Comparison of gravimetric and seismic constraints on the structure of the Aegean lithosphere in the forearc of the Hellenic subduction zone in the area of Crete

The island of Crete is located in the forearc of the Hellenic subduction zone, where the African lithospheric plate is subducting beneath the Eurasian one. The depth of the plate contact as well as the internal structure of the Aegean plate in the area of Crete have been a matter of debate. In this study, seismic constrains obtained by wide-angle seismic, receiver function and surface wave studies are discussed and compared to a 3D density model of the region.The interface between the Aegean continental lithosphere and the African one is located at a depth of about 50 km below Crete. According to seismic studies, the Aegean lithosphere in the area of Crete is characterised by strong lateral, arc–parallel heterogeneity. An about 30 km thick Aegean crust is found in central Crete with a density of about 2850 kg/m³ for the lower Aegean continental crust and a density of about 3300 kg/m³ for the mantle wedge between the Aegean crust and the African lithosphere. For the deeper crust in the area of western Crete two alternative models have been proposed by seismic studies. One with an about 35 km thick crust and another one with crustal velocities down to the plate contact. A grid search is performed to test the consistency of these models with gravimetric constraints. For western Crete a model with a thick lower Aegean crust and a density of about 2950 kg/m³ is favoured. The inferred density of the lower Aegean crust in the area of Crete correlates well with S-wave velocities obtained by surface wave studies.Based on the 3D density model, the weight of the Aegean lithosphere is estimated along an E–W oriented profile in the area of Crete. Low weights are found for the region of western Crete.     

Crustal and upper mantle structure of the Mediterranean area derived from surface-wave data

A summary of results based mainly on the inversion of available surface-wave dispersion data is given for the Mediterranean area both for crustal and upper mantle structure. The results are presented on maps outlining the regionalization of the crust and the lithosphere-asthenosphere system in the area. It is possible to distinguish several types of crust with average S-wave velocities in the range 2.8–3.8 km s⁻¹ and thickness varying from a minimum of about 10–16 km, in the Western Mediterranean, to a maximum of about 50 km (including a possible transitional layer) beneath the Ionian Sea. The average properties of the crust and of the lithospheric part of the mantle indicate a possible continuous structure extending from North Africa through the Ionian Sea to the Adriatic Sea, characterized by the presence of a transitional layer at the crust-mantle boundary. Strong lateral variations are present in the lithosphere-asthenosphere system both in thickness, from 30 km in the Western Mediterranean, to about 130 km, under the Alps, and in S-wave velocity, from 4.1–4.2 km s⁻¹ up to 4.7 km s⁻¹. The relatively high position of low resistivity material that seems to characterize the Mediterranean area agrees fairly well with the shallower average top of the asthenosphere found in this area from the study of the elastic properties. The usefulness of combining seismological and electromagnetic studies is stressed.     

Comparison of gravimetric and seismic constraints on the structure of the Aegean lithosphere in the forearc of the Hellenic subduction zone in the area of Crete

The island of Crete is located in the forearc of the Hellenic subduction zone, where the African lithospheric plate is subducting beneath the Eurasian one. The depth of the plate contact as well as the internal structure of the Aegean plate in the area of Crete have been a matter of debate. In this study, seismic constrains obtained by wide-angle seismic, receiver function and surface wave studies are discussed and compared to a 3D density model of the region.The interface between the Aegean continental lithosphere and the African one is located at a depth of about 50 km below Crete. According to seismic studies, the Aegean lithosphere in the area of Crete is characterised by strong lateral, arc–parallel heterogeneity. An about 30 km thick Aegean crust is found in central Crete with a density of about 2850 kg/m³ for the lower Aegean continental crust and a density of about 3300 kg/m³ for the mantle wedge between the Aegean crust and the African lithosphere. For the deeper crust in the area of western Crete two alternative models have been proposed by seismic studies. One with an about 35 km thick crust and another one with crustal velocities down to the plate contact. A grid search is performed to test the consistency of these models with gravimetric constraints. For western Crete a model with a thick lower Aegean crust and a density of about 2950 kg/m³ is favoured. The inferred density of the lower Aegean crust in the area of Crete correlates well with S-wave velocities obtained by surface wave studies.Based on the 3D density model, the weight of the Aegean lithosphere is estimated along an E–W oriented profile in the area of Crete. Low weights are found for the region of western Crete.     

