Depth Of Lithosphere-asthenosphere Boundary Research Articles

Lithospheric weakening by a small-scale plume and its geodynamic implications

The process of small-scale mantle plume emplacement within the continental lithosphere influences the dynamic topography and architecture of the lithosphere-asthenosphere boundary (LAB). Furthermore, plume emplacement alters the rheology of the crust and lithospheric mantle through melt-driven, mechanical and thermal weakening. Previous studies of plume emplacement within the lithosphere have described these outcomes, albeit without considering plume-driven weakening. In this study, we quantitatively evaluated the effects of small-scale plume-driven predefined weakening of the continental lithospheric mantle and crust on dynamic topography and LAB depth changes using a series of two-dimensional numerical models. Model calculations showed that small plume-driven predefined weakening led to short-wavelength dynamic topography and LAB depth reduction through both mechanical and thermal erosion of the lithospheric mantle by the plume. An increase in the degree of weakening decreased the lithospheric strength and reduced the LAB depth, thereby transforming the short-wavelength topography from depressions to elevations as the extensional kinematics shifted to compression. Our model findings explain the topography, lithospheric strength, and LAB depth constrained from natural small-scale plume emplacement zones.

Crustal and lithospheric mantle structure below the Indian shield based on 3-D constrained gravity inversion and Deep Neural Network approach: Geological implications

The Indian shield region comprises of several Archean cratons, Proterozoic mobile belts, Proterozoic-Phanerozoic rift basins, and volcanic provinces and has been significantly modified by various tectono-thermal events and the supercontinental cycles. These overprinting processes and evolution can be understood, to some extent by characterizing the lithosphere structure. In this study, we carried out seismically constrained 3-D nonlinear gravity inversion using tesseroids in the spherical coordinate system to delineate the Moho geometry for the Indian shield. Further, the supervised machine learning approach using Deep Neural Network (DNN) is implemented on the geopotential data to obtain Lithosphere-Asthenosphere Boundary (LAB) across the Indian shield. The inversion results show that the Moho depth varies between 33 and 60 km below the Indian shield with a mean difference of ∼ −0.3 km and a standard deviation of ∼4.9 km relative to the Moho from receiver function data. For the DNN model, the estimated LAB depth values range between 95 and 280 km below the Indian shield. The model gave rise to the coefficient of determination R2 of 0.93, Mean Absolute Error of ∼14 km, and Root Mean Square Error of ∼22 km between the observed and predicted LAB depths. Apart from the general agreement of Moho and LAB depth estimates with previous seismological studies, the present study provides better resolved information of lithosphere structure across the Indian shield. The modelled lithosphere structure therefore will be useful to integrate with the geochronological data in order to to understand the Proterozoic tectonic evolution of the Indian Shield.

Probabilistic Assessment of Antarctic Thermomechanical Structure: Impacts on Ice Sheet Stability

AbstractUncertainty in present‐day glacial isostatic adjustment (GIA) rates represents at least 44% of the total gravity‐based ice mass balance signal over Antarctica. Meanwhile, physical couplings between solid Earth, sea level and ice dynamics enhance the dependency of the spatiotemporally varying GIA signal on three‐dimensional variations in mantle rheology. Improved knowledge of thermomechanical mantle structure is therefore required to refine estimates of current and projected ice mass balance. Here, we present a Bayesian inverse method for self‐consistently mapping shear‐wave velocities from high‐resolution adjoint tomography into thermomechanical structure using calibrated parameterisations of anelasticity at seismic frequency. We constrain the model using regional geophysical data sets containing information on upper mantle temperature, attenuation and viscosity structure. Our treatment allows formal quantification of parameter covariances, and naturally permits propagation of material parameter uncertainties into thermomechanical structure estimates. We find that uncertainty in steady‐state viscosity structure at 150 km depth can be reduced by 4–5 orders of magnitude compared with a forward‐modeling approach neglecting covariance between viscoelastic parameters. By accounting for the dependence of apparent viscosity on loading timescale, we find good agreement between our estimates of mantle viscosity beneath West Antarctica, and those derived from satellite GPS. Direct access to temperature structure allows us to estimate lateral variations in lithosphere‐asthenosphere boundary (LAB) depth, geothermal heat flow (GHF), and associated uncertainties. We find evidence for shallow LAB depths (63 ± 13 km), and high GHF (76 ± 7 mW m−2) beneath West Antarctica that, combined with low asthenospheric viscosities, indicate a highly dynamic response to ice mass loss.

