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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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

2D integrated modelling approach was applied to determine the lithospheric structure along transect HT-1 located in the Carpathian-Pannonian Basin{European platform region. Our approach combines simultaneous interpretation of surface heat flow, topography, gravity and geoid data. All available geophysical and geological data were used to create an initial model that has been afterwards modified by trial and error method until reasonable fit was obtained between input data and model predictions. The main focus of our study was the position and shape of the lithosphere-asthenosphere boundary (LAB). In the Pannonian Basin the modelled LAB is at depths of about 80-90 km and rapidly dips towards the Western Carpathians where its depth reaches values 145 to 150 km. Beneath the European platform the LAB depth is about 135-140 km. We can observe a slight lithospheric root under the Western Carpathians. This lithospheric thickening is interpreted as a small remnant of a subducted slab. This result is in a good agreement with the previous lithospheric models in the Carpathian-Pannonian Basin. [ABSTRACT FROM AUTHOR]

Recent seismic observations challenge the conventional view that tectonic plates are driven by slab pull and ridge push forces. The observations indicate that the asthenosphere flows faster and in a different direction than the plate above. Previous mantle convection models argued that pressure‐driven flow, in combination with a non‐Newtonian upper mantle, can account for these observations. We expand those models by introducing a formulation that allows for the development of weak plate margins. Weak plate margins lead to an increase in the ratio of slab‐driven to pressure‐driven asthenosphere flow, but pressure‐driven flow remains active and increases with plate margin strength. Locally, the asthenosphere can drive plates and can change flow direction with depth. Furthermore, a non‐Newtonian upper mantle allows for a hysteresis effect where, depending on initial conditions, single‐plate and plate tectonic modes can exist at the same parameter conditions. Plain Language Summary: It is generally thought that tectonic plates drive motion in the Earth's rocky interior. Recent observations have challenged this view as they indicate that interior motion can drive tectonic plates. Models of coupled tectonics and interior flow are used to address this discrepancy. The models reveal that the question of "does plate tectonics drive interior flow or does interior flow drive plate tectonics" may be ill founded as both possibilities may be active at the same time. The balance between the two drivers is found to depend on plate margin strength. The models also reveal that different tectonic modes can exist under the same physical conditions. This indicates a planet's initial state can determine if it will or will not have plate tectonics. Key Points: Asthenosphere flow velocities can exceed tectonic plate velocityAsthenosphere flow can rotate with depth away from plate flow directionsHysteresis exists between plate tectonic and stagnant lid modes of mantle convection [ABSTRACT FROM AUTHOR]

A model of the geological evolution of the Japan and Okhotsk seas was developed based on radioisotope age, mineralogical, and isotope-geochemical study of the Late Mesozoic–Cenozoic volcanic rocks. The geodynamic settings of volcanic stages were determined as follows: (1) the Late Cretaceous continental-margin (calc-alkaline), (2) the Eocene transform-margin (adakite) in the Sea of Okhotsk, (3) the Miocene–Pliocene marginal sea (alkaline basaltic) in the Sea of Japan, and (4) the Pliocene–Pleistocene island arc (calc-alkaline) in the southern Sea of Okhotsk. It has been established that parental magmas were derived from subcontinental lithosphere, oceanic asthenosphere, and lower-mantle plume-continental (CAB) and plume-oceanic (OIB) sources. Geodynamic settings changed from the Late Cretaceous subduction to the Maastrichtian–Pliocene transform margin. The transform margin setting involved destruction and extension, maximum oceanic-margin spreading (end of the Early Miocene–beginning of the Middle Miocene), and post-spreading lower mantle plume upwelling (the Middle Miocene – Pliocene). It was completed by the resumption of the Pliocene–Pleistocene subduction of the Pacific plate beneath the Eurasian continent. [ABSTRACT FROM AUTHOR]

