SUMMARYWe present a seismic ambient noise tomography of the Cape Verde archipelago, located in the Atlantic Ocean, approximately 600 km west of Senegal. We used 38 seismic broad-band stations that continuously recorded for 10 months, in order to construct the first 3-D model of Sv-wave velocities for the crust and uppermost mantle beneath the Cape Verde region. We started by computing phase cross-correlations for vertical component recordings using all possible inter-island station pairs. Next, a time–frequency phase-weighted stack was applied to obtain robust Rayleigh-wave group-velocity dispersion curves in the period band between 10 and 24 s. Group-velocity maps at different periods are obtained by inverting the dispersion curves. We then inverted the group-velocity maps to obtain the 3-D shear wave velocity structure of the crust and uppermost mantle beneath Cape Verde. The final 3-D model extends from 8 km down to 23 km and has a lateral resolution of about 50 km. The crust in the southwestern sector, encompassing Fogo, presents lower S-wave velocities that may be caused by the presence of melt pockets and/or hydrothermal fluids circulation. The uppermost mantle beneath the northwestern sector is characterized by higher S-wave velocities in agreement with previous results obtained from Ps and Sp receiver functions. Those high-velocity anomalies can reflect non-altered crust or remnants of magma chambers or solidified basaltic intrusions, which fed the volcanism in these islands. Our maps revealed the presence of crustal underplating across the entire archipelago, yet stronger beneath the groups Santo Antão—São Vicente—São Nicolau and Fogo—Santiago—Maio.

Volcanic hotspot islands are thought to be surface manifestations of mantle plumes that rise from the core–mantle boundary. When mantle plumes approach the surface, their mostly vertical rise must be deflected into near-horizontal flow beneath tectonic plates. This creates an opportunity to constrain their dynamics and their interactions with lithospheric plates and mid-ocean ridges. Seafloor observations have been used to propose that a focused flow in the asthenosphere transports plume heat to the nearest mid-ocean ridge, where it efficiently dissipates through formation of lithosphere. Here we present imaging results from a seismological survey of a proposed plume-to-ridge flow channel between the Réunion hotspot and the Central Indian Ridge. Rayleigh-wave tomography and shear-wave splitting confirm the presence of a channelized flow of shallow asthenosphere, eastward from the hotspot to the spreading ridge. At a larger scale, a deeper reservoir of hot asthenosphere fills vast tracts of the Indian Ocean basin east and north of Réunion Island. Its flows, decoupled from overlying lithospheres, are also directed towards the Central Indian Ridge but extend well beyond, tapped but not significantly depleted by the spreading ridge. Based on seismic and geochemical observations, we suggest that this hidden heat reservoir is generated and driven by the mantle plume, which buffers more heat near the surface than expected. A large, deep reservoir of asthenosphere flows eastward from the Réunion hotspot and interacts with the Indian spreading ridge, suggests seismological imaging.

This is IPGP contribution number 3700. Numerical computations were performed on the S-CAPAD platform, IPGP, France. M.S. acknowledges the Spanish MISTERIOS project CGL2013-48601-C2-1-R.

We present a high-resolution 3-D lithospheric model of the Indian plate region down to 300 km depth, obtained by inverting a new massive database of surface wave observations, using classical tomographic methods. Data are collected from more than 550 seismic broadband stations spanning the Indian subcontinent and surrounding regions. The Rayleigh wave dispersion measurements along similar to 14,000 paths are made in a broad frequency range (16-250 s). Our regionalized surface wave (group and phase) dispersion data are inverted at depth in two steps: first an isotropic inversion and next an anisotropic inversion of the phase velocity including the SV wave velocity and azimuthal anisotropy, based on the perturbation theory. We are able to recover most of the known geological structures in the region, such as the slow velocities associated with the thick crust in the Himalaya and Tibetan plateau and the fast velocities associated with the Indian Precambrian shield. Our estimates of the depth to the Lithosphere-Asthenosphere boundary (LAB) derived from seismic velocity V-sv reductions at depth reveal large variations (120-250 km) beneath the different cratonic blocks. The lithospheric thickness is similar to 120 km in the eastern Dharwar, similar to 160 km in the western Dharwar, similar to 140-200 km in Bastar, and similar to 160-200 km in the Singhbhum Craton. The thickest (200-250 km) cratonic roots are present beneath central India. A low velocity layer associated with the midlithospheric discontinuity is present when the root of the lithosphere is deep.

