Images Of Upper Mantle Research Articles

Gravity effect of Alpine slab segments based on geophysical and petrological modelling

Abstract. In this study, we present an estimate of the gravity signal of the slabs beneath the Alpine mountain belt. Estimates of the gravity effect of the subducting slabs are often omitted or simplified in crustal-scale models. The related signal is calculated here for alternative slab configurations at near-surface height and at a satellite altitude of 225 km. We apply three different modelling approaches in order to estimate the gravity signal from the subducting slab segments: (i) direct conversion of upper mantle seismic velocities to density distribution, which are then forward calculated to obtain the gravity signal; (ii) definition of slab geometries based on seismic crustal thickness and high-resolution upper mantle tomography for two competing slab configurations – the geometries are then forward calculated by assigning a constant density contrast and slab thickness; (iii) accounting for compositional and thermal variations with depth within the predefined slab geometry. Forward calculations predict a gravity signal of up to 40 mGal for the Alpine slab configuration. Significant differences in the gravity anomaly patterns are visible for different slab geometries in the near-surface gravity field. However, different contributing slab segments are not easily separated, especially at satellite altitude. Our results demonstrate that future studies addressing the lithospheric structure of the Alps should have to account for the subducting slabs in order to provide a meaningful representation of the geodynamic complex Alpine area.

Upper mantle tomography beneath the Pacific Northwest interior

The Pacific Northwest has undergone extensive tectonic and magmatic construction in the Cenozoic, and many clues to its history are in a well-resolved upper mantle structure. We present new P, S, and VP/VS models that benefit from improved algorithms and additional data from recently deployed flexible array experiments in the region of the Columbia River Basalts, western Snake River Plain, and Idaho Batholith. We refine the geometries of previously imaged structures and identify new anomalies beneath the Pacific Northwest. A clear separation is resolved between the Wallowa and Idaho structures that extends from the base of the crust to the depth extent of these structures. The separation follows the Precambrian margin of North America, possibly indicating separate tectonic histories for the two structures. A shallow fast anomaly beneath Idaho Batholith may be responsible for the observed curvature of the Yellowstone track around this batholith. And a strong and unusually deep (180–250 km) slow anomaly is present east of the apparent Juan de Fuca slab gap. Resolving these structures well is important in understanding the origin of the lithospheric fragments in the mantle, and key to any reconstruction of the evolution of the Pacific Northwest-Farallon-Kula-Juan de Fuca tectonic system. Of most relevance, understanding the origin of the imaged high-velocity mantle structures provides strong constraint on their density, that otherwise would be largely unknown, which controls the evolution of vertical and horizontal stresses acting at the base of the lithosphere and hence the evolution of topography. Also, resolving the volumes of partially molten upper mantle shapes our understanding on the origin of the abundant volcanism in the Pacific Northwest interior.

Upper mantle tomography of the Western Junggar: Implications for its geodynamic evolution

A high-resolution 3-D P-wave velocity model of the upper mantle beneath the Western Junggar is determined by inverting a large number of relative travel-time residuals of teleseismic events. Our results reveal a prominent high-velocity (high-V) anomaly beneath the western Junggar basin and a low-velocity (low-V) anomaly beneath the Darbut belt. The NE-SW striking high-V anomaly at depths of 50–200 km beneath the western Junggar basin dips toward the northwest, which represents a fossil oceanic slab. The low-V anomaly may reflect an intraoceanic arc-related terrain that has experienced strong metasomatism by hot upwelling asthenosphere in the late Paleozoic, resulting in adakitic intrusions and concentration of porphyry CuAu deposits in the Darbut belt. These results shed new light on the geodynamic evolution of the Western Junggar and formation mechanisms of the adakites and porphyry CuAu deposits.