Global volcanic arc magma composition correlates with thickness of both arc crust (Moho depth) and arc lithosphere (LAB depth): A revealing message on arc basement histories and arc magmatism

Past fifty years have seen mounting publications on the genesis of volcanic arc magmas. While details remain debated, it is generally agreed that arc magmas result from slab-dehydration induced mantle wedge melting followed by crustal level differentiation of varying extent and sophistication. Two recent arc magma studies deserve particular attention because they attempt to discuss globally unifying controls on arc magma composition. Both Harvard study (Turner and Langmuir, 2015a,b) and Rice study (Farner and Lee, 2017) show correlations of arc magma composition with crustal thickness and both ascribe the crustal thickness as the principal control on their observed magma compositional variations, yet the physical role of the crustal thickness in their interpretations is markedly different because of (1) the ambiguous use of “crust” and (2) their different magma compositional ranges chosen in discussion. The Harvard study only uses basaltic samples corrected to MgO = 6.0 wt.% to discuss mantle processes and interprets the arc crustal thickness as restricting the mantle wedge melting, i.e., the extent of melting decreases with increasing crustal thickness. The Rice study uses samples of all compositions (basaltic to rhyolitic), whose extent of differentiation increases with increasing crustal thickness, interpreted as Moho-crossing mantle wedge melts travelling greater vertical distance with greater degree of cooling and erupting more evolved compositions above thicker crust than melts erupted above thinner crust without need of invoking mantle wedge processes. We commend these efforts and approve their different approaches but emphasize that the unifying understanding of global arc magmatism requires clearly defined Moho (the base of the crust) and LAB (the lithosphere-asthenosphere boundary) and their intrinsic controls on mantle wedge melting (Harvard Study model) and crustal level magma differentiation (Rice Study model) beneath global arcs. In this study, we use chemical compositions of 36,945 global arc volcanic samples provided by the Rice study together with the literature data on seismic Moho and LAB depths of these sample locations to establish (1) the correlation of crustal thickness interval averaged magma composition with crustal thickness and (2) the correlation of lithosphere thickness interval averaged magma composition with lithosphere thickness. These correlations reaffirm our understanding that the lithospheric mantle must exist beneath volcanic arc crust with a globally averaged LAB/Moho depth ratio of 3.26 ± 0.63, i.e., the arc crust is on average about 31.8% ± 6.1% of the lithosphere thickness. This knowledge forms a solid constraint essential for models of global arc magmatism.

Undulating electrical lithosphere–asthenosphere boundary beneath Northeast China; as revealed by long–period magnetotelluric data

The geometry of continental lithosphere–asthenosphere boundary (LAB) significantly affects the movements of tectonic plates and flow of the upper mantle. However, the properties and geometry of this boundary beneath tectonically active regions in Phanerozoic continents are mysterious and elusive. Ninety–three long–period magnetotelluric (LMT) observations across Northeast China (NEC) provide a unique opportunity to image the shape of the LAB. Probabilistic resistivity–depth model inversion was employed to obtain the one–dimensional resistivity structure beneath each LMT site. The depth of the electrical LAB is constrained by the most probable interface at which the resistivity changes rapidly from high to low. The inversion results show that the electrical LAB is undulating beneath NEC. The North–South Gravity Lineament marks the transition from deep to shallow LAB. The thickest lithosphere occurs beneath the Great Xing'an Range (GXAR), where the LAB is ∼200 km deep. The electrical lithosphere is relatively thin in the east, with the thinnest lithosphere (∼50 km) being found beneath the Changbai Mountain volcano. We speculate that melt depletion, together with possible mantle downwelling are the likely causes for the formation of the thick GXAR lithosphere. The unresolved lithosphere beneath the northern Songliao Basin could be ascribed to the strong attenuation of electromagnetic waves in highly conductive sedimentary layers or the weak difference in the resistivity of the lithosphere and asthenosphere. The lithosphere steps that can form the edge-driven convection in the asthenosphere may play key roles in forming the widely-spread intraplate magmatism in NEC.