The P- and S -wave receiver functions and dispersion curves of the fundamental Rayleigh wave are used to study the lithosphere within the Central Anatolian Plateau. The results for eight broadband seismic stations are presented. It is established that within the plateau, the crust with a thickness of about 35 km is underlain by the mantle lid with its bottom at a depth of about 60 km. The velocities of longitudinal ( Vp ) and shear ( Vs ) waves in this layer are at most 7.6 and 4.5 km/s, respectively, and the Vp / Vs ratio is close to 1.7 (i.e., by 6% lower than in the standard IASP91 and PREM models). Such a low velocity ratio is characteristic of rocks having high orthopyroxene content. Beneath the high-velocity mantle lid, the S -wave velocity decreases to 4.0–4.2 km/s and the Vp / Vs ratio is close to its standard value (1.8). At most stations, the P -wave receiver functions do not contain seismic phase P 410 s , which is formed at the global seismic boundary at a depth of 410 km. The seismic boundary at a depth of 410 km is related to the olivine-spinel phase transformation, and its absence can indicate the anomalously low olivine content and high basalt content. This anomaly is probably associated with the subduction of a large amount of oceanic crust during the closure of the Tethys. The results of the study overall indicate the high informativity of the used method.

We present exact solutions of the Earth-flattening problem, properties of these solutions, and their expansions into power series in the reciprocal of the Earth's radius. Although the exact Earth-flattening transformation for SH vibrations has been known for more than a quarter of a century, all attempts to find its analogue for P-SV vibrations have not been successful. Various approximation methods were used instead. We considered inhomogeneous media and succeeded in deriving the exact flattening transformation based on the following result. Equations of linear elastodynamics for plane, cylindrically, and spherically layered media are transformed to a certain matrix Sturm-Liouville form. In all these cases the part of the differential operator that does not depend on wavenumber is represented as the composition of two first-order differential operators. By virtue of this representation, the respective spherical operator reduces to a plane one through a matrix transformation. This change offers the possibility of computing Rayleigh-like vibrations of a cylindrically or spherically symmetric body by methods developed for plane layered media.

P and S receiver functions (PRFs and SRFs, respectively) for 21 broad-band seismograph stations of the India Meteorological Department (IMD) illuminate lithosphere and the underlying mantle of some previously poorly sampled regions of the Indian sub-continent. Our analysis demonstrates that the Archean and Early Proterozoic lithospheric keel of the Indian shield has been reworked by younger processes. We find very low S-wave velocities in the uppermost mantle (from 4.0 to 4.3 km/s) to the north of the Deccan Volcanic Province (Kutch region and Aravalli Craton) (1), in the south (Southern Granulite Terrain and Sri Lanka) (2) and in the north-east (Gangetic Plane, Bengal Basin and Singhbuhm Craton) (3). The anomalies 1 and 2 may extend into the transition zone. Early arrivals of the S410p seismic phase are indicative of anomalously high Vp/Vs ratio (~ 1.9) in the upper mantle of the low-velocity regions, whereas late arrivals in the western Himalaya, Ladakh and western Tibet are consistent with the previously found indications of anomalously low Vp/Vs ratio. A transition from the high-S-velocity mantle lid to a layer of slightly lower velocity is seen in part of the data but a straightforward interpretation of this transition as the lithosphere–asthenosphere boundary is problematic. A mafic S velocity in the upper crust and a pronounced low-S-velocity layer in the lower crust beneath the eruptive center is practically the only specific feature in the lithosphere that may be linked to the Deccan Traps. A separation in depths between the 410-km and 660-km discontinuities varies laterally in a range from 240 to 270 km. The largest uplift of the 410-km discontinuity (up to 390 km) is observed beneath the foothills of the Himalaya where it is caused by cooling of the transition zone by the ongoing continental collision.