P-wave upper-mantle tomography of the Tanlu fault zone in eastern China

The Tanlu fault zone is the most significant active fault in eastern China, which generated the great 1668 Tancheng earthquake (M 8.5). It is still unclear whether or not there is a link between the great earthquake generation and the upper-mantle structure. To address this issue, we study P-wave upper-mantle tomography beneath eastern China using 44,047 teleseismic P-wave arrival times. Our results show that at depths <150 km, high-velocity (high-V) anomalies appear west of the Tanlu fault zone, whereas low-velocity (low-V) anomalies are visible east of the fault zone. Strong lateral heterogeneities are revealed along the fault zone. At depths of 230–470 km, northwest of the Tanlu fault zone, there are obvious low-V anomalies that may reflect hot and wet mantle upwelling, whereas to the east high-V anomalies are visible, which may reflect the detached Eurasian lithosphere (downwelling). In the mantle transition zone (MTZ), both high-V and low-V anomalies are revealed, and the widespread high-V anomalies may reflect the stagnant Pacific slab. Beneath the hypocenter of the 1668 Tancheng earthquake, intermittent low-V anomalies are revealed in the upper mantle down to the MTZ depth, which may reflect hot and wet mantle upwelling flow. Integrating the present results with previous findings, we deem that the Tancheng earthquake was affected by fluids from the hot and wet mantle upwelling associated with the lithospheric delamination. Complicated mantle convection, including both upwelling and downwelling flows, may occur under the Tanlu fault zone in the big mantle wedge above the stagnant Pacific slab in the MTZ.

Hunting for the Tristan mantle plume – An upper mantle tomography around the volcanic island of Tristan da Cunha

The active volcanic island Tristan da Cunha, located at the southwestern and youngest end of the Walvis Ridge – Tristan/Gough hotspot track, is believed to be the surface expression of a huge thermal mantle anomaly. While several criteria for the diagnosis of a classical hotspot track are met, the Tristan region also shows some peculiarities. Consequently, it is vigorously debated if the active volcanism in this region is the expression of a deep mantle plume, or if it is caused by shallow plate tectonics and the interaction with the nearby Mid-Atlantic Ridge. Because of a lack of geophysical data in the study area, no model or assumption has been completely confirmed.We present the first amphibian P-wave finite-frequency travel time tomography of the Tristan da Cunha region, based on cross-correlated travel time residuals of teleseismic earthquakes recorded by 24 ocean-bottom seismometers. The data can be used to image a low velocity structure southwest of the island. The feature is cylindrical with a radius of ∼100 km down to a depth of 250 km. We relate this structure to the origin of Tristan da Cunha and name it the Tristan conduit. Below 250 km the low velocity structure ramifies into narrow veins, each with a radius of ∼50 km. Furthermore, we imaged a linkage between young seamounts southeast of Tristan da Cunha and the Tristan conduit.

Upper mantle tomography in the northwestern Pacific region using triplicated P waves

AbstractWe conducted delay time tomography of the upper mantle beneath the northwestern Pacific using P data from NorthEast China Extended SeiSmic Array, F‐net, and nearby available stations. To improve resolution and accuracy in the vicinity of mantle discontinuities, we extracted traveltimes of both initial and secondary phases observed at triplication distances by using a waveform fitting technique. Compared with the model obtained by using only the initial phase, the resolution just above the 410 km discontinuity is especially improved, and low‐velocity anomalies beneath the Changbai Volcano are clearly observed down to the 410 km discontinuity. Compared with previous models, low‐velocity anomalies atop the 410 are more pronounced. The results of this study together with the previous receiver function analysis provide further support that we have hot material beneath the Changbai Volcano.

Global mantle heterogeneity and its influence on teleseismic regional tomography

A new global P-wave tomography model is determined using a flexible-grid parameterization. This new model better reveals the mantle structure under the polar regions than the previous tomographic models. The subducting slabs are generally imaged clearly as high-velocity (high-V) zones. The young slabs are still subducting in the upper mantle and the mantle transition zone (MTZ), whereas the old and ancient slabs are either stagnant in the MTZ or have subducted down to the lower mantle, even reaching the core-mantle boundary. Low-velocity (low-V) anomalies are generally revealed in the mantle under the hotspot regions. It seems that a variety of mantle upwelling (plumes) exist. Some strong plumes are visible in the whole mantle under the long-living hotspots, such as those in south-central Pacific, Africa, Hawaii and Iceland, whereas weak plumes are visible in only some depth range under the minor hotspots. Under the intraplate volcanoes in East Eurasia, Bering Sea and West Alaska, significant low-V anomalies are revealed in the upper mantle, which may reflect hot and wet upwelling associated with corner flows in the big mantle wedge (BMW) above the stagnant Pacific slab in the MTZ and perhaps deep slab dehydration as well. The subduction-triggered magmatism in the BMW may be a new class of mantle plumes. We also used the new global model to investigate the influence of whole-mantle heterogeneity on the determination of upper-mantle tomography under Japan with a teleseismic inversion method. The results show that the mantle heterogeneities outside the target volume of regional tomography can cause significant changes (~0.2-0.4s) to the observed relative travel-time residuals of a distant earthquake. The pattern of regional tomography remains the same even after correcting for the whole-mantle heterogeneity, but there are some changes in the amplitude of velocity anomalies in the regional tomography. Hence it is necessary to correct for the mantle heterogeneities outside the target volume so as to obtain a better regional tomography.

