Lithosphere-asthenosphere System Research Articles

A new 3D S-velocity model for the lithosphere-asthenosphere system of the Arabian Peninsula and the Iranian plateau

A new 3D S-velocity model for the lithosphere-asthenosphere system of the Arabian Peninsula and the Iranian plateau

Lateral and vertical variations of seismic anisotropy in the lithosphere–asthenosphere system underneath Central Europe from long-term splitting measurements

SUMMARY Backazimuthal variations in the shear wave splitting of core-refracted shear waves (SKS, SKKS and PKS phases, jointly referred to as XKS) at the Black Forest Observatory (BFO, Southwest Germany) indicate small-scale lateral and partly vertical variations of the seismic anisotropy. However, existing anisotropy studies and models for the nearby Upper Rhine Graben (URG) area in the northern Alpine foreland are mostly based on short-term recordings and by this suffer from a limited backazimuthal coverage and averaging over a wide or the whole backazimuth range. To identify and delimit laterally confined anisotropy regimes in this region, we carry out XKS splitting measurements at six neighbouring (semi-)permanent broad-band seismological recording stations (interstation distance 10–80 km). We manually analyse long-term (partly > 20 yr) recordings to achieve a sufficient backazimuthal coverage to resolve complex anisotropy. The splitting parameters (fast polarization direction $\phi $, delay time $\delta t$) are determined in a single- and multi-event analysis. We test structural anisotropy models with one layer with horizontal or tilted symmetry axis and with two layers with horizontal symmetry axes (transverse isotropy). To account for lateral variations around a single recording site, modelling is compared for the whole and for limited backazimuth ranges. Based on this, we provide a 3-D block model with spatial variation of anisotropic properties. Based on delay times > 0.3 s and missing discrepancies between SKS and SKKS phases, which do not support lower mantle anisotropy, the found anisotropy is placed in the lithosphere and asthenosphere. The spatial distribution as well as the lateral and backazimuthal variations of the splitting parameters confirm lateral and partly vertical variations in anisotropy. On the east side of the URG, we suggest two anisotropic layers in the Moldanubian Zone (south) and one anisotropic layer in the Saxothuringian Zone (north). In the Moldanubian Zone, a change of the fast polarization directions is observed between the east and the west side of the URG, indicating different textures. At the boundary between the two terranes, an inclined anisotropy is modelled which may be related with deformation during Variscan subduction. Regarding the observation of numerous null measurements and inconsistent splitting parameters, especially (southwest of BFO) in the southern URG, different hypothesis are tested: scattering of the seismic wavefield due to small-scale lateral heterogeneities, a vertical a-axis due to a vertical mantle flow related to the Kaiserstuhl Volcanic Complex, as well as a different preferred orientation of the olivine crystals (not A-type, but C-type) due to specific ambient conditions (high temperature, water content).

Deep Structure of the Baikal Rift Zone and Central Mongolia

The upper mantle and the transition zone of the Baikal rift zone (BRZ) are studied. The observations are analyzed using 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. 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 to a depth of 200 km with a lithospheric inclusion between low-velocity layers.

Viscoelastic earthquake cycle model for the Caribbean subduction zone in northwestern Colombia: Implications of coastal subsidence for seismic/tsunami hazards

Recent interplate coupling results from the inversion of GPS data in northwestern Colombia have shed light on the seismogenic and tsunami potential along the Caribbean coast. The identified locked region on the subduction interface could generate an Mw8.0 earthquake every 600 years, followed by a tsunami. While observed horizontal velocities have been reproduced successfully by the elastic coupling model, vertical velocities remain unexplained and differ, both in their signs and magnitudes, from the model prediction. To explain 3-dimensional velocities, particularly rapid coastal subsidence, we evaluate the viscoelastic response of the lithosphere-asthenosphere system to an earthquake cycle at the Caribbean subduction zone. We confirm that the role of viscous relaxation on interseismic deformation is critical when the recurrence interval is longer than the asthenosphere relaxation time. Moreover, fitting observed crustal motions requires a strong lithosphere (60–100 km) consistent with the depth of the lithosphere-asthenosphere boundary. Our results support the hypothesis of the Caribbean as a locus of seismic and tsunami hazards but do not resolve the vertical motion paradox at different time scales since the interseismic crustal motions recover completely the coseismic and postseismic displacements caused by the potential mainshock. Supplementary geological investigation is essential to validate our current interpretation and resolve the remaining gaps.

