Vinnik, Vinnik | Institute of Physics of the Earth, Russian Academy of Sciences (original) (raw)

Papers by Vinnik, Vinnik

Journal of Geophysical Research: Solid Earth, 2018

The crustal structure in the interior of Greenland is largely unknown because of its remote locat... more The crustal structure in the interior of Greenland is largely unknown because of its remote location below the up to 3.4-km-thick ice sheet. We present a model of the crustal velocity structure in central-eastern Greenland based on simultaneous inversion of P and S receiver functions for data acquired at 23 broadband stations between the coast and the center of the ice sheet. The area is believed to mainly include Precambrian basement and includes a part covered by Tertiary volcanic rocks and some sedimentary basins. Our results show a westward deepening Moho from less than 20 km at the coast to 50 km below central Greenland. Crustal S wave velocities are generally 3.75 km/s through the whole crust which may be relatively small for Precambrian areas, and V p /V s is generally around 1.73, although slightly higher in central Greenland. In the coastal area we observe anomalously low velocities at the top of the crust. In the volcanic area south of Scoresbysund Fjord this layer has very high V p /V s (>2), which indicates a high mafic content and the presence of water-filled cracks in the basaltic material. In the north, outside the volcanic area, V p /V s is normal and the low-velocity layer probably is instead related to the presence of sedimentary basins. At stations in the center of our study area we find low V s and high V p /V s in the lower crust. Based on the Moho topography, our results do not support Airy type isostasy as explanation of the high topography in eastern Greenland. Plain Language Summary Crustal structure in interior Greenland is largely unknown due to its inaccessibility. The origin of the mountains at the rim of Greenland is enigmatic, and cannot be explained by standard plate tectonic processes. We present the first crustal structure profile across the mountains and to the centre of Greenland which is needed for constraining the possible mechanisms that formed the mountains. We conclude that crustal isostasy cannot explain their presence.

Geophysical Journal International, 2007

Shear wave splitting in the seismic SKS phase provides a unique possibility to judge on deformati... more Shear wave splitting in the seismic SKS phase provides a unique possibility to judge on deformations at depths inaccessible for direct observations. Fast S wave polarization direction in collisional belts is often parallel to the trend of the belt, although deformations of the mantle lithosphere in low-angle thrusts would lead to the fast polarization direction normal to the trend of the belt. These considerations suggested that the upper mantle in collisional belts is decoupled from the crust. However, SKS technique is notable by a poor depth resolution, and usually it assumes that the fast polarization direction is the same at any depth, which is hard to justify. Here, to investigate depth dependent azimuthal anisotropy in the mantle, we invert jointly P receiver functions and SKS particle motions at a number of seismograph stations. The technique involves azimuthal filtering of the receiver functions and provides a criterion to discriminate between the effects of azimuthal anisotropy and lateral heterogeneity of isotropic medium. A search for the optimum models is conducted with a technique similar to simulated annealing. Testing with synthetics demonstrates that this approach is robust. The results for 10 seismograph stations in the Tien Shan, the world's most active intracontinental collisional belt in Central Asia, reveal a pronounced change in the patterns of azimuthal anisotropy at a depth around 100 km. In the mantle lithosphere (at depths less than 100 km), anisotropy is relatively weak and fast wave polarization direction varies laterally in a broad range. This layer is not necessarily decoupled from the crust: its anisotropy can be a combined effect of present day thrusting and of deformations of the geologic past. In the lower layer (asthenosphere) the average azimuth of fast wave polarization is close to the trend of the belt, whereas magnitude of S wave anisotropy is stable and large (between 5 and 6 per cent). This anisotropy is a likely result of recent uniaxial shortening at right angle to the trend of the belt. At some stations the data require anisotropy in the crust. There is no evidence for anisotropy at depths exceeding 150-250 km.

Geophysical Journal International, 2009

To understand deep structure and processes beneath southern Africa, we apply the doublestacking v... more To understand deep structure and processes beneath southern Africa, we apply the doublestacking 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 ∼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 ∼140 ± 20 km is present at most locations. The minimum S velocity in the LVZ is ∼4.5 km s −1. 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.

Geophysical Journal International, 1991

We have developed a technique for the inversion of teleseismic S-waveforms in terms of azimuthal ... more We have developed a technique for the inversion of teleseismic S-waveforms in terms of azimuthal anisotropy in the upper mantle. We test different models of the Earth upper mantle by transforming the observed horizontal components into a synthetic vertical component and comparing this with the observed vertical component. The optimum model is found by minimizing the difference between the synthetic vertical component and the observed one. Using this method, we explore the possibility of constraining the distribution of azimuthal anisotropy with depth. We present examples of seismic observations where the data are clearly in favour of an anisotropic model. These observations can be interpreted in terms of two anisotropic layers with different directions of fast velocity axes.

