Seismic Lithosphere-asthenosphere Boundary Research Articles

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.

The lithospheric structure beneath the northeastern Tibetan Plateau and western North China Craton from S receiver function imaging and its implications for lateral growth of the plateau

The mantle lithospheric structures beneath the northeastern Tibetan Plateau (NETP) and adjacent areas provide critical information about the role of the mantle lithosphere in the lateral growth of the plateau. In this study, we construct a comprehensive image of the lithospheric structure across the NETP and western North China Craton (NCC) using the S-wave receiver function method based on the dense seismic array of ChinArray II. We detect one or multiple negative conversions at the depth ranging from 80 to 200 km across most of the study region, which is associated with the seismic lithosphere-asthenosphere boundary (LAB) or mid-lithospheric discontinuity (MLD) within the lithospheric mantle. The main features include that the Beishan Block, West Qinling Orogen, and Qilian Orogen present a relatively shallow LAB at a depth range of 120–140 km, while the Longxi Basin and Alashan Block have the similar lithospheric structure with a deep LAB at depth > 150 km and some MLDs within the lithospheric mantle. We find that the Qilian Orogen in the western NETP overlying a thin mantle lithosphere has been significantly elevated, whereas the Longxi Basin in the eastern NETP with a thick mantle lithosphere lacks both high elevations and significant crustal thickening. It implies that the thick mantle lithosphere beneath the eastern NETP and western NCC likely prevents the plateau from invading the Longxi Basin, while the thin mantle lithosphere across the western NETP and Beishan Block favors outward growth of the Qilian Orogen. Our seismic images provide new insights into the lithospheric heterogeneities and their roles in the lateral growth of the Tibetan Plateau.

Delineation of Average 1-D Shear Velocity Structure below North India by Surface Wave Dispersion Study

During 2014–16, a semi-permanent network of four 3-component broadband seismographs was operational in the Rajasthan craton and Aravalli mobile belt in the NW Indian shield. The reliable and accurate broadband data from 16 selected regional Indian earthquakes of Mw 5.5–7.8 from this seismic network enabled to estimate the group velocity dispersion characteristics and one-dimensional regional shear velocity structure of northern India, covering the region below north India between the entire Himalaya (from the Pakistan Himalaya in west to the Burmese arc in the east) and Rajasthan (including Aravalli mobile belt). First, Rayleigh- (at 7–87s) and Love- (at 7–82s) wave group velocity dispersion curves were measured and then these curves were inverted to estimate the crustal and upper mantle structure below north India. It is observed that group velocities are of variable nature within the region. This could be attributed to the complex crust-mantle structure in the study region resulted from the magmatism episodes associated with the Proterozoic collision, 65 Ma Deccan volcanism and the Himalayan collision. The best model in the study region reveals a two-layered crust, with a 15-km thick upper-crust (UC) of average shear velocity (Vs) of 3.12 km/s and a 25-km thick lower-crust(LC) of average Vs of 3.44 km/sec. The modeling detects a drop in Vs (∼1-2%) at 79–120 km depths, underlying north India, representing the probable seismic lithosphere-asthenosphere boundary (LAB) at 79 km depth. A geothermal gradient extrapolated from the surface heat flow (∼74 mW/m2) shows that such a gradient would intercept CO2-bearing mantle peridotite solidus at 100 km depth, and thus could signal the presence of small amounts of partially melted magma below 100 km depth. Therefore, this 1–2% drop in Vs could be attributed to the presence of carbonatite melts in the upper mantle related to magmatic episode of 65 Ma Deccan plume activity as also suggested by existing geological and seismological evidence.

Inferred rheological structure and mantle conditions from postseismic deformation following the 2010 Mw 7.2 El Mayor-Cucapah Earthquake

