by tectonic events resulting from depressurization during the Last Glacial MaximumThe hydrothermal event at 10.5–11.6 ka was caused by increased decompression melting of the upper mantle during the Last Glacial Maximum [ABSTRACT FROM AUTHOR]

The 1,900‐km‐long Sumatran fault plays an important role in accommodating oblique plate motion between the Indo‐Australian and Sunda plates. Although fault geometry, kinematics, and seismic activity are well‐studied, the onset of dextral motion on the Sumatran fault and uplift of adjacent Barisan Mountains is unclear; this hinders an understanding of the late Cenozoic evolution of Sumatra and the forearc region. In this study, we use low‐temperature thermochronology to measure cooling histories of rocks within and outside of the Sumatran fault. An accelerated exhumation within the fault zone began at ∼2 Ma. The Barisan Mountains may have experienced uplift and associated river incision in the late Miocene to Pliocene. The fault systems in the forearc region were inferred to be kinematically linked to the Andaman Sea and Sunda Strait and accommodated early relative plate convergence, then relocated the strike‐slip component of deformation to the Sumatran fault at ∼2 Ma. Plain Language Summary: The 1,900‐km‐long trench‐parallel Sumatran fault plays an important role in accommodating the oblique plate motion between the Indian Ocean lithosphere and Sunda continental crust. Although fault geometry, kinematics, and seismic activity are well‐studied, the onset of motion on the Sumatran fault and uplift of adjacent Barisan Mountains is unclear, hindering an understanding of the late Cenozoic evolution of Sumatra and forearc region. Fault movement with dip‐slip components affects bedrock exhumation rates of the fault zone, which should be recorded by low‐temperature thermochronologic data. We present new apatite and zircon (U‐Th)/He data collected from the Sumatran fault zone and adjacent Barisan Mountains to examine their exhumation histories, which has previously been unconstrained. We then discuss the driving mechanism based on structural, sedimentary and chronologic observations. Our AHe and ZHe data help to provide timing constraints on the late Cenozoic tectonic and landscape evolution of the Sumatran fault and Barisan Mountains, which in turn can be related to broader plate tectonic setting. Key Points: A Pleistocene accelerated exhumation is identified within the Sumatran fault zoneThe Barisan Mountains may have experienced uplift and rapid exhumation in the late Miocene to PlioceneForearc faults likely accommodated early oblique plate motion and deformation relocated to the Sumatran fault around 2 Ma [ABSTRACT FROM AUTHOR]

Previous works used marine magnetic survey data and interpreted magnetic anomalies related to seaward‐dipping reflectors (SDRs) as oceanic crust or SDR age constraints in the South Atlantic Ocean. However, we advise against using SDR‐related magnetic anomalies to constrain ages because of the mostly low‐dipping geometry causing the overlapping of rocks extruded during periods of different polarities. The data are difficult to interpret correctly because of the SDR compositional heterogeneity, sedimentary intercalations, variations in package thickness with different compositions and magnetic polarities, and the absence of a link between the SDR magmatic position and age. SDR‐related magnetic anomalies are mainly caused by magnetic susceptibility contrasts rather than remanent magnetism. Due to this complexity, the SDR geometry and variable composition make SDR‐related magnetic anomalies challenging to use for age constraints. The Pelotas Basin SDRs age cannot be estimated by magnetostratigraphy, and this method likely cannot constrain the age of any SDR wedge. Plain Language Summary: Since the 1960s, an increased understanding of oceanic crust chronology has quickly evolved into a field of study called magnetostratigraphy. Basaltic lavas extruded in a certain time interval acquire a positive or negative magnetic polarity according to the magnetic polarity on Earth during the extrusion. From the mid‐ocean ridge to the continental oceanic boundary, the continuous and progressive accretion of the oceanic crust provides age constraints for the sea bottom. However, on volcanic passive margins, the presence of volcanic rocks with wedge geometries is abundant. These volcanic rocks are called seaward‐dipping reflectors (SDRs) because of their wedge‐shaped geometry toward the ocean and are usually identified by seismic reflection surveys. The presence of these volcanic wedges makes the identification of the oceanic–continental crustal boundary a challenging task. Some magnetic anomalies that some researchers state as related to a regular vertical oceanic crust are SDR‐related. The SDR‐related magnetic anomalies are not related to that age because SDR‐related magnetic anomalies are mainly caused by magnetic susceptibility contrasts rather than remanent magnetism. We used seismic lines and magnetic data from the Pelotas Basin to illustrate how these magnetic anomalies are not age‐related. Key Points: Some magnetic anomalies interpreted as related to vertical fabric oceanic crust are related to seaward‐dipping reflectors (SDRs)Seaward‐dipping reflectors are different from typical oceanic crust in geometry and compositional heterogeneitySDR‐related magnetic anomalies should not be used to constrain ages [ABSTRACT FROM AUTHOR]

Data from three co‐located wide‐angle ocean‐bottom seismograph experiments were used to make a new two‐dimensional tomographic image of the East Pacific Rise at 9°N. The upper half of the crustal magmatic system has a different seismic structure than the lower half, presumably corresponding with differences in melt storage and crustal cooling. The upper region is narrow (∼4.5 km wide at 2–3 km depth bsf) and estimated to contain higher melt fractions; the lower region broadens with depth and contains lower melt fractions. The shape of the system suggests steeply‐dipping isotherms and relatively stronger hydrothermal circulation in the upper gabbroic section of the crust, with moderately flattening isotherms and less circulation in the lower gabbroic section. Plain Language Summary: Along mid‐ocean ridges two lithospheric plates separate, inducing mantle upwelling and pressure‐release melting. The melt buoyantly rises toward the surface where it enters a crustal magmatic system. Hydrothermal circulation and conductive heat loss cool the magmatic system and the melts solidify to form new oceanic crust. A seismological study is used to determine the size and internal structure of the crustal magmatic system, the amount of melt in the mantle as it enters the crust, and the final destination of the melt recorded by crustal thickness and crustal structure. Key Points: A new seismic tomographic image of the East Pacific Rise reveals the average structure of the sub‐ridge magmatic systemThe upper half of the magmatic system is narrow, melt‐rich, and flanked by steeply‐dipping sub‐solidus isothermsThe lower half has lower melt fractions and widens with depth as isotherms begin to flatten [ABSTRACT FROM AUTHOR]

The extension of back‐arc basins and formation of marginal seas following the subduction of oceanic lithosphere are usually attributed to the rollback of subducting slabs and distorted mantle convection. However, for the Japan Sea, the largest marginal sea in the northwestern Pacific, its opening is unlikely only resulted from the subduction of the Pacific plate because of the coeval Philippine plate subduction and the arcuate arc volcanic zone. Therefore, the present‐day thermal structure in the uppermost mantle, which can be directly constrained by strong Pn‐wave attenuation, plays a vital role in understanding the Japan Sea opening. Here, we observe two belts of strong Pn attenuation beneath the Japan Sea; their strikes are generally consistent with local Pn anisotropy and the retreat directions of the Pacific and Philippine trenches. Hence, there seem to be two divergent mantle flows in the uppermost mantle, pushing a "double door" for the Japan Sea opening. Plain Language Summary: The western Pacific is one of the most active regions of global tectonics. For the Japan Sea, the largest marginal sea in the northwest Pacific, its formation mechanism is still controversial. The Japan Sea may have experienced a complex evolution process driven by the superposition of multiple mechanisms, and finally produced a diamond‐shaped ocean. The present thermal structure in the uppermost mantle, which can be directly constrained by strong Pn‐wave attenuation, plays a vital role in understanding the Japan Sea opening. In this study, we construct a high‐resolution broadband Pn attenuation model for the uppermost mantle beneath the Japan Sea. Two strong Pn attenuation belts are observed in this region, with their strikes generally consistent with the local Pn velocity anisotropy and the opening directions of the Japan Sea. Therefore, two divergent mantle flows likely exist in the uppermost mantle, pushing the opening of the Japan Sea like a "double door." These mantle flows could be part of mantle convection in a big mantle wedge, where ascending hot materials from the deep mantle not only feed volcanoes in northeastern Asia but also thicken the back‐arc oceanic crust. Key Points: A high‐resolution broadband Pn attenuation model is obtained beneath the Japan SeaHot mantle materials ascend to feed the volcanoes in NE Asia, intruding into and thickening the oceanic crust in the Japan SeaDivergent mantle flows likely push the fan‐shaped rotational opening of the Japan Sea [ABSTRACT FROM AUTHOR]

