Mantle Shear Wave Velocity Structure Research Articles

Plume-lithosphere interaction beneath southwestern Africa – Insights from multi-mode Rayleigh wave tomography

A stationary mantle plume heats the overlying lithosphere from below, generating melts and causing lithospheric erosion. When the lithosphere moves away from the plume, it can be re-established by melt residues that have been since attached, partly due to cooling, at the base of the lithosphere. The Etendeka flood basalt region in northwest Namibia used to overlie the Tristan da Cunha mantle plume in the Early Cretaceous and thus provides an ideal place to examine plume-lithosphere interaction. Here we determine the upper mantle shear wave velocity structure of southern Africa down to 400 km depth by waveform inversion of multi-mode Rayleigh waves in order to find the imprints left by the Tristan da Cunha mantle plume. Thick lithosphere with high shear wave velocities is observed beneath the Congo and Kalahari Cratons, extending down to 200 km depth with the largest thickness beneath the Limpopo Belt. Along the landfall of the Walvis Ridge and Damara Belt, our model reveals a thick lithosphere down to 100–200 km depths. The thick lithosphere seems to constitute the Congo Craton, but the thick Congo cratonic lithosphere is extended farther south than observed in most of the previous models. We propose that the lithosphere along the landward extension of the Walvis Ridge was affected by magmatic processes related to the plume, but has been partially reconstructed since then by melt depletion and successive cooling. A high velocity anomaly beneath the northwest coast of Namibia down to a depth of ∼80 km coincides with distribution of the Etendeka basalt at the surface and the seaward-dipping reflectors at the continental margin. At sub-lithospheric depths, a low-velocity anomaly (at 100–200 km depth) and a high-velocity anomaly (at 250–350 km depth) can be detected. The origin of these anomalies could be related to an ongoing edge-driven convection process.

Upper Mantle Shear Wave Velocity Structure of Southeastern China: Seismic Evidence for Magma Activities in the Late Mesozoic to the Cenozoic

AbstractThe Fujian‐Taiwan Strait‐Taiwan region, as the active eastern margin of Eurasia since the Mesozoic, has experienced the subduction and retreat of the Paleo‐Pacific plate from the Middle Mesozoic, the breakup of the continental margin in the early Cenozoic, and then the collision with the Philippine Sea plate in the late Cenozoic. To better understand the tectonic evolution process, a high‐resolution 3‐D shear wave velocity model of the crust and upper mantle was obtained from surface wave tomography through a direct inversion method. Intermediate‐ to long‐period Rayleigh wave dispersion curves, extracted from teleseismic surface wave with the traditional two‐station method, were combined with the short‐period dispersion curves obtained from ambient noise cross correlation to obtain broader band surface wave data. Two groups of large‐scale low‐velocity bodies are found to be located in the upper mantle of Fujian and the Taiwan Strait and are inferred to be closely related to the magmatic activities caused by the Mesozoic subduction and Cenozoic extension, respectively. The low‐velocity anomalies in Fujian are concentrated in the west Cathaysia block in deep regions and split into small bodies in the uppermost mantle, while the low‐velocity body in the strait is divided into northern and southern parts, corresponding to the two‐stage Cenozoic extension. The velocity gradient was also calculated to estimate the lithospheric thickness, which is only 60–80 km, and the lithosphere‐asthenosphere boundary also features strongly horizontal variations and discontinuities. Our results provide important seismological constraints to investigate the geological and tectonic processes of this region.

Crustal and uppermost mantle shear wave velocity structure beneath the Middle East from surface wave tomography

