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An introduction to the European Geotraverse Project: First results and present plans
Abstract
The European Geotraverse (EGT) is an international, multidisciplinary project focused on a north-south orientated lithospheric profile, 4000 km long and of varying width, extending from northernmost Scandinavia to North Africa. This profile consists of three interlinking Segments (Northern, Central, and Southern) comprising a continuous succession of tectonic provinces ranging from the oldest Precambrian areas of the Baltic Shield to the currently active area of the Western Mediterranean. The broad aim of the EGT Project is to obtain a better three-dimensional picture of the structure, state, and composition of the continental lithosphere to use as a basis for an understanding of its evolution and dynamics. All of the 12 major projects that constitute the EGT “Joint Programme” have now been initiated, and several of these projects are nearing completion.
... Between Alps-Provence and Corsica, a highly thinned continental crust on the deep part of the margins underlies the Ligurian Sea, while an oceanic-type crust is present in its narrow central part. Corsica and Sardinia islands are underlain by a "typically" Variscan bowl-shaped continental crust (Galson and Mueller, 1986). ...
... Especially, Monte Doria is located on the northern -continental side -of the Capo delle Mele -Cap Corse transform fault zone, a major crustal feature linked to a sharp down faulting of Moho. Indeed, an abrupt deepening of 6 km was registered between two shots distant of 3 km during the Geotraverse Project (Galson and Mueller, 1986). ...
... much higher than those found in south-central Canada. Furthermore, we have determined that the upper-crustal velocities (5-20 km depth range) beneath the NE Alpha Ridge complex are much higher than those reported from (1) other regions of the North American craton (Berry 1973;Mereu et al. 1986) (2) the Pre-cambrain Shields of Australia (Finlayson 1982;Finlayson & Mathur 1984), India (Kaila 1982), USSR (Jentsch 1979), Finland (Luosto et al. 1984), Scandinavia (Hirschleber et al. 1975;Galston & Mueller 1986) and Saudi Arabia (Mooney et af. 1985) (3) the Phanerozoic mountain belts of western North America (Cumming et al. 1979;Monger et al. 1985;Prodehl 1979;Prodehl & Pakiser 1980;Walter & Mooney 1982;Leaver et af. ...
... 1984), eastern North America (Taylor & Toksoz 1979;Unger et al. 1987), Europe (Bamford et af. 1976;Mooney & Prodehl 1978;Miller et al. 1982;Galston & Mueller 1986;Deichmann et al. 1986) and Asia (Sapin et af. 1985;Kaila 1982;Teng et al. 1985) (4) the intracontinental rift regions of North America, Europe, USSR and Africa (see reviews by Mooney et al. 1983 andOlsen 1983). ...
SUMMARYA crustal-scale seismic refraction survey has been conducted in the Arctic Ocean across the NE Alpha Ridge complex and its northern flank within the SE Makarov Basin. The data from the four reversed profiles have been analysed using 1-D and 2-D synthetic seismogram modelling schemes. Upper crustal velocities beneath these parts of the Alpha Ridge complex and Makarov Basin are surprisingly uniform at 5.0–5.2 km s−1, increasing smoothly and rapidly to ≈6.5 km s−1 at depths of only ≈8 km. Below this level the velocities continue to increase relatively smoothly, reaching values of ≈7.0 km s−1 at depths ranging from 14 to 19 km. Mantle-type velocities of ≳8.0 km s−1 are found at depths of 36–44 km beneath some regions of the Alpha Ridge complex and 21–25 km beneath the SE Makarov Basin. Based on these results and other geophysical and geological data from the Arctic Ocean we interpret the Alpha Ridge complex as an Icelandic-type structure generated by Mantle Plume activity, and we suggest that the region of Makarov Basin surveyed is underlain by a thick oceanic crustal section.
... Finally I report the Vp cross-section along the same profile obtained from the local P-wave earthquake tomographic investigation by Diehl et al. (2009), which also in this case shows a similar trend regarding fast or slow areas than the surrounding zones. (Galson and Mueller, 1986;Blundell et al., 1992), which crosses the Central Alps from North to the South along 9.3°E. In the European domain the Moho discontinuity goes from values around 30 km in the northern part to values of 60 km in the middle of the Central Alps, again expressing a crustal root. ...
Passive seismological investigations typically image the Earth’s crust with direct P-waves or ambient noise correlation yielding S-wave information. While the first method requires local earthquakes to achieve high resolution, in the second method the depth penetration strongly depends on the recording network’s aperture.
In this thesis I develop a new inversion method and implement the related software in
which teleseismic P-to-S converted waves (receiver functions) are exploited to construct a fully 3-D structural and shear-wave velocity model of the crust. This method does not require local earthquakes, nor a large aperture seismic network, but a dense array of 3-component sensors with a station spacing similar to the expected crustal thickness. This new technique is first applied to the Central Alps, a tectonically complex area where imaging in 3-D is of pivotal interest.
