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The Southwestern United States experiences active deformation, seismicity, and magmatism, remarkable in an intraplate setting. The Basin and Range and Colorado Plateau (CP) are inferred to differ in lithospheric thickness, but modeling geophysical properties of the lithosphere, in particular the depth of the Lithosphere‐Asthenosphere Boundary (LAB), across the entirety of the region, has proved challenging. Here, we introduce a new model of 1‐D depth profiles in shear wavespeed, determined through a probabilistic joint inversion of information from Sp receiver functions and Rayleigh wave phase velocity. From these profiles we quantify the locations and Vs contrast of wavespeed gradients that represent boundaries such as the Moho, the LAB, and intralithospheric discontinuities. We infer a lithosphere that is thinner and lower in Vs in the Basin and Range. In the CP and farther north, the LAB is more gradual, deeper, and intermittently observed. We also observe Mid‐Lithospheric Discontinuities (MLDs) near the boundaries between the CP, Wyoming Craton, and Northern Basin and Range, as well as within the Craton. When both an MLD and LAB are observed, the Vs gradient associated with the LAB is narrower than expected. Finally, we image Positive Velocity Gradients beneath areas of thinner lithosphere, which are consistent with recent global observations that have been attributed to the base of a partially molten zone below the lithosphere. Overall, the picture of the lithosphere‐asthenosphere system that emerges is one of considerable structural complexity with a strong dependency on tectonic regime and geological history. Plain Language Summary: We examine the properties of, and processes that shape, the lithosphere—the rigid outermost layer of the Earth, which participates in plate tectonics. Our focus is the southwestern United States where in fact the lithosphere does not behave like a simple, homogeneous, plate. Instead, in the Basin and Range Province, widespread deformation, frequent earthquakes, and recent volcanic activity are observed, all of which are rare for a location removed from plate boundaries. In the neighboring Colorado Plateau and the regions north and west of it, the lithosphere behaves more rigidly but volcanism is also observed. To understand how the lithosphere and mantle below vary, we produce a model of seismic wavespeed using data from surface waves, which travel along the Earth's surface, and converted body waves, which interact with interior boundaries such as the bottom of the lithosphere. This model enables us to find the depths of major boundaries and understand how the speed of seismic waves changes across them. We consistently observe decreases in seismic wavespeed that we associate with the bottom of the lithosphere or structures within the lithosphere. We also note wavespeed increases below the lithosphere that might indicate partial melting of rocks deep within the Earth. Key Points: We perform a joint inversion for Vs in the upper mantle of the Southwestern U.S., using data from body wave scattering and surface wavesThe lithosphere varies regionally: it is flat and thin in the Basin and Range, thicker in the Colorado Plateau, and complex in cratonic regionsMid‐Lithospheric Discontinuities are observed in cratonic regions, associated with sharp Vs gradients at the base of the lithosphere [ABSTRACT FROM AUTHOR]

The features of the occurrence and propagation of transverse deformation waves in the elastic lithosphere–viscous asthenosphere system under nonisothermal conditions are studied in the approximation of a thin layer. In this case, the phase transition at the boundary of the layers plays an essential role due to temperature and pressure disturbances. The qualitative and quantitative properties of the propagation of disturbances of thermomechanical fields have been studied in the long-wave approximation. It is shown that, due to the energy supply from the nonisothermal asthenosphere, weakly attenuated wave packets can occur, which spread over thousands of kilometers with a characteristic speed of about 100 km/year. This makes it possible to consider them as a possible trigger mechanism for the massive emission of methane from frozen sedimentary rocks into the atmosphere. [ABSTRACT FROM AUTHOR]

The asthenosphere is commonly defined as an upper mantle zone with low velocities and high attenuation of seismic waves, and high electrical conductivity. These observations are usually explained by the presence of partial melt, or by a sharp contrast in the water content of the upper mantle. Low viscosity asthenosphere is an essential ingredient of functioning plate tectonics. We argue that a substantial component of asthenospheric weakening is dynamic, caused by dislocation creep at the base of tectonic plates. Numerical simulations of subduction show that dynamic weakening scales with the surface velocity both below the subducting and the overriding plate, and that the viscosity decrease reaches up to two orders of magnitude. The resulting scaling law is employed in an apriori estimate of the lateral viscosity variations (LVV) below Earth's oceans. The obtained LVV help in explaining some of the long‐standing as well as recent problems in mantle viscosity inversions. Plain Language Summary: The motion of lithospheric plates at the Earth's surface is enabled by a weak underlying layer—the asthenosphere. The origin of this low viscosity layer is still subject of discussion. Presence of water or partial melt were proposed as possible reasons of its reduced viscosity. Another mechanism that may lead to weakening is non‐linear deformation. Rheological description of asthenospheric material includes dislocation creep, a deformation mechanism that depends on the velocity contrast between the lithospheric plate and underlying mantle—the faster the plates are, the weaker the underlying layer becomes and vice versa. Here we argue that a substantial component of asthenospheric weakening is dynamic, caused by this deformation mechanism. We evaluate numerical models of subduction including dislocation creep and derive a relation between the surface velocity of oceanic plates and the magnitude of the underlying asthenospheric viscosity. This allows us to estimate how the viscosity varies under different oceanic plates on Earth, which is otherwise hard to constrain. Our results indicate that the asthenosphere below the Pacific plate should be particularly weak. Key Points: Substantial component of asthenospheric weakening is dynamic, caused by dislocation creep at the base of tectonic platesDynamic weakening scales with the surface velocity both below the subducting and the overriding plateThe resulting scaling law is employed in an apriori estimate of the lateral viscosity variations below Earth's oceans [ABSTRACT FROM AUTHOR]

The transition area between the Pannonian Basin and the East Carpathians is the subject of considerable tectonic research in Central Europe because of its geological diversity, especially for Lithosphere–Asthenosphere Boundary determinations. The major objective of this study is to investigate the Earth’s lithospheric structure and in particular to determine the Lithosphere–Asthenosphere Boundary from the Pannonian Basin, through the Transylvanian Basin to the Carpathian Bend area. We recorded six new deep magnetotelluric soundings and used two archive measurements to complete the information and to put additional constraints on the depth of the Lithosphere–Asthenosphere Boundary. In the part of the discussions we provide a brief overview of the existing various methodologies and Lithosphere–Asthenosphere Boundary determinations for the wider Carpathian–Pannonian region and Europe and comparison with new magnetotelluric results. The lowest Lithosphere–Asthenosphere Boundary depth was detected in the Pannonian Basin (~50 km). Our results indicate in addition that the Lithosphere-Asthenosphere Boundary beneath the Transylvanian Basin (~70–80 km) is not thick and much thinner than those of the European and Moesian Platforms. The additional geophysical information (geomagnetic data and phase tensor results) detected the presence of deep well conductive zones towards the Pannonian Basin, the Bogdan Vodă–Dragoș Vodă fault and the East Carpathians. This could be explained by the elevated position of the well conductive asthenosphere in the Pannonian Basin and deep and presumably fluid rich tectonic zones associated with the Bogdan Vodă–Dragoș Vodă fault and the East Carpathians. The phase tensors highlighted that the most complex tectonic zones are present in the vicinity of the East Carpathians (where the average Lithosphere–Asthenosphere Boundary depth is >100 km), which is in line with the relatively young age of the mountain belt and the very complex nature of collisional orogens. [ABSTRACT FROM AUTHOR]

Sublithospheric diamonds and the inclusions they may carry crystallize in the asthenosphere, transition zone, or uppermost lower mantle (from 300 to ∼800 km), and are the deepest minerals so far recognized to form by plate tectonics. These diamonds are distinctive in their deformation features, low nitrogen content, and inclusions of these major mantle minerals: majoritic garnet, clinopyroxene, ringwoodite, CaSi perovskite, ferropericlase, and bridgmanite or their retrograde equivalents. The stable isotopic compositions of elements within these diamonds (δ11B, δ13C, δ15N) and their inclusions (δ18O, δ56Fe) are typically well outside normal mantle ranges, showing that these elements were either organic (C) or modified by seawater alteration (B, O, Fe) at relatively low temperatures. Metamorphic minerals in cold slabs are effective hosts that transport C as CO3 and H as H2O, OH, or CH4 below the island arc and mantle wedge. Warming of the slab generates carbonatitic melts, supercritical aqueous fluids, or metallic liquids, forming three types of sublithospheric diamonds. Diamond crystallization occurs by movement and reduction of mobile fluids as they pass through host mantle via fractures—a process that creates chemical heterogeneity and may promote deep focus earthquakes. Geobarometry of majoritic garnet inclusions and diamond ages suggest upward transport, perhaps to the base of mantle lithosphere. From there, diamonds are carried to Earth's surface by eruptions of kimberlite magma. Mineral assemblages in sublithospheric diamonds directly trace Earth's deep volatile cycle, demonstrating how the hydrosphere of a rocky planet can connect to its solid interior. Sublithospheric diamonds from the deep upper mantle, transition zone, and lower mantle host Earth's deepest obtainable mineral samples. Low-temperature seawater alteration of the ocean floor captures organic and inorganic carbon at the surface eventually to become some of the most precious gem diamonds. Subduction transports fluids in metamorphic minerals to great depth. Fluids released by slab heating migrate, react with host mantle to induce diamond crystallization, and may trigger earthquakes. Sublithospheric diamonds are powerful tracers of subduction—a plate tectonic process that deeply recycles part of Earth's planetary volatile budget. [ABSTRACT FROM AUTHOR]

The upper mantle and the transition zone of the Baikal rift zone (BRZ) are studied. The observations are analyzed from P-wave receiver functions. It is found that in the BRZ central and northeastern part, the P410s converted seismic phase is preceded by a precursory wave with negative polarity which is formed in the low S-wave velocity layer at a depth of 350–410 km. A similar precursory wave with low S-wave velocity and negative polarity is formed at a depth of 600–660 km. The low-velocity layers are interpreted as resulting from the hydration of wadsleyite and ringwoodite during the subduction of the Pacific lithosphere. A similar study of the mantle in Central Mongolia found no expected signs of hydration. The modeling of the lithosphere–asthenosphere system in Central Mongolia by joint inversion of the body wave receiver functions and surface wave dispersion curves reveals a very thin lithospheric lid beneath Khangai and a thick layered asthenosphere down to a depth of 200 km with a lithospheric inclusion between low-velocity layers. [ABSTRACT FROM AUTHOR]

Abstract The asthenosphere is commonly defined as an upper mantle zone with low velocities and high attenuation of seismic waves, and high electrical conductivity. These observations are usually explained by the presence of partial melt, or by a sharp contrast in the water content of the upper mantle. Low viscosity asthenosphere is an essential ingredient of functioning plate tectonics. We argue that a substantial component of asthenospheric weakening is dynamic, caused by dislocation creep at the base of tectonic plates. Numerical simulations of subduction show that dynamic weakening scales with the surface velocity both below the subducting and the overriding plate, and that the viscosity decrease reaches up to two orders of magnitude. The resulting scaling law is employed in an apriori estimate of the lateral viscosity variations (LVV) below Earth's oceans. The obtained LVV help in explaining some of the long‐standing as well as recent problems in mantle viscosity inversions.

