U. Luosto

Seismic and geoelectric evidence for collisional and extensional events in the Fennoscandian Shield implications for Precambrian crustal evolution

Extensive deep seismic and electromagnetic sounding programs have resulted in tens of deep refraction seismic profiles, normal-incidence reflection profiles, several hundred magnetotelluric soundings, and several magnetovariational array observations. Results are compiled into four maps: (1) Moho depth; (2) thickness of the lower crustal high-velocity layer (Vp > 7 km/s); (3) depth to the upper interface of the high-velocity layer; and (4) distribution of upper and middle crustal electrical conductors. The Moho depth is approximately 40 km in the Archaean Domain and varies between 40 and 65 km in the Svecofennian Domain. Most of the thickness variation (0–24 km) is concentrated in the lower crustal high-velocity layer. Electrically, the crust is formed of several rather homogeneous, either resistive or conductive, blocks that are bounded by upper and middle crustal conductive zones. The cratonized Archaean crust experienced an extensional phase during which a thin, mafic, lower crustal high-velocity layer was produced in association with mafic dykes (2.2–2.1 Ga) and well-conducting volcanogenic and metasedimentary belts (2.5–2.0 Ga). The Moho of the Archaean Domain was, consequently, reformed in the Early Proterozoic. Major thickening of the crust (50–65 km) occurred when a Svecofennian island arc and the Archaean craton collided. The Svecofennian orogeny (1.76–1.9 Ga) resulted in a collage of metasedimentary rocks squeezed between crustal blocks of the island arc. The internal terrane boundaries are seen as good reflectors and/or inclined or vertical conductors that extend at least to the middle crust. The thick high-velocity (> 7 km/s) lower crust is a combination of mafic lower crust of the island arc crustal blocks and mafic additions from either delamination of the lithosphere after the collision or from a period of Late Svecofennian subduction. The collisional boundary is also preserved as a discontinuous conductive zone between the Archaean and Svecofennian Domains. The thickened crust was subsequently thinned to 42 km along E—W-striking zones during an extensional period characterized by the intrusion of anorogenic rapakivi granitoid batholiths, coeval mafic dykes, and gabbro-anorthosite bodies (1.6–1.5 Ga). The crust of the central part of the shield has since remained virtually intact.

The crustal structure along the POLAR Profile from seismic refraction investigations

Abstract 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-Kittila 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.

Seismic velocities and Q-factors in the uppermost crust beneath the SVEKA profile in Finland

The SVEKA deep seismic sounding profile in Central Finland was shot in 1981 to study the crustal structure of the Fennoscandian shield in the contact zone between the Archaean and Svecokarelian provinces. Apart from P-and S-waves, well developed Rayleigh surface waves (R-waves) with periods of 0.5–2.0 s were recorded from all the shot points. In the frequency band 0.5–2.5 Hz the R-waves were clearly observed on the entire 325 km long profile. Amplitudes of P-, S- and R-waves corrected for charge size were used to study the attenuation of waves and Q-factors. From the records of body and R-waves the distributions of seismic velocities and quality factors were determined using the dynamic ray-tracing method (2-D modelling) and the reflectivity method (1-D modelling). In the 2-D model of the uppermost crust, the P-wave velocities vary from 5.8–5.9 km/s at the top to 6.2–6.3 km/s at a depth of 6 km. The velocity ratio solVpVs varies with depth and distance from 1.67 to 1.74, reaching its maximum value of 1.8–2.0 in the uppermost 200–500 m and values of 1.76–1.80 in the uppermost 1 km. The quality factors Qp and Qs vary in the uppermost 1 km from 20 to 120, reaching a higher value of 140 in the marginal zone of the Archaean basement. In the depth range of 1–2 km Q-values of 80–400 were found. In the depth range 2–6 km Q-values of 300–800 were found for the Svecokarelian province and 200–400 for the Archaean. The low values of Vs and Q in the uppermost 1 km are thought to be the result of rock fracturing. The crack density, ϵ, was estimated for the uppermost crust along the profile. For the SVEKA profile we found ϵ-values of 0.35–0.54 in the uppermost 200 m, 0.10–0.32 at a depth of 0.2-1.0 km and ϵ < 0.1 for depths of 1–4 km. The comparison of our seismic data with results of theoretical and laboratory investigations implies that the uppermost crustal rocks are saturated. The zones of strong attenuation of R-waves coincide with faults and schist zones which are well known from surface geology.

Crustal structure of the baltic shield based on off-FENNOLORA refraction data

Abstract Since the early 1960s, deep seismic sounding experiments have been carried out on the Baltic Shield. In this study, we will mainly concentrate on the results obtained from two international profiles. Sveka and Baltic, carried out in Finland in 1981 and 1982. Results from these profiles are shown and discussed, and compared with those obtained from the FENNOLORA and from the other recent refraction profiles of the Baltic Shield in Fennoscandia. According to the results from Sveka and Baltic, and average crustal velocity is 6.6–6.7 km/s, which is rather high. Several distinct reflection boundaries have been found within the crust. In the lower part of the crust, a high-velocity layer with a P-wave velocity of 7.0–7.5 km/s has been found in some cases. In addition, the results indicate that the crustal structure has a clear block-like character, different blocks being separated from each other by deep fractures. The crustal thickness in the Baltic Shield is about 45 km on average, whereas around the Ladoga-Bothnian Bay zone in Central Finland, it is about 10 km thicker than this. Thus, there is a large-scale depression in the Moho boundary in the central part of the Baltic Shield.