The petrology, geochronology and geochemistry of the mafic enclaves in the Mid‐Late Triassic Jiefangyingzi pluton from Chifeng area, southern Inner Mongolia, in China are studied to reveal their petrogenetic relationship with the host pluton. Furthermore, the coeval magmatic assemblage and its petrogenesis on the northern margin of the North China craton (NCC) are studied synthetically to elucidate their tectonic setting and the implications for the destruction of the NCC. Zircon U‐Pb dating reveals that the mafic enclaves formed at 230.4 ± 2.2 Ma, which is similar to the age of the host pluton. The most basic mafic enclaves belong to weak alkaline rocks, and they display rare earth element (REE) and trace element normalized patterns and trace element compositions similar to those of ocean island basalt (OIB). In addition, they have positive ∊Nd(t) values (+3.84 to +4.94) similar to those of the Cenozoic basalts on the northern margin of the NCC. All of these geochemical characteristics suggest that the basic mafic rocks originated from the asthenosphere. Petrological and geochemical studies suggest that the Jiefangyingzi pluton and the intermediate mafic enclaves were formed by the mixing of the asthenosphere‐derived and crust‐derived magmas in different degrees. The Mid‐Late Triassic magmatic rocks on the northern margin of the NCC could be classified into three assemblages according to their geochemical compositions: alkaline series, weak alkaline–sub‐alkaline series and sub‐alkaline series rocks. Petrogenetic analyses suggest that the upwelling of the asthenosphere played an important role in the formation of these Mid‐Late Triassic magmatic rocks. Basing on an analysis of regional geological data, we suggest that the northern margin of the NCC underwent destruction due to the upwelling of the asthenosphere during the Mid‐Late Triassic, which was induced by the delamination of the root of the collisional orogeny between Sino‐Korean and Siberian paleoplates in Late Permian. [ABSTRACT FROM AUTHOR]

We performed waveform inversion of the P waveforms recorded by our BroadBand Ocean Bottom Seismometers (BBOBSs) deployed in the Northwestern Pacific. Consequently, the depth profile of the P velocity of the oceanic upper mantle, which has not been well resolved by previous surface wave or receiver function analyses, was revealed. We considered the azimuthal anisotropy in the lithosphere, which significantly improved the variance reduction from 34% to 44%. The resulting P model exhibited higher and lower velocities in the lithosphere and asthenosphere, respectively. The velocity contrast was found to be second/third of that observed in the previous S models; however the obtained model appeared to have some trade‐off with the VS structures in the vicinity of the source. We compared our P model with the previous S model obtained using our BBOBSs and obtained the VP/VS model. The resulting VP/VS model has two notable features. First, the lithosphere is characterized by a rapid increase in the VP/VS values with depth, which implies chemical stratification. Second, the VP/VS values in the vicinity of the lithosphere‐asthenosphere boundary (LAB) are larger than the synthetic values of any major upper mantle mineral predicted by considering the anharmonic effects, which suggests the effects of anelasticity or melt. Key Points: We inverted P waveforms recorded by our BBOBSs to obtain the depth profile of the P velocity beneath the older Pacific plateCombining our P model with a previous S model revealed two notable features in Vp/VS structuresThe obtained Vp/Vs model suggests chemical stratification in the lithosphere and the effects of anelasticity or melt at the LAB [ABSTRACT FROM AUTHOR]

Deep seismic studies of the upper mantle conducted in Russia with the use of nuclear explosions and laboratory studies of mantle materials at high pressure and temperature have revealed new structural and petrophysical features of the upper mantle beneath Northern Eurasia. These features are hard to explain within the framework of current understanding of the structure of the continental lithosphere. For example, in the region of the asthenosphere distinguished according to the thermal field, no corresponding decrease in seismic velocities has been detected, but instead low-velocity layers have been identified at a depth of 100–150 km within the lithosphere. However, the asthenosphere may be distinguished by structural features of the seismic boundaries. This means that it is represented by a layer of elevated plasticity without partial melting. The laboratory studies also indicate that seismic velocities do not depend on the composition of mantle rocks and are controlled primarily by their temperature. This made it possible to infer the temperature regime of the upper mantle from seismic data. Calculations carried out on this basis have shown that the thickness of the lithosphere beneath the Siberian Craton does not vary and is 300–350 km. These data are inconsistent with evaluations of the lithosphere bottom depth based on the heat flow at the surface. This discrepancy may be explained by a stronger effect of deep fluids on the heat flows and on petrophysical parameters of mantle rocks. At depths where mechanical properties of rocks drastically change and their porosity increases, the density of the deep fluids decreases, and the fluids release much heat. As a result, low-velocity domains (plumes), which may contain partial melts, are formed and the surface heat flow increases. This may explain how low-velocity layers can be formed at a depth of 100–150 km and why the temperatures determined from seismic data differ from those derived from the heat flow data. Deep fluids also initiate physical and chemical transformations in mantle rocks, for instance, produce depleted material that has a reduced density but is characterized by the unvarying seismic velocity. Deviations from linear relationship between seismic velocity in a material and its density is also detected at integrated interpretations of seismic and gravimetric data. Physicochemical transformations of rocks in areas where deep fluid flows are focused can also account for the origin of complex reflective boundaries identified in the lithosphere. [ABSTRACT FROM AUTHOR]