Subducted oceanic crust is enriched in free silica. Although being one of the silica polymorphs at lower‐mantle pressures, niccolite‐type phase (Nt‐phase) has not been documented in multicomponent metabasaltic or metasediment compositions relevant to subducting oceanic crust. Here, we report the formation of an Al‐rich Nt‐phase (∼24.4 to 32.4 wt% Al2O3), coexisting with Al‐depleted bridgmanite (∼6.4 to 7.6 wt% Al2O3), δ‐phase, and iron‐rich phase in model hydrated basalts over the pressure‐temperature range of 84–113 GPa and 1,800–2,200 K. Infrared spectroscopy of a pure synthetic Al‐rich Nt‐phase shows OH bending and stretching vibrations at high pressures, indicative of its hydrous nature. This study suggests that Al‐rich Nt‐phase can serve as a potential water carrier in subducted oceanic crust to the deep lower mantle. Plain Language Summary: Water is transported into Earth's interior via subduction of hydrated lithospheric plates. The water distribution in peridotitic and basaltic portion of subducted hydrated slabs is heterogeneous, which is dependent on the stability of water‐bearing minerals under the imposed pressure‐temperature and local mineralogical conditions. The aim of this study is to provide an understanding regarding the host for water transport within the subducted oceanic crust in the deep lower mantle. To this end, we performed high pressure‐temperature experiments on model hydrated basaltic rocks in a laser‐heated diamond anvil cell. Compared with the formation of CaCl2‐type silica in dry oceanic basalts from previous studies, a new water‐bearing Al‐rich niccolite‐type silica has been discovered in the present study. Our results imply that Al‐rich niccolite‐type silica can potentially carry water to the deep lower mantle in subducted oceanic crust. Key Points: Chemical reactions between water and a model basalt were investigated in laser‐heated diamond anvil cells under lower mantle conditionsA new Al‐rich niccolite‐type silica containing ∼24.4 to 32.4 wt% Al2O3 was identified, coexisting with Al‐depleted bridgmanite, δ‐phase, and iron‐rich phaseInfrared spectroscopy showed that Al‐rich niccolite‐type silica contains structural water [ABSTRACT FROM AUTHOR]

Here we report the first set of thallium (Tl) isotope data in alkaline rocks from the North China Craton to constrain the nature of recycled materials in the metasomatized subcontinental lithospheric mantle. Samples from the Hekanzi and Saima alkaline complexes display Tl isotope compositions (ε205Tl) identical to the present‐day upper mantle and continental crust, suggesting that neither the recycled low‐temperature altered oceanic crust nor the pelagic sediments were involved in their sources. The volcanic rocks from the Liaodong‐Jinan region, whose source was previously proposed to contain recycled low‐temperature altered oceanic crust, also display modern mantle‐like Tl isotope composition, suggesting a significant Tl‐loss in the recycled oceanic crust during the subduction due to the dehydration process. Our Tl isotope data provide complementary constraints to the Sr‐Nd‐Hf‐O isotopes on the nature of metasomatic agent in the subcontinental lithospheric mantle. Plain Language Summary: Recycling of crustal materials into Earth's mantle by subduction causes mantle heterogeneities. The sources of some alkaline rocks contain recycled crustal materials. Pelagic sediments, low‐temperature altered oceanic crust, and continental crust have sharp differences in thallium isotope compositions. Combined with Sr‐Nd‐Hf‐O isotopes, whole‐rock Tl isotopes place a better constraint on the metasomatic sources. Key Points: Thallium isotope compositions of the alkaline rocks were analyzed to track crustal recycling in the subcontinental lithospheric mantleThallium isotope suggested that low‐temperature altered oceanic crust and pelagic sediments were not involved in the investigated sourcesThe O‐Nd‐Hf‐Tl isotope data for the Liaodong‐Jinan volcanics reveal a significant Tl‐loss in the subducted slab during dehydration process [ABSTRACT FROM AUTHOR]

The compositional similarity between high‐Mg andesite‐dacite from accretionary orogens and bulk continental crust (CC) provides an opportunity to unravel the CC formation paradox. Compositional data from a global compilation of Quaternary magmatic arcs indicate the presence of low‐Mg series (LMS) and high‐Mg series (HMS). The LMS show trends of crystal fractionation and can be subdivided into high Ba/Th and high La/Sm groups, which likely originate from fluid‐ and sediment melt‐modified mantle wedge, respectively. In contrast, the HMS have variably mixed compositions (e.g., high Mg#, Ba/Th, and La/Sm) and can be explained by partial melting of mélange diapirs rising into the mantle wedge, which are mixtures of subducted sediment, eroded arc crust or CC, buoyant oceanic crust, and peridotite. We, therefore, propose a two‐step process for creating CC involving extraction of LMS from the mantle followed by re‐melting of recycled LMS in the mantle to generate HMS and thus CC. Plain Language Summary: Continental crust (CC) is extracted from the mantle primarily by subduction‐related magmatism in accretionary orogens. However, igneous rocks sourced from the mantle have low SiO2 (basalt), whereas average CC has high SiO2 (andesite) and high MgO. To resolve this paradox, we analyzed Quaternary subduction‐related igneous rocks worldwide and find that they can be divided into low‐Mg series (LMS) and high‐Mg series (HMS). We compare existing compositional data for LMS and HMS and find that LMS are dominated by basalt whereas HMS are dominated by andesite and similar to the average CC. Petrogenetic analyses reveal that LMS likely originate from the mantle and represent primitive CC, whereas HMS likely originate from recycled crustal materials in the mantle and represent mature CC. We propose a two‐step process for creating CC, involving extraction of LMS from the mantle followed by re‐melting of recycled LMS to generate HMS and thus CC. Our results provide a new solution to the CC formation paradox. Key Points: Low‐Mg series (LMS) arc igneous rocks originate from fluid‐/sediment melt‐modified mantle wedge, followed by crystal fractionationHigh‐Mg series (HMS) arc igneous rocks likely originate from recycled crustal materials as mélange diapirs rising into the mantle wedgeContinental crust is formed by extracting LMS from the mantle followed by re‐melting of recycled LMS in the mantle wedge to generate HMS [ABSTRACT FROM AUTHOR]

Geophysical surveys of the oceanic crust indicate that hydrothermal circulation universally occurs in seismic layer 2, which results in a low seismic velocity and high VP/VS due to the occurrence of cracks. However, the anomalously low VP/VS observed at the layer 2/3 transition cannot be explained by the crack model, because the effective medium theory predicts an increase in VP/VS due to crack development. In this study, we present the first evidence that shows the low VP/VS in the oceanic crust is caused by epidotization due to upward fluid flow in hydrothermal systems. Simultaneous measurements of elastic wave velocity and porosity of epidosites collected by the Oman Drilling Project show that quartz precipitation and spheroidal pores results in low VP/VS, in contrast to diabases that contain thin cracks. The presence of spheroidal pores in epidosites is supported by CT imaging, and is consistent with predictions from the effective medium model. Plain Language Summary: The low velocity and high VP/VS structure in the upper oceanic crust ("seismic layer 2") is generally associated with the development of fluid‐filled thin cracks. However, the anomalously low VP/VS ratios that have been observed in the layer 2/3 transition cannot be explained by the classical thin crack model. We provide the first direct evidence of low VP/VS ratios in epidotized rocks that are associated with the occurrence of spheroidal pores and quartz precipitation. Our theoretical calculations show that both the crack geometry and alteration minerals are required to explain the anomalously low VP/VS ratios in the transition zone. Knowledge of these unique physical properties of epidotized rocks will facilitate the detection of fluid upwelling zones in the oceanic crust, which are possibly linked to the hydrothermal ore resources. Key Points: Epidosites are characterized by low VP/VS due to the presence of spheroidal poresLow VP/VS anomaly in the layer 2/3 transition can be explained by alterations and pore development during epidotizationThe unique characteristics of epidosites may be useful to detect the fluid upwelling zones in the oceanic crust [ABSTRACT FROM AUTHOR]

The Nazca Ridge (NR) was formed near the interaction of a hotspot mantle plume and an active spreading center. We use active‐source wide‐angle seismic data to obtain 2‐D Vp and Vs tomographic models, and hence the Poisson's ratio (ν) structure beneath the NR. Results show a ∼2 km thick seismic layer 2A with ν values of 0.25–0.32 in the uppermost crust interpreted as pillow basalts with a low degree of fracturing and/or hydrothermal alteration. The 2A/B boundary layer presents ν values of 0.27–0.29 consistent with pillow basalts/sheeted dykes units. A ∼3 km layer 2B overlies a ∼10 km layer 3 with ν values of 0.24–0.3 at the 2/3 boundary layer. The lowermost layer 3 presents ν values of 0.28 ± 0.02 suggesting an increase in Mg content (≥10% wt). The NR crust (∼15 km thick) requires an increment of the asthenospheric mantle potential temperature in ∼100°C formed by passive adiabatic decompression melting. Plain Language Summary: The interaction of a hot mantle plume with an overlying moving oceanic plate results in hotspot volcanic activity, and the formation of large volcanoes (hotspot tracks). This process is enhanced in a setting where the hotspot mantle plume is located near an active spreading center or mid ocean ridge such as the East Pacific Rise or Middle Atlantic Ridge. The Nazca Ridge (NR) is a hotspot track located off South America at present and was formed 30–45 Ma by the interaction of the Easter Island hotspot mantle plume with the Farallon‐Nazca plate near the East Pacific Rise. Our geophysical results show that the NR presents an anomalously thick oceanic crust (∼15 km thick), which is about 2.5 times thicker than normal crust of the Pacific Ocean. The rocks at the base of the crust presents likely an increase in magnesium content. This is consistent with the intrusion of voluminous magmatic material during the interaction of the hotspot plume with the oceanic plate at anomalously high temperatures compared with the surrounding mantle. Key Points: The seismic P‐ and S‐wave structures from wide‐angle seismic data yield a Poisson's ratio model beneath the Nazca Ridge (NR) prior to subductionThe NR presents a lower crust with Poisson's ratios of 0.28 ± 0.02 consistent with a high concentration of MgThe thick NR crust (∼15 km) is consistent with an elevation of the asthenospheric mantle potential temperature in ∼100°C [ABSTRACT FROM AUTHOR]

