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Sound velocities and densities of Al‐bearing superhydrous phase B (Al‐bearing SuB) were investigated up to 24 GPa and 1300 K by synchrotron X‐rays combined with ultrasonic interferometry techniques in a multi‐anvil apparatus. We found that Al + H incorporation decreases the adiabatic bulk modulus and shear modulus of SuB. Our results however suggest that this effect is less important than that of temperature. The presence of hydrous pyrolite with ∼10 wt.% Al‐bearing SuB in cold subducting slabs could explain up to ∼2% high velocity anomalies at the bottom of the mantle transition zone (500–660 km) while it turns into ∼7.7% low velocity anomalies below 660 km. Al‐bearing SuB is reportedly stable at mantle temperatures, where it could account for ∼12.2% low velocity anomalies beneath subduction zones in the uppermost lower mantle. Plain Language Summary: Among dense hydrous magnesium silicates, Superhydrous phaseB (SuB) is believed to hold large amount of water and transport it to the deep mantle. Recent studies showed that Al can largely increase the thermal stability region of SuB, suggesting this phase may also be an important carrier of water in the mantle transition zone (MTZ) and uppermost lower mantle (ULM). Here we investigated the sound velocities of Al‐bearing SuB up to 24 GPa and 1300 K using ultrasonic interferometry combined with synchrotron X‐ray techniques in a multi‐anvil apparatus. We found that Al + H incorporation decreases adiabatic bulk modulus and shear modulus of SuB. Our results however suggest that this effect is less important than that of temperature. Based on our elasticity data, we calculated velocity of a hydrated pyrolite composition, containing up to 10% of Al‐SuB along temperature profiles corresponding to cold slab and hot mantle at the depths of the MTZ and the ULM. We found that the presence of hydrous pyrolite may explain high velocity anomalies up to 660 km depth at MTZ while low velocity anomalies appear below 660 km along cold slab geotherm. Nevertheless, low velocity anomalies are generated in the whole depth range along a normal geotherm. Key Points: We constrained the Al + H incorporation on sound velocities of superhydrous phase B (SuB) up to 24 GPa and 1300 KAl + H incorporation which promoted by increased temperature significantly affect velocities of SuBAccumulation of Al‐bearing SuB can explain velocity contrast with various magnitude owing to the change of Al + H content with temperature [ABSTRACT FROM AUTHOR]

It was recognized that two magmatic belts in the Lhasa‐Tengchong terrane formed due to the Mesozoic‐Cenozoic Tethyan evolution. Still, their spatiotemporal variations of magmatic flare‐ups/lulls are rarely discussed. Here we use the new U‐Pb and Lu‐Hf isotopic data of captured zircons and a comprehensive data set to show that the flare‐up of northern magmatic belt has peak ages of 110 Ma in central and northern Lhasa and 120 Ma in eastern Tengchong, possibly related to the tectonic transition from Meso‐ and Neo‐Tethyan double subduction to Neo‐Tethyan single subduction. For the southern magmatic belt, the flare‐ups at 100–85 Ma and 65–45 Ma in eastern southern Lhasa indicate obvious juvenile crustal growth, while flare‐ups at 75–45 Ma in western southern Lhasa and Tengchong record ancient crustal reworking. Such flare‐up variations in the southern magmatic belt possibly resulted from asynchronous changes in the Neo‐Tethyan slab dip. Plain Language Summary: The Tethyan slab subduction and following collision of India and Asia produced two magmatic belts in the Lhasa‐Tengchong terrane. How did the magmatic flare‐ups or high‐flux episodes and sources spatiotemporally vary? We combine the new U‐Pb and Lu‐Hf isotopic data of captured zircons (xenocrystic zircons caught by volcanics from country rocks) and a comprehensive data set to show that the peak ages of early Cretaceous flare‐up of the northern magmatic belt are ca.10 Ma younger in central and northern Lhasa than in eastern Tengchong, possibly related to the diachronous closure of the Meso‐Tethys and flat subduction of the Neo‐Tethys. The southern magmatic belt in Tengchong and western southern Lhasa exhibit 75–45 Ma flare‐ups featured by the reworking of ancient crust, different from the eastern southern Lhasa with the flare‐ups at 100–85 Ma and 65–45 Ma which are dominated by the juvenile crustal growth. Such flare‐up variations may be related to changes of the Neo‐Tethyan slab dip. Key Points: The Neo‐Tethyan arc has asynchronous magmatic flare‐ups and diverse magmatic sourcesThe spatiotemporal changes of slab dip triggered the asynchronous flare‐ups of the Neo‐Tethyan arc [ABSTRACT FROM AUTHOR]

The detachment (i.e., break‐off) of down‐going subducting oceanic slabs is a major geodynamic event with far‐reaching consequences, one of which is the reduction of the slab pull force acting on the trailing plate. We investigate the motion of the Sinai Microplate where a recent (∼1 Myr ago) slab break‐off occurred along its sole converging plate boundary (Cyprian Arc) with the overriding Anatolia Microplate. Based on new bathymetric mapping, high‐resolution seismic reflection imaging, geodetic and earthquake data, we show that Sinai is actively moving in a northwest direction with respect to Nubia. Our results indicate that despite the recent slab break‐off, Sinai has and is still being pulled (or pushed) toward the overriding Anatolia Microplate. The continued convergence possibly occurs because of a persistent slab pull force, a suction force induced by the down‐going detached slab and/or by the upper mantle flow induced by the Afar Plume. Plain Language Summary: Subduction of the oceanic lithosphere into the mantle exerts the prime force that drives plate tectonic motions. The termination of subduction due to the tearing of the down‐going oceanic slab is, therefore, an important geodynamic phenomenon with far‐reaching kinematic consequences. Here we show that despite the recent tearing of the leading oceanic slab of the Sinai Microplate, convergence motion between Sinai and Anatolia Microplates continued without substantial changes. The continued convergence possibly occurs because of a persistent slab pull force, a suction force induced by the down‐going detached slab and/or by the motion of the upper mantle flow induced by the Afar Plume. Key Points: Within the eastern Mediterranean, the Sinai Microplate is moving in a northwest direction with respect to the Nubian PlateThe Sinai Microplate is still converging with the overriding Anatolia Plate, despite the recent break‐off of the leading subducting slab [ABSTRACT FROM AUTHOR]

The 180° curvature of the Banda arc at the eastern end of the Java‐Banda subduction zone reflects complicated geodynamic processes. A detailed investigation of its anisotropic structure would reveal its subduction dynamics, further resolving the controversial issue on how the highly arcuate Banda arc formed. We apply tilting‐axis anisotropic tomography to obtain a high‐resolution 3‐D anisotropic model beneath the Java‐Banda region. Our results show significant differences between Java and Banda in the pattern of anisotropy in both the subducting slab and its surrounding mantle, which reflect two distinctly different deformation modes in the two domains. Our results support the single‐slab subduction model for the Banda region. In addition, trench‐normal and upright fast‐velocity‐planes appear in the deep upper mantle, which may indicate material migrations in the big mantle wedge. Fast‐velocity‐planes in the shallow mantle exhibit a toroidal distribution, reflecting past counter‐clockwise rotation and asthenospheric material extrusion. Plain Language Summary: How the great curvature of the Banda arc formed remains in debate. Previous studies have proposed a single slab model and a double slab model for this subduction zone. Seismic anisotropy in the subducting slab and its surrounding mantle could provide clues on which model is correct. In this work, we perform the first tomographic inversion for tilting‐axis anisotropy to reveal the 3‐D anisotropic structure of the Java‐Banda subduction zone. The anisotropic features in the subducted slab and its surrounding mantle revealed by our anisotropic tomography support the single‐slab subduction model for the formation of the Banda arc. Key Points: Significant anisotropic differences exist between Java and Banda in both the mantle and slabSlab subduction is the main cause of seismic anisotropy in the Java‐Banda arcOur results support the single slab model for the Banda subduction zone [ABSTRACT FROM AUTHOR]

Recent observations revealed the existence of ice slabs and aquifers on the Greenland ice sheet (GrIS). Both affect the ice sheet's hydrology: ice slabs facilitate runoff and aquifers modulate drainage to the bed. However, their climatic drivers and history remain unclear, as most observations cover only two decades. Here, we present a model simulation of the evolution of GrIS ice slabs and aquifers (1980–2020), evaluated using radar measurements. The results show that accumulation, melt and rain rates are good predictors for the spatial distribution of ice slabs and aquifers. Both features were already present in the late 1980s, and their extent remained relatively constant until the beginning of this century, after which increased melt led to their expansion. We show that almost any transect from the coast to the ice‐sheet interior will cross either an ice slab region, or an aquifer, or both. Plain Language Summary: The Greenland ice sheet is covered by a layer of old, porous snow called firn, which has the capacity to prevent surface melt runoff. The firn can however, form thick ice slabs or store large amounts of water underground. These change the fate of surface melt by respectively enhancing runoff and draining water to the ice sheet's bed. While observations have shown where these phenomena occur, it is still unknown what the exact atmospheric conditions are that lead to their formation. Here, we tackle this problem using computer simulations. Our results show that the amount of snowfall, melt and rain determine whether ice slabs or aquifers can form. The results also suggest that ice slabs and aquifers are more abundant than previously assumed, and that their observed expansion started only in the early 2000s, and they will continue to expand if the climate gets warmer and melt and rain become more abundant. Key Points: Accumulation, melt and rain are good predictors of ice slab and firn aquifer locationsThe expansion of modeled ice slabs and aquifers on the Greenland ice sheet started in the early 2000sWe show that, in between the ablation zone and the accumulation zone, there are always ice slabs and/or aquifers present [ABSTRACT FROM AUTHOR]

The Cascadia margin is an unusual subduction zone characterized by the downdip movement of young and thin oceanic plates, where mantle flow and intraslab deformation are still unclear. Here we present new anisotropic tomography of the Cascadia subduction zone, in which the hexagonal symmetry axis of anisotropy is tilting rather than horizontal or vertical as assumed in previous studies of seismic anisotropy. Subduction‐induced entrained and toroidal flows under the Cascadia margin are discriminated well by the spatial relationship between tilting‐axis anisotropy and slab geometry. The obliquely entrained flow is trapped in a narrow zone (<100 km wide) above and below the subducting slab and reaches ∼200 km depth, which is surrounded by large‐scale sub‐horizontal toroidal flow. The intraslab anisotropy is trench‐normal above 80 km depth but changes to trench‐parallel at 100–400 km depths, which may reflect fossil anisotropy overprinted by deep deformation beneath the arc, or joint effect of serpentinization and hydrous faulting. Plain Language Summary: The slab deformation and mantle flow under the Cascadia margin are controversial. We obtain new anisotropic tomography of the Cascadia subduction zone, which can reveal tilting 3‐D fast velocity directions (FVDs) and planes. Because the observed trench‐normal FVD in the Cascadia margin can be explained by both 3‐D toroidal and 2‐D entrained flows, we discriminate the two types of mantle flow with 3‐D FVDs that contain dip angle information. The 2‐D entrained flow above and below the subducting slab is trapped in a narrow zone of ∼100 km wide and is surrounded by large‐scale sub‐horizontal toroidal flow. The slab anisotropy above 80 km depth is consistent with that in the Juan de Fuca plate before subduction, but it is reshaped beneath the arc due to the slab deformation or phase transition. Key Points: Tilting‐axis anisotropy can reconcile contradictory assumptions of azimuthal and radial anisotropiesEntrained flow occurs within ∼100 km depth above and below the subducting slab and is surrounded by toroidal flowFossil anisotropy in the slab is overprinted beneath the arc due to intraslab deformation or phase transition [ABSTRACT FROM AUTHOR]