Subduction zones are fundamental features of Earth's mantle convection and plate tectonics, but mantle flow and pressure around slabs are poorly understood because of the lack of direct observational constraints on subsurface flow. To characterize the linkages between slabs and mantle flow, we integrate high‐resolution representations of Earth's lithosphere and slabs into a suite of global mantle convection models to produce physically plausible present‐day flow fields for Earth's mantle. We find that subduction zones containing wide, thick, and long slabs dominate regional mantle flow in the neighboring regions and this flow conforms to patterns predicted by simpler regional subduction models. These subduction zones, such as Kuril‐Japan‐Izu‐Bonin‐Mariana, feature prismatic poloidal flow coupled to the downgoing slab that rotates toward toroidal slab‐parallel flow near the slab edge. However, other subduction zones, such as Sumatra, deviate from this pattern because of the competing influence of other slabs or longer‐wavelength mantle flow, showing that upper mantle flow can link separate subduction zones and how flow at subduction zones is influenced by broader scale mantle flow. We find that the non‐linear dislocation creep reduces the coupling between slab motion and asthenospheric flow and increases the occurrence of non‐ideal flow, in line with inferences derived from seismological constraints on mantle anisotropy. Key Points: We embed detailed representations of Earth's subduction zones within global convection models to predict mantle flow at subduction zonesSome subduction zones conform to ideal models, with upper mantle flow coupled to downgoing slabs, while others deviate from this patternDislocation creep partially decouples upper mantle flow from slab motion, increasing the fraction of non‐ideal flow [ABSTRACT FROM AUTHOR]

Abstract Subduction zones are fundamental features of Earth's mantle convection and plate tectonics, but mantle flow and pressure around slabs are poorly understood because of the lack of direct observational constraints on subsurface flow. To characterize the linkages between slabs and mantle flow, we integrate high‐resolution representations of Earth's lithosphere and slabs into a suite of global mantle convection models to produce physically plausible present‐day flow fields for Earth's mantle. We find that subduction zones containing wide, thick, and long slabs dominate regional mantle flow in the neighboring regions and this flow conforms to patterns predicted by simpler regional subduction models. These subduction zones, such as Kuril‐Japan‐Izu‐Bonin‐Mariana, feature prismatic poloidal flow coupled to the downgoing slab that rotates toward toroidal slab‐parallel flow near the slab edge. However, other subduction zones, such as Sumatra, deviate from this pattern because of the competing influence of other slabs or longer‐wavelength mantle flow, showing that upper mantle flow can link separate subduction zones and how flow at subduction zones is influenced by broader scale mantle flow. We find that the non‐linear dislocation creep reduces the coupling between slab motion and asthenospheric flow and increases the occurrence of non‐ideal flow, in line with inferences derived from seismological constraints on mantle anisotropy.

Post-collisional volcanism contains important clues for understanding the processes that prevail in orogenic belts, including those in the mantle and the uplift and collapse of continents. Here we report new geochronological and geochemical data for a suite of post-collisional Miocene to Pleistocene volcanic rocks from northwest Iran. Four groups of volcanic rocks can be distinguished according to their geochemical and isotopic signatures, including: (1) Miocene depleted lavas with high Nd and Hf but low Pb and Sr isotopic ratios, (2) less depleted lavas with quite variable Pb isotopic composition, (3) lavas with non-radiogenic Nd and Hf isotopic values, but highly radiogenic Sr and Pb isotopic composition, and (4) Pleistocene adakitic rocks with depleted isotopic signatures. The isotopic data reveal that the Miocene rocks are derived from asthenospheric and highly heterogeneous sub-continental lithospheric mantle sources. Evidence suggests that the lithospheric mantle contains recycled upper continental material and is isotopically similar to the enriched mantle two (EMII) end-member. Analysis of Sr-Nd-Pb-Hf-O isotopes in both mineral and rock groundmass, in conjunction with energy-constrained assimilation and fractional crystallization (EC-AFC) numerical modeling, demonstrates that the incorporation of continental crust during magma fractionation via AFC had an insignificant impact on the isotopic composition of the Miocene lavas. Moreover, adakites are the youngest rocks and show a geochemical signature consistent with the partial melting of a young and mafic continental lower crust. Both seismological data and geochemical signatures on these Miocene to Pleistocene volcanic rocks indicate the initiation of asthenospheric upwelling and orogen uplift in the Arabia-Eurasia collision zone, which occurred after slab break-off, following the Neotethyan closure. [ABSTRACT FROM AUTHOR]

Rapid plate motion, alongside pronounced variations in age and thickness of the Australian continental lithosphere, makes it an excellent location to assess the relationship between seismic anisotropy and lithosphere‐asthenosphere dynamics. In this study, SKS and PKS shear‐wave splitting is conducted for 176 stations covering the transition from the South Australian Craton to eastern Phanerozoic Australia. Comparisons are made with models of lithospheric thickness as well as numerical simulations of mantle flow. Splitting results show uniform ENE‐WSW aligned fast directions over the Gawler Craton and broader South Australian Craton, similar to the orientation of crustal structures generated during an episode of NW‐SE directed compression and volcanism ∼1.6 billion years ago. We propose that heat from volcanism weakened the lithosphere, aiding widespread lithospheric deformation, which has since been preserved in the form of frozen‐in anisotropy. Conversely, over eastern Phanerozoic Australia, fast directions show strong alignment with the NNE absolute plate motion. Overall, our results suggest that when the lithosphere is thin (<125 km), lithospheric contributions are minimal and contributions from asthenospheric anisotropy dominate, reflecting shear of the underlying mantle by Australia's rapid plate motion above. Further insights from geodynamical simulations of the regional mantle flow field, which incorporate Australian and adjacent upper mantle structure, predict that asthenospheric material would be drawn in from the south and east toward the fast‐moving continental keel. Such a mechanism, alongside interactions between the flow field and lithospheric structure, provides a plausible explanation for smaller‐scale anomalous splitting patterns beneath eastern Australia that do not align with plate motion. Plain Language Summary: The Australian continent is moving rapidly northward at around 7–8 cm per year. As the continent moves, it is expected to shear or deform the warmer and weaker layer of the Earth below, called the mantle. The actual pattern of deformation within the mantle can be investigated by studying how seismic waves are polarized as they pass through this material. Results show that for one of the geologically oldest regions in Australia, an area in South Australia, the deeper part of the continent here was substantially deformed 1.6 billion years ago. This deformation was likely aided by volcanism that occurred at the same time that would have warmed and weakened the material, making it easier to deform. This material has since cooled and strengthened over time, freezing in the ancient pattern of deformation. Meanwhile in eastern Australia, the continental material here has a much younger geological age (<550 million years old). The results from this region instead show agreement with the present‐day direction of shear due to the fast northward motion of the Australian continent, as initially expected. Key Points: Fossilized anisotropy dominates where lithosphere is old and thick, with deformation of the Gawler Craton from ∼1.6 Ga preservedAsthenospheric shear relating to Australia's fast plate motion dominates where lithosphere is young and thinLithospheric thickness variations likely induce further deviations in mantle flow [ABSTRACT FROM AUTHOR]

Abstract Rapid plate motion, alongside pronounced variations in age and thickness of the Australian continental lithosphere, makes it an excellent location to assess the relationship between seismic anisotropy and lithosphere‐asthenosphere dynamics. In this study, SKS and PKS shear‐wave splitting is conducted for 176 stations covering the transition from the South Australian Craton to eastern Phanerozoic Australia. Comparisons are made with models of lithospheric thickness as well as numerical simulations of mantle flow. Splitting results show uniform ENE‐WSW aligned fast directions over the Gawler Craton and broader South Australian Craton, similar to the orientation of crustal structures generated during an episode of NW‐SE directed compression and volcanism ∼1.6 billion years ago. We propose that heat from volcanism weakened the lithosphere, aiding widespread lithospheric deformation, which has since been preserved in the form of frozen‐in anisotropy. Conversely, over eastern Phanerozoic Australia, fast directions show strong alignment with the NNE absolute plate motion. Overall, our results suggest that when the lithosphere is thin (

The lithosphere--asthenosphere boundary (LAB), which can be seismically detected, stabilizes plate tectonics. Several conflicting hypotheses have been proposed as the causes of LAB discontinuity, such as the contribution of hydrated minerals, mineral anisotropy, and partial melts. The petit-spot melts ascending from the asthenosphere, owing to subducting plate flexures, support the partial melting at the LAB. Here, we observed the lava outcrops of six monogenetic volcanoes formed by petit-spot volcanism in the western Pacific. Thereafter, we determined the 40Ar/39Ar ages, major and trace element compositions, and Sr, Nd, and Pb isotopic ratios of the petit-spot basalts. The 40Ar/39Ar ages of two monogenetic volcanoes were ca. 2.6 Ma (million years ago) and ca. 0 Ma, respectively. The isotopic compositions of the western Pacific petit-spot basalts suggest their geochemically similar melting sources. They were likely derived from a mixture of high-μ (HIMU) mantle-like and enriched mantle (EM) -1-like components related to carbonatitic/carbonated materials and recycled crustal components. A mass balance-based melting model implied that the characteristic trace element composition (i.e., Zr, Hf, and Ti depletions) of the western Pacific petit-spot magmas could be explained by the partial melting of garnet lherzolite with a small degree of carbonatite melt flux with crustal components. This result confirms the involvement of carbonatite melt and recycled crust in the source of petit-spot melts and provides an implication for the genesis of tectonic-induced volcanism with similar geochemical signatures to those of petit-spots. [ABSTRACT FROM AUTHOR]

