Comparison of the crustal structures of the Barents Sea and the Baltic Shield from seismic data

doi:10.1016/S0040-1951(00)00079-2

Tectonophysics

Volume 321, Issue 4, 30 June 2000, Pages 429-447

https://doi.org/10.1016/S0040-1951(00)00079-2 Get rights and content

Abstract

A compilation of the available seismic refraction and wide-angle reflection data from the Barents Sea and the Baltic Shield and a comparison of the crustal structures of both areas are presented. Seismic cross-sections and velocity–depth models of the crust have been analyzed.

Six areas differing in crustal structure are revealed and outlined in the Barents Sea region as a result of the velocity–depth models analysis. Three main layers are distinguished in the central area: the upper layer with an average velocity of 3.5 km/s overlies two layers with velocities of 5.5 and 6.8 km/s. The average thickness of the crust is 33 km. The crustal structure in the area is anomalous, as compared to the marginal areas of the sea, and is distinguished by the absence of the 6.2 km/s velocity layer.

Combined seismic cross-sections for the coastal and marine seismic profiles show that the main seismic boundaries with velocities of 6.2–6.5, 6.8–7.2 and 8.0–8.2 km/s can be confidently interpolated from the northern part of the Baltic Shield to the southern part of the Barents Sea. The junction of their crusts is marked by a deepening of the crystalline basement with a seismic velocity of 6.2 km/s. The sedimentary cover thickness increases to 15–20 km, and the total thickness of the crust is reduced northward from 40–60 to 28–30 km.

A comparison of the models shows that the Barents Sea Basin and the Baltic Shield appreciably differ in the total thickness of the earth's crust: under the basin, it is just about 10 km less than on the shield. Both regions have an important similarity: seismic velocities in the lower crystalline crust (7.0 km/s) are similar. Their fundamental difference is that the upper part of the crystalline crust with a seismic velocity of 6.0–6.5 km/s, characteristic of the Baltic Shield, is absent in the central Barents Sea Basin, and a thick sedimentary layer with velocities to 5.8 km/s exists instead of it. The anomalous crustal structure in the central part of the Barents Sea is assumed to be of a riftogenic nature.

Introduction

A study of the deep structure of the Barents Sea Basin is of great importance for solving fundamental, as well as practical, problems. Currently, this region has attracted increasing interest from geologists and geophysicists because of the development of commercial oil and gas deposits. In spite of a considerable number of geophysical surveys carried out there, many problems of the structure and geological evolution of the Barents Sea floor remain unsolved. One such problem is the relation of the structures and processes in the thick sedimentary cover of the Barents Sea Basin to the deep structure and geodynamics of the consolidated crust and the upper mantle.

Studies of the crustal structure of the Barents Sea region have been carried out by the Russian, American, German, Norwegian and French expeditions. To date, a large amount of seismic refraction and wide-angle reflection data has been collected (Fig. 1).

The seismic refraction measurements by the western researchers (Eldholm and Ewing, 1971, Eldholm and Talwani, 1977, Hinz and Schluter, 1978, Houtz and Windisch, 1977, Renard and Malod, 1974, Sundvor, 1971, Sundvor, 1974) were carried out mainly by the sonobuoy technique, which enabled a study of only the upper crust, and data on the boundaries with seismic velocities more than 6.0 km/s are practically unavailable. For example, layers with seismic velocities of 1.85, 2.2, 2.65, 3.2, 3.7, 4.2, 4.7, and 5.2 km/s were revealed on the southwestern Barents Sea shelf (Eldholm and Ewing, 1971). At other sonobuoy stations on the shelf (Renard and Malod, 1974), the layers with seismic velocities ranging from 2.4 to 5.4 km/s could be identified. For refracting boundaries with velocities of 2.4, 3.1, 4.3, 5.3, and 6.2–6.4 km/s, structural cross-sections and contour maps were presented for the western and central Barents Sea (Eldholm and Talwani, 1977). In the western Barents Sea, the layers with seismic velocities of 2.2–2.8, 3.0–3.4, 3.6–4.8, 5.2–5.6, and 5.8–6.2 km/s were identified and assumed to correspond to the Tertiary, Cretaceous–Jurassic, Mesozoic sediments, Caledonian or Rhiphean basement and Precambrian metamorphics, respectively (Hinz and Schluter, 1978). About 300 velocity solutions from the western Barents Sea sonobuoy data were combined to construct the velocity models (Houtz and Windisch, 1977). The main geological provinces of this region are described by five different velocity–depth models.

A two-ship refraction survey on the Norwegian continental shelf (Sundvor, 1971) has revealed a stratified sedimentary cover with average seismic velocities of 1.85, 2.20, 2.55, 3.25, and 3.90 km/s, probably of Cenozoic and Mesozoic ages. The deeper layer with a velocity of 5.25 km/s is assigned to the basement. It has been suggested that this layer is the seaward continuation of Caledonian rocks of the land. The depth of this layer ranges from 2 to 5 km.