High‐temperature ionic conductivity in olivine single crystals has been measured in the [100], [010], and [001] crystallographic orientations as a function of pressure from 2 to 10 GPa, temperature from 1450 to 2180 K, and H2O content from 20 to 580 wt. ppm using multianvil presses with in situ impedance analyses. The experimental results yield an activation energy, activation volume, and H2O content exponent of 250–405 kJ/mol, 3.2–5.3 cm3/mol, and 1.3 ± 0.2, respectively, for the high‐temperature ionic conduction regime. Olivine ionic conductivity has negative pressure and positive temperature dependences and is significantly enhanced by H2O incorporation. The [001] direction is more conductive than the [100] and [010] directions. The H2O‐enhanced ionic conductivity may contribute significantly to the electrical conductivity profile in the asthenosphere, especially in the regions under relatively high‐temperature and low‐pressure conditions. Key Points: Pressure, temperature, and water content dependences of olivine ionic conductivity are obtainedOlivine ionic conductivity is dramatically enhanced by water incorporationOlivine ionic conductivity may contribute significantly to the bulk conductivity of olivine in the asthenosphere [ABSTRACT FROM AUTHOR]

A model of the geological evolution of the Sea of Okhotsk is presented based on data from radioisotope age determinations and the mineral and isotope-geochemical composition of Late Mesozoic–Cenozoic volcanic rocks. We discuss the probable interrelationship between gas geochemical manifestations of gas fluxes with anomalous methane concentrations and the formation of gas hydrates, along with the occurrence of volcanic processes in the Sea of Okhotsk, fault zones, and different geological structures of basement and sedimentary deposits, landslides, and earthquakes. As a result of the studies, the nature of each volcanic stage has been determined. These include the Late Cretaceous continental-marginal belt stage (calc-alkaline), the Eocene transform-marginal (adakite) stage, and the Pliocene-Pleistocene island arc stage in the southern part of the Sea of Okhotsk. The sources of magma generation have been recognized, namely, the lithosphere subcontinental, asthenosphere oceanic, and plume-oceanic (OIB). The change in geodynamic regimens was traced back from the Late Cretaceous subduction regimen to the Maastrichtian–Datian transform-marginal, which continued as far as the Pliocene and ended at the resumption of the Pliocene–Pleistocene subduction of the Pacific Plate beneath the Eurasian continent. This involved the processes of destruction of the subduction plate, lithosphere and asthenosphere diapirism, and lower mantle plume upwelling. In the periods of geodynamic, seismotectonic and volcanic activities, gas migrated along the fault zones from the depth to the surface, together with different volcanic substrates ascending through the upper and lower mantle. Gas contained CO2, CH4, H2, He, N2, O2, and superheated steam (Н2О). Gas plays an important dynamic and physicochemical role in the evolution of the Sea of Okhotsk. At the present stage, gas fluxes migrating from the depth to the surface manifest as gas bubbles penetrating from the bottom sediments into the water column, with some portion of gas penetrating into the atmosphere. In the areas with gas fluxes, the fields containing anomalous concentrations of hydrocarbons, gas hydrate, carbon dioxide, hydrogen, and helium result in the formation of gas hydrates and associations of authigenic minerals and various geochemical elements. [ABSTRACT FROM AUTHOR]