Geophysical Journal International, 2007

The fate of the mantle lithosphere of the Indian Plate in the India-Eurasia collision zone is not... more The fate of the mantle lithosphere of the Indian Plate in the India-Eurasia collision zone is not well understood. Tomographic studies reveal high P velocity in the uppermost mantle to the south of the western Himalaya, and these high velocities are sometimes interpreted as an image of subducting Indian lithosphere. We suggest that these high velocities are unrelated to the ongoing subduction but correspond to a near-horizontal mantle keel of the Indian shield. In the south of the Indian shield upper-mantle velocities are anomalously low, and relatively high velocities may signify a recovery of the normal shield structure in the north. Our analysis is based on the recordings of seismograph station NIL in the foothills of the western Himalaya. The T component of the P receiver functions is weak relative to the Q component, which is indicative of a subhorizontally layered structure. Joint inversion of the P and S receiver functions favours high uppermost mantle velocities, typical of the lithosphere of Archean cratons. The arrival of the Ps converted phase from 410 km discontinuity at NIL is 2.2 s earlier than in IASP91 global model. This can be an effect of remnants of Tethys subduction in the mantle transition zone and of high velocities in the keel of the Indian shield. Joint inversion of SKS particle motions and P receiver functions reveals a change in the fast direction of seismic azimuthal anisotropy from 60 • at 80-160 km depths to 150 • at 160-220 km. The fast direction in the lower layer is parallel to the trend of the Himalaya. The change of deformation regimes at a depth of 160 km suggests that this is the base of the lithosphere of the Indian shield. A similar boundary was found with similar techniques in central Europe and the Tien Shan region, but the base of the lithosphere in these regions is relatively shallow, in agreement with the higher upper-mantle temperatures. The ongoing continental collision is expressed in crustal structure: the crust beneath NIL is very thick (58 ± 2 km), and the S velocity in the intermediate and lower crust is around 4.0 km s −1. This anomalously large velocity and thickness can be explained by scraping off the lower crust, when the Indian lithosphere underthrusts the Himalaya.

Geophysical Journal International, 1993

In the horizontal components of GRF records, particle motions of the teleseismic S waves in the p... more In the horizontal components of GRF records, particle motions of the teleseismic S waves in the period range between 5 and 20s are usually elliptic. We correct the particle motions for the effect of azimuthal anisotropy at GRF and explain the residual ellipticity by assuming that it is produced at the source side of the wave path by interference of two waves with orthogonal polarizations and differing traveltimes. The estimates of polarization direction of the fast wave and time delay between the waves can be found by analysing the residual particle motion for groups of closely spaced seismic events. The technique was applied to about 40 GRF records of events from the northern and northwestern Pacific. In the source regions the S-wave pulses propagate in subduction zones. It is found that the surface projections of polarization of the fast wave of the shallow events coincide approximately with the strikes of the corresponding island arcs. This regularity can be explained by anisotropy in the subduction zones provided that the a axis of olivine is parallel to the plane of subduction. The fast direction for the deep events in the sea of Japan and the sea of Okhotsk is close to the strike of the Kurile island arc. The time delay between the fast and the slow waves for the deep events is smaller than for the shallow Kurile events (0.9 s versus 1.4 s).

Geophysical Journal International, 1996

Geophysical Journal International, 1984

Seismic anisotropy has been previously studied at depths usually not exceeding 100 or 150 km. In ... more Seismic anisotropy has been previously studied at depths usually not exceeding 100 or 150 km. In this paper we present a method of analysis of seismic records which is very sensitive to azimuthal anisotropy and is applicable at almost any depth range. The idea of the method is to detect and analyse the SH-component of the waves, converted from P to S in the mantle. The procedure of record processing includes frequency filtering, axis rotation, transformation of the record to a standard form, stacking the standardized SH-component records of many seismic events, and the harmonic analysis of amplitude as a function of the direction of wave propagation. When applied to the long-period records of NORSAR the procedure detected a converted wave with the properties implying the possibility of its propagation in a transversely isotropic medium with a horizonta2 axis of symmetry. Our preferred model postulates anisotropy of-1 per cent in a layer 50 km thick at the base of the upper mantle.