Subduction transforms azimuthal anisotropy in the Juan de Fuca plate

To clarify the internal structure of the oceanic lithosphere-asthenosphere system, here we determine a new 3-D model of azimuthal anisotropic shear-wave velocity (Vs) down to ∼200 km depth from the Juan de Fuca (JdF) mid-ocean ridge to the Cascadia subduction zone, by inverting newly measured teleseismic fundamental mode Rayleigh-wave phase and amplitude data at periods of 25–100 s. The JdF lithosphere is clearly imaged as a high-velocity anomaly from the ridge to the subduction zone. Distinct azimuthal anisotropies are revealed in the JdF lithosphere before and after its subduction. Obvious trench-normal fast-velocity directions (FVDs) exist in the JdF plate beneath the Pacific Ocean, whereas the subducting JdF slab beneath the Cascadia margin generally exhibits trench-parallel FVDs. We propose that the subduction-induced structure and mineral alignment in the JdF slab transform the fossil fabrics in the JdF plate formed at the mid-ocean ridge and altered during the plate motion and cooling.

Seismic attenuation tomography of Eastern Europe from ambient seismic noise analysis

SUMMARY The Eastern-Europe region (EER), is a complex geotectonic area that captures part of the Alpine-Himalayan Orogen, the subduction of multiple NeoTethys Branches and part of the East European Craton. It is one of the most exciting geological areas in Europe due to a diversity of tectonic processes acting within it: extensional basin evolution, oceanic subduction, post-collisional volcanism, as well as active crustal deformation associated with the push of the Adria plate or the pull of the actively detaching Vrancea slab. This makes EER an excellent natural laboratory to study the behaviour of the lithosphere–asthenosphere system in a heterogeneous tectonic setting. To investigate the lateral heterogeneity and physical properties of the crust in the EER, we use ambient seismic noise data recorded by the vertical components of broad-band stations that have been operational between 1999 and 2020 in Eastern Europe and surrounding regions. We used this significant amount of data and the latest processing techniques of the ambient seismic noise field based on the continuous wavelet transform to compute cross-correlations between various station pairs, turning every available seismic station into a virtual source. The coda of the interstation cross-correlograms were used to determine coda quality factors (Qc) of Rayleigh waves in four different period ranges (3.0–5.0, 5.0–10.0, 10.0–20.0 and 20.0–30.0 s) and to invert them in the 2-D space, constructing the highest resolution attenuation tomography of the region. Our results reveal high attenuation features throughout the northeast Pannonian region, the Bohemian Massif, the East Carpathians and the Moesian Platform. Nevertheless, our findings do not emphasize a close correlation between the depth of sedimentary basins and attenuation features identified at longer periods. In addition, Qc variations are larger at short periods, indicating higher heterogeneity in the uppermost crust of Eastern Europe. Our findings demonstrate the higher efficiency of noise correlation approaches relative to earthquake data analyses investigating Qc at low frequencies.

Seismic Architecture of the Lithosphere-Asthenosphere System in the Western United States from a Joint Inversion of Body- and Surface-wave Observations: Distribution of Partial Melt in the Upper Mantle

