Upper-mantle Discontinuities Research Articles

Thermochemistry of the Mantle Transition Zone Beneath the Western Pacific

AbstractThe Earth's mantle transition zone has significant control on material flux between upper and lower mantle, thus constraining its properties is imperative to understand dynamic processes and circulation patterns. Global seismic data sets to study the transition zone typically display highly uneven spatial distribution. Therefore, complementary geometries are essential to improve knowledge of physical structures, thermochemistry, and impact on convection. Here, we present a new automated approach utilizing machine learning to analyze large seismic data sets, and derive high‐resolution maps of transition zone discontinuity properties. Seismic measurements from ScSScS precursors are integrated with mineralogical modeling to constrain thermochemistry of the western Pacific subduction zone. Our models map recent subduction patterns through the transition zone, indicating stagnation of slabs and accumulation of basalt at its base, and interaction between stagnant slabs and plumes. These results suggest that the thermochemical properties of upper mantle discontinuities can provide high‐resolution images of mantle circulation patterns.

The mapping of shallow upper mantle discontinuities using teleseismic P-wave autocorrelation

The mapping of shallow upper mantle discontinuities using teleseismic P-wave autocorrelation

On the detection of upper mantle discontinuities with radon-transformed receiver functions (CRISP-RF)

SUMMARYSeismic interrogation of the upper mantle from the base of the crust to the top of the mantle transition zone has revealed discontinuities that are variable in space, depth, lateral extent, amplitude and lack a unified explanation for their origin. Improved constraints on the detectability and properties of mantle discontinuities can be obtained with P-to-S receiver function (Ps-RF) where energy scatters from P to S as seismic waves propagate across discontinuities of interest. However, due to the interference of crustal multiples, uppermost mantle discontinuities are more commonly imaged with lower resolution S-to-P receiver function (Sp-RF). In this study, a new method called CRISP-RF (Clean Receiver-function Imaging using SParse Radon Filters) is proposed, which incorporates ideas from compressive sensing and model-based image reconstruction. The central idea involves applying a sparse Radon transform to effectively decompose the Ps-RF into its underlying wavefield contributions, that is direct conversions, multiples, and noise, based on the phase moveout and coherence. A masking filter is then designed and applied to create a multiple-free and denoised Ps-RF. We demonstrate, using synthetic experiment, that our implementation of the Radon transform using a sparsity-promoting regularization outperforms the conventional least-squares methods and can effectively isolate direct Ps conversions. We further apply the CRISP-RF workflow on real data, including single station data on cratons, common-conversion-point stack at continental margins and seismic data from ocean islands. The application of CRISP-RF to global data sets will advance our understanding of the enigmatic origins of the upper mantle discontinuities like the ubiquitous mid-lithospheric discontinuity and the elusive X-discontinuity.

Alteration in the mantle transition zone structure beneath Sikkim and adjoining Himalaya in response to the Indian plate subduction

We map the depth variations of 410 km and 660 km discontinuities (d410 and d660) beneath the Sikkim Himalaya and adjoining regions using 18816 P receiver functions. The average variation of d410 and d660 for the entire study region is found to be 395±4 km and 652±5 km, respectively, with the mantle transition zone (MTZ) thickness being ∼255 km. A major upliftment in the upper mantle discontinuities is seen in the southeastern portion of the study region close to Shillong plateau, where the average depth of d410 and d660 is 373±4 km and 648±6 km, respectively. Overall, 90% of the grids have an uplifted transition zone. An upliftment of ∼20 km in d410 is observed beneath the central Nepal Himalaya and east of the Bhutan Himalaya. For the remaining grids, the d410 is elevated by ∼10 km. For d660, ∼15 km of elevated topography is observed in Central Nepal Himalaya and the northern part of Arunachal Himalaya, while the remaining areas have a relatively flat d660. The upliftment in the upper mantle discontinuities reflects presence of a thick Indian lithosphere beneath the study region. Furthermore, clear positive and negative conversions are observed between 450 km and 550 km depths, reflecting a highly heterogeneous nature of the MTZ. In addition, strong negative amplitude conversions corresponding to ∼350 km depth suggest presence of low velocity zones in the northernmost parts of Himalaya and southern Tibet. The low velocity zones may reflect compositional changes in response to the relict slab dynamics.