The rate of ocean‐crust production exerts control over sea level, mantle heat loss, and climate. Different strategies to account for incomplete seafloor preservation have led to differing conclusions about how much production rates have changed since the Cretaceous, if at all. We construct a new global synthesis of crust production along 18 mid‐ocean ridges for the past 19 Myr at high temporal resolution. We find that the global production rate during 6–5 Ma was only 69%–75% of the 16–15 Ma interval. The reduction in crust production is mostly due to slower seafloor spreading along almost all ridge systems. While the total ridge length has varied little since 19 Ma, some fast‐spreading ridges have grown shorter and slow‐spreading ridges grown longer, amplifying the spreading‐rate changes. Our production curves represent a new data set for investigating the forces driving plate motions and the role of tectonic degassing on climate. Plain Language Summary: The question of whether the speeds of tectonic plates vary over time is controversial but has big‐picture implications for our understanding of the forces inside the Earth that drive the plates, the role of volcanoes in controlling climate change over millions of years, and the rise and fall of sea level. At mid‐ocean ridges, two plates move apart, and the volcanic rocks that comprise the ocean crust are created. Magnetic minerals in the rocks record their age of formation and therefore the relative speeds of the diverging plates. However, this record is incomplete because seafloor is destroyed at subduction zones. We used the preserved seafloor magnetic record to calculate diverging plate speeds over the past 19 million years. We find that the relative plate speed at almost all divergent plate boundaries has slowed down, with a major inflection point at 15–16 Myr. As a result, the rate at which new ocean crust is created also slowed down, by roughly 35%. We speculate that there is not a single explanation for the nearly global slowdown in plate speeds but rather several unrelated tectonic events, such as the emergence of the Andes. Key Points: A new global synthesis of seafloor‐spreading rates at high temporal resolution is presentedSince 15 Ma, spreading rate decreased along 15 of 18 major ridge systems, with a total global reduction of 37%High‐resolution reconstructions, new data for eastern Pacific, and astronomical ages allow more detail than previous studies [ABSTRACT FROM AUTHOR]

Permeability controls energy and matter fluxes in deep‐sea hydrothermal systems fueling a 'deep biosphere' of microorganisms. Here, we indirectly measure changes in sub‐seafloor crustal permeability, based on the tidal response of high‐temperature hydrothermal vents at the East Pacific Rise 9°50'N preceding the last phase of volcanic eruptions during 2005–2006. Ten months before the last phase of the eruptions, permeability decreased, first rapidly, and then steadily as the stress built up, until hydrothermal flow stopped altogether ∼2 weeks prior to the January 2006 eruption phase. This trend was interrupted by abrupt permeability increases, attributable to dike injection during last phase of the eruptions, which released crustal stress, allowing hydrothermal flow to resume. These observations and models suggest that abrupt changes in crustal permeability caused by magmatic intrusion and volcanic eruption can control first‐order hydrothermal circulation processes. This methodology has the potential to aid eruption forecasting along the global mid‐ocean ridge network. Plain Language Summary: Permeability governs how fluids such as water and melt flow beneath the seafloor, and is influenced by diverse magmatic, volcanic, and tectonic forces as new oceanic crust is formed and altered over time. Despite being a master variable controlling the fluxes of energy and matter in deep‐sea hydrothermal systems, permeability remains a poorly constrained hydrologic parameter of the oceanic crust. Here, we indirectly measure changes in crustal permeability for the first time, using the tidal response of high‐temperature hydrothermal vents on the East Pacific Rise at 9°50'N preceding the last phase of volcanic eruptions in 2005–2006. We show that abrupt changes in crustal permeability caused by magmatic intrusion and volcanic eruption can control first‐order hydrothermal circulation processes. Key Points: Hydrothermal vent fluid temperature data modeled with ocean tides reveal permeability changes associated with seafloor volcanic eruptionsChanges in crustal permeability caused by magmatic intrusion and volcanic eruption control first‐order hydrothermal circulation processesThis methodology expands the potential of forecasting seafloor eruptions along the global mid‐ocean ridge network [ABSTRACT FROM AUTHOR]

The density structure of the cratonic lithospheric mantle (CLM) remains debated. We suggest that one important reason for which many geodynamic studies favor neutrally buoyant CLM is that they adopted separate reference frames when estimating the isostatic effects of continental and oceanic lithosphere, while instead a globally consistent one should be used. This reflects a misconception that continental crust perfectly balances the surrounding oceanic lithosphere. Using a unified global reference frame with recent constraints on crustal properties and residual topography, we show that assuming neutrally buoyant CLM leads to prominent negative residual topography (∼−1.3 km) and positive residual gravity (∼354 mGal) within cratons relative to oceans, neither of which can be explained by the effects of the convecting mantle. This requires the CLM, especially that with thick keels, to be less compositionally buoyant and denser than previously thought, a conclusion supporting recent observations on CLM deformation. Plain Language Summary: The density of mantle lithosphere beneath ancient crusts is traditionally thought to be the same as the ambient mantle, an assumption challenged by recent observations. We find that these debates resulted from an assumed force balance between continental crust and surrounding oceanic plates, which results in the topography of continents and oceans being evaluated against different reference levels. In reality, this assumption may not hold due to the indefinite density structure of the mantle below the continental crust. Using a globally consistent reference frame, we show that the observed continental topography is too low if supported entirely by crustal buoyancy, an inference further supported by gravity. This requires the continental mantle lithosphere to be less compositionally buoyant and denser than previously thought, a conclusion that supports recent observations of lithosphere deformation beneath ancient crusts. Key Points: The tectosphere hypothesis was challenged by recent observationsNeither topography nor gravity over cratons could be explained by pure crustal isostasyCratonic lithospheric mantle is 0.5%–1.2% denser than the ambient asthenosphere [ABSTRACT FROM AUTHOR]

Oceanic transform faults are inferred to be weak relative to surrounding oceanic crust and primarily slip aseismically. Neither their weakness nor tendency to creep are well‐explained. We test the effects of fault‐rock evolution on oceanic transform fault frictional strength and stability using direct‐shear experiments (at room temperature, 10 MPa normal stress, and fluid‐saturated conditions) on dolerite from the East Pacific Rise and natural fault rocks from the exhumed Southern Troodos Transform, Cyprus. Dolerites and cemented breccias are frictionally strong (μ = 0.52–0.85) and velocity‐weakening (strength decreases with increasing slip velocity, characteristic of earthquakes). In contrast, matrix‐rich chlorite‐bearing fault breccias and gouges are frictionally weak (μ = 0.25–0.48) and velocity‐strengthening (characteristic of stable creep). This transition implies that seismic behavior is controlled by degree of damage and alteration, such that earthquakes can nucleate within relatively intact oceanic crust, whereas fault segments of increased damage and chlorite content tend to slip aseismically. Plain Language Summary: Oceanic transform faults are plate boundary faults where motion of oceanic lithosphere is dominantly horizontal and parallel to tectonic motion. Fewer and smaller earthquakes than expected occur along these faults, which are also considered weak structures. The reasons for their weakness and lack of large earthquakes are puzzling. To understand these characteristics, we conducted laboratory deformation experiments using rocks collected from the ocean floor (near Hess Deep in the Pacific) and from an ancient transform fault (in Cyprus). We sheared cylindrical samples, holding one half in place and sliding the other over it, creating laboratory equivalents of geological faults. We find that dolerite, one of the primary rock‐types of the oceanic crust, is strong and capable of starting earthquakes. In contrast, we find that already damaged and altered rocks, found within natural faults (containing an increased proportion of the mineral chlorite), are weak. The same damage and alteration responsible for the weakness also prevents earthquakes. Our findings suggest that variations in the size and number of earthquakes on oceanic transform faults is controlled most of all by how damaged the existing rock is and how much alteration to weak, water‐bearing secondary minerals such as chlorite has occurred along fault planes. Key Points: Dolerite hydrothermally cemented at greenschist facies is strong and velocity‐weakening at 10 MPa normal stress and room temperatureNatural fault rocks comprising chlorite‐rich, fractured, and comminuted metadolerite are frictionally weak and velocity‐strengtheningAlong oceanic transform faults, in the mafic oceanic crust, increased damage and alteration can explain fault weakness and aseismic slip [ABSTRACT FROM AUTHOR]