SUMMARY We have constructed a 3-D shear wave velocity (Vs) model for the crust and uppermost mantle beneath the Middle East using Rayleigh wave records obtained from ambient-noise cross-correlations and regional earthquakes. We combined one decade of data collected from 852 permanent and temporary broad-band stations in the region to calculate group-velocity dispersion curves. A compilation of >54 000 ray paths provides reliable group-velocity measurements for periods between 2 and 150 s. Path-averaged group velocities calculated at different periods were inverted for 2-D group-velocity maps. To overcome the problem of heterogeneous ray coverage, we used an adaptive grid parametrization for the group-velocity tomographic inversion. We then sample the period-dependent group-velocity field at each cell of a predefined grid to generate 1-D group-velocity dispersion curves, which are subsequently inverted for 1-D Vs models beneath each cell and combined to approximate the 3-D Vs structure of the area. The Vs model shows low velocities at shallow depths (5–10 km) beneath the Mesopotamian foredeep, South Caspian Basin, eastern Mediterranean and the Black Sea, in coincidence with deep sedimentary basins. Shallow high-velocity anomalies are observed in regions such as the Arabian Shield, Anatolian Plateau and Central Iran, which are dominated by widespread magmatic exposures. In the 10–20 km depth range, we find evidence for a band of high velocities (>4.0 km s–1) along the southern Red Sea and Arabian Shield, indicating the presence of upper mantle rocks. Our 3-D velocity model exhibits high velocities in the depth range of 30–50 km beneath western Arabia, eastern Mediterranean, Central Iranian Block, South Caspian Basin and the Black Sea, possibly indicating a relatively thin crust. In contrast, the Zagros mountain range, the Sanandaj-Sirjan metamorphic zone in western central Iran, the easternmost Anatolian plateau and Lesser Caucasus are characterized by low velocities at these depths. Some of these anomalies may be related to thick crustal roots that support the high topography of these regions. In the upper mantle depth range, high-velocity anomalies are obtained beneath the Arabian Platform, southern Zagros, Persian Gulf and the eastern Mediterranean, in contrast to low velocities beneath the Red Sea, Arabian Shield, Afar depression, eastern Turkey and Lut Block in eastern Iran. Our Vs model may be used as a new reference crustal model for the Middle East in a broad range of future studies.

Crustal and Upper Mantle Shear Wave Velocity Structure of Botswana: The 3 April 2017 Central Botswana Earthquake Linked to the East African Rift System

AbstractRayleigh wave group and phase velocity measurements obtained from ambient noise and earthquake data at 51 broadband stations were used to construct the first 3‐D crustal and upper mantle shear wave velocity model of Botswana. The model shows low crustal velocities associated with the Passarge and Nosop sedimentary basins, whereas the Kaapvaal, Zimbabwe, Maltahohe, and Congo Cratons are recognized by high mantle velocities. The lowest upper mantle shear wave velocity, beneath northeastern Botswana, is associated with the southwestern branch of the East African Rift System. This low‐velocity mantle anomaly appears to be linked to the crust of the Okavango Rift Zone and the location of the 3 April 2017 6.5 earthquake in central Botswana. We suggest that fluids or melt at the base of the crust from the southward continuation of the East African Rift Zone triggered the intraplate earthquake in an extensional tectonic setting.

Seismological evidence of the Hales discontinuity in northeast India

The crust and upper mantle shear wave velocity structure beneath the northeast India is estimated by joint inversion of Rayleigh wave group velocity and receiver function, calculated from teleseismic earthquakes data recorded at nine broadband seismic stations. The Assam valley and the Shillong-Mikir plateau are the two important tectonic blocks in the northeast India, which are surrounded by the Himalayan collision zone in the north, Indo-Burma subduction zone in the east and by the Bengal basin in the south. The joint inversion followed by forward modeling reveal crustal thicknesses of 30–34 km beneath the Shillong plateau, 36 km beneath the Mikir hills and 38–40 km beneath the Assam valley with an average shear wave velocity (Vs) of 3.4–3.5 km/s. The estimated low upper mantle shear wave velocity (Vsn) 4.2–4.3 km/s may be due to the rock composition or grain size or increased temperature and partial melt (<1%) in the upper mantle, or an effect of all. Also, we report for the first time, the existence of the Hales discontinuity at depths 56–74 km with Vs ∼4.4–4.6 km/s. Variable depth of the Hales discontinuity may be explained by the geotherm and/or addition of Cr3+ and Fe2+ in the spinel-garnet system.

