We present new results on the structure resulting from Palaeoproterozoic terrane accretion and later formation of one of the aulacogens in the East European Platform. Seismic data has been acquired along the 530-km-long, N–S-striking EUROBRIDGE'97 traverse across Sarmatia, a major crustal segment of the East European Craton. The profile extends across the Ukrainian Shield from the Devonian Pripyat Trough, across the Palaeoproterozoic Volyn Block and the Korosten Pluton, into the Archaean Podolian Block. Seismic waves from chemical explosions at 18 shot points at approximately 30-km intervals were recorded in two deployments by 120 mobile three-component seismographs at 3–4 km nominal station spacing. The data has been interpreted by use of two-dimensional tomographic travel time inversion and ray trace modelling. The high data quality allows modelling of the P- and S-wave velocity structure along the profile. There are pronounced differences in seismic velocity structure of the crust and uppermost mantle between the three main tectonic provinces traversed by the profile: (i) the Pripyat Trough is a ca. 4-km-deep sedimentary basin, fully located in the Osnitsk–Mikashevichi Igneous Belt in the northern part of the profile. The velocity structure is typical for a Precambrian craton, but is underlain by a ca. 5-km-thick lowest crustal layer of high velocity. The development of the Pripyat Trough appears to have only affected the upper crust without noticeable thinning of the whole crust; this may be explained by a rheologically strong lithosphere at the time of formation of the trough. (ii) Very high seismic velocity and Vp/Vs ratio characterise the Volyn Block and Korosten Pluton to a depth of 15 km and probably also the lowest crust. The values are consistent with an intrusive body of mafic composition in the upper crust that formed from bimodal melts derived from the mantle and the lower crust. (iii) The Podolian Block is close to a typical cratonic velocity structure, although it is characterised by relatively low seismic velocity and Vp/Vs ratio. A pronounced SW-dipping mantle reflector from Moho to at least 70 km depth may represent the Proterozoic suture between Sarmatia and Volgo–Uralia, the structure from terrane accretion, or a later shear zone in the upper mantle. The sub-Moho P-wave seismic velocity is high everywhere along the profile, with the exception of the area above the dipping reflector. This velocity change further supports a plate tectonic origin of the dipping mantle reflector. The profile demonstrates that structure from Palaeoproterozoic plate tectonic processes are still identifiable in the lithosphere, even where younger metamorphic equilibration of the crust has taken place.

The large-scale seismic experiment POLONAISE '97 (POlish Lithospheric ONsets-An International Seismic Experiment) was carried out in May 1997 in Poland, Lithuania, and Germany. Its main purpose was to investigate the structure of the crust and the uppermost mantle in the region of the Trans European Suture Zone (TESZ) that lies between the East European Craton (EEC) and the Palaeozoic Platform. This paper covers the interpretation of seismic data along the NW-SE-trending, 180-km-long profile P5 located on the EEC. The recordings were of a high quality with seismic energy clearly visible along the whole profile. We have not found waves refracted below the upper crust in first arrivals. In the NW part of the profile, we have delineated a high-velocity body with the P-wave velocity in the range of 6.5-6.75 km/s in the upper crust. It corresponds to the Ketrzyn anorthosite massif within the Mazury complex. The Mazowsze massif is rather uniformly characterized by P-wave velocities 5.9-6.05 and 6.2-6.35 km/s in two layers, respectively. Sufficient S-wave data were available to estimate the V-p/V-s ratio (as well as the Poisson ratio), being 1.80 (0.277) in the high-velocity body and 1.67 (0.220) in the upper crust.Apart from the 2-D model along the profile, results of 3-D modelling in the area of the P5 profile are presented. Using off-line recordings, we got P-wave velocity field up to 8 km/s below the P5 profile at the depth of about 40 kin as well as horizontal extent of the high-velocity body. (C) 2002 Elsevier Science B.V. All rights reserved.

