Lower mantle dynamics with the post-perovskite phase change, radiative thermal conductivity, temperature- and depth-dependent viscosity

The new post-perovskite phase near the core-mantle boundary has important ramifications on lower mantle dynamics. We have investigated the dynamical impact arising from the interaction of temperature- and depth-dependent viscosity with radiative thermal conductivity, up to a lateral viscosity contrast of 10⁴, on both the ascending and descending flows in the presence of both the endothermic phase change at 670 km depth and an exothermic post-perovskite transition at 2650 km depth. The phase boundaries are approximated as localized zones. We have employed a two-dimensional Cartesian model, using a box with an aspect-ratio of 10, within the framework of the extended Boussinesq approximation. Our results for temperature- and depth-dependent viscosity corroborate the previous results for depth-dependent viscosity in that a sufficiently strong radiative thermal conductivity plays an important role for sustaining superplumes in the lower mantle, once the post-perovskite phase change is brought into play. This aspect is especially emphasized, when the radiative thermal conductivity is restricted only to the post-perovskite phase. These results revealed a greater degree of asymmetry is produced in the vertical flow structures of the mantle by the phase transitions. Mass and heat transfer between the upper and lower mantle will deviate substantially from the traditional whole-mantle convection model. Streamlines revealed that an overall complete communication between the top and lower mantle is difficult to be achieved.     

Upper mantle lateral heterogeneities and magnetotelluric daily variation data

We use telluric and magnetic data of the diurnal variation recorded in Europe, Australia and North America to study the magnetotelluric tensor in the 6h–24h period range. We use associate directions and we eliminate the effects of deviation of telluric currents. We thus obtain for each observatory reliable phases and apparent resistivity values representative of the neighbouring stratified substratum. It appears that the values obtained in the four European observatories (Saint-Maur, France; Ebro, Spain; Toledo, Spain; Nagycenk, Hungary) give similar results and that these results are different from those obtained either in Tucson (USA) or in Watheroo (Australia).Using Bostick transform we interpret these phase and apparent resistivity values in terms of conductivity of the upper mantle. We discuss then the conductivity heterogeneities in terms of change either in temperature, or partial melting or percentage of fluids of the upper mantle: at depths of about 300 km, the upper mantle appears to be 100 °C hotter under Australia than under Europe; the probable presence of fluids at depths about 100 km in the southwestern North America upper mantle appears to be responsible for the high observed conductivities. All these conductivity values are coherent with tomography results from Woodhouse and Dziewonsky: high (low) conductivities are cohernet with low (high)seismic wave velocities.     

Water distribution across the mantle transition zone and its implications for global material circulation

Various methods for inferring the water distribution in Earth's mantle are reviewed including geochemical and geophysical methods. The geochemical approach using the water contents of basalts shows that the water content in the source regions of ocean island basalt is generally larger than that of the source region of mid-ocean ridge basalt, but the location of the source regions of ocean island basalts is poorly constrained. Geophysical approaches have potential of providing constraints on the spatial distribution of water but their usefulness depends critically on the sensitivity of geophysical observations to water content relative to other factors, in addition to the resolution of geophysical observations. Existing experimental data on the influence of water on seismologically observable properties and on electrical conductivity are reviewed. Frequently used seismological observations such as the anomalies in seismic wave velocities and of the topography on the mantle discontinuities are only weakly sensitive to water content but more sensitive to other factors such as the major element chemistry and temperature for a typical range of water contents. In contrast, electrical conductivity is highly sensitive to water content and only modestly sensitive to other factors such as temperature, oxygen fugacity and major element chemistry. Models of electrical conductivity–depth profiles are constructed where the influence of hydrogen and iron partitioning among coexisting minerals and of the depth variation in oxygen fugacity are incorporated. It is shown (i) that the electrical conductivity varies more than two orders of magnitude for a plausible range of water content in the mantle (~ 10 ppm wt to ~ 1 wt.%) and (ii) that if water content is constant with depth, there will be a drop in electrical conductivity at ~ 410-km. Although the resolution is not as high as seismological observations, geophysically inferred electrical conductivity distributions generally show higher conductivity in the mantle transition zone than the upper mantle, suggesting that the water content in the transition zone is higher than that in the upper mantle with some lateral variations. Implications of inferred water distribution are discussed including the possible partial melting near 410-km and its role in global water circulation.     