... 1), направлен� ных на изучение глубинного строения литосферы этой крупной структуры Восточно�Евро� пейской платформы [Prodehl, Kaminsky, 1984;Husebye et al., 1986;Grad, Luosto, 1987, 1993Guggisberg et al., 1987Guggisberg et al., , 1991Kinck, Husebye, 1988;Grad et al., 1991;Sharov, 1991;Structure…, 1991]. Lund, 1979;Drummond , Collins, 1986;Galson, Mueller, 1986;Gug� gisberg, 1986;Корхонен и др., 1986;Guggisberg et al., 1991;The European Geotravers, 19925 Полар 440 198541-48 6,6 8,2-8,4 Luosto et al., 1989Luosto, Huvonen, 2001 6 Финлеп 300 1979 49 6,6 8,4 Luosto et al., 1983;Korhonen et al., 1984Korhonen et al., 7 Печенга -Ловно 230 1960Korhonen et al., -1961Korhonen et al., 38-42 6,4 8,1-8,3 Литвиненко, 1963Земная…, 19788 Печенга -Ковдор -Костомукша 550 1981-198241-49 6,4-6,5 8,1 Азбель и др., 1986Azbel et al., 1989;Литосфера…, 1989;Чеку� нов и др., 1993;Mitrofanov et al., 1998Mitrofanov et al., 9 Ковдор -Кировск 140 1975Mitrofanov et al., 35-42 6,4 -Панасенко, Шаров, 1977Ша� ров, 199310 Кировск -Дальние Зеленцы 190 1983---Шаров, 1989, 199311 Кировск -Костомукша 350 198740-45 6,5 8,1-8,2 Шаров, 1989, 1993 12 Никель -Умбозеро -Ручьи 600 1984 36-44 6,6 8,2 ...
В монографии на основе анализа и обобщения всей совокупности имеющейся сейсмической информации разработаны обоснованные современные представления о строении земной коры и верхней мантии Северной Европы. Наиболее детальные комплексные геолого-геофизические исследования, заверенные сверхглубоким бурением, проведены в последние годы в Баренцевом Евро&Арктическом регионе. Строение типовых структурных блоков Фенноскандинавско&
го щита и области его сочленения с Баренцевоморским шельфом изучены комплексом сейсмических методов. Построены сейсмогеологические модели земной коры отдельных геотектонических провинций, которые показывают, что кристаллическая кора является мозаично-блоковой средой. В отдельных блоках установлено локальное развитие волновода в верхней части разреза. Составлены карта рельефа поверхности Мохоровичича и скоростные модели верхней мантии.
В низах коры выявлены участки развития высокоскоростного слоя, который заполняет глубокие прогибы в рельефе поверхности Мохоровичича. На базе совместного многопланового анализа геолого-геофизических данных разработаны новые представления о строении и эволюции региона. Проведено сопоставление данных по глубинному строению литосферы Фенноскандинавского щита с другими кристаллическими щитами, делается вывод, что верхняя мантия достаточно сложная и неоднородная по латерали, а астеносфера в классическом понимании отсутствует.
Книга рассчитана на широкий круг исследователей глубинного строения континентальной литосферы, интересующихся докембрийской геологией, а также студентов-геологов и геофизиков старших курсов.
Работа подготовлена при финансовой поддержке Российского фонда фундаментальных исследований
по проекту № 14-05-00535-а.
The best&documented modern concepts of the earth crust and the upper mantle structure of Northern Europe, based on analysis of available seismic information, were developed. The most detailed integrated geological and geophysical studies and superdeep drilling were conducted in the past few years in the Barents Euro-Arctic region. The structure of the type structural blocks of
the Fennoscandian Shield and the Fennoscandian Shield-Barents Sea shelf jointing zones was studied using integrated seismic methods. Seismo-geological models of the earth crust of individual geotectonic provinces were constructed. They show that the crystalline crust is a mosaic&block medium. A waveguide was proved to evolve locally in the upper portion of the rock columns of some blocks. A map of the M-boundary surface relief was produced and velocity models of the upper mantle were constructed. High-velocity layer zones, which fill deep sags in the M-boundary surface relief, were revealed in the lower portions of the crust. New concepts of the structure and evolution of the region, based on comprehensive analysis of geological and geophysical data, were developed. Data on the deep lithospheric structure of the Fennoscandian Shield were correlated with those on other crystalline shields. The author has concluded that the upper mantle is laterally complex and heterogeneous and that there is no asthenosphere by the classical definition.
The book is meant for scientists who study the deep structure of the continental lithosphere, Precambrian geologists and senior geology and geophysics students.
... Velocity structure has been extensively mapped through several refraction profiles crossing the region of study. The FENNOLORA profile lies just west of the array (Galson and Mueller, 1986;Guggisberg and Berthelsen, 1987;Guggisberg et al., 1991;Lund, 1987) (Fig. 1), while the POLAR profile runs east of the array, crossing the Granulite belt and the PV suture (Luosto et al., 1989). Other profiles include FINLAP, which extends east from FENNOLORA (Luosto et al., 1983) and several profiles in Russia (Azbel et al., 1989;Glaznev et al., 1989). ...