Most paleo‐reconstruction models agree that the South China Sea (SCS) opens within a continental domain located at the trailing edge of the subducting plate of a vanished ocean named Proto South China Sea (PSCS). Existing models do not agree on the age, size, and location of the putative oceanic domain, thus hindering the reconstruction of the plate circuit in the distant past. In this study, we test the relevance and implication of the opening of the SCS in the downgoing plate with a series of thermomechanical simulations which appear to impact the possible age of the PSCS. Our findings suggest that the oceanic domain must be stronger than the continental margin for the slab pull force to be transmitted to the continental margin, resulting in continental breakup. Our simulations support a Cretaceous age, rather than an Eocene, for at least part of the PSCS sea‐floor spreading. Plain Language Summary: Most scientists agree that the South China Sea formed within the China margin through the subduction of an ancient oceanic plate, known as the Proto South China Sea (PSCS), beneath Borneo. Today, this oceanic plate has nearly vanished from the Earth's surface. Only a few scattered fragments found on the islands of Borneo and Palawan confirm its existence. The history of an ocean that no longer exist can be approach by physically driven models. In this study, we use numerical simulations of subduction zones to evaluate the mechanical conditions required for the PSCS subduction to be responsible of the South China Sea formation. We find that the subduction of old oceanic plate are more likely to generate new basin opening within continental margin. Combining our results with the limited age derived from the remaining fragments of the PSCS seafloor, we propose that this oceanic plate formed at least 65 million years ago, probably around 100 million years. Key Points: The slab pull such as the one of the Proto South China Sea (PSCS) may drive spreading within the continental margin of the lower plateSimulations show that the PSCS domain must be stronger than the China continental margin in order to localize the breakup on the continentA minimum Cretaceous age is required for at least part of the PSCS oceanic lithosphere [ABSTRACT FROM AUTHOR]

The mechanism behind the significant Izu‐Bonin‐Mariana (IBM) trench advance is still controversial. We conduct slab subduction numerical models that reproduce the spatio‐temporal tectonic evolution of the Philippine Sea region to investigate whether slab pull from the Ryukyu subduction zone can cross the weakened Philippine Sea Plate and act on the IBM trench. Model results show that the lithospheric strengthening and weakening effects cancel out each other during the rift stage so that the slab pull from the Ryukyu Trench can transmit through the weak fossil spreading centers and intra‐arc rifts and drive the Izu‐Bonin Trench's advance. In contrast, lithospheric weakening overwhelms lithospheric strengthening and impedes stress transfer in the back‐arc spreading stage, suggesting that the slab pull cannot directly pull the Mariana Trench to advance at present. Our study indicates that extreme rheological parameters are not needed for the IBM trench advance, despite the Philippine Sea Plate is weakened. Plain Language Summary: Understanding why IBM (Izu‐Bonin‐Mariana) has the globe's most significant trench advance affects not only our understanding of subduction dynamics and lithospheric deformation mechanisms, but also our knowledge on slab rheology, which is key to understanding many fundamental geodynamic questions. A number of models have been proposed to explain the IBM trench advance. These models can be broadly classified into two categories: one is single‐slab subduction, and the other is double‐slab subduction. Single‐slab subduction models usually require extreme rheological parameters that seem inconsistent with those inferred from slab curvature and gravity observations, while double‐slab subduction models need to consider whether the slab‐pull force can transmit through the weak Philippine Sea Plate. The Philippine Sea Plate is strongly weakened due to extensive back‐arc extensional deformations. We conduct numerical models that reproduce the first‐order tectonic evolution of the IBM Subduction Zone to solve this dispute. The model results show that slab pull from the Ryukyu Trench can pass through the weakened Philippine Sea Plate and act on the IBM Trench. Thus, the IBM Trench advance does not require special rheological parameters as suggested by single‐slab models. Key Points: Slab pull can pass through the fossil young spreading centers and active intra‐arc rifts, but not active spreading centersExtremely strong slabs or strong subduction‐interface coupling are not needed to explain the IBM trench advanceThe Mariana Trench advance may be driven by the advance of the Izu‐Bonin Trench at its north [ABSTRACT FROM AUTHOR]

We conduct a P‐wave receiver function analysis of the Bransfield Strait (West Antarctica) to determine the lithospheric structure of this back‐arc basin, thanks to 31 temporary and permanent stations. Our main finding is a 15 km tear of the Phoenix slab, coinciding with the location of the 2020–2021 Orca earthquake swarm's epicenters. Teleseismic wave modeling reveals that the two major earthquakes occurred at the base of the crust, suggesting that the swarm could have been triggered by active underplating driven by mantle flow through the slab tear. There is evidence for such an underplating layer at least under Deception Island and for a widespread low velocity zone in the mantle wedge probably undergoing partial melting. We found average crustal thickness (30.5 ± 1.0 km) and Vp/Vs (1.81 ± 0.04) values close to average extended continental crust, although results in the South Shetland Islands are significantly more heterogeneous than in the Antarctic Peninsula. Plain Language Summary: With more seismic stations than ever before, we investigate the structure of the earth beneath the Bransfield Strait (West Antarctica), a region located behind a subduction zone which features active volcanism and extension of the crust. We imaged for the first time in the region a gap in the sinking Phoenix plate which coincides with the position of the 2020–2021 Orca earthquake series. We located the two major earthquakes at the base of the crust, so we link seismicity to the flow of hot materials through the gap in the Phoenix plate and its accumulation at the base of the crust. We imaged this layer below Deception Island, and also identified a widespread area where melting probably occurs. The average thickness and seismic velocities of the crust resemble standard values of areas undergoing extension, although the South Shetland Islands are highly heterogeneous. Key Points: Slab tearing occurs along an ancient fracture zone in the subducted Phoenix plateActive magmatic underplating could trigger the Orca earthquake swarm [ABSTRACT FROM AUTHOR]

We use airborne accumulation radar data acquired over the Greenland Ice Sheet between 2002 and 2018 to identify changes in ice slab extent and thickness. We show that ice slabs several meters thick were already present at least as early as 2002. Between 2012 and 2018, they expanded by 13,400–17,600 km2 ${\mathrm{k}\mathrm{m}}^{2}$ inland, or 37%–44%. Our results document that the extremely warm summer of 2012 produced near‐surface ice layers at higher elevations, enabling ice slabs to develop with only moderate melting in the following summers. With repeat flights along a transect in southwest Greenland, we show that moderate melting primarily causes slab thickening through uniform accretion on top of the ice slabs, while large melting events can also trigger localized accretion below existing ice slabs. Plain Language Summary: Above the equilibrium line elevation, seasonal snow is not entirely removed by summer melting. As a result, firn—an interannual layer made of old snow and refrozen meltwater—builds up. Firn holds the potential to buffer sea level rise by trapping liquid water within its pore space. However, surface melting has increased in recent decades, making large quantities of water available to percolate into the firn where it refreezes, eventually creating meters‐thick ice slabs that hinder future percolation. We mapped ice slabs in the subsurface firn (0–20 m depth) by using airborne radar surveys and show that they have expanded inland and thickened from 2002 to 2018. Once formed, ice slabs continue to thicken, even under moderate melt conditions. Recent increases in the area drained by surface rivers on the ice sheet match well with the ice slabs extent, so we conclude that ice slabs will be an important control on the future runoff area of the ice sheet. Key Points: Ice slabs were already present in the early 2000s in southwest, central‐west and north GreenlandIce slabs expanded inland from 2002 to 2018 and thickened by accretion to both their tops and undersidesNear‐surface ice layers support subsequent ice slab development [ABSTRACT FROM AUTHOR]

The Anatolian Plateau, currently experiencing rapid uplift and westward escape, records both the termination of oceanic subduction and the conversion to continental collision. The crustal response to the transition of the subduction environment from eastern to western Anatolia can be inferred by the seismic velocity and attenuation structures. With this study, we construct a broadband Lg‐wave attenuation model for the Anatolian Plateau and use it to constrain lateral crust heterogeneities linked to this transition. Crustal Lg attenuation links late Cenozoic magmatism with asthenospheric upwelling by characterizing the lithospheric thermal structure. The widely distributed strong attenuation observed in eastern Anatolia may be related to the crustal partial melting due to mantle upwelling after the delamination and subsequent break‐off of the Bitlis slab. Lithospheric dripping in central Anatolia likely facilitates the mantle flows through the window between the Cyprus and Aegean slabs, which results in the piecemeal low QLg ${Q}_{\mathit{Lg}}$ anomaly in central Anatolia. Plain Language Summary: Different parts of the Anatolian Plateau are in different evolution stages between oceanic subduction and continental collision and currently undergoing plateau uplift and tectonic escape. The regional seismic velocity and attenuation can be used to characterize crustal partial melting and lateral heterogeneity, which can further identify the underlying subduction process. In this study, we construct a high‐resolution broadband Lg‐wave attenuation model for the Anatolian Plateau. Strong Lg attenuation in Anatolia correlates well with late Cenozoic magmatism distributions and can be an indicator of high temperature or partial melting in the crust. Combined with previous studies, we suggest that the mantle upwelling induced by the delamination of the Bitlis slab is likely reworking the crust in eastern Anatolia and is the cause of widespread thermal anomalies there. The lithospheric dripping process in central Anatolia may facilitate the mantle flows through the window between the Cyprus and Aegean slabs, and results in a piecemeal low QLg ${Q}_{\mathit{Lg}}$ anomaly pattern in central Anatolia. Key Points: A high‐resolution broadband Lg‐wave attenuation model is constructed for the Anatolian PlateauWidespread strong attenuation in the eastern Anatolian crust is likely related to slab delaminationThe circular‐shaped attenuation anomaly may result from slab tearing and lithospheric dripping beneath central Anatolia [ABSTRACT FROM AUTHOR]

The thermal conductivity of minerals is a fundamental parameter in understanding the evolution and dynamics of the Earth. Wadsleyite, the major mineral in the mantle transition zone (MTZ), can contain abundant water. However, how water affects its thermal conductivity remains unknown. Here, we predicted the thermal conductivity of dry and hydrous wadsleyite at high pressure and temperature (P‐T) by combining non‐equilibrium molecular dynamics and machine learning potential trained with data from first‐principles calculations. We found that the thermal conductivity of wadsleyite is anisotropic and is reduced by ∼10% in the P‐T conditions of the MTZ by the presence of 0.81 wt.% water. The heat flow toward the slab tends to follow the direction with the lowest thermal conductivity due to the lattice‐preferred orientation of wadsleyite and olivine. Both hydration and thermal‐conductivity anisotropy slow down the heating of slabs, allowing hydrous minerals and metastable olivine to survive in the deeper mantle. Plain Language Summary: The subducted slab was heated by conduction from the ambient mantle. Minerals' thermal conductivity is crucial for us in evaluating the temperature of the slab. Wadsleyite, the high‐pressure polymorph of olivine, can contain significant amounts of water in the wet mantle transition zone and subducting slabs. However, the thermal conductivity of hydrous wadsleyite remains unknown. First‐principles calculations of the thermal conductivity of hydrous wadsleyite are very expensive. Here, we apply machine learning to break the limits of computational costs and obtain the thermal conductivity of dry and hydrous wadsleyite at high temperatures and pressures. We found that water remarkably reduces the thermal conductivity of wadsleyite. The lattice‐preferred orientation and thermal‐conductivity anisotropy of olivine and wadsleyite would have further slowed the heating of the slab. Low temperatures facilitate the survival of hydrous minerals and metastable olivine. Thus, more water could be transported to the deeper Earth by subducting slabs, and the dehydration of hydrous minerals and the transformation of metastable olivine, which may trigger the deep earthquakes, would occur at deeper depths. Key Points: A machine learning potential for dry and hydrous wadsleyite approaches the accuracy of first‐principles calculationsThe thermal conductivity of wadsleyite is anisotropic with the lowest thermal conductivity along [001] direction and decreases with waterMore water could reach the deep mantle and deep‐focus earthquakes could occur deeper due to hydration and lattice‐preferred orientation [ABSTRACT FROM AUTHOR]