The DSS studies in the Barents Sea by the Russian researchers were started in 1960–1962 (Litvinenko, 1968) and concentrated in the eastern part of the region. The principal results were obtained in 1976 along the K23 profile running from the Kola Peninsula shore towards the Franz Joseph Land Archipelago (Fig. 1). Observations were made by a network of ocean bottom seismographs (OBS) spaced within about 50 km. Charges of 135 kg of explosives as sources of seismic waves were fired at an interval of 3–5 km along the profile. A system of reversed and overlapped travel-time curves was obtained. Interpretation of these data revealed a significant vertical and horizontal heterogeneity of the crust and the uppermost mantle (Davydova et al., 1985, Davydova and Mikhota, 1986, Pavlenkova, 1986, Tulina, 1986).

In subsequent years, the DSS studies in the central and eastern parts of the Barents Sea were continued by many academic and industrial expeditions. The results of these investigations are published only partially and mostly in Russian (Elnikov, 1982, Gubaidulin et al., 1993, Merklin, 1985, Merklin, 1987, Neprochnov, 1980, Neprochnov et al., 1984, Pavlenkin, 1981, Zverev et al., 1986). All available DSS profiles are shown in Fig. 1 and are discussed in the paper.

The first DSS study on the Baltic Shield was carried out in 1958–1959 along the Kem–Uchta profile (Litvinenko, 1984) and continued along the Barents Sea–Pechenga–Lovno profile (Litvinenko, 1968). In the 1980s, the deep seismic research of the Baltic Shield was continued along a series of international and domestic profiles FENNOLORA (Guggisberg et al., 1991), SVEKA (Grad and Luosto, 1987, Luosto et al., 1984), BABEL (BABEL Working Group, 1993), BALTIC (Luosto et al., 1990), POLAR (Luosto et al., 1989, Walther and Flueh, 1993), Blue Road (Lund, 1979), FINLAP (Luosto et al., 1983), Kemi-Kajaani (Yliniemi, 1991), RUBIN (Azbel et al., 1991) and others (Fig. 2). Forty DSS profiles with a total length of more than 20 000 km have been executed. However, the profiles are distributed non-uniformly and have a different data reliability.

The Baltic Shield is dipping under the Barents Sea and occupies its significant part. Undoubtedly, the major tectonic processes on the shield and in the sea basin are interrelated. The transition between these two tectonic structures is not clear and has been the subject of studies.

In this work, we present the results of compilation and joint analysis of the existing seismic data on the deep structure of the earth's crust and the uppermost mantle of the Barents Sea Basin and the Baltic Shield. The analysis of the Barents Sea data has allowed us to suggest a new structural division of the region. A comparison of 1D and 2D seismic models for the Barents Sea Basin and the Baltic Shield has revealed an important similarity as well as a fundamental difference in the crystalline part of their crusts. From this, we can conclude that the riftogenic processes have played an important role in the formation of the earth's crust in the region.

Section snippets

Seismic wave field

The seismic wave field is described for the K23 profile as the best studied to date (Davydova et al., 1985, Davydova and Mikhota, 1986). The profile crosses the major tectonic structures of the Barents Sea (from the south to the north): the Baltic Shield margin, the Central Barents Rise, and the North Barents Basin. The actual seismic record sections have been reported previously by Davydova et al. (1985), Davydova and Mikhota (1986), Tulina (1986) and Morozova et al. (1995). The examples of

Seismic wave field

The record sections for the Baltic Shield DSS profiles are published in (Luosto and Korhonen, 1986, Luosto et al., 1990, Luosto, 1991). The examples of record sections for the POLAR profile are presented in Fig. 7. In all the profiles, the refracted waves with apparent velocities of 6.0–6.2 km/s are registered as first arrivals up to a distance of 100–120 km from shot points. Reflected waves corresponding to the boundaries at depths of 3–7 km are often recorded. Refracted waves with velocities of

1D and 2D crustal models

Fig. 10 shows the averaged seismic models of the earth's crust for three typical areas of the region investigated: (1) the southern area of the Barents Sea adjacent to the Baltic Shield, (2) the central area of the sea, and (3) the central area of the Baltic Shield.

It is clear from a comparison of the models that the central Barents Sea Basin (Model 2) and the Baltic Shield (Model 3) differ appreciably in the total thickness of the earth's crust: under the basin, it is about 10 km less than on

Conclusion

This paper presented an analysis of seismic refraction profiles in the Barents Sea and the Baltic Shield. The main results of the study are as follows:

(1) The compilation and joint analysis of seismic data of the Barents Sea Basin collected by the Russian, American, German, Norwegian and French expeditions have resulted in a new structural division of the region. Six areas with different structures of the crust were distinguished here.

The southern area, to about 150 km from the Kola Peninsula,

Acknowledgements

This work has been done as part of the joint research project of the Institute of Seismology of the University of Helsinki, the Geophysical Observatory and the Institute of Geosciences and Astronomy of the University of Oulu, and the Shirshov Institute Oceanology of the Russian Academy of Sciences. It was supported by the Academy of Finland, the Russian Academy of Sciences, and the Ministry of Science and Technology of the Russian Federation (project ‘Tectonosphere’).