Deep seismic structure of the Earth's crust along the Baltic Sea profile

Abstract In summer of 1989 the Institute of Oceanology of the Russian Academy of Sciences (Moscow) for the first time used a new large volume airgun for deep seismic sounding (DSS) in the Baltic Sea. The experiment was carried out during the 23-rd cruise of R/V Professor Shtockman. The wave-field analysis showed clear P and S wave arrivals, including Moho reflections. The Moho reflections were distinctly observed at distances exceeding 100 km. The P waves can be followed to a maximum distance of 368 km. Two-dimensional raytracing modelling of the data revealed a pronounced topography of the Moho along the BALTIC SEA profile: a depression in the middle of the profile at a depth of 45 km is bounded by 2–3 km steps and uplifts on both sides. Reflections off a sub-Moho boundary at a depth of 60 km have also been recorded. The upper mantle velocities vary between 7.8 km/s under the depression to 8.1 km/s to the side of it. The crustal P waves velocities define three layers: 5.8–6.3, 6.3–6.7 and 6.9–7.1 km/s. All crustal velocity boundaries bend over the Moho depression and reach their deepest points in the central part of the profile. The velocities in the crust also change laterally, reaching their minimum within the central depression. The crustal thickness along the profile varies notably between 38 and 45 km. A comparison with the FENNOLORA and the BABEL B profiles indicates that the Moho depression found along the BALTIC SEA profile correlates with similar depressions on the other profiles.

Wide-angle reflection migration technique with an example from the POLAR profile (northern Scandinavia)

Abstract The 440 km long POLAR profile runs approximately SW to NE across the Baltic Shield from the Karelian Province through the Lapland Granulite Belt and the Kola Peninsula Province in northern Finland and northern Norway. The migration method developed by V.N. Pilipenko (Physics of the Earth, N1, 1983, 36–42) was applied to the wide-angle reflection data obtained along the profile. To prepare the data for the migration, P- and S-waves were analysed with more attention to the distribution of real reflectors in the crust and on the relation between the P- and S-wave velocities. Depth-migrated images of the crustal structure were obtained for both the P- and S-wave fields. This new analysis revealed structural features of the crust which had not before been imaged. The most impressive features are several inclined boundaries in the upper crust dipping northeast beneath the Lapland Granulite Belt. Another newly found element is a lower crustal boundary at a depth of 35 km in the Karelian Province which is well traced in the P-wave migration image but is not observed in the S-wave image. This boundary corresponds to a strong change in the V P / V S ratio in the lower crust which may be interpreted in terms of mafic crustal composition. We have found that this migration technique can be applied to deep seismic sounding data resulting in new opportunities for deep crustal seismic studies. Nevertheless, more and denser observations are necessary to continue the improvement of the resolution and the reliability of the structure images.

Crustal structures of Eastern Fennoscandia

The area of the Eastern Fennoscandian Shield is quite well covered by deep seismic sounding profiles, most of which were shot in the last ten years. In general the velocities of the seismic waves in the crust increase continuously with depth, but intense reflections also indicate the presence of distinct velocity boundaries. The velocities of the refracted waves in the various profiles resemble each other but the depths to the boundaries vary considerably. Low-velocity zones were found in the uppermost crust along most of the profiles. The Eastern Karelia profiles also exhibited large low-velocity zones in the lower part of the crust. High-velocity bodies found in the uppermost crust in Eastern Fennoscandia are thought to be associated with mafic intrusions and mafic metavolcanic rocks. The depth to the Moho shows considerable lateral variation, and deep depressions in this boundary are thought to be connected with large fracture zones. High Pn-wave velocities of about 8.5 km/s are in many cases associated with these depressions.

A teleseismic study of the lithospheric structure across the Ladoga-Bothnian Bay zone in Finland

Abstract In order to study teleseismic arrivals times across the Ladoga-Bothnian Bay zone, a proposed suture zone that separates the Karelian region (2.7 Ga) from the Svecofennian region (1.8 Ga), a seismic array of 22 event recorders was deployed along the SVEKA profile in Central Finland. The P-travel-time delays are shown to vary by less than ± 0.2 s along the profile and consistent delay patterns are observed for the various earthquakes. The travel-time delays delineate zones that appear to correlate with different geological terrains. A 2-D inversion of the data yields a tomographic image of the crust and lithosphere across the Ladoga-Bothnian Bay zone where 1% velocity variations are resolved. The southwestern Proterozoic crust is slower (0.15 s) than the northeastern Archean crust, in agreement with a relatively thinner (about 6 km) Archean crust. A subcrustal lower-velocity ( − 0.8 %) dome-like structure is present beneath the Kuhmo block (northern part of the profile) and is bordered by a southwest-dipping higher-velocity ( + 0.8 %) body that seems to connect to the surface trace of the Kuhmo-Suomussalmi Greenstone belt.

Tomographic study of crustal velocity structure in southern Finland

Abstract An inverse tomographic problem of the construction of a 3-D velocity structure has been solved for depths of 10, 20, 30, and 40 km using the P-wave arrivals registered by the seismic array in southern Finland and on the DSS lines Sveka (1981) and Baltic (1982). The interpreted results verify and enlarge DSS data obtained earlier for a high-velocity structure of the crust and explain some small lateral velocity variations. The concordant tomographic and DSS data specify a 3-D model for a complex geological structure in the region.