The seismic anisotropy observed in Earth's lowermost mantle (D″ layer) is thought to originate from the lattice‐preferred orientation (LPO) of mantle materials induced by plastic deformation. Ferropericlase is the second most abundant mineral in the lower mantle, and its LPO induced by deformation may significantly contribute to the seismic anisotropy in the D″ layer. The slip system(s) of ferropericlase controlling the LPO in the D″ layer is still controversial, owing to technical difficulties in conducting high‐pressure deformation experiments. We conducted shear deformation experiments on MgO under pressures of up to 120 GPa and reported the LPO after the deformation experiments. Our results indicate that {100}<011> is the dominant slip system in the D″ layer. The LPO developed during shear deformation would result in shear wave splitting with VSH > VSV, consistent with seismic observations in the regions expected to accommodate large shear strain by subducted slabs in the D″ layer. Plain Language Summary: Heat from Earth's core induces active thermal convection involving plastic flow in the Earth's solid mantle. Owing to plastic flow, minerals with elastic anisotropy in the mantle are thought to be aligned in certain directions (lattice‐preferred orientation, LPO) and generate the seismic anisotropy observed in the lowermost mantle. In particular, the LPO of ferropericlase, the second most abundant mineral in the lower mantle, has garnered attention as a likely candidate for attributing the seismic anisotropy of the lowermost mantle. To quantitatively interpret the dynamics of mantle convection in the lowermost mantle from seismic observations, it is essential to understand the LPO patterns of ferropericlase during deformation under the lowermost mantle conditions (for instance, extremely high‐pressure conditions at 1.2 million atmospheric pressure). However, owing to technical difficulties in conducting high‐pressure deformation experiments, there have been uncertainties in the deformation‐induced LPO of ferropericlase in the lowermost mantle. We conducted shear deformation experiments of MgO under the lowermost mantle pressure for the first time and demonstrated that the LPO of MgO produced by shear deformation is indeed consistent with the seismic anisotropy observed in parts of the lowermost mantle expected to accommodate large shear deformation by subducted slabs. Key Points: Strain rate controlled shear deformation experiments on MgO were conducted under lowermost mantle pressure (120 GPa) for the first timeLarge strain deformation experiments on MgO show that the {100}<011> slip is dominant in the D″ layerA {100}<011> slip of MgO by shear deformation results in VSH > VSV shear splitting, consistent with seismic observations in the D″ layer [ABSTRACT FROM AUTHOR]

Geological processes in Southern Patagonia are affected by the Patagonian slab window, formed by the subduction of the Chile Ridge and subsequent northward migration of the Chile Triple Junction. Using shear wave splitting analysis, we observe strong splitting of up to 2.5 s with an E‐W fast direction just south of the triple junction and the edge of the subducting Nazca slab. This region of strong anisotropy is coincident with low uppermost mantle shear velocities and an absence of mantle lithosphere, indicating that the mantle flow occurs in a warm, low‐viscosity, 200–300 km wide shallow mantle channel just to the south of the Nazca slab. The region of flow corresponds to a volcanic gap caused by depleted mantle compositions and absence of slab‐derived water. In most of Patagonia to the south of this channel, splitting fast directions trend NE‐SW consistent with large‐scale asthenospheric flow. Plain Language Summary: Slab windows represent openings or gaps in the downgoing slab, allowing the mantle to flow through the plane of the slab from one side of the subduction zone to the other. The subduction of a spreading ridge beneath South America forms a gap in the subducting slab below Patagonia, presenting an opportunity to investigate the influence of slab windows on mantle flow and geological processes. Although this region has been poorly instrumented in the past, the deployment of new seismic instruments and available data allow us to study how the mantle seismic velocity varies with direction in the region. From the directional dependence of seismic velocity, we can infer the direction of mantle flow. We observe a change from N‐S to E‐W mantle flow in the northern part of the slab window, near the edge of the subducting Nazca plate. The flow occurs in a warm, low viscosity shallow mantle channel corresponding to a gap in activity along the volcanic arc. Key Points: Shear wave splitting indicates strong anisotropy with an E‐W fast direction just south of the Chile Triple Junction and the edge of the subducting Nazca slabThe splitting and shear wave velocity structure suggest eastward shallow mantle flow in a 200–300 km wide channel around the edge of the Nazca slabIn most of southernmost Patagonia, splitting shows NE‐SW fast directions consistent with large‐scale asthenospheric flow [ABSTRACT FROM AUTHOR]

Subduction zones transport water into Earth's deep interior through slab subduction. Serpentine minerals, the primary hydration product of ultramafic peridotite, are abundant in most subduction zones. Characterization of their high‐temperature elasticity, particularly their anisotropy, will help us better estimate the extent of mantle serpentinization and the Earth's deep water cycle. Lizardite, the low‐temperature polymorph of serpentine, is stable under the P‐T conditions of cold subduction slabs (<260°C at 2 GPa), and its high‐temperature elasticity remains unknown. Here we report ab initio elasticity and acoustic wave velocities of lizardite at P‐T conditions of subduction zones. Our static results agree with previous studies. Its high‐temperature velocities are much higher than previous experimental‐based lizardite estimates with chrysotile but closer to antigorite velocities. The elastic anisotropy of lizardite is much larger than that of antigorite and could better account for the observed large shear‐wave splitting in some cold slabs such as Tonga. Plain Language Summary: Serpentine minerals are crucial water carriers in subduction zones. Their layered structure is responsible for their strong elastic anisotropy. However, their elastic properties, especially the anisotropy at high‐temperature conditions remain unclear. In this study, we determined the elasticity of lizardite, the low‐temperature polymorph of serpentine, at P‐T conditions of subduction zones using ab initio calculations. The resulting velocities agree with previous calculation studies but are much higher than previous experimentally‐based estimates. Lizardite has significant shear wave anisotropy. The large elastic anisotropy could account for the observed shear‐wave splitting in the subducting slabs. Key Points: The elastic properties of lizardite are determined at P‐T conditions corresponding to subduction zones using ab initio calculationsThe elastic anisotropy of lizardite for S wave is almost double that of antigoriteThe large elastic anisotropy of lizardite can account for the observed shear‐wave splitting in the subducting slabs near trenches [ABSTRACT FROM AUTHOR]

Two large low velocity provinces (LLVPs) are observed in Earth's lower mantle, beneath Africa and the Pacific Ocean, respectively. The maximum height of the African LLVP is ∼1,000 km larger than that of the Pacific LLVP, but what causes this height difference remains unclear. LLVPs are often interpreted as thermochemical piles whose morphology is greatly controlled by the surrounding mantle flow. Seismic observations have revealed that while some subducted slabs are laterally deflected at ∼660–1,200 km, other slabs penetrate into the lowermost mantle. Here, through geodynamic modeling experiments, we show that rapid sinking of stagnant slabs to the lowermost mantle can cause significant height increases of nearby thermochemical piles. Our results suggest that the African LLVP may have been pushed more strongly and longer by surrounding mantle flows to reach a much shallower depth than the Pacific LLVP, perhaps since the Tethys slabs sank to the lowermost mantle. Plain Language Summary: Seismic observations have revealed the two large low velocity provinces (LLVPs) beneath Africa and the Pacific Ocean in Earth's lowest mantle. Growing evidence indicates that the maximum height of the African LLVP is up to ∼1,000 km taller than that of the Pacific LLVP, but the underlying cause of such a large difference in height remains uncertain. Geodynamic modeling studies have shown that the morphology of LLVPs is greatly controlled by their interactions with surrounding mantle flow which is mainly driven by subduction of cold slabs. Seismic observations have shown that while some subducted slabs flatten or stagnate at ∼660–1,200 km, other slabs arrive at the deepest mantle. With geodynamic modeling experiments, we show that catastrophic sinking of subducted slabs into the lower mantle can cause strong upwelling flows that in turn greatly increase the height of nearby LLVP‐like piles, while other piles can be less affected. This work suggests that the African LLVP may be enduring stronger nearby mantle flow to rise by the slab‐driven surrounding mantle flow than the Pacific LLVP. The African LLVP may start rising since the sinking of the Tethys slabs to the lowermost mantle. Key Points: Continuous sinking of voluminous subducted slabs to the lower mantle can push nearby large low velocity province (LLVP)‐like piles to rise significantlyEpisodic large increase of height is only possible for piles that are nearly neutrally buoyant and are relatively unstableThe African LLVP may have been strongly pushed by the Tethys slabs and reach a much larger height than the Pacific LLVP [ABSTRACT FROM AUTHOR]

Single‐crystal elasticity of both α‐ and β‐orthopyroxene was determined up to 20 GPa and 300 K by Brillouin scattering. Using the derived full elastic moduli (Cij), we investigated the contribution of the metastable pyroxene to the seismically observed 3%–5% low‐velocity anomalies along the subducting slab in the top transition zone. Our modeled results show that a harzburgite wedge with a 1000‐K colder geotherm and metastable α‐orthopyroxene and olivine displays compressional (VP) and shear‐wave (VS) velocities 3.0%–3.6(6)% and 2.0%–2.8(6)% lower than the surrounding mantle at 410–460 km depth, respectively. At deeper depth up to 520 km, VP and VS of this metastable wedge with β‐orthopyroxene and olivine are 3.6%–4.4(6)% and 2.8%–4.3(6)% lower than the pyrolitic mantle, respectively. The presence of both metastable orthopyroxene and olivine instead of metastable olivine alone helps better explain the origin of the low‐velocity anomalies within the subduction slab in the top transition zone. Plain Language Summary: Subducting slab plays a significant role in transportation the surface material to the Earth's deep interior. It is normally imaged as a high‐velocity body compared to the surrounding mantle. However, seismic studies detected the existence of 3%–5% low compressional‐wave velocity anomalies accompanied with strong seismic shear‐wave anisotropies within the subducting slab at the top transition zone in various locations of the Earth which cannot be explained by the presence of metastable olivine alone. Besides olivine, orthopyroxene could also remain metastable in the coldest harzburgite layer of the slab. Here we report experimental results on the single‐crystal elasticity of both α‐ and β‐orthopyroxene up to 20 GPa and 300 K. These experimental results allow us to provide a comprehensive evaluation on the velocity profiles of the coldest harzburgite layer of the slab. We found that the coldest harzburgite layer with 22–37 vol.% orthopyroxene and 62–78 vol.% olivine has the VP and VS 3.0%–4.4(6)% and 2.0%–4.5(6)% lower than those of the pyrolitic mantle in the mantle transition zone, respectively. The observed 3%–5% low‐velocity anomalies within the slab in the top transition zone should be explained by the metastable orthopyroxene and olivine instead of metastable olivine alone. Key Points: Single‐crystal elasticity of α‐ and β‐orthopyroxene was determined to 20 GPa and shows anomalous change across the phase transitionThe obtained results were used to model the velocity profiles of the coldest harzburgite layer of the sinking subduction slabWe find that the low‐velocity anomalies within the slab at the top transition zone should be caused by metastable orthopyroxene and olivine [ABSTRACT FROM AUTHOR]