Author(s): Dickinson-Lovell, H; Huang, MH; Freed, AM; Fielding, E; Burgmann, R; Andronicos, C | Abstract: The 2010Mw7.2ElMayor-Cucapah earthquake provides a unique target of postseismic study as deformation extends across several distinct geological provinces, including the coldMesozoic arc crust of the Peninsular Ranges and newly formed, hot, extending lithosphere within the Salton Trough. We use five years of global positioning system measurements to invert for afterslip and constrain a 3-D finite-element model that simulates viscoelastic relaxation. We find that afterslip cannot readily explain far-field displacements (more than 50 km from the epicentre). These displacements are best explained by viscoelastic relaxation of a horizontally and vertically heterogeneous lower crust and upper mantle. Lower viscosities beneath the Salton Trough compared to the Peninsular Ranges and other surrounding regions are consistent with inferred differences in the respective geotherms. Our inferred viscosity structure suggests that the depth of the Lithosphere/Asthenosphere Boundary (LAB) is ~65 km below the Peninsular Ranges and ~32 km beneath the Salton Trough. These depths are shallower than the corresponding seismic LAB. This suggests that the onset of partial melting in peridotite may control the depth to the base of the mechanical lithosphere. In contrast, the seismic LAB may correspond to an increase in the partial melt percentage associated with the change from a conductive to an adiabatic geotherm.

Lithospheric discontinuities beneath the U.S. Midcontinent – signatures of Proterozoic terrane accretion and failed rifting

Seismic discontinuities between the Moho and the inferred lithosphere–asthenosphere boundary (LAB) are known as mid-lithospheric discontinuities (MLDs) and have been ascribed to a variety of phenomena that are critical to understanding lithospheric growth and evolution. In this study, we used S-to-P converted waves recorded by the USArray Transportable Array and the OIINK (Ozarks–Illinois–Indiana–Kentucky) Flexible Array to investigate lithospheric structure beneath the central U.S. This region, a portion of North America's cratonic platform, provides an opportunity to explore how terrane accretion, cratonization, and subsequent rifting may have influenced lithospheric structure. The 3D common conversion point (CCP) volume produced by stacking back-projected Sp receiver functions reveals a general absence of negative converted phases at the depths of the LAB across much of the central U.S. This observation suggests a gradual velocity decrease between the lithosphere and asthenosphere. Within the lithosphere, the CCP stacks display negative arrivals at depths between 65 km and 125 km. We interpret these as MLDs resulting from the top of a layer of crystallized melts (sill-like igneous intrusions) or otherwise chemically modified lithosphere that is enriched in water and/or hydrous minerals. Chemical modification in this manner would cause a weak layer in the lithosphere that marks the MLDs. The depth and amplitude of negative MLD phases vary significantly both within and between the physiographic provinces of the midcontinent. Double, or overlapping, MLDs can be seen along Precambrian terrane boundaries and appear to result from stacked or imbricated lithospheric blocks. A prominent negative Sp phase can be clearly identified at 80 km depth within the Reelfoot Rift. This arrival aligns with the top of a zone of low shear-wave velocities, which suggests that it marks an unusually shallow seismic LAB for the midcontinent. This boundary would correspond to the top of a region of mechanically and chemically rejuvenated mantle that was likely emplaced during late Precambrian/early Cambrian rifting. These observations suggest that the lithospheric structure beneath the Reelfoot Rift may be an example of a global phenomenon in which MLDs act as weak zones that facilitate the removal of cratonic lithosphere that lies beneath.

Group velocity dispersion characteristics and one-dimensional regional shear velocity structure of the eastern Indian craton

In the past three years, a semi-permanent network of fifteen 3-component broadband seismographs has become operational in the eastern Indian shield region occupying the Archean (∼2.5–3.6Ga) Singhbhum-Odisha craton (SOC) and the Proterozoic (∼1.0–2.5Ga) Chotanagpur Granitic Gneissic terrane (CGGT). The reliable and accurate broadband data for the recent 2015 Nepal earthquake sequence from 10 broadband stations of this network enabled us to estimate the group velocity dispersion characteristics and one-dimensional regional shear velocity structure of the region. First, we measure fundamental mode Rayleigh- and Love-wave group velocity dispersion curves in the period range of 7–70 s and then invert these curves to estimate the crustal and upper mantle structure below the eastern Indian craton (EIC). We observe that group velocities of Rayleigh and Love waves in SOC are relatively high in comparison to those of CGGT. This could be attributed to a relatively mafic-rich crust-mantle structure in SOC resulting from two episodes of magmatism associated with the 1.6Ga Dalma and ∼117Ma Rajmahal volcanisms. The best model for the EIC from the present study is found to be a two-layered crust, with a 14-km thick upper-crust (UC) of average shear velocity (Vs) of 3.0km/s and a 26-km thick lower-crust (LC) of average Vs of 3.6km/s. The present study detects a sharp drop in Vs (∼−2 to 3%) at 120–260km depths, underlying the EIC, representing the probable seismic lithosphere-asthenosphere boundary (LAB) at 120km depth. Such sharp fall in Vs below the LAB indicates a partially molten layer. Further, a geothermal gradient extrapolated from the surface heat flow shows that such a gradient would intercept the wet basalt solidus at 88–103km depths, suggesting a 88–103km thick thermal lithosphere below the EIC. This could also signal the presence of small amounts of partial melts. Thus, this 2–3% drop in Vs could be attributed to the presence of partial melts in the upper mantle related to the earlier volcanic episodes viz. back-arc volcanism associated with the Archean/Proterozoic subduction, 1.6Ga Dalma volcanism, and ∼117Ma Rajmahal volcanism. The main result of our modeling provides evidences for the absence of Keel or thick lithosphere below the EIC.