• Large Fe isotopic variation for continental intraplate basalts from SE China. • A two-stage melting process explains both heavy and light Fe isotope data. • Early-stage low-degree melt preserves heavy Fe isotopes of carbonated pyroxenite. • Further decompression and mixing with SCLM melt causes light Fe isotope signature. • Fe isotopes provide insights into generation of continental intraplate basalts. Iron isotope studies on ocean island basalts (OIBs) and mid-ocean ridge basalts (MORBs) have disclosed the contribution of pyroxenite lithologies in the generation of basaltic magmas. Whether Fe isotopic compositions of continental intraplate basalts can be applied to trace lithological heterogeneity within the mantle source regions remains poorly investigated. To explore the systematics of stable Fe isotopes as a potential probe of lithological heterogeneity in the source of continental intraplate basalts, we present twenty-four Fe isotope data on a suite of well-characterized Cenozoic basalts from SE China. The samples show a large range of Fe isotope values (δ56Fe = +0.09‰ to +0.20‰), which correlate with SiO 2 , CaO/Al 2 O 3 , Ti/Eu, Hf/Hf*, Zr/Nb, Dy/Yb, La/Yb, Nb/Y, K/La, Sr/Ce, δ66Zn and estimated equilibrium pressures. The samples with the highest δ56Fe values represent early-stage low-silica basalts with moderately enriched Sr-Nd isotope ratios and high δ66Zn values, while the late-stage high-silica basalts display a broad δ56Fe decrease with increasing 87Sr/86Sr and with decreasing ε Nd and δ66Zn. We demonstrate that the heaviest Fe isotope signatures cannot be derived from a pure peridotite source and require a pyroxenite component in the source. Combined with other geochemical proxies, we show that the heaviest basalt compositions are consistent with low-degree melts produced by the adiabatic decompression of a carbonated pyroxenite-bearing asthenospheric mantle. Mixing of these hybrid melts with melt produced from in-situ melting of the subduction-modified sub-continental lithospheric mantle (SCLM) reproduces the Fe-isotope variability of the late-stage basalts. Our results demonstrate the significance of lithological heterogeneity in the mantle source of continental intraplate basalts and highlight the subsequent imprint of mantle lithosphere in the evolution of their Fe isotope compositions. Finally, this study exemplifies the potential of the Fe-Zn stable isotope pair for mantle geochemistry; coupled with traditional radiogenic isotopes and major and trace element concentrations, they formed an efficient tool to trace the nature and contribution of the mantle source components in the formation of intraplate basalts. [ABSTRACT FROM AUTHOR]

Recent geophysical observations report the presence of a very weak and thin upper asthenosphere underneath subducting oceanic plates at convergent margins. Along these margins, trench migrations are significantly slower than plate convergence rates. We use numerical models to assess the role of a weak upper asthenospheric layer on plate and trench motions. We show that the presence of this layer alone can enhance an advancing trend for the motion of the plate and hamper trench retreat. This mechanism provides a novel and alternative explanation for the slow rates of trench migration and fast‐moving plates observed globally at natural subduction zones. Plain Language Summary: Plate tectonics relies on the concept of a rigid surface layer (the lithosphere) fragmented in a series of major and minor tectonic plates moving over a weaker and buoyant layer of asthenospheric material. The motion and deformation of the lithosphere are primarily driven by the sinking of the colder and denser oceanic lithosphere through time into the Earth's interior at subduction zones. A range of geophysical regional studies has recently brought considerable attention to the detailed structure of the asthenosphere at these zones. These studies report the presence of a thin and even weaker asthenospheric layer at the base of the subducting lithosphere. We investigate the role of this layer on subduction dynamics using geodynamics numerical models. Our models apply knowledge of physics, chemistry, mathematics, and geology and study the dynamics of the Earth's deep interior and their feedback on surface deformation. Our numerical results demonstrate that a very weak and thin upper asthenosphere affects the dynamics of sinking plates. It acts as a slippery base for the motion of the plate significantly increasing its speed and deformation. Our research provides a novel explanation to interpret some of the subduction phenomena at the regional scale observed on Earth, which have proven numerical difficulties until now. Key Points: Numerical models show that a very weak and thin upper asthenosphere layer can alone reduce trench retreat and enhance plate motionThe impact of this effect depends on the relative contrast between the effective stiffness of the lithosphere and the underlying mantleThis regional mechanism explains the high convergence and low trench migration rates observed globally at natural subduction zones [ABSTRACT FROM AUTHOR]