Satellite gravity data for mapping lithospheric structure of Precambrian tectonic blocks in Africa: The advantages and limitations

In this study, we use the Precambrian tectonic blocks in southern Africa to examine the utility of satellite gravity data in mapping the lithospheric structure beneath the Precambrian terrains in Africa. For this, we focus on the Archean – Paleoproterozoic Congo – Tanzania – Bangweulu, Zimbabwe – Kaapvaal – Niassa cratons, Paleoproterozoic – Neoproterozoic Trans-Southern Africa orogen, the Neoproterozoic early Cambrian Lufilian arc, and Paleoproterozoic – Neoproterozoic Irumide metacraton. We use the Bouguer gravity anomalies of the World Gravity Model 2012 (WGM 2012). Our goal is to estimate the thickness of the lithosphere (hence the depth to lithosphere – asthenosphere boundary (LAB)) beneath these Precambrian tectonic blocks. We use two approaches including the two-dimensional (2D) radially averaged power spectral analysis (henceforth the spectral analysis), and 2D forward gravity modeling (henceforth the 2D forward gravity model). Using the spectral analysis, we are able, to a large extent, image the Congo – Tanzania – Bangweulu craton, as been underlain by thick lithosphere with the LAB depth of ∼250 km. Differently, using this method, we find no anomalously thick lithosphere beneath the Zimbabwe – Kaapvaal craton, although some of the 2D forward gravity models show relatively thick lithosphere beneath this craton. The only segment of the Trans-Southern Africa orogen we find to have thinner lithosphere (∼150 km) using the spectral analysis is the Mesoproterozoic – Neoproterozoic Damara – Ghanzi – Chobe orogen. We find the “system” limitations in using satellite gravity data for imaging Precambrian lithospheric structure to be the selection of linear segments in the spectral curves, selection of the size of sub-windows during the analysis, and selection of density values during the 2D forward gravity modeling. We find the “environmental” limitations to be the modification of Precambrian lithospheric structure by younger tectonic events.

Evidence for a very thick Kaapvaal craton root: Implications for equilibrium fossil geotherms in the early continental lithosphere

The presence of diamonds of Archaean age within the cratonic lithosphere is surprising because their formation implies a cool early mantle lithosphere despite the early Earth's hotter state. There remains a widely (albeit not exclusively) held concept that the prevailing geothermal gradient within Archaean cratons was not much different from today. This ‘Archaean paradox’ is supported by pressure and/or temperature (PT) estimates of potentially ancient diamond inclusion (DI) falling close to modern cratonic geotherms. Solutions to the paradox include more effective Archaean heat loss mechanisms (e.g. heat pipes), deeper lithospheric mantle roots or the early lithosphere forming in a disequilibrium state.Here we demonstrate the effects of a thermally evolving Earth on equilibrium geotherms within the Kaapvaal craton of South Africa. We modelled geotherms from the present-day conditions backward in time to 3.0 Ga at thermal equilibrium in the lithosphere by adjusting three dominant variables: (i) time-corrected crustal heat production; (ii) a secularly cooling mantle potential temperature (Tp) in the underlying convecting mantle; and, for illustrative purposes, (iii) variable depth of the lithosphere-asthenosphere boundary (LAB) or variable heat flow at the LAB. Computed model geotherms were compared to DI PT data from the Kaapvaal craton, as well as PT conditions recorded in mantle xenoliths of variable age. It is demonstrated that equilibrium geotherms in the lithosphere cannot be reconciled with PT conditions recorded in Kaapvaal craton DI nor with those recorded in Proterozoic xenoliths if either the lithosphere has remained of constant thickness or held a constant basal heat flux since 3.0 Ga. Rather, provided that PT conditions recorded by DI from the Kaapvaal craton reflect equilibration along fossil ancient equilibrium geotherms, it is required that the early lithosphere was substantially thicker than what is preserved today. The inferred former thickness of Kaapvaal craton could have reached up to ∼350 km if the prevailing mantle Tp at the time of diamond encapsulation was high and they are of Archaean age. If, however, the Archaean asthenosphere Tp was more modest or if the diamonds formed later than the Archaean the inferred LAB depths are shallower (∼250-275 km), but still substantially deeper than at the present day.Although only the Kaapvaal craton has the necessary DI PT constraints for this exercise, the fundamental nature of radioactive heat production rate and mantle cooling mean that our findings could imply that Earth's continental lithosphere in general attained its maximal vertical extent early in the Archaean and that cratons have been variably eroded and weakened since that time.