The main characteristics of the lithosphere-asthenosphere system in Italy and surrounding regions

We present a summary of the available information on Rayleigh-wave dispersion data for the Italian area. The dispersion curves can be grouped into two main families according to their shape: one with an almost linear trend in the period range from 40 to 150 sec the other with a relatively flat portion in the period range from 40 to 80 sec. The inversion of these data shows that there are strong lateral variation in the lithosphere system: a very rapid transition in the lithospheric thickness, from about 130 km to about 30 km, as well as in the S-wave velocity of the lithosphere and asthenosphere.Some cross-sections are described in order to have a more direct insight on the gross features of the upper mantle.An attempt is made to discuss the properties of the upper mantle taking into account the tectonic evolution of the Mediterranean.     

Crust and upper mantle structures beneath Northeast China from receiver function studies

P-wave and S-wave receiver function analyses have been performed along a profile consisted of 27 broadband seismic stations to image the crustal and upper mantle discontinuities across Northeast China. The results show that the average Moho depth varies from about 37 km beneath the Daxing’anling orogenic belt in the west to about 33 km beneath the Songliao Basin, and to about 35 km beneath the Changbai mountain region in the east. Our results reveal that the Moho is generally flat beneath the Daxing’anling region and a remarkable Moho offset (about 4 km) exists beneath the basin-mountain boundary, the Daxing’anling-Taihang Gravity Line. Beneath the Tanlu faults zone, which seperates the Songliao Basin and Changbai region, the Moho is uplift and the crustal thickness changes rapidly. We interpret this feature as that the Tanlu faults might deeply penetrate into the upper mantle, and facilitate the mantle upwelling along the faults during the Cenozoic era. The average depth of the lithosphere-asthenosphere boundary (LAB) is ~80 km along the profile which is thinner than an average thickness of a continental lithosphere. The LAB shows an arc-like shape in the basin, with the shallowest part approximately beneath the center of the basin. The uplift LAB beneath the basin might be related to the extensive lithospheric stretching in the Mesozoic. In the mantle transition zone, a structurally complicated 660 km discontinuity with a maximum 35 km depression beneath the Changbai region is observed. The 35 km depression is roughly coincident with the location of the stagnant western pacific slab on top of the 660 km discontinuity revealed by the recent P wave tomography.     

Heat flow in Eastern Egypt: The thermal signature of a continental breakup

The Red Sea is a modern example of continental fragmentation and incipient ocean formation. Heat flow data have been collected from eastern Egypt to provide information relating to the mode and mechanism of Red Sea opening. Preliminary heat flow data, including new data reported here, are now available from twenty-five sites in eastern Egypt and one site in western Sinai. A pattern of low to normal heat flow (35–55 mW m⁻²) inland with high heat flow (75–100 mW m⁻²) in a zone within 30 to 40 km of the coast is indicated.Moderately high heat flow (around 70 mW m⁻²) is indicated for the Gulf of Suez. The coastal zone thermal anomaly appears continuous with high heat flow previously reported for the Red Sea shelf. Heat production data indicate that the coastal thermal anomaly is not primarily related to crustal radiogenic heat production. The effects of rapid erosion may contribute to the anomaly, but are not thought to be the primary cause of the anomaly. If the anomaly is caused by lateral conduction from hot, extended, offshore lithosphere, the extension must have been active for the last 30 Ma or so, and a minimum of 100% extension is indicated. Alternatively, the anomaly is primarily caused by high mantle heat flow causing lithospheric thinning, centred beneath the Red Sea. The Red Sea is probably underlain by dominantly basic crust, formed either by intrusion into attenuated continental crust or sea-floor spreading, and for most purposes the crust formed in these two modes of extension may be essentially indistinguishable. Fission-track ages from eastern Egypt indicate that uplift started prior to, or at latest at the time of initial Red Sea opening, and this result, together with thermo-mechanical considerations, suggests an active asthenospheric upwelling beneath the Red Sea and high temperature in the lithosphere prior to extension.     