Within the framework of the EUROBRIDGE project, the P-wave velocity structure along the coast of southeastern Sweden was determined from 2-D forward modeling of seismic refraction and wide-angle reflection airgun data on a profile named the Coast Profile. Previously obtained results of the FENNOLORA Profile were also used.The studied traverse extends from the ca. 1.90-1.86 Ga Svecofennian orogenic domain in the north to the Blekinge-Bornholm region (BBR) in the south where the dominant crust-forming granitoid rocks were emplaced between ca. 1.78 and 1.72 Ga. In the intervening area, the crust has ages between these extremes, the rocks of the different age groups and lithologies mostly being arranged in roughly EW-trending belts. However, there are also substantially younger shear zones and granite intrusions.The obtained results show a crustal structure typical of shield areas, with velocities of ca. 5.9-6.5 km/s in the upper and 6.5-6.9 km/s in the lower parts of the final model. The crustal levels below approximately 40 km have velocities above 7 km/s. The Moho is situated at depths of ca. 36 km beneath the BBR and 40-42 krn beneath most of the modeled part of the Svecofennian Orogen. The thickest now identified crust is about 52 km. It forms a wide belt along the southern edge of the latter domain. The variations of crustal thickness do not appear to occur smoothly but rather stepwise. The particulars of the resultant Moho topography are described.In a genetic interpretation, it is suggested that the metamorphic island-arc environment in the southern part of the Svecofennian Domain developed by a subduction-related process ending ca. 1.85 Ga ago, but that similar developments, including mafic underplating, continued still later, until at least ca. 1.77 Ga, farther to the south. Between ca. 1.55 and 1.45 Ga, the BBR and adjoining area were affected by largely granitoid magmatism associated with foliation and the formation of major zones of shearing. Together, the various episodes of underplating, subduction and crustal stacking appear to have been responsible for the varying depths and present step-like topographies of Moho and the crust. (C) 2001 Elsevier Science BN. All rights reserved.

The EUROBRIDGE deep seismic sounding (DSS) profile is a key component of a EUROPROBE project to examine Palaeoproterozoic processes of continental collision and crustal accretion. Its purpose is to establish the deep lithospheric structure of the East European Craton between the exposed Proterozoic and Archaean complexes of the Baltic and Ukrainian Shields. In 1994 a DSS experiment was recorded across the Baltic Sea from Västervik (Sweden) to Shventoji (Lithuania). We report on EUROBRIDGE'95, the first onshore stage of the seismic profile. It is 280km long, recorded from NW to SE on the Lithuanian part of the East European Platform, traversing the Proterozoic West Lithuanian Granulite Domain (WLG) and East Lithuanian Belt (EL) terranes. Explosive shots of up to 1000kg TNT were detonated at 10 shotpoints (SP01–SP10) at intervals of about 30km. Arrivals were recorded at 76 3-component seismograph stations with an average station spacing of 3.5km, providing high quality records. A 11th shot of 3000kg (SP00) was fired in the Baltic Sea close to Gotland. Raytracing analysis of refracted and reflected P-waves has been used to determine a 2-dimensional seismic velocity model for the crust and uppermost mantle below EUROBRIDGE'95. The thickness of the Phanerozoic sedimentary cover decreases from 2.2km in the north–west near the Baltic Sea coast to 0.4km at the south–east end of the profile near the Lithuania/Belarus border. Crust in the north–west and central part of profile consists of two major layers with a thickness of about 44km, increasing to 50km and three layers in the south–east. Crystalline upper crust is about 20km thick, thinning in the south–east, with P-wave velocities of 6.0–6.3km/s. A very weak low velocity zone, with a velocity contrast of 0.1–0.2km/s, occurs at 8–13km depth below the north–west and central part of the profile only. Lower crust exhibits velocities of commonly 6.5–6.9km/s, and thickens to the south–east with P-wave velocities up to 7.0km/s in the deepest parts. Crystalline crust is characterised by low velocity gradients and small velocity contrasts at most seismic boundaries. Major lateral changes in crustal velocity structure at all depths can be spatially correlated with the WLG–EL boundary determined from near surface geological information. Very strong reflections from the Moho boundary are observed. The mantle P-wave velocity immediately below the crust is 8.2–8.35km/s. A reflector in the lower lithosphere at a depth of almost 70km was found below Lithuania. Reflectivity modelling of the Gotland shot data suggests that this interface is absent offshore, where mantle velocities are lower. The DSS model supports the interpretation of the WLG and EL as terranes of Proterozoic age forming part of Fennoscandia. Later modification of crustal structure may have occurred, possibly by a mantle-heating episode centred on the Baltic Sea area.