Relative arrival-time upper-mantle tomography and the elusive background mean

SUMMARY The interpretation of seismic tomographic images of upper-mantle seismic wave speed structure is often a matter of considerable debate because the observations can usually be explained by a range of hypotheses, including variable temperature, composition, anisotropy, and the presence of partial melt. An additional problem, often overlooked in tomographic studies using relative as opposed to absolute arrival-times, is the issue of the resulting velocity model’s zero mean. In shield areas, for example, relative arrival-time analysis strips off a background mean velocity structure that is markedly fast compared to the global average. Conversely, in active areas, the background mean is often markedly slow compared to the global average. Appreciation of this issue is vital when interpreting seismic tomographic images: ‘high’ and ‘low’velocityanomaliesshouldnotnecessarilybeinterpreted,respectively,as‘fast’and‘slow’ compared to ‘normal mantle’. This issue has been discussed in the seismological literature in detail over the years, yet subsequent tomography studies have still fallen into the trap of mis-interpreting their velocity models. I highlight here some recent examples of this and provide a simple strategy to address the problem using constraints from a recent global tomographic model, and insights from catalogues of absolute traveltime anomalies. Consultation of such absolute measures of seismic wave speed should be routine during regional tomographic studies, if only for the benefit of the broader Earth Science community, who readily follow the red = hot and slow, blue = cold and fast rule of thumb when interpreting the images for themselves.

Tectonic regionalization without a priori information: A cluster analysis of upper mantle tomography

Global mantle tomography can be improved through better use of data and application of more accurate wave propagation methods. However, few techniques have been developed for objective validation and exploration of the resulting tomographic models. We show that cluster analysis can be used to validate and explore the salient features across such models. We present a cluster analysis of a global upper mantle radially anisotropic model SEMum developed using full waveform tomography and the Spectral Element Method. Applied to SEMum down to 350 km depth, the cluster analysis reveals that absolute shear wave velocity (Vs) depth profiles naturally group into families that correspond with known surface tectonics. This allows us to construct a global tectonic regionalization based solely on tomography, without the help of any a priori information. We find that the profiles of stable platforms and shields consistently exhibit a mid-lithospheric low velocity zone (LVZ) between 80 and 130 km depth, while the asthenosphere is found at depths greater than 250 km in both regions. This global intra-continental-lithosphere low velocity zone agrees with recent receiver function studies and regional tomographic studies. Furthermore, we identify an anomalous oceanic region characterized by slow shear wave speeds at depths below 150 km. Hotspots are found preferentially in the vicinity of this anomalous region. In the Pacific Ocean, where plate velocities are largest, these regions have elongated shapes that align with absolute plate motion, suggesting a relationship between the location of hotspots and small-scale convection in the oceanic upper mantle.

Three‐dimensional P wave attenuation and velocity upper mantle tomography of the southern Apennines–Calabrian Arc subduction zone

We propose a 3‐D crust–upper mantle seismic attenuation (QP) model of the southern Apennines–Calabrian Arc subduction zone together with a 3‐D velocity (VP) model. The QP model is calculated from relative t* using the spectral ratio method and the VP from traveltime data. The final data set used for the inversion of the VP model consists of 2400 traveltime arrivals recorded by 34 short‐period stations that are part of the Italian National Seismic Network, and for the QP model, 2178 Pn phases recorded by a subset of 32 stations. Traveltimes and waveforms come from 272 intermediate‐depth Calabrian slab events. This 3‐D model of attenuation, together with the 3‐D velocity model, improves our knowledge of the slab/mantle wedge structure and can be a starting point in determining the physical state of the asthenosphere (i.e., its temperature, the presence of melt and/or fluids) and its relation to volcanism found in the study area. Main features of the QP and VP models show that the mantle wedge/slab, in particular, the area of highest attenuation, is located in a volume underlying the Marsili Basin. The existence and shape of this main low‐QP (and low‐VP) anomaly points to slab dehydration and fluid/material flow, a process that may explain the strong geochemical affinities between the subduction‐related magmas from Stromboli and Vesuvius. Other interesting features in the models are strong lateral variations in QP and VP that are put in relation with known important tectonic structures and volcanic centers in the area.