Wave propagation of crustal phases, in particular Lg, near the ARCESS array in northern Norway is examined and compared to propagation characteristics near the NORESS array in southern Norway. Relying on array analysis, we show that ray-theory explanations of arrivals in local and regional seismograms are very useful in deducing the composition of the crustal wave trains. Frequency-wavenumber anal-ysis is applied to the array data to identify the arrivals; then a composite of array beams is used to approximate each event. Record sections of composite seismograms are constructed for different directions from ARCESS to study propagational char-acteristics with distance and azimuth and then compared to composite-seismogram record sections from NORESS to study different regional characteristics. To model the observed character of Lg and Rg, synthetic record sections are constructed by wavenumber integration in velocity models representative of structure in the different regions. The results show that Rg propagation distance varies significantly between regions and that the velocity gradient in the lower crust is the dominant factor in specifying Lg characteristics in the Baltic shield region. Lg is dominated by several discrete arrivals representing multiple Moho reflections, or by multiple Moho reflec-tions and turning waves, with each multiple confined to a small distance range so that only one multiple set dominates at each distance. Furthermore, we demonstrate that incomplete knowledge of the propagation characteristics leads to mislocations of local and regional events and inhibits source-depth discrimination of events. We identify three earthquakes in the ARCESS data set based on source depths. These are constrained by phase velocities and depth phases, none of which were properly iden-tified by the automatic array-detection software.
... Extending the 1982-1985 seismic refraction experiments of the European Geotraverse (EGT) (Egger et al., 1988) south of Sardinia into Tunisia had been a part of the EGT Joint Programme since its inception (Galson and Mueller, 1986;Morelli and Nicolich, 1990). Continuing the nearly north-south orientation of the main profile made sense because the structures of western Tunisia can be in facî easily sampled along a north-south inline profile. ...
... The most detailed refraction profiles crossing the region of study are the FENNOLORA profile, which runs just west of ARCIFSS (Galson andMueller, 1986: Guggisberg andBerthelsen, 1987;Guggisberg et al., 1991: Lund, 1987 ( Figure 1) and the POLAR Profile, which runs just east of ARCESS and crosses the Granulite belt and the PV suture (Luosto et al, 1989). Other profiles include FINLAP, which extends east from FENNOIORA (luosto et al, 1983) and profiles in Russia (Azbel et al., 1989: Glaznev et al., 1989. ...
Wave propagation of crustal phases near the ARCESS array is examined and compared to characteristics near the NORESS array, with emphasis on Lg and its composition. f-k analysis is applied to the data, to identify arrivals in the records, then a composite of array beams is used to approximate each event. Record sections of composite-seismograms are constructed for different directions from ARCESS, to study propagational characteristics with distance and azimuth. Near surface velocity under NORESS and ARCESS is obtained through inversion of Rg wave dispersion curves. A composite ARCESS record section is compared to a NORESS record section of events located in the Caledonides. The differences that emerge are that Lg in the Caledonian region, north of NORESS, is dominated by discrete arrivals representing Moho reflections, with the order of reflection increasing with distance. In the Archean ARCESS region, however, Lg is dominated by turning waves, also with the order of reverberation increasing with distance, but with each confined to a small distance range. Rg wave propagation in the ARCESS region is much more efficient than in the Caledonian NORESS region, as Rg is observed to 400 km distance at ARCESS, but only to 200 km distance at NORESS. Synthetic record sections are constructed by wavenumber integration in order to model the observed character of Lg and Rg in the two regions. The dominating factor in the difference of Lg characteristics turns out to be the velocity gradient in the lower crust. A few earthquakes near ARCESS are also studied. Their depths can be constrained on the basis of phase velocities and depth phases. For reliable phase identification, however, the composition of Lg in the region needs to be accurately known.
... Seismology currently offers the most powerful means for mapping the physical properties of the earth, leading towards the understanding of present and past geodynaxnical processes. Crustal velocity structures have traditionally been mapped only along selected profile lines using refraction and wide-angle reflection data (e.g., in projects such as FENNOLORA, Galson & Mueller, 1986). Although such experiments give detailed information on vertical velocity variations the resolution of lateral heterogeneities is restricted to the profile line. ...
Pioneering work on mapping the Scandinavian crust commenced in the early 1960s and since then numerous profiling surveys have been undertaken, particularly as part of the on-going EUGENO-S project. However, the most significant contribution to mapping crustal structural details came from the M.V. Mobil Search cruises in the Skagerrak and off the West coast of Norway (16 s TWT reflection profiling). All past and present crustal profiling results have been integrated to produce detailed maps of Moho depths and crustal thicknesses for South Scandinavia. The thinnest crust is found in the North Sea and Skagerrak (approximately 20 km), while East-central Sweden features very thick crust (approximately 50 km). Other interesting features are the apparent correlation between crustal thinning and sedimentation/subsidence, magmatic activity, earthquake occurrences and the tectonic age of the crust. Moho depths and the crustal thicknesses clearly reflect the tectonic evolution and the present structural features of the region investigated.
... The seismic structure of the crust beneath sites 1-25 was deduced from results along the Fennoscandian seismic refraction profile of 1979 (Guggisberg 1986;Galson & Mueller 1986;Guggisberg & Berthelsen 1987). Crustal thickness varies from about 35 to 55 km and the lower crust has greater thickness where the Moho is deeper. ...