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

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

Ambient noise autocorrelation scheme for imaging theP-wave reflectivity of the lithosphere

SUMMARYAmbient noise autocorrelations can be used to reconstruct the seismic reflection response of the Earth structure beneath single stations using continuous recordings without the need for either active sources or earthquakes. In the last decade, this technique has emerged as an inexpensive approach with the potential to provide similar information to that from the classical receiver function (RF) analysis. Previous studies have located and mapped discontinuities at different crustal depths with ambient noise autocorrelations by applying different processing techniques. An ambient noise autocorrelation function (ACF) provides the body-wave reflectivity of the local structure, assuming a homogeneous distribution of noise sources. However, an effective method design is required in order to determine a reliable reflection response. Here, we review the theory behind the ambient noise ACF method and design a workflow to obtain the P-wave reflectivity with a special focus on the Moho depth. In particular, we calculate a smooth function to fit and subtract the zero-lag component in the time domain, that is, the large-amplitude signal near 0 lag time in the ACF. The zero-lag component can interfere with the reflection component, so its removal allows us to increase the frequency band to use. We bandpass filter the ACFs between 1 and 6 s. We also derive and apply a phase shift correction in the ACFs due to the integration of a homogeneously distributed noise field dominated by distant sources from deep below, such as teleseismic sources. Both linear and nonlinear, phase-weighted stacks are used. Linear stacking is used to identify the main interfaces since it ensures the linearity of the processing steps; nevertheless, nonlinear, phase-weighted stacking help validate the coherent signals. We test and apply our method to continuous vertical recordings from three stations in Ireland and five stations in different cratons and obtain clear P-wave reflection from the Moho and other crustal and upper-mantle discontinuities in most cases. However, noise coming from local heterogeneities, non-homogeneous distribution of the ambient noise sources or instrumental noise is also expected. Therefore, additional, a priori information is desirable to help identify key phases in single ACFs. We compute synthetic ACFs using P-wave velocity (VP) models from controlled-source profiles in Ireland. The relatively complex ACF traces obtained at the stations in Ireland show a close data-synthetic match for the Moho and mid-crustal discontinuities. The ACF traces from the stations in different cratons are directly compared with receiver functions showing overall agreement and offering complementary information on the origin of the signal.

Variable source depth beneath the Indian Ocean geoid low area: Insights from L1 and L2 norm-based scaling power spectrum inversion

Indian Ocean geoid low, situated at the south of the Indian peninsula, is an intriguing feature. Hitherto, several interpretations have been put forth to explain the genesis and causative depths of the anomaly. In this work, we investigate the scaling spectral inversion approach utilizing gravity data to estimate the causative depth and nature of the source from this region. In practice, crustal or sub-surface heterogeneities follow scaling or fractal characteristics, and comparative studies across various regions suggest that the scaling power spectrum method provides better insights into the sub-surface source distribution. An optimum fitting and comparison between the modeled and observed power spectrum have been attained by utilizing two norm penalties e.g. L1 and L2 norms for various combinations of depth and scaling exponents. Equally important, the L1 norm misfit function yields better results when compared to the L2 norm function. We propose that estimated source depths (̴80 km and 120 km, respectively) are perfect examples of a half-space model with a source following scaling or fractal characteristics (scaling exponent value of ̴1.78) along the upper mantle boundary. However, the scaling spectrum windowing analysis of the studied region reveals a variable depth to long wavelength sources as 1014 km, 431 km, and 94 km, which conform well with the recently published depth range. Further, the weight of low and high wavenumbers indicates that the causative source depths might be due to the above-stated depth range rather than a single model fit. The study has demonstrated a good case for wider use of L1 norm-based power spectrum inversion for source depth estimation from potential field data. We further suggest that the causative source for the Indian Ocean geoid low may extend deeper than upper mantle discontinuities.