Seismic evidence of crustal heterogeneity beneath the northwestern Deccan volcanic province of India from joint inversion of Rayleigh wave dispersion measurements and P receiver functions

The northwestern Deccan volcanic province (NWDVP) of India, encompassing the Saurashtra peninsula and the adjoining Gulf of Cambay, is investigated through joint inversion of surface wave dispersion measurements and teleseismic P receiver functions, to estimate the crustal and shallow upper mantle shear wave velocity (Vs) structure. The Mw∼7.7 Bhuj earthquake and the post Bhuj regional events, recorded during the period 2001–2010 at 7 stations along 37 source-receiver paths were used along with 35 teleseismic events. A joint curve fitting inversion technique is applied to obtain a best fit for the fundamental mode Rayleigh wave group velocity dispersion curves for time periods 5–50s and high quality crustal P wave receiver functions obtained at each station. Significant crustal heterogeneity is observed within the study region with the average crustal Vs ranging from 3.5km/s to 3.8km/s with the paths cutting across the Gulf of Cambay exhibiting large reduction in shear wave velocities. Utilizing the average crustal Vs≈3.66km/s estimated for Saurashtra, together with the average crustal P wave velocity (Vp)≈6.54km/s derived independently through deep seismic sounding studies, yields a bulk Vp/Vs ratio of 1.786 or an equivalent crustal Poisson’s ratio of 0.271. A major contribution to the high Poisson’s ratio comes from the 12 to 16km thick lower crustal layers with shear velocities ranging from 3.8km/s to 4.19km/s suggesting widespread magmatic underplating due to emplacement of mafic cumulates in the lower crust. The shallow uppermost mantle shear velocities are in the range 4.2–4.5km/s averaging 4.36km/s, which is less than that observed for the Indian shield, indicating the effects of residual thermal anomaly. The variation in the crustal Vs, high Poisson’s ratios and low upper mantle shear velocities reflect the thermal and compositional effects of the Deccan volcanism which are manifested in terms of pervasive presence of mafic dykes, volcano plutonic complexes, lower crustal mafic cumulates and possible partial melt due to elevated geotherms. The significant crustal alterations are possibly a result of weakening of the lithosphere by a mantle thermal source, leading to a thin and weak lithosphere which facilitated the Deccan volcanism through the rift zones of northwestern India.

Upper mantle shear wave velocity structure beneath northern Victoria Land, Antarctica: Volcanism and uplift in the northern Transantarctic Mountains

The Transantarctic Mountains (TAMs) are the largest non-compressional mountain range on Earth, and while a variety of uplift mechanisms have been proposed, the origin of the TAMs is still a matter of great debate. Most previous seismic investigations of the TAMs have focused on a central portion of the mountain range, near Ross Island, providing little along-strike constraint on the upper mantle structure, which is needed to better assess competing uplift models. Using data recorded by the recently deployed Transantarctic Mountains Northern Network, as well as data from the Transantarctic Mountains Seismic Experiment and from five stations operated by the Korea Polar Research Institute, we investigate the upper mantle structure beneath a previously unexplored portion of the mountain range. Rayleigh wave phase velocities are calculated using a two-plane wave approximation and are inverted for shear wave velocity structure. Our model shows a low velocity zone (LVZ; ∼4.24 km s−1) at ∼160 km depth offshore and adjacent to Mt. Melbourne. This LVZ extends inland and vertically upwards, with more lateral coverage above ∼100 km depth beneath the northern TAMs and Victoria Land. A prominent LVZ (∼4.16–4.24 km s−1) also exists at ∼150 km depth beneath Ross Island, which agrees with previous results in the TAMs near the McMurdo Dry Valleys, and relatively slow velocities (∼4.24–4.32 km s−1) along the Terror Rift connect the low velocity anomalies. We propose that the LVZs reflect rift-related decompression melting and provide thermally buoyant support for the TAMs uplift, consistent with proposed flexural models. We also suggest that heating, and hence uplift, along the mountain front is not uniform and that the shallower LVZ beneath northern Victoria Land provides greater thermal support, leading to higher bedrock topography in the northern TAMs. Young (0–15 Ma) volcanic rocks associated with the Hallett and the Erebus Volcanic Provinces are situated directly above the imaged LVZs, suggesting that these anomalies are also the source of Cenozoic volcanic rocks throughout the study area.