Steady-state heat conduction modelling was carried out to calculate the crustal temperature field and thermal lithosphere thickness in the Baltic shield. The radiogenic sources at the surface were fixed depending on the age of the crust. Below the uppermost 10 kilometres, a relationship between heat generation and P-wave velocity was applied and seismic structure was used to define individual crustal blocks of specific thermal parameters. Small-scale surface heat-flow density anomalies are interpreted as lateral variations of heat generation within the upper part of the crust, whereas the large anomaly in the southern part of the shield is attributed to an anomalously high mantle heat-flow density. The results are shown on maps outlining mantle heat-flow density, Moho temperature and thermal lithosphere thickness. A relationship between subcrustal temperature and Moho depth was found, except for southern Sweden. The lithospheric thickness is found to exceed 200 km in the Bothnian Gulf–northern-central Finland and part of the Kola peninsula, where temperatures at the Moho are less than 500 °C; the temperature gradient at the top of the mantle has, on the average, a value of 7.5 mK m−1 and the mantle heat-flow density varies from 19 to 25 mW m−2. Towards the south, the lithospheric thickness decreases until it attains a value lower than 100 km under southern Sweden. In this latter area, the Moho temperature and mantle heat-flow density are remarkably high, 700°–900°C and 30–45 mW m−2, respectively, as well as the temperature gradient, which amounts to 10 mK m−1.
... Seismology currently offers the most powerful means of mapping the physical properties of the earth, leading towards the understanding of present and past geodynamic processes. Crustal velocity structures have traditionally been mapped only along selected profile lines using refraction and wide-angle reflection data (e.g., in projects such as FENNOLORA (Galson and Mueller, 1986)). Although such experiments give detailed information on vertical velocity variations, the resolution of lateral heterogeneities is restricted to the profile line. ...
Pn and Sn velocity estimates are derived for the sub-Moho structure beneath Fennoscandia using a tomographic conjugate gradient scheme. Observational data stem from local earthquake recordings of crustal phases Pg, Sg (Lg) and the sub-crustal phases Pn and Sn by the Fennoscandian seismograph network. Unknowns are Pn and Sn velocities and event hypocenter parameters. Crustal thicknesses are assumed as known and are not estimated, whereas velocities are allowed to vary. The most prominent P and S velocity anomalies are found in the northwest (inland Lofoten) and southwest (Møre) of Norway and, for shear waves, also in central Finland. There is a good correlation between the velocity anomalies in Norway and observed negative Bouguer anomalies, indicating the presence of low-density sub-Moho material in these areas. Geologically the Pn and Sn velocity anomalies appear to be associated with geodynamic processes tied to the opening of the Norwegian-Greenland Sea.
... Table 1. Overview of seismic refraction and reflection experiments in Norway # Name/Location References 1 NSDP84-3 Fichler and Hospers, 1990 2 NSDP84-2 Fichler and Hospers, 1990 3 NSDP84-4 Fichler and Hospers, 1990 4 NSDP84-1 Fichler and Hospers, 1990 5 Southern Norway, CANOBE Cassell et al., 1983 6 Fedje-Grimstad Sellevoll, 1968; Kanestrøm and Nedland, 1975 7 Oslo Rift, Falkum-Orud Tryti and Sellevoll, 1977 8 Oslo Rift, Falkum- Tryti and Sellevoll, 1977 Semestad 9 Flora-Åsnes Sellevoll and Warrick, 1971; Kanestrøm and Nedland, 1975 10 Trondheim-Oslo Kanestrøm Haugland, 1971a and 1971b 11 Årsund-Otta Mykkeltveit, 1980 12 Crustal transect off Planke et al., 1991 Norway, a 13 Crustal transect off Planke et al., 1991 Norway, b 14 Blue Norma Avedik et al., 1984 15 Blue Road Hirschleber, 1975 16 Crustal Transect Kodaira et al., 1995; across the Lofoten Eldholm et al., 1994 margin 17 Lofoten Drivenes et al., 1984 18 Lofoten-Vester˙ alen Sellevoll, 1968; Kanestrøm, 1971; Sellevoll, 1983 19 Tromsø-Muonio Sellevoll et al., 1964 20 FENNOLORA Galson and Mueller, 1986; Lund, 1987 21 POLAR Profile Luosto et al., 1989 ...