Geophysical Journal International, 2005

To study the deep structure of Iceland, we conducted S receiver function analysis for almost 60 l... more To study the deep structure of Iceland, we conducted S receiver function analysis for almost 60 local broad-band seismograph stations of the Hotspot, ICEMELT and SIL networks. The structure was investigated separately for the central region of Iceland containing the neovolcanic zone and two peripheral regions to the east and west. S-toP converted phases from uppermantle discontinuities were detected by stacking recordings of several tens of teleseismic events. The analysis reveals previously unknown details. Magnitude and depth extent of the low S velocity anomaly in the upper mantle beneath Iceland are much larger than reported in earlier studies. Clear S-toP converted phases are obtained from the discontinuity at a depth of 80 ± 5 km, separating the high-velocity mantle lid from the underlying low S velocity layer. This discontinuity can be interpreted as a chemical boundary between dry harzburgite in the upper layer and wet peridotite underneath. Beneath peripheral parts of Iceland, we detect a boundary at a depth of 135 ± 5 km with S velocity increasing downwards. This boundary may correspond to the onset of melting in wet peridotite at a potential temperature of around 1400 • C. Models of melting induced by CO 2 are not incompatible with our observations. The seismic data demonstrate effects that may be caused by azimuthal anisotropy in the upper mantle. There are indications of a second low S velocity layer to the NNE of Iceland, with the top near 480 km depth, similar to one recently detected beneath the Afro-Arabian hotspot.

Geophysical Journal International, 1994

Geophysical Journal International, 1984

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Geophysical Journal International, 2000

Seismic strati¢cation of the upper mantle is investigated by applying two complementary technique... more Seismic strati¢cation of the upper mantle is investigated by applying two complementary techniques to the records of the Graefenberg array in southern Germany. The anisotropic P receiver function technique (Kosarev et al. 1984; Vinnik & Montagner 1996) is modi¢ed by using summary seismic events instead of individual events and di¡erent weighting functions instead of the same function for the harmonic angular analysis of the SV and T components of the Pds phases. The summary events provide better separation of the second azimuthal harmonic than the individual events. The parameters of the second harmonics of SV and T thus evaluated should be similar if they re£ect the e¡ects of azimuthal anisotropy. This can be used as a criterion to identify the anisotropy. To detect the Sdp phases and their azimuthal variations caused by azimuthal anisotropy we have developed a stacking technique, which can be termed the S receiver function technique. It includes axis rotation to separate interfering P and S arrivals, determination of the principal (M) component of the S-wave motion, deconvolution of the P components of many recordings by their respective M components and stacking of the deconvolved P components with weights depending on the level of noise and the angle between the M direction and the backazimuth of the event. Both techniques yield consistent results for the Graefenberg array. As indicated by the P receiver functions, the upper layer of the mantle between the Moho and 80 km depth is anisotropic with dV s /V s around 0.03 and the fast direction close to 20 0 clockwise from north. The fast direction of anisotropy below this layer is around 110 0. The boundary between the upper and the lower anisotropic layers is manifested by the detectable Pds and Sdp converted phases. Shear wave splitting in SKS is strongly dominated by azimuthal anisotropy in the lower layer (asthenosphere).

Geophysical Journal International, 2001

Indications of lower mantle discontinuities have been debated for decades, but still little is kn... more Indications of lower mantle discontinuities have been debated for decades, but still little is known about their properties, and their origins are enigmatic. In our study broad-band recordings of deep events are examined for the presence of signals from the lower-mantle discontinuities with a novel technique. We deconvolve vertical component of the P-wave coda in the period range around 10 s by the S waveform and stack many deconvolved traces with moveout time corrections. In synthetic seismograms for an earth model without lower mantle discontinuities, the strongest signal thus detected in the time window of interest is often s'410'P phase (generated as S and reflected as P from the '410 km' discontinuity above the source). In actual seismograms there are other phases that can be interpreted as converted from S to P at discontinuities in the lower mantle beneath the seismic source. We summarize the results of processing the seismograms (1) of deep events in Sunda arc at seismograph stations in east Asia, (2) deep Kermadec-Fiji-Tonga events at the J-array and FREESIA networks in Japan and stations in east Asia, and (3) deep events in the northwest Pacific region (Mariana, Izu-Bonin and the Japan arc) recorded at stations in north America. In our data there are indications of discontinuities near 860-880, 1010-1120, 1170-1250 and 1670-1800 km depths. The clearest signals are obtained from the discontinuity at a depth of 1200 km. We argue that the '900', '1200' and '1700 km' discontinuities are global, but laterally variable in both depth and strength. Seismic stratification of the lower mantle may have bearings on the patterns of subduction, as revealed by tomographic models.