Nature of heterogeneity of the upper mantle beneath the northern Philippine Sea as inferred from attenuation and velocity tomography

We investigated the physical properties in the upper mantle beneath the Philippine Sea using a theoretical relation derived by Karato [Mapping water content in the upper mantle. Subduction factory, AGU Monograph, in press]. From the attenuation model of Shito and Shibutani [Phys. Earth Planet. Interact., in press] and the velocity model of Widiyantoro et al. [Earth Planet. Sci. Lett. 173 (1999) 91], observed attenuation and velocity anomalies were evaluated to explain the temperature, water content, and chemical heterogeneities in the target area. The results indicate that the observed anomalies in the shallower regions (50–200 km) may be due to chemical composition effects (e.g., concentration of iron), in addition to the temperature and water content anomalies. In contrast, for the deep upper mantle (300–400 km), the observations can be explained by only the effects of high water content (10–50 times higher than the average mantle). These inferred properties of the mantle are consistent with the tectonic history of the Philippine Sea region, which has had a long history of subduction and active magmatism.     

Temperature profiles in the earth of importance to deep electrical conductivity models

Deep in the Earth, the electrical conductivity of geological material is extremely dependent on temperature. The knowledge of temperature is thus essential for any interpretation of magnetotelluric data in projecting lithospheric structural models. The measured values of the terrestrial heat flow, radiogenic heat production and thermal conductivity of rocks allow the extrapolation of surface observations to a greater depth and the calculation of the temperature field within the lithosphere. Various methods of deep temperature calculations are presented and discussed. Characteristic geotherms are proposed for major tectonic provinces of Europe and it is shown that the existing temperatures on the crust-upper mantle boundary may vary in a broad interval of 350–1,000°C. The present work is completed with a survey of the temperature dependence of electrical conductivity for selected crustal and upper mantle rocks within the interval 200–1,000°C. It is shown how the knowledge of the temperature field can be used in the evaluation of the deep electrical conductivity pattern by converting the conductivity-versustemperature data into the conductivity-versus-depth data.     

Constraints on the dynamics of mantle plumes from uplift of the Hawaiian Islands

The ∼0.2 mm/yr uplift of Hawaiian islands Lanai and Molokai and Hawaiian swell topography pose important constraints on the structure and dynamics of mantle plumes. We have formulated 3-D models of mantle convection to investigate the effects of plume-plate interactions on surface vertical motions and swell topography. In our models, the controlling parameters are plume radius, excess plume temperature, and upper mantle viscosity. We have found that swell height and swell width constraints limit the radius of the Hawaiian plume to be smaller than 70 km. The additional constraint from the uplift at Lanai requires excess plume temperature to be greater than 400 K. If excess plume temperature is 400 K, models with plume radius between 50 and 70 km and upper mantle viscosity between 10²⁰ and 3×10²⁰ Pa s satisfy all the constraints. Our results indicate that mantle plume in the upper mantle may be significantly hotter than previously suggested. This has important implications for mantle convection and mantle melting. In addition to constraining plume dynamics, our models also provide a mechanism to produce the observed uplift at Lanai and Molokai that has never been satisfactorily explained before.     

3D shear-wave velocity structure of the Eifel plume, Germany

The Quaternary Eifel volcanic fields, situated on the Rhenish Massif in Germany, are the focus of a major interdisciplinary project. The aim is a detailed study of the crustal and mantle structure of the intraplate volcanic fields and their deep origin. Recent results from a teleseismic P-wave tomography study reveal a deep low-velocity structure which we infer to be a plume in the upper mantle underneath the volcanic area [J.R.R. Ritter et al., Earth Planet. Sci. Lett. 186 (2001) 7-14]. Here we present a travel-time investigation of 5038 teleseismic shear-wave arrivals in the same region. First, the transverse (T) and radial (R) component travel-time residuals are treated separately to identify possible effects of seismic anisotropy. A comparison of 2044 T- and 2994 R-component residuals demonstrates that anisotropy does not cause any first-order travel-time effects. The data sets reveal a deep-seated low-velocity anomaly beneath the volcanic region, causing a delay for teleseismic shear waves of about 3 s. Using 3773 combined R- and T-component residuals, an isotropic non-linear inversion is calculated. The tomographic images reveal a prominent S-wave velocity reduction in the upper mantle underneath the Eifel region. The anomaly extends down to at least 400 km depth. The velocity contrast to the surrounding mantle is depth-dependent (from −5% at 31-100 km depth to at least −1% at 400 km depth). At about 170-240 km depth the anomaly is nearly absent. The resolution of the data is sufficient to recover the described features, however the anomaly in the lower asthenosphere is underestimated due to smearing and damping. The main anomaly is similar to the P-wave model except the latter lacks the ‘hole’ near 200 km depth, and both are consistent with an upper mantle plume structure. For plausible anhydrous plume material in the uppermost 100 km of the mantle, an excess temperature as great as 200-300 K is estimated from the seismic anomaly. However, 1% partial melt reduces the required temperature anomaly to about 100 K. The temperature anomaly associated with the deeper part of the plume (250 to about 450 km depth) is at least 70 K. However, this estimate is quite uncertain, because the amplitude of the shear-wave anomaly may be larger than the modelled one. Another possibility is water in the upwelling material. The gap at 170-240 km depth could arise from an increase of the shear modulus caused by dehydration processes which would not affect P-wave velocities as much. An interaction of temperature and compositional variations, including melt and possibly water, makes it difficult to differentiate quantitatively between the causes of the deep-seated low-velocity anomaly.     