Nature of the seismic lithosphere‐asthenosphere boundary within normal oceanic mantle from high‐resolution receiver functions

AbstractReceiver function observations in the oceanic upper mantle can test causal mechanisms for the depth, sharpness, and age dependence of the seismic wave speed decrease thought to mark the lithosphere‐asthenosphere boundary (LAB). We use a combination of frequency‐dependent harmonic decomposition of receiver functions and synthetic forward modeling to provide new seismological constraints on this “seismic LAB” from 17 ocean‐bottom stations and 2 borehole stations in the Philippine Sea and northwest Pacific Ocean. Underneath young oceanic crust, the seismic LAB depth follows the ∼1300 K isotherm but a lower isotherm (∼1000 K) is suggested in the Daito ridge, the Izu‐Bonin‐Mariana trench, and the northern Shikoku basin. Underneath old oceanic crust, the seismic LAB lies at a constant depth ∼70 km. The age dependence of the seismic LAB depth is consistent with either a transition to partial‐melt conditions or a subsolidus rheological change as the causative factor. The age dependence of interface sharpness provides critical information to distinguish these two models. Underneath young oceanic crust, the velocity gradient is gradational, while for old oceanic crust, a sharper velocity gradient is suggested by the receiver functions. This behavior is consistent with the prediction of the subsolidus model invoking anelastic relaxation mediated by temperature and water content, but is not readily explained by a partial‐melt model. The Ps conversions display negligible two‐lobed or four‐lobed back azimuth dependence in harmonic stacks, suggesting that a sharp change in azimuthal anisotropy with depth is not responsible for them. We conclude that these ocean‐bottom observations indicate a subsolidus elastically accommodated grain‐boundary sliding (EAGBS) model for the seismic LAB. Because EAGBS does not facilitate long‐term ductile deformation, the seismic LAB may not coincide with the conventional transition from lithosphere to asthenosphere corresponding to a change in the long‐term rheological properties.

Upper mantle structure around the Trans-European Suture Zone obtained by teleseismic tomography

Abstract. The presented study aims to resolve the upper mantle structure around the Trans-European Suture Zone (TESZ), which is the major tectonic boundary in Europe. The data of 183 temporary and permanent seismic stations operated during the period of the PASsive Seismic Experiment (PASSEQ) 2006–2008 within the study area from Germany to Lithuania was used to compile the data set of manually picked 6008 top-quality arrivals of P waves from teleseismic earthquakes. We used the TELINV nonlinear teleseismic tomography algorithm to perform the inversions. As a result, we obtain a model of P wave velocity variations up to about ±3% with respect to the IASP91 velocity model in the upper mantle around the TESZ. The higher velocities to the east of the TESZ correspond to the older East European Craton (EEC), while the lower velocities to the west of the TESZ correspond to younger western Europe. We find that the seismic lithosphere–asthenosphere boundary (LAB) is more distinct beneath the Phanerozoic part of Europe than beneath the Precambrian part. To the west of the TESZ beneath the eastern part of the Bohemian Massif, the Sudetes Mountains and the Eger Rift, the negative anomalies are observed from a depth of at least 70 km, while under the Variscides the average depth of the seismic LAB is about 100 km. We do not observe the seismic LAB beneath the EEC, but beneath Lithuania we find the thickest lithosphere of about 300 km or more. Beneath the TESZ, the asthenosphere is at a depth of 150–180 km, which is an intermediate value between that of the EEC and western Europe. The results imply that the seismic LAB in the northern part of the TESZ is in the shape of a ramp dipping to the northeasterly direction. In the southern part of the TESZ, the LAB is shallower, most probably due to younger tectonic settings. In the northern part of the TESZ we do not recognize any clear contact between Phanerozoic and Proterozoic Europe, but further to the south we may refer to a sharp and steep contact on the eastern edge of the TESZ. Moreover, beneath Lithuania at depths of 120–150 km, we observe the lower velocity area following the boundary of the proposed paleosubduction zone.