The evolution of the western Kunlun‐Pamir region involved the opening and closing of several branches of the Paleo‐Tethys Ocean, although the specific timing of these events is poorly constrained. Here, we present a synthesis of sedimentary, magmatic, and metamorphic records associated from the Mazar‐Kangxiwa suture zone in the western Kunlun‐Pamir that is generally regarded as the main Paleo‐Tethys Ocean suture. These data show that the Paleo‐Tethyan oceanic basin opened at ca. 340 Ma and closed by ca. 250 Ma, and there is no record of a magmatic arc between ca. 300–250 Ma. The absence of a magmatic arc was a result of oceanic crust underthrusting, rather than oceanic subduction, which is consistent with a narrow back‐arc basin. Our study provides an important example of how an oceanic basin opened and closed without oceanic subduction, and highlights a potential mechanism to account for the absence of a magmatic arc. Plain Language Summary: The closure of oceanic basins is generally believed to result from oceanic subduction associated with development of arc magmatism. However, there are no magmatic arcs associated with the closure of some oceanic basins in the Paleo‐Tethys realm. In order to understand the detailed history of ocean basins in the Paleo‐Tethys oceans, we carry out an integrated in situ analysis of zircon U‐Pb age and Hf‐O isotopes, along with whole‐rock geochemistry for sedimentary, magmatic, and metamorphic records from the Mazar‐Kangxiwa suture zone in the western Kunlun‐Pamir that is widely regarded as the main Paleo‐Tethys Ocean suture. Our data reveals that there is no record of a magmatic arc associated with Paleo‐Tethys Ocean closure during the Permian (ca. 300–250 Ma). This Paleo‐Tethyan oceanic basin was closed by oceanic crust underthrusting, rather than oceanic subduction. Thus, not all Paleo‐Tethyan oceanic basins were closed by subduction with development of arc magmatism. We propose that widespread oceanic crust underthrusting accounts for the common absence of a magmatic arcs in oceanic basins of the Paleo‐Tethys realm. Key Points: There is no record of a magmatic arc during oceanic basin closure in the Mazar‐Kangxiwa suture zone between ca. 300–250 MaThis Paleo‐Tethyan oceanic basin opened at ca. 340 Ma and closed by ca. 250 MaOceanic crust underthrusting was a potential mechanism to account for oceanic basin closure and the absence of a magmatic arc [ABSTRACT FROM AUTHOR]

Fractures and faults in igneous oceanic crust affect hydrothermal circulation and hydration of the oceanic plates. Seismic observations have provided information about lithospheric fabrics, but direct measurements of oceanic crust's elastic properties were not made. We report determinations of compressibility of the upper igneous crust (of 3.6 Ma) of the Juan de Fuca plate, using observed formation fluid pressure oscillations in sealed boreholes associated with passing surface waves from distant earthquakes. We determine an azimuthal variation of formation‐matrix compressibility by a factor of ∼5, with the crust being most compressible in the plate‐spreading direction across the structural fabric inherited from crustal creation. This is equivalent to a seismic compressional wave speed anisotropy of ∼50%–60%, much greater than that of standard seismic measurements (typically <20%). This likely reflects a previously unresolved degree of fracturing of the uppermost oceanic crust, consistent with existing observations that suggest a high degree of hydraulic permeability anisotropy. Plain Language Summary: Hydrothermal circulation in the oceanic crust plays fundamental roles in the exchange of water, heat, and chemical constituents between the oceanic hydrosphere and lithosphere, in the hydration of the crust and upper mantle, and in the support of the subseafloor microbial biosphere. Water is carried primarily in faults and fractures created at the time of seafloor spreading, and thus constraints on their distribution and orientation are important. Such information has been provided by active‐source seismic surveys, and results have shown that compressional wave speeds in the directions of spreading—thus across local faults and fractures—are typically <20% lower than those along the structural fabric. Here, we report more direct determinations of the anisotropy at the 3.6‐Ma Juan de Fuca plate, using formation fluid pressure variations in sealed subseafloor boreholes caused by seismic surface waves from distant earthquakes. These observations are sensitive to formation compressibility of the upper couple of hundreds of meters of the igneous crust. The observed azimuthal variation in fluid pressurization reveals a contrast in compressibility across versus along the structural trend of roughly a factor of five, and a much higher degree of seismic anisotropy (>50%) than that determined from standard seismic velocity measurements. Key Points: Offshore borehole monitoring shows formation fluid pressure variations caused by Rayleigh waves from distant large earthquakesPressure records reveal highly anisotropic oceanic crust, with the formation being most compressible in the plate‐spreading directionThe anisotropy surpasses values determined from standard seismic measurements and implies denser fracturing of uppermost oceanic crust [ABSTRACT FROM AUTHOR]

Phosphorus is an essential element for life, and the phosphorous cycle is widely believed to be a key factor limiting the extent of Earth's biosphere and its impact on remotely detectable features of Earth's atmospheric chemistry. Continental weathering is conventionally considered to be the only source of bioavailable phosphorus to the marine biosphere, with submarine hydrothermal processes acting as a phosphorus sink. Here, we use a novel 29Si tracer technique to demonstrate that alteration of submarine basalt under anoxic conditions leads to significant soluble phosphorus release, with an estimated ratio between phosphorus release and CO2 consumption (∑PO43−/∑CO2) of 3.99 ± 1.03 µmol mmol−1. This ratio is comparable to that of modern rivers, suggesting that submarine weathering under anoxic conditions is potentially a significant source of bioavailable phosphorus to planetary oceans and that volatile‐rich Earth‐like planets lacking exposed continents could develop robust biospheres capable of sustaining remotely detectable atmospheric biosignatures. Plain Language Summary: It is conventionally thought that continents above sea level are required in order for habitable planets to support a robust biosphere. We use experimental geochemistry and a simple model of biological cycling to show that this is incorrect, significantly expanding the possible range of planets that may host surface biospheres that would be detectable through telescope observations. Key Points: Continental weathering is conventionally considered to be the only phosphorus source to planetary biospheresWe experimentally demonstrate that weathering of ocean crust under anoxic conditions releases significant amounts of bioavailable phosphorusHabitable planets (including the earliest Earth) without subaerial continents may support significant biogenic gas fluxes to the atmosphere [ABSTRACT FROM AUTHOR]

The element mercury (Hg) can develop large mass‐independent fractionation (MIF) (Δ199Hg) due to photo‐chemical reactions at Earth's surface. This results in globally negative Δ199Hg for terrestrial sub‐aerially‐derived materials and positive Δ199Hg for sub‐aqueously‐derived marine sediments. The mantle composition least affected by crustal recycling is estimated from high‐3He/4He lavas from Samoa and Iceland, providing an average of Δ199Hg = 0.00 ± 0.10, Δ201Hg = −0.02 ± 0.0.09, δ202Hg = −1.7 ± 1.2; 2SD, N = 11. By comparison, a HIMU‐type lava from Tubuai exhibits positive Δ199Hg, consistent with altered oceanic crust in its mantle source. A Samoan (EM2) lava has negative Δ199Hg reflecting incorporation of continental crust materials into its source. Three Pitcairn lavas exhibit positive Δ199Hg which correlate with 87Sr/86Sr, consistent with variable proportions of continental (low Δ199Hg and high 87Sr/86Sr) and oceanic (high Δ199Hg and low 87Sr/86Sr) crustal material in their mantle sources. These observations indicate that MIF signatures offer a powerful tool for examining atmosphere‐deep Earth interactions. Plain Language Summary: While Earth's mantle is continuously chemically and isotopically stirred by convection, some ocean island lavas preserve isotopic anomalies. Their most likely origin is the recycling of crustal material into Earth's mantle by subduction. A question is then whether these crustal materials originate from the ocean or the continents. By using mercury stable isotopic compositions, which have specific signatures in ocean and continent materials, we identify whether these anomalies are due to continental or oceanic crustal material in various ocean island basalts. Key Points: The Hg isotopic composition of the primitive mantle was determined by analyzing lavas from the Samoa and Iceland hotspotsKey samples from the canonical mantle end member were analyzed to track crustal recycling in the mantleWe demonstrate the presence of recycled oceanic and continental materials in the source of ocean island basalts [ABSTRACT FROM AUTHOR]