To test the relationship between plate age and uppermost mantle structure for the Juan de Fuca and Gorda plates, as well as the mode of lithospheric cooling, we produce a new 3D oceanic shear wave speed (VS) model. This model is constructed using a new high‐quality Rayleigh wave phase speed data set across the plates and is based on a novel thermo‐seismic hybrid parameterization. In the lithosphere, we find that the VS structure is primarily (68% of study area) governed by conductive cooling. Exceptions are observed, suggesting advective heat influx along ridges and hydrothermal circulation through cracks near the subduction trench and transform faults. In the asthenosphere of the Juan de Fuca plate, no simple monotonic relationship between seismic structure and plate age exists, but instead there is a non‐monotonic slow‐fast‐slow VS pattern from west to east. The two slow asthenospheric VS anomalies indicate the presence of partial melt beneath both the mid‐ocean ridge and the subduction trench. The partial melt beneath the trench is enigmatic. Plain Language Summary: Previous studies have demonstrated that the uppermost mantle structure across oceanic plates varies significantly with plate age. To explore the relation between the uppermost mantle structure and the plate age of small and young oceanic plates in higher resolution, we generate a new 3D shear wave speed (VS) model across the entire Juan de Fuca and Gorda plates using seismic surface wave data. Our model shows that conductive cooling is the primary cause controlling the lithospheric structure in most of the Juan de Fuca and Gorda plates. Near plate boundaries, other causes such as advective heat transport, plate deformation, and mantle hydration may have an additional influence. This suggests that conductive cooling is a valid assumption for young plate ages and is significant on spatial scales as small as the resolution of our model (80 km). In the underlying asthenosphere, our VS model shows a slow VS near the mid‐ocean ridge, fast enough to be considered melt‐free near the age of 4.5 Ma, and slow again near the trench. The mechanism behind this slow‐fast‐slow pattern is still enigmatic. Key Points: A shear wave speed model for the Juan de Fuca and Gorda plates is constructed using a novel thermo‐seismic hybrid parameterizationTwo thirds of lithospheric structure agrees with plate age, with deviations attributable to heat advection or plate deformationMelt may be present beneath the Cascadia trench near a depth of 100 km, but its source is unclear [ABSTRACT FROM AUTHOR]

We have estimated the geochemical equilibration depth and temperature of the widespread Neogene alkaline basalts in the Cordillera of Alaska, Northwest Canada, and in Mexico using geobarometry on bulk compositions that have been minimally differentiated in upward transit. The method has uncertainties of about ±10 km and <70 °C. The regional averages of geochemical equilibration depth for 12 sites in Alaska vary from 50 ± 10 to 84 ± 2 km, somewhat broader than those from the Cordillera in western Canada, western USA, and Mexico. There are no associations of depth with terranes or geological provinces. The final equilibration depth of lavas with the surrounding mantle is concluded to be where partial melt percolating from greater depths' ponds at the lithosphere–asthenosphere boundary (LAB) until it becomes gravitationally unstable and moves upward in conduits. The top of the low velocity zone from seismic receiver functions, taken to be the LAB in regions of Alaska where Neogene volcanism occurs, varies from 60 to 85 km, covering the range of geochemical equilibration depths of the alkaline lavas. A mean lava equilibration depth of 65 ± 10 km occurs in 24 of 36 alkaline volcanic centers from Alaska to Mexico, and several other global locations, suggesting the LAB may be controlled to a first order by the change in H2O storage capacity and viscosity across the garnet–spinel peridotite phase change at this depth. The scatter and variation in equilibration depths and temperatures are a factor of 2 greater than the recognized uncertainties, and are not yet explained. [ABSTRACT FROM AUTHOR]

'Transitional kimberlite' is a collective term previously used to classify rocks occurring in southern Africa that show bulk rock geochemical and Sr–Nd isotope features intermediate between (cratonic) lamproites and kimberlites. However, it is now well established that detailed petrographic and mineral chemical criteria represent a more robust approach towards the classification of kimberlites, lamproites and related rocks. Here, we re-assess the classification of southern African 'transitional kimberlites' by combining new petrographic observations and mineral compositional results for samples from six localities (Leicester, Frank Smith, Wimbledon, Melton Wold, Droogfontein, and Silvery Home) straddling the southwestern margin of the Kaapvaal Craton. These new data indicate that Leicester and Frank Smith are archetypal kimberlites, whereas Wimbledon, Melton Wold, Droogfontein, and Silvery Home represent bona fide olivine lamproites. We combine the mineral chemical results with new (Wimbledon) and existing bulk rock trace element and Nd–Hf isotope compositions, and emplacement ages, to assess whether the previously documented trends in Nd–Hf isotope vs time for these 'transitional kimberlites' constrain their petrological evolution. Modal groundmass mineralogy, bulk rock K/La and chromite compositions, the latter being a proxy for primitive melt composition, are linearly correlated with emplacement age and initial Nd–Hf isotope compositions. These observations suggest derivation of both older lamproites (181–115 Ma) and younger kimberlites (114–93 Ma), from a common evolving source. The temporal evolution of Nd–Hf isotope compositions in these rocks converge to values typical of archetypal Cretaceous kimberlites elsewhere in the Kaapvaal Craton, but are clearly different from the isotopic compositions of on-craton Kaapvaal lamproites (previously known as orangeites). This observation distinguishes the petrogenesis of the Wimbledon, Melton Wold, Droogfontein, and Silvery Home lamproites from those of 'typical' Kaapvaal lamproites. We hypothesize that progressive consumption of enriched and hence fertile K-bearing components in a common sub-lithospheric (i.e. convective mantle) source beneath the southwestern margin of the Kaapvaal Craton might represent a plausible scenario to explain the temporal evolution of petrographic and geochemical traits of the examined lamproites and kimberlites. A source in the lithospheric mantle is considered at odds with the contrasting location of the current localities as they occur both off- and on-craton. Migration of the African plate between 180 and 90 Ma over a relatively stationary convective mantle (plume?) source is not compatible with the spatial–temporal distribution of 'transitional kimberlites'. Instead, we invoke viscous coupling between an upper asthenospheric source and the lithosphere to reconcile a single evolving source with the geographic and age distribution of these rocks. This work supports the hypothesis that olivine lamproites occurring in intra-continental settings share similar genetic features with kimberlites. [ABSTRACT FROM AUTHOR]

Mantle convection plays a key role in magmatism and volcanism on Earth. The final distribution of deep mantle material upwelled into the asthenosphere cannot be clearly tracked using seismic imaging techniques. Where mid-ocean ridges and plumes interact, the along-ridge variations in plume-affected basalts constrain the spatial extent of the plume-related flow in the asthenosphere. These variations are helpful for revealing convection throughout the mantle from the core-mantle boundary (CMB) to the bottom of the lithosphere. In this study, regional geophysical data, as well as the results of geochemical independent component analysis of radiogenic Sr–Nd-Pb isotopes of South Mid-Atlantic Ridge (SMAR) basalts, were used to analyze the distribution characteristics of the plume-affected asthenosphere beneath the South Atlantic Ocean. We determined that the ridge scope of the Ascension plume-influenced SMAR segments is bounded by the Ascension transform fracture (~ 7.5°S) to the north and the Bode Verde transform fracture (~ 11.1°S) to the south, while the Saint Helena plume-contaminated SMAR segments are bounded by the Cardno transform fracture (~ 14.2°S) to the north and the Trinidade transform fracture (~ 20.8°S) to the south. Furthermore, we determined that the melt extraction process taking place between the mantle plume and ridge system may weaken the plume-related geochemical signals of these plume-affected MORBs. Our results suggest that the distribution of plume-related asthenosphere under the South Atlantic is influenced by large transform faults that block the propagation of the plumes along the bottom of the lithosphere, as well as the propagation of plume-affected materials along the ridge system. [ABSTRACT FROM AUTHOR]

[Display omitted] • Low density craton-like core lithosphere in Zagros. • Arabian lithosphere underthrusts beneath Eurasia (Iran) due to density excess. • Cold and relatively dense sub-lithospheric body beneath the Iranian plateau. The tectonic evolution of the Arabia-Eurasia collision zone is enigmatic due to our poor knowledge of the compositional structure of the Zagros keel and the sub-lithospheric mantle beneath the Iranian plateau. Here, we present a thermochemical model for the upper mantle, starting from the Arabian plate toward the Zagros keel (in SW Iran) and the Iranian plateau to the Kopet Dagh Mountains in NE Iran. The crustal and (sub) lithospheric mantle structure is constrained using receiver functions and high-resolution seismic tomography. The crustal density is obtained by converting local Vp velocity profiles to density. The presumed mantle composition/density of different tectonic units has been inferred from the composition of mantle-derived xenoliths in Pleistocene alkaline basalts of NE Iran, mantle xenoliths from Eastern Arabia, and integrated geophysical-petrological modeling of seismic tomography as well as Bouguer and geoid anomalies. Our results indicate that the lower part of the lithosphere in the Arabian plate is ∼ +25 kg/m3 denser than the sub-lithospheric mantle, which allows underthrusting of the Arabian lithosphere beneath the Iranian plateau. The Zagros core–an ultra-positive seismic velocity anomaly in the tomographic models– is modeled assuming a refractory "craton-like" composition, leading to a ∼-60 kg/m3 density contrast with its underlying fertile lithospheric mantle. We hypothesize that at least the upper ∼100 km of the hydrous Arabian lithosphere was compositionally cratonic before the collision-related shortening processes. In contrast, the anhydrous Iranian lithosphere has been refertilized during asthenosphere upwelling and mantle flows. The Iranian plateau is underlain by a cold sub-lithospheric body characterized by ∼ +25 kg/m3 density contrast with respect to the ambient asthenosphere. [ABSTRACT FROM AUTHOR]

Abstract —This work presents the results of GPS observations (2003–2020) carried out in PrimorskyKrai (aka the Primorie) and Khabarovsk Krai in southeastern Russia. The objectives of our study were to obtain displacement velocities, to test the relation of current velocities with seismicity, with the specific features of the geological structure of the Primorie, and to study the rheological parameters of the crust and asthenosphere at the continental margin. This paper analyzes the results of measurements made in the Primorie at the Central Sikhote-Alin fault. The study includes the effects of the Tohoku-Oki earthquake in Japan with magnitude М = 9 which occurred on March 11, 2011. The zone of coseismic and postseismic displacements extends to a distance of over 1000 km from the epicenter. The postseismic decay of the long-term horizontal and vertical displacements allow us to estimate the relaxation time for the elastic-viscous model. With the decay times of 4 to 8 years, the viscosity of the lower layer in the two-layer rheological model is estimated at 8 × 1018–3 × 1019 Pa s. Using the bending model of the Earth's surface and the bottom of the Sea of Japan, we estimated the thickness of the elastic upper part of the Earth's crust in the continent–ocean contact zone at 20–25 km. [ABSTRACT FROM AUTHOR]