Modeling Lithospheric Thickness Along the Conjugate South Atlantic Passive Margins Implies Asymmetric Rift Initiation

AbstractThe lithospheric architecture of passive margins is crucial for understanding the tectonic processes that caused the breakup of Gondwana. We highlight the evolution of the South Atlantic passive margins by a simple thermal lithosphere‐asthenosphere boundary (LAB) model based on onset and cessation of rifting, crustal thickness, and stretching factors. We simulate lithospheric thinning and select the LAB as the T = 1,330°C isotherm, which is calculated by 1D advection and diffusion. Stretching factors and margin geometry are adjusted to state‐of‐the‐art data sets, giving a thermal LAB model that is especially designed for the continental margins of the South Atlantic. Our LAB model shows distinct variations along the passive margins that are not imaged by global LAB models, indicating different rifting mechanisms. For example, we model up to 200 km deep lithosphere in the South American Santos Basin and shallow lithosphere less than 60 km in the Namibe Basin offshore Africa. These two conjugate basins reflect a strong asymmetry in LAB depth that resembles variations in margin width. In a Gondwana reconstruction, we discuss these patterns together with seismic velocity perturbations for the Central and Austral Segments of the margins. The shallow lithosphere in the Namibe Basin correlates with signatures of the Angola Dome, attributed to epeirogenic uplift in the Neogene, suggesting an additional component of post‐breakup lithospheric thinning.

A shear-wave velocity model for the Scandinavian lithosphere from Rayleigh waves and ambient noise - Implications for the origin of the topography of the Scandes mountain range

We present a new 3D shear-wave velocity model and Moho map of Scandinavia, which is based on the inversion of the merged phase dispersion curves from ambient noise and earthquake-generated Rayleigh waves. A classic two step inversion scheme is used where first maps of phase velocities at different periods are derived, and then a 1D transdimensional Bayesian method is applied to determine the VSV-depth structure. We assess the question of what compensates for the unusual high Scandes mountains and aim to identify the different tectonic domains of the adjacent continental lithosphere (Baltic Shield).While the southern Scandes lacks a pronounced crustal root, we observe a crustal root below the northern Scandes that is decreasing towards the central Scandes. A ∼10 km thick high-density lower crustal layer is present below the northern Scandes and generally thickening to the east below the Baltic Shield. The lithosphere-asthenosphere boundary (LAB) below the Scandes is deepening as well from west to east with a sharp step and a strong VSV decrease with depth of 9% in the north and of 5.5% in the south. The LAB of the thinner lithosphere is at 150 km depth in the north and varies from 90 to 120 km depth in the south. Both LAB steps coincide with the mountain front. The central area shows rather smoothly varying structures (170 km LAB depth, −4% VSV with depth) towards the east and no clear spatial match with the front. We infer therefore distinct uplift mechanisms along the Scandes. The southern Scandes might sustain their topography due to dynamic support from the mantle, while the northern Scandes experience both crustal and mantle lithosphere isostasy. In both cases, we suspect a dynamic support from small-scale edge-driven convection that developed at the sharp lithospheric steps.Beneath the Archean Karelia craton in northern Finland, we find low-velocity areas below 150 km depth while a 250 km deep lithospheric keel is imaged below the Paleoproterozic southern Finland. The Norrbotten craton in northern Sweden can be identified at mantle depths as a unit different from the Karelia craton, Scandes and Paleoproterozic central Sweden.