Rayleigh wave dispersion measurements reveal low‐velocity zones beneath the new crust in the Gulf of California

AbstractRayleigh wave tomography provides images of the shallow mantle shear wave velocity structure beneath the Gulf of California. Low‐velocity zones (LVZs) are found on axis between 26 and 50 km depth beneath the Guaymas Basin but mostly off axis under the other rift basins, with the largest feature underlying the Ballenas Transform Fault. We interpret the broadly distributed LVZs as regions of partial melting in a solid mantle matrix. The pathway for melt migration and focusing is more complex than an axis‐centered source aligned above a deeper region of mantle melt and likely reflects the magmatic evolution of rift segments. We also consider the existence of solid lower continental crust in the Gulf north of the Guaymas Basin, where the association of the LVZs with asthenospheric upwelling suggests lateral flow assisted by a heat source. These results provide key constraints for numerical models of mantle upwelling and melt focusing in this young oblique rift.

Upper mantleSwave velocity structure of the East Anatolian-Caucasus region

Geodynamic processes occurring in the upper mantle such as slab break off and lithosphere delamination often result in high rates of lithospheric deformation and rapid tectonic uplift of large areas. The continent-continent collision zone between Arabia and Eurasia has been widely studied in this context, but several different viable geodynamic models exist to explain the uplift and deformation of the Anatolian Plateau and the Caucasus Mountains. We have imaged the uppermost mantle shear wave velocity structure of the East Anatolian-Caucasus region using surface wave tomography to better understand the regional tectonic activity since the onset of the collision between the Arabian and Eurasian Plates. Furthermore, we used our tomographic models to better understand the processes, which are responsible for the formation of the 2 km high plateau and the widespread volcanism in eastern Turkey, as well as reactivation of deformation and deep seismicity in the eastern Greater Caucasus. Our model of regional upper mantle shear wave velocity structure supports subduction of the northern and southern branches of Neo-Tethys lithosphere between Eurasia and Gondwana and suggests a possible underthrusting of the Kura Basin lithosphere beneath the Greater Caucasus.

The uppermost mantle shear wave velocity structure of eastern Africa from Rayleigh wave tomography: constraints on rift evolution

An expanded model of the 3-D shear wave velocity structure of the uppermost mantle beneath eastern Africa has been developed using earthquakes recorded by the AfricaArray East African Seismic Experiment in conjunction with data from permanent stations and previously deployed temporary stations. The combined data set comprises 331 earthquakes recorded on a total of 95 seismic stations spanning Kenya, Uganda, Tanzania, Zambia and Malawi. In this study, data from 149 earthquakes were used to determine fundamental-mode Rayleigh wave phase velocities at periods ranging from 20 to 182 s using the two-plane wave method, and then combined with the similarly processed published measurements and inverted for a 3-D shear wave velocity model of the uppermost mantle. New features in the model include (1) a low-velocity region in western Zambia, (2) a high-velocity region in eastern Zambia, (3) a low-velocity region in eastern Tanzania and (4) low-velocity regions beneath the Lake Malawi rift. When considered in conjunction with mapped seismicity, these results support a secondary western rift branch striking southwestwards from Lake Tanganyika, likely exploiting the relatively weak lithosphere of the southern Kibaran Belt between the Bangweulu Block and the Congo Craton. We estimate a lithospheric thickness of ∼150–200 km for the substantial fast shear wave anomaly imaged in eastern Zambia, which may be a southward subsurface extension of the Bangweulu Block. The low-velocity region in eastern Tanzania suggests that the eastern rift branch trends southeastwards offshore eastern Tanzania coincident with the purported location of the northern margin of the proposed Ruvuma microplate. Pronounced velocity lows along the Lake Malawi rift are found beneath the northern and southern ends of the lake, but not beneath the central portion of the lake.

Upper mantle shear wave velocity structure beneath the East African plateau: evidence for a deep, plateauwide low velocity anomaly

SUMMARY The shear wave velocity structure of the upper mantle beneath the East African plateau has been investigated using teleseismic surface waves recorded on new broadband seismic stations deployed in Uganda and Tanzania, as well as on previously deployed stations in Tanzania and Kenya. Rayleigh wave phase velocities at periods between 20 and 182 s, measured with a two-plane wave method, have been used to create phase velocity maps, and dispersion curves extracted from the maps have been inverted to obtain a quasi-3-D shear wave velocity model of the upper mantle. We find that phase velocities beneath the Tanzania Craton and areas directly north and west of the craton are faster, at all periods, than those beneath the Western and Eastern branches of the East African Rift System. At periods 400 K. Consequently, it is likely that the velocity reduction reflects a combination of thermal and compositional changes, and also possibly the presence of partial melt. The width and thickness of the low velocity anomaly is greater than typically expected for a plume head and is more easily attributed to an upward continuation of the lower mantle African superplume structure into the upper mantle.