Traces of the crustal units and the upper-mantle structure in the southwestern part of the East European Craton

Abstract. The presented study is a part of the passive seismic experiment PASSEQ 2006–2008, which took place around the Trans-European Suture Zone (TESZ) from May 2006 to June 2008. The data set of 4195 manually picked arrivals of teleseismic P waves of 101 earthquakes (EQs) recorded in the seismic stations deployed to the east of the TESZ was inverted using the non-linear teleseismic tomography algorithm TELINV. Two 3-D crustal models were used to estimate the crustal travel time (TT) corrections. As a result, we obtain a model of P-wave velocity variations in the upper mantle beneath the TESZ and the East European Craton (EEC). In the study area beneath the craton, we observe up to 3% higher and beneath the TESZ about 2–3% lower seismic velocities compared to the IASP91 velocity model. We find the seismic lithosphere–asthenosphere boundary (LAB) beneath the TESZ at a depth of about 180 km, while we observe no seismic LAB beneath the EEC. The inversion results obtained with the real and the synthetic data sets indicate a ramp shape of the LAB in the northern TESZ, where we observe values of seismic velocities close to those of the craton down to about 150 km. The lithosphere thickness in the EEC increases going from the TESZ to the NE from about 180 km beneath Poland to 300 km or more beneath Lithuania. Moreover, in western Lithuania we find an indication of an upper-mantle dome. In our results, the crustal units are not well resolved. There are no clear indications of the features in the upper mantle which could be related to the crustal units in the study area. On the other hand, at a depth of 120–150 km we indicate a trace of a boundary of proposed palaeosubduction zone between the East Lithuanian Domain (EL) and the West Lithuanian Granulite Domain (WLG). Also, in our results, we may have identified two anorogenic granitoid plutons.

From underplating to delamination-retreat in the northern Apennines

Recordings of teleseismic earthquakes from a dense set of temporary and permanent broadband seismic stations reveal the lithospheric structure of the northern Apennines and support the scenario of a retreating detachment within the mid-crust. Lithospheric delamination appears crucial to the formation and evolution of the Apennines orogen. Receiver-function (RF) stacks outline a continuous west-dipping Ps converted phase from a positive velocity jump that we interpret as the top of the lower crust and mantle of the Adria continental lithosphere, which is descending into the shallow mantle. The correlation of seismicity with two RF profiles across the northern Apennines suggests distinct stages of lithospheric delamination. Active penetration of the detachment into the Adria lithosphere seems evident in the south/east, with induced shallow-mantle flow facilitated by slab dehydration. Penetration of the detachment in the north/west seems to have arrested, and is possibly marked by crustal underplating. This layer atop the Apennines slab is visible only down to 80 km depth and suspends above an oppositely-dipping paired positive/negative Ps converted phase in stacked receiver functions. The break in the west-dipping Adria lithosphere conflicts with a westward-subduction scenario continuous from the Oligocene. Lateral changes of deep structure and seismicity along the northern Apennines suggest that underplating of crustal material and delamination-retreat are distinct mechanisms active today in the western and eastern sectors, respectively, of the northern Apennines. Negative Ps-pulses at 100–120 km depth help to define a seismic lithosphere–asthenosphere boundary (LAB), but cross-cut a volume of high-velocity mantle rock, as inferred from tomographic models. We hypothesize that this seismic LAB is a rheological discontinuity that affects the frequency band of seismic body waves, but not the long-term viscous response that governs the evolution and eventual detachment of the continental slab.