• We find high seismic attenuation in the mantle beneath the Central Appalachians. • Small-scale convection has persisted since the lithosphere was lost. • The results support delamination as the cause of volcanism at this passive margin. The passive margin of the eastern coast of the United States is known to be geologically active, with recently rejuvenated topography, intraplate seismicity, and volcanism of Eocene age. This study uses seismic data from the Mid-Atlantic Geophysical Integrative Collaboration (MAGIC) experiment to constrain lateral variations in the attenuation of teleseismic P waves beneath the central Appalachian Mountains to shed light on the structure and dynamics of the upper mantle at this "active" passive margin. We use a Monte Carlo approach to estimate variations in attenuation along with both data and model uncertainties. The quality factor of the upper mantle dramatically decreases over a distance of less than 50 km on the western side of the central Appalachian Mountains, where a low-velocity anomaly has been previously inferred. Extrinsic factors such as scattering or focusing are rejected as explanations for the observations on the basis of finite-difference waveform modeling experiments. The peak in attenuation beneath the crest of the Appalachian Mountains requires that near- to super-solidus conditions occur in the upper mantle and is co-located with volcanism of Eocene age. Our preferred interpretation is that the attenuation reflects the removal of the mantle lithosphere via delamination beneath the mountains, followed by ongoing small-scale convection. [ABSTRACT FROM AUTHOR]

Methods for a spherical harmonic analysis and synthesis of global gravitational and lithospheric structure models are applied to compile the mantle and sub-lithospheric mantle gravity maps. Both gravity maps are then interpreted and assessed by means of their accuracy. The mantle gravity map exhibits a gravitational signature that mainly reflects a thermal state of the lithospheric mantle. This is particularly evident over the oceanic lithosphere, with gravity lows along mid-oceanic spreading ridges. The increasing gravity signal with the ocean-floor age is attributed to conductive cooling of the oceanic lithosphere. Gravity lows extend along continental rift systems. Gravity lows also mark active convergent tectonic margins (in Pacific, Mediterranean, and Caribbean). The old, cold and tectonically stable cratonic mantle is typically characterized by gravity highs. A thermal signature of upwelling mantle under mid-oceanic spreading ridges clearly manifests (by gravity lows) also in the sub-lithosphere mantle gravity map. Nevertheless, the overall signature of conductive cooling is less pronounced in this gravity map, and a thermal signature of the asthenosphere under most of the continental lithosphere is weak. This indicates that a lateral thermal gradient within the asthenosphere tends to be weaker than within the overlying lithospheric mantle. The most pronounced feature in this gravity map is the signature of subducted slabs in West Pacific, marked by gravity highs. An antipodal signature of two large low shear-velocity provinces in both mantle gravity maps is absent, while its long-wavelength pattern could clearly be recognized in the free-air gravity map. We explain this finding by the fact that gravity-stripping procedures applied in this study superpose a gravitational signature of an intermediate layer, in this case the lithospheric mantle and the asthenosphere, over a much weaker signature of deeper mantle density heterogeneities. Moreover, the interpretational quality of both mantle gravity maps is considerably worsen by the LITHO1.0 lithospheric model uncertainties, especially within a more complex structure of the continental lithosphere. As a result, some spatial features in presented gravity maps could be artefacts rather than a real gravity signal. Despite accuracy limitations of currently available lithospheric density models, such types of gravity maps provide a useful information for various purposes in geophysics, among others gravimetric interpretations of Earth's inner structure or a separation of gravitational signals from different sources. In geodesy, a primary motivation is related to a compilation of Earth's synthetic density model based on the condition of fulfilling the total mass budget for testing numerical techniques applied in gravimetric forward modelling by means of solving Newton's volume integral. [ABSTRACT FROM AUTHOR]

Throughout the oceanic asthenosphere there exists a zone of seismic velocities that are lower than would be predicted. The cause of this low velocity zone (LVZ), whether partial melt or solid-state mechanisms, has been debated for decades. We investigate the LVZ by considering seismic and magnetotelluric data from tectonically stable, ∼70 Myr old lithosphere in the central Pacific Ocean. We utilise recent experimental advances on the influence of partial melt and seismic attenuation. Results show that the LVZ is characterised by a small volume of interconnected melt and by low hydrogen contents. Beneath the LVZ, the asthenosphere does not contain partial melt but has high hydrogen contents and low attenuation values, indicating large grain sizes and/or low oxygen fugacities. To explain these observations, we propose that a small amount of unextractable melt is trapped in the asthenosphere after melting at mid-ocean ridges. • A small amount of melt is unextractable after mid-ocean ridge melting. • Seismic low velocity zone (LVZ) is caused by this partial melt. • Peridotite in the LVZ has low water contents. • Below the LVZ the asthenosphere is melt-free and hydrogen-rich. • Seismic attenuation is lower than predicted in the deep asthenosphere. [ABSTRACT FROM AUTHOR]