南海北部珠江口—琼东南盆地地壳速度结构与几何分层

基于南海北部大陆边缘珠江口—琼东南盆地深水区实施的14条近垂直深反射地震探测叠加速度谱,利用Dix公式将叠加速度剖面转换为地壳层速度剖面,并利用时深转换方法构建了深度域地壳层速度模型,综合各地壳速度剖面分析了南海北部大陆边缘珠江口与琼东南盆地不同深度层次的P波速度变化趋势以及地壳几何分层特征.结果表明,琼东南盆地区可分为4～8 km沉积层（V_P为1.7～4.7 km/s）、4～10 km厚的上地壳层（V_P为5.2～6.3 km/s）、5 km〗左右的下地壳层（V_P为6.4～7.0 km/s）以及2～6 km厚的高速下地壳底层（V_P＞7.0 km/s）.V_P＞7.0 km/s下地壳高速层的存在被认为是岩石圈伸展、下地壳底部底辟构造或者是残存的原始华夏下地壳基性层的地震学指示;综合研究区地球物理探测成果构建了跨越华南大陆与南海北部陆坡区剖面莫霍和岩石圈底界图像,揭示出岩石圈上地幔在华南大陆与南海北部大陆边缘的减薄特征.     

中国大陆及其邻区岩石层挠曲强度变化和均衡补偿机制

利用中国大陆以及最近与俄罗斯拼接的中亚地区的布格重力和地形数据，采用响应函数和互相关函数技术，结合弹性板挠曲理论，研究了中国大陆及其邻近地区（２５°Ｎ－７０°Ｎ，５０°Ｅ－１４０°Ｅ）的岩石层强度，用现代均衡的观点探讨了研究区的均衡补偿机制．研究结果表明，整个中国大陆及其邻近地区的有效弹性厚度Ｔｅ值为３５ｋｍ．其中，青藏地区Ｔｅ值为８５ｋｍ．西伯利亚西部至乌拉尔Ｔｅ值为７５ｋｍ，天山、塔里木盆地Ｔｅ值为４０ｋｍ，帕米尔高原Ｔｅ值为２５ｋｍ左右，西伯利亚中部、蒙古以及中国东北北部Ｔｅ值为４０ｋｍ，华北Ｔｅ值为２５ｋｍ，华南Ｔｅ值为４０ｋｍ．均衡补偿对于波长大于几千米的地形负荷为局部补偿，对几百公里到１１００ｋｍ的地形负荷为局部补偿和岩石层强度所支持，短波长地形负荷基本上由岩石层强度所支持．     

Seismicity in the vicinity of Ross Island, Antarctica

Earthquakes in the Ross Sea, Antarctica, are of two types: volcanic, and those which appear to be of tectonic origin. Volcanic events in the vicinity of Ross Island are associated exclusively with Mount Erebus, Ross Island; this volcano erupts regularly, generating several earthquakes per day whose characteristics are quite distinct from non-volcanic events. These nonvolcanic earthquakes are recognizable by their distinct P- and S-wave arrivals, and a lack of the high frequency, often monochromatic character typical of Erebus events.One hundred fifty-seven tectonic microearthquakes (M < 2.0) were recorded in 1983 and 1984 by the ten station network on Ross Island; these events were located using the least-squares routine, HYPOELLIPSE. Of these events, 106 have RMS residual traveltime errors of less than or equal to 0.6 seconds; they are clustered in the vicinity of Ross Island, but are not restricted to it. There is a linear trend of epicenters cutting across the island and continuing northward. Most activity seems to center beneath Mount Terra Nova, between Mount Erebus and Mount Terror. Mean depth for events is 8.2 km; however, depths are rather evenly distributed over a range of 0 to 25 km.Modelling based on Bouger gravity anomalies and seismic refraction studies indicates a depth to the Moho of about 40 km beneath the continent, shallowing to 27 km beneath the Ross Sea. This 27 km depth is approximately equal to the lower limit of the tectonic seismicity detected by the Erebus network; hence, events are of crustal origin. These data suggest, with the rift-type geochemistry of Erebus' magma, that the Ross Sea is a site of active crustal extension and rifting.     

云南壳幔S波速度结构与强震的构造背景

本文选取云南及周边65个台站记录到的47个地震事件,利用相匹配滤波技术分离出了基阶Rayleigh面波信号.选取与震中处于同一大圆弧上的两个台站,利用双台格林函数法获取了台间相速度频散,频散的周期范围在10～80 s之间.从2000个波形记录中提取了152个台站对之间的相速度频散,最后,利用台间的相速度频散反演得到云南...     