Teleseismic body waves from seismic broadband and short periodstations were used to investigate the crustal structure of Norwaythrough inversion of the receiver functions. The Moho depths ofthe Baltic Shield are quite well known from previous studiesincluding seismic experiments and spectral ratio technique.However, the results on the details of the crustal structure areinconsistent. This study provided more detailed crustalstructure information at 16 locations than previously known andgenerally confirmed Moho depth results obtained in earlier studies. Significant differences are seen at a few sites. The Moho for the various sites was found at depths between 28 and 44 km. In summary, the crustal thicknessincreases from the West Coast of Norway, away from thecontinental margin, towards the centre of the Baltic Shield andfrom Southwest to the Northeast. This corresponds to theincreasing age of the crust. The P velocities in the crust atmost sites show a gradual increase from about 6.0 to 7.1 km/s, withou
The Icelandic plume, a major convective upwelling, has had a considerable influence on the geological evolution of the North Atlantic region. Direct manifestations of this major convective upwelling include positive residual depth anomalies and long wavelength free-air gravity anomalies, both of which reach from Baffin Island to Norway and from Newfoundland to Svalbard. Signifi cant shear wave velocity anomalies, observed in full-waveform tomographic models between 100 km and 200 km depth, show the Icelandic plume has a complex, irregular planform. These anomalies suggest about fi ve horizontal fi ngers radiate away from the central plume conduit. The best imaged fingers lie beneath the British Isles, southern Scandinavia and Greenland, extending ~1,000 km from the Icelandic plume. It is proposed that these radial miscible fi ngers develop due to the Saffman-Taylor instability, a fluid dynamical phenomenon which occurs when a less viscous fluid is injected into a more viscous fluid. Mobility ratio (i.e. the ratio of fluid viscosities), Peclet number (i.e. the ratio of advective and diffusive transport rates) and thickness of the horizontal layer into which the fluid is injected, together control the presence of fi ngering due to the Saffman-Taylor instability. Estimates for the Icelandic plume suggest the mobility ratio is at least 15, the Peclet number is ~ 2 x 10⁴, and the asthenospheric channel thickness is 100 ± 50 km. Appropriately scaled laboratory experiments play a key role in developing a quantitative understanding of the spatial and temporal evolution of mantle plume planforms. My results prove that the presence or absence of radial miscible fi ngering due to the Saffman-Taylor instability is controlled by changes in mobility ratio, Peclet number and horizontal layer thickness. At large horizontal thicknesses, gravity has an increasingly important influence and acts to damp the production of radial viscous miscible fi ngers. Observed values from the Icelandic plume suggest the fluid dynamics may be more complex than the Saffman-Taylor instability alone. Additional processes, such as interaction with the base of the lithospheric plate, along with the Saffman-Taylor instability, may be the origin of the fi ngers.
In July 1985, deep seismic measurements were carried out in close European-Tunisian cooperation along the extension of the main European Geotraverse (EGT) seismic refraction line through Tunisia up to the Saharian platform. The execution and results of this experiment are described and presented in this paper. Eight shots from seven different shotpoints were recorded by 120 mobile stations deployed in a network of nine reversed profile segments, with a total surveyed length exceeding 1300 km. The 6-km/s isoline is found everywhere at great depth, usually between 10 and 15 km. The main feature of the crust, sediments excluded, is its low average velocity (6 km/s), with no clear evidence for any high-velocity lower crust, except maybe in the Kairouan area. For the most part of continental Tunisia, the Moho depth varies between 30 and 35 km, with a maximum depth of 37 km in the Kasserine area. To the north and northeast, the continental crust merges into the thinned crust of the Sardinian Channel and Pelagian Sea. This crust has a typically higher mean velocity and a minimum thickness of only 13 km in the central Sardinian Channel, where the Moho depth is 21 km. In the upper mantle, we derive consistent velocity values in the 7.9–8.1-km/s range. Offsets observed in Pn-wave travel time curves may indicate steps in the Moho beneath the Tellian chains. Finally, observations of two sea shots at large distance (250–500 km) reveal the presence of an upper-mantle reflector under central Tunisia, at a depth of 87 km, with an unusually high apparent velocity of 9.4 km/s below.
For the first time a continuous coverage with modern seismic refraction data has been achieved on a line from North Germany to North Italy. From this data we derived a detailed crustal model which ties together most of the scattered earlier data. A tectonic cross section can be constructed which includes the Caledonides and the Variscides of Germany as well as the young Alpine orogene of Switzerland and North Italy.
The European Geotraverse (EGT) crosses along a 4000 km profile from the North Cape to Tunisia the following main suture zones:
the Tornquist-Teisseyre zone between the Baltic Shield and the Variscan realm,
the transition zones between Rhenohercynian and Saxothuringian as well as between Saxothuringian and Moldanubian zones in the Variscan part of central Europe, and
the collision zone between the European continent and the Adriatic microplate.
Some structural aspects of these suture zones are described.
From a variety of seismic refraction and reflection profiling results, a new detailed Moho depth map for the Fennoscandian part of the Baltic Shield has been compiled. A complementary crystalline crustal thickness map was compiled for Denmark, the Skagerrak Sea, offshore western Norway and adjacent areas where sedimentary strata reach maximum thicknesses of 10 km and even more. These maps are discussed in relation to tectonic evolution and crustal ages of Fennoscandia, seismic velocities, Bouguer gravity and seismicity. Areas deviating from the proposed age-thickness relationships are the Kola Peninsula (Archean) and the Oslo Rift (Permian). The average crustal thickness in the former area is around 44 km, while the latter area is associated with a Moho elevation of 3–5 km. Maximum crustal thicknesses in excess of 50 km have been found in southwestern and Central Finland and correlate with a crust of Proterozoic age. The Caledonides of western Norway do not exhibit, as elsewhere, a crustal root. Sub-Moho Pn and Sn seismic velocities are relatively small in that area and Bouguer gravity values are strongly negative. Intracrustal P-velocity variations do not seem to reflect geological age or specific tectonic features. Although the seismicity is moderate in Fennoscandia, most of the earthquake activity and, in particular, the largest earthquakes are confined to areas where the crustal thinning is most pronounced.