Imprints of subducted Palaeo-Tethys oceanic lithosphere on upper-mantle discontinuities and the formation of the Emeishan large igneous province

SUMMARYGenerally, the upwelling of a mantle plume is suggested to have led to the formation of the Emeishan large igneous province (ELIP). However, this idea has been challenged by recent geological and seismological studies. In this investigation, three profiles of velocity perturbations crossing the ELIP are drawn following previous tomographic studies, and common conversion point (CCP) stacking of receiver functions in the ELIP is carried out. A slab-like high-velocity structure is revealed, which may be associated with the lithosphere of the Palaeo-Tethys Ocean (LPTO) subducting toward the northeast. CCP stacking of receiver functions shows that the LPTO generated imprints on the 410- and 660-km discontinuities. A comprehensive analysis suggests that the LPTO may have induced large-scale mantle upwelling, which contributed to the formation of the ELIP.

Regularizing the 3-D teleseismic wavefield for receiver function imaging using a radial basis function

SUMMARY Receiver function is important for imaging crustal and upper-mantle discontinuities. However, sparsely scattered stations could introduce imaging artefacts or misinterpretations for complex structures. We regularize a 3-D teleseismic wavefield to reduce these artefacts using radial basis function interpolation. First, we evaluated the feasibility of wavefield regularization with several typical models using synthetic data. The results demonstrate the high reliability of our method for recovering local 2-D and 3-D structures, even when seismic stations intentionally miss 95 per cent of a uniform fine grid. Then, we applied this method to sparsely deployed stations in Northeast China. The waveforms reconstructed from surrounding stations show good consistency with the observed waveforms; furthermore, the imaging results using the regularized data are highly comparable with the reference results obtained by using a dense 2-D seismic array of 60 stations (with a spacing of 10–17 km), even though our input data were mainly contributed by only 9 stations (with an average spacing of ∼80 km). Our results show better continuity of the 3-D topography of the subsurface compared with that obtained by the traditional method. Our regularization method could significantly improve the spatial resolution of receiver function imaging for both sparse and dense distributions of seismic stations, especially for imaging relatively complex structures with lateral variations.

Weak and vanishing upper mantle discontinuities generated by large-scale lithospheric delamination in the Longmenshan area, China

A large amount of high-quality teleseismic data is used for common conversion point (CCP) stacking of receiver functions in the Longmenshan area and near-by region. The results show that a large-scale high-velocity anomaly (LHVA) or lithospheric delamination can modify the structure of upper mantle discontinuities or weaken the phase boundary of olivine, which is a very important finding and can be used to assess stagnant slabs in the mantle transition zone globally. The deepening region of the 660 km discontinuity beneath the Songpan-Ganzi terrane identified in this study might be generated by the LHVA.

Detecting lithospheric discontinuities beneath the Mississippi Embayment using S-wave receiver functions

SUMMARY Identifying upper-mantle discontinuities in the Central and Eastern US is crucial for verifying models of lithospheric thinning and a low-velocity anomaly structure beneath the Mississippi Embayment. In this study, S-wave receiver functions (SRFs) were used to detect lithospheric boundaries in the embayment region. The viability of SRFs in detecting seismic boundaries was tested before computing them using the earthquake data. A careful analysis using a stochastic noise and coda model on the synthetics revealed that a negative velocity contrast could be detected with certainty at low to moderate noise levels after stacking. A total of 31 518 SRFs from 688 earthquakes recorded at 174 seismic stations including the Northern Embayment Lithospheric Experiment, EarthScope Transportable Array and other permanent networks were used in this study. Common depth point stacks of the SRFs in 1° × 1° bins indicated a continuous and broad S-to-P converted phase (Sp) arrival corresponding to a negative velocity contrast at depths between 50 and 100 km. The observed negative Sp phase is interpreted as a mid-lithospheric discontinuity (MLD), and several possible origins of the velocity drop corresponding to the MLD are explored. After quantitative analysis, a combination of temperature, water content and melt content variations are attributed to explain the observed MLD in this study. The observations and interpretations in this study support the previous claims of an MLD in the Central and Eastern US and provide a possible mechanism for its origin.