A new seismic refraction and wide-angle reflection experiment in the Teisseyre–Tornquist Zone (TTZ) in Poland was conducting during July 1993 as an international Polish, Finnish, German and Swedish co-operation. The TTZ profile running in an SE–NW direction was exactly located in the central part of the Mid-Polish Trough in the zone of maximum subsidence (from Upper Permian to Upper Cretaceous). 19 shot points were placed along the 450 km profile. The interval between the shot points was about 25 km. 31 explosions (90 to 1000 kg of dynamite) were recorded in two deployments using 135 modern three-component seismic stations, with spacing of the recording sites of about 1.7 km. In this area, the depth of the consolidated basement, with a velocity of about 5.7–5.8 km/s, is 5 to 12 km deep. Nevertheless, the P-wave velocity is very low (Vp<6.1 km/s) down to depths of 15–20 km. Below this complex, velocities of 6.5–6.6 km/s and 6.9–7.2 km/s were found and the thicknesses of the corresponding layers are 8–11 km and 9–14 km respectively. The total thickness of the crust varies from 35 to 41 km. The results are discussed in combination with other seismic results from this region, which were obtained from deep seismic sounding and deep near-vertical reflection profiles as well as surface wave studies.

The BABEL marine seismic experiment has been carried out to investigate the lithospheric structure and antecedent tectonic signatures of the Baltic Shield, including the Archaean-Proterozoic collisional structure in the northern part of the Gulf of Bothnia.Lithospheric seismic-reflection streamer data and simultaneously recorded wide-angle reflection and refraction data collected in the Gulf of Bothnia as part of the BABEL project have been used for 3-D modelling. The distribution of land stations around the Gulf provides a good 3-D ray coverage of the PMP reflection data recorded at the eight stations in the area and allows an estimation of strikes and dips of the Moho boundary in the area. The traveltimes of reflected phases are calculated using a method that utilizes the finite-difference solution of the eikonal equation. The Moho wide-angle-reflection (PMP) traveltimes are modelled using an inversion method. A 2-D model from the Gulf of Bothnia extended into the third dimension is used as an initial model. During the inversion the velocity is kept constant and only the Moho boundary is allowed to vary. To estimate the strike of the Moho boundary and the stability of the inversion, two initial models with different strikes are examined.\The results indicate that the Moho depth in the Gulf of Bothnia undulates and has a maximum depth of 55 km in the south, rising to 42 km in the north, The Moho depth variations seem to be step-like. This change in the Moho depth coincides with the location of the presumed fossil subduction zone in the area. The crustal-thickness variations seem to be well approximated by a nearly 2-D structure striking parallel to a postulated subduction zone immediately to the south of the Skellefte area, The presence of the step at the crust/mantle boundary can be interpreted as a result of a plate-collision event at about 2 Ga.