A new P-wave velocity model is derived from wide-angle data collected in 1984 and 1986 during the European Geotraverse (EGT) projects EUGENO-S and EUGEMI. The data have been thoroughly reinterpreted and modelled with a travel time inversion algorithm. We have used model grid nodes approximately every 25 km horizontally with a resulting model that is smoother than previously derived models. Despite the lack of fine details in the model we clearly image the thickness of the crystalline crust which varies between 25 and 45 km along the 600-km-long profile. We present the final model together with the velocity parameter resolution which demonstrates more clearly than in previous studies which parts of the model are well constrained by the available data. We interpret the crystalline crust below the sediments of the Rinkøbing Fyn High and the Norwegian–Danish Basin to be of Sveconorwegian type, a continuation of the Baltic Shield southwest of the Sorgenfrei–Tornquist Zone, a tectonic lineament previously thought of as the boundary between the Precambrian and Palaeozoic Europe. The crust is some 10–15 km thinner here than in the shield proper and in our seismic model we find evidence that the crustal thinning occurs almost entirely in the lower crust (P-wave velocities between 6.5–7.0 km/s). The observed crustal structure shows clear signs of extension, estimated to be at least 20% along the profile.
The structure of the Earth's crust under the northern margin of the Swiss Alps was determined by means of seismic refraction and reflection measurements. The profile runs northeast from Montreux on Lake Geneva along the Helvetic nappes on the southern edge of the Molasse basin to the Rhine valley south of Lake Constance with two shotpoints 165 km apart. We derived a two-dimensional P-wave velocity model from these data. Using data from two fans perpendicular to the profile at a distance of 110 km from the shotpoints we migrated the overcritical Moho reflections laterally to their proper position.The complicated sedimentary structure was mainly taken from various exploration surveys and earlier shallow research. Main features of the crust are: the 3-km-thick upper basement has an average velocity of 6.0 km/s, which is followed by 2 km with a reduced velocity of 5.6 km/s. At 10 km depth the velocity increases to 6.15 km/s. No significant structural changes occur in the depth range between 10 and 26 km with an average velocity of 6.2 km/s. At a depth of 26 km in the west and 24 km in the east the velocity increases to 6.6 km/s for about 3 km followed by a low-velocity zone of 6.4 km/s with an average thickness of 6 km. The crust—mantle boundary is reached at 34 km depth under shotpoint Jaunpass and deepens to 35 km near shotpoint Saentis. The southeastward downdip of the Moho observed on the PmP fans migrates the derived deeper crustal structure laterally to the northwest by about 9 km from the surface position of the profile. The velocity-depth structure derived from the refraction and wide-angle reflection data agrees well with a short near-vertical reflection profile near shotpoint Jaunpass.
A series of geological cross sections based on data from the new geological map of Tunisia coupled with stratigraphic and structural studies indicate that a folded, post-Triassic sedimentary cover was separated from the basement by a decollement zone located in Triassic evaporitic deposits. Four depositional basins were formed during the Triassic and Liassic in an extensional regime, on a presumably thinned continental crust. Geological fieldwork, studies of metamorphic grade and gravimetric and magnetic surveys indicate that their depocentres shifted with time from basin to basin, finally resulting in today's Tellian and Tunisian troughs to the north, Gafsa Trough to the west of Gabes and the Tuniso-Libyan Trough to the east. Subsidence in the latter trough commenced at the end of the Cretaceous and is still continuing today. There are four well-defined seismic stratigraphic sequences comprising Mesozoic and Cenozoic sedimentary rocks, each with distinctive lithoseismic characteristics. We display the distribution and structure of these seismic sequences in a series of cross sections. In addition, we note that the regional tectonic style of Tunisia has been greatly influenced by a conjugate strike-slip fault system. The role of wrench faulting has been well-documented in the Kasserine and other areas by detailed structural studies.
A seismic refraction profile across Langeland (Denmark) obtained from land stations recording airgun shots allowed to resolve upper crustal velocities to a depth of 8 km. The profile traverses the proposed Caledonian Deformation Front and the Ringkoebing-Fyn High. The Ringkoebing-Fyn High is about 10 km wide and the top basement lies less than 2 km below the surface. Basement velocities as high as 6.4 km/s, at depths between 6 and 8 km, can be best explained by compositional changes between adjoining basement units to the north and south. South of the Ringkoebing-Fyn High another high velocity basement unit is encountered and most probably represents a basement affected by the Caledonian orogeny. Along this profile on Langeland the positions of the Caledonian Deformation Front and the northern limit of the Zechstein deposits coincide.