Mantle Upwelling Beneath the Cathaysia Block, South China

AbstractA large low‐velocity anomaly (LLVA) beneath the Cathaysia Block in South China that may be associated with mantle upwelling was revealed by seismic tomography. However, seismological and geological studies indicate that ascending mantle must have a higher temperature than the surrounding mantle, and a low‐velocity anomaly alone is not indicative of mantle upwelling. The receiver function technique has been used as a robust tool to characterize the topographic relief of the upper mantle discontinuities, which are closely related to temperature variations and mantle upwelling. In this study, we extracted 25,187 receiver functions and performed common‐conversion‐point stacking of those receiver functions in South China and across the location of the LLVA beneath the Cathaysia Block. The results show that the 410 km discontinuity is depressed and that the 660 km discontinuity is elevated at the location of the LLVA. The structure of the mantle transition zone seems to be the passage of an upwelling mantle plume, which implies that the LLVA is a signature of mantle upwelling.

Seismic images of crust and upper mantle in Central Iran (from Jiroft to Ashtian) via teleseismic waves

A detailed knowledge of the thickness of crust and upper mantle structure is important for understanding a plate tectonics and geodynamics in the region. We use body wave for detecting details of the subsurface structure. The information in this research is collected from a seismic linear profile that extends across the Sanandaj-Sirjan metamorphic zone in seismic states of Central Iran and Zagros. We compute P receiver functions to investigate crustal and upper mantle discontinuities. We use teleseismic events (mb ≥ 5.5, 30° < Δ < 95°) registered between 1996 and 2018 and recorded at 10 short-period stations with 3 components and 17 broadband stations with high signal to noise ratio. The observed depth of Moho in the study area is approximately 50 km and rises to 70 km at the end of the seismic linear profile beneath Sanandaj-Sirjan zone. In Central Iran, depths discontinuities in the transition zone are shown by the reference model of deviation, which can be attributed to the convergence of Arabian plate with the Central Iran plateau. Also, the study area was identified as geothermal susceptibility by SUNA and this observation was confirmed.

Upper-mantle discontinuities beneath Australia from transdimensional Bayesian inversions using multimode surface waves and receiver functions

SUMMARYRadially anisotropic S-wave structures under the permanent seismic stations in Australia are reconstructed using multimode surface waves (SWs) and receiver functions (RFs) in a framework of the Bayesian inference. We have developed a fully nonlinear method of joint inversions incorporating P-RFs and multimode Rayleigh and Love waves, based on the transdimensional Hierarchical Bayesian formulation. The method allows us to estimate a probabilistic Earth model taking account of the complexity and uncertainty of Earth structure, by treating the model parameters and data errors as unknowns. The Parallel Tempering algorithm is employed for the effective parameter search based on the reversible-jump Markov Chain Monte Carlo method. The use of higher modes enables us to enhance the sensitivity to the depth below the continental asthenosphere. Synthetic experiments indicate the importance of higher mode SWs for the better recovery of radial anisotropy in the whole depth range of the upper mantle. The method is applied to five Global Seismographic Network stations in Australia. While the S-wave models in eastern Australia show shallow lithosphere–asthenosphere boundary (LAB) above 100 km depth, those in central and Western Australia exhibit both mid-lithosphere discontinuities (MLDs) and LAB. Also, seismic velocity jumps equivalent to the Lehmann discontinuity (L-D) are found in all seismic stations. The L-D under the Australian continents is found at around 200–300 km depth, depending on locations. Radial anisotropy in the depth range between LAB and L-D tends to show faster SH anomalies, which may indicate the effects of horizontal shear underneath the fast-moving Australian plate.