The subduction of serpentinized slabs is the dominant process to transport "water" into Earth's mantle, and plays a pivotal role for subduction dynamics. Antigorite, the most abundant serpentine mineral in subduction settings, may imprint a seismic signature on serpentinized slabs, making them seismically distinguishable from the dry, non‐serpentinized ones. However, the complete single‐crystal elasticity of antigorite has not been experimentally constrained at high pressures, hindering the use of seismological approaches to detect serpentinization in subducting slabs. Here, we report the full elastic stiffness tensor of antigorite by single‐crystal Brillouin spectroscopy and X‐ray diffraction up to 7.71(5) GPa. We use our results to model seismic properties of antigorite‐bearing rocks and show that their seismological detectability depends on the geometrical relation between seismic wave paths and foliation of serpentinized rocks. In particular, we demonstrate that seismic shear anisotropy shows low sensitivity to serpentinization for a range of relevant geometries. Plain Language Summary: The subduction of serpentinized slabs plays a key role in the deep recycling of water into the Earth's interior. Antigorite is the main serpentine mineral in subducting slabs, and the most important carrier of water. Antigorite‐bearing rocks are predicted to have a distinct seismic signature, potentially allowing them to be detected with seismological approaches. However, our current knowledge on seismic properties of antigorite‐bearing rocks is limited, mostly hampered by a lack of experimental constraints on single‐crystal elasticity of antigorite at relevant pressures. In this study, state‐of‐the‐art techniques were employed to produce the first experimental description of the complete high‐pressure elasticity of antigorite single crystals. Our experimental data set was implemented in the modeling of seismic properties of antigorite‐bearing rocks at pressures relevant for subduction. Our results were used to discuss the relation between seismic wave path and shear wave anisotropy in serpentinized slabs, and challenge the use of shear wave splitting as a proxy for serpentinization in slabs. Key Points: Single‐crystal elasticity of antigorite at high pressures is determined by Brillouin spectroscopy and X‐ray diffraction experimentsSeismic signature of serpentinized slabs is constrained in a relevant composition‐pressure spaceSerpentinization in slabs may be undetectable through shear wave anisotropy [ABSTRACT FROM AUTHOR]

The subducting African Plate in the easternmost Mediterranean is actively tearing and deforming beneath the Anatolian Plate as the margin transitions from long‐lived subduction to collision. In central Anatolia, the subducting slab is characterized by both lateral and vertical slab tears. We investigate patterns of mantle flow around the edges of a contorting and fragmenting African slab segment, called the Cyprean slab, using measurements of shear wave splitting. We observe three distinct regions of coherent shear wave splitting that correlate with the segmentation boundaries of the Cyprean slab. Regionally coherent mantle flow occurs near both the eastern and western the edges of the slab. These regions of coherent splitting are separated by an area of null splitting that encompasses the Central Anatolian Volcanic Province near the easternmost edge of the slab. The null measurements likely result from mantle upwelling due to the displacement of asthenosphere from the vertical Cyprean slab. Plain Language Summary: The subducting African Plate in the easternmost Mediterranean has been tearing beneath central Turkey as the region switches from subduction to collision. This area provides one of the best chances to study how tearing of a subducting plate impacts patterns of mantle flow during the last stages of subduction. We study mantle flow around the edges of the fragmented Cyprean slab using measurements of shear wave splitting, which provide information on the direction of mantle flow via the fast splitting direction and delay time. We see three distinct regions of mantle dynamics that relate to the boundaries of the Cyprean slab. We observe coherent flow near both the eastern and western edges of the slab that align with background regional dynamics. The two regions with coherent flow are separated by an area of anomalously absent splitting. This likely indicates that mantle is flowing upward next to the vertical Cyprean slab segment in central Anatolia, including just below the Central Anatolian Volcanic Province. Key Points: Shear wave splitting in central Anatolia shows evidence of mantle upwelling around the Cyprean slabWeak toroidal return flow exists along the western edge of the Cyprean slab and not along the eastern boundaryBackground mantle flow driving the Anatolian Plate is seen at many Continental Dynamics: Central Anatolian Tectonics stations [ABSTRACT FROM AUTHOR]

A number of geophysical observations reveal that the underthrusting Indian continental plate beneath the southern Tibetan Plateau is laterally torn and segmented, with contrasting subduction angles along strike. However, the mechanism of such kind of slab tearing remains unclear. Here, a series of 3‐D high‐resolution numerical models are conducted, which suggest that the lateral variation (either gradually or abruptly changing) of boundary convergence rate plays a critical role in the tearing of underthrusting continental slab. The abruptly changing convergence with a large lateral contrast could lead to slab tearing all by itself, whereas the gradually changing convergence should be combined with pre‐existing weakness in the underthrusting block. The structures and properties of overriding block play secondary roles. Finally, we propose that Indian slab tearing could be attributed to the time‐dependent and lateral variation of convergence rate during the Indian‐Asian collision and the pre‐existing Indian lithospheric weakness. Plain Language Summary: Horizontal slab tearing associated with the final slab break‐off occurs in many places of the global subduction‐collision system, the mechanism of which is easier for understanding and is mainly driven by the negative buoyancy of the subducted slab. However, for the underthrusting Indian continental plate beneath the southern Tibetan Plateau, the slab tearing and fragmentation occurs along several vertical planes perpendicular to the collision zone, which is identified by a number of geophysical explorations. It is rather difficult for a coherent subducting slab to tear along these sub‐vertical planes due to the lack of major forces in the along‐strike direction. Thus, the mechanism of Indian slab tearing is widely debated and remains unclear. In this study, many speculations have been tested by conducting a series of 3‐D high‐resolution numerical models. The model results indicate that the Indian slab tearing may be resulted from the time‐dependent and lateral variation of convergence rate between the Indian‐Asian collision and the resulting rotation of Indian continent, as well as the pre‐existing weakness within the Indian plate. Alternatively, the structures and properties of overriding Tibetan plate only play secondary roles. Key Points: Systematic 3‐D numerical models are conducted to study the mechanism of Indian slab tearing along vertical planes beneath Tibetan PlateauLateral variation of boundary convergence rate (either abruptly or gradually changing) is generally required for such slab tearingIndian slab tearing may be attributed to the gradually changing collision rate along strike and the pre‐existing lithospheric weakness [ABSTRACT FROM AUTHOR]

The Patagonian slab window has been proposed to enhance the solid Earth response to ice mass load changes in the overlying Northern and Southern Patagonian Icefields (NPI and SPI, respectively). Here, we present the first regional seismic velocity model covering the entire north‐south extent of the slab window. A slow velocity anomaly in the uppermost mantle indicates warm mantle temperature, low viscosity, and possibly partial melt. Low velocities just below the Moho suggest that the lithospheric mantle has been thermally eroded over the youngest part of the slab window. The slowest part of the anomaly is north of 49°S, implying that the NPI and the northern SPI overlie lower viscosity mantle than the southern SPI. This comprehensive seismic mapping of the slab window provides key evidence supporting the previously hypothesized connection between post‐Little Ice Age anthropogenic ice mass loss and rapid geodetically observed glacial isostatic uplift (≥4 cm/yr). Plain Language Summary: A gap in the subducting plate beneath Patagonia has enabled hotter, less viscous mantle material to flow underneath South America. Icefields in the Austral Andes above the gap in the plate have recently been shrinking, removing weight that had caused the continent to flex downward. We use seismic data to image the subsurface structure and find very low seismic velocity within and around the gap, as well as thinning of the rigid South American lithosphere overlying the gap. The low mantle velocity implies that mantle viscosity is also low beneath the shrinking icefields, and low viscosity enables the region to rebound upwards. Key Points: We observe anomalously low upper mantle shear wave velocities (<4.1 km/s) in the Patagonian slab windowThe lithospheric mantle has been thermally eroded over the youngest part of the slab windowLow viscosities in the slab window link observed rapid geodetic uplift to geologically recent ice mass loss in Patagonia [ABSTRACT FROM AUTHOR]

Subduction of the Nazca plate results in the uneven distributions of earthquakes and arc volcanoes along the South America's western margin. Here, we construct a high‐resolution shear‐wave velocity model from immediately offshore to the backarc in South America, using advanced full‐wave ambient noise tomography. Our new model confirms and provides further constraints on three major features, including (a) extensive low‐velocity anomalies within the continental crust, (b) two high‐velocity flat slab segments located beneath southern Peru and central Chile, and (c) complex slab geometry at flat‐to‐normal transitional subduction. The flat slab segments roughly correlate with the volcanic gaps but not with the seismicity gaps. We suggest that variations of slab geometry along strike and down dip have significantly modified the flow patterns within the mantle wedge. Subduction of oceanic ridges has altered the slab dehydration processes, which can influence the distribution of arc volcanism and the occurrence of intermediate‐depth earthquakes. Plain Language Summary: The subduction of the oceanic Nazca plate beneath the South American continent results in abundant earthquakes and arc volcanoes, which are unevenly distributed along the margin. There is a lack of intermediate‐depth earthquakes and arc volcanoes beneath Peru and central Chile/Argentina. How the properties of the subducted slab influence the distribution patterns of earthquakes and arc volcanoes remains as a fundamental question to be answered. In this study, we use an advanced seismological imaging method to construct a high‐resolution seismic velocity model within the South American margin. Our new model reveals two flat subduction regions beneath southern Peru and central Chile, which roughly correlate with the observed gaps of arc volcanoes but not of the earthquakes. Our model suggests that the slab could be either torn or distorted when transiting from flat to normal subduction. We propose that the variations of slab geometry have significantly modified the mantle flow patterns within the mantle wedge, resulting in the uneven distributions of arc volcanoes. The subduction of the Nazca Ridge and the Juan Fernandez Ridge has played a critical role in controlling the occurrence of the intermediate‐depth earthquakes. Key Points: Strong low‐velocity anomalies distribute extensively within the continental crust along latitudes of 12°–28°S at 15–35 km depthsThe subducting slab is divided into four segments along the margin including two normal sections and two nearly flat sectionsThe two flat slab segments beneath southern Peru and central Chile correlate well with the volcanic gaps but not with the seismic gaps [ABSTRACT FROM AUTHOR]

The state‐of‐stress within subducting oceanic plates controls rupture processes of deep intraslab earthquakes. However, little is known about how the large‐scale plate geometry and the stress regime relate to the physical nature of the deep intraslab earthquakes. Here we find, by using globally and locally observed seismic records, that the moment magnitude 7.3 2021 East Cape, New Zealand earthquake was driven by a combination of shallow trench‐normal extension and unexpectedly, deep trench‐parallel compression. We find multiple rupture episodes comprising a mixture of reverse, strike‐slip, and normal faulting. Reverse faulting due to the trench‐parallel compression is unexpected given the apparent subduction direction, so we require a differential buoyancy‐driven stress rotation, which contorts the slab near the edge of the Hikurangi plateau. Our finding highlights that buoyant features in subducting plates may cause diverse rupture behavior of intraslab earthquakes due to the resulting heterogeneous stress state within slabs. Plain Language Summary: A key type of tectonic boundary is where two plates collide with one sinking into the mantle beneath. These subduction zones generate the world's largest earthquakes. Quantifying stress in the subducting plate ("slab") is important because slabs drive the global plate‐tectonic system, and large earthquakes can occur within them. These earthquakes can cause strong shaking, and when occurring near cities, can lead to damage. However, mapping stress is challenging as we cannot directly "see" inside deep slabs. Our best indications of slab stress come from earthquakes themselves. A magnitude 7.3 earthquake north of New Zealand in 2021 generated a distinct pattern of seismic waveforms at seismometers installed worldwide. We used these seismic records to probe the earthquake, providing a new view of stress in subduction zones. We found the earthquake generated both vertical and horizontal motions along faults, driven by compressional and extensional stresses deep within the slab. The compressional part is oriented 90 degrees from the subduction direction, which is opposite to the usual compression in subduction zones. This unusual direction of compression can be explained by subduction of a thickened and buoyant part of the Pacific plate, known as the Hikurangi plateau. Key Points: A moment magnitude 7.3 2021 East Cape, New Zealand intraslab earthquake comprised multiple rupture episodes with different faulting stylesThe complex rupture comprises components of shallow trench‐normal extension and unexpectedly, deep trench‐parallel compression in slabThe trench‐parallel compression likely reflects stress rotation at a buoyancy contrast that drives slab contortion [ABSTRACT FROM AUTHOR]