Quantitative evaluation of the physical state of the upper mantle, including mapping temperature variations and the possible distribution of partial melt, requires accurately characterizing absolute seismic velocities near seismic discontinuities. We present a joint inversion for absolute but discontinuous models of shear-wave velocity (Vs) using 4 types of data: Rayleigh wave phases velocities, P-to-s receiver functions, S-to-p receiver functions, and Pn velocities. Application to the western United States clarifies where upper mantle discontinuities are lithosphere-asthenosphere boundaries (LAB) or mid-lithospheric discontinuities (MLD). Values of Vs below 4 km/s are observed below the LAB over much of the Basin and Range and below the edges of the Colorado Plateau; the current generation of experimentally based models for shear-wave velocity in the mantle cannot explain such low Vs without invoking the presence of melt. Large gradients of Vs below the LAB also require a gradient in melt-fraction. Nearly all volcanism of Pleistocene or younger age occurred where we infer the presence of melt below the LAB. Only the ultrapotassic Leucite Hills in the Wyoming Craton lie above an MLD. Here, the seismic constraints allow for the melting of phlogopite below the MLD.

Lithosphere structure beneath the eastern Dharwar craton kimberlite field, India, inferred from joint inversion of surface wave dispersion and receiver function data

Cratons are the oldest parts of continents with cold and thick, high velocity, depleted lithosphere, which remained stable for billions of years. However, many of them are susceptible to deformation and erosion under various geological settings, such as near subduction zones and above mantle plumes. The role of mantle plumes in eroding the overlying lithosphere remains elusive, particularly for the fast-moving cratons. In this study, we examine the plume-lithosphere interaction in the Dharwar Craton located in South India by comparing the lithospheric property derived from the diamondiferous kimberlite xenoliths with its present-day shear velocity structure computed from collocated seismological data. The lithosphere-asthenosphere system beneath a 200 km long profile is explored at 13 locations through joint inversion of receiver function and surface wave phase dispersion data. The velocity model suggests 165 ± 15 km thick present-day lithosphere, marginally lower than 180 ± 20 km derived from the kimberlite xenoliths which erupted around 1100 Myr. The velocity increases from the Moho to ∼100 km depth, possibly due to the spinel-garnet peridotite transformation. The cold cratonic lithosphere of Eastern Dharwar craton (EDC) is manifested in very high shear velocities, up to >4.7 km/s indicating harzburgite-dunite composition, correlating well with the composition inferred from analysis of xenolith data. Our result suggests no significant thermal alteration to the lithosphere of the eastern Dharwar Craton despite being affected by three mantle plumes in the last 90 Myr.

Active tectonics of the Circum-Pannonian region in the light of updated GNSS network data

The Pannonian basin is an extensional back-arc basin that has undergone neotectonic inversion and is currently shortening. The understanding and quantification of present-day deformation processes during this inversion are still incomplete. To this end, we investigate the active deformation of the Circum-Pannonian region via the interpolation of GNSS-derived velocity field and the derivation of the strain rate fields. For the interpolation of the velocity field, we use ordinary kriging, a strochastic interpolation method. Our results show that estimating a strain rate field that is virtually free of short-wavelength noise requires the scaling of the velocity uncertainties, i.e. assuming a minimum standard deviation of 1 mm/yr in our case. The deformation of the Circum-Pannonian region is defined by the 2–3 mm/yr, NNE-directed motion of the Dinarides, and by the 0.5–1.5 mm/yr, WSW to SSW directed motion of the eastern areas (European foreland, East Carpathians, South Carpathians, Transylvanian basin). These opposite-sense motions define a large-scale, on average NE-SW shortening and transpression-type deformation in the Dinarides as well as in the Pannonian basin, while the East and South Carpathians undergo regional N–S extension. Neotectonic structures generally show good agreement with the strain rate field, for example in the Dinarides, Eastern Alps, or in the western Pannonian basin. However, the presence of fault-parallel shortening or biaxial shortening along sinistral neotectonic structures in the central and eastern Pannonian basin show some discrepancy between current geodetic and observed neotectonic deformation. The vertical velocity field shows dominantly 100 and 1000 km wavelength signals, the former is probably related to the response of the Pannonian lithosphere-asthenosphere system to neotectonic basin inversion, while latter can possibly be explained by far-field subsidence patterns induced by the mantle response to melting of the Fennoscandian ice sheet during the current interglacial period.