Geophysical Journal International, 2002

Azimuthal anisotropy in the upper mantle of many continental regions is documented by Swave split... more Azimuthal anisotropy in the upper mantle of many continental regions is documented by Swave splitting measurements with SKS techniques. Here we present observations of splitting of the SKS seismic phase at seismograph stations in the Siberian platform, where few such data were known previously. The parameters of splitting are coherent: the fast direction everywhere is around 150 • , and the delay of the slow split wave is close to 1.0 s. These observations provide no constraints on the distribution of anisotropy with depth. However, the Siberian platform is remarkable in that it is covered by a network of long-range profiles, where P waves from nuclear explosions are recorded at epicentral distances of 2000 km and more. Depending on epicentral distance these waves sample the upper mantle from the Moho to the transition zone. Two profiles run approximately parallel to the fast direction of the azimuthal anisotropy, whereas the directions of the two others are intermediate between the fast and slow. We examine the observed P traveltimes for their dependence on the azimuth and epicentral distance. With the available data on elastic anisotropy in mantle xenoliths, the values of P-wave anisotropy for horizontal propagation can be used to evaluate S-wave splitting for vertical propagation. It appears that the upper mantle between the Moho and 150 km depth is responsible for not more than about 30 per cent of the large-scale effect in the SKS phase. The major effect is accumulated in a broad low-velocity zone, the top of which is found at a depth of 150 km. Anisotropy within this zone can be caused by recent mantle flow. A similar distribution with depth might explain discrepancies between the estimates of azimuthal anisotropy from phase velocities of surface waves and SKS splitting in North America and South Africa.

Earth and Planetary Science Letters, 2008

To understand deep structure of the western Himalaya, Ladakh and western Tibet, we conduct an int... more To understand deep structure of the western Himalaya, Ladakh and western Tibet, we conduct an integrated analysis of teleseismic body wave recordings from the linear array of 16 portable broadband seismographs along a profile running through the Himalaya to the Karakoram. The database includes P and S receiver functions, teleseismic P and S residuals and observations of shear wave splitting in SKS. The analysis reveals beneath the Himalaya a cold region at a depth of several hundred kilometers. This anomaly can be caused by remnants of Tethys subduction and, perhaps, is unrelated to the ongoing process. The present day process is reflected in velocities beneath the lesser Himalaya, intermediate between those of the crust and the upper mantle. This observation can be explained by scraping off the ductile lower crust of the underthrusting Indian plate and accumulation of the high-velocity crustal material in the frontal region of the thrust zone. A representative value of the crustal thickness at the profile is 65 km, and at most stations there is no evidence for a low-velocity layer at mid-crustal depths, which makes crustal channel flow unlikely. Seismic waves in the upper 200 km of the mantle are faster than in global IASP91 model, and the structure includes a lowvelocity layer sandwiched between two high-velocity layers. We interpret the lower layer as an image of subducted mantle lithosphere of the Indian plate. Shear wave splitting is different in the south and the north. In the south the fast direction of anisotropy is normal to the trend of the Himalaya and can be interpreted as an effect of the NE motion of the Indian lithosphere. In the north the fast direction is oriented E-W and can be explained by the fabric left in the presently extinct subduction zones. In eastern Tibet a similar analysis of the P and S receiver functions reveals upper mantle with the S velocities about 5% lower than in the west. The mantle in the vicinity of station LSA (Lhasa) contains a low S velocity layer between 160-km and 230-km depths with a velocity reduction of 0.2 km/s, underlain by the Lehmann discontinuity.

Earth and Planetary Science Letters, 2012

We investigate the crust, upper mantle and mantle transition zone of the Cape Verde hotspot by us... more 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 510km 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.

Earth and Planetary Science Letters, 2004

Earth and Planetary Science Letters, 2007

A thin, lowS velocity layer atop the 410-km discontinuity is an intriguing feature of the upper m... more A thin, lowS velocity layer atop the 410-km discontinuity is an intriguing feature of the upper mantle with important implications for geodynamics, but relevant seismic data are few. By applying S receiver function technique to more than 50 globally distributed stations, in 10 regions we obtain evidence for a negative discontinuity at a depth of about 350 km. In most cases, the low velocity is found beneath Precambrian platforms, in association with either Mesozoic or Cenozoic mantle plumes. This relationship suggests dehydration of water-bearing silicates as a possible reason for the low velocity, but contradicts the predictions of the transition-zone-water-filter model of Bercovici and Karato (Nature 425, 39-44, 2003). The presence of the low velocity beneath some Mesozoic traps, in spite of plate motions, implies the possibility of coupling of the continental lithosphere and the underlying upper mantle up to a depth of ∼400 km.

Earth and Planetary Science Letters, 2011

ABSTRACT P and S receiver functions from seismograph stations in the Indian shield, Western Himal... more ABSTRACT 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.