Estimation of source localization and central depth of anisotropy beneath an active seismic region plays an important role in understanding the deformation mechanism and current configuration of tectonics. Though shear wave splitting measurements are capable of deciphering the mantle dynamics and deformation patterns, they are constrained from analysing the actual depth of anisotropy or heterogeneity, which may have large influences in interpreting the geodynamics of a particular region. The present study fills this gap of estimating the source localization and depth of seismic anisotropy beneath the Shillong Plateau and Himalayan foredeep region through a well‐established spatial coherency analysis of splitting parameters based upon the Fresnel zone concept. The model suggests a central depth of heterogeneity at around 100 km beneath the Shillong Plateau mass, coinciding with the lithosphere‐asthenosphere boundary and strengthens our implication that the asthenospheric drag at the base of the Indian lithosphere is responsible for absolute plate motion of India, is the major source of anisotropy and controls the deformation patterns of the Shillong mass. In a similar manner, the spatial coherency analysis predicts a central depth of anisotropy at around 150 km beneath the Himalayan foredeep region. The asthenospheric force corresponding to this particular depth is suggested as the major source of anisotropy for this region. [ABSTRACT FROM AUTHOR]

Vertical-incidence seismic reflection profiles generated from global phases for 16 earthquakes recorded by stations of the Transportable Array (TA) show distinctive patterns of P-wave reflectivity in the uppermost mantle beneath the central and eastern United States. The overall distribution of reflections identified objectively using the sign test statistic applied to bootstrapped stacks is consistent with a westward increase in depth of the lithosphere-asthenosphere boundary (LAB) from roughly 110 to 250 km that is marked within the lower lithosphere by piecewise continuous segments of elevated horizontal P-wave reflectivity. For some profiles, the onsets of zones of increased reflectivity closely match depths corresponding to the maximum negative S-wave velocity gradients found by surface-wave tomography, suggesting that P-wave reflectivity can be used to help characterize properties of the lower lithosphere. We suggest that the vertical change in horizontal reflectivity straddles the lithosphere-asthenosphere transition, encompassing a broad zone of layering caused by increased strain in the lower lithosphere as well as drag-induced flow in the asthenosphere. Some of the lines also show waveforms that fall within the depth range of arrivals identified as midlithospheric discontinuities (MLDs) in overlapping Sp receiver-function profiles. The reflection waveforms observed in the TA lines are mostly multicyclic with a mix of polarities indicating a layered transition, consistent with previous observations and model studies that show the breakup of single Sp waveforms into a series of less prominent, shorter-period P-wave reflections as the dominant frequency of incident energy is increased. • We investigate structure of the lithosphere beneath the central and eastern U.S. • PKPdf-generated reflections at near-vertical incidence observed to 75 s (∼ 275 km). • The lower lithosphere is marked by an increase in horizontal P-wave reflectivity. • Elevated horizontal P-wave reflectivity correlates with negative gradients in V s. • Consistent with strain in lower lithosphere & drag-induced flow in asthenosphere. [ABSTRACT FROM AUTHOR]

[Display omitted] • The gabbro was originated from metasomatized lithospheric mantle. • The hornblende gabbro formed from mixing of asthenosphere and lithospheric mantle. • The slab breakoff propagated eastward under the Lhasa terrane in Paleocene-Eocene. Past geodynamic events can be reconstructed using the spatial and temporal patterns of geochemical variability in igneous rocks. Here we apply this principle to define the timing and geometry of breakoff of the Neo-Tethyan slab after the India-Asia collision. Two episodes of mafic magmatism (∼53 Ma and ∼ 48 Ma) are preserved in the Quxu area of the southern Lhasa terrane (SL). We present their whole-rock geochemistry, zircon U-Pb ages, and in situ zircon Hf isotope ratios. The early gabbro (∼53 Ma) exhibits typical arc basalt characteristics, including enrichment of large-ion lithophile elements (LILEs), depletion of high-field strength elements (HFSEs), and radiogenic Nd and zircon Hf isotopes, implying that they were formed by partial melting of metasomatized lithospheric mantle. In comparison to the early gabbro, the late hornblende gabbro (∼48 Ma) contains higher contents of HFSEs (e.g., Nb, Zr) and relatively less radiogenic Nd and zircon Hf isotopes, suggesting interaction between the enriched sub-continental lithospheric mantle and asthenospheric mantle. It is likely that upwelling asthenosphere reached the central SL around 50 Ma and that the Neo-Tethyan slab tore off under this area at this time. The spatial and temporal distribution of mafic rocks across the whole southern Lhasa terrane show that this geochemical transition and hence, presumably, the tearing off of the Neo-Tethyan slab, propagated eastward along the collision zone throughout Paleocene-Eocene time. [ABSTRACT FROM AUTHOR]

Here, we present crustal and lithospheric structure at six broadband stations below the Palghar swarm activity region, Maharashtra, India, which has produced 7786 events of ML0.4–4.7 during November 2018–March 2020. The crustal thicknesses are modelled through the Differential Evolution (DE) waveform inversion of radial P-receiver functions (PRFs), while lithospheric thicknesses are modelled using the joint inversion of PRFs and fundamental model surface wave group velocity dispersion data (SWD). This swarm activity has produced 7786 events of ML0.4–4.7 during November 2018–March 2020. On an average, our modelled crustal velocities from the PRF modelling through the DE waveform inversion delineate a crustal seismic velocity model similar to an oceanic crustal model near the Western Ghat continental margin. Our modelled Moho depths range from 39.5 to 42 km, with an average crustal thickness of (40.5 ± 1) km, while lithospheric thicknesses vary from 108 to 120 km, with an average lithospheric thickness of (114 ± 5) km. Our modelling reveals on an average a gently north dipping Moho and a sharply SSE dipping lithosphere over an area of 44 × 44 km underlying the Palghar region where the presence of pockets of CO2-rich carbonatite partial melts at 108–120 km depths is also inferred. The average crustal model as derived from the joint inversion of PRFs and SWD suggests a low Vs-layer at 5–20 km depth correlating well with the crustal moderately conductive zones as derived from the modelling of magneto-telluric data wherein most of the micro-earthquakes have occurred below the Palghar region. Thus, we propose that the occurrences of earthquakes at 0–5 km depths might have been triggered by rain or meteoric water while the occurrences of deeper earthquakes (at 5–20 km depths) could be triggered by the presence of aqueous fluids. Some earthquakes are also occurring between 20 and 32 km depths, which might have been triggered by the presence of aqueous fluids or entrapped volatile CO2 emanating from the presence of pockets of CO2-rich carbonatite partial melts beneath the Palghar swarm activity region. [ABSTRACT FROM AUTHOR]

Small‐scale convection beneath the oceanic plates has been invoked to explain off‐axis nonplume volcanism, departure from simple seafloor depth‐age relationships, and intraplate gravity lineations. We deployed 30 broadband ocean bottom seismometer stations on ∼40 Ma Pacific seafloor in a region notable for gravity anomalies, measured by satellite altimetry, elongated parallel to plate motion. P‐wave teleseismic tomography reveals alternating upper mantle velocity anomalies on the order of ±2%, aligned with the gravity lineations. These features, which correspond to ∼300°–500°K lateral temperature contrast, and possible hydrous or carbonatitic partial melt, are—surprisingly—strongest between 150 and 260 km depth, indicating rapid vertical motions through a low‐viscosity asthenospheric channel. Coherence and admittance analysis of gravity and topography using new multibeam bathymetry soundings substantiates the presence of mantle density variations, and forward modeling predicts gravity anomalies that qualitatively match observed lineations. This study provides observational support for small‐scale convective rolls beneath the oceanic plates. Plain Language Summary: Covered by kilometers of water and therefore hard to access, Earth's oceanic tectonic plates have several features we cannot explain. Among these are linear undulations ("rolls") in the strength of gravitational acceleration at the sea surface. Using data from a rare underwater seismic experiment, we have produced 3‐D maps of seismic properties of the Earth's sub‐surface in a location of clear gravity rolls. We find linear blobs of fast and slow material in the mantle beneath the oceanic plate, parallel to the gravity features. These represent cold sinking and warmer rising material, revealing a highly dynamic convective system underneath the plate, which has long been theorized but not previously directly observed at this scale. Key Points: We use ocean bottom seismometer data to image elongated upper mantle P‐wave velocity anomalies, of order ±2%, striking parallel to free air gravity lineationsThese features extend >200 km deep, requiring thermal anomalies and small‐fraction partial melts related to volatiles in the asthenosphereAnomalies are inferred to arise from small‐scale convective cells beneath the plate with a planform parallel to absolute plate motion [ABSTRACT FROM AUTHOR]

Abstract Small‐scale convection beneath the oceanic plates has been invoked to explain off‐axis nonplume volcanism, departure from simple seafloor depth‐age relationships, and intraplate gravity lineations. We deployed 30 broadband ocean bottom seismometer stations on ∼40 Ma Pacific seafloor in a region notable for gravity anomalies, measured by satellite altimetry, elongated parallel to plate motion. P‐wave teleseismic tomography reveals alternating upper mantle velocity anomalies on the order of ±2%, aligned with the gravity lineations. These features, which correspond to ∼300°–500°K lateral temperature contrast, and possible hydrous or carbonatitic partial melt, are—surprisingly—strongest between 150 and 260 km depth, indicating rapid vertical motions through a low‐viscosity asthenospheric channel. Coherence and admittance analysis of gravity and topography using new multibeam bathymetry soundings substantiates the presence of mantle density variations, and forward modeling predicts gravity anomalies that qualitatively match observed lineations. This study provides observational support for small‐scale convective rolls beneath the oceanic plates.

We investigate the dependence of the Gutenberg-Richter b parameter on the crustal thickness quantified by the Moho depth, for nine different regional catalogs. We find that, for all the catalogs considered in our study, the b-value is larger in areas presenting a thicker crust. This result appears in apparent contradiction with previous findings of a b decreasing with the focal depth. However, both the results are consistent with acoustic emission experiments, indicating a b-value inversely proportion to the applied differential stress. Our results can be indeed interpreted as the signature of a larger stress concentration in areas presenting a thinner crust. This is compatible with the scenario where postseismic deformation plays a central role in stress concentration and in aftershock triggering. [ABSTRACT FROM AUTHOR]

The Adria microplate has the particular feature to be involved in two subduction systems with slab dipping in opposite directions, one toward west beneath the Apennines and the other to the east beneath the Dinarides. The deep structure of Adria and the shape and characteristics of the slabs have mainly been studied through seismic tomography. However, the uncertainty about the presence and dimensions of tear and windows along the Apennines and the Dinarides slabs is still large. An instrument that can be used to draw mantle flows and to support the possible presence of slab windows or tears is the detection of seismic anisotropy, in particular core phases shear wave splitting. In this paper, to give more light to the structure of Adria slabs and possible mantle circulation beneath this microplate, we benefit from data recorded by seismic stations located along a profile running across the central Adriatic from the Apennines to the edge of the Panonnian basin. The new measurements, together with previous findings, show an evident change of the anisotropic properties when moving along the profile. The distribution of SKS-splitting measurements in the Apennines strongly agree with previous measurements that already described the toroidal flow generated by the slab rollback of the Calabrian arc. In addition, the N-S and NE-SW directions found beneath the Apulia are in agreement with those attributed previously in the outer northern Apennines, to a proper typical pattern of the mantle beneath Adria, which is undeformed by the slab retreat. The pattern of the anisotropy in the Dinarides region shows lateral and vertical variations that together with recent tomographic images that better define the slab window allow us to speculate as follows: the new SKS measurements, interpreted in terms of mantle deformation and flows, agree with the geodynamic model that justifies the mantle circulation beneath Adria with the presence of slab windows in both the Apennines and Dinarides slabs.