In late 1989, wide-angle reflection and refraction profiles were shot in the Baltic Sea, the Bothnian Sea and the Gulf of Bothnia in order to study the lithospheric structure of Archaean and Proterozoic domains. The present study concerns the crustal structure and velocity variations beneath the Gulf of Bothnia area. Data collected at six land stations, recording marine airgun shots fired along BABEL line 2 in the Gulf of Bothnia, have been interpreted. The results from traveltime inversion show an upper crustal thickness of 13-22 km with P- and S-wave velocities varying from 5.5 to 6.5 and 3.3 to 3.7 km/s, respectively. The middle crust has a thickness of 13-15 km, with P-wave velocity varying from 6.3 to 6.9 km/s and S-wave velocity from 3.6 to 3.9 km/s. The lower crustal layer with thickness ranging from 9 to 20 km shows a high P- and S-wave velocity of approximately 7.5 and 4.4 km/s, respectively. The P- and S-wave velocities for the upper mantle are about 8.0 and 4.5 km/s. Calculated Poisson's ratio ranges from 0.20 to 0.24 for the upper crust and 0.25 to 0.28 for the middle and lower crust, and 0.29-0.31 for the upper mantle, respectively. The Moho depth varies between 42 and 55 km along the profile. The Moho topography shows a smooth undulation underneath the central part of the Gulf of Bothnia. A steep offset of about 8 km occurs beneath the southernmost part of the profile. Northward-dipping reflectors have been recorded in the lower crust. These lower crustal reflections line up perfectly with the northward-dipping upper mantle reflectors displayed in the vertical reflection section from the same line.

Short period (SP) Rayleigh wave dispersion was determined along linear seismic arrays in southern Sweden, using data from the Fennolora and the Eugeno-S seismic refraction projects on the Baltic Shield. The study area consists of crystalline and metamorphic igneous rocks of Precambrian age and has essentially no sedimentary cover. The dispersion curves were classified into dispersion regions and the regional (background) shear velocity structure was determined down to 2–3 km or more. Noise studies were undertaken in order to separate trends caused by the heterogeneous medium from the experimental uncertainties. It was shown that standard errors of the data in the 1–3 Hz band were predominantly significantly smaller than the conventional estimate based on the standard deviation. Thus, the estimates of the shear velocity resolution could be significantly improved, Q-values were determined from the SP Rayleigh waves in each dispersion region. In a broad sense similar Q-values were found in the area, except the northwestern corner that had significantly lower values. Here the lowest shear velocities were also found. However, a clear trend from higher Q in the southeast to lower Q in the northwest could be seen in the 1.7–3.3 Hz averages. The Q-values were inverted to obtain preliminary Qβ-structures. The Qβ-value of the upper km of the crust was 110 in the dominating part of the area and 52 in the northwestern corner. The lowest Q-values could be modelled with the same Qβ-structure below 1 km as in the other parts of the area.

Using short-period Rayleigh-wave dispersion data recorded along refraction lines, the shear-velocity background structure of the uppermost part of the crystalline in the southern Baltic Shield was found. On a regional scale the dispersion was grouped into dispersion regions and inverted to shear-velocity models down to about 2-3 km depth. The inferred P-velocity models are given. The models separated naturally into categories in close agreement with the large-scale geology of the area investigated. The regional shear-velocity structure varied strongly over the area and the uppermost few hundred metres of the crust. At deeper levels the structure showed a normal variation. The variation in these uppermost layers could be explained by the variation of seismic velocities with pressure in pre-stressed rock, due to partial closure of crack porosity. As there was no significant sedimentary cover in the area, this superficial layer could be attributed to the weathering layer of crystalline rocks. On a local characteristic horizontal scale of about 6 km, the lateral heterogeneities were studied using the dispersion in the 1.5-2.5 Hz band. With a perturbation method the local variations were mapped into the background shear-velocity structure along the EUGENO-S profile IV array. The local shear-velocity models were constrained to be increasing functions of depth and to satisfy the values and slopes of the known regional dispersion. Most of the 48 models could be mapped using a reference model constructed from the profile IV average model. The models defined a strongly heterogeneous weathering layer with a thickness of 0.4-0.5 km. The uppermost crustal shear background velocities correlated in detail with the local surface geology. The velocities formed two distinct populations composed essentially of granites (population I) and granitic gneisses (population II) with beta(I) = 3.19 +/- 0.08 and beta(II) = 2.84 +/- 0.15 km s-1, respectively. These velocities correspond to an in situ crack-porosity density of 0.1-0.5%.