Journal of Geophysical Research: Solid Earth, 2018

The crustal structure in the interior of Greenland is largely unknown because of its remote locat... more The crustal structure in the interior of Greenland is largely unknown because of its remote location below the up to 3.4-km-thick ice sheet. We present a model of the crustal velocity structure in central-eastern Greenland based on simultaneous inversion of P and S receiver functions for data acquired at 23 broadband stations between the coast and the center of the ice sheet. The area is believed to mainly include Precambrian basement and includes a part covered by Tertiary volcanic rocks and some sedimentary basins. Our results show a westward deepening Moho from less than 20 km at the coast to 50 km below central Greenland. Crustal S wave velocities are generally 3.75 km/s through the whole crust which may be relatively small for Precambrian areas, and V p /V s is generally around 1.73, although slightly higher in central Greenland. In the coastal area we observe anomalously low velocities at the top of the crust. In the volcanic area south of Scoresbysund Fjord this layer has very high V p /V s (>2), which indicates a high mafic content and the presence of water-filled cracks in the basaltic material. In the north, outside the volcanic area, V p /V s is normal and the low-velocity layer probably is instead related to the presence of sedimentary basins. At stations in the center of our study area we find low V s and high V p /V s in the lower crust. Based on the Moho topography, our results do not support Airy type isostasy as explanation of the high topography in eastern Greenland. Plain Language Summary Crustal structure in interior Greenland is largely unknown due to its inaccessibility. The origin of the mountains at the rim of Greenland is enigmatic, and cannot be explained by standard plate tectonic processes. We present the first crustal structure profile across the mountains and to the centre of Greenland which is needed for constraining the possible mechanisms that formed the mountains. We conclude that crustal isostasy cannot explain their presence.

Geophysical Journal International, 2007

Shear wave splitting in the seismic SKS phase provides a unique possibility to judge on deformati... more Shear wave splitting in the seismic SKS phase provides a unique possibility to judge on deformations at depths inaccessible for direct observations. Fast S wave polarization direction in collisional belts is often parallel to the trend of the belt, although deformations of the mantle lithosphere in low-angle thrusts would lead to the fast polarization direction normal to the trend of the belt. These considerations suggested that the upper mantle in collisional belts is decoupled from the crust. However, SKS technique is notable by a poor depth resolution, and usually it assumes that the fast polarization direction is the same at any depth, which is hard to justify. Here, to investigate depth dependent azimuthal anisotropy in the mantle, we invert jointly P receiver functions and SKS particle motions at a number of seismograph stations. The technique involves azimuthal filtering of the receiver functions and provides a criterion to discriminate between the effects of azimuthal anisotropy and lateral heterogeneity of isotropic medium. A search for the optimum models is conducted with a technique similar to simulated annealing. Testing with synthetics demonstrates that this approach is robust. The results for 10 seismograph stations in the Tien Shan, the world's most active intracontinental collisional belt in Central Asia, reveal a pronounced change in the patterns of azimuthal anisotropy at a depth around 100 km. In the mantle lithosphere (at depths less than 100 km), anisotropy is relatively weak and fast wave polarization direction varies laterally in a broad range. This layer is not necessarily decoupled from the crust: its anisotropy can be a combined effect of present day thrusting and of deformations of the geologic past. In the lower layer (asthenosphere) the average azimuth of fast wave polarization is close to the trend of the belt, whereas magnitude of S wave anisotropy is stable and large (between 5 and 6 per cent). This anisotropy is a likely result of recent uniaxial shortening at right angle to the trend of the belt. At some stations the data require anisotropy in the crust. There is no evidence for anisotropy at depths exceeding 150-250 km.

Geophysical Journal International, 2009

To understand deep structure and processes beneath southern Africa, we apply the doublestacking v... more To understand deep structure and processes beneath southern Africa, we apply the doublestacking 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 ∼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 ∼140 ± 20 km is present at most locations. The minimum S velocity in the LVZ is ∼4.5 km s −1. 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.

Geophysical Journal International, 1991

We have developed a technique for the inversion of teleseismic S-waveforms in terms of azimuthal ... more We have developed a technique for the inversion of teleseismic S-waveforms in terms of azimuthal anisotropy in the upper mantle. We test different models of the Earth upper mantle by transforming the observed horizontal components into a synthetic vertical component and comparing this with the observed vertical component. The optimum model is found by minimizing the difference between the synthetic vertical component and the observed one. Using this method, we explore the possibility of constraining the distribution of azimuthal anisotropy with depth. We present examples of seismic observations where the data are clearly in favour of an anisotropic model. These observations can be interpreted in terms of two anisotropic layers with different directions of fast velocity axes.