Abstract High Mg diorites provide important insights into crust-mantle interaction during tectonic evolution of orogenic belt. An integrated study including zircon U-Pb dating, whole-rock geochemistry and isotopic compositions of Sr and Nd was carried out on high Mg diorites of the Wulong pluton located in the narrowest part of the South Qinling Belt. Crystallized ages of 218-210 Ma are obtained in combination with the published data. These rocks are characterized with relatively high MgO (2.76–7.17%), Cr (68.7-310 ppm) and Ni (21.3–76.3 ppm) contents. Most diorites exhibit high Sr/Y and La/Yb ratios with negligible Eu anomaly. Isotopic compositions of whole-rock show enriched attributes (87Sr/86Sr(i) = 0.7050–0.7071, ε Nd (t) = −7.56 to −3.45). Geochemical models indicate that these rocks did not result from melting of subducted slab, magma mixing, fractional crystallization or melts derived from delaminated lower crust reacted with mantle. Instead, high Mg diorites were generated through basaltic melts derived from asthenosphere mantle assimilating crustal components with crystallized phase of garnet-bearing amphibolite (AFC). Together with regional geological setting of the South Qinling Belt, we propose that the parental magma of these high Mg diorites was probably originated from upwelled asthenosphere mantle at the base of thinned crust after delamination of thickened crust during Late Triassic. The continental crust of the South Qinling Belt was thickened as a result of collision with the Yangtze Block during Middle Triassic. Then eclogitized lower crust collapsed and foundered into asthenosphere mantle due to partial melting of thickened crust during Late Triassic. In response, the upwelling and underplating of asthenosphere mantle took place at the base of continental crust. Moreover, upwelling of hot mantle may provide the heat source for migmatites and initiate the doming of Foping area. Highlights • High Mg diorites of the Wulong pluton are formed at 218-210 Ma. • High Mg diorites are generated through AFC process at lower crust. • Parental magma of diorites originated from asthenosphere mantle. • Upwelling of asthenosphere mantle promotes the doming of Foping area. [ABSTRACT FROM AUTHOR]

Highlights • Upper mantle anisotropy structure revealed beneath Bass Strait, SE Australia. • Large delay times (>2.5 s) in vicinity of recent intraplate volcanism. • Postulated Precambrian microcontinent appears to be defined by fast polarisation directions oriented approximately NE. • Overall complex anisotropy pattern that seldom correlates with magnetic anomalies and absolute plate motion. Abstract We investigate the structure of the upper mantle using teleseismic shear wave splitting measurements obtained at 32 broadband seismic stations located in Bass Strait and the surrounding region of southeast Australia. Our dataset includes ∼366 individual splitting measurements from SKS and SKKS phases. The pattern of seismic anisotropy from shear wave splitting analysis beneath the study area is complex and does not always correlate with magnetic lineaments or current N-S absolute plate motion. In the eastern Lachlan Fold Belt, fast shear waves are polarized parallel to the structural trend (∼N25E). Further south, fast shear wave polarization directions trend on average N25–75E from the Western Tasmania Terrane through Bass Strait to southern Victoria, which is consistent with the presence of an exotic Precambrian microcontinent in this region as previously postulated. Stations located on and around the Neogene-Quaternary Newer Volcanics Province in southern Victoria display sizeable delay times (∼2.7 s). These values are among the largest in the world and hence require either an unusually large intrinsic anisotropy frozen within the lithosphere, or a contribution from both the lithospheric and asthenospheric mantle. In the Eastern Tasmania Terrane, nearly all observed fast directions are approximately NW-SE. Although part of our data set strongly favours anisotropy originating from "fabric" frozen in the lithospheric mantle, a contribution from the asthenospheric flow related to the present day plate motion is also required to explain the observed splitting parameters. We suggest that deviation of asthenospheric mantle flow around lithospheric roots could be occurring, and so variations in anisotropy related to mantle flow may be expected. Alternatively, the pattern of fast polarisation orientations observed around Bass Strait may be consistent with radial mantle flow associated with a plume linked to the recently discovered Cosgrove volcanic track. However, it is difficult to characterise the relative contributions to the observed splitting from the lithospheric vs. asthenospheric upper mantle due to poor backazimuthal coverage of the data. [ABSTRACT FROM AUTHOR]