Oceanic plateau crust is thicker and hotter than the surrounding "normal" oceanic crust, causing differential subsidence and subsequent strain between the two domains. Here, we show that recently active sedimentary faults that have been imaged adjacent to the Rio Grande Rise, an oceanic plateau in the western South Atlantic, have extensions that can penetrate at least ∼1.5 km into the upper oceanic crust. Our high‐resolution seismic velocity model indicates that these fault zones significantly reduce seismic velocities and increase inferred porosity and permeability. We also present physical property‐constrained fluid flow models that predict substantially increased fluxes along these pathways compared to crust not affected by faulting. As these phenomena are effectively enabled by the proximity of an oceanic plateau, we suggest that similar geologic settings globally may also exhibit elevated levels of tectonic and hydrothermal activity, with implications for the hydration of mature oceanic crust and the oceanic crustal biosphere. Plain Language Summary: Oceanic crust slowly sinks as it cools after its formation at midocean spreading centers, but it does so more slowly than usual at oceanic plateaus, where anomalous magmatism has left it relatively thick and buoyant. The different rates of sinking can induce faults that rupture the transitionary crust between the plateau and the surrounding oceanic crust. The Rio Grande Rise, located in the western South Atlantic Ocean, is an oceanic plateau at whose fringes recently active faults have been imaged in the sediments that cover the crust. We trace those faults at least 1.5 km deep into the crust and provide additional evidence for recent fault activity. We also show that fluids likely flow along those pathways, spilling into the ocean in much larger volumes than in normal oceanic crust. Faulting and associated fluid flow is likely also very active in crust bordering other oceanic plateaus globally, affecting the composition of the crust and the habitat of living organisms. We estimate the affected area to make up close to 10% of all oceanic crust on Earth. Key Points: Recently, active extensional faults adjacent to the Rio Grande Rise penetrate at least ∼1.5 km into the crustThe imaged faults significantly increase fluid circulation within the upper crust and fluid exchange with the oceanCrust adjacent to oceanic plateaus and other major bathymetric features may release an estimated 43 km3 of fluid into the oceans globally [ABSTRACT FROM AUTHOR]

Based on the C‐14 data of Druffel et al. (2021, https://doi.org/10.1029/2021gl092904) along the Eastern Pacific Rise, dissolved organic carbon (DOC) in the deep ocean is old and thus refractory. Their data in combination with previous He‐3 data indicate that the source of this aged DOC is from the hot waters emanating from hydrothermal vents along the ridge axis. The isotopic and structural composition of the source DOC is unknown, which requires a concerted effort by the marine organic chemistry community to elucidate these chemical forms. Plain Language Summary: The age of the dissolved organic carbon (DOC) in the deep ocean has been ascertained by the C‐14 radioisotope, and it is older than 6,000 years. The only significant source that can account for this is seawater passing through the ocean crust and spewing from hydrothermal vents. However, the chemical composition of this refractory or difficult to degrade DOC is unknown. Key Points: Hydrothermal vent fields are a source of old and refractory dissolved organic carbon (DOC) to the deep oceanThere is scant information on the concentration and chemical structures of DOC from ventsRadiocarbon data are needed on DOC emanating from the hot waters of hydrothermal vents [ABSTRACT FROM AUTHOR]

At non‐volcanic passive continental margins, seismic techniques often failed to uniquely define the nature of crustal domains. Here, we overcome this problem by studying the structure and composition of the continent‐ocean transition (COT) in the Southwest Sub‐basin of the South China Sea, using P and S wave seismic tomography and Vp/Vs ratios, providing unique constraints on lithology. Throughout the image domain, we can rule out large areas of exhumed mantle as Vp/Vs ratios are always <1.9 in the shallow basement layer. Instead, the COT is characterized by extended and fragmented continental crust, and possibly mafic aggregation at the bottom of the crust. In concert with observations from multichannel seismic reflection data, seismic velocities and Vp/Vs ratios suggest that the oldest oceanic crust was formed by starved magmatism, causing rugged basement, thin crust, nearly absent lower crust, and moderately serpentinized mantle below. Our results reveal that rifting occurred without un‐roofing continental mantle. Plain Language Summary: Traditional P wave velocity is not able to reveal the detailed rock types in the crust or lithosphere, which are crucial for analyzing the structures and geodynamics in non‐volcanic continental margins. In this study, we choose the South China Sea as a case study area, using an effective method to image the Vp/Vs ratio, unraveling the rock type distribution of basalt, gabbro/diabase, serpentinized peridotite in our model from top to bottom. We analyzed the variation of the crustal nature and composition from the continent‐ocean transition zone (COT) to the oceanic basin, revealing that the COT is characterized by magmatic intruded continental crust, while the oldest oceanic domain shows a thin oceanic crust overlying mantle that experienced hydration. Our study provides an integrated seismic study which may become a paradigmatic case study to investigate the blurred area of the COT. Key Points: Vp/Vs ratio obtained by seismic tomography from OBS data is a sensitive tool to constrain lithologies in oceanic basinsLow Vp/Vs ratios in the South China Sea rule out the occurrence of a wide continent‐ocean transition zone of exhumed mantleA magmatic‐affected continent‐ocean transition zone transferring to a thin oceanic domain was revealed [ABSTRACT FROM AUTHOR]

Evolution of the Arctic Ocean lithosphere has involved multiple stages of opening with crustal stretching and thinning prior to lithospheric breakup. Mapping lateral variations in lithospheric rheology can help unravel the detailed tectonic history of the Arctic. Here we perform a wavelet analysis of gravity and bathymetry data to map the effective elastic thickness (Te) of the lithosphere in the Arctic. The low overall Te suggests that large shear stresses and serpentinization weakened the lithosphere at Arctic passive margins during a multistage opening process. Moderately low Te values along the Gakkel Ridge imply a relatively cold ultraslow‐spreading center compared to typical mid‐ocean ridges. Plain Language Summary: Geological exploration and sampling in the Arctic Ocean are difficult due to sea ice cover and frigid weather. Gravity and bathymetry data obtained using remote sensing methods can be used to estimate the effective elastic thickness (Te) of the lithosphere, which is a proxy for the strength of the tectonic plates. We draw a new Te map of the Arctic Ocean by combining the latest data with recent crustal structure models. Te is low on the edge of the oceanic basins, suggesting that the lithosphere was weakened by mechanical rifting and mantle metamorphism in the opening of the Arctic Ocean. The seafloor spreading centers are generally thought to be hot and weak, but the Gakkel Ridge shows moderately low Te, revealing unique mechanical and geothermal properties of this slowest spreading center in the world. Key Points: The Arctic effective elastic thickness (Te) map is obtained by spectral analysisSedimentary correction for vertical density variation improves the Te estimationPre‐breakup lithospheric extension caused low Te along passive margins [ABSTRACT FROM AUTHOR]

During the last two decades, quasi‐periodic long‐term slow slip events (SSEs) of magnitude up to Mw7.5 have been observed about every 4 years in the Guerrero Seismic Gap, Mexico. We present numerical simulations of the long‐term SSE cycles along the 3D slab geometry of central Mexico. Our model accounts for the hydrated oceanic crust in the framework of rate‐and‐state friction and captures the major source characteristics of the long‐term SSEs occurring between 2001 and 2014, as inferred from geodetic observations. Synthetic surface deformation calculated from simulated fault slip is in good agreement with the cumulative GPS displacements. Our results suggest that the flat‐slab segment of the Cocos plate aids the large magnitudes and long recurrence interval of the long‐term SSEs. We conclude that 3D slab geometry is an important factor in improving our understanding of the physics of slow slip events. Plain Language Summary: Slow slip events (so‐called "silent earthquakes") have been detected worldwide in circum‐Pacific subduction zones, such as Cascadia and southwest Japan. Long‐term slow slip events occur about every 4 years in the Guerrero Seismic Gap (Mexico) where tectonic plate movement is largely accommodated by aseismic slip and no large earthquakes have been observed since 1911. We build a numerical model incorporating a realistic 3D geometry of the subducting slab and lab‐derived friction laws to investigate the physics of these slow slip events. The simulated events have slip patterns, magnitudes, and recurrence intervals comparable with the observed ones. Our study demonstrates that plate geometry is an important factor to account for when studying the initiation, propagation and arrest of slow slip. Key Points: We model sequences of long‐term slow slip events in the Guerrero Seismic Gap using a geometrically flexible three‐dimensional (3D) boundary integral methodOur model reproduces the source characteristics and surface deformation of the four long‐term slow slip events (SSEs) inferred from geodetic observationsThe flat segment of the Cocos plate likely aids the large magnitudes and long recurrence interval of the slow slip events in Guerrero [ABSTRACT FROM AUTHOR]