Long-wavelength character of subducted slabs in the lower mantle

The high resolution seismic tomography provides the detailed information about the fate of the subducted plates in the mantle. While the slab deformation in the transition zone is quite complex and differs among the individual subduction zones, it could be generally concluded that most slabs seem to penetrate into the lower mantle. The character of the fast seismic velocity anomalies, however, changes when the slabs cross the 670 km discontinuity. While in the upper mantle tomography resolves relatively narrow plate-like structures, in the lower mantle it rather indicates thick blobs of seismically fast material. Such thickening of the downgoing slabs is traditionally attributed to the effect of the viscosity increase in the lower mantle. Here we study the deformation of the slabs in the lower mantle in a viscous model with a non-linear composite rheology including the diffusion creep, the dislocation creep and a power-law stress limiter. We perform numerical simulations of the subduction process in a 2D Cartesian domain and concentrate especially on the effect of the change of the rheological parameters across the 670 km boundary on the slab deformation in the upper part of the lower mantle. The thickening or the buckling of the plate is indeed observed in the models with relatively low yield stress (0.1 GPa) of the stress-limiting mechanism and with a viscosity increase at the 670 km discontinuity. Further, we show that both the major mantle phase transitions and the strength of the crust layer have significant consequences for the creation of the buckling instabilities in the lower mantle.

Global upper-mantle tomography with the automated multimode inversion of surface andS-wave forms

SUMMARY We apply the Automated Multimode Inversion of surface and S-wave forms to a large global data set, verify the accuracy of the method and assumptions behind it, and compute an S vvelocity model of the upper mantle (crust‐660 km). The model is constrained with ∼51 000 seismograms recorded at 368 permanent and temporary broadband seismic stations. Structure of the mantle and crust is constrained by waveform information both from the fundamentalmode Rayleigh waves (periods from 20 to 400 s) and from S and multiple S waves (higher modes). In order to enhance the validity of the path-average approximation, we implement the automated inversion of surface- and S-wave forms with a three-dimensional (3-D) reference model. Linear equations obtained from the processing of all the seismograms of the data set are inverted for seismic velocity variations also relative to a 3-D reference, in this study composed of a 3-D model of the crust and a one-dimensional (1-D), global-average depth profile in the mantle below. Waveform information is related to shear- and compressional-velocity structure within approximate waveform sensitivity areas. We use two global triangular grids of knots with approximately equal interknot spacing within each: a finely spaced grid for integration over sensitivity areas and a rougher-spaced one for the model parametrization. For the tomographic inversion we use LSQR with horizontal and vertical smoothing and norm damping. We invert for isotropic variations in S- and P-wave velocities but also allow for S-wave azimuthal anisotropy—in order to minimize errors due to possible mapping of anisotropy into isotropic heterogeneity. The lateral resolution of the resulting isotropic upper-mantle images is a few hundred kilometres, varying with data sampling. We validate the imaging technique with a ‘spectral-element’ resolution test: inverting a published global synthetic data set computed with the spectral-element method using a laterally heterogeneous mantle model we are able to reconstruct the synthetic model accurately. This test confirms both the accuracy of the implementation of the method and the validity of the JWKB and path-average approximations as applied in it. Reviewing the tomographic model, we observe that low-S v-velocity anomalies beneath mid-ocean ridges and backarc basins extend down to ∼100 km depth only, shallower than according to some previous tomographic models; this presents a close match to published estimates of primary melt production depth ranges there. In the seismic lithosphere beneath cratons, unambiguous high velocity anomalies extend to ∼200 km. Pronounced low-velocity zones beneath cratonic lithosphere are rare; where present (South America; Tanzania) they are neighboured by volcanic areas near cratonic boundaries. The images of these low-velocity zones may indicate hot material—possibly of mantle-plume origin—trapped or spreading beneath the thick cratonic lithosphere.

Asthenospheric flow and origin of volcanism in the Baikal Rift area

The origin of low-volume, hotspot-like volcanism often observed in continental rift areas is debated, as is the nature of the flow in the mantle beneath. In this paper we assemble seismic constraints on the mantle flow below the Baikal Rift Zone. We combine new evidence from upper-mantle tomography and from a radially anisotropic shear-velocity profile measured beneath and SE of the rift with published shear-wave-splitting constraints on azimuthal anisotropy. Fast directions of azimuthal anisotropy are perpendicular to the rift on both of its sides. Stable Siberian-Platform lithosphere on the northern side extends down to 200 km; on the southern side, the lithosphere is only 60–70 km thick. This asymmetry, with the thick cratonic root forming a barrier to convective flow, inhibits a mid-ocean-ridge-like flow pattern. The radially anisotropic profile yields evidence for horizontal flow in the upper asthenosphere (60–100 km) SE of the rift but no evidence for vertical flow. These results are consistent with horizontal asthenospheric flow NW to SE perpendicular to the rift, from beneath the thick Siberian towards the thin Baikal–Mongolian lithosphere. When asthenosphere ascends from the 200-km depth beneath the craton to shallower depths beneath the rift, decompression melting of embedded veins of enriched rock can produce the hotspot-like basaltic volcanism observed. The occurrence of such veins of enriched mantle in sub-cratonic asthenosphere has been predicted by geochemical mantle models; we propose that sub-horizontal asthenospheric flow from beneath cratons is a common cause for hotspot-like volcanism in continental rift zones.