Using gravity maps recently published in Italy and Switzerland we have modelled a profile running along the northern part of the Southern Segment of the European Geotraverse. The profile stretches from the Ligurian coast east of Genova to the Swiss Molasse Basin. The gravity modelling took into consideration geophysical cross sections and density contrasts from various sources. For all models we kept the parameters for the upper crust constant and varied the options for the gravity effect of the lower crust. The upper mantle was considered to be homogeneous. Based on the data available at the time of writing we make the following conclusions. 1.(1) The major contribution to the regional anomalies stems from the topography of the crust-mantle boundary.2.(2) In the southern part of the profile the observed Bouguer anomalies can be modelled by a “step” in the Moho beneath the Ligurian coast.3.(3) In the northern part of the profile a good fit between the observed and calculated Bouguer anomalies can be obtained by introducing another Moho “jump” beneath the Insubric Line.4.(4) Two models satisfy the measured profile; one with a single, the other with a double crust-mantle transition.
The major objective of FENNOLORA, the Fennoscandian Long Range Seismic Project of 1979, was the determination of lower lithospheric and upper mantle structure down to depths in excess of 400 km below the Fennoscandian Shield. In the component represented by this study, data recorded at stations in southern Sweden from three shotpoints, one in northern Germany (profile WN) and two separated by 300 km in southern Sweden (profiles BN and CS), are used to derive a two-dimensional lithosphere structure model extending over 600 km. From north to south, the profile crosses the Svecofennides, the Småland-Värmland Granite Belt, Paleozoic cover rocks lying unconformably on the Fennoscandian Shield, the Baltic Sea (where there were no stations) and into the Caledonian tectonic province of northern Germany where one shotpoint but no stations were located. Interpretation of the three record sections was based on a two-dimensional ray tracing procedure which included the calculation of asymptotic ray theory synthetic seismograms. Lack of data from 0–150 km, for the shotpoint in Germany to the first station in southern Sweden, precluded derivation of any detailed crustal structure for this region. Crustal thickness was determined to be 32 km. A rapid increase in velocity from 8.0 to 8.35 km/s at about a depth of 50 km is underlain by a low velocity zone extending to a depth of 70 km. The reversed profiles BN and CS in southern Sweden are fundamentally different. Interpretation shows a difference in crustal thickness of about 12 km, with the northern segment having a deeper Moho, a feature resulting from a thicker lower crust and a crust-mantle transition zone rather than a rapid increase in velocity. In the southern segment of the reversed profile, a crustal low velocity zone of limited extent is indicated. The most significant aspect of the interpretation is the suggestion that the change in crustal structure between the two shotpoints in southern Sweden occurs within a transition zone of limited lateral extent, less than a few tens of kilometers. We suggest that the model transition zone is a fundamental lithosphere boundary associated with the juxtaposition of the Småland-Värmland Granite Belt and the Svecofennides.
A seismic refraction and wide-angle reflection experiment was carried out in the summer of 1984 in southern Sweden, Denmark and northernmost Germany. The EUGENO-S project (European GEotraverse Northern segment—Southern part) was part of the European Geotraverse. Observations were made along five profiles reaching a total length of 2100 km. In addition to 51 explosions, an airgun array was operated along the offshore parts of the profiles, giving signals to distances greater than 200 km. Three profiles cross the southwestern edge of the Baltic Shield, and two run along the edge, one on each side.The results show that in this area the Moho discontinuity is a sharp boundary. The crustal thicknesses vary significantly, between 26 km and 47 km. In some places the transition zones are narrow, in others they are broad. The variations are mostly in the upper crystalline layer and in the sediments. The sedimentary thicknesses are between 0 and 10 km. The general relationship is such that thick sediments correlate with thin upper crystalline crust and shallow Moho depths. The thickness of the lower crust (approximately 20 km) is quite uniform throughout the region of the edge of the Baltic Shield. A well-known gravity anomaly in Denmark around Silkeborg is explained by high-density and high-velocity material anomalously close to the bottom of the sediments. A number of regional lineaments known from geological studies and reflection profiles can be followed through the crust into the upper mantle. Some pieces of shield crust have become detached, and are now found in the Ringkøbing-Fyn basement high in the middle of Denmark.
Lithospheric temperature distributions, Moho heat flow and the thermal thickness of the lithosphere along the European geotraverse were numerically calculated for a simple 3-D geothermal model. Relatively low crustal temperatures dominate the Baltic shield (TM 400–500°C), while high Moho temperatures of up to 900–1000°C are to be expected in the Alpine-Mediterranean area. Also the expected heat flow from below the crust may vary from less than 20 mW m−2 in northern Europe, to over 50–60 mW m−2 in the south. The results for two different approaches to assess the lithospheric temperature field are given, compared and discussed in terms of the data reliability.