Inference of velocity structures of oceanic crust and upper mantle from surface waveform fitting

SUMMARYInversion for seismological structures of the oceanic lithosphere–asthenosphere system is important to understand the mechanisms of plate tectonics. Previous models of the oceanic upper mantle have been primarily obtained via global tomography using surface waveforms. However, besides scarcity of waveform data in the oceanic regions, difficulties in fitting phases for shorter-period components in the previous global tomography have yielded resultant models that possess poor resolutions above $\sim \, 50$ km depth. Recent developments of broad-band ocean-bottom seismometer (BBOBS) arrays provide larger amount of seismic data with epicentral distances of <20°. In this study, we develop an appropriate method to fully utilize the information contained in the shorter-period components of BBOBS arrays. We first fit the envelopes without phase information to analyse the shorter-period components (8–60 s) which are generally unavailable in the conventional phase fitting. We then use the resultant model as our initial model for waveform inversion of the longer periods (12.5–200 s) to fit the phase, which allows us to infer a continuous structure model from the crust to the asthenosphere. We demonstrate the validity of this combined envelope-fitting and waveform inversion method by analysing the waveform data from a BBOBS array that was deployed in the Northwestern Pacific and has recorded events in the vicinity of the Japan Trench to obtain the average velocity structure between the event and station arrays. We independently resolve the crustal compressional and shear wave velocities, and thickness by analysing the envelopes, which minimizes biases in the subsequent waveform inversion. We also find that the waveform inversion improves the resolution in the asthenosphere. Our results suggest that further extension of this method should improve our knowledge of the oceanic lithosphere–asthenosphere system.

Possible Seismogenic-Trigger Mechanism of Activation of Glacier Destruction, Methane Emission and Climate Warming in Antarctica

A seismogenic-trigger mechanism is proposed for the rapid activation of the destruction of cover and shelf glaciers in West Antarctica at the end of the 20th and the beginning of the 21st centuries, accompanied by the release of methane from the underlying hydrate-bearing sedimentary rocks and consequent rapid climate warming. This mechanism is associated with the action of deformation waves in the lithosphere-asthenosphere system, resulting from the strongest earthquakes occurring in the subduction zones surrounding Antarctica – Chile and Kermadec-Macquarie. Disturbances in the lithosphere are transmitted over long distances of the order of 3000 km, and the additional stresses associated with them, which come to Antarctica several decades after earthquakes, lead to a decrease in the adhesion of glaciers to underlying rocks, accelerated sliding of glaciers and the development of faults in them. This process, in turn, results in a reduction of pressure on the underlying sedimentary layers containing gas hydrates, which lead to methane emission and climate warming. The considered hypothesis leads to the conclusion that in the coming decades the processes of destruction of glaciers and climate warming in Antarctica will speed-up due to an unprecedented increase in the number of strongest earthquakes in the subduction zones of the South Pacific Ocean in the late 20th and early 21st centuries.

Trigger Mechanisms of Gas Hydrate Decomposition, Methane Emissions, and Glacier Breakups in Polar Regions as a Result of Tectonic Wave Deformation

Trigger mechanisms are proposed for gas hydrate decomposition, methane emissions, and glacier collapse in polar regions. These mechanisms are due to tectonic deformation waves in the lithosphere–asthenosphere system, caused by large earthquakes in subduction zones, located near the polar regions: the Aleutian arc, closest to the Arctic, and the Antarctica–Chilean and Tonga–Kermadec–Macquarie subduction zones. Disturbances of the lithosphere are transmitted over long distances (of the order of 2000–3000 km and more) at a speed of about 100 km/year. Additional stresses associated with them come to the Arctic and Antarctica several decades after the occurrence of seismic events. On the Arctic shelf, additional stresses destroy the microstructure of metastable gas hydrates located in frozen rocks at shallow depths, releasing the methane trapped in them and leading to filtration and emissions. In West Antarctica, these wave stresses lead to decreases in the adhesions of the covered glaciers with underlying bedrock, sharp accelerations of their sliding into the sea, and fault occurrences, reducing pressure on the underlying rocks containing gas hydrates, which leads to their decomposition and methane emissions.