A sharp cratonic lithosphere–asthenosphere boundary beneath the American Midwest and its relation to mantle flow

Beneath the American Midwest, S-to-P (Sp) converted wave imaging and multi-mode surface wave tomography identify a north-trending transition in seismic structure at 150–250 km depth. To the east of this American Midwest transition (AMT), the lithosphere–asthenosphere boundary (LAB) is imaged as a 1–2% Sp/Sv amplitude arrival at 200–240 km depth, consistent with the depth of negative shear velocity and azimuthal anisotropy gradients imaged by surface wave tomography. To the west of the AMT, Sp conversions are much shallower at 150–190 km depth and are much weaker (<0.7%) or absent. Azimuthal anisotropy constrained by surface wave tomography also changes across the AMT, with stronger anisotropy to the east of the transition beneath the thicker lithospheric root. We suggest that the seismic changes across the AMT can be explained by considering the effects of asthenospheric flow beneath the leading edge of the thick lithospheric root. The mantle flow is dominantly driven by the drift of the North America plate. Locally higher flow velocities are expected where the asthenosphere is forced to flow beneath the thicker root. This mantle underflow could create a sharper seismic LAB east of the AMT via two effects. First, the local increase in flow velocities could steepen the thermal gradient at the base of the lithosphere, and hence the isotropic velocity contrast. Second, the increased strain rate along edge of the lithosphere could enhance the magnitude of azimuthal anisotropy. Our results suggest that seismically detectable LAB sharpness variations could be used to constrain geographic variations in coupling between plates and mantle convection. A secondary result is the image of a Mid-Lithospheric Discontinuity arrival at 80–110 km depth that is found primarily to the east of the AMT. This arrival is interpreted as produced by a layer of low-velocity metasomatic minerals that have accumulated since the >1.8 Ga creation of the lithosphere.

PandSreceiver function analysis of seafloor borehole broadband seismic data

[1] The crustal and lithospheric structure of the normal oceanic plates is investigated using converted wave techniques (P and S receiver functions (RFs) and novel stacking analysis techniques without deconvolution) applied to the data from two seafloor borehole broadband seismic stations located in the central Philippine Sea and in the northwest Pacific ocean. We observe sufficient energy from at least two discontinuities within the error bounds, one from the crust-mantle (Moho) boundary and the other from the seismic lithosphere-asthenosphere boundary (LAB). Synthetic seismograms for seafloor stations show that the water reverberations interfere with the vertical component of seismograms but to a lesser extent with the radial part of P receiver functions. On the other hand, S receiver functions are devoid of such effects since all the multiples and converted waves are separated in time by the primary S wave in time. Waveform modeling of RFs shows that the crustal thicknesses of the western Philippine Sea plate and northwest Pacific plate are ∼7–8 km, and that depths of LAB are 76 ± 1.8 km and 82 ± 4.4 km, respectively, with an abrupt Vs drop at LAB of ∼7%–8%, as reported by Kawakatsu et al. (2009). The LAB depth for the eastern Philippine plate is found to be ∼55 km. To confirm the robustness of this observation, we further analyze vertical and radial components of the data without deconvolution for P wave backscattered reflection phases and P-to-S converted phases. The result indicates that the reflected/converted phases from Moho and LAB are observed at timings consistent with the receiver function results. The effect of seismic anisotropy for observed RFs is also investigated.

Three-dimensional wavefield imaging of data from the USArray: New constraints on the geometry of the Farallon slab