In the autumn of 1989 a co-operative experiment involving 12 research institutions in northwestern Europe collected 2268 km of deep seismic reflection profiles in the Gulf of Bothnia and the Baltic Sea. The 121 litre airgun array used for this profiling was also recorded by 62 multicomponent land stations to provide coincident refraction surveys, fan-spreads, and 3-D seismic coverage of much of the Gulf of Bothnia. We thus have potentially both high-resolution impedance contrast images as well as more regional 3-D velocity models in both P- and S-waves. In the Bothnian Bay a south-dipping, non-reflective zone coincides with the conductive Archaean-Proterozoic boundary onshore in Finland. Between the Bothnian Bay and Bothnian Sea observed reflectivity geometries and velocity models at Moho depths suggest structures inherited from a 1.9 Ga subduction zone; the upper crust here appears to have anomalously low velocity. Within the Bothnian Sea, reflectivity varies considerably beneath the metasedimentary/granitoid rocks of the Central Svecofennian Province (CSP) and the surrounding metavolcanic-arc rocks. Numerous dipping reflectors appear throughout the metavolcanic crust, whereas the CSP has little reflectivity. Wide-angle reflections indicate that the metasedimentary crust of the Bothnian Basin is 10 km thicker than the neighbouring Svecofennian subprovinces. Near the Aland archipelago Rapakivi granite plutons exhibit bright reflections, a contrast to the usual non-reflective plutons elsewhere in western Europe. Additional dipping reflections deep in the crust of this area may support models of rifting and crustal thinning during emplacement of the 1.70-1.54 Ga Rapakivi granites. Coeval gabbroic/anorthositic magmatism may explain the high reflectivity and high velocity of these plutons. The c. 1.25 Ga mafic sills and feeder dykes of the Central Scandinavian Dolerite Group also produce clear reflections on both near- and far-offset seismic sections. Continued modelling will produce better velocity models of the crust and better constrained contour maps of crustal thickness in this part of the Baltic shield.

In 1989 the BABEL Working Group collected 2268 km of near-vertical reflection data in the Baltic and Bothnian Seas. As an integrated part of the field survey, the marine airgun shots were recorded by 64 multicomponent land stations. In this paper results are presented from interpretation of profiles B and A in the Baltic Sea, extending from the Aland Archipelago (Finland) into the Bay of Lubeck (Germany). In the shield part of the profiles northeast of the Sorgenfrei-Tornquist Zone, crustal reflectivity is observed at all levels and its termination in depth coincides largely with the crust-mantle boundary. The wide-angle data indicate a three-layer crust with velocities of 6.1-6.4, circa 6.6, and 6.9-7.2 km s-1. The Moho is found between 40-48 km depth, corresponding to 12-15 s TWT. In the northeastern part of profile A and the southern part of profile B, steeply northeast-dipping reflections are found at all crustal levels. The tectonic inversion of the Sorgenfrei-Tornquist Zone is clearly imaged above a thickened, high-velocity lowermost crust (7.1-7.4 km s-1). At depth the Sorgenfrei-Tornquist Zone widens and displays some asymmetry that is believed to be indicative of crustal shortening across the zone. An undulating Moho is observed along profile A where the lateral variability in structure and velocity field primarily is in the lower crust. Beneath the Skurup Basin south of the Sorgenfrei-Tornquist Zone, no intracrustal discontinuities are seen in the wide-angle data and a highly reflecting lowermost crust between 8 and 10 s TWT corresponds to a zone between 25 and 31 km depth with high velocity gradient (6.7-7.1 km s-1). A bright upper mantle reflection at 12 s TWT below the Skurup Basin can be explained by a velocity increase from 7.8 to 8.2 km s-1. Southwest-dipping reflections in the basement of the Mon High, an eastward continuation of the Ringkobing-Fyn basement High, indicate that the Caledonian Deformation Front is located at least 50 km further north than previously believed. The crust below the Mon High is 38 km thick with high velocities (7.1-7.4 km s-1) in the lower crust. In the North German Lowlands, the crystalline crust below the 10 km thick post-Caledonian sedimentary sequence is only 20 km thick and has velocities between 6.0 and 6.9 km s-1. It is hypothesized that during the Caledonian evolution, Baltica's Pre-Cambrian crust protruded into the docking Avalonian terrain as a major crustal flake structure.