Geophysical Journal International, 2007

The fate of the mantle lithosphere of the Indian Plate in the India-Eurasia collision zone is not... more The fate of the mantle lithosphere of the Indian Plate in the India-Eurasia collision zone is not well understood. Tomographic studies reveal high P velocity in the uppermost mantle to the south of the western Himalaya, and these high velocities are sometimes interpreted as an image of subducting Indian lithosphere. We suggest that these high velocities are unrelated to the ongoing subduction but correspond to a near-horizontal mantle keel of the Indian shield. In the south of the Indian shield upper-mantle velocities are anomalously low, and relatively high velocities may signify a recovery of the normal shield structure in the north. Our analysis is based on the recordings of seismograph station NIL in the foothills of the western Himalaya. The T component of the P receiver functions is weak relative to the Q component, which is indicative of a subhorizontally layered structure. Joint inversion of the P and S receiver functions favours high uppermost mantle velocities, typical of the lithosphere of Archean cratons. The arrival of the Ps converted phase from 410 km discontinuity at NIL is 2.2 s earlier than in IASP91 global model. This can be an effect of remnants of Tethys subduction in the mantle transition zone and of high velocities in the keel of the Indian shield. Joint inversion of SKS particle motions and P receiver functions reveals a change in the fast direction of seismic azimuthal anisotropy from 60 • at 80-160 km depths to 150 • at 160-220 km. The fast direction in the lower layer is parallel to the trend of the Himalaya. The change of deformation regimes at a depth of 160 km suggests that this is the base of the lithosphere of the Indian shield. A similar boundary was found with similar techniques in central Europe and the Tien Shan region, but the base of the lithosphere in these regions is relatively shallow, in agreement with the higher upper-mantle temperatures. The ongoing continental collision is expressed in crustal structure: the crust beneath NIL is very thick (58 ± 2 km), and the S velocity in the intermediate and lower crust is around 4.0 km s −1. This anomalously large velocity and thickness can be explained by scraping off the lower crust, when the Indian lithosphere underthrusts the Himalaya.

Geophysical Journal International, 1993

In the horizontal components of GRF records, particle motions of the teleseismic S waves in the p... more In the horizontal components of GRF records, particle motions of the teleseismic S waves in the period range between 5 and 20s are usually elliptic. We correct the particle motions for the effect of azimuthal anisotropy at GRF and explain the residual ellipticity by assuming that it is produced at the source side of the wave path by interference of two waves with orthogonal polarizations and differing traveltimes. The estimates of polarization direction of the fast wave and time delay between the waves can be found by analysing the residual particle motion for groups of closely spaced seismic events. The technique was applied to about 40 GRF records of events from the northern and northwestern Pacific. In the source regions the S-wave pulses propagate in subduction zones. It is found that the surface projections of polarization of the fast wave of the shallow events coincide approximately with the strikes of the corresponding island arcs. This regularity can be explained by anisotropy in the subduction zones provided that the a axis of olivine is parallel to the plane of subduction. The fast direction for the deep events in the sea of Japan and the sea of Okhotsk is close to the strike of the Kurile island arc. The time delay between the fast and the slow waves for the deep events is smaller than for the shallow Kurile events (0.9 s versus 1.4 s).

Geophysical Journal International, 1996

Geophysical Journal International, 1984

Seismic anisotropy has been previously studied at depths usually not exceeding 100 or 150 km. In ... more Seismic anisotropy has been previously studied at depths usually not exceeding 100 or 150 km. In this paper we present a method of analysis of seismic records which is very sensitive to azimuthal anisotropy and is applicable at almost any depth range. The idea of the method is to detect and analyse the SH-component of the waves, converted from P to S in the mantle. The procedure of record processing includes frequency filtering, axis rotation, transformation of the record to a standard form, stacking the standardized SH-component records of many seismic events, and the harmonic analysis of amplitude as a function of the direction of wave propagation. When applied to the long-period records of NORSAR the procedure detected a converted wave with the properties implying the possibility of its propagation in a transversely isotropic medium with a horizonta2 axis of symmetry. Our preferred model postulates anisotropy of-1 per cent in a layer 50 km thick at the base of the upper mantle.