华北地区地壳上地幔S波三维速度结构

利用华北地区大型流动地震台阵的记录资料,采用近震和远震联合成像方法,得到了水平分辨率0.5°×0.5°、深至600km的S波速度结构.研究结果表明,上地壳S波速度结构与地表地质构造基本一致,燕山—太行山山脉均呈现高速异常,延庆—怀来盆地、大同盆地表现为低速异常,华北盆地内部的拗陷和隆起分别呈现低速和高速.唐山地区中地壳、山西裂陷盆地中下地壳存在明显的低速异常,可能分别与流体和热物质作用有关,有利于形成孕育强震的地质构造环境.90km的速度结构图像依然与地表的构造特征有较大的相关性,可能说明深部结构对地表构造有一定的控制作用.燕山隆起区岩石圈的厚度可达120～150km左右,华北盆地的岩石圈厚度可能在80km左右,太行山地区的岩石圈厚度介于两者之间.山西裂陷盆地上地幔低速层较厚,反映了该区不稳定的构造环境造成了地幔热物质的上涌.华北盆地下方220～320km出现的高速异常体,可能揭示了华北盆地上地幔仍然存在拆沉后残留的难熔、高密度的古老岩石圈地幔.研究区东部地幔转换带呈低速异常,推测可能与太平洋板块俯冲至该区下方地幔转换带前缘120°E左右的俯冲板块相变脱水有关.     

Anisotropic tomography of the Atlantic Ocean

We present the first regional three-dimensional model of the Atlantic Ocean with anisotropy. The model, derived from Rayleigh and Love wave phase velocity measurements, is defined from the Moho down to 300 km depth with a lateral resolution of about 500 km and is presented in terms of average isotropic S-wave velocity, azimuthal anisotropy and transverse isotropy.The cratons beneath North America, Brazil and Africa are clearly associated with fast S-wave velocity anomalies. The mid-Atlantic ridge (MAR) is a shallow structure in the north Atlantic corresponding to a negative velocity anomaly down to about 150 km depth. In contrast, the ridge negative signature is visible in the south Atlantic down to the deepest depth inverted, that is 300 km depth. This difference is probably related to the presence of hot-spots along or close to the ridge axis in the south Atlantic and may indicate a different mechanism for the ridge between the north and south Atlantic. Negative velocity anomalies are clearly associated with hot-spots from the surface down to at least 300 km depth, they are much broader than the supposed size of the hot-spots and seem to be connected along a north-south direction.Down to 100 km depth, a fast S-wave velocity anomaly is extenting from Africa into the Atlantic Ocean within the zone defined as the Africa superswell area. This result indicates that the hot material rising from below does not reach the surface in this area but may be pushing the lithosphere upward.In most parts of the Atlantic, the azimuthal anisotropy directions remain stable with increasing depth. Close to the ridge, the fast S-wave velocity direction is roughly parallel to the sea floor spreading direction. The hot-spot anisotropy signature is striking beneath Bermuda, Cape Verde and Fernando Noronha islands where the fast S-wave velocity direction seems to diverge radially from the hot-spots.The Atlantic average radial anisotropy is similar to that of the PREM model, that is positive down to about 220 km, but with slightly smaller amplitude and null deeper. Cratons have a lower than average radial anisotropy. As for the velocities, there is a difference between north and south Atlantic. Most hot-spots and the south-Atlantic ridge are associated with positive radial anisotropy perturbation whereas the north-Atlantic ridge corresponds to negative radial anisotropy perturbation.     

Three-dimensional crustal and upper mantle structure beneath Chalkidiki (northern Greece)

We invert teleseismic relative residuals recorded in the Thessaloniki seismological network, for the crustal and upper mantle structure beneath Chalkidiki (Northern Greece) where extension is observed to have occurred since Eocene time. After conducting several tests to insure the reliability of the results, a low-velocity anomaly (5–8%) is observed which is located beneath Chalkidiki for the top two layers (0–35 km and 35–85 km); this anomaly is probably related to the fact that the crust is thicker here than beneath the neighboring basins. For the two other layers, with depths between 85 and 145 km and between 145 and 205 km, relatively low velocities (3–5%) are observed beneath the Thermaico Gulf and the Kavala Basin. These results are compared with Lyberis's (1985) [8] calculated shallowing of the isotherms due to extension and thinning of the lithosphere. We conclude that the velocity anomalies that we observe are likely to be due to the intrusion of hot material from the asthenosphere into the gap in the extended lithosphere.     