Small‐scale intraslab heterogeneity is well documented seismically in multiple subduction zones, but its nature remains elusive. Previous efforts have been mostly focusing on the scattering strength at intermediate depth (<350 km), without constraining its evolution as a function of depth. Here, we illustrate that the inter‐source interferometry method, which turns deep earthquakes into virtual receivers, can resolve small‐scale intraslab heterogeneity in the mantle transition zone. The interferometric waveform observations in the Japan subduction zone require weak scattering (<1.0%) within the slab below 410 km. Combining with previous studies that suggest high heterogeneity level (∼2.5%) at intermediate depth, we conclude that the small‐scale intraslab heterogeneity weakens as slabs subduct. We suggest that the heterogeneities are caused by intraslab hydrous minerals, and the decrease in their scattering strength with depth reveals processes associated with dehydration of subducting slabs. Plain Language Summary: Deep earthquakes often generate surprisingly lengthy surface shaking, due to wave trapping and guiding along laminar small‐scale scatterers within subducting slabs. The nature of these scatterers is not well understood, although they have been observed in multiple subduction zones above 350 km. Here, by turning some deep earthquakes into virtual sensors closer to our target, we find that the small‐scale scatterers within the slab core fade substantially as slab subducts, from ∼2.5% above 350 km to <1% below 410 km. The fading signal suggests that the scatterers are caused by heterogeneous hydration of the slab in the outer rise that decreases as the slab core dehydrates. Key Points: We apply inter‐source interferometry to probe the small‐scale intraslab heterogeneity in the mantle transition zone beneath the Japan SeaThe deep intraslab scattering level needs to be weaker than at intermediate depth to fit the high‐frequency interferometric waveformsThe decrease of intraslab scattering with depth reveals the processes associated with slab dehydration [ABSTRACT FROM AUTHOR]

Oxyhydroxides like δ‐(Al,Fe)OOH may stabilize "water" in Mid‐Ocean Ridge Basalt (MORB) subducted into the Earth's lower mantle. The single‐crystal elasticity of δ‐(Al,Fe)OOH has not been experimentally constrained, hampering an accurate evaluation of the seismic detectability of this high‐pressure solid solution, and the presence of "water," in the deep Earth. Here, we report the first experimental single‐crystal elasticity results of δ‐(Al0.97Fe0.03)OOH measured by X‐ray diffraction and Brillouin spectroscopy. We use our results to compute seismic properties of hydrous and anhydrous MORB at pressures and temperatures expected in slabs at shallow lower mantle conditions. We show that hydrous MORB is less dense than anhydrous MORB, but has faster aggregate seismic velocities. This suggests that hydration in MORB has an effect on velocities opposite to that observed in other lithologies, and further indicates that hydration of MORB increases the seismic contrast to the background mantle. Plain Language Summary: Water can be delivered into the Earth's mantle via subduction of hydrous phases, playing an active role in global‐scale geological processes. Hence, tracking hydrous phases during subduction is pivotal to understand the geological evolution of our planet. At lower mantle depths, oxyhydroxides such as δ‐(Al,Fe)OOH are the primary hosts of water in subducted oceanic crust. Here, we investigated the single‐crystal elasticity of δ‐(Al0.97Fe0.03)OOH at pressures consistent with subduction in the upper mantle and transition zone. Our experimental results were used to evaluate the impact that hydration has on the physical properties of oceanic crust subducted into the lower mantle. For this purpose, we modeled density and aggregate properties of hydrous and anhydrous oceanic crust subducted into the lower mantle. Our modeling shows that hydration induces a reduction in the density of the modeled lithologies, but increases their aggregate velocities. Our results suggest that hydration may influence the subducting behavior of oceanic crust as well as their detectability through seismology. Key Points: Brillouin spectroscopy and X‐ray diffraction provide first constraints on the single‐crystal elasticity of δ‐(Al,Fe)OOH at mantle pressuresHydration decreases density but increases aggregate velocities of MORB in the shallow lower mantleHydration of MORB increases the seismic velocity contrast with the background mantle [ABSTRACT FROM AUTHOR]

The Pamir plateau protrudes ∼300 km between the Tajik‐ and Tarim‐basin lithosphere of Central Asia. Whether its salient location and shape are caused by forceful indentation of a promontory of Indian mantle lithosphere is debated. We present a new local‐seismicity and focal‐mechanism catalog, and a P‐wave velocity model of the eastern part of the collision system. The data suggest a south‐dipping Asian slab that overturns in its easternmost segment. The largest principal stress at depth acts normal on the slab and is orientated parallel to the plate convergence direction. In front (south) of the Asian slab, a volume of mantle with elevated velocities and lined by weak seismicity constitutes the postulated Indian mantle indenter. We propose that the indenter delaminates and overturns the Asian slab, underthrusts the Tarim lithosphere along a compressive transform boundary, and controls the location and shape of the Pamir plateau. Plain Language Summary: The Pamir plateau stands out between the Tajik basin to the west and the Tarim basin to the east. Its location and shape are either caused by a part of the Indian continent that protrudes below Pamir's crust, or thinned lithosphere of a former Asian basin existed in the place of the Pamir and subducted during the collision of India with Asia. Our new seismological data show that the Asian slab—a displaced part or slice of the Tarim‐Tajik‐basin lithosphere—is overturned beneath the eastern Pamir. A zone of high seismic velocities, indicative of a relatively cold and rigid mantle lithosphere, occurs in front (south) of the Asian slab. A seismically active zone with low seismic velocities is squeezed between this structure and the Tarim lithosphere. Together, these observations trace the northern and eastern margin of the Indian mantle indenter that predefines the shape of the Pamir plateau. Key Points: New local earthquake catalog and seismic P‐wave velocity model for the eastern PamirElevated velocities outline the northern and eastern margin of the Indian mantle indenter beneath the Pamir plateauThe indenter overturns the eastern end of the Asian slab and terminates along a transform margin against the Tarim block [ABSTRACT FROM AUTHOR]

A new seismic model for crust and upper mantle of the south Central Andes is derived from full waveform inversion, covering the Pampean flat subduction and adjacent Payenia steep subduction segments. Focused crustal low‐velocity anomalies indicate partial melts in the Payenia segment along the volcanic arc, whereas weaker low‐velocity anomalies covering a wide zone in the Pampean segment are interpreted as remnant partial melts. Thinning and tearing of the flat Nazca slab is inferred from gaps in the slab along the inland projection of the Juan Fernandez Ridge. A high‐velocity anomaly in the mantle below the flat slab is interpreted as relic Nazca slab segment, which indicates an earlier slab break‐off triggered by the buoyancy of the Juan Fernandez Ridge during the flattening process. In Payenia, large‐scale low‐velocity anomalies atop and below the re‐steepened Nazca slab are associated with the re‐opening of the mantle wedge and sub‐slab asthenospheric flow, respectively. Plain Language Summary: Taking advantage of the abundant information recorded in seismic waveforms, we imaged the seismic structure of the crust and upper mantle beneath central Chile and western Argentina, where the oceanic Nazca slab is subducting beneath the South American plate. The subducted Nazca slab is almost flat at a depth of 100–150 km in the north of the study area below the Pampean region, where the Juan Fernandez seamount ridge is subducting as part of the Nazca slab. The slab steepens again in the south in the Payenia region. Our model reveals pronounced low‐velocity anomalies within the Pampean flat slab along the inland projection of the Juan Fernandez Ridge, indicating that the Pampean flat slab is thinned or even torn apart. A high‐velocity anomaly is imaged beneath the flat slab, representing a former slab segment that was broken off during the slab flattening process and was overridden by the advancing young slab. Our model suggests a causal relationship between the oceanic ridge subduction and the flat slab formation. In the Payenia region, the slab re‐steepening resulted in the re‐establishment of the mantle wedge and induced hot mantle flow below the slab, which are characterized by low‐velocity anomalies in the model. Key Points: A new seismic model for the crust and upper mantle beneath central Chile and western Argentina is presentedThinning and tearing within the Pampean flat slab is detected along the inland projection of the Juan Fernandez RidgeA relic slab is imaged beneath the Pampean flat slab, reflecting slab break‐off during the flattening process [ABSTRACT FROM AUTHOR]

Phase A (Mg7Si2O8(OH)6) is one of the important dense hydrous magnesium silicates in subducting slab, because it forms after the breakdown of antigorite serpentine and could be the dominant hydrous phase in the upper‐mantle deep slab for the water transportation into deep Earth. In this study, the compressional (P) and shear (S) wave velocities of phase A were measured at simultaneous pressure and temperature conditions up to 11 GPa and 1073 K. Combined with elastic properties of olivine and pyroxenes, we calculate the hydration effect on the velocities of harzburgite lithology in cold subduction zones throughout the depth ranges where phase A is thermodynamically stable. Our calculations suggest that the hydration increases both P‐ and S‐wave velocities of harzburgite; at ∼5 wt% hydration, its seismic detectability is enhanced by 1%–1.5% in velocity contrasts relative to its anhydrous counterpart. Plain Language Summary: Oceanic plate reacts with sea water and forms several hydrous minerals, including antigorite. When cold oceanic plate went down into deep Earth, which has high pressure and high temperature, antigorite transforms into several high‐pressure hydrous minerals. Among those, phase A, an experimentally synthesized hydrous mineral, is believed to be the dominant hydrous phase. A technique called ultrasonic interferometry was used in the laboratory to simulate the propagation of seismic wave in the small sample. We used this technique to measure the speed of sound of phase A at conditions related to the upper mantle. Combined with data of other minerals, we can calculate the wave speed of hydrous harzburgite layer in the oceanic plate and compare the results with those seismic researches. We found that hydrous harzburgite with 5 weight percent of water has higher seismic wave speed than the dry one, by about 1%–1.5%. Key Points: Direct measurement of P‐ and S‐wave velocities of phase A at mantle pressures and temperaturesFormation of phase A increases both VP and VS of cold subduction slabs but decreases the VP/VS ratioHydration of harzburgite increases the velocity contrasts between slabs and ambient mantle [ABSTRACT FROM AUTHOR]

Phase A (Mg7Si2O8(OH)6) is one of the important dense hydrous magnesium silicates in subducting slab, because it forms after the breakdown of antigorite serpentine and could be the dominant hydrous phase in the upper‐mantle deep slab for the water transportation into deep Earth. In this study, the compressional (P) and shear (S) wave velocities of phase A were measured at simultaneous pressure and temperature conditions up to 11 GPa and 1073 K. Combined with elastic properties of olivine and pyroxenes, we calculate the hydration effect on the velocities of harzburgite lithology in cold subduction zones throughout the depth ranges where phase A is thermodynamically stable. Our calculations suggest that the hydration increases both P‐ and S‐wave velocities of harzburgite; at ∼5 wt% hydration, its seismic detectability is enhanced by 1%–1.5% in velocity contrasts relative to its anhydrous counterpart. Plain Language Summary: Oceanic plate reacts with sea water and forms several hydrous minerals, including antigorite. When cold oceanic plate went down into deep Earth, which has high pressure and high temperature, antigorite transforms into several high‐pressure hydrous minerals. Among those, phase A, an experimentally synthesized hydrous mineral, is believed to be the dominant hydrous phase. A technique called ultrasonic interferometry was used in the laboratory to simulate the propagation of seismic wave in the small sample. We used this technique to measure the speed of sound of phase A at conditions related to the upper mantle. Combined with data of other minerals, we can calculate the wave speed of hydrous harzburgite layer in the oceanic plate and compare the results with those seismic researches. We found that hydrous harzburgite with 5 weight percent of water has higher seismic wave speed than the dry one, by about 1%–1.5%. Key Points: Direct measurement of P‐ and S‐wave velocities of phase A at mantle pressures and temperaturesFormation of phase A increases both VP and VS of cold subduction slabs but decreases the VP/VS ratioHydration of harzburgite increases the velocity contrasts between slabs and ambient mantle [ABSTRACT FROM AUTHOR]