Interaction among neighbouring rectangular finite strike slip faults in a linear viscoelastic half space representing lithosphere-asthenosphere system

There are seismically active regions consisting of fault system with a number of neighbouring earthquake faults. A movement across any one of them may affect the nature of stress accumulation near the others. Mathematical models may be developed to study these interactions and the pattern of interseismic stresses during the aseismic period in between two consecutive seismic events. In this paper, the lithosphere-asthenosphere system is being represented by a linear viscoelastic half space. The material of the half space is expected to possess the properties of both Maxwell and Kelvin type materials. It is assumed that the system is under a steady shear stress generated by some tectonic phenomena. For obtaining the solution for displacement, strain and stresses from the resulting boundary value problem, Integral transform, Green’s function techenique and correspondence principle have been used. Appropriate estimates of the model parameters were used in carrying out the numerical computations for investigating the nature of interactions among the faults.

Melting conditions and mantle source composition from probabilistic joint inversion of major and rare earth element concentrations

The chemical composition of erupted basalts provides a record of the thermo-chemical state of their source region and the melting conditions that lead to their formation. Here we present the first probabilistic inversion framework capable of inverting both trace and major element data of mafic volcanic rocks to constrain mantle potential temperature, depth of melting, and major and trace element source composition. The inversion strategy is based on the combination of (i) a two-phase multi-component reactive transport model, (ii) a thermodynamic solver for the evolution of major elements and mineral/liquid phases, (iii) a disequilibrium model of trace element partitioning and (iv) an adaptive Markov chain Monte Carlo algorithm. The mechanical and chemical evolution of melt and solid residue are therefore modelled in an internally- and thermodynamically-consistent manner.We illustrate the inversion approach and its sensitivity to relevant model parameters with a series of numerical experiments with increasing level of complexity. We show the benefits and limitations of using major and trace element compositions separately before demonstrating the advantages of a joint inversion. We show that such joint inversion has great sensitivity to mantle temperature, pressure range of melting and composition of the source, even when realistic uncertainties are assigned to both data and predictions. We further test the reliability of the approach on a real dataset from a well-characterised region: the Rio Grande Rift in western North America. We obtain estimates of mantle potential temperature (∼1340 °C), lithospheric thickness (∼60 km) and source composition that are in excellent agreement with numerous independent geochemical and geophysical estimates. In particular, this study suggests that the basalts in this region originated from a moderately hot upwelling and include the contribution from a slightly depleted source that experienced a small degree of melt or fluid metasomatism. This component is likely associated with partial melting of the lower portions of the lithosphere. The flexibility of both the melting model and inversion scheme developed here makes the approach widely applicable to assessing the thermo-chemical structure and evolution of the lithosphere-asthenosphere system and paves the way for truly joint geochemical-geophysical inversions.

KineDyn: Thermomechanical forward method for validation of seismic interpretations and investigation of dynamics of rifts and rifted margins

Physical processes interact during rifting to produce a range of crustal and sedimentary architectures and subsidence histories. Methodologies to analyze these processes combine techniques such as cross-section restoration, backstripping and numerical simulations. Despite the valuable outcomes of each method, intrinsic limitations related to particular interactions between kinematics and thermo-mechanics leave open questions on how seismic interpretation is consistent in terms of thermomechanical aspects of lithospheric extension. In this work, we present a new modelling technique, KineDyn, to simulate basement architecture, heat-flow, subsidence and sedimentation patterns along a given basin-scale seismic reflection profile in 2D. KineDyn is the fusion of dynamic thermomechanical approach and kinematic forward modelling of faulting and excludes drawbacks of conventional methods while preserving their valuable outcomes. In our approach, faults are initially controlled by prescribed initial locations, slips and timings, while the rest of the model is resolved in a fully dynamic mode, with non-linear visco-elasto-plastic rheology. KineDyn allows dynamic simulation of the response of the lower crust and mantle to different patterns of faulting in time and space and direct comparison to seismic observations. Thus, it gives, in effect, the same results as existing section restoration techniques (i.e. the potential history of faulting) and forward modelling techniques (i.e. the likely history of sedimentation, thinning, heat flow and subsidence), while simultaneously taking into account non-linear interactions between processes occurring during rifting. In addition, comparison of the model outcomes to lithospheric-scale seismic observations gives insight into the relationship between shallow crustal structures and deeper geodynamic processes in the lithosphere-asthenosphere system, which shape rifts.