The dispersion of short-period fundamental-mode Rayleigh waves (Rg) was measured along linear arrays on the Baltic Shield. The data used is a selected set of 176 records along three refraction profiles in southern Sweden. The phase velocity, determined in the 0.2-3.5 Hz band, could be grouped into a number of dispersion regions. Each regional dispersion showed a high internal consistency and the slowness as function of frequency was almost linear in the 1-3 Hz band. Linear regression was used to separate the effects of the medium from the experimental uncertainties. Systematic studies of the noise-to-signal ratio showed that the standard deviation of the dispersion, within each region and the above frequency band, essentially reflected the lateral heterogeneities. From the dispersion shear velocity models were inverted down to about 2-3 km, in one case down to 6 km. The S-wave velocity was weakly constrained to a non-decreasing function of depth with decreasing gradient. The P-wave velocity was constrained using recent results on Poisson's constant ranging from 0.28 to 0.25 in the 0-2 km interval. The density was constrained using geological knowledge of the area. Using these constraints, the S-wave velocity increased rapidly with depth in the first few hundred metres of the crust. The models could be naturally grouped into three sets with similar character, in close agreement with the large-scale surface geology. The highest velocities were found in the east, in the Smaland-Varmland Granitic Belt and the smallest in the northwest, in the Sveconorwegian crust.

Before the deposition of a Proterozoic cover and the repeated Proterozoic reworking of the older rocks, the presently exposed Archaean areas in northern Sweden formed part of a coherent craton. In the present study, we have used SmNd isotopic analyses of Proterozoic granitoids and metavolcanics to delineate the Archaean palaeoboundary. In a regional context, the transition from strongly negative ϵNd(t) values in the northeast to positive values in the southwest is distinct, and approximately defines the border of the old craton. The Archaean palaeoboundary extends in a WNW direction, and is subparallel to the longitudinal axis of the Skellefte sulphide ore district but it is situated ∼ 100 km farther to the north. The ∼ 1.9 Ga old granitoids on the two sides of the palaeoboundary were all formed in compressional environments, but those situated to the north have higher contents of LILE and LREE at similar contents of Si. This indicates that they were generated in an area with thicker crust and supports the location of the Archaean-Proterozoic palaeoboundary. There is no simple correlation between the Archaean palaeoboundary, as defined by the isotopic results, and any of the major fracture systems as interpreted from regional geophysical measurements. Reflection seismic work indicates that juvenile volcanic-arc terrains to the south have been thrust onto the Archaean craton. Possible thrust faults have been identified from aeromagnetic measurements. Rifting of the Archaean craton created a passive margin ∼ 2.0 Ga ago. Spreading shifted to convergence with subduction beneath the Archaean continent ∼ 1.9 Ga ago. Subsequently, the resulting juvenile volcanic arc collided with the old continent, and the Archaean palaeoboundary as existing today was formed by a collision characterized by overthrusting. The boundary then was disturbed by later deformation predominantly along NNE-trending fracture systems.

This paper demonstrates that - under favorable conditions - by using multichannel recording and subsequent stacking of adjacent records marine airgun shots have been detected at offset distances up to 700 km, the maximum offset at which we attempted to record data. Besides a powerful airgun array, a low noise environment at the recording site and the elimination of static shifts are the prerequisites to obtain refracted and reflected arrivals from the crust and upper mantle at such large offsets. Primary arrivals detected at offsets between 400 and 700 km image the upper mantle from 70 to about 120 km depth. Stacking of neighbouring shots and/or receivers successfully increases the signal-to-noise ratio, if the traces have been corrected for offset differences, which requires knowledge of the apparent phase velocities. The data presented here were collected in autumn 1989 during the BABEL Project on the Baltic Shield.