Evidence for a lithospheric step and pervasive lithospheric thinning beneath southern New England, northeastern USA

AbstractIn this study, we use data from the SEISConn seismic experiment to calculate Sp receiver functions in order to characterize the geometry of upper-mantle structure beneath southern New England (northeastern United States). We image robust negative-velocity-gradient discontinuities beneath southern New England that we interpret as corresponding to the lithosphere-asthenosphere boundary (LAB) and identify a well-defined step of 15 km in LAB depth at a longitude of 73°W, which we interpret to be the boundary between Laurentian and Appalachian lithosphere, although the offset may be larger if the putative LAB phase is reinterpreted to be a mid-lithospheric discontinuity. We infer that the lithosphere throughout the region is substantially thinner than elsewhere in the continental interior, consistent with regional tomographic studies and previously published Sp receiver function results. The presence of thinned lithosphere suggests that the low-velocity Northern Appalachian Anomaly (NAA) in the upper mantle may extend as far south as coastal Connecticut. The presence of regionally thinned lithosphere and a step in lithospheric thickness suggests that inherited structure may be preserved in present-day lithosphere, even in the presence of more recent dynamic processes associated with the NAA.

Thermal Modification of the Northwest Indian Shield: As Evidenced by Integrated Geopotential Modelling

Abstract The architecture of the continental lithospheric mantle is vital for understanding the wide-ranging processes that govern the creation of Earth’s continents. The present investigation delineates the crustal and lithospheric boundaries underneath northwest India, which provides essential constraints as an imprint of tectonomagmatic events under the distinct tectonic setting. This study integrates three data sets (gravity, geoid, and topography data) and thermal parameters in a three-dimensional inversion algorithm to northwest India. Inversion results are constrained by available passive seismic crustal thickness, which we are using as a priori information. Our modelling results show crust down to a depth of 33 to 39 km in the Saurashtra and Kachchh region, which increases gradually to about 40 km beneath the Jaisalmer region. The deepest Moho at about 42-45 km is found under the Aravalli-Delhi fold belt and Indo-Gangetic plain. A comparable crustal thickness of 41-44 km is observed beneath the Bundelkhand craton. The mean crustal density varies laterally between 2850 and 2890 kg/m-3 in the study area. The lithosphere-asthenosphere boundary (LAB) is 115-145 km deep under Saurashtra and Kachchh regions, which remains comparable under the Barmer-Pokharan region (130-160 km). A moderate LAB depth under the Aravalli-Delhi fold belt (140-180 km) and relatively deeper under the Bundelkhand craton (175 to 185 km) reaches over 200 km below the Indo-Gangetic plain. The thinned lithosphere beneath the Kachchh, Saurashtra, and Barmer-Pokhran regions, crisscrossed by two different rift systems, together with the dominance of the volcanic centres of two major magmatic episodes, and the possible presence of a low-velocity zone in the subcrustal upper mantle region suggest a genetic link between the evolution and the multistage thermal modification of the mantle lithosphere of this region.

Adjoint Waveform Tomography of South America

AbstractWe used 3D spectral‐element seismic wave simulations and data from 112 earthquakes and 1,311 seismic stations, totalizing 20,884 unique ray paths, to construct an adjoint waveform tomographic model of South America. We performed 23 conjugate‐gradient iterations using exponentiated phase (EP) measurements. Our final model (SAAM23, South American Adjoint Model—iteration 23) shows a 50% decrease in the EP misfit relative to its 3D starting model. We further assessed the phase misfit reduction by using cross‐correlation travel‐time measurements of 53 earthquakes not included in the inversion. We estimated SAAM23 resolution using point‐spread function tests and density coverage analysis. The Nazca Slab is well imaged and is shown to be continuous in the 300–500 km depth range. Beneath northern South America, the slab traverses the mantle transition zone and continues into the lower mantle. In the central and southern part of South America, the slab appears to flatten near the 650 km discontinuity before continuing into the lower mantle. In the stable Precambrian platform, both cratons (Amazonian and São Francisco), as well as covered cratonic blocks beneath the intracratonic Paraná and Parnaíba basins (Paranapanema and Parnaíba, respectively), show high velocities at lithospheric depths. The seismic Lithosphere/Asthenosphere boundary (LAB) agrees well with published values obtained by S‐wave receiver functions. In the Amazonian craton, the positive lithospheric S‐wave velocity anomalies and LAB depth increase with the average age of the geochronological provinces. No lithospheric high‐velocity anomalies were found beneath the Río de la Plata Craton.