Upper mantle structure of the Apulian plate from Rayleigh waves

Phase velocities of Rayleigh waves for the Adriatic Sea area are obtained in the period range 25–190 sec along the path (l'Aquila-Trieste) AQU-TRI and 20–167 sec along the path (Trieste-Bari) TRI-BAI.The phase velocities are systematically higher than the known values for the surrounding regions. The data inversion indicates the presence of a lithosphere typical of stable continental areas with clear high-velocity lid (V _s 4.6 km/sec) overlying a well developed low velocity zone (V _s 4.2 km/sec).P. F. Geodinamica C.N.R., Roma Pubbl. N. 189.     

中国东部海域及邻区岩石层地幔的P波速度结构与构造分析

利用中国大陆东部及台湾地区、日本和琉球群岛的地震观测数据,通过体波地震层析技术反演了中国东部海域及其邻近地区的P波速度结构.以此为依据分析了不同地区的岩石层性质和深部动力学条件,探讨了中朝与扬子块体、扬子与华夏块体在海区的深部边界及其构造属性,揭示出菲律宾海板块与欧亚大陆的碰撞以及板片俯冲下沉、弧后扩张作用对中国东部海域岩石层结构的影响.结果表明,中国东部海域的岩石层地幔存在明显的横向非均匀性,它们与区域构造的形成演化有一定的联系.中国大陆东部的五莲－青岛断裂与朝鲜半岛西缘断裂、济州岛南缘断裂共同构成中朝和扬子块体的边界,江绍断裂向东延伸至朝鲜半岛南端成为分隔扬子和华夏块体的边界;东海陆架与冲绳海槽的岩石层结构差异明显,东海陆架具有中国东部地区的岩石层特征,属于欧亚大陆向海域的延伸;冲绳海槽的岩石层强烈减薄,为大陆向大洋过渡的区域;沿着日本－琉球－台湾俯冲带,菲律宾海板块俯冲下沉引起的地幔扰动对中国东部海域产生了较大的影响,欧亚大陆与菲律宾海板块之间的相互碰撞导致台湾地区岩石层明显增厚.     

东海重磁地震综合探测剖面研究

依据东海南部一条自东海陆架直至菲律宾海盆的重磁地震综合探测剖面,采用地壳结构重磁地震综合反演解释系统开展岩石圈结构的综合研究,制作了岩石圈结构地学断面图. 结果表明基隆凹陷为一中新生代沉积凹陷,沉积厚度达到14km;冲绳海槽盆地为弧后扩张地堑型盆地,地壳厚度仅为14.5km;菲律宾板块沿北西西方向向欧亚板块俯冲,莫霍面急剧下插,导致冲绳海槽的形成与发展;岩石圈厚度约为80km,但在370km处仅为53km,在450km至540km处岩石圈厚度大于100km.     

Location of zones of anomalously high s-wave attenuation in the upper crust near ruapehu and ngauruhoe volcanoes, New Zealand

Many earthquakes within the crust near Ruapehu and Ngauruhoe volcanoes, recorded at epicentral distances less than 20 km on vertical seismometers, show S-waves of lower dominant frequency than the P-waves. A large number also have amplitudes in the S-group less than those of the P-waves. Whereas the reduced amplitude of S-waves relative to that of P-waves can be a source mechanism effect, the corresponding reduction in dominant frequency should be independent of the source radiation pattern. The most plausible reason for such a reduction in dominant S-wave frequency is that the waves have passed through a zone of partially molten rock. The data are therefore interpreted in terms of the presence of magma in restricted zones near the volcanoes.Using ray paths from 232 hypocentres to three permanent seismograph stations, together with paths from three additional earthquakes to one permanent and two temporary stations, an interpretation in three dimensions has been made of the source of the anomalous attenuation at depths between 2 and 10 km below datum (Ruapehu Crater Lake). Wave paths which lie largely at depths shallower than 2 km cannot be used, as almost all such paths show evidence of enhanced S-wave attenuation, and this is attributed to the presence of superficial pyroclastic and unconsolidated laharic material within 2 km of the surface.At Ruapehu, the data suggest the presence of three principal intrusions, one underlying much of the southwest slopes and reaching as far east as Crater Lake, one beneath the eastern side of the Summit Plateau, and one beneath part of the northeast slopes of the volcano. All three are essentially vertical or steeply dipping structures, detectable to a depth of between 7 and 9 km. The first appears to extend to within about 5 km of the surface, whereas the other two have intruded to within 2 or 3 km. Other, less well-defined, and comparatively small bodies exist beneath both the western and eastern slopes of Ruapehu.In the Ngauruhoe area, few earthquakes have occurred and all have been at depths less than 6 km. Therefore, only shallow attenuating areas can be defined. A small area of anomalous S-wave attenuation occurs beneath the northwest slopes of Ngauruhoe, and another, elongated, body appears to coincide with a fault zone west of the volcano. Both of these lie at depths of about 3 km below datum (less than 2 km below surface in one locality).Finally, areas of high attenuation, at depths of 4–5 km below datum, appear to define a narrow east-west zone about 6 km long in the immediate area of Whakapapa village. Other zones exist east of the volcanic axis, defining a line which cuts the axis on the north east slopes of Ruapehu, at a point where a parasite crater formed a few thousand years ago.     