Geophysical Journal International, 2005

To study the deep structure of Iceland, we conducted S receiver function analysis for almost 60 l... more To study the deep structure of Iceland, we conducted S receiver function analysis for almost 60 local broad-band seismograph stations of the Hotspot, ICEMELT and SIL networks. The structure was investigated separately for the central region of Iceland containing the neovolcanic zone and two peripheral regions to the east and west. S-toP converted phases from uppermantle discontinuities were detected by stacking recordings of several tens of teleseismic events. The analysis reveals previously unknown details. Magnitude and depth extent of the low S velocity anomaly in the upper mantle beneath Iceland are much larger than reported in earlier studies. Clear S-toP converted phases are obtained from the discontinuity at a depth of 80 ± 5 km, separating the high-velocity mantle lid from the underlying low S velocity layer. This discontinuity can be interpreted as a chemical boundary between dry harzburgite in the upper layer and wet peridotite underneath. Beneath peripheral parts of Iceland, we detect a boundary at a depth of 135 ± 5 km with S velocity increasing downwards. This boundary may correspond to the onset of melting in wet peridotite at a potential temperature of around 1400 • C. Models of melting induced by CO 2 are not incompatible with our observations. The seismic data demonstrate effects that may be caused by azimuthal anisotropy in the upper mantle. There are indications of a second low S velocity layer to the NNE of Iceland, with the top near 480 km depth, similar to one recently detected beneath the Afro-Arabian hotspot.

Geophysical Journal International, 1994

Geophysical Journal International, 1984

Iirstitirtc ofPliysics of tlrc Eartli. Acadeniy of Scierices of the USSR, Moscoicl. USSR

Geophysical Journal International, 2000

Seismic strati¢cation of the upper mantle is investigated by applying two complementary technique... more Seismic strati¢cation of the upper mantle is investigated by applying two complementary techniques to the records of the Graefenberg array in southern Germany. The anisotropic P receiver function technique (Kosarev et al. 1984; Vinnik & Montagner 1996) is modi¢ed by using summary seismic events instead of individual events and di¡erent weighting functions instead of the same function for the harmonic angular analysis of the SV and T components of the Pds phases. The summary events provide better separation of the second azimuthal harmonic than the individual events. The parameters of the second harmonics of SV and T thus evaluated should be similar if they re£ect the e¡ects of azimuthal anisotropy. This can be used as a criterion to identify the anisotropy. To detect the Sdp phases and their azimuthal variations caused by azimuthal anisotropy we have developed a stacking technique, which can be termed the S receiver function technique. It includes axis rotation to separate interfering P and S arrivals, determination of the principal (M) component of the S-wave motion, deconvolution of the P components of many recordings by their respective M components and stacking of the deconvolved P components with weights depending on the level of noise and the angle between the M direction and the backazimuth of the event. Both techniques yield consistent results for the Graefenberg array. As indicated by the P receiver functions, the upper layer of the mantle between the Moho and 80 km depth is anisotropic with dV s /V s around 0.03 and the fast direction close to 20 0 clockwise from north. The fast direction of anisotropy below this layer is around 110 0. The boundary between the upper and the lower anisotropic layers is manifested by the detectable Pds and Sdp converted phases. Shear wave splitting in SKS is strongly dominated by azimuthal anisotropy in the lower layer (asthenosphere).

Geophysical Journal International, 2001

Indications of lower mantle discontinuities have been debated for decades, but still little is kn... more Indications of lower mantle discontinuities have been debated for decades, but still little is known about their properties, and their origins are enigmatic. In our study broad-band recordings of deep events are examined for the presence of signals from the lower-mantle discontinuities with a novel technique. We deconvolve vertical component of the P-wave coda in the period range around 10 s by the S waveform and stack many deconvolved traces with moveout time corrections. In synthetic seismograms for an earth model without lower mantle discontinuities, the strongest signal thus detected in the time window of interest is often s'410'P phase (generated as S and reflected as P from the '410 km' discontinuity above the source). In actual seismograms there are other phases that can be interpreted as converted from S to P at discontinuities in the lower mantle beneath the seismic source. We summarize the results of processing the seismograms (1) of deep events in Sunda arc at seismograph stations in east Asia, (2) deep Kermadec-Fiji-Tonga events at the J-array and FREESIA networks in Japan and stations in east Asia, and (3) deep events in the northwest Pacific region (Mariana, Izu-Bonin and the Japan arc) recorded at stations in north America. In our data there are indications of discontinuities near 860-880, 1010-1120, 1170-1250 and 1670-1800 km depths. The clearest signals are obtained from the discontinuity at a depth of 1200 km. We argue that the '900', '1200' and '1700 km' discontinuities are global, but laterally variable in both depth and strength. Seismic stratification of the lower mantle may have bearings on the patterns of subduction, as revealed by tomographic models.