Lithospheric structure beneath the boundary region of North China Craton and Xing Meng Orogenic Belt from S-receiver function analysis

The boundary region of the North China Craton (NCC) and Xing-Meng Orogenic Belt (XMOB), including the northern boundary of the NCC and the southern XMOB, is the ideal area to investigate the spatially uneven lithospheric deformation and associated tectonic evolution in the boundary region of the craton and orogen. The depths and structures of the lithosphere–asthenosphere boundary (LAB) and mid-lithosphere discontinuity (MLD) are the key information to describe the nature and deformation pattern of the lithosphere, and thus can provide basic constraints on the associated tectonic processes and mechanism of the lithospheric deformation. Based on waveform data collected from 226 broadband seismic stations, high resolution spatial variations in the LAB and MLD depths were obtained by using the wave equation migration method for the S-receiver functions. The migrated images show a lateral variation in the LAB depth from 100–120 km in the northern boundary of the NCC to 120–140 km in the southern XMOB. The imaged LAB within the NCC is much shallower than that revealed by mantle xenoliths enclosed in the nearby Early Paleozoic kimberlites (~180 km), suggesting that the cratonic lithosphere of the area may have undergone significant thinning during the Phanerozoic evolution. Furthermore, a coherent MLD is identified at depths of 70–90 km beneath the northeastern NCC, which corroborates the speculation that the MLD probably existed in the eastern NCC before lithospheric destruction in the Mesozoic. The spatial variations in the LAB and MLD depths probably reflect complex lithospheric deformation and thinning around the boundary region of NCC and XMOB. The properties, including the gradient thickness and S-wave velocity contrast, of the two discontinuities further indicate that lithospheric deformation and thinning might be related to partial melting induced by the Big Mantle Wedge associated with the subducting Paleo-Pacific plate and its stagnation during the Late Mesozoic.

Lithosphere\u2013asthenosphere boundary beneath the Sea of Japan from transdimensional inversion of S-receiver functions

The evolution history of the Sea of Japan back-arc basin remains under debate, involving the opening of sub-basins such as the Japan and Yamato Basins. Detailed knowledge of the lithospheric structure will provide the key to understanding tectonic history. This study identifies the lithosphere–asthenosphere boundary (LAB) beneath the Sea of Japan back-arc basin using S-receiver functions (S-RFs). The study area, including the Japan and Yamato Basins, has been instrumented with broadband ocean-bottom seismometers (OBSs). S-RFs from these OBSs show negative Sp phases preceding the direct S arrivals, suggesting the LAB. The S-RFs also show abnormally reduced amplitudes. For further qualitative interpretation of these findings, we conduct transdimensional Bayesian inversion for S-wave velocity models. This less-subjective Bayesian approach clarifies that the low-velocity seafloor sediments and damped deconvolution contribute to the amplitude reduction, illuminating the necessity of such considerations for similar receiver function works. Inverted velocity structures show a sharp velocity decrease at the mantle depths, which we consider the LAB. The obtained LAB depths vary among sites: ~ 45 km beneath the Japan and Yamato Basins and ~ 70 km beneath the Yamato Rise, a bathymetric high between the two basins. The thick lithosphere beneath the Yamato Rise most likely reflects its continental origin. However, the thickness is still thin compared to that of eastern Asia, suggesting lithosphere extension by rifting. Notably, the Japan and Yamato Basins show a comparable lithospheric thickness, although the crustal thickness beneath the Yamato Basin is known to be anomalously thick. This consistency in the lithospheric thickness implies that both basins undergo similar back-arc opening processes.

Western Gondwana imaged by S receiver-functions (SRF): New results on Moho, MLD (mid-lithospheric discontinuity) and LAB (lithosphere-asthenosphere boundary)