Location and characteristics of the X-discontinuity beneath SW Morocco and the adjacent shelf area usingP-wave receiver functions

SUMMARYReceiver function approaches have proven to be valuable for the investigation of crustal and upper mantle discontinuities whose sharp changes in seismic velocities cause wave conversions. While the crustal and mantle transition zone discontinuities are largely understood, the X-discontinuity at 250–350 km depth is still an object of controversial debate. The origin and global distribution of this structure with a velocity jump of 1.5–4.8 % for compressional and shear waves is still unexplained. Although the crustal and mantle transition zone discontinuities beneath SW Morocco and surroundings have been investigated, only a few studies observed the X-discontinuity and place the depth at 260–370 km beneath the region of western Morocco. In order to better locate and characterize the X-discontinuity beneath southwest Morocco, we create P-wave receiver functions using data recorded by the Morocco–Münster array and detect the X-discontinuity at apparent depths of 285–350 km. In the western part of our study region we find apparent depths of ∼ 310–340 km. The eastern part of the study area appears more complex: we locate two velocity jumps at apparent depths of around 285–295 km and 330–350 km in the northeast, and in the southeast we find a discontinuity at apparent depths of 340–350 km. Due to the large depth range and the twofold appearance of the X-discontinuity, we suggest that two different phase transitions cause the X-discontinuity beneath SW Morocco. The velocity contrasts at larger depths likely point to the coesite–stishovite phase transition occurring in deep eclogitic pools. The shallower depths can be explained by the transition from orthoenstatite to high-pressure clinoenstatite which requires the reaction between eclogite and peridotite to form orthopyroxene-rich peridotite. This reaction is likely related to previously proposed small-scale mantle upwellings beneath SW Morocco. Since both phase transitions require eclogite occurrence, the location of the X-discontinuity in this region can be used to indicate the location of recycled oceanic crust.

Receiver Function Velocity Analysis Technique and Its Application to Remove Multiples

AbstractResults of P wave receiver function (RF) analysis (e.g., H‐κ stacking, common conversion point (CCP) stacking, and migration) depend on the velocity models employed. Converted phases of crustal and upper mantle discontinuities are often interfered by multiple reflections from shallower structures, adding further complexity to the interpretation of P wave RF images. In this study, we propose a receiver function velocity analysis technique (RFVAT) that can constrain the velocity structure above discontinuities and remove multiple reflections arising from target discontinuities. By scanning the amplitude spectra of the Ps and PpPs phases, we can obtain precise arrival times of these phases for interfaces above 250 km (errors < 0.13 s) and derive the average velocities and thicknesses of the layers between interfaces in the crust and upper mantle. Synthetic experiments are used to demonstrate that crustal and upper mantle discontinuities can be coherently identified and that multiple reflections can be effectively removed in the CCP stacking images of P wave RFs using the RFVAT even with low signal‐to‐noise ratio data. By applying the proposed RFVAT to real P wave RF data from the northeastern part of the North China Craton, we achieve an improvement in CCP stacking image of lithospheric discontinuities, especially for the lithosphere‐asthenosphere boundary (LAB). Our P wave RF images reveal distinct lateral variations of the LAB depth. The lithospheric thickness extends to ~120 km beneath the central orogenic belt and decreases sharply to ~80 km at the west edge of the eastern block, consistent with previous S wave RF studies.