Melt extraction from the partially molten mantle is among the fundamental processes shaping the solid Earth today and over geological time. A diversity of properties and mechanisms contribute to the physics of melt extraction. We review progress of the past ∼25 years of research in this area, with a focus on understanding the speed and style of buoyancy-driven melt extraction. Observations of U-series disequilibria in young lavas and the surge of deglacial volcanism in Iceland suggest this speed is rapid compared to that predicted by the null hypothesis of diffuse porous flow. The discrepancy indicates that the style of extraction is channelized. We discuss how channelization is sensitive to mechanical and thermochemical properties and feedbacks, and to asthenospheric heterogeneity. We review the grain-scale physics that underpins these properties and hence determines the physical behavior at much larger scales. We then discuss how the speed of melt extraction is crucial to predicting the magmatic response to glacial and sea-level variations. Finally, we assess the frontier of current research and identify areas where significant advances are expected over the next 25 years. In particular, we highlight the coupling of melt extraction with more realistic models of mantle thermochemistry and rheological properties. This coupling will be crucial in understanding complex settings such as subduction zones. Mantle melt extraction shapes Earth today and over geological time. Observations, lab experiments, and theory indicate that melt ascends through the mantle at speeds ∼30 m/year by reactively channelized porous flow. Variations in sea level and glacial ice loading can cause significant changes in melt supply to submarine and subaerial volcanoes. Fluid-driven fracture is important in the lithosphere and, perhaps, in the mantle wedge of subduction zones, but remains a challenge to model. [ABSTRACT FROM AUTHOR]

By using P and S wave receiver functions and P and S wave travel time residuals, we have found velocity models for 16 seismograph stations in Eastern Anatolia. Our study is focused mainly on the mantle lithosphere, asthenosphere and transition zone. The volcanism and uplift of the Eastern Anatolia Plateau are thought to be related to the Bitlis slab break off and delamination of the continental lithosphere. Sinking cold slab and lithospheric drips can reduce temperature in the mantle transition zone (MTZ) by up to a few hundred degrees C. However, our analysis of seismic data provides no robust evidence of significant cooling of the transition zone. In the mantle immediately above the 410-km discontinuity there is a pronounced low S wave velocity layer that may be a source of the volcanism in the study region. Another low velocity layer is present at the base of the MTZ. The obtained S wave velocity models of the upper mantle can be divided into three groups. In the first group, the lithosphere—asthenosphere boundary (LAB) is at a depth of ~ 60 km. In the second group, the LAB is at a depth from 90 to 100 km. In the third group, the mantle lithosphere is practically absent. On a scale of our analysis there is no clear correspondence between the obtained mantle velocity models and the volcanism (< 23 Ma) exposed at the surface. Only the models of the first group are well represented in the neighboring Central Anatolian Plateau. [ABSTRACT FROM AUTHOR]

Eastern Asia is a prime location for the study of intracontinental tectono‐magmatic activity. For instance, the origin of widespread intraplate volcanism has been one of the most debated aspects of East Asian geological activity. Measurements of attenuation of teleseismic phases may provide additional constraints on the source regions of volcanism by sampling the upper mantle. This study uses data from three seismic arrays to constrain lateral variations in teleseismic P wave attenuation beneath the Central Asian Orogenic Belt and the North China Craton. We invert relative observations of attenuation for a 2‐D map of variations in attenuation along with data and model uncertainties by applying a Hierarchical Bayesian method. As expected, low attenuation is observed beneath the Ordos block. High attenuation is observed beneath most of the volcanoes (e.g., the Middle Gobi volcano, the Bus Obo volcano, and the Datong volcano) in the study area, and estimated asthenospheric Qp values span from 95 to 200. These values are within the range of globally average asthenosphere. We infer that these volcanoes may tap melt from ambient asthenosphere and occur where the lithosphere is thin, which is consistent with previous petrologic studies. More complex mantle drivers of volcanism are not rejected but are not needed to explain eruptions in this area. In contrast, at the Xilinhot‐Abaga volcanic site, the observed low attenuation (as low as beneath the Ordos block) excludes a typical shallow melting column. Fluids from the subducted Pacific plate may initiate the deep melting and would be consistent with petrological constraints. Plain Language Summary: Seismic attenuation, the loss of energy in a seismic wave as it passes through a medium, provides complementary constraints on the Earth's interior to those provided by the more common studies of seismic velocity. We present the first map of the attenuation experienced by the first arriving P phase from distant earthquakes in the Central Asian Orogenic Belt and North China Craton. The P phase from distant earthquakes travels nearly vertically beneath the stations that record them, and so variations in their attenuation reflect variations in the attenuation below the stations. We find low attenuation beneath the Ordos Block and high attenuation beneath most of the volcanic sites in the study region (e.g., the Middle Gobi volcano, the Bus Obo volcano, and the Datong volcano), where our results suggest that the asthenosphere is as about as attenuating as normal asthenosphere across the globe. We infer most of the volcanism in this portion of Eastern Asia taps melt from the ambient asthenosphere where the lithosphere is thin. This hypothesis is consistent with previous petrologic studies and does not require invoking unusual conditions such as a mantle plume. Counterintuitively, low attenuation is observed beneath the Xilinhot‐Abaga volcanic site, which excludes a shallow melting column beneath most volcanoes and requires a deep source. Taken together, our results support the hypothesis that lithospheric thickness is a primary driver of variations in the composition of erupted lavas. Key Points: The teleseismic P wave attenuation is obtained in the Central Asian Orogenic Belt and North China CratonMost volcanism in study area may tap melt from ambient asthenosphere and occur where the lithosphere is thinA thick lithosphere and a deep magma source are inferred beneath the Xilinhot‐Abaga volcanic site [ABSTRACT FROM AUTHOR]

Late Mesozoic magmatism in southeastern China has been widely considered to be related to the subduction of the Paleo-Pacific Plate. However, it remains controversial whether mafic rocks are derived from the lithosphere or the asthenosphere. Here we present a comprehensive study on mafic dikes from Fujian Province in southeastern China, aiming to understand their source. Two types of mafic rocks have been recognized based on their trace-element features. Type-I rocks show arc-like trace-elemental characteristics, while type-II rocks are distinguished by their relatively flat patterns in primitive-mantle-normalized trace-element diagram. Despite such differences between two types of rocks, these mafic dikes show two trends in the plots of 87Sr/86Sr(i) versus La/Nb, which can be explained by the influences of crustal contamination and melt-lithospheric mantle interaction, respectively. 87Sr/86Sr(i), La/Nb, Sr/Y and Zr/Y ratios of type-I rocks are significantly correlated to the thickness of the underlying lithosphere, and the signals of lithosphere are clearer with increasing lithospheric thickness. This highlights the important influences of melt-lithosphere interaction during their formation. Such observations also indicate that these mafic rocks are more likely to have been originated from the asthenosphere rather than the lithospheric mantle. [ABSTRACT FROM AUTHOR]

Abstract—Analysis of P- and S-receiver functions for 19 seismic stations on the Balkan Peninsula has been performed. Half of the stations are in Bulgaria. The crustal thickness varies from 28–30 to 50 km. The ratio of longitudinal and shear wave velocities in the upper crust reaches 2.0 in some places. In the southwest of the study area, the 410-km seismic boundary is uplifted by 10 km relative to nominal depth. The elevation may be caused by hydration and/or cooling of the mantle transition zone under the influence of the Hellenic subduction zone. A low S-wave velocity layer related to the 410-km boundary may be located atop this boundary. In the northwestern part of the study area this layer is present in spite of the absence of the 410-km boundary. A similar paradox has been previously noted in central Anatolia. Indications of a low-velocity layer are present at a depth exceeding 410 km. The simultaneous inversion of the receiver functions of the two types (P and S) and the Rayleigh wave phase velocities reveals a large (7–9%) decrease in the S-wave velocity in the upper mantle of southern Bulgaria and northern Greece. The thickness of the low-velocity layer (asthenosphere) is about 50 km. The lithosphere-asthenosphere boundary (LAB) is at depths of 40 to 60 km. In terms of tectonics, this zone is characterized as the South Balkan extension system. To the north of 43° N, the S-wave velocity in the upper mantle is usually at least 4.4 km/s and the LAB is not detected or is detected at a depth of over 80 km. The SKS analysis of azimuthal anisotropy reveals lateral zoning in the upper mantle that is correlated to velocity zoning. Probably, the mechanically weak low-velocity mantle of the South Balkan system is easily deformed, and the azimuth of the fast direction of anisotropy (20°) indicates the direction of extension. At the northern stations, the fast direction (about –30°) may be a reflection of an older process. [ABSTRACT FROM AUTHOR]

The paper presents the results of isotope-geochronological and petrological-geochemical study of young volcanic rocks in the Geghama highland (Central Armenia), which were formed at the Pliocene–Early Quaternary stage of the Late Cenozoic post-collisional magmatism of the Lesser Caucasus. The boundaries of the area of volcanic activity manifested during this stage were established. The total duration (3.5–1.9 Ma) of the Pliocene–Early Quaternary stage, the time range of its main phases, and the scale and nature of eruptions were determined. Petrological and geochemical data indicate that the studied young volcanic rocks from Central Armenia belong to the mildly alkaline series and are represented by a continuous association of (trachy-)basalt–mugearite–latite–trachyte–rhyolite. The geochemical evolution of parental basaltic melts was mainly controlled by the fractional crystallization with Cpx as the main cumulus phase. Crustal assimilation and mixing of magmas were of limited importance: their possible contribution was recorded only in the most silicic rocks. The deep source responsible for magma generation under the studied part of the Lesser Caucasus at the Pliocene–Pleistocene boundary was represented by the asthenospheric mantle that was enriched during previous long-term (tens of millions of years) subduction of the Neotethys slab. Melting occurred in the Grt-peridotite stability zone; the composition of derived melts was geochemically similar to E-MORB basalts. An important feature of the regional mantle source was the notable admixture of subduction component. The generalization of the obtained petrological–geochemical and isotope–geochemical data on the young igneous rocks formed at different stages of Late Cenozoic magmatism at the Geghama highland allowed us to trace the evolution of the main parameters (mineral, chemical, and isotopic composition, depth of location, degree of melting) of mantle reservoirs responsible for magma generation beneath Central Armenia at the post-collisional stage (from the Late Miocene to the Holocene). [ABSTRACT FROM AUTHOR]