Global variations in geomagnetic intensity can provide dating markers for sedimentary records, which is particularly helpful in precisely reconstructing climate changes. However, there are few continuous relative paleointensity (RPI) records derived from sediments spanning the past 11 Myr, and those that exist generally bear a high degree of uncertainty. Therefore, independent evidence from other continuous paleointensity records is needed to verify the existing information. Here, we analyzed magnetic anomaly profiles from the East Pacific Rise and the Southeast Indian Ridge and discovered correlative tiny wiggles in the stacked profiles from three different areas. A comparison with a previous magnetic study on the South Atlantic Ridge and the synthetic profile from RPI records generated for the past ~8 Myr confirmed that the small wavelength anomalies result from global geomagnetic intensity fluctuations. The newly calibrated timescales from selected marine magnetic anomalies could provide a potential dating reference for sedimentary strata. Plain Language Summary Ocean crust at the mid-ocean ridge acquires a thermoremanent magnetization proportional to the intensity and parallel to the geomagnetic field when it was formed. Therefore, the marine magnetic anomaly profiles over the ocean crust provide an independent method to reconstruct past intensity variations of Earth's dipole. At present, there are few continuous records of geomagnetic intensity spanning longer time intervals. Here, we analyzed marine magnetic anomaly profiles from the ocean crust in the East Pacific Rise and the Southeast Indian Ridge over the past 11 Myr, and discovered coherent short-wavelength anomalies in the stacked profiles from three different areas. A comparison with a previous marine magnetic anomaly study on the South Atlantic Ridge and the synthetic profile from relative paleointensity records for the past ~8 Myr confirmed that the tiny wiggles result from global geomagnetic intensity fluctuations. The newly calibrated timescales based on the age of selected marine magnetic anomalies could provide a potential dating reference for sedimentary records. [ABSTRACT FROM AUTHOR]

Buoyant trench sediment generates an isostatic restoring force that opposes the slab pull of subducting plates; however, the influence of this force on subduction dynamics is poorly understood. Here, we performed three‐dimensional free subduction simulations adopting a variety of sediment distributions along a trench to investigate the correlation between trench motions and heterogeneous buoyant forces. Two endmembers, sediment‐rich and sediment‐starved centers, induced convex and concave trenches, respectively. The trench curvatures obtained from the natural subduction zones are well constrained by our models over wide magnitudes (1–9 km) and wavelengths (100–400 km) of deficient and excess sediments. Conversely, a uniform sediment distribution leads to an extremely narrow range of trench curvatures. These results imply that trench sediment contributes significantly to deviated curvatures along trench strikes. Finally, we observed stress localization at shallow slab hinges that lie beneath abundant sediments, clarifying the relationship between sediment thickness and subduction earthquakes. Plain Language Summary: The role of trench sediment in subduction dynamics as a lubricant or restoring force against denser oceanic crust remains controversial. Buoyant sediment induces a restoring force and variations in sediment thickness may lead to laterally complicated force balances. However, the effect of variations in sediment distribution along strikes on subduction dynamics is underexplored. We conducted three‐dimensional free subduction modeling to analyze the factors that affect trench morphology and stress states within the subducting slab. Our results show that trench motion is strongly affected by differences in local isostatic restoring forces associated with sediment thickness variations. Locally thin trench sediment leads to a more rapid trench retreat and vice versa. The resultant differential trench motion along strike yields complicated trench shapes which exhibit a wide range of curvatures. By comparing the trench curvatures derived from our models to those obtained from natural subduction zones, we confirmed that the models with differential sediment distribution explained the patterns observed in major subduction zones. In contrast, models with uniform sediment thickness did not reflect natural trench curvature. Our study provides a fresh perspective on the trench shape of subduction zones (e.g., the Tonga‐Kermadec) with highly variable along‐strike sediment thickness. Key Points: We performed three‐dimensional free subduction simulations of trenches with variable along‐strike sediment distributionsLocally excessive and deficient trench sediments induce convex and concave trenches, respectivelyThe models with differential sediment distributions explain a wide range of trench curvatures of modern subduction zones [ABSTRACT FROM AUTHOR]

Deep tectonic tremors occur at the downdip extent of the seismogenic zone due to fluid processes. Beneath the northeastern Kii Peninsula, southwestern Japan, there is an along‐dip bimodal distribution of tremor. However, no constraint exists on the structures controlling that distribution. We extract detailed seismological structures from multi‐band receiver functions and evaluate conditional differences in the distribution. To achieve high resolution images along the plate interface, we utilize records of regional deep‐focus earthquakes from the Pacific slab. Cross‐section images show the subducting oceanic plate with depth‐dependent phases along the bimodal distribution, revealing a conspicuous plate interface at the updip portion and an inconspicuous interface below the mantle wedge at the downdip portion. This indicates that episodic tremors occur in the high pore‐fluid plate interface below the impermeable forearc crust, and that continual tremors occur at the permeable mantle wedge corner, owing to continuous fluid supply from the oceanic crust. Plain Language Summary: Deep slow earthquakes have mainly been detected at the deeper extent of estimated large‐slip regions of large‐scale regular earthquakes in the Nankai subduction zone, southwestern Japan. Epicenters of tectonic tremors are also downdip‐aligned. However, some clusters of continual tremor with frequent small bursts were found at further downdip portions beneath the northeastern Kii Peninsula. The complexity of the bimodal tremor distribution poses a structural question regarding whether the tectonic tremor occurs below a mantle wedge or below the continental crust. We utilize a receiver function method that surveys subsurface velocity boundaries and evaluate detailed seismological structures around the plate interface using a multi‐band analysis. Furthermore, regional deep‐focus earthquake records are effectively utilized for receiver function mapping. The high‐frequency cross section exhibits depth dependence of plate‐interface phases, which demarcates active regions of updip events and downdip continual tremor, thus revealing that episodic tremor occurs below the continental crust and continual tremor occurs at the mantle wedge corner. The high‐contrast updip interface reveals that a large amount of fluid is confined at the plate interface below the impermeable forearc crust, which may lead to active episodic slow earthquakes at updip portions. Key Points: Multi‐band receiver functions from global and regional deep events provide detailed plate‐interface structures in the Kii PeninsulaHigh‐frequency images show a clear depth dependence for the subducting plate interfaceAlong‐dip difference in overriding plate materials and permeability explains bimodal distribution of tectonic tremor [ABSTRACT FROM AUTHOR]

The quantification of topographic growth at convergent margins is of primary importance to assessing the linkages between tectonic processes and landscape evolution. Traditionally, this task has relied on the applicability of conventional paleobotanical and isotopic methods to estimate paleoelevations, which is not always straightforward. Here, we use recent calibrations based on trace elements of arc‐related magmatic rocks to estimate crustal thickening and surface uplift of the northern Colombian Andes during the early Andean orogeny at ca. 70‐60 Ma. Increased Sr/Y and (La/Yb)N ratios of arc‐related intrusives suggest a ∼20 km crustal thickening that was probably accompanied by an isostatically compensated topographic uplift of up to ∼2 km along the proto‐Central Cordillera and the Santa Marta Range. This kilometer‐scale uplift was coeval with a regional shift from marine to continental deposition in foreland basins and was triggered by the collision of the Caribbean oceanic plateau. Plain Language Summary: Quantifying the history of mountain building is primordial for our knowledge of how Earth's landscape, and the associated effects on climate and biodiversity, evolves in response to tectonic processes. As thickening of the Earth's crust is compensated in most orogenic systems by topographic growth (i.e., isostatic compensation), we can estimate how high mountains were in ancient times by tracking changes in crustal thickness. Here, we use trace element Sr/Y and La/Yb ratios of magmatic rocks, which allow documenting a crustal thickening of ∼20 km in the northern Colombian Andes at ∼70–60 Ma, that was likely accompanied by a topographic uplift of up to ∼2 km. Such period of mountain growth is consistent with the transition from marine to continental accumulation settings in foreland basins. Our results demonstrate the primary role that the collision of thick oceanic crust plays on mountain building in convergent margins. Key Points: The Colombian Andes underwent ∼20 km of crustal thickening at ∼70‐60 Ma, suggested by an increase in Sr/Y and (La/Yb)N ratios of arc rocksA surface uplift up to ∼2 km of the proto‐Central Cordillera was accompanied by a regional transition from marine to fluvial settingsCollision of thick and buoyant oceanic crust contributes to mountain building along convergent margins such as the Colombian Andes [ABSTRACT FROM AUTHOR]