Abstract In the last 10 Ma, the northwest Barents Sea passive margin and Svalbard archipelago experienced tectonic reactivation including regional uplift, volcanism and fault movements. Previous studies have variously attributed this reactivation to the establishment of continuous seafloor spreading between the North Atlantic and Arctic oceans within the Fram Strait, mantle convection, or a lithospheric response to climate-driven processes such as fluvial and glacial erosion in a late Pliocene-Pleistocene time (∼1-3 Ma). The relative magnitudes, and the physical relationship between surface processes, plate boundaries and mantle flow, has remained unclear. In this work, I jointy interpret seismological models of the upper mantle, gravity anomalies, rates of glacial isostatic adjustment (GIA) and the spatiotemporal characteristics of earthquakes with the aim to i) constrain the thermal state and rheological structure of the upper mantle beneath Svalbard, ii) infer the magnitude of vertical motion in the late Cenozoic time related to various processes, iii) better understand the likely causes of the tectonic and volcanic activity. Most part of the northwest Barents Sea passive margin including Svalbard is underlain by a thin continental lithosphere (∼50–80 km) and a normal thickness of crust (30–35 km). The geophysical data indicate an average excess mantle temperature of ∼40–100°C beneath western Svalbard compared to the central Barents Sea. The peak of the mantle temperature anomaly coincides with the location of Quaternary basalt eruptions. The predicted long-wavelength (>420 km) non-isostatic uplift of ∼600-800 m correlates with the distribution of pre-glacial uplift previously inferred from geological data in the northwest Barents Sea. The gravity anomalies within an intermediate range of wavelengths (30–420 km) allows me to estimate topography changes due to erosion by ice streams to be ∼200–500 m. I conclude that a transient temperature anomaly in the asthenosphere northeast of Jan Mayen, combined with the northwest absolute plate motion of Eurasia, is the likely primary cause of the late Cenozoic tectonic reactivation in Svalbard. [ABSTRACT FROM AUTHOR]

We investigate the presence of the quasi-Love wave (qL) at 51 seismic stations of a temporary seismic network across the western Arabia-Eurasia collision zone. We quantify the intensity of the qL observations from the April 12, 2014 Solomon Islands earthquake by calculating the peak-to-peak amplitude ratios of the qL and Love waves, and compare them with predicted qL intensities from previous shear-wave splitting results. We determine the polarity, timing, and period-dependence of the qL observations within the period range of 50–100 s. Our analysis reveals that the qL observations at stations in the Zagros and Alborz mountain belts exhibit opposite characteristics. In contrast to the Alborz stations, the intensity of qL observations at the Zagros stations exhibits relatively negligible dependence on the period, while their receiver-scatterer distances are considerably period-dependent. We approximately locate the anisotropic gradients that generate the qL waves. Our results suggest that a lithospheric gap is responsible for the shallow and abrupt variation in the belt-parallel trend of fast-axis orientations in the westernmost part of the Zagros. Additionally, the period/depth dependence of the anisotropic gradients along the boundary between the central Zagros and central Iran provides insight into the variation in the downward dip of the Arabian lithosphere. The anisotropic gradient located to the north of the Doruneh fault in eastern Iran indicates its role as a major shear zone and lithospheric boundary. Finally, we observe that the spatial distribution of the anisotropic gradient in northeastern Iran matches the higher strain-rate areas in the Kopet Dagh Mountains, suggesting coupling between the lithospheric mantle and crust in that region. [ABSTRACT FROM AUTHOR]

Seismic P- and S-wave velocities of the lower lithosphere and underlying asthenosphere at the SW margin of the East European Craton in northern Poland were obtained with different seismic techniques: seismic refraction, P-residuals of the first arrivals from teleseismic earthquakes, P-wave receiver function, and inversion of the Rayleigh surface wave dispersion curves, the last two using data collected in the passive seismic experiment “13 BB star”. The uniform array consisted of 13 stations deployed in a 120 km in diameter area. Below the depth of 180-220 km a decrease of about 6% of the S-wave velocity is interpreted as a thermal gradient zone corresponding to a lithosphere-asthenosphere transition. The average mantle velocities down to a depth of 300 km beneath the array are relatively high, exceeding values for other Precambrian cratons by 0.1-0.2 km/s, and cannot be modeled by reasonable mantle peridotite compositions in the lithospheric part of the profile. We suggest that significant peridotite anisotropy could explain the misfit between measured and calculated seismic velocities in the lithosphere. [ABSTRACT FROM AUTHOR]