Distributions of slabs within Earth's mantle are increasingly used to reconstruct past subduction zones, based on first‐order assumptions that slabs sink vertically after slab break‐off, and thus delineate paleo‐trench locations. Non‐vertical slab motions, which occur prior to break‐off, represent a potentially significant source of error for slab‐based plate reconstructions, but are poorly understood. We constrain lateral migration of the Indian slab and overlying India‐Asia collision zone by comparing tomographically imaged mantle structure with plate‐kinematic constraints. Following coupling of the Indian and Australian plates at the onset of collision, ∼1,000 km lateral migration of the Indian slab was driven by vertical subduction of the Australian slab. The sinking behaviors of individual slabs do not evolve in isolation, but instead influence, or are influenced by, other slabs in the same plate network. Hence, lateral slab migrations may be determined by interpreting the sinking behavior of slabs collectively, and with respect to plate kinematics. Plain Language Summary: To understand the links between plate tectonics and mantle processes, researchers must determine how tectonic plates have moved with respect the evolving mantle through geological time. To overcome this problem, recent studies use the locations of subducted slabs in the deep mantle to reconstruct plate motions, based on the hypothesis that slabs sink vertically through the mantle, and therefore mark the surface locations of past subduction zones. Here, we test slab sinking hypotheses, and their use in plate reconstruction modeling, by investigating the sinking kinematics of the subducting Indian and Australian slabs during the India‐Asia collision. Our analysis indicates that since onset of collision at ∼45–40 Ma, the Indian slab migrated laterally, ∼1,000 km northwards through the mantle, driven by subduction of the neighboring Australian slab. We arrive at this new interpretation because we interpret Indian and Australian slab kinematics collectively, and with respect to India‐Australia plate motions. Our study shows that the sinking behavior of one slab can influence that of another slab in the same network. Slab‐based plate reconstructions should therefore interpret slabs of the same network collectively, and with respect to plate motions, in order to constrain non‐vertical slab motions and avoid potentially significant plate reconstruction errors. Key Points: Subduction of Australian oceanic lithosphere drove northward motion of coupled India‐Australia plate since onset of collision at 45–40 MaBuoyant Indian continent stalled subduction of Indian slab whilst Australian slab subduction drove motion of coupled India‐Australia plateApproximately 1,000 km north lateral migration of Indian slab occurred to maintain compatibility with plate kinematics of coupled India‐Australia plate [ABSTRACT FROM AUTHOR]

The extensive fast seismic anomalies in the mantle transition zone beneath East Asia are often interpreted as stagnant Pacific slabs, and a reason for the widespread tectonics since the Mesozoic. Previous hypotheses for their formation mostly emphasize vertical resistances to slab penetration or trench retreat. In this study, we investigate the origin of these stagnant slabs using global‐scale thermal‐chemical models with data‐assimilation. We find that subduction of the Izanagi‐Pacific mid‐ocean ridge marked the transition of mantle flow beneath western Pacific from being surface‐driven Couette‐type flow to pressure‐driven Poiseuille‐type flow, a result previously unrealized. This Cenozoic westward mantle wind driven by the pressure gradient independently explains seismic anisotropy in the region. We conclude that the mantle wind is the dominant mechanism for the formation of stagnant slabs by advecting them westward while the pressure gradient holds them in the transition zone. Plain Language Summary: The wide‐spread flat lying fast seismic anomalies in the mantle transition zone are often referred to as stagnant slabs. The mechanisms for their formation are debated. In this study we use Earth‐like global models to simulate slab geometry variation and reproduced the East Asian stagnant slabs. By running different tests we find that most previously proposed mechanisms are not the key reason, including the phase transformation at 660 km, viscosity increase in the lower mantle, seafloor age variation, and trench retreat. Instead, a westward mantle wind controlled the formation of stagnant slabs. This mantle wind occurred after the Izanagi‐Pacific mid‐ocean ridge began to subduct, and is driven by a long‐lasting horizontal pressure gradient induced by previously subducted slabs. Without this mantle wind the slabs sink directly into the lower mantle. Seismic anisotropy models further confirm the dominant role of this upper‐mantle flow in forming stagnant slabs. Key Points: Global thermal‐chemical subduction models with data‐assimilation reproduced the evolution of East Asian stagnant slabsThe key mechanism for slab stagnation is the westward mantle wind driven by continuous former Izanagi and Tethyan subductionAnisotropy calculations confirm the predominant upper‐mantle depth of the mantle wind since the early Cenozoic [ABSTRACT FROM AUTHOR]

The thermal stability of hydrous minerals in Earth's deep interior is key to understanding the evolution and physicochemical states of the planet. The recently discovered pyrite‐type (Py) FeO2Hx (x ≤ 1) phase, which can be transformed from α/ε‐FeOOH at ∼80 GPa, is believed to be a crucial candidate in transporting water and hydrogen to the lowermost mantle through subducting slabs. Here, we examined the stability and decomposition behavior of FeOOH through a set of shock‐recovery experiments up to ∼70 GPa and ∼2,750 K. Our results show that FeOOH partially decomposes to iron oxides Fe2O3 and Fe3O4 at 35–70 GPa and 1,150–2,750 K, which indicates that H2O and O2 are released during the decomposition of FeOOH in subducting slabs. The released H2O and O2 may have altered the physical and chemical properties of the surrounding mantle and contributed to the oxidation of surface Earth. Plain Language Summary: The mineral goethite (α‐FeOOH), a primary component of rusts and bog iron ores, is widespread on our planet. Early studies show that FeOOH polymorphs could transport water and hydrogen to the lowermost mantle when the minerals are transformed into hydrogen‐bearing iron peroxides at ∼1,800 km depths. However, the thermal stability of FeOOH at the mid‐lower mantle equivalent pressure‐temperature conditions is poorly known. Here, we used a shock‐recovery approach to examine the stability of FeOOH at pressures up to ∼70 GPa and temperatures to ∼2,750 K. Our results show that FeOOH partially decomposes to form hematite and magnetite, releasing water and oxygen at the equivalent pressure‐temperature conditions of a subducting slab in the mid‐lower mantle, thus indicating a narrower FeOOH‐stable region in the lower mantle than previously proposed. The released oxygen via FeOOH decomposition may have provided extra oxidation power for the long‐term oxidation of Earth's surface. Key Points: The thermal stability of FeOOH has been investigated by shock recovery experiments up to ∼70 GPa and ∼2,750 KFeOOH partially decomposes and releases water and oxygen in the mid‐lower mantleThe deoxygenation of FeOOH is a potential sporadic oxygen source for the Great Oxidation Event of Earth [ABSTRACT FROM AUTHOR]

Seismic observations indicate accumulation of subducted slabs in the mantle transition zone in many subduction zones. By systematically conducting 2‐D numerical experiments, we demonstrate that a weak layer or zone beneath the spinel‐to‐post‐spinel phase transition leads to horizontally deflected (stagnant) slab structures in the mantle transition zone, which is consistent with recent studies of 3‐D global mantle convection models. Trench retreat velocity, Clapeyron slope and the viscosity contrast between the lower mantle and mantle transition zone also affect horizontally deflected slab formation. By considering grain size dependent viscosity and grainsize evolution for slabs going through the phase change in the lower mantle, our models with a dynamically generated weak zone beneath the phase boundary indicate that the geometry and viscosity reduction of the weak zone is strongly affected by grain growth rate. A smaller grain growth rate results in a thicker and wider weak zone that promotes deflected slab formation. Plain Language Summary: When oceanic plates subduct into the deep mantle, some of them penetrate from the upper mantle to the lower mantle directly, others are horizontally deflected over a long distance in the mantle transition zone above 670‐km depth as indicated by recent seismic observations. In this study, by conducting numerical experiments, we demonstrate that this phenomenon could be explained by presence of a weak layer or zone in the lower mantle immediately beneath the mantle transition zone. The weak zone is generated due to grainsize reduction after phase change of mantle minerals. By considering grain size dependent viscosity and grainsize evolution for subducted slabs going through the phase change in the lower mantle, our models with a dynamically generated weak zone beneath the phase boundary indicate that the geometry and viscosity reduction of the weak zone is strongly affected by the grain growth rate. Our results also show that trench retreat velocity, phase change properties and the viscosity contrast between the lower mantle and mantle transition zone also affect the horizontally deflected slab formation. Key Points: A weak layer or zone beneath the spinel‐to‐post‐spinel phase transition leads to deflected slab structures in the mantle transition zoneTrench retreat velocity and the viscosity contrast between the upper and the lower mantle also affect horizontally deflected slab formationThe weak zone is controlled by the grainsize reduction following the phase transition and subsequent grain growth rate [ABSTRACT FROM AUTHOR]

Deep‐focus earthquakes provide insight into how subducting slabs deform over a range of spatial and temporal scales as they descend into the mantle. This study uses a 4D source imaging approach to determine centroid locations of the 2015 Mw 7.9 Bonin Islands deep‐focus earthquake and its aftershock sequence. Imaged sources of the mainshock show a complex rupture, but one that is compatible with a sub‐horizontal rupture plane. Previously undetected early aftershocks are imaged down to depths of approximately 750 km and represent the first reported earthquakes that initiate in the lower mantle. These events and a previously reported group of shallower distal aftershocks occur at the lower and upper boundaries of an imaged slab segment that deforms as it penetrates into the lower mantle. We hypothesize that mainshock failure allowed gravitational settling of the slab segment to occur which produced the distal aftershock sequences. Plain Language Summary: The causes of earthquakes below depths of approximately 60 km inside the Earth have been debated for nearly a century. This study reports on the first detected earthquakes with source locations in the lower mantle. These observations provide new insights into the mechanisms that can produce earthquakes deep inside the Earth. Key Points: A 4D back‐projection method is used to image the 2015 Mw 7.9 Bonin Islands deep‐focus earthquake and its aftershock sequenceEarly aftershocks detected down to ∼750 km depth are the first reported events that initiate in the lower mantleDistal aftershocks are hypothesized to occur at the top and bottom boundaries of a settling slab segment in response to the mainshock [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]

Deep‐focus earthquakes provide insight into how subducting slabs deform over a range of spatial and temporal scales as they descend into the mantle. This study uses a 4D source imaging approach to determine centroid locations of the 2015 Mw 7.9 Bonin Islands deep‐focus earthquake and its aftershock sequence. Imaged sources of the mainshock show a complex rupture, but one that is compatible with a sub‐horizontal rupture plane. Previously undetected early aftershocks are imaged down to depths of approximately 750 km and represent the first reported earthquakes that initiate in the lower mantle. These events and a previously reported group of shallower distal aftershocks occur at the lower and upper boundaries of an imaged slab segment that deforms as it penetrates into the lower mantle. We hypothesize that mainshock failure allowed gravitational settling of the slab segment to occur which produced the distal aftershock sequences. Plain Language Summary: The causes of earthquakes below depths of approximately 60 km inside the Earth have been debated for nearly a century. This study reports on the first detected earthquakes with source locations in the lower mantle. These observations provide new insights into the mechanisms that can produce earthquakes deep inside the Earth. Key Points: A 4D back‐projection method is used to image the 2015 Mw 7.9 Bonin Islands deep‐focus earthquake and its aftershock sequenceEarly aftershocks detected down to ∼750 km depth are the first reported events that initiate in the lower mantleDistal aftershocks are hypothesized to occur at the top and bottom boundaries of a settling slab segment in response to the mainshock [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]