The crust near an extinct mid‐ocean ridge provides unique constraints on how its accretion and deformation responded to the cessation of spreading. Here we present crustal thickness and Vp/Vs measurements beneath 11 Ocean Bottom Seismograph sites that cross the South China Sea's extinct spreading axis. We find that the oceanic crust, which generally had only slight thickness changes once spreading started, abruptly thins at sites close to the extinct ridge axis. Abnormally high Vp/Vs ratios are obtained at several sites south of the ridge, indicating the presence of serpentine. These observations imply that, in its final stage, spreading changed to an ultraslow accretion style. As the axial crust thinned, normal faults and/or detachment faults began to form. Water could penetrate more deeply through these faults, and large fault slip could raise ultramafic peridotites to near or at the seafloor, creating favorable conditions for their enhanced serpentinization. Plain Language Summary: Mid‐ocean ridges can both form and die. As a ridge dies, what happens to melt generation beneath the ridge axis? Clues exist in the oceanic crust near an extinct ridge that was created during the ridge's final stages of seafloor spreading. We explore a well‐known extinct spreading center within the South China Sea. Using ocean bottom seismometers and the receiver function technique, we determine the thickness of the magmatic crust, and a parameter (Vp/Vs) that reflects the structure of the underlying crust. We find that, approaching the extinct spreading axis, the crust, typically uniform in thickness, thins significantly. This indicates that the melt supply decreases relative to spreading rate during the final stages of spreading. The Vp/Vs ratio at three sites nearest to the extinct spreading axis is abnormally high. Our inference is that the spreading rate and magma production has decreased to the point where large crustal faults can develop at several locations along the ridge. These faults provide pathways for seawater to reach and react with the uppermost mantle, forming minerals with higher Vp/Vs ratios which are then uplifted to the seafloor as the faults grow. Key Points: Ocean Bottom Seismograph receiver functions constrain crustal structure near the extinct ridge of the South China SeaThinned crust indicates reduced spreading rate and magma supply during the final stage of spreadingAnomalously high Vp/Vs ratios are likely due to serpentine along faults formed in response to slowing spreading [ABSTRACT FROM AUTHOR]

The Great Serpentinite Belt of eastern Australia is a ∼1500 km long dismembered ophiolite assumed to be Cambrian based on studies of small (typically <50 m2) exotic meta‐igneous inclusions despite contrasting ages (Cambrian—Devonian) and complex P‐T histories. To overcome these issues, we studied a ∼18 km2 coherent block of dismembered ophiolite that provides robust geological context to sampling the ophiolite. Zircon U‐Pb‐Hf‐O isotope and trace analyses from three plagiogranite dykes cutting massive gabbro confirm ∼283–277 Ma ages and a mantle source. As a result, we argue older Cambrian to Devonian plagiogranite and subducted blocks were inherited from previous subduction events in eastern Australia. These findings allow us to match the Great Serpentinite Belt with the contemporary Dun Mountain ophiolite (New Zealand) and the Koh ophiolite (New Caledonia), thus supporting a new, integrated Pacific Gondwana margin paleogeography involving multiple arcs and subduction zones. Plain Language Summary: Ophiolites are fragments of oceanic crust and mantle that have been thrusted onto continents by tectonics. Ophiolites provide important records of oceanic lithosphere and for assessing the timing of significant tectonic events. Previous studies of the Great Serpentinite Belt of eastern Australia, established a ∼530 Myr age. However, studies focused on small (typically < 50 m2) exotic fault bounded blocks of ophiolitic material of varying geological ages and complex metamorphic histories. By focusing on an intact 18 km2 fragment of oceanic crust with reliable geological relationships and low degrees of metamorphism, our results show this ophiolite is far younger (∼280 Myr old). This age overlaps with ophiolites in New Caledonia and New Zealand on what was the paleo‐Pacific Gondwana margin. This new discovery leads to a new paleogeography for this period and improves geological links between eastern Australia, New Zealand, and New Caledonia. Key Points: A coherent 18 km2 ophiolite block within the Great Serpentinite Belt, eastern Australia is Early Permian in ageCambrian to Ordovician ophiolite inclusions (often <50 m2) do not date the main ophiolite but are inherited from previous subduction eventsA new reconstruction of the Early Permian east Gondwana paleogeography that incorporates coeval ophiolites in New Zealand and New Caledonia [ABSTRACT FROM AUTHOR]

Seismological studies have revealed heterogeneities at scales of 10–1,000 km in the lower mantle. For the first time, we demonstrate here that seismic interferometry of ambient noise can be used to detect faint scattered waves. We locate scattering structures at depths of ~900–1,000 km from P‐to‐P scattered wave signals in stacks of vertical cross‐correlograms from dense seismic arrays in northeast China. Independently, we show that this scattering structure produces S‐to‐P converted signals in the recordings of a deep earthquake. We constrain the thickness (H), the contrasts of the shear wave speed (δVs), and density (δρ) of the anomaly using 1‐D synthetics. Our modeling of both the reflected and converted signals indicates that H = 10–20 km, δVs = −7.2%, and δρ = 0.6%, which are similar to values reported previously and consistent with the interpretation that the scattering structure is a fragment of the basaltic oceanic crust associated with the subducted Izanagi plate. Plain Language Summary: Analysis of surface waves in ambient noise has been widely applied to image the structure of the crust and mantle. However, it is more difficult to extract the signals of body waves in the deep mantle from ambient noise. We explore signals by cross‐correlation using ground motion recordings from dense seismic arrays in northeast China. We identify a scattering signal at time ~200 s. By analyzing and comparing the results of conversion waves generated by deep earthquakes, we confirm that this scattering signal represents a kind of body wave reflected by a small‐scale structure in the mid‐mantle at depths of ~900–1,000 km. The successful extraction of the weak body waves generated by scatterers in the lower mantle provides a potential method for the global detection of fine‐scale heterogeneities in the mantle, which will provide a better understanding of the composition and structure of the mantle, and the geodynamic processes in the deep earth. Key Points: Seismic interferometry is a promising technique for mapping small‐scale heterogeneities in the lower mantle far from subduction zonesWe locate scattering structures at depths of ~900–1,000 km from P‐to‐P scattered wave signals beneath northeast ChinaScatterers in the lower‐mantle that generate the observed SP and PP phases are probably associated with the mid‐ocean ridge basalt [ABSTRACT FROM AUTHOR]

There is compelling evidence for subsurface water oceans among the three outer Galilean satellites and evidence for an internal magma ocean in the innermost moon, Io. Tidal forces from Jupiter periodically deform these bodies, causing heating and deformation that, if measured, can probe their interior structures. In addition to Jupiter‐raised tides, each moon also raises tides on the others. We investigate moon‐moon tides for the first time in the Galilean moons and show that they can cause significant heating through the excitation of high‐frequency resonant tidal waves in their subsurface oceans. The heating occurs both in the crust and ocean and can exceed that of other tidal sources and radiogenic decay if the ocean is inviscid enough. The resulting tidal deformation can be used to constrain subsurface ocean thickness. Our understanding of the thermal‐orbital evolution and habitability of the Jovian system may be fundamentally altered as a result. Plain Language Summary: The three icy Galilean moons, Europa, Ganymede, and Callisto, are thought to contain liquid water oceans beneath their surface, while the innermost moon Io may contain an internal ocean of magma. Jupiter's gravity stretches and squeezes these moons as they orbit the gas giant, heating their interiors through friction. It is essential to understand this process, known as tidal heating, given the unique geophysical structure of ocean worlds and their potential for habitability. In addition to Jupiter, each moon also raises tides on the others, a process that is usually neglected as Jupiter's gravitational attraction is many times larger than that due to the adjacent moons. Here, we show that these moon‐moon tides cannot in fact be neglected when considering tides as an energy source because they can excite these subsurface oceans near their natural frequencies. By modeling subsurface tidal currents, we find that the corresponding resonant response of the ocean manifests itself through the generation of fast flowing tidal waves, which can release significant amounts of heat into the oceans and crusts of Io and Europa. Our understanding of how ocean worlds in compact systems evolve over time may be altered by the existence of moon‐moon tidal resonances. Key Points: Thick subsurface oceans can generate resonant tidal waves in response to moon‐moon tidal forcingEnhanced crustal and oceanic energy dissipation due to tidal resonances may alter the thermal‐orbital evolution of the Jovian systemMeasuring moon‐moon tidal deformation can help constrain the thickness of subsurface oceans [ABSTRACT FROM AUTHOR]