Thermal Structure of the Ionian Slab

We propose a thermal model of the subducting Ionian microplate. The slab sinks in an isothermal mantle, and for the boundary conditions we take into account the relation between the maximum depth of seismicity and the thermal parameter L_th of the slab, which is a product of the age of the subducted lithosphere and the vertical component of the convergence rate. The surface heat-flux dataset of the Ionian Sea is reviewed, and a convective geotherm is calculated in its undeformed part for a surface heat flux of 42 mW m^–2, an adiabatic gradient of 0.6 mK m^–1, a mantle kinematic viscosity of 10¹⁷ m² s^–1 and an asthenosphere potential temperature of 1300°C. The calculated temperature-depth distribution compared to the mantle melting temperature indicates the decoupling limit between lithosphere and asthenosphere occurs at a depth of 105 km and a temperature of 1260°C. A 70–km thick mechanical boundary layer is found. By considering that the maximum depth of the seismic events within the slab is 600 km, a L_th of 4725 km is inferred. For a subduction rate equal to the spreading rate, the corresponding assimilation and cooling times of the microplate are about 7 and 90 Myr, respectively. The thermal model assumes that the mantle flow above the slab is parallel and equal to the subducting plate velocity of 6 cm yr^–1, and ignores the heat conduction down the slab dip. The critical temperature, above which the subduced lithosphere cannot sustain the stress necessary to produce seismicity, is determined from the thermal conditions governing the rheology of the plate. The minimum potential temperature at the depth of the deepest earthquake in the slab is 730°C.     

A double partial melt zone in the mantle beneath mid-ocean ridges

For a lherzolite mantle with about 0.1 wt.-percent CO₂ or less, and a CO₂/H₂O mole ratio greater than about one, the mantle solidus curve in P-T space will have two important low-temperature regions, one centered at about 9 kbar (30 km depth) and another beginning at about 28 kbar (90 km depth). It is argued that the depth of generation of primary tholeiitic magmas beneath ridge crests is about 9 kbar, and that the geotherm changes from an adiabatic gradient at greater pressures to a strongly superadiabatic gradient at lesser pressures. Such a ridge geotherm would intersect the solidus at two separate depth intervals corresponding to the two low-temperature regions on the solidus. With increasing age and cooling of the lithosphere, the shallow partial melt zone would pinch out and the thickness of the deep partial melt zone would decrease. With increasing depth in a mature oceanic lithosphere, the rock types would consist of depleted harzburgite from directly beneath the crust to about 30 km depth, fertile spinel lherzolite from about 30 km to 50–60 km, and fertile garnet lherzolite from about 50–60 km to the top of the deep partial melt zone at about 90 km.     

Crustal and upper mantle structure of stable continental regions in North America and northern Europe

From an analysis of many seismic profiles across the stable continental regions of North America and northern Europe, the crustal and upper mantle velocity structure is determined. Analysis procedures include ray theory calculations and synthetic seismograms computed using reflectivity techniques. TheP wave velocity structure beneath the Canadian Shield is virtually identical to that beneath the Baltic Shield to a depth of at least 800 km. Two major layers with a total thickness of about 42 km characterize the crust of these shield regions. Features of the upper mantle of these region include velocity discontinuities at depths of about 74 km, 330 km, 430 km and 700 km. A 13 km thickP wave low velocity channel beginning at a depth of about 94 km is also present.A number of problems associated with record section interpretation are identified and a generalized approach to seismic profile analysis using many record sections is described. TheS wave velocity structure beneath the Canadian Shield is derived from constrained surface wave data. The thickness of the lithosphere beneath the Canadian and Baltic Shields is determined to be 95–100 km. The continental plate thickness may be the same as the lithospheric thickness, although available data do not exclude the possibility of the continental plate being thicker than the lithosphere.