超高压下氧化物电导率测量及下地幔电导率的讨论

在金刚石对顶砧压机上,对一些氧化物进行了0—30 GPa 压力范围下的电导率测量,发现所测氧化物可按其电导性质为两类:过渡族元素氧化物和非过渡族元素氧化物前者平均电导率较高且有正压效应,后者平均电导率较低且压力效应不明显.过渡族元素氧化物的高导性质对讨论下地幔的电导事和化学模式具有重要意义.结合前人的研究成果,半定量地估计了700 km 处下地幔的电导率.     

Seismic scattering at the top of the mantle Transition Zone

We document strong seismic scattering from around the top of the mantle Transition Zone in all available high resolution explosion seismic profiles from Siberia and North America. This seismic reflectivity from around the 410 km discontinuity indicates the presence of pronounced heterogeneity in the depth interval between 320 and 450 km in the Earth’s mantle. We model the seismic observations by heterogeneity in the form of random seismic scatterers with typical scale lengths of kilometre size (10-40 km by 2-10 km) in a 100-140 km thick depth interval. The observed heterogeneity may be explained by changes in the depths to the α-β-γ spinel transformations caused by an unexpectedly high iron content at the top of the mantle Transition Zone. The phase transformation of pyroxenes into the garnet mineral majorite probably also contributes to the reflectivity, mainly below a depth of 400 km, whereas we find it unlikely that the presence of water or partial melt is the main cause of the observed strong seismic reflectivity. Subducted oceanic slabs that equilibrated at the top of the Transition Zone may also contribute to the observed reflectivity. If this is the main cause of the reflectivity, a substantial amount of young oceanic lithosphere has been subducted under Siberia and North America during their geologic evolution. Subducted slabs may have initiated metamorphic reactions in the original mantle rocks.     

How flat is the lower-mantle temperature gradient?

The temperature gradient in the lower mantle is fundamental in prescribing many transport properties, such as the viscosity, thermal conductivity and electrical conductivity. The adiabatic temperature gradient is commonly employed for estimating these transport properties in the lower mantle. We have carried out a series of high-resolution 3-D anelastic compressible convections in a spherical shell with the PREM seismic model as the background density and bulk modulus and the thermal expansivity decreasing with depth. Our purpose was to assess how close under realistic conditions the horizontally averaged thermal gradient would lie to the adiabatic gradient derived from the convection model. These models all have an endothermic phase change at 660 km depth with a Clapeyron slope of around −3 MPa K⁻¹, uniform internal heating and a viscosity increase of 30 across the phase transition. The global Rayleigh number for basal heating is around 2×10⁶, while an internal heating Rayleigh number as high as 10⁸ has been employed. The pattern of convection is generally partially layered with a jump of the geotherm across the phase change of at most 300 K. In all thermally equilibrated situations the geothermal gradients in the lower mantle are small, around 0.1 K km⁻¹, and are subadiabatic. Such a low gradient would produce a high peak in the lower-mantle viscosity, if the temperature is substituted into a recently proposed rheological law in the lower mantle. Although the endothermic phase transition may only cause partial layering in the present-day mantle, its presence can exert a profound influence on the state of adiabaticity over the entire mantle.     

How is the European Lithosphere Imaged by Magnetotellurics?