We study the Moho, the mid-lithospheric discontinuity (MLD), and the lithosphere-asthenosphere boundary (LAB) from southern Africa to northern Arabia, from Archean cratons to active rifts, at 1° resolution using our comprehensive new database of shear-wave receiver functions (SRFs). The good agreement between the Moho depth obtained from our SRFs and published P-wave receiver function (PRF) results provides confidence that our images of deeper lithospheric discontinuities are robust, including boundaries not normally visible on PRFs. We map the Moho and a deeper negative velocity gradient (NVG) almost everywhere we have data coverage. Our synthetic tests and comparisons of SRFs processed with and without deconvolution, and with varying filter parameters, indicate the observed NVG represents earth structure, not a processing artifact. Depth comparisons with seismic tomography and tectonothermal age studies suggest the NVG represents the MLD beneath Archean cratons but represents the LAB beneath non-cratonic regions. Both preserved crustal thickness and lithospheric thickness in the Nubia-Somalia-Arabia plates are statistically thinner for Phanerozoic and late Proterozoic terranes and older regions reactivated during these eras, than for cratons not reworked since the early Proterozoic or Archean. In contrast, NVG depth is uniform for all tectonothermal ages, though with a possible increase in amplitude with age. The equivalence of NVG depth and LAB depth in Phanerozoic lithosphere suggests that low-wavespeed compositions are frozen into the lithosphere as it thickens by cooling, forming our observed MLD at the present day.

Lithosphere as a constant-velocity plate: Chasing a dynamical LAB in a homogeneous mantle material

While the lateral limits of tectonic plates are well mapped by seismicity, the bottom boundary of the lithosphere, the uppermost rigid layer of the Earth comprising both crust and shallow mantle, remains elusive. The lithosphere is usually viewed as consisting of cold, rigid, internally undeformed blocks that translate coherently. The base of the lithosphere, designated as the lithosphere-asthenosphere boundary (LAB), may thus be characterised by different physical fields: temperature, viscosity, strain rate and velocity. The LABs as defined by these different fields are investigated here using thermo-mechanical models of plate and upper mantle dynamics, either in a transient subduction or in a steady-state plate-driven set-up. Mantle material is modelled as homogeneous in composition with a viscosity that depends on temperature, pressure and strain rate. In such a system, the thermo-mechanical transition between lithosphere and asthenosphere occurs over a finite depth interval in temperature, strain rate and velocity. We propose that the most useful dynamical LAB is defined as the base of a “constant-velocity” plate (i.e. the material translating at constant horizontal velocity). The bottom part of this plate deforms at strain rates comparable to those in the underlying asthenosphere mantle: the translating block is not fully rigid. Thermal structure exerts a major control on this dynamical LAB, which deepens with increasing plate age. However, the surface plate velocity, the asthenospheric flow geometry and magnitude also influence the depth of the dynamical LAB, as well as the thickness of the deformed region at the base of the constant-velocity plate. The mechanical transitions from lithosphere to asthenosphere adjust when mantle dynamics evolves. The dynamical and thermo-mechanical LABs occur within a thermal lithosphere-asthenophere gradual transition, similar to the one imaged by geophysical proxies. The concept of a constant-velocity plate can be extended to a constant-velocity subducting slab, which also deforms at its borders and drags the surrounding mantle. This framework is relevant to quantify mass transport within the Earth's mantle.

Lithospheric model along transect HT-1 across Western Carpathians and Pannonian Basin based on 2D integrated modelling

2D integrated modelling approach was applied to determine the lithospheric structure along transect HT-1 located in the Carpathian-Pannonian Basin–European platform region. Our approach combines simultaneous interpretation of surface heat flow, topography, gravity and geoid data. All available geophysical and geological data were used to create an initial model that has been afterwards modified by trial and error method until reasonable fit was obtained between input data and model predictions. The main focus of our study was the position and shape of the lithosphere-asthenosphere boundary (LAB). In the Pannonian Basin the modelled LAB is at depths of about 80–90 km and rapidly dips towards the Western Carpathians where its depth reaches values 145 to 150 km. Beneath the European platform the LAB depth is about 135–140 km. We can observe a slight lithospheric root under the Western Carpathians. This lithospheric thickening is interpreted as a small remnant of a subducted slab. This result is in a good agreement with the previous lithospheric models in the Carpathian-Pannonian Basin.