The Northern Appalachian Anomaly (NAA), a region of exceptionally low seismic velocities in the asthenosphere beneath southern New England and easternmost New York State, has been interpreted as a site of mantle upwelling. We synthesize a combination of new and previously published data that indicates the following: (1) The upwelling has eroded or delaminated the lithosphere in a localized region centered in southern Vermont that we call the "Green Mountains Anomaly." Forty‐second period Rayleigh wave phase velocities, which have peak sensitivity at lithospheric depths, are slow in this region, and S wave receiver functions are dominated by shallow (60‐km depth) mantle structures indicating reduced velocities. Thermal springs and sites with anomalously high concentrations of mantle‐derived helium‐3 are concentrated at the borders of the Green Mountains anomaly, perhaps due to stress concentrations produced by geologically recent, uncompensated delamination of the lower lithosphere. (2) S wave receiver functions indicate intense (>10%), shallow (60‐km depth) short wavelength structures along the southern and western edges of the NAA, indicating that the lithosphere there is being intensely altered, possibly by a combination of shearing and introduction of volatiles. And (3) Notwithstanding the localized thinning and alteration, the NAA lithosphere as a whole does not appear to have experienced pervasive heating, for the compressional wave quality factor of QP = 870 inferred from the decay rate of Po waves is very significantly above the QP ≈ 60 value previously reported for the NAA asthenosphere. Plain Language Summary: At depths between 100 and 300 km (60–180 miles), the Earth beneath southern New England and easternmost New York State (USA) is unusually hot, so much so that the rock is at or near its melting point. This region has been nicknamed the Northern Appalachian Anomaly, or NAA, for short. One previously published explanation is that convection currents in the Earth are bringing up hot material from deeper depths. We follow up on this idea by examining whether the upwelling is heating up and eroding the lithosphere, the roughly100‐km‐thick (60‐mile‐thick) layer of cool rock just below the Earth's surface, and causing pieces of it to fall off (delaminate) and sink. We find evidence for erosion and delamination only beneath a region in and around the Green Mountains (Vermont, USA). There, seismic waves are unusually slow and spring temperatures are unusually warm, both of which are signatures of heat being close to the Earth's surface. The lithosphere above the rest of the NAA is less affected by the upwelling, except near its southern and western edges, where some alteration may be beginning. Key Points: Slow Rayleigh wave speeds, thermal springs, and helium‐3 anomalies indicate that the lithosphere beneath the Green Mountains has thinnedStrong midlithospheric reflectivity in shear receiver functions indicate lithospheric alteration at the edges of asthenospheric upwellingNormal Po and So attenuation in the lithosphere of southern New England indicates that the alteration and thinning are very localized [ABSTRACT FROM AUTHOR]

Recent seismic observations, focused on mantle flow below the Pacific plate, indicate the presence of two shear layers in the Earth's asthenosphere. This is difficult to explain under the classic assumption of asthenosphere flow driven by plate shear from above. We present numerical mantle convection experiments that show how a power law rheology, together with dynamic pressure gradients, can generate an asthenosphere flow profile with a near constant velocity central region bounded above and below by concentrated shear layers (a configuration referred to as plug flow). The experiments show that as the power law dependence of asthenosphere viscosity is increased from 1 to 3, maximum asthenosphere velocities can surpass lithosphere velocity. The wavelength of mantle convection increases and asthenosphere flow transitions from a linear profile (Couette flow) to a plug flow configuration. Experiments in a 3D spherical domain also show a rotation of velocity vectors from the lithosphere to the asthenosphere, consistent with seismic observations. Global mantle flow remains of whole mantle convection type with plate and asthenosphere flow away from a mid-ocean ridge balanced by broader return flow in the lower mantle. Our results are in line with theoretical scalings that mapped the conditions under which asthenosphere flow can provide an added plate driving force as opposed to the more classic assumption that asthenosphere flow is associated with a plate resisting force. [ABSTRACT FROM AUTHOR]