Crustal seismic structure and anisotropy of Madagascar and southeastern Africa using receiver function harmonics: interplay of inherited local heterogeneities and current regional stress

SummaryThis study investigates the seismic structure and anisotropy in the crust beneath Madagascar and southeastern Africa, using receiver functions. The understanding of seismic anisotropy is essential for imaging past and present deformation in the lithosphere–asthenosphere system. In the upper mantle, seismic anisotropy mainly results from the orientation of olivine, which deforms under tectonic (fossil anisotropy) or flow processes (in the asthenosphere). In the crust, the crystallographic alignment of amphiboles, feldspars (plagioclase) or micas or the alignment of heterogeneities such as fractures, add to a complex geometry, which results in challenges to understanding the Earth's shallow structure. The decomposition of receiver functions into backazimuth harmonics allows to characterize orientations of lithospheric structure responsible for azimuthally varying seismic signals, such as a dipping isotropic velocity contrasts or layers of azimuthal seismic anisotropy. By analysing receiver function harmonics from records of 48 permanent or temporary stations this study reveals significant azimuthally varying signals within the upper crust of Madagascar and southeastern Africa. At 30 stations crustal anisotropy dominates the harmonics while the signature of a dipping isotropic contrast is dominant at the remaining 18 stations. However, all stations’ backazimuth harmonics show complex signals involving both dipping isotropic and shallow anisotropic contrasts or more than one source of anisotropy at shallow depth. Our calculated orientations for the crust are therefore interpreted as reflecting either the average or the interplay of several sources of azimuthally varying signals depending of their strength. However, comparing information between stations allows us to draw the same conclusions regionally: in both southern Africa and Madagascar our measurements reflect the interplay between local, inherited structural heterogeneities and crustal seismic anisotropy generated by the current extensional stress field imposed by the southward propagation of the East-African Rift System. A final comparison of our crustal orientations with SKS orientations attributed to mantle deformation further probes the interplay of crustal and mantle anisotropy on SKS measurements.

Creeping effect across a buried, inclined, finite strike-slip fault in an elastic-layer overlying an elastic half-space

The process of stress-strain accumulation near earthquake faults during the aseismic period has become a subject of research during the last few decades. It is noted that seismic waves, generated by an earthquake, result in a considerable disturbance in a seismic region causing a movement of the free surface. Such ground movements are not observed during the aseismic period. But a slow quasi-static aseismic surface displacement of the order of few cms. per year or less can be observed during the aseismic period which indicates a slow sub-surface process of stress-strain accumulation. Keeping this in view we here consider an aseismically creeping, buried, finite strike-slip fault inclined to the vertical at an arbitrary angle. The fault is situated in an elastic layer over an elastic half-space representing the lithosphere-asthenosphere system. An analytical study for displacement, stress and strain due to creeping effect has been carried out for a buried, finite, inclined fault. The solutions for displacement, stress and strain are then found before the onset of fault slip and then superpose the effect of fault slip using Laplace transform and suitable mathematical techniques of Green’s function. The analytical results and the graphical presentations show that the inclination of the fault and the velocities of the fault movement has noticeable effect on displacements, stresses and strains.