We suggest a theory of linear elastodynamics for the cases of isotropic continuously layered media of three kinds: plane, spherically, and cylindrically layered. The known transformations of the general solution lead to its decomposition into axially symmetrical solutions. On separation of variables, we obtain a second-order two-dimensional matrix operator for P-SV vibrations and a scalar operator for SH vibrations. The two-dimensional operator reduces to a matrix Sturm-Liouville form which is a composition of explicitly represented first-order operators in the case where the spectral parameter vanishes. This is the result of the theory. By using this result, we find canonical forms of the operator, which follows from various matrix transformations. We show that, given a fixed frequency and a plane, spherically, or cylindrically layered medium, the local interpretation problem for a given operator is solvable. This result is necessary for solving the inverse problem of seismology; besides, it provides for an exact solution of the Earth-flattening problem for Rayleigh waves.

We examine a boundary value problem for Rayleigh, or P-SV, vibrations occurring in elastic layered media of four types: flat layered half-space, spherically layered sphere, cylindrically layered circular cylinder, and flat centrally symmetric disk. In the previous research the authors, together with their colleagues, represented the equations governing the vibrations in the matrix Sturm-Liouville form with a nonsymmetric potential. Boundary conditions were prescribed at the surface of the body; it was also assumed that displacements approached zero at infinity for a half-space and were limited at the axis of a cylinder, at the center of a sphere, or at the center of a disk. In this paper we represent the boundary condition at the free surface as a linear combination of the vector solution and its first derivative. We prove that there exists a relationship between the matrix coefficient of this linear combination and the potential entering the Sturm-Liouville equation. This relationship implies that the determinant of the difference between two certain matrix functions remains constant at any argument value. The first function is the matrix impedance of the solution. The second function is such that the sum of its derivative and its square is the potential entering the Sturm-Liouville equation. This difference is more smooth than the impedance and the matrix potential. We prove that its elements are found from Volterra's integral equation of the second kind with a kernel depending on the matrix potential. Equations governing vibrations in bodies of different symmetry have a common structure; hence they can be treated as cases of the general matrix boundary value problem.

We investigate the crust, upper mantle and mantle transition zone of the Cape Verde hotspot by using seismic P and S receiver functions from several tens of local seismograph stations. We find a strong discontinuity at a depth of similar to 10 km underlain by a similar to 15-km thick layer with a high (similar to 1.9) Vp/Vs velocity ratio. We interpret this discontinuity and the underlying layer as the fossil Moho, inherited from the pre-hotspot era, and the plume-related magmatic underplate. Our uppermost-mantle models are very different from those previously obtained for this region: our S velocity is much lower and there are no indications of low densities. Contrary to previously published arguments for the standard transition zone thickness our data indicate that this thickness under the Cape Verde islands is up to similar to 30 km less than in the ambient mantle. This reduction is a combined effect of a depression of the 410-km discontinuity and an uplift of the 660-km discontinuity. The uplift is in contrast to laboratory data and some seismic data on a negligible dependence of depth of the 660-km discontinuity on temperature in hotspots. A large negative pressure-temperature slope which is suggested by our data implies that the 660-km discontinuity may resist passage of the plume.Our data reveal beneath the islands a reduction of S velocity of a few percent between 470-km and 510-km depths. The low velocity layer in the upper transition zone under the Cape Verde archipelago is very similar to that previously found under the Azores and a few other hotspots. In the literature there are reports on a regional 520-km discontinuity, the impedance of which is too large to be explained by the known phase transitions. Our observations suggest that the 520-km discontinuity may present the base of the low-velocity layer in the transition zone. (C) 2011 Elsevier B.V. All rights reserved.