Constraining the characters of the upper mantle discontinuities beneath the NE margin of the Tibetan Plateau with a dense broadband seismic array

Based on observations from a dense broadband seismic array located along the northeastern (NE) margin of the Tibetan Plateau in southeastern Gansu Province, we use receiver functions (RFs) to pick the arrival times of P-to-S converted waves and bin the traces in different grids according to the piercing points of the 410 and 660 km discontinuities in the upper mantle. The depths of the two discontinuities are estimated by the ray tracing method with the IASP91 velocity model and a 3-D tomography model. The results indicate the following: (1) The arrival times of the P410s and P660s converted phases are delayed by approximately 1 s than those predicted by the IASP91 model. The mantle transition zone (MTZ) is thicker than that in the global model. (2) The synchronous lags in the P410s and P660s arrival times are consistent with low-velocity anomalies in the upper mantle, which are believed to result mainly from the eastward migration of materials beneath the NE margin of the Tibetan Plateau. (3) Combined with previous tomography results, the depression of the ‘660’ discontinuity and the thickened MTZ are somewhat consistent with the big mantle wedge (BMW) model. However, due to data limitations, more studies are required to explore the BMW in the future.

Mantle Structure in the Central Mediterranean Region From P and S Receiver Functions

AbstractWe investigate the upper mantle discontinuities in the central Mediterranean region by applying the P and S receiver function techniques on waveforms recorded at broadband stations located around the Tyrrhenian basin. P and S wave velocity profiles (down to 300‐km depth) are calculated with joint inversion of P and S receiver functions. We could identify the Moho, lithosphere‐asthenosphere boundary, and an underlying low‐velocity layer between ~60‐ and ~200‐km depth. The low‐velocity layer is interpreted as asthenospheric material, and its lower boundary is identified below the western Ionian and Tyrrhenian basins as a sharp Lehmann discontinuity. Although the stations are located on different lithospheric domains we find a strong correlation between Moho and the lithosphere‐asthenosphere boundary depths, which suggests ubiquitous coupling of the crust and lithospheric mantle, consistently with the southward opening of the Tyrrhenian basin. The Tyrrhenian and western Ionian basins present thinning of the transition zone of ~14 km, as inferred from a reduced P660s‐P410s differential time. Below the southern Apennines we observe a standard differential time that implies an average mantle transition zone thickness. We explain these mantle transition zone thickness variations as due to temperature heterogeneity linked to the area's subduction history. Finally, under central Europe (the location of the deep S‐to‐P conversion points) two strong signals from nonstandard discontinuities within the mantle transition zone are observed. These signals can be explained as being generated at the boundaries of high seismic velocity layers that are spatially correlated with stagnant slabs in the transition zone detected by seismic tomography.

Mantle transition zone structure beneath the Central Asian Orogenic Belt

Over the past decades, there have been hot debates in the geodynamic community regarding to the deep evolution mechanisms of Cenozoic volcanism in the Central Asian Orogenic Belt (CAOB). Of all the constraints available, high resolution structure of upper mantle discontinuities, especially the discontinuities at depths of 410 km (D410) and 660 km (D660), is of the most important, which may provide reliable clues on the magma channel as well as its evolution. In this work, we adopt the common conversion point stacking technique with teleseismic radial P-wave receiver functions to examine the D410 and D660 discontinuities. The primary results exhibit that the major characteristics of the mantle transition zone (MTZ) obtained by different velocity models are largely consistent. Obviously elevated D410 and slightly depressed D660 are observed beneath the Hannuoba Volcano, suggesting possible delamination of the local lithosphere deposited at the D410. This process may induce upwelling of the asthenospheric materials filling the space left by the delaminated lithosphere, and subsequently trigger volcanic eruptions. Strong depressions are observed at both D410 and D660 depths beneath west of the Dariganga Volcano, and the depression of D660 is more pronounced. It leads to the apparently thickened MTZ, indicating the presence of cold material at the D660. This cold material is speculated as a stagnant slab from the subducted Pacific Plate or the remains of a detached island arc system from the collision and formation of the CAOB. Slightly thinner MTZ is found beneath the Hentey Mountains and the Middle Gobi Volcano. Apparently, this thinner MTZ is not significant enough to support the existence of high thermal anomalies, which may rule out the possibility of large-scale hot material upwelling from either the local MTZ or even the lower mantle.

Towards full-waveform inversion for upper-mantle discontinuities on the global scale (solicited)

Towards full-waveform inversion for upper-mantle discontinuities on the global scale (solicited)