I review recent investigations on the electrical conductivity of the lithosphere and asthenosphere in Europe. The principal method in the reviewed studies is the magnetotelluric method, but in many cases other electromagnetic methods (e.g., magnetovariational profilings and geomagnetic depth soundings) have provided additional information on subsurface conductivity or have been the primary method. The review shows that the magnetotelluric method has been used, and is being used, in all kinds of environments and for many different processes shaping the crust and lithosphere. The crust is very heterogeneous, both with respect to the scale of conductive/resistive features and interpretations: research targets vary from Archaean palaeostructures to ongoing processes. The European database of the depth to the lithosphere-asthenosphere boundary (LAB) in Europe is updated, and a new map showing lateral variations of the depth of LAB is provided. The compilation shows that (1) the Phanerozoic European lithosphere, with considerable variations (45–150 km), is much thinner than the Precambrian European lithosphere, (2) the Trans-European Suture Zone is a major electrical border in Europe separating electrically (as well as geophysically and geologically in general) two quite different settings, (3) the thinnest lithosphere is found under the extensional Pannonian Basin (45–90 km), (4) in most of the East European Craton there are no indications of a high conductivity zone in upper mantle. In many regions there is no information at all on upper mantle conductivity, which calls for pan-European projects to operate arrays of simultaneously recording instruments with long recording periods (2–8 months) and dense spatial sampling (20–50 km).     

On the origin of El Chichón volcano and subduction of Tehuantepec Ridge: A geodynamical perspective

The origin of El Chichón volcano is poorly understood, and we attempt in this study to demonstrate that the Tehuantepec Ridge (TR), a major tectonic discontinuity on the Cocos plate, plays a key role in determining the location of the volcano by enhancing the slab dehydration budget beneath it. Using marine magnetic anomalies we show that the upper mantle beneath TR undergoes strong serpentinization, carrying significant amounts of water into subduction. Another key aspect of the magnetic anomaly over southern Mexico is a long-wavelength (∼ 150 km) high amplitude (∼ 500 nT) magnetic anomaly located between the trench and the coast. Using a 2D joint magnetic-gravity forward model, constrained by the subduction P–T structure, slab geometry and seismicity, we find a highly magnetic and low-density source located at 40–80 km depth that we interpret as a partially serpentinized mantle wedge formed by fluids expelled from the subducting Cocos plate. Using phase diagrams for sediments, basalt and peridotite, and the thermal structure of the subduction zone beneath El Chichón we find that ∼ 40% of sediments and basalt dehydrate at depths corresponding with the location of the serpentinized mantle wedge, whereas the serpentinized root beneath TR strongly dehydrates (∼90%) at depths of 180-200 km comparable with the slab depths beneath El Chichón (200-220 km). We conclude that this strong deserpentinization pulse of mantle lithosphere beneath TR at great depths is responsible for the unusual location, singularity and, probably, the geochemically distinct signature (adakitic-like) of El Chichón volcano.     

Evidence of electrical anisotropic structures in the lower crust and the upper mantle beneath the Rhenish Shield

We present the results of a long-period electromagnetic investigation of the crust and upper mantle beneath the Rhenish Shield in Western Germany. The magnetotelluric phase data reveal a frequency-dependent regional strike that varies only smoothly across the array. At short periods (t<100 s) the striking of the maximum phase splitting is N45°E, which can be explained with an electrical anisotropic lower crust. At long periods (t>1000 s) there is a consistent striking in WE direction, which provides strong evidence of an anisotropic structure in the upper mantle, too. Geomagnetic data were also used, whereas we reference the magnetic field components of all sites to an arbitrarily chosen field site. That provides a direct view of the anomalous current flow. We show that in case of a non-one-dimensional electrical substructure of the reference site all magnetic transfer functions of the other sites can be affected by lateral conductivity contrasts beneath the reference site. A simple method to remove such effects is introduced. Applying this method the magnetic data show a distinct anomaly in the northwestern part of the area. Finally, we present a 3D model of the conductivity structure beneath the Rhenish Shield which contains a superposition of two anisotropic structures in the lower crust (2000 S) and the upper mantle (20?000 S), respectively, and a local anomaly in the upper crust (4000 S).     