The Nature of the Lithosphere‐Asthenosphere Boundary

AbstractPlate tectonic theory was developed 50 years ago and underpins most of our understanding of Earth's evolution. The theory explains observations of magnetic lineations on the seafloor, linear volcanic island chains, large transform fault systems, and deep earthquakes near deep sea trenches. These features occur through a system of moving plates at the surface of the Earth, which are the surface expression of mantle convection. The plate consists of the chemically distinct crust and some amount of rigid mantle, which move over a weaker mantle beneath. However, exactly where the transition between stronger and weaker mantle occurs and what determines and defines the plate are still debated. In the classic definition the plate is defined thermally, by the geotherm‐adiabat intersection, where the plate is the conductively cooling part of the mantle convection system. Many observations such as heat flow, seafloor bathymetry, seismic imaging, and magnetotelluric (MT) imaging are consistent with general lithospheric thickening with age, which suggests that temperature is an important factor in determining lithospheric thickness. However, while age averages give a good indication of overall properties, the range of lithospheric thicknesses reported is large for any given tectonic age interval, suggesting greater complexity. A number of observations including sharp discontinuities from teleseismic scattered waves and active source reflections and also strong anomalies from surface and body wave tomography and MT imaging cannot be explained by a purely thermal model. Another property or process is required to explain the anomalies and sharpen the boundary. Many subsolidus models have been proposed, although none can universally explain the variety of independent global observations. Alternatively, a small amount of partial melt can easily satisfy a range of observations. The presence of melt could also weaken the mantle over geologic timescales, and it would therefore define the lithosphere‐asthenosphere boundary (LAB). The location of melt is important to mantle dynamics and the LAB, although exactly where and exactly how much melt exists in the mantle are debated. Asthenospheric melt interpretations include a variety of forms: in small or large melt triangles beneath spreading ridges, in channels, in layers, along a permeability boundary leading to the ridge, at a depth of neutral buoyancy, punctuated, or pervasively over broad areas and either sharply or gradually falling off with depth. This variability in melt character or geometry may explain the previously described variability in LAB depths. The LAB is likely highly variable laterally as are the locations, forms, and amounts of melt, and the LAB is likely dynamic, dictated by small‐scale convection and the dynamics of melt generation and migration. A melt‐defined, dynamic LAB and a weak asthenosphere have broad implications for our understanding of Earth systems and planetary habitability. A weak asthenosphere caused by volatiles or melt could enable plate tectonic style convection, allow multiple scales of convection, and dictate the driving forces of the system. A better understanding of plate tectonics has broad implications for life on Earth. These include mitigating natural disasters caused by plate motions including volcanoes, earthquakes, and tsunamis. In addition, uplift and subsidence of the tectonic plates affects the sea level, impacting the level of the paleo‐oceans and potentially affecting climate change estimates through geologic time. Finally, plate tectonic processes shape the surface morphology of the planet, making continents that enable our existence on land and the ocean basins that hold our free‐surface water. Remarkably, despite large amounts of material transfer into and out of the mantle, and multiple scales of convection, plate tectonics has maintained a hydrosphere over billions of years that is favorable for life.

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.

Lithospheric layering beneath northern Central Africa by S-to-P receiver functions

Using a total of 4520 S-to-P receiver functions (SRFs) recorded by 19 broadband seismic stations belonging to the Libyan Center for Remote Sensing and Space Science and the Incorporated Research Institutions for Seismology Data Management Center, we imaged the depth of the lithosphere–asthenosphere boundary (LAB) beneath northern Central Africa. This boundary occurs over the depth range of 57–124 km which we imaged in consecutive circular bins (radius of 2°) using a high number of receiver functions. The mean depth of the discontinuity is 80 ± 17 km, which is significantly shallower than the global average of ~250 km, commonly found beneath ancient cratons. The SRFs in the study area produced 156 bins with observable arrivals from the LAB. All the stacked traces were plotted along eight latitudinal profiles from 20°N to 35°N. The observed depth of the LAB increases systematically toward the northern central part of the study area from approximately 67 km–120 km. The apparent depth of the LAB increases from 70 to 90 km from 21°N to 28°N and then further increases to 120 km from 28°N to 34°N. These depth variations are extreme beneath the northern central part of Libya. The LAB depth beneath the Hoggar volcanic province area is relatively low (~57 km) compared to other areas. This study provides new constraints on regional-scale tectonic processes such as lithospheric stretching or thinning, due to partial melting beneath the study region that may be an effect of the LAB topography in this part of northern Central Africa.