Measurement of anisotropy advances our understanding of mantle dynamics by linking remote seismic observations to local deformation state through constraints from mineral physics. The Pacific Northwest records the largest depth‐integrated anisotropic signals across the western United States but the depths contributing to the total signal are unclear. We used the amplitudes of orthogonally polarized P‐to‐S converted phases from the mantle transition zone boundaries to identify anisotropy within the ∼400–700 km deep layer. Significant anisotropy is found near slab gaps imaged by prior tomography. Focusing of mantle flow through slab gaps may lead to locally elevated stress that enhances lattice preferred orientation of anisotropic minerals within the transition zone, such as wadsleyite. Plain Language Summary: Earth's mantle convects like a fluid over geological time and it organizes mineral fabrics resulting in directional dependence of seismic velocities, that is seismic anisotropy. There is abundant evidence for flow‐induced seismic anisotropy at depths above about 400 km, but it is less clear if anisotropy is developed in the mantle transition zone at about 400–700 km deep. Here, we use seismic waves generated from the bottom and top of the transition zone to constrain anisotropy within the layer. Localized evidence of strong anisotropy is found beneath the Pacific Northwest near locations where prior imaging studies show gaps between subducted oceanic plate fragments. We propose that focused flow through constrictions like slab gaps may cause seismic anisotropy in the mantle transition zone. Key Points: PSH/PSV amplitude ratios between P660s and P410s are useful for constraining transition zone anisotropyEvidence for up to 2% mantle transition zone anisotropy is found beneath the Pacific Northwest of the USMantle flow between slab fragments may enhance development of transition zone anisotropy [ABSTRACT FROM AUTHOR]

The release of CO2 from subducted slabs is an important process of the global carbon cycle. Previous studies have demonstrated that metamorphic reactions, carbonatitic melts, carbonate dissolutions, and hydrous carbonatitic liquids are effective CO2 recycling mechanisms in hot and warm thermal regimes, whereas the fate of subducted carbonates in cold subduction zones remains poorly understood. Here, we investigate the phase relations of subducted ophicarbonate at pressures beyond sub‐arc depths. The presence of complex quench textures indicates that hydrous carbonatitic liquids formed at temperatures as low as ∼850°C at depths of ∼300–440 km, which corresponds to cold thermal regimes. Water released by the decomposition of dense hydrous magnesium silicate phases can substantially affect the stability of subducted carbonates, which facilitates the formation of hydrous carbonatitic liquids. This provides a mechanism for scavenging CO2 from cold slabs and may promote the formation of metasomatized domains in the deep mantle. Plain Language Summary: More than half a gigaton of CO2 is transported into the Earth each year, contributing to the evolution of oxygen fugacity of the deep mantle. Recycling CO2 from subducted slabs to the atmosphere help balance the global long‐term carbon cycle. Processes known as carbon removal from subducted slabs to arc volcanoes include metamorphic reaction, carbonatitic melt, hydrous carbonatitic liquid, and fluid dissolution, all of which are restricted to hot and warm subduction zones. In cold subduction zones, available studies show that carbon can be transported to great depths, rather than supply arc volcanoes. Here, we conduct high‐pressure experiments to investigate the phase transitions of ophicarbonate, important water‐ and carbon‐bearing rock in subduction zones. We find hydrous carbonatitic liquids at temperatures as low as ∼900°C at 5–10 GPa, and ∼850°C at 15 GPa, which correspond to typical cold subduction zones. We suggest that water released by the decomposition of hydrous minerals strongly affects the stability of carbonates, facilitating the formation of hydrous carbonatitic liquids. Therefore, dehydration‐induced carbonatitic liquid may provide a mechanism for the removal of CO2 from subducted slabs at great depths in cold thermal regimes. Key Points: Phase relations of subducted ophicarbonates were investigated under subduction zone thermal regimes at 5–15 GPaHydrous carbonatitic liquids were observed at temperatures as low as ∼850°CHydrous carbonatitic liquids may be a key mechanism for scavenging CO2 from subducting slabs in cold thermal regimes [ABSTRACT FROM AUTHOR]

Deep focus earthquakes within the Tonga‐Fiji subduction system account for about two‐thirds of the global total and provide significant constraints on slab deformation. The factors controlling the intense deformation remain unclear. Here, we use two‐dimensional, time‐dependent geodynamic models to study the morphology, stress state, and thermal structure of the Tonga‐Fiji subduction zone. The results, consistent with tomographic images and focal mechanisms, demonstrate that collision between a relic slab from the Vanuatu Trench and the Tonga slab may control the steeper dip of the Tonga slab and earthquakes in the mantle transition zone. We suggest that the magnitude 8.2 and 7.9 earthquakes in 2018 mostly ruptured within the warm rim of the Tonga slab and occurred beneath the folding relic slab with high temperatures of at least ∼900°C and ∼1100°C, respectively. The findings support the hypothesis that local slab temperature likely controls rupture of deep earthquakes. Plain Language Summary: Deep earthquakes at the depths between 300 and 700 km are controversial as the increase of temperature and pressure with depth will promote ductile flow and inhibit brittle failure. Approximately two‐thirds of the deep earthquakes globally are located below the North Fiji Basin which provides significant constraints on the deformation of the Tonga slab. However, the factors controlling the intense deformation remain unclear. Here, a time‐evolving mechanical model is used to study the morphology, state‐of‐stress, and thermal structure of the Tonga‐Fiji subduction zone. The models reproduce the high‐resolution Tonga slab morphology and can better fit the focal mechanisms of the deep earthquakes, which demonstrate that collision between a relic slab detached from the Vanuatu Trench and the Tonga slab leads to the nearly vertical Tonga slab and controls the occurrences of the deep earthquakes in the mantle transition zone. We suggest that two large deep earthquakes with moment magnitude (Mw) 8.2 and 7.9 in 2018 occurred within the warm rim of the Tonga slab and beneath the high‐temperature folded relic slab, respectively, supporting the hypothesis that local slab temperature probably controls the rupture details of deep earthquakes. Key Points: Geodynamic models reproduce the high‐resolution tomographic images of the Tonga slabCollision between a relic and Tonga slabs controls the intense deformation within the two slabsSimulated thermal structures predict deep earthquake doublet (Mw 8.2 and 7.9) occurs in the warm slab areas [ABSTRACT FROM AUTHOR]

Many convergent orogens, such as the eastern European Alps, display an asymmetric doubly vergent wedge geometry. In doubly vergent orogens, deepest exhumation occurs above the retro‐wedge. Deep‐seismic interpretations depict the European plate dipping beneath the Adriatic, suggesting the pro‐wedge location on the north side of the orogen. Our new thermochronometer data across the Eastern Alps confirm distinct shifts in the locus of exhumation associated with orogen‐scale structural reorganizations. Most importantly, we find a general Mid‐Miocene shift in exhumation (in the Tauern Window and the Southern Alps) and focus of modern seismicity across the Southern Alps. Taken together, these observations suggest a subduction polarity reversal at least since the Mid‐Miocene such that the present‐day pro‐wedge is located on the south side of the Alps. We propose a transient tectonic state of a slow‐and‐ongoing slab reversal coeval with motion along the Tauern Ramp, consistent with a present‐day northward migration of drainage divides. Plain Language Summary: When tectonic plates collide, they bend downwards and form two lithospheric wedges dipping in opposite directions, such as in the Eastern Alps. We present new crustal cooling data along a transect in the Eastern Alps confirming that surface rocks across the central Tauern Window originated from the deepest structural levels along the transect. South of the Tauern Window rocks were exhumed from higher depths compared to those north of it and were exhumed more recently, while seismic activity is also focused across the Southern Alps. These observations suggest a subduction polarity reversal because they are inconsistent with the original southern and northern locations of overriding and subducting plates, respectively, >15 million years ago. This interpretation is contrary to lithosphere‐scale tomography that shows no change in subduction polarity. Therefore, we propose a transient tectonic state, that is, a slow‐and‐ongoing subduction polarity reversal that initiated when Tauern Window rocks began their steep ascent to the surface along a deep‐seated fault known as the Tauern Ramp. This study bridges observations in the mantle, crust and on the surface over geologic time. Key Points: Thermochronologic data in the Eastern Alps is consistent with a transient tectonic state toward complete slab reversalThe pro‐wedge has switched from north to south of the Periadriatic Fault along TRANSALPMid‐Miocene motion along the Tauern Ramp is the consequence of slab‐reversal [ABSTRACT FROM AUTHOR]

Buoyancy forces associated with subducting lithosphere control the dynamics of convergent margins. In the postcollisional stage these forces are significantly reduced, yet mountain building and seismicity are ongoing, albeit at lower rates. We leverage advances of a newly developed seismo‐thermo‐mechanical modeling approach to simulate tectonic and seismicity processes in a self‐driven subduction and continental collision setting. We demonstrate that the rearrangement of forces due to slab breakoff, in the postcollisional stage, causes bending and rollback of the residual slab, suction forces, and mantle traction at the base of the upper plate, while stress coupling transfers to the shallow crust. Our results provide an explanation for the postcollisional evolution of the Central Alps, where the so‐called Slab Rollback Orogeny model explains the slow yet persistent upper plate advance, the height of the mountain range, and a seismicity pattern consistent with the different tectonic regimes throughout the orogen. Plain Language Summary: A long‐standing debate in tectonophysics evolves around whether vertical or horizontal forces are the primary drivers of mountain building processes. Here we explore this problem using 2‐D numerical models in a generic subduction and continental collision setting. Our results show how the postcollisional evolution of the orogen is controlled by a slow, but persistent, sinking and bending of the postbreakoff residual slab. The resulting seismicity pattern shows a broad pattern of different style of faulting, which are consistent with the local tectonic regimes. We find good correlations between our numerical results and the previously conflicting tectonic observations in the Central Alps and the adjacent foreland basin. Our results thus support the hypothesis that the postbreakoff remaining slab exerts a first‐order control on the motions and deformations of collisional orogens. Key Points: Two‐dimensional seismo‐thermo‐mechanical modeling is performed to investigate the driving forces during the postcollisional stagesPost‐breakoff residual slab proves sufficient to sustain slab rollback and retreat of the entire orogenThe proposed scenario is consistent with the postcollisional seismotectonic evolution of the central European Alps [ABSTRACT FROM AUTHOR]