Geophysical Journal International, 2002

Azimuthal anisotropy in the upper mantle of many continental regions is documented by Swave split... more Azimuthal anisotropy in the upper mantle of many continental regions is documented by Swave splitting measurements with SKS techniques. Here we present observations of splitting of the SKS seismic phase at seismograph stations in the Siberian platform, where few such data were known previously. The parameters of splitting are coherent: the fast direction everywhere is around 150 • , and the delay of the slow split wave is close to 1.0 s. These observations provide no constraints on the distribution of anisotropy with depth. However, the Siberian platform is remarkable in that it is covered by a network of long-range profiles, where P waves from nuclear explosions are recorded at epicentral distances of 2000 km and more. Depending on epicentral distance these waves sample the upper mantle from the Moho to the transition zone. Two profiles run approximately parallel to the fast direction of the azimuthal anisotropy, whereas the directions of the two others are intermediate between the fast and slow. We examine the observed P traveltimes for their dependence on the azimuth and epicentral distance. With the available data on elastic anisotropy in mantle xenoliths, the values of P-wave anisotropy for horizontal propagation can be used to evaluate S-wave splitting for vertical propagation. It appears that the upper mantle between the Moho and 150 km depth is responsible for not more than about 30 per cent of the large-scale effect in the SKS phase. The major effect is accumulated in a broad low-velocity zone, the top of which is found at a depth of 150 km. Anisotropy within this zone can be caused by recent mantle flow. A similar distribution with depth might explain discrepancies between the estimates of azimuthal anisotropy from phase velocities of surface waves and SKS splitting in North America and South Africa.

Earth and Planetary Science Letters, 2008

To understand deep structure of the western Himalaya, Ladakh and western Tibet, we conduct an int... more To understand deep structure of the western Himalaya, Ladakh and western Tibet, we conduct an integrated analysis of teleseismic body wave recordings from the linear array of 16 portable broadband seismographs along a profile running through the Himalaya to the Karakoram. The database includes P and S receiver functions, teleseismic P and S residuals and observations of shear wave splitting in SKS. The analysis reveals beneath the Himalaya a cold region at a depth of several hundred kilometers. This anomaly can be caused by remnants of Tethys subduction and, perhaps, is unrelated to the ongoing process. The present day process is reflected in velocities beneath the lesser Himalaya, intermediate between those of the crust and the upper mantle. This observation can be explained by scraping off the ductile lower crust of the underthrusting Indian plate and accumulation of the high-velocity crustal material in the frontal region of the thrust zone. A representative value of the crustal thickness at the profile is 65 km, and at most stations there is no evidence for a low-velocity layer at mid-crustal depths, which makes crustal channel flow unlikely. Seismic waves in the upper 200 km of the mantle are faster than in global IASP91 model, and the structure includes a lowvelocity layer sandwiched between two high-velocity layers. We interpret the lower layer as an image of subducted mantle lithosphere of the Indian plate. Shear wave splitting is different in the south and the north. In the south the fast direction of anisotropy is normal to the trend of the Himalaya and can be interpreted as an effect of the NE motion of the Indian lithosphere. In the north the fast direction is oriented E-W and can be explained by the fabric left in the presently extinct subduction zones. In eastern Tibet a similar analysis of the P and S receiver functions reveals upper mantle with the S velocities about 5% lower than in the west. The mantle in the vicinity of station LSA (Lhasa) contains a low S velocity layer between 160-km and 230-km depths with a velocity reduction of 0.2 km/s, underlain by the Lehmann discontinuity.

Earth and Planetary Science Letters, 2012

We investigate the crust, upper mantle and mantle transition zone of the Cape Verde hotspot by us... more 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 510km 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.

Earth and Planetary Science Letters, 2004

Earth and Planetary Science Letters, 2007

A thin, lowS velocity layer atop the 410-km discontinuity is an intriguing feature of the upper m... more A thin, lowS velocity layer atop the 410-km discontinuity is an intriguing feature of the upper mantle with important implications for geodynamics, but relevant seismic data are few. By applying S receiver function technique to more than 50 globally distributed stations, in 10 regions we obtain evidence for a negative discontinuity at a depth of about 350 km. In most cases, the low velocity is found beneath Precambrian platforms, in association with either Mesozoic or Cenozoic mantle plumes. This relationship suggests dehydration of water-bearing silicates as a possible reason for the low velocity, but contradicts the predictions of the transition-zone-water-filter model of Bercovici and Karato (Nature 425, 39-44, 2003). The presence of the low velocity beneath some Mesozoic traps, in spite of plate motions, implies the possibility of coupling of the continental lithosphere and the underlying upper mantle up to a depth of ∼400 km.

Earth and Planetary Science Letters, 2011

ABSTRACT P and S receiver functions from seismograph stations in the Indian shield, Western Himal... more ABSTRACT 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.