The problem of the development of thermomechanical waves in the system consisting of two horizontal layers with rheology of a linearly elastic medium for the upper layer (lithosphere) and a viscous fluid for the lower layer (asthenosphere) is considered with regard to phase transition on their common boundary. The exact solution to the problem is found and its properties as functions of the parameters are studied. It is shown that for the characteristic physical parameters of the lithosphere and asthenosphere there exist solutions in the form of moderately damped strain tectonic waves and a geophysical interpretation of the results obtained is given. [ABSTRACT FROM AUTHOR]

Abstract The style of convective force transmission to plates and strain‐localization within and underneath plate boundaries remain debated. To address some of the related issues, we analyze a range of deformation indicators in southern California from the surface to the asthenosphere. Present‐day surface strain rates can be inferred from geodesy. At seismogenic crustal depths, stress can be inferred from focal mechanisms and splitting of shear waves from local earthquakes via crack‐dependent seismic velocities. At greater depths, constraints on rock fabrics are obtained from receiver function anisotropy, Pn and P tomography, surface wave tomography, and splitting of SKS and other teleseismic core phases. We construct a synthesis of deformation‐related observations focusing on quantitative comparisons of deformation style. We find consistency with roughly N‐S compression and E‐W extension near the surface and in the asthenospheric mantle. However, all lithospheric anisotropy indicators show deviations from this pattern. Pn fast axes and dipping foliations from receiver functions are fault‐parallel with no localization to fault traces and match post‐Farallon block rotations in the Western Transverse Ranges. Local shear wave splitting orientations deviate from the stress orientations inferred from focal mechanisms in significant portions of the area. We interpret these observations as an indication that lithospheric fabric, developed during Farallon subduction and subsequent extension, has not been completely reset by present‐day transform motion and may influence the current deformation behavior. This provides a new perspective on the timescales of deformation memory and lithosphere‐asthenosphere interactions.

Teleseismic earthquakes data recorded by 8 broadband digital seismic stations deployed in the Kumaun-Garhwal and adjoining area of NW Himalaya are analyzed to investigate the seismic anisotropy in the upper mantle. Shear-wave splitting parameters (ɸ and ∂t) derived from the analysis of core-refracted SKS/SKKS phases provide first hand information about seismic anisotropy and deformation in the upper mantle beneath the region. A total, 209 good measurements of seismic anisotropy have been obtained from the two years of collected high-quality teleseismic data in the study region. The analysis shows considerable strength of anisotropy (delay time(∂t) 0.6–2.8 s) with average ENE–WSW oriented fast polarization orientation at most of the stations while other one either along with the absolute plate motion (APM) of the Indian plate or parallel to strike of the Himalayan mountain belt. The Fast axis direction observed at stations close to between the MBT and MCT aligns parallel to the strike of local geological faults and orthogonal to absolute plate motion (APM) direction of the Indian plate. The large variation in the splitting strength and fast polarization trends around the Kumaun-Gharwal and adjacent in NW Himalaya have invoked both lithospheric and asthenospheric anisotropy.

Simultaneous inversion of P and S receiver functions and of dispersion curves of Rayleigh waves for 16 seismograph stations provides insight into structure beneath the Caucasus and the Caspian Basin up to a depth of 700 km. Crustal thickness of the Caucasus ranges from 30 to 50 km. An anomalously high velocity ratio of the P and S waves (2.0 and more) is observed systematically in the upper crust. The upper mantle at most locations can be split into the upper high S wave velocity layer (4.5–4.8 km/s, litospheric mantle) and the underlying low S velocity (4.0–4.2 km/s) asthenosphere. The depth to the lithosphere—asthenosphere boundary (LAB) ranges from 90 to 145 km. Under the East Caucasus the depth to the 410 km boundary is close to the standard (IASP91) value, whereas the 660 km boundary is lowered on the average by 10 km. The sinking of the 660 km boundary may be caused by cooling and/or hydration of the lower transition zone by the subducted Neo-Tethys plate. Under the western margin of the Caspian Basin structure of the upper mantle resembles a subduction zone: the low-velocity (Vs < 4.2 km/s) asthenosphere which lies immediately beneath the Moho boundary is underlain at a depth of 140 km by a layer of (subducted) high-velocity lithosphere. The S wave receiver functions indicate that the 410 km boundary beneath the Caspian Basin is lowered by about 10 km. This may be an effect of elevated by 100°C temperature. An uplift of the 410 km boundary is found beneath the Scythian platform. [ABSTRACT FROM AUTHOR]

The tomographic inversion of shear wave splitting data for upper mantle anisotropy has been a longstanding challenge. This is due to the ray-based approximation of classical approaches and the near-vertical incidence of the core-mantle converted phases such as SKS that are often used. Recent developments include the calculation of finite-frequency sensitivity kernels for SKS splitting intensity observations, which allows us to accurately take into account the sensitivity to anisotropic structure with depth. A requirement of this tomographic technique is a dense station spacing, which results in overlapping sensitivity kernels at depth and allows for the localization of anisotropic structure. This is satisfied by a growing number of temporary seismic deployments, which motivates the desire to image anisotropic complexities with depth. Here, we introduce and make available a toolbox for the MATLAB environment that facilitates the application of finite-frequency splitting intensity tomography to dense seismic arrays. Our implementation includes several key features, including: 1) A forward calculation of splitting intensities and sensitivity kernels for a complex anisotropic model space. 2) Consideration of the dominant period of the wave, allowing for multiple-frequency analysis, as well as the incoming wave's non-vertical incidence. 3) The inversion can be based on a classical gradient descent, on a form of the conjugate gradient method known as the BFGS algorithm, or on a gradient-informed stochastic reversible jump algorithm, allowing for a data-driven parametrization of the model space. 4) Importing splitting intensity measurements from waveforms processed in SplitRacer allows for fast pre-processing of large data sets due to its fully automatic design. To illustrate our method, we present both synthetic tests and an application to real data. We apply our inversion procedure to data from the Swath-D network, which densely covers the transition of the Central to the Eastern Alps. Previous studies showed evidence for an abrupt lateral change of layered seismic anisotropy that had been attributed to an opening for channeled asthenospheric flow. Using an SKS splitting intensity tomography approach, we can confirm previous inferences while providing additional constraints on the distribution of anisotropy laterally and with depth. • We present a new MATLAB toolbox for anisotropy tomography using splitting intensity. • The approach (partially) overcomes the limited depth resolution of XKS-splitting. • We test and discuss several different approaches for solving the inverse problem. • We demonstrate its applicability on a dense seismic network in the European Alps. • Our results indicate shallow asthenospheric flow beneath the Eastern Alps. [ABSTRACT FROM AUTHOR]

Abstract Widespread recent volcanic rocks occur across the Cordillera landward of the current/recent volcanic arc Mexico to Alaska and most other subduction backarcs. We conclude that most are produced by partial melt in the upper asthenosphere where two conditions are met: (1) thin lithosphere, shallow hot asthenosphere. Most of the Cordillera is uniformly hot with thin lithosphere such that the hot asthenosphere extends up to a sufficiently shallow depth to intersects the wet solidus; (2) wet upper asthenosphere. There is substantial water in the upper asthenosphere that reduces the solidus sufficiently for partial melting. We integrate geochemical analyses that constrain the partial melt source temperature and depth, seismic velocities that define the upper mantle temperature and partial melt zones, and seismic receiver functions that define the lithosphere‐asthenosphere boundary (LAB). Geochemical data for the Canadian Cordillera are integrated with data from the western United States. Upper mantle xenoliths indicate a dry strong lithosphere, 250 ppm, is indicated by the volcanics, facilitating small‐scale convection. Geochemical equilibration averages ∼1,350°C, at ∼65 km. Receiver functions also define the LAB at ∼65 km for the western Cordillera, deeper in the eastern US Cordillera, and asthenosphere tomography velocities indicate ∼1,350°C. Low velocities above ∼150 km suggest a few percent partial melt that percolates upward and ponds at the base of the lithosphere until enough accumulates to locally penetrate upward. The 65‐km depth may be controlled by the spinel‐garnet phase transition. Mechanisms are discussed for the spatial distribution of recent volcanics.

Abstract The evolution of ocean lithosphere and asthenosphere are fundamental to plate tectonics, yet high resolution imaging is rare. We present shear wave velocity and azimuthal anisotropy models for the upper mantle from Rayleigh wave group velocities from local earthquake and ambient noise at 15–40‐s period recorded by the Passive Imaging of the Lithosphere Asthenosphere Boundary experiment at the equatorial Mid‐Atlantic Ridge covering 0–80‐Myr‐old seafloor. We find slow velocities along the ridge, with faster velocities beneath older seafloor. We image a fast lid (25–30‐km thick) beneath the ridge that increases to 50–60 km beneath older seafloor. Punctuated, ∼100 km wide low velocity anomalies exist off‐axis. There are multiple layers of azimuthal anisotropy, including (i) a lithosphere (20–40 km depth) characterized by strong anisotropy (4.0%–7.0 %) with fast‐axes that rotate from ridge subparallel toward the absolute plate motion/spreading direction at distances >60 km from the ridge, and (ii) weak anisotropy (1.0%–2.0%) at >40 km depth. Our results are consistent with conductive cooling of lithosphere, although with some complexities. Thickened lithosphere beneath the ridge supports lateral conductive cooling beneath slow‐spreading centers. Undulations in lithospheric thickness and slow asthenospheric velocities are consistent with small scale convection and/or partial melt. Lithospheric anisotropy can be explained by vertical flow and a contribution from either fluid or mineral filled cracks organized melt beneath the ridge and plate motion induced strain off axis. Deep azimuthal anisotropy is suggestive of upwelling beneath the ridge and three‐dimensional flow possibly caused by small scale convection off‐axis.