Seismic evidence for a plume-modified oceanic lithosphere–asthenosphere system beneath Cape Verde

SUMMARYWe determine a new 3-D shear wave velocity (Vs) model down to 400 km depth beneath the Cape Verde hotspot that is far from plate boundaries. This Vs model is obtained by using a new method of jointly inverting P- and S-wave receiver functions, Rayleigh-wave phase-velocity data and S-wave arrival times of teleseismic events. Two Vs discontinuities at ∼15 and ∼60 km depths are revealed beneath volcanic islands, which are interpreted as the Moho discontinuity and the Gutenberg (G) discontinuity. Between the north and south islands, obvious high-Vs anomalies exist in the uppermost mantle down to a depth of ∼100–150 km beneath the Atlantic Ocean, whereas obvious low-Vs anomalies exist in the uppermost mantle beneath the volcanic islands including the active Fogo volcano. These low-Vs anomalies merge into a significant column-like low-Vs zone at depths of ∼150–400 km beneath the Cape Verde swell. We propose that these features in the upper mantle reflect a plume-modified oceanic lithosphere–asthenosphere system beneath the Cape Verde hotspot.

3D S-velocity model of the lithosphere-asthenosphere system beneath the Bering Sea

The 3D S-velocity structure for the Bering Sea region is determined from dispersion analysis (Rayleigh waves), from which the most conspicuous features of the crust and upper mantle (from 0 to 400 km depth) will be revealed. In the depth range from 0 to 5 km, this model shows the distribution of the sedimentary basins present in the study area, in terms of S-velocity. For the Bering Shelf, the S-velocity values decrease southward, indicating the presence of deep sedimentary basins in the southern Bering Shelf. In the depth range from 5 to 30 km, the S-velocity model shows clearly the division of the Bering Sea basin in three sub-basins: the Aleutian Basin, the Bowers Basin and the Komandorsky Basin, produced by the Bowers Ridge and the Shirshov Ridge. In this model, the low S-velocity pattern determined for the Bering Shelf confirms that its crust is similar to a continental-type crust, no an oceanic crust. The Moho map determined in the present study, from the 3D S-velocity model, is the first Moho map calculated for the Bering Sea region. In this map, the crust beneath the Bering Shelf shows thicker thickness than a typical oceanic crust, from which is concluded that this crust must be considered as a transitional crustal structure. In the depth range from 30 to 100 km, the Aleutian Basin shows a pattern of high S-velocity that can be correlated with the origin of this basin, because this basin is considered as formed by the entrapment of a piece of Pacific plate. In the depth range from 45 to 60 km, the low S-velocity pattern shown for the eastern Aleutian arc is associated to the active arc volcanoes present in this region. From the S-velocity model, the asthenosphere beneath the study area is firstly determined from ~ 100 to ~ 180 km depth. For the depth range from 80 to 400 km, the high S-velocity pattern determined for the eastern Aleutian arc, allows the modeling (imaging) of the subducting Pacific slab, in terms of S-velocity. This pattern of high S-velocity is not visible beneath the western Aleutian arc, because the Pacific plate is not subducting beneath the Bering Sea in the western Aleutian arc.

A geophysical perspective on the lithosphere–asthenosphere system from Periadriatic to the Himalayan areas: the contribution of gravimetry

The structure of the lithosphere–asthenosphere system as imaged from the complementary inversion of seismological and gravimetric data can provide significant clues to understand the processes of orogeny and cratonization and to better formulate the theory of plate tectonics within a sound geodynamic context. In this paper, we review some studies sharing a common synergic methodology that uses the geometry of cellular shear-wave velocity models of the crust and uppermost mantle to constrain a 3D density model obtained by means of linear inversion of gravimetric data. These studies, which were applied to selected areas along an ideal stream running from the Periadriatic region to the Himalayas, despite having different resolution, show some basic common features in the density distribution of the upper mantle whose unveiling may be precluded to gravity models achieved by assuming velocity–density relationships. In fact, one of the main conclusions of the discussed studies is that lithospheric slabs under the investigated orogenic belts, as identified by seismic wave velocity, are, as a rule, not marked by high density. This fact casts serious doubts about the effectiveness of slab-pull and confirms one of the key issues of polarized plate tectonics, mostly fuelled by tidal drag.