We compare equilibrium climate sensitivity (ECS) estimates from pairs of long (≥800‐year) control and abruptly quadrupled CO2 simulations with shorter (150‐ and 300‐year) coupled atmosphere‐ocean simulations and slab ocean models (SOMs). Consistent with previous work, ECS estimates from shorter coupled simulations based on annual averages for years 1–150 underestimate those from SOM (−8% ± 13%) and long (−14% ± 8%) simulations. Analysis of only years 21–150 improved agreement with SOM (−2% ± 14%) and long (−8% ± 10%) estimates. Use of pentadal averages for years 51–150 results in improved agreement with long simulations (−4% ± 11%). While ECS estimates from current generation U.S. models based on SOM and coupled annual averages of years 1–150 range from 2.6°C to 5.3°C, estimates based longer simulations of the same models range from 3.2°C to 7.0°C. Such variations between methods argues for caution in comparison and interpretation of ECS estimates across models. Plain Language Summary: Precise definition and estimation of equilibrium climate sensitivity (ECS) continues to challenge model intercomparison. While annual analyses of years 1–150 of coupled atmosphere‐ocean models agree with slab ocean model simulations, they underestimate coupled ECS estimates from multicentennial to millennial scale simulations. However, long‐term ECS estimates can be largely recovered through a combination of (1) ignoring the first 50 years of abrupt 4 times preindustrial CO2 simulation dominated by early timescales of ocean response and (2) using pentadal (5‐year) averages instead of annual ones for years 51–150. This variation between methods argues for reconsideration of ECS estimation and application acknowledging that slab ocean estimates systematically ignore potential sources of enhanced sensitivity and simulations longer than 150 years are necessary for precise estimation of the long‐term trend. Key Points: Equilibrium climate sensitivity (ECS) estimates for a single coupled model can vary by more than 1°C (20%) depending on analysis methodECS estimates from ≥300‐year coupled simulations from current U.S. models range from 3.1°C to 7.0°C, another method giving 2.7°C to 5.3°CAnalysis of years 21–150 agrees with slab ocean ECS, but pentadal analysis of years 51–150 reduces bias against long, coupled simulations [ABSTRACT FROM AUTHOR]

Several hypotheses have been proposed to explain intriguing circular shear wave splitting patterns in the Pacific Northwest, invoking either 2‐D entrained flows or 3‐D return flows. Here, we present some hitherto unidentified, depth‐dependent anisotropic signatures to reconcile different conceptual models. At depths shallower than 200 km, the fast propagation directions of seismic waves to the west of the Rocky Mountain are aligned sub‐parallel to the subduction direction of the Juan de Fuca and Gorda Plates. This pattern is consistent with previous onshore/offshore shear wave splitting measurements and indicates that 2‐D entrained flows dominate at shallower depths. From 300 to 500 km, two large‐scale return flows are revealed, one circulating around Nevada and Colorado and the other running around the edge of the descending Juan de Fuca slab. These observations suggest the development of toroidal‐mode mantle flows, driven by the fast rollback of the narrow, fragmented Juan de Fuca and Gorda slabs. Key Points: The fast directions are aligned sub‐parallel to the subduction direction of the Juan de Fuca Plates at depths shallower than 200 kmTwo large‐scale return flows are revealed, circulating around the edges of the descending Juan de Fuca and Gorda slabsThe uppermost lower mantle beneath the Cascadian Subduction Zone is dominated by a NE‐SW oriented flow field [ABSTRACT FROM AUTHOR]

The presence of water in minerals generally alters their physical properties. Ringwoodite is the most abundant phase in the lowermost mantle transition zone and can host up to 1.5–2 wt% water. We studied high‐pressure lattice thermal conductivity of dry and hydrous ringwoodite by combining diamond‐anvil cell experiments with ultrafast optics. The incorporation of 1.73 wt% water substantially reduces the ringwoodite thermal conductivity by more than 40% at mantle transition zone pressures. We further parameterized the ringwoodite thermal conductivity as a function of pressure and water content to explore the large‐scale consequences of a reduced thermal conductivity on a slab's thermal evolution. Using a simple 1‐D heat diffusion model, we showed that the presence of hydrous ringwoodite in the slab significantly delays decomposition of dense hydrous magnesium silicates, enabling them to reach the lower mantle. Our results impact the potential route and balance of water cycle in the lower mantle. Plain Language Summary: The physical properties of minerals are determined by the interaction of atoms in the crystal lattice. Water can be incorporated into the crystal structure and alter its behavior. Ringwoodite is a high‐pressure mineral that can host large quantities of water and is expected to be abundant in the lower part of Earth's mantle transition zone, a region ranging from 520 to 660‐km depth. Here we studied ringwoodite thermal conductivity, describing how effectively heat is transported through solids. Based on our measurements we determined that water in ringwoodite significantly slows down heat propagation. We performed computer simulations to investigate the large‐scale implications of our findings. For this purpose, we modeled a cold oceanic plate, entirely made of ringwoodite, which is surrounded by warm mantle. The delayed heat transport is sufficient to maintain low temperatures in the inner part of the oceanic plate and potentially preserve the hydrous minerals for an extended period of time. Key Points: Ringwoodite thermal conductivity is reduced by 40% due to the presence of 1.73 wt% water in the crystal structureLower thermal conductivity of hydrous ringwoodite might delay the breakdown of hydrous phases hosted in a subducting slabHydrous ringwoodite acts as a heat propagation barrier, supporting preservation of hydrous minerals down to the lower mantle [ABSTRACT FROM AUTHOR]

The Andean subduction zone is an excellent place to study deformation within a subducting slab as a function of depth, owing to the varying and well‐resolved geometry of the subducting Nazca slab beneath South America. Here we combine the results of source‐side shear wave splitting with the latest regional tomography model to isolate intraslab raypaths and determine the spatial distribution of anisotropy within the Nazca slab. We observe that in the upper mantle, the intraslab anisotropy appears strongest where the slab is most contorted, suggesting a strong link between anisotropy and subduction‐related slab deformation. We identify a second source of anisotropy (δt∼ 1 s) within the subducting slab at lower mantle depths (660–800 km). The surrounding mantle and transition zone appear largely isotropic, with deep anisotropy concentrated within the slab as it deforms while entering the higher‐viscosity lower mantle. Plain Language Summary: Few observations exist of how a tectonic plate deforms as it descends deep into the Earth's interior at a subduction zone. Carefully selected seismic waves that mostly travel through this subducting plate, or slab, provide some of the most direct measurements of how the slab behaves as it sinks through the upper mantle (0–410 km) and the mantle transition zone (410–660 km). Studying the polarization of seismic waves allows us to detect and infer the pattern of deformation within the Earth's interior. Using this technique, we find that the Nazca slab in the Andean subduction zone in South America has undergone internal deformation during the process of subduction, in particular where the slab's 3‐D shape changes. Furthermore, we find that the deeper Nazca slab (≥660 km) appears to undergo further deformation as it interacts with the stiffer uppermost lower mantle. Key Points: We find a notable presence of anisotropy within the Nazca slab in both the upper and lower mantleUpper mantle slab anisotropy is strongest where the slab geometry is most contorted and deformedWidespread anisotropy is observed at the base of Nazca slab where it penetrates the higher‐viscosity lower mantle [ABSTRACT FROM AUTHOR]

Observations of remote dynamic triggering of intermediate and deep (I&D) earthquakes can potentially clarify the faulting process but have not been systematically compiled. We relocate earthquakes following the two largest recent deep earthquakes with a multiple‐event location algorithm and compile a comprehensive catalog of dynamically triggered I&D earthquakes from 1964 to 2015. We find 119 large I&D earthquakes that likely dynamically triggered aftershocks at distances between two mainshock fault lengths and 400 km during the first 4 days following these mainshocks. Dynamic triggering is more common for deep than for intermediate depth earthquakes and is overrepresented in areas with low seismicity that are adjacent to slabs, which suggest that subducting slabs in the transition zone are surrounded by material that is critically stressed but warm enough to inhibit rupture initiation. The passage of seismic waves may initiate runaway ductile shear failure, which nucleates over a period of minutes to days. Plain Language Summary: Seismic waves from earthquakes consist of dynamic stress and strain, which can trigger other earthquakes at close distances as well as at locations far away from the original earthquakes. We compile a comprehensive catalog of dynamically triggered intermediate (100–300 km depth) and deep (300–700 km depth) earthquakes from 1964 to 2015 on a global scale. We also focus on particularly significant dynamic triggering that occurred after two large deep earthquakes, and we obtain earthquake locations in both cases with higher resolution than standard global catalogs. We find that dynamic triggering is relatively common within 400 km distance and within the first 5 hr after the original earthquakes occurred. Our data set shows that dynamic triggering is more common for deep earthquakes than for intermediate earthquakes. Furthermore, at the greatest depths, the dynamically triggered earthquakes tend to be located in regions of little or no other seismicity. This observation implies that deep subducting slabs are surrounded by "thermal halos" where the temperature is high enough to prohibit the onset of earthquakes generally but are prone to hosting triggered earthquakes after the passage of high amplitude seismic waves. Key Points: Deep earthquakes are much more likely to be dynamically triggered than intermediate depth earthquakesDeep earthquakes tend to be triggered in mostly aseismic regions in the thermal halo surrounding the slab corePropagating seismic waves may induce thermal runaway processes in ductile shear zones [ABSTRACT FROM AUTHOR]

A total of 431 well‐defined and 632 null shear–wave splitting measurements obtained from 115 broadband seismic stations located in the eastern Himalayan syntaxis and adjacent areas is largely inconsistent with predicted fast orientations by absolute plate motion models. Spatial coherency analysis of the splitting parameters suggests that the observed azimuthal anisotropy is mostly located in the upper asthenosphere or the transitional layer between the lithosphere and asthenosphere, and the disagreement between the fast orientations and regional tectonic fabrics suggests an insignificant lithospheric contribution to the observed anisotropy. The observations may be attributed to flow systems that are driven by the westward rollback of the Indian slab beneath the Indo‐Burma block and are modulated by a previously revealed gap between the northward and eastward subducting slabs of the Indian plate. Key Points: Seismic anisotropy may be explained by a rollback of the Indian slab and modulation by a slab gapThe rotational pattern of fast orientations in SE Tibetan Plateau may be attributed to flow in the mantle wedge escaped from the slab gapN‐S anisotropy in NE India and Burma block and E‐W anisotropy in Indochina may reflect sub‐slab trench parallel flow from slab rollback [ABSTRACT FROM AUTHOR]

We analyzed new recordings of SPdKS seismic waveforms from a global set of broadband seismograms and horizontal tiltmeters from the Hi‐net array in Japan from 26 earthquakes in the Central American region. The anomalous waveforms are consistent with the presence of at least three ultralow‐velocity zones (ULVZs), on the core‐mantle boundary beneath northern Mexico and the southeastern United States. These ULVZs ring an area of high seismic wave speeds observed in tomographic models that has long been associated with past subduction. Waveform modeling using the PSVaxi method suggests that the ULVZs have S and P wave velocity decreases of 40% and 10%, respectively. These velocity decreases are likely best explained by a partially molten origin where the melt is generated through melting of mid‐ocean ridge basalt atop the subducted slab. Plain Language Summary: We use a set of seismic observations recorded globally to investigate the lower mantle beneath Central America. The deepest mantle in this region has been associated with the final resting place of subducted slab material from subduction that initiated approximately 200 million years ago. This ancient subducted material is associated with high seismic wave speeds in the lowermost mantle just above the core‐mantle boundary. We find that patches of highly reduced seismic wave speeds, referred to as ultralow‐velocity zones (ULVZs), appear to be associated with the border of the high wave speed region, along the border of the subducted slab material. These ULVZ patches are consistent with being regions of partial melt. A possible scenario for their creation is that mid‐ocean ridge basalt (MORB), comprising the crust of the subducted slab material, has a low melting point at conditions in the deep earth and may be melting as the slabs reach the bottom of the mantle. Previous experimental work has suggested that MORB will likely partially melt in the deep mantle, yet little evidence for the existence of MORB partial melt has previously been found. Key Points: Subducted slab material resting on the core‐mantle boundary is ringed by ultralow‐velocity zonesA partially molten origin to these ultralow‐velocity zones is likelyMelting of mid‐ocean ridge basalt may give rise to ultralow‐velocity zones [ABSTRACT FROM AUTHOR]