The depth to the lithosphere‐asthenopshere boundary (LAB) provides a strong control on the temperature and strength of the lithosphere and its susceptibility to tectonic deformation. We probe the nature of the LAB by deriving the depth and source of mantle‐xenolith bearing alkaline lavas aged 28 Ma to 8 ka in the Canadian Cordillera (CC). We document a striking coincidence of the average depth of equilibration of these lavas (65 ± 5 km) with the seismically observed LAB both in the CC and in western USA, and the change in H2O storage capacity at the spinel‐garnet transition of fertile peridotite under the geothermal gradient of hot thin back arc regions. The convergence of these factors show the LAB of hot back arcs is hygrometrically controlled at this phase boundary, limiting the H2O to <150 ppm in the mantle lithosphere. Intraplate regions with a shallower LAB require more H2O and/or less fertile lithosphere. Plain Language Summary: The surface of the earth is made of tectonic plates that vary in thickness and temperature, and governs how they deform under the forces of plate tectonics. The base of the plate is marked by a change in the speeds of seismic waves in the mantle indicates small amounts of melt below certain depths. We show a narrow depth range at which magmas stall en route to the surface beneath British Columbia and Yukon that correlates with the base of the plate in this region. In this and other continental margins, the depth to the base of the plate is controlled by the combined effects of a change in mineralogy and H2O storage capacity of mantle peridotite that determines whether or not melts are stable under a certain temperature. Key Points: Alkaline lavas in the Canadian Cordillera equilibrated at or below a lithosphere‐asthenosphere boundary (LAB) of 65 ± 5 km depthThe spinel‐garnet peridotite transition changes H2O storage capacity in fertile lithosphere to control the LAB depth in hot back arcsEquilibration depths of alkaline lavas constrain a maximum of ∼150 ppm H2O in mantle lithosphere at the LAB of most intraplate regions [ABSTRACT FROM AUTHOR]

Widespread recent volcanic rocks occur across the Cordillera landward of the current/recent volcanic arc Mexico to Alaska and most other subduction backarcs. We conclude that most are produced by partial melt in the upper asthenosphere where two conditions are met: (1) thin lithosphere, shallow hot asthenosphere. Most of the Cordillera is uniformly hot with thin lithosphere such that the hot asthenosphere extends up to a sufficiently shallow depth to intersects the wet solidus; (2) wet upper asthenosphere. There is substantial water in the upper asthenosphere that reduces the solidus sufficiently for partial melting. We integrate geochemical analyses that constrain the partial melt source temperature and depth, seismic velocities that define the upper mantle temperature and partial melt zones, and seismic receiver functions that define the lithosphere‐asthenosphere boundary (LAB). Geochemical data for the Canadian Cordillera are integrated with data from the western United States. Upper mantle xenoliths indicate a dry strong lithosphere, <50 ppm H2O. A wet asthenosphere source, >250 ppm, is indicated by the volcanics, facilitating small‐scale convection. Geochemical equilibration averages ∼1,350°C, at ∼65 km. Receiver functions also define the LAB at ∼65 km for the western Cordillera, deeper in the eastern US Cordillera, and asthenosphere tomography velocities indicate ∼1,350°C. Low velocities above ∼150 km suggest a few percent partial melt that percolates upward and ponds at the base of the lithosphere until enough accumulates to locally penetrate upward. The 65‐km depth may be controlled by the spinel‐garnet phase transition. Mechanisms are discussed for the spatial distribution of recent volcanics. Plain Language Summary: There are widespread recent volcanic rocks in most of the North American Cordillera from Mexico to Alaska and most 200–1,000 km wide global subduction zone backarcs. We interpret them to result mainly from partial melting in upper wet asthenosphere where there is small‐scale solid‐state convection. There is substantial geochemical modification and differentiation in transit to the surface through the dry strong lithosphere. Partial melt is inferred to pond and chemically equilibrate at the lithosphere‐asthenosphere boundary which is near 65 km for most of the Cordillera from geochemical and seismic structure constraints. This depth may be stabilized by the spinel‐garnet phase transformation. The boundary in most areas is at about 1,350°C which strongly constrains upper mantle and crust temperatures, with the Cordillera Moho generally at 800°C–850°C. Local concentrations of volcanics may result from upward movement into the asthenosphere melt window. Key Points: Similar mafic backarc volcanics are widespread in the Cordillera from Mexico to AlaskaMost of Cordillera is uniformly hot with base of the lithosphere at ∼1,350°C and ∼65 km from geochemical equilibria, seismic tomography, and receiver functionsVolcanics produced mainly by partial melting in wet asthenosphere, ponding at base of dry lithosphere; depth controlled by the spinel‐garnet phase change [ABSTRACT FROM AUTHOR]

Abstract—This review discusses the major milestone results yielded by the regional seismological studies of the deep structure of the Earth's crust and mantle beneath the Baikal rift since the 1960s to present. It also includes data from recent global models covering a depth interval to below 400 km rarely considered in regional studies. The main focus of the review is laid on the intercomparison of various velocity models of the region, sometimes substantially contradicting each other, which determines the pertinence of this work. In particular, there has been no consensus among different authors as to the crustal thinning beneath the Baikal rift, the thickness of the anomalous mantle layer and the lithosphere. For establishing the causes of the revealed discrepancies, the review briefly compares the inversion methods used in the studies and their resolution. Separate discussion is dedicated to anisotropic properties of the upper-mantle material which is studied from splitting of SKS-waves and from phase and group velocity dispersion data of surface waves. The Conclusions section presents the additional information which can be used for verifying a particular model: there are the results of the studies of thermal, gravitational, geomagnetic, and geoelectric fields and some geological data. The implications of geophysical data covered by the review for the ongoing discussion on the origin of lithospheric extension in the Baikal rift zone are analyzed. It is shown that most of the data (low surface heat flow values and temperatures in the mantle, fairly large bottom depths of the lithospheric magnetic sources, the estimates of the lithospheric thickness from gravimetric and geoelectric data) including purely seismological results (the absence of a general thinning of the crust and lithosphere along the entire rift axis) as well as some geological data contradict the hypotheses of active rifting. However, the existing deep structure models are inconclusive for the ultimate choice between the hypotheses explaining the rift formation by purely passive or mixed mechanisms. The solution of this question requires further, more detailed geophysical study. Thus, the presented review of the deep structure of the Earth's crust and mantle of the Baikal rift provides the framework for assessing the results of the previous geophysical studies and outlining the prospects of the future research. [ABSTRACT FROM AUTHOR]

The evolution of ocean lithosphere and asthenosphere are fundamental to plate tectonics, yet high resolution imaging is rare. We present shear wave velocity and azimuthal anisotropy models for the upper mantle from Rayleigh wave group velocities from local earthquake and ambient noise at 15–40‐s period recorded by the Passive Imaging of the Lithosphere Asthenosphere Boundary experiment at the equatorial Mid‐Atlantic Ridge covering 0–80‐Myr‐old seafloor. We find slow velocities along the ridge, with faster velocities beneath older seafloor. We image a fast lid (25–30‐km thick) beneath the ridge that increases to 50–60 km beneath older seafloor. Punctuated, ∼100 km wide low velocity anomalies exist off‐axis. There are multiple layers of azimuthal anisotropy, including (i) a lithosphere (20–40 km depth) characterized by strong anisotropy (4.0%–7.0 %) with fast‐axes that rotate from ridge subparallel toward the absolute plate motion/spreading direction at distances >60 km from the ridge, and (ii) weak anisotropy (1.0%–2.0%) at >40 km depth. Our results are consistent with conductive cooling of lithosphere, although with some complexities. Thickened lithosphere beneath the ridge supports lateral conductive cooling beneath slow‐spreading centers. Undulations in lithospheric thickness and slow asthenospheric velocities are consistent with small scale convection and/or partial melt. Lithospheric anisotropy can be explained by vertical flow and a contribution from either fluid or mineral filled cracks organized melt beneath the ridge and plate motion induced strain off axis. Deep azimuthal anisotropy is suggestive of upwelling beneath the ridge and three‐dimensional flow possibly caused by small scale convection off‐axis. Plain Language Summary: Mid‐ocean ridges are the locations where new oceanic lithosphere is formed. The process by which material upwells before cooling and moving off‐axis is an important question in Earth science. There are only few high resolution seismic images of this process. We deployed 39 broadband ocean‐bottom seismometers at the equatorial Mid‐Atlantic Ridge. We present a shear wave velocity and azimuthal anisotropy model for the upper mantle using a combination of local earthquake events and ambient noise data. We find slower velocities along the ridges that increase as a function of seafloor ages. Although the lithospheric thickness increases with seafloor age as predicted by thermal models, there is also greater complexity. We find punctuated slow velocity anomalies off‐axis in the asthenosphere. Depth inversion of anisotropy suggests a multilayer anisotropy model is required to interpret the results. Based on the combination of three‐dimensional shear wave velocity and azimuthal anisotropy, we suggest that in addition to conductive cooling, small scale convection is required to explain our observation, and evolution ocean lithosphere is more complex than typically thought. Key Points: We find slow velocities along the ridge and faster velocities off axis in the lithosphere, generally consistent with conductive coolingWe find punctuated low velocity anomalies off‐axis that are likely formed by upwellings due to small scale convection and partial meltMultiple layers of azimuthal anisotropy are required owing to lithospheric cracking, organized melt, upwelling, and/or plate motions [ABSTRACT FROM AUTHOR]