This paper presents preliminary results from direct imaging of P to S conversion data from the Earthscope Transportable Array (TA). Input data are receiver function estimates from the Earthscope Automated Receiver Function Survey (EARS). The waveforms from EARS were first stacked to produce composite events from 60 different source regions defined by a radial grid with a center in the western United States (U.S.). The composite waveforms were imaged with a three-dimensional, prestack migration method using a plane wave decomposition and an inverse generalized Radon transform algorithm to yield estimates of radial and transverse component scattering potential from each composite event. Data deficiencies limited this processing to 50 of the composite events. The migrated results from these 50 events were stacked to produce a final three-dimensional image volume examined in this paper through three-dimensional visualization techniques. The crust-mantle boundary, the 410 km, and 660 km discontinuities are imaged. An erratic negative conversion at the depth that has been suggested as the seismic lithosphere-asthenosphere boundary is observed in these results, but I avoid interpreting this feature as it may be biased by interference with first-order, free-surface multiples from the Moho. The most significant new feature imaged by this technique is a series of east-dipping conversions directly above the 410 km discontinuity. This feature is imaged in the region immediately above the transition zone across the entire western U.S., although it becomes partly obscured in Arizona by interference with what I interpret as sediment reverberations that contaminate some stations in southern California and the Rio Grande Rift. Overlaying these results with published tomography models suggests that this feature is linked to the Juan de Fuca–Farallon slab. I follow this hypothesis to build a three-dimensional model of the top of the Farallon slab that honors the new data and results from published tomography models. The model clarifies that these dipping features are consistent with a bending plate model of the slab. Using a simple kinematic model of the geometry of the slab window linked to the widening of the San Andreas system, I argue that the same dipping features seen under Nevada are consistent with those seen to the north, suggesting that there is a continuous, relatively smooth slab-related boundary layer under most of the western U.S.

Imaging the seismic lithosphere-asthenosphere boundary of the oceanic plate

[1] The seismic lithosphere-asthenosphere boundary (LAB) or G discontinuity, a seismologically characterized abrupt drop in wave speed in the uppermost mantle, is one of the key issues in current geodynamics. Although plate tectonics started as a theory for the ocean, reports on LAB for normal oceanic regions are scarce due to paucity of seismic data, and whether or not the oceanic LAB grows with age is the key issue to be resolved. We conduct a systematic survey for the oceanic LAB using S-to-p converted seismic waves along three margins of oceanic plates whose crustal age ranges from ∼10 Myr to ∼130 Myr, and we observe laterally continuous oceanic LAB images. The thickness of the oceanic plate estimated from LABs increases with the plate age, though scattered, suggesting that the evolution of oceanic lithosphere is predominantly governed by temperature and that the oceanic seismic LAB represents a boundary that grows with age.

Europe from the bottom up: A statistical examination of the central and northern European lithosphere–asthenosphere boundary from comparing seismological and electromagnetic observations

The Lithosphere–Asthenosphere Boundary (LAB) is a fundamental boundary in the plate tectonic paradigm — it is the most pervasive on the planet, yet comparatively it is one we know little about. Defined initially on the basis of the mechanical response of the Earth to loading, its usage has become ubiquitous across the geosciences but the natural differences in its definition, due to differences in thermal, physical and chemical parameters, cause confusion. To advance this debate, comparisons are made both qualitatively and quantitatively, between the delineation of the LAB for Europe based on seismological and electromagnetic observations. We examine statistically, using robust methods, the LABs derived from independent datasets and methods. Essentially, all definitions of the LAB, as an impedance contrast from receiver functions (sLABrf), a seismic anisotropy change (sLABa) and an increase in conductivity from magnetotellurics (eLAB), are consistent with a deeper LAB beneath Precambrian Europe, and a shallower LAB beneath Phanerozoic Europe, with some exceptionally deep regions in Phanerozoic Europe, such as the Alps. All three LABs increase in depth significantly and rapidly at a location consistent with the surface expression of the Trans-European Suture Zone (TESZ) which separates Precambrian Europe to the north and east from Phanerozoic Europe in the centre. Two of the definitions, sLABrf and eLAB, are consistent for Phanerozoic Europe with mean values of 90–100 km, compared to an average sLABrf of 135 km. A different two, sLABa and sLABrf, are consistent for Precambrian Europe with mean values of 170–180 km compared to an eLAB mean of 250 km. The deeper eLAB depths for Precambrian Europe are more consistent with body wave and surface wave tomography models than are the sLABa and sLABrf ones, suggesting that the lower lithosphere beneath Precambrian Europe between 170 to 250 km is responding to shearing on the base of the lithosphere from plate driving forces. Taken together the definitions provide strong constraints on the nature of the LAB. For example, the electrical eLAB beneath the TESZ is anomalously thick, to possibly the transition zone, whereas sLABa and sLABrf seismological definitions would place it at far shallower depths. Given the sensitivity of electrical conductivity to partial melt and water content, the thick eLAB beneath the TESZ excludes interpretation of the seismic LAB in that location in terms of a thermal structure that would induce partial melting or of hydration.