Seismic velocity structure and depth‐dependence of anisotropy in the Red Sea and Arabian shield from surface wave analysis

We investigate the lithospheric and upper mantle shear wave velocity structure and the depth‐dependence of anisotropy along the Red Sea and beneath the Arabian Peninsula using receiver function constraints and phase velocities of surface waves traversing two transects of stations from the Saudi Arabian National Digital Seismic Network. Frequency‐dependent phase delays of fundamental‐mode Love and Rayleigh waves, measured using a cross‐correlation procedure, require very slow shear velocities and the presence of anisotropy to depths of at least 180 km in the upper mantle. Linearized inversion of these data produce path‐averaged 1D radially anisotropic models with ∼4% anisotropy in the lithosphere and across the lithosphere‐asthenosphere boundary (LAB). Models with reasonable crustal velocities in which the mantle lithosphere is isotropic cannot satisfy the data. The lithosphere, which ranges in thickness from about 70 km near the Red Sea coast to about 90 km beneath the Arabian Shield, is underlain by a pronounced low‐velocity zone with shear velocities as low as 4.1 km/s. Forward models of azimuthal anisotropy, which are constructed from previously determined shear wave splitting estimates, can reconcile surface and body wave observations of anisotropy. The low shear velocities extend to greater depth than those observed in other continental rift and oceanic ridge environments. The depth extent of these low velocities combined with the sharp velocity contrast across the LAB may indicate the influence of the Afar hot spot and the presence of partial melt beneath Arabia. The anisotropic signature primarily reflects a combination of plate‐ and density‐driven flow associated with rifting processes in the Red Sea.

Small‐scale upper mantle convection and crustal dynamics in southern California

We present numerical modeling of the forces acting on the base of the crust caused by small‐scale convection of the upper mantle in southern California. Three‐dimensional upper mantle shear wave velocity structure is mapped to three‐dimensional density structure that is used to load a finite element model of instantaneous upper mantle flow with respect to a rigid crust, providing an estimate of the tractions acting on the base of the crust. Upwelling beneath the southern Walker Lane Belt and Salton Trough region and downwelling beneath the southern Great Valley and eastern and western Transverse Ranges dominate the upper mantle flow and resulting crustal tractions. Divergent horizontal and upward directed vertical tractions create a tensional to transtensional crustal stress state in the Walker Lane Belt and Salton Trough, consistent with transtensional tectonics in these areas. Convergent horizontal and downward directed vertical tractions in the Transverse Ranges cause approximately N–S crustal compression, consistent with active shortening and transpressional deformation near the “Big Bend” of the San Andreas fault. Model predictions of crustal dilatation and the forces acting on the Mojave block compare favorably with observations suggesting that small‐scale upper mantle convection provides an important contribution to the sum of forces driving transpressional crustal deformation in southern California. Accordingly, the obliquity of the San Andreas fault with respect to plate motions may be considered a consequence, rather than a cause, of contractional deformation in the Transverse Ranges, itself driven by downwelling in the upper mantle superimposed on shear deformation caused by relative Pacific–North American plate motion.

Surface wave tomography of the upper mantle beneath the Reykjanes Ridge with implications for ridge–hot spot interaction