P receiver functions from 23 stations of the SASE experiment in southern Africa are inverted simultaneously with SKS waveforms for azimuthal anisotropy in the upper mantle. Our analysis resolves the long‐standing issue of depth dependence and origins of anisotropy beneath southern Africa. In the uppermost mantle we observe anisotropy with a nearly E‐W fast direction, parallel to the trend of the Limpopo belt. This anisotropy may be frozen since the Archean. At a depth of 160 km the fast direction of anisotropy changes to 40° and becomes close to the recent plate motion direction. This transition is nearly coincident in depth with activation of dominant glide systems in olivine and with a pronounced change in other properties of the upper mantle. Another large change in the fast direction of anisotropy corresponds to the previously found low‐S‐velocity layer atop the 410‐km discontinuity.

P and S receiver functions from seismograph stations in the Indian shield, Western Himalaya, Ladakh and Tibet are processed with a method which provides estimates of the P and S velocities and their ratio as a function of depth. The time difference between the P660s and P410s phases in the north of the Indian shield and the Lesser Himalaya is 1.0–1.5s larger than the normal 24s. This is an effect of a low temperature with implication that the consumed material of the Indian shield has reached the transition zone. The waveforms of the P410s and S410p phases at some stations in the Indian shield are indicative of a thin (a few tens of kilometers) low S velocity layer atop the 410-km discontinuity, which is usually related to mantle upwelling. The mantle S velocity under the Indian shield at depths less than 180km is 4.4–4.5km/s, much lower than the 4.7km/s, typical for Precambrian shields. We explain this low S velocity mainly by a recent (Tertiary?) metasomatic alteration of the high-velocity mantle keel. Beneath the western Himalaya, Ladakh and western Tibet (but not eastern Tibet) the S velocity in the mantle at depths less than 100–150km is around 4.7km/s, Vp/Vs is anomalously low, and we argue that this high-velocity layer is a remnant of the mantle lithosphere of the northern Greater India. At most locations in the Indian shield high S velocities (3.5km/s and more) are dominant in the middle and lower crusts, and the elevated S velocity is accompanied by an increased Vp/Vs ratio (1.8–2.1 versus the standard 1.73). In the foothills of the Himalaya, the crust is 50–55km thick and consists almost entirely of a high-S-velocity (3.7km/s and more) rock with the increased Vp/Vs ratio in the middle and the standard Vp/Vs ratio in the lower crust. This observation suggests that the upper crust of the Indian plate is scraped off in the collision zone, whereas the high-velocity lower crust is subducted jointly with the mantle lithosphere. The high velocities are responsible for the P-wave teleseismic travel time anomaly of ~1s relative to Ladakh. Under the Himalaya the Vp/Vs ratio in the crust is normal, which suggests a change in composition relative to the crust of the Indian shield. Under Ladakh and Tibet the anomalously high Vp/Vs ratio in the crust is observed again. Beneath Tibet our analysis reveals a low-velocity crustal zone of partial melt between the 20-km and 45-km depths. Previously, the 45-km discontinuity was interpreted as the effect of eclogitization.

The analysis of rock anisotropy revealed by seismic waves provides fundamental constraints on stress‐strain field in the lithosphere and asthenosphere. Nevertheless, the anisotropic models resolved for the crust and the upper mantle using seismic waves sometimes show substantial discrepancies depending on the type of data analyzed. In particular, at several permanent stations located in Africa, previous studies revealed that the observations of SKS splitting are accounted for by models with a single and homogeneous anisotropic layer whereas 3‐D tomographic models derived from surface waves exhibit clear anisotropic stratification. Here we tackle the issue of depth‐dependent anisotropy by performing joint inversion of receiver functions (RF) and SKS waveforms at four permanent broadband stations along the East African Rift System (EARS) and also on the Congo Craton. For three out of the four stations studied, stratified models allow for the best fit of the data. The vertical variations in the anisotropic pattern show interesting correlations with changes in the thermomechanical state of the mantle associated with the lithosphere‐asthenosphere transition and with the presence of hot mantle beneath the Afar region and beneath the EARS branches that surround the Tanzanian Craton. Our interpretation is consistent with the conclusion of earlier studies that suggest that beneath individual stations, multiple sources of anisotropy, chiefly olivine lattice preferred orientation and melt pocket shape preferred orientation in our case, exist at different depths. Our study further emphasizes that multiple layers of anisotropy must often be considered to obtain realistic models of the crust and upper mantle.