The elusive lithosphere–asthenosphere boundary (LAB) beneath cratons

The lithosphere–asthenosphere boundary (LAB) is a first-order structural discontinuity that accommodates differential motion between tectonic plates and the underlying mantle. Although it is the most extensive type of plate boundary on the planet, its definitive detection, especially beneath cratons, is proving elusive. Different proxies are used to demarcate the LAB, depending on the nature of the measurement. Here we compare interpretations of the LAB beneath three well studied Archean regions: the Kaapvaal craton, the Slave craton and the Fennoscandian Shield. For each location, xenolith and xenocryst thermobarometry define a mantle stratigraphy, as well as a steady-state conductive geotherm that constrains the minimum pressure (depth) of the base of the thermal boundary layer (TBL) to 45–65 kbar (170–245 km). High-temperature xenoliths from northern Lesotho record Fe-, Ca- and Ti-enrichment, grain-size reduction and globally unique supra-adiabatic temperatures at 53–61 kbar (200–230 km depth), all interpreted to result from efficient advection of asthenosphere-derived melts and heat into the TBL. Using a recently compiled suite of olivine creep parameters together with published geotherms, we show that beneath cratons the probable deformation mechanism near the LAB is dislocation creep, consistent with widely observed seismic and electrical anisotropy fabrics. If the LAB is dry, it is probably diffuse (> 50 km thick) and high levels of shear stress (> 2 MPa or > 20 bar) are required to accommodate plate motion. If the LAB is wet, lower shear stress is required to accommodate plate motion and the boundary may be relatively sharp (≤ 20 km thick). The seismic LAB beneath cratons is typically regarded as the base of a high-velocity mantle lid, although some workers infer its location based on a distinct change in seismic anisotropy. Surface-wave inversion studies provide depth-constrained velocity models, but are relatively insensitive to the sharpness of the LAB. The S-receiver-function method is a promising new seismic technique with complementary characteristics to surface-wave studies, since it is sensitive to sharpness of the LAB but requires independent velocity information for accurate depth estimation. Magnetotelluric (MT) observations have, for many decades, imaged an “electrical asthenosphere” layer at depths beneath the continents consistent with seismic low-velocity zones. This feature is most easily explained by the presence of a small amount of water in the asthenosphere, possibly inducing partial melt. Depth estimates based on various proxies considered here are similar, lending confidence that existing geophysical tools are effective for mapping the LAB beneath cratons.

Inverse Monte Carlo simulation of the lithospheric thermal regime in the Fennoscandian Shield using xenolith-derived mantle temperatures

Monte Carlo inversion was applied in 2-dimensional conductive steady-state thermal simulation of a 600-km long lithosphere transect in the Fennoscandian Shield. The thermal regime in the mantle was constrained with thermobarometric data derived from kimberlite-hosted mantle xenoliths in eastern Finland, which suggest an average temperature of 1250±50°C at 208 km, but no partial melting down to at least 240 km. A priori models were generated from probability distributions assigned to thermal conductivity and heat production rate values of the 38 domains in the model, as well as the mantle heat flow density used as the lower boundary condition in the simulations. The forward problem was solved with a finite difference code, and the modification of the a priori distributions into a posteriori distributions was done using the Metropolis algorithm as the acceptance rule. The two-stage inversion, first using the measured surface heat flow values as a fitting object, and second, re-sampling the obtained a posteriori models but using xenolith-derived mantle temperature data as a fitting object, results in a considerable improvement in the resolution and average values of temperature and heat flow density in the model. The improvement can be seen in the model results to a distance of about 400 km from the xenolith area. The obtained results support a scheme that mantle heat flow is low in the thickest lithosphere area of the Fennoscandian Shield and about 10±1 mW m −2 at 200 km, and 13±1 mW m −2 in the uppermost subcrustal mantle. The inversion results suggest that there is no partial melt-bearing asthenosphere under the transect at depths shallower than at least 250 km. The seismic lithosphere-asthenosphere boundary is at depths of 110–170 km on the transect and corresponds to about 1100°C temperature in our model. This transition can be related to a zone of rheological weakening but not to the onset of melting. The mantle temperature and heat flow value in the eastern Finland kimberlite province are in agreement with models where small-scale convection (solid state creep) transports heat to the base of the lithosphere at about 250 km depth.