We modeled fundamental mode Love and Rayleigh waves to study the seismic properties of the upper mantle beneath the Reykjanes Ridge. These waves were generated by regional earthquakes occurring in the North Atlantic to the south of Iceland and were recorded by stations located on Iceland. Over 12,000 measurements of the phase, group arrival time, and amplitude of narrow‐pass‐filtered waveforms (over the period range of 9.5–100 s) were used to solve for mantle shear wave velocity structure and anisotropy. In a vertical plane oriented normal to the ridge axis, the velocity structure contains a broad and deep low‐velocity zone in the upper mantle beneath the ridge. A joint analysis of the seismic structure with gravity data reveals that the low velocities are consistent with elevated temperatures (∼75°). Our study shows that plume material spreads broadly outward beneath the Reykjanes Ridge from Iceland and is not confined to a narrow lithospheric channel. At distances &gt;200 km from the ridge, shear wave anisotropy indicates a predominant horizontal alignment of the fast axes of anisotropic crystals (mainly above 50 km depth), which can be interpreted as past, horizontal, ridge‐perpendicular flow. Within ±200 km of the ridge the anisotropy indicates a general vertical alignment of the fast axes or an alignment such that the fast axes point along the ridge. The transition to this type of anisotropy coincides with the appearance of increased hot spot–ridge interaction ∼20 Myr ago, indicating that plume flow has largely disrupted mantle flow beneath the ridge since that time.

Spectra of mantle shear wave velocity structure

SUMMARY We applied the stochastic method of Gudmundsson, Davies & Clayton (1990) (which was applied to ISC P-wave data) to teleseismic ISC S-wave data to obtain an independent estimate of mantle structure. We inverted the variance of S-wave traveltime residuals of bundles of rays to obtain a description of the spectrum of lateral heterogeneity as a function of depth through the mantle. The technique yields robust estimates of the traveltime scattering power (the product of a characteristic scalelength of heterogeneity and the mean square of slowness perturbations). We can estimate the characteristic scalelength (half-width), from the autocovariance; which can be reconstructed from the spectra. Hence by division, we can estimate the root mean square slowness. By extrapolating the variance of bundles of rays to bundles of zero cross-sectional area we can also estimate the scale-incoherent signal (which is a plausible estimate of the noise in the data), which is removed from the data. We find that most of the structure generating shear wave traveltime residuals is located in the uppermost mantle. About half of the structure is short scale (harmonic degree 1 > 50). The large-scale structure (1 < 50) has a half-width of about 500 km in the upper half of the mantle. This S-wave half-width is consistent with the P-wave half-widths determined by Gudmundsson et al. (1990). The S-wave half-width in the lower half of the mantle is poorly constrained. It varies from 500 to 3000km, which spans the better constrained value of 1200 km found by Gudmundsson et al. (1990) for P-waves. The incoherent scatter suggests that the signal-to-noise ratio of the S-wave data set is around 1.5. Assuming that the compressional and shear wave velocity variations are correlated then the signal weighted value of the ratio d In (y.)ld In (y,) is =2, as also found in normal mode studies. This is much larger than the value of ~0.8-1.4 suggested by laboratory experiments undertaken at atmospheric pressure. There is no evidence of periodicity in the traveltime autocovariance; this suggests little or no periodicity in the underlying convection. The short half-width through most of the mantle suggests high Rayleigh number convection, with its attendant small-scale structures. The power decreases by an order of magnitude or more in going from the upper mantle to the lower mantle, the same as found by Gudmundsson et al. (1990) for P-waves. This large difference suggests either a change in convective regime and/or a difference in the temperature sensitivity of elastic constants in both layers. The increased short-scale structure at the top of the mantle suggests that a large part of the seismic signature at this boundary is compositional, since one would expect a red spectrum for a thermal boundary layer. The derived spectra between 1 = 10 and I = 50 are similar in shape to spectra from the mantle convection simulations of Glatzmaier (1988) with a Rayleigh number of 106-107, which would suggest layered convection, if the comparison is valid.

Comparison of the upper mantle shear wave velocity structure of the Indian Shield and the Tibetan Plateau and tectonic implications

The upper mantle velocities of both the Indian Shield and Tibetan Plateau are constrained to a depth of 400 km by means of the travel times and waveforms of long-period SH waves recorded at distances of 10-30 deg, as well as some SS waveforms. S-wave travel times and waveforms, however, indicate that mean velocity at 70-250 km below the central and northern Tibetan Plateau is 4 percent or more slower than that below the Indian Shield. These results indicate that the structure of Tibet is not that of a shield, and imply that the structure of Tibet is not that of a shield, and imply that the Indian plate is not underthrusting the whole of the Tibetan Plateau at the present time.