Seismic recordings of IRIS/IDA/GSN station CMLA and of several temporary stations in the Azores archipelago are processed with P and S receiver function (PRF and SRF) techniques. Contrary to regional seismic tomography these methods provide estimates of the absolute velocities and of the Vp/Vs ratio up to a depth of ~300km. Joint inversion of PRFs and SRFs for a few data sets consistently reveals a division of the subsurface medium into four zones with a distinctly different Vp/Vs ratio: the crust ~20km thick with a ratio of ~1.9 in the lower crust, the high-Vs mantle lid with a strongly reduced Vp/Vs velocity ratio relative to the standard 1.8, the low-velocity zone (LVZ) with a velocity ratio of ~2.0, and the underlying upper-mantle layer with a standard velocity ratio. Our estimates of crustal thickness greatly exceed previous estimates (~10km). The base of the high-Vs lid (the Gutenberg discontinuity) is at a depth of ~80km. The LVZ with a reduction of S velocity of ~15% relative to the standard (IASP91) model is terminated at a depth of ~200km. The average thickness of the mantle transition zone (TZ) is evaluated from the time difference between the S410p and SKS660p, seismic phases that are robustly detected in the S and SKS receiver functions. This thickness is practically similar to the standard IASP91 value of 250km, and is characteristic of a large region of the North Atlantic outside the Azores plateau. Our data are indicative of a reduction of the S-wave velocity of several percent relative to the standard velocity in a depth interval from 460 to 500km. This reduction is found in the nearest vicinities of the Azores, in the region sampled by the PRFs, but, as evidenced by SRFs, it is missing at a distance of a few hundred kilometers from the islands. We speculate that this anomaly may correspond to the source of a plume which generated the Azores hotspot. Previously, a low S velocity in this depth range was found with SRF techniques beneath a few other hotspots.

P>To understand deep structure and processes beneath southern Africa, we apply the double-stacking version of the S receiver function (SRF) technique to the recordings of the South African Seismic Experiment. In this technique the wavefields of S and SKS are separated by space-time filtration, and the receiver functions are constructed separately for the S and SKS seismic phases. The results are consistent with those obtained from the SRFs of the permanent stations BOSA and LBTB. Evidence for a reduced S velocity atop the 410-km discontinuity is present in both SRFs and P receiver functions: the S velocity contrast at the 410-km discontinuity is similar to 40 per cent larger than the norm, and there are observations of S350p and P350s seismic phases from the negative 350-km discontinuity beneath the Kaapvaal craton. The S350p and P350s phases display a dependence on the azimuth, which can be caused by anisotropy in the layer atop the 350-km discontinuity. This dependence is consistent with observations of shear wave splitting in SKS. There are observations of the S450p phase from the 450-km negative discontinuity in the transition zone. The most anomalous transition zone is found close to the region where the Kalahari craton was located in the Mesozoic. Lateral variations of the S velocity are found beneath the 660-km discontinuity. Teleseismic S and P traveltime residuals with respect to IASP91 model are evaluated from the traveltimes of the P410s and Pp410s seismic phases. In the uppermost mantle these residuals as well as S410p traveltimes require a reduced V-P/V-S ratio (around 1.75 +/- 0.01 versus 1.8 in IASP91), an effect of the depletion of the mantle lithosphere in basaltic material. In the models obtained by a joint inversion of the receiver functions and the teleseismic traveltime residuals the low velocity zone (LVZ) with the onset of low velocity at a depth of similar to 140 +/- 20 km is present at most locations. The minimum S velocity in the LVZ is similar to 4.5 km s<SU-1</SU. The LVZ in our models is consistent with the S velocity, V-P/V-S velocity ratio and depth range of high-temperature lerzholites in the mantle xenoliths from southern Africa. Our most intriguing finding is a very low quality factor Q(S) (on the order of a few tens) in the upper mantle. We interpret high attenuation, the LVZ and the low S velocity atop the 410-km discontinuity as the effects of plume-like phenomena in the upper mantle.