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Most of the East European Craton lacks surface relief; however, the amplitude of topography at the top of the basement exceeds 20 km, the amplitude of topography undulations at the crustal base reaches almost 30 km with an amazing amplitude of ca. 50 km in variation in the thickness of the crystalline crust, and the amplitude of topography variations at the lithosphere–asthenosphere boundary exceeds 200 km. This paper examines the relative contributions of the crust, the subcrustal lithosphere, and the dynamic support of the sublithospheric mantle to maintain surface topography, using regional seismic data on the structure of the crystalline crust and the sedimentary cover, and thermal and large-scale P- and S-wave seismic tomography data on the structure of the lithospheric mantle. For the Precambrian lithosphere, an analysis of Vp/Vs ratio at 100, 150, 200, and 250 km depths does not show any age-dependence, suggesting that while Vp/Vs ratio can be effectively used to outline the cratonic margins, it is not sensitive to compositional variations within the cratonic lithosphere.
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In recent years the epicontinental ocean of the northern Barents Sea and the continental margin around Svalbard have been examined through wide angle seismic programs (Sellevoll et al., 1991; Neprochnov et al., 2000; Breivik et al., 2002, 2003; Ritzmann et al., 2002; Ritzmann and Jokat, 2003; Ritzmann et al., 2004).
The western Barents Sea and the Svalbard archipelago share a common history of Caledonian basement formation and subsequent sedimentary deposition. Rock formations from the period are accessible to field study on Svalbard, but studies of the near offshore areas rely on seismic data and shallowdrilling. Offshore mapping is reliable down to the Permian sequence, but multichannel reflection seismic data do not give a coherent picture of older stratigraphy. A survey of 10 Ocean Bottom Seismometer profiles was collected around Svalbard in 1998. Results show a highly variable thickness of pre-Permian sedimentary strata, and a heterogeneous crystalline crust tied to candidates for continental sutures or major thrust zones. The data shown in this paper establish that the observed gravity in some parts of the platform can be directly related to velocity variations in the crystalline crust, but not necessarily to basement or Moho depth. The results from three new models are incorporated with a previously published profile, to produce depth-to-basement and -Moho maps south of Svalbard. There is a 14 km deep basement located approximately below the gently structured Upper Paleozoic Sørkapp Basin, bordered by a 7 km deep basement high to the west, and 7–9 km depths to the north. Continental Moho-depth range from 28 to 35 km, the thickest crust is found near the island of Hopen, and in a NNW trending narrow crustal root located between ∼19°E and 20°E, the latter is interpreted as a relic of westward dipping Caledonian continental collision or major thrusting. There is also a basement high on this trend. Across this zone, there is an eastward increase in the V_P, V_P/V_S ratio, and density, indicating a change towards a more mafic average crustal composition. The northward basement/Moho trend projects onto the Billefjorden Fault Zone (BFZ) on Spitsbergen. The eastern side of the BFZ correlates closely with coincident linear positive gravity and magnetic anomalies on western Ny Friesland, apparently originating from an antiform with high-grade metamorphic Caledonian terrane. A double linear magnetic anomaly appears on the BFZ trend south of Spitsbergen, sub-parallel to and located 10–50 km west of the crustal root. Based on this correlation, it is proposed that the suture or major thrust zone seen south of Svalbard correlates to the BFZ. The preservation of the relationship between the crustal suture, the crustal root, and upper mantle reflectivity, challenges the large-offset, post-collision sinistral transcurrent movement on the BFZ and other trends proposed in the literature. In particular, neither the wide-angle seismic data, nor conventional deep seismic reflection data south of Svalbard show clear signs of major lateral offsets, as seen in similar data around the British Isles.
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Comparison of the crustal structures of the Barents Sea and the Baltic Shield from seismic data

Abstract

Introduction

Section snippets

Seismic wave field

Seismic wave field

1D and 2D crustal models

Conclusion

Acknowledgements

Tectonophysics

Tectonophysics

Tectonophysics

Mar. Geol.

Tectonophysics

Tectonophysics

Earth Planet. Sci. Lett.

Mar. Geol.

Precamb. Res.

The analysis and interpretation of wave fields on Soviet and Finnish DSS profiles

Velocity cross section of the crust along the Pechenga-Umbozero-Ruchyi profile

Integrated seismic studies of the Baltic Shield using data in the Gulf of Bothnia region

Geophys. J. Int.

Deep structure of the southeastern Barents Sea according to DSS data

Marine geophysical survey in the southwestern Barents Sea

J. Geophys. Res.

Sediment distribution and structural framework of the Barents Sea

Geol. Soc. Am. Bull.

Initial report about geophysical and geological work in 8th cruise of the R/V Professor Shtokman

Seismic models of the crust of the Baltic shield along the Sveka profile in Finland

Ann. Geophys.

Profiles along the northeastern and eastern framing of the shield

Barents Sea continental Margin Sonobuoy data

Geol. Soc. Am. Bull.

The feature of the deep cross-section of the earth's crust of the northwestern part of the Kola Peninsula and the southern part of the Barents Sea