During magnetic storms, solar‐magnetosphere‐ionosphere‐Earth interactions give rise to geomagnetically induced currents (GICs) in man‐made technological conductors such as power grids, gas pipelines, and railway networks with potentially damaging outcomes. Generally, electrically conductive regions of the Earth are assumed to be less at risk to GICs than resistive ones, since induced electric fields associated with GICs are linearly related to given magnetic source fields via Earth's impedance. Here, we show that magnetic source fields associated with storms can be enhanced by secondary electromagnetic (EM) induction in Earth's electrically conductive asthenosphere and that this previously neglected effect can give rise to larger electric fields close to the lithosphere‐asthenosphere boundary in regions where the conductance of the asthenosphere is higher. Our analysis of data from the 30 October 2003 "Halloween" and 8 September 2017 storms shows that the magnitudes of electric fields from both storms are affected by lithospheric plate thickness and asthenosphere conductance (conductivity‐thickness product) and that they are 5 times larger in southern Sweden (>5 V/km for the 30 October 2003 "Halloween" storm) than in central Scotland. Our results provide insight into why Sweden experienced a storm‐related power outage in 2003, whereas Scotland did not. Plain Language Summary: Space weather is generated by changes in the rate at which the Sun emits high‐velocity, charged particles (plasma) that are collectively referred to as the "solar wind." Just as extreme atmospheric perturbations cause meteorological storms whose high‐velocity winds can damage ground‐based infrastructure, extreme electromagnetic perturbations of Earth's magnetosphere and ionosphere due to fluctuations in solar‐wind pressure cause magnetic storms that drive hazardous currents that are quasi‐DC (direct current) through ground‐based linear conductors—power networks, gas pipelines, and railways. These currents, referred to as "geomagnetically induced currents" or "GICs," can trip electricity transformers, corrode gas pipelines, and cause railway signal failure. GIC hazards depend not only on external factors but also on the Earth itself, because the external magnetic source fields interact with Earth's deep electrical conductivity—that is, electric fields that drive surficial GICs are induced deeper within the Earth. Here, we investigate how the regionally variable thickness of Earth's quasi‐rigid outer, electrically resistive shell—the "tectonic plate" or "lithosphere"—and region‐dependent conductance of the underlying electrically conductive layer known as the "asthenosphere" modify surface electric fields making some regions of the world inherently more vulnerable to extreme space weather events than others. Key Points: A novel study of coupling between external storm time geomagnetic fields and Earth's deep electrical conductivity structure is presentedTectonic plate thickness and electrical conductivity of the underlying asthenosphere are shown to affect regional vulnerability to space weatherElectric fields are derived for the 8 September 2017 and 30 October "Halloween" 2003 magnetic storms in central Scotland and southern Sweden [ABSTRACT FROM AUTHOR]

Abstract A seismic discontinuity known as the G‐discontinuity, which has been prevailingly detected in the oceanic mantle, is considered to provide evidence of a sharp lithosphere‐asthenosphere boundary (LAB). We investigate whether this sharp LAB is explained by grain boundary premelting, which causes a solid‐state weakening of rock from just below the solidus temperature. We use an experimentally derived mechanical model describing the effect of premelting and calculate vertical profiles of VS and QS−1 for various sets of oceanic geotherm and solidus values. Based on the realistic volatile distribution and solidus profile obtained from a model of fractional decompression melting, we show that even when the oceanic upper mantle is melt free, geotherm sufficiently approaches the solidus at various plate ages, resulting in a steep reduction in VS due to the anelastic effect of premelting. A small amount of volatile adsorbed to grain boundary plays an important role in this result. The predicted seismic discontinuity sharpness (mostly ∼15 km) is consistent with seismological observations (≤15 km), predicted discontinuity depth (along ∼1300°C isotherm) is deeper than observations (along 1100°C isotherm), and predicted velocity reduction amplitude (∼1%–2%) is considerably smaller than observations (≥5%). Because the predicted velocity reduction amplitude is subject to uncertainty resulted from the limited experimental data, further study is needed for the mechanical effect of premelting. We further discuss a discrepancy of seismological results from receiver function and SS precursor studies and from surface wave studies, making it difficult to explain both data consistently.

Abstract The oceanic lithosphere is a primary component of the plate tectonic system, yet its evolution and its asthenospheric interaction have rarely been quantified by in situ imaging at slow spreading systems. We use Rayleigh wave tomography from noise and teleseismic surface waves to image the shear wave velocity structure of the oceanic lithosphere‐asthenosphere system from 0 to 80 My at the equatorial Mid‐Atlantic Ridge using data from the Passive Imaging of the Lithosphere‐Asthenosphere Boundary (PI‐LAB) experiment. We observe fast lithosphere (VSV > 4.4 km/s) that thickens from 20–30 km near the ridge axis to ~70 km at seafloor >60 My. We observe several punctuated slow velocity anomalies (VSV 400 km from the ridge. We observe a high velocity lithospheric downwelling drip beneath 30 My seafloor that extends to 80–130 km depth. The asthenospheric slow velocities likely require partial melt. Although melt is present off axis, the lack of off‐axis volcanism suggests the lithosphere acts as a permeability boundary for deeper melts. The punctuated and off‐axis character of the asthenospheric anomalies and lithospheric drip suggests small‐scale convection is active at a range of seafloor ages. Small‐scale convection and/or more complex mantle flow may be aided by the presence of large offset fracture zones and/or the presence of melt and its associated low‐viscosities and enhanced buoyancies.

The original continental crust of the deep-sea basins in the Sea of Okhotsk region was considerably reworked by tectonic and magmatic processes, whereby it has become substantially denser while less thick. The subcrustal lithospheric mantle of the South Okhotsk basin is characterized by the high velocities of longitudinal seismic waves (8.4 km/s) and higher density values, accordingly. A poly-astenospheric model is proposed for the South Okhotsk deep-water basin: it comprises three asthenospheric layers separated by hardened layers. The roof of the upper mantle asthenosphere in the basins is observed at shallow depths (70–75 km), showing low electrical resistivity (r=3–10 Om×m). The asthenospheric layers contain the substantial amounts of magmatic melts (11–16 %), which significantly reduce their viscosity. Emplacement of basic-ultrabasic melts in the Earth's crust and subcrustal lithospheric mantle (i.e. dense mass redistribution, and consolidation of the crust and mantle), as well as the lower viscosity asthenospheric layers facilitate the isostatic subsidence during the formation of deep-sea basins. The deep structure of the continental Pannonian basin, characterized by a high heat flow, is compared to the deep structure of the neotectonic deep-water depression in the Sea of Okhotsk region.

This study presents an improved approach to common‐conversion point stacking of converted body waves that incorporates scattering kernels, accurate and efficient measurement of stack uncertainties, and an alternative method for estimating free surface seismic velocities. To better separate waveforms into the P and SV components to calculate receiver functions, we developed an alternative method to measure near‐surface compressional and shear wave velocities from particle motions. To more accurately reflect converted phase scattering kernels in the common‐conversion point stack, we defined new weighting functions to project receiver function amplitudes only to locations where sensitivities to horizontal discontinuities are high. To better quantify stack uncertainties, we derived an expression for the standard deviation of the stack amplitude that is more efficient than bootstrapping and can be used for any problem requiring the standard deviation of a weighted average. We tested these improved methods on Sp phase data from the Anatolian region, using multiple band‐pass filters to image velocity gradients of varying depth extents. Common conversion point stacks of 23,787 Sp receiver functions demonstrate that the new weighting functions produce clearer and more continuous mantle phases, compared to previous approaches. The stacks reveal a positive velocity gradient at 80–150 km depth that is consistent with the base of an asthenospheric low‐velocity layer. This feature is particularly strong in stacks of longer period data, indicating it represents a gradual velocity gradient. At shorter periods, a lithosphere‐asthenosphere boundary phase is observed at 60–90 km depth, marking the top of the low‐velocity layer. Plain Language Summary: This paper presents a new method that more accurately incorporates the physics of seismic scattering into how the wave records are combined to form images of gradients in seismic velocity structure. This method was tested on data from the Anatolian region, where the asthenosphere is known to have low seismic wave velocities, consistent with high mantle temperatures and possibly small fractions of partial melt, as suggested by the presence of volcanic fields at the surface. However, the depth of the asthenospheric low‐velocity layer is not well known. In this study, we locate this low‐velocity mantle layer by applying the newly developed imaging method to seismic shear waves that convert to compressional waves at the velocity gradients that mark the layer boundaries. This study is the first to clearly resolve both the lower and upper margins of the asthenosphere for the whole region. The top of the layer corresponds to the lithosphere‐asthenosphere boundary at 60–90 km depth, and this velocity gradient is localized in depth. However, the bottom boundary, which lies at depths of 80–150 km, occurs over a broader depth range. Key Points: A new approach to common conversion point stacking based on scattering kernels is developedA fast and accurate quantification of the standard deviation of the weighted stack average is derivedAn anomalously low‐velocity asthenosphere beneath Anatolia is indicated by velocity discontinuities imaged with multifrequency data [ABSTRACT FROM AUTHOR]

A seismic discontinuity known as the G‐discontinuity, which has been prevailingly detected in the oceanic mantle, is considered to provide evidence of a sharp lithosphere‐asthenosphere boundary (LAB). We investigate whether this sharp LAB is explained by grain boundary premelting, which causes a solid‐state weakening of rock from just below the solidus temperature. We use an experimentally derived mechanical model describing the effect of premelting and calculate vertical profiles of VS and QS−1 for various sets of oceanic geotherm and solidus values. Based on the realistic volatile distribution and solidus profile obtained from a model of fractional decompression melting, we show that even when the oceanic upper mantle is melt free, geotherm sufficiently approaches the solidus at various plate ages, resulting in a steep reduction in VS due to the anelastic effect of premelting. A small amount of volatile adsorbed to grain boundary plays an important role in this result. The predicted seismic discontinuity sharpness (mostly ∼15 km) is consistent with seismological observations (≤15 km), predicted discontinuity depth (along ∼1300°C isotherm) is deeper than observations (along 1100°C isotherm), and predicted velocity reduction amplitude (∼1%–2%) is considerably smaller than observations (≥5%). Because the predicted velocity reduction amplitude is subject to uncertainty resulted from the limited experimental data, further study is needed for the mechanical effect of premelting. We further discuss a discrepancy of seismological results from receiver function and SS precursor studies and from surface wave studies, making it difficult to explain both data consistently. Plain Language Summary: Since the discovery of plate tectonics, the Earth's upper mantle has been known to consist of the rigid lithosphere and the underlying soft asthenosphere. However, the reason why rocks are soft in the asthenosphere remains an open question. In this study, we present a new idea that rock in the asthenosphere is softened by grain boundary premelting, which causes a solid‐state weakening of rock from just below melting temperature. Recent seismological observations suggest that the lithosphere‐asthenosphere boundary (LAB) is sharp because it reflects seismic waves. Our recent experimental studies clarified premelting's mechanical effects, which can be used to predict seismic wave velocity vertical profiles in the lithosphere‐asthenosphere system. We show that a steep velocity reduction occurs by the premelting's mechanical effects; however, the predicted depth is slightly deeper and the predicted velocity drop is considerably smaller than observed. To address this discrepancy, further experimental and observational studies are needed. There is a prospect that premelting can explain diverse observations with respect to the asthenosphere, such as seismic low velocity and high attenuation, strong seismic anisotropy, low viscosity, and very small or no melt fraction. Therefore, further study of premelting is important to clarify the origin of the asthenosphere. Key Points: A mechanical model of premelting predicts a seismic discontinuity without invoking discontinuous distribution of melt or waterPremelting caused by the volatiles remaining after decompression melting can predict weak asthenosphere even in a presently melt‐free mantleQuantitative comparisons between premelting model predictions and seismological observations of lithosphere‐asthenosphere boundary are presented [ABSTRACT FROM AUTHOR]