Intraplate seismicity in SE Brazil: stress concentration in lithospheric thin spots

SUMMARY Intraplate seismicity has generally poor correlation with surface geological patterns. Except for major extensional features, such as aborted continental rifts, which may act as weak zones, it is usually difficult to find simple geology based models to explain differences in seismic activity in stable continental regions. Seismicity in Brazil is clearly not uniform and a few areas of higher activity have been identified. However, the seismic areas show almost no correlation with the main geological provinces, which is typical of other intraplate settings. A recent uppermantle tomography study in SE and central Brazil, using approximately 8500 P-wave and 2000 PKP-wave arrivals recorded in 59 sites since 1992, has mapped P-wave velocity anomalies from lithospheric depths down to 1300 km. In this region, higher seismic activity occurs preferentially in areas with low P-wave velocities at 150‐250 km depth. The low P-wave velocities are interpreted as shallower asthenosphere. In such areas, a hotter geotherm will reduce the strength of the lithospheric upper mantle causing most of the intraplate forces to be concentrated in the brittle upper crust. The low-velocity anomalies coincide with Late Cretaceous provinces of alkaline intrusions. The proposed ponding of the Trindade plume head beneath lithospheric thin spots is consistent with our tomography results, suggesting that plume effects may have helped to preserve lithosphere/asthenosphere topography. Although other factors are also important, the present data show that stress concentrations resulting from lithosphere/asthenosphere topography should play an important role in explaining the intraplate seismicity in the Brazilian platform.

New images of the Earth's upper mantle from measurements of surface wave phase velocity anomalies

The sensitivity of a surface wave to the lateral structure of the Earth's mantle depends on the local structure and thickness of the crust; particularly at short periods, the strong lateral heterogeneity of the crust has a significant nonlinear effect on the surface wave “sensitivity functions.” Thus far, this effect has not been quantified, and tomographic studies of the upper mantle based on surface wave observations have not taken it into account. On the basis of a Jeffreys‐Wentzel‐Kramers‐Brillouin (JWKB) description of surface wave propagation we find and discuss the lateral variations of the sensitivity of Love and Rayleigh waves to mantle seismic anomalies, associated with a realistic model of the crust [Mooney et al., 1998]. We then use the resulting laterally varying sensitivity kernels to determine a new tomographic image of horizontally and vertically polarized shear velocity in the upper mantle from Love and Rayleigh wave phase velocity measurements (periods from 35 to 300 s). In terms of large‐scale patterns of high and low velocity our model is similar to previously published ones, confirming, among other features, the anomalous anisotropy underlying the central Pacific [Ekström and Dziewonski, 1998]; we find, however, that the lateral dependence of the surface wave sensitivity kernels has a significant effect on the absolute amplitude of the anomalies, which should not be neglected as we attempt to obtain higher‐resolution images of the upper mantle.

Slow-ridge/hotspot interactions from global gravity, seismic tomography and87Sr/86Sr isotope data

SUMMARY Among the mantle hotspots present under oceanic areas, a large number are located on—or close to—active oceanic ridges. This is especially true in the slow-spreading Atlantic and Indian oceans. The recent availability of worldwide gravity grids and the increasing coverage of geochemical data sets along active spreading centres allow a fruitful comparison of these data with global geoid and seismic tomography models, and allow one to study interactions between mantle plumes and active slow-spreading ridges. The observed correlations allow us to draw preliminary conclusions on the general links between surficial processes, which shape the detailed morphology of the ridge axes, and deeper processes, active in the upper mantle below the ridge axial domains as a whole. The interactions are first studied at the scale of the Atlantic (the Mid-Atlantic Ridge from Iceland to Bouvet Island) from the correlation between the zero-age free-air gravity anomaly, which reflects the zero-age depth of the ridge axis, and Sr isotopic ratios of ridge axis basalts. The study is then extended to a more global scale (the slow ridges from Iceland to the Gulf of Aden) by including geoid and uppermantle tomography models. The interactions appear complex, ranging from the eVect of large and very productive plumes, almost totally overprinting the long-wavelength segmentation pattern of the ridge, to that of weaker hotspots, barely marking some of the observables in the ridge axial domain. Intermediate cases are observed, in which hotspots of medium activity (or whose activity has gradually decreased) located at some distance from the ridge axis produce geophysical or geochemical signals whose variation along the axis can be correlated with the geometry of the plume head in the upper mantle. Such observations tend to preclude the use of a single hotspot/ridge interaction model and stress the need for additional observations in various plume/ridge configurations.