Based on recordings from the northern part of the Baltic Sea-Black Sea profile and EUGENO-S profile 4, 2-D seismic models have been constructed for a profile across the southwestern part of the East European Platform from southern Sweden to northern Poland. The thickness of the crust along the profile varies from 33 to 47 km. The Moho P-wave velocity is 8.0–8.3 km/s. The uppermost mantle has a fine structure with alternating layers of higher and lower velocities. Down to a depth of about 120 km three alternating high- and low-velocity layers have been modelled. The velocities vary between 8.2 and 8.7 km/s in the high-velocity layers and 8,0 and 8.5 km/s in the low-velocity layers. A tendency of the depth of the Moho and the depth of the uppermost mantle seismic boundaries to increase towards the southeast is observed.

Plate tectonics provides the linking framework for all tectonic and magmatic activity seen today, but it is not known when plate tectonics first developed on Earth. New deep seismic reflection and coincident refraction profiles across an exposed, 1.89-Gyr-old volcanic arc complex show a 10-km-thick offset in the Moho and bivergent reflectors in the crust, which were most probably created by plate convergence, subduction and accretion during the Early Proterozoic. Hence, plate tectonic models seem to be applicable for at least the second half of Earth's history.

Seismic refraction investigations along a 440-km long profije on the northern Baltic Shield have resolved the crustal structure in this area of Archaean to Early Proterozoic lithosphere formation. The profile, called the POLAR Profile, extends approximately along a SW-NE-oriented line from the Karelian Province in northern Finland across the Lapland Granulite Belt and the Kola Peninsula Province to the Varanger Peninsula in northeastern Norway. At six shotpoints, large explosions (200–1680 kg), and at three shotpoints, small explosions (80 kg) were detonated and recorded at an average station spacing of 2 km, providing high-quality record sections. A two-dimensional cross section of the crust was obtained by forward modelling using ray-tracing techniques. High-velocity bodies are found in the upper crust related to the Karasjok-Kittilä Greenstone Belt and the Lapland Granulite Belt. They extend to a depth of 6–13 km. In the Karelian Province in the southwest, a low-velocity zone was found between the depths of 8 and 14 km. The middle crust shows a slight increase in the average velocities from the southwest to the northeast, and a small velocity jump is found along a mid-crustal boundary between 18 and 21 km. The thickness of the middle crust varies between 16 and 18 km. The lower crust and the crust-mantle boundary (Moho) show considerable lateral variation. The top of the lower crust lies between 26 and 33 km, while its thickness decreases from 21 km in the southwest to 10–14 km beneath the Lapland Granulite Belt and the Inari Terrain, reaching 20 km again in the extreme northeast. The velocities also change laterally. The thin lower crust is characterized by rather low velocities (6.8–6.9 km/s), whereas in the southwest and northeast the velocities (6.9–7.3 km/s) resemble more typical shield structures. The Moho is found at 47 km in the Karelian Province, rises to 40 km beneath the Lapland Granulite Belt and descends to 46 km in the northeastern part of the Kola Peninsula Province. The upper mantle velocities at the Moho range from 8.1 km/s in the region of the thin crust, to 8.5 km/s and more beneath the Karelian Province. It is tempting to suggest that the anomalous lower crust underlying the Lapland Granulite Belt and the Inari Terrain may represent the remnants of an Early Proterozoic back-arc basin that was active prior to the 2.0 to 1.9 Ga plate convergence event, during which the Lapland Granulite Belt was thrust onto the Archaean basement of the Karelian Province. Another explanation is to assume that the velocity reduction in the anomalous lower crust was caused by a rather pronounced uplift of this region following the 1.9-Ga collision event.

The POLAR Profile Transect Display attempts to integrate all the existing aeromagnetic, gravimetric, geological and geochemical investigations with the results of the deep seismic and electromagnetic studies carried out along the POLAR Profile. Our description is based on the stage of joint interpretation reached at the Second Earth Science Study Centre.