Lateral variation in upper mantle viscosity: role of water

Differences in the viscosity of the earth's upper mantle beneath the western US (∼10¹⁸-10¹⁹ Pa s) and global average values based on glacial isostatic adjustment and other data (∼10²⁰-10²¹ Pa s) are generally ascribed to differences in temperature. We compile geochemical data on the water contents of western US lavas and mantle xenoliths, compare these data to water solubility in olivine, and calculate the corresponding effective viscosity of olivine, the major constituent of the upper mantle, using a power law creep rheological model. These data and calculations suggest that the low viscosities of the western US upper mantle reflect the combined effect of high water concentration and elevated temperature. The high water content of the western US upper mantle may reflect the long history of Farallon plate subduction, including flat slab subduction, which effectively advected water as far inland as the Colorado Plateau, hydrating and weakening the upper mantle.     

Geochemical constraints on the model of the composition and thermal conditions of the Moon according to seismic data

A new method of reconstruction of the temperature profile in the lunar mantle from the velocities of seismic P- and S-waves for different models of chemical composition is developed. The procedure of the solution of an inverse problem is realized with the help of the minimization of the Gibbs free energy and the equations of state of a mantle substance, taking into account phase transformations, anharmonicity, and the effects of inelasticity. The geophysical and geochemical constraints on composition and temperature distribution in Moon’s mantle are established. The upper mantle can be composed of olivine pyroxenite, depleted by low-volatile oxides (∼2 wt % of CaO and Al₂O₃). On the contrary, the lower mantle must be enriched by low-volatile oxides (∼4–6 wt % of CaO and Al₂O₃). Its composition can be represented by a mineral association of the olivine + clinopyroxene + garnet or olivine + orthopyroxene + clinopyroxene + garnet type, which is close in composition to pyrolite. The temperature distribution at depths 50–1000 km are approximated by the equation: T(°C) = 351 + 1718[1–exp (−0.00082H)]. The constraints inferred make it possible to conclude that the published values of the velocities of P- and S-waves for the lunar mantle, obtained by processing the data of seismic experiments of the Apollo lunar mission are inconsistent with each other at depths below 300 km. Otherwise, the variations in the velocities of P- and S-waves disturb the symmetry between the petrological model (composition), the temperature profile, and the seismic profile.     

Upper mantle P wave velocity structure of the northern part of China and Mongolia

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Shear localisation in upper mantle peridotites

Upper mantle peridotite bodies at the earth's surface contain relict structures and microstructures which provide direct information on the role and the mechanisms of shear localisation in the upper mantle. Deformation which occurred at high temperatures (T>950±50°C) is relatively homogeneous within domains ranging in scale from a few kilometres to a few tens of kilometres. Below 950±50°C strain is localised into centimetre to several hundred metre wide shear zones which commonly contain hydrated mylonitic peridotites. The microstructures developed in the peridotites suggest there is a correlation between the occurrence of shear localisation and the occurrence of strain softening and brittle deformation processes. The most important strain softening processes are inferred to be structural and reaction induced softening. Structural softening processes include dynamic recrystallisation and strain-induced transitions from dislocation creep to some form of grain-size-sensitive (GSS) creep. Reaction induced softening is related to the formation of fine grained polyphase reaction products which deform by GSS creep and the formation of weak sheet silicates such as phlogopite, chlorite, talc and antigorite. From experimental studies these softening processes and brittle deformation processes are inferred to occur mainly at temperatures less than about 910±160°C. This temperature range is inferred to be a significant rheological transition in the upper mantle. Below 910±160°C deformation during orogenesis may be accommodated by an anastomosing network of hydrated mylonitic shear zones with a distinct, perhaps weak, rheology. At higher temperatures strain is accommodated in much wider deformation zones.On the scale of the lithosphere the degree of localisation may be different to that determined at the scale of the periodotite massif. An anastomosing network of hundred metre wide mylonitic shear zones forming 0.05–0.3 by volume fraction of the mantle lithosphere atT<950°C could accommodate inhomogeneous or homogeneous bulk deformation depending on the spatial distribution and ordering of the mylonite zones. The higher temperature deformation at deeper levels in the mantle could be markedly inhomogeneous being concentrated in shear zones with widths in the range of 2–20 km, alternatively these zones may widen significantly during deformation, resulting in a decrease in the degree of localisation with increasing bulk strain.     