We investigate earthquake distribution and focal mechanisms associated with the 2018 Kīlauea volcano eruption in Hawaii. Our high‐precision earthquake relocations delineate an aseismic zone bounded by two subhorizontal bands of seismicity at 3.5 and 7 km depths beneath the eastern south flank, both of which are dominated by the shallow‐dipping reverse faulting during the 2018 activity. We interpret the deeper seismicity as related to the basal décollement that separates the volcanic edifice from the oceanic crust. The shallower seismicity is a feature exhibited in the recent activity and, which we propose, reveals a detachment that either represents the contact between Mauna Loa and Kīlauea volcanoes or coincides with the onland extension of the base of the Hilina slump. We suggest that large earthquakes, such as the 1975 Mw 7.7 and the 2018 Mw 6.9 mainshocks, are capable of triggering failures of both the basal décollement and the shallower surface. Plain Language Summary: In 2018, Kīlauea volcano in Hawaii experienced an eruption that changed the magmatic behavior in the past few decades. In this study, we use the earthquake data recorded by Hawaiian Volcano Observatory to study the seismic activity during the 2018 Kīlauea volcano eruption. We improve earthquake location accuracies using high‐precision seismic data and velocity model and find the earthquake distribution shows different characteristics from the background seismicity in the southern portion of the volcano. A planar seismicity layer at about 7 km depth, also observed in previous location catalogs, is positioned along the interface between the volcanic edifice and the oceanic crust. A shallower layer at 3.5 km depth is a new feature of the recent activity, which we propose represents the boundary between Mauna Loa and Kīlauea volcanoes or the onland extension of a large submarine landslide. We suggest that large earthquakes are strong enough to trigger displacements at both surfaces. The discovery of the new shallower plane will also impact our understanding of slow earthquakes in the volcano. Key Points: A high‐precision relocation catalog is available for the earthquakes during the 2018 Kīlauea eruptionA new band of shallow microearthquakes at 3.5 km depth is revealed beneath the south flank of Kīlauea volcanoThis shallow layer may coincide with either the contact between Mauna Loa and Kīlauea volcanoes or the base of the Hilina slump [ABSTRACT FROM AUTHOR]

Recycling of oceanic lithosphere and serpentinized peridotites into the mantle leads to dehydration melting and volcanic arcs. However, the mechanism of the occurrence of long‐term volcanic gap in subduction zones remains poorly understood. Two‐dimensional thermomechanical numerical models focusing on resisting the transport of the major hosts of hydrous minerals to mantle depths, show that thin oceanic crust together with rheologically weak and buoyant serpentinized mantle could result in hydrated lithologies piling up in the accretionary wedge and no or sparse occurrence of arc magmatism. This scenario may occur during subduction of oceanic lithosphere formed at (ultra)slow‐spreading margins characterized by thin crust and extensive mantle serpentinization, which is a plausible explanation for the "arc gaps" in the Alps and southern Tibet. Plain Language Summary: Volcanic arcs in subduction zones are widespread and are mainly attributed to slab dehydration and subsequent mantle hydrous melting. However, the magmatic history of the Alps and southern Tibet is characterized by two periods of "arc gap" (i.e., no volcanism) that lasted for 40–50 Myr and 10–30 Myr, respectively, with no or only very sparse magmatism and simultaneous exhumation of serpentinized mantle slivers. Our numerical models indicate that thin oceanic crust, such as that formed at (ultra)slow‐spreading centers, together with an extensively serpentinized lithosphere facilitates the removal of the subducting crust at shallow depths resulting in limited arc magmatism. Key Points: The influence of abyssal and slab‐bending serpentinites on magmatism in subduction zones are numerically investigatedAmagmatic subduction zones may occur where abyssal serpentinites are abundantModel results can explain sparse magmatism during subduction in the Alps and southern Tibet [ABSTRACT FROM AUTHOR]

Carbon recycling efficiency of arc (CREA) is an important parameter to assess the recycling of slab carbon into Earth's deep interior. Although previous studies observed variable degrees of recycled slab carbon at global arcs, the CREA value of any individual subduction zone has not been obtained due to the loose constraints on carbon budget in altered oceanic crust (AOC). Here, through estimates of carbon input by both sediments and AOC at DSDP Site 543 and recycled carbon output from major volcanoes in the Central‐Northern Lesser Antilles, we show an extremely efficient carbon recycling case, with the CREA value reaching 100±27%. Nearly complete slab carbon release at sub‐arc depth implies little carbon has been lost in the forearc region or subducted into the deep mantle in this subduction zone. Our results highlight strongly variable CREA on a global scale, which must be considered in the modeling of global deep carbon cycle. Plain Language Summary: Subduction zones are the major channel to deliver crustal carbon into the mantle. However, the fate of crustal carbon at a variety of depth (e.g., forearc, sub‐arc, beyond arc) in the subduction channel is poorly quantified, which is an obstacle to our understanding of the deep carbon cycle. Here, we assessed the carbon recycling efficiency in the Central‐Northern Lesser Antilles arc by comparing the carbon input flux (estimated from data of the subducting slab recovered by DSDP drillings) with carbon output flux (estimated based on volcanic emission data). We found that the subducted crustal carbon was nearly completely recycled by arc volcanism in the Central‐Northern Lesser Antilles. This implies little carbon loss within the forearc region and little carbon subducted into the deep mantle in the Central‐Northern Lesser Antilles, which differs from some other subduction zones (e.g., the Izu‐Bonin‐Mariana and Central America) that show low carbon recycling efficiencies. Our discovery highlights that the carbon recycling in subduction zone is highly variable on a global scale. Key Points: Subducting input and volcanic output fluxes of carbon at the Central‐Northern Lesser Antilles arc were constrained by instrumental dataComparison shows that slab carbon is efficiently recycled through volcanic emissions at this subduction zoneThis implies little carbon loss at the forearc region as well as little carbon subduction into the deep mantle in this individual subduction zone [ABSTRACT FROM AUTHOR]

Over the first billion years of Mercury's history, mantle melting and surface volcanism produced a secondary magmatic crust varying spatially in composition and mineralogy. By combining geochemical mapping from MESSENGER with laboratory experiments on partial melting, we translate the surface mineralogy into lateral variations of surface density and calculate the degree of mantle melting required to produce surface rocks. If lateral density variations extend through the whole crust, the local crustal thickness correlates well with the degree of mantle melting. Low‐degree mantle melting produced a thin crust below the northern volcanic plains (19±3 km), whereas high‐degree melting produced the thickest crust in the ancient high‐Mg region (50±12 km), refuting the hypothesis of an impact origin for that region. The thickness‐melting correlation has also been observed for the oceanic crust on Earth and might be a common feature of secondary crust formation on terrestrial planets. Plain Language Summary: Mercury's crust has a complex structure resulting from a billion years of volcanism. The surface variations in chemical composition have been identified from orbit by the spacecraft MESSENGER. Combining these measurements with laboratory experiments on partial melting, we estimate which variations in surface density and degree of mantle melting are required to produce surface rocks. If the surface density is representative of the deep crustal density, more than one half of crustal thickness variations in the Northern Hemisphere are explained by lateral variations in mantle melting. The crust is thin below the magnesium‐poor northern volcanic plains, whereas the thickest crust is found in the magnesium‐rich region located at middle northern latitudes in the Western Hemisphere. The magnesium‐rich region is thus not due to an early impact but rather to extensive mantle melting. The thickness‐melting relation has also been observed for the oceanic crust on Earth and might be a common feature of terrestrial planets. Key Points: Maps of surface density, degree of mantle melting, and crustal thickness are inferred from geochemical and geodetic dataWe find a high correlation between lateral variations in Mercury's crustal thickness and mantle melt productionThe crust is thickest in the ancient high‐Mg region [ABSTRACT FROM AUTHOR]

Seafloor spreading magnetic anomalies formed at mid‐ocean ridges initially display strong amplitudes that decay within the first 10 million years as a result of pervasive hydrothermal circulation and alteration. The amplitudes do not vary much for older oceanic crust, suggesting that the thickening sediments hinder heat advection. Here we show, however, that a systematic loss of ~20% in the amplitude of the anomalies arises between the outer rise and the trench on old ocean crust approaching the Japan and Kuril subduction zones. We interpret this decay as reflecting the opening of normal faults and fissures caused by extension on the outer flexural rise and the subsequent renewed circulation of seawater into the oceanic crust, resulting in additional alteration of the magnetic minerals. This interpretation is supported by higher heat flow and seismic velocity changes observed toward the trench. Plain Language Summary: Seafloor spreading magnetic anomalies formed at mid‐ocean ridges initially display strong amplitudes that decrease within the first 10 million years as a result of the widespread circulation of hot seawater within the oceanic crust and the resulting alteration of its magnetic minerals. The amplitudes do not vary much for older oceanic crust, suggesting that the thickening sediments hinder the free exchange of seawater between the crustal aquifer and overlying ocean. Here we show, however, that a systematic loss of ~20% in the amplitude of the anomalies appear between the outer rise, an elevation caused by the flexure of the plate entering subduction, and the trench on old ocean crust approaching the Japan and Kuril subduction zones. We interpret this decrease as reflecting the opening of faults and cracks caused by extension at the top of the bent oceanic plate and the subsequent renewed circulation of seawater into the oceanic crust, resulting in additional alteration of the magnetic minerals. This interpretation is supported by higher heat flow and seismic velocity changes observed toward the trench. Key Points: Decaying seafloor spreading magnetic anomalies are observed before subduction in Japan‐Kuril subduction zoneThe damping shape decaying pattern corresponds to increasing heat flow and the hydration of the oceanic crust approaching the trenchesThe extra alteration of magnetic minerals by renewed water circulation from the flexure‐related opening of normal faults cause the decay [ABSTRACT FROM AUTHOR]