The scales of mantle convection

Seismic, topographic and gravity data show that there are two important scales of mantle convection. These are associated with spherical harmonic degrees l =2 and 6. The l =2 pattern corresponds to the pattern of subduction cooling since the breakup of Pangea. Most hotspots and ridges occur in the half of the globe unaffected by this cooling and all large igneous provinces were generated over this part of the mantle. The l =2 distribution of upwellings and downwellings is likely to be a long-lived feature of the Earth; cold regions of the mantle repeatedly attract continents and subduction reinforces the coldness. An l =1 pattern of convection is probably related to supercontinents and their breakup. Degree 6 convection shows up in the spectrum of hotspots and upper mantle tomography and in the correlation of tomography and topography with the geoid. Cratons, with their deep cold keels, control the l =6 pattern, and may even cause it, by their role in establishing lateral temperature gradients and relief at the top of the convecting mantle. Subduction zones reinforce the craton pattern. Downwellings preferentially occur under cratons; upwellings, and hotspots, occur at complementary locations. Supercontinents, and their associated subduction zones ( l =1) constantly assemble and reassemble in the African-Atlantic hemisphere and the continental fragments, in the dispersed state, periodically settle into the polar band of geoid lows ( l =2 and 6) that now includes the Americas, Antarctica, Australia and India. If cratons control the l =6 pattern of convection (and many patterns of l =6 are possible; e.g., sectoral, zonal, checker-board) then upper-mantle convection may reorganize roughly every 30 Ma as cratons move about. The hotspot spectrum is probably related to lithospheric extension as well as to broad upwellings. A third and smaller scale of convection, order 400–1000 km in dimension, is just below the resolution of global tomography but shows up in the gravity field (geoid) and topography. This scale is controlled by the depth of an endothermic phase change which tends to stratify mantle convection, and the thickness of the upper mantle low viscosity zone. Convective domains of this dimension are also implied by the scales of chemical homogeneity, lengths of rifts, ridges and seamount chains, fracture zone spacing and mid-ocean ridge segmentation. The temperature, geochemistry and fertility of these upper-mantle scale domains is controlled by their previous history of subduction, continental insulation or refrigeration, and processing by ridges. Fertile, or volatile-rich, or hot, cells can be mistaken for plumes. Hotspot swells are typically of the dimension that we argue is a characteristic upper mantle scale rather than a deep mantle plume scale. Geochemical domains of various sizes exist in the upper mantle. They are broad-scale features, rather than point sources, as in plume theories. Lithospheric dynamics and geometric focusing, not mantle dynamics, control the dimensions of so-called hotspot eruptives.

Crust and upper mantle tomography in Tibet using surface waves

We have obtained maps of lateral variations of structure in the crust and lithosphere in Tibet and surrounding regions by inversion of single station measurements of fundamental mode Rayleigh wave phase velocities in the period range 25 ‐ 100 sec, for earthquakes located in central Asia and observed at teleseismic digital stations in Eurasia. Phase velocity maps are obtained using a tomographic method without a priori constraints on region boundaries. The lateral variations of S velocity reveal a symmetric pattern on each side of crustal low velocity zone centered on the Chang Tang region (north central Tibet). This zone is bounded by rather steep velocity gradients to the south and east, about 250 and 400 km north and west of the frontal thrusts of the Himalayas and Lungmen Shan, respectively. The northern boundary of the zone crosses the northern limit of the Tibet plateau (Altyn Tagh and Kunlun) extending into the southwest Tarim and Qaidam basins. The low crustal velocities are consistent with a thick, hot crust in the central part of Tibet, consistent with Quaternary volcanism in Chang Tang. The steep gradients towards higher velocities in the south and in the north are compatible with the documented continental subduction in the south, and suggest that a similar situation might exist in the north.