华南陆缘高热流区的壳幔温度结构与动力学背景

华南陆缘是我国重要的矿产、地热资源区.晚中生代以来,在太平洋板块西向俯冲,地幔热对流活动共同作用下,该区出现多期岩浆-热事件和大规模爆发式成矿作用.在前人研究基础上,本文利用地表热流观测资料、地震剪切波资料、重力位球谐系数,计算了壳-幔温度结构,分析了动力学背景.计算结果表明：华南陆缘东南沿海地带,地壳10 km以浅温度达200℃以上,居里点温度475℃,莫霍面平均温度550℃.地壳浅层较热,花岗岩中放射性元素衰变放热是地壳浅层地下水热活动的重要热源,但地壳总体温度不高,为"冷壳热幔"型热结构.地幔中,90 km深度,温度950~1250℃;120 km深度,温度1050~1400℃;150 km深度,温度1200~1450℃;220 km深度,温度1500~1700℃."热"岩石圈底界深度在110~150 km之间,西深东浅.岩石圈内,地幔应力场为挤压-伸展相间格局;岩石圈之下,地幔应力场为一个以南昌为中心、长轴NE-SW向的椭圆.分析认为,晚中生代以来,太平洋板块的西向俯冲,导致华南陆缘在区域性SE向地幔对流背景上叠加局域性不稳定热扰动,在175~85Ma期间,上地幔物质向上流动,形成不同的岩浆活动高峰期.同时,岩石圈地幔受俯冲洋壳流体的影响,含水量高,黏度小,在地幔流切向应力场作用下,岩石圈底界由西向东"波浪"状减薄.现今岩石圈之下仍具备地幔小尺度热对流温度条件,但除地表浅层外,地壳整体温度不高,岩石圈构造稳定.     

基于电性结构模型的青藏高原东缘上地幔热结构研究

研究青藏高原东缘地区的深部物质结构对于理解青藏高原的隆升及扩张机制具有重要的科学意义.本文将青藏高原东缘实测大地电磁测深剖面反演所得的岩石圈电性结构模型与高温高压岩石物理实验测得的上地幔矿物和熔融体导电性定量关系相结合,通过Hashin-Shtrikman(HS)边界条件建立上地幔电导率与温度、熔融百分比等参数的定量关系,在此基础上计算得到了青藏高原东缘上地幔热结构及熔融百分比分布模型.研究结果表明在青藏高原东缘地区通过大地电磁测深方法所探测到的上地幔低阻体可以解释为由高温作用所产生的局部熔融区域.其中,松潘—甘孜地块上地幔高导体对应的温度介于1300～1500℃之间,熔融百分比可高达10%,支持前人将松潘—甘孜地块内部的低阻体解释为局部熔融的观点.龙门山断裂带以东、四川盆地西缘的上地幔高导体温度介于1200～1400℃之间,熔融百分比介于1%～5%左右,表明扬子克拉通的西缘可能正在经历一定程度的活化作用.龙门山断裂带下方的上地幔高阻体温度介于1100℃附近,基本没有发生局部熔融,具有较冷的刚性块体特征,与该区域频发的地震活动相吻合.四川盆地东部的扬子上地幔温度介于800～900℃之间,没有发生局部熔融,符合古老稳定的克拉通块体的基本特征.     

Seismic structure of the upper mantle beneath the western Philippine Sea

We investigate the seismic structure of the western Philippine Sea using two sets of seismological observations: ScS reverberations, which provide the layering framework for a regional upper mantle model, and observations of frequency-dependent phase delays for direct S waves, surface-reflected phases (sS, SS, sSS), and surface waves (R₁, G₁), which constrain the velocity and anisotropy structure within the layers. The combined data set, comprising 17 discontinuity amplitudes and layer travel times from the ScS-reverberation stack and more than 1000 frequency-dependent phase delays, was inverted for a path-averaged, radially anisotropic model. Mineralogical estimates of the bulk sound velocity and density are incorporated as complementary constraints. The final model, PHB3, is characterized by a 11.5-km thick crust, an anisotropic lid bounded by a sharp negative G discontinuity at 89 km, an anisotropic low-velocity layer extending to 166 km, a subjacent high-gradient region, and transition-zone discontinuities at depths of 408 km, 520 km and 664 km. The lid is slower than in a comparable model for the Tonga–Hawaii corridor (PA5), but also significantly thicker, requiring a compositional variation between the two regions. We explore the hypothesis that the thickness of the oceanic lid is controlled by the melting depth at the spreading centers during crust formation, and that the thicker crust and lid in the Philippine Sea results from deeper melting owing to a higher potential temperature and perhaps a higher water content in the upper mantle.