In the Tunisian Atlas nothing is known about the nature of the acoustic basement, and in the Saharan cratonic part, only a few deep exploration wells have reached the Precambrian below a thick unfolded Palaeozoic sequence. North of the craton, a major set of faults (W-E or WNW-ESE) represents the southern limit of palaeo-Tethys; thick sequences of marine Late Carboniferous and Permian were developed below the Jeffara and Chott Jerid. Triassic evaporites are thick below the Tunisian Atlas, which has enabled detachment of the cover from the basement and folding into “plis de couverture” structures. Large diapiric intrusions are known, especially in the northwestern Sillon Tunisien (Tunisian trough). From east to west there is a progressive variation from the stable platform on the eastern side to the unstable shelf of western Tunisia. Between these a zone of tectonic instability, named the North-South Axis, corresponds to a deep fault system that acted as a positive suture, characterized by tilted blocks, gaps in sedimentation and reduced or condensed sequences. In the north, the unstable shelf of western Tunisia grades to the deep furrows where the sediments now forming the Tellian and Numidian nappes were deposited. These nappes overthrust the autochthonous foreland during the Miocene orogeny. The main folding took place in the Pliocene and Early Pleistocene. Recent tectonic activity occurs along the main fault zones.
One-dimensional, steady-state heat conduction models in the crust and upper mantle for 24 areas along the European Geotraverse (EGT) show that temperatures are higher beneath young geological regions; at the Moho they range from 450° to 825°C. Heat flowing out from the mantle varies from 18 to 72 mW/m2 and the contribution of crustal heat-flow density, evaluated by the correlation between seismic velocity and the abundance of radioactive elements, accounts for about 48% of the surface heat-flow density. The temperature gradient below Moho (from about 5 to 23 mK/m) is low where the surface heat-flow density reaches its minimum values. The depth at which the geotherms reach 0.85 of the mantle melting temperature is greater in the Precambrian and Caledonian areas, with a maximum value of about 195 km under the Early Proterozoic Scandinavian Shield. In the Hercynian and Alpine areas, this depth is generally lower. Higher values (maximum 150 km) coincide with the orogenic belts and the continental block of Sardinia and Corsica; under the Ligurian Sea and the Sardinian Channel, the lithospheric thickness decreases remarkably to about 50 km. Globally, the lithosphere thickness is controlled by the mantle heat-flow density which decreases, according to a power law, with an increase in the lithosphere-asthenosphere depth.
As part of the European Geotraverse Project (EGT) a number of international cooperative seismic refraction surveys were carried out along the N-S segment from the Ligurian Sea, across the islands of Corsica and Sardinia, to the Sardinian Channel between Sardinia and North Africa, from 1982 to 1985. For all the experiments offshore explosions served as energy sources ranging from 200 to 1125 kg. The main result is that the crust in the Ligurian Sea is of transitional type with a thickness of only 18 km. The two continental blocks of Corsica and Sardinia have a maximum crustal thickness of 33 and 34 km with average crustal velocities of 6.4 and 6.3 km/s, respectively. Other results are that the internal crustal structure as derived from the available data in both islands is rather undifferentiated with only local evidence for a more steplike interface in the lower crust (Corsica 6.3 to 6.6 km/s; Sardinia 6.5 to 6.8 km/s). There are also indications of a slight velocity inversion under the southwestern part of Sardinia. Farther south in the Sardinian Channel the crustal thickness decreases again to 20 km.A significant feature is the observed Pn velocity. This parameter increases from the relatively low value of 7.5 km/s in the north (Ligurian Sea), to 7.7 km/s and 7.9 km/s for Corsica and Sardinia and finally to 8.0 km/s under the Sardinian Channel. These values reflect the tectonic evolution of that part of the Mediterranean Sea.
A seismic reflection profile has been shot in Värmland, southwestern Sweden, across two major tectonic zones, the Protogine Zone and the Mylonite Zone. The crustal bedrock units separated by the tectonic zones are clearly distinguished in the seismic profile by changes in the reflection character in the upper 5–6 km. The Protogine Zone is represented by a 15–20 km wide band of dipping structures. The Mylonite Zone dips too steeply to be seen directly and a lack of clear reflectors at depth makes a continuation down through the crust hypothetical. In the uppermost 5–6 km between the two tectonic zones, stronger and more continuous reflections can be seen, and are interpreted as being connected with “hyperite” intrusions, which have been mapped at the surface. The lower crust shows generally little reflectivity, with only short and weak reflectors. An exception occurs at mid-crustal depths where a number of clearly recognizable bands of reflected energy are suggested to be major shear zones formed during Sveconorwegian-Grenvillian thrusting towards the east. A slight increase in reflectivity in the depth range 42–48 km correlates well with the Moho depth determined from refraction seismic studies.The possible relationships between data acquisition parameters and the observed scarcity and low strength of reflections from the deeper crust have been considered in some detail. It is shown that the use of 28 Hz single geophones instead of 10 Hz geophone strings along a major part of the profile did not reduce the signal-to-noise ratio for deep reflections. However, the varying source characteristics, in combination with the different quality of the recordings, made computation of the residual static corrections difficult, which may have resulted in a partly destructive stacking of seismic traces. In this way, the low reflectivity of the deeper crust in this area is further brought out in the stacked seismic sections.
Electromagnetic soundings have been made in order to construct a geoelectrical (conductivity) model of the crust along the European Geotraverse (EGT) POLAR Profile. Forty magnetotelluric (MT) soundings, eighteen audiomagnetotelluric (AMT) soundings and ten magnetohydrodynamic (MHD) soundings were made on the main POLAR Profile (POLAR I) and ten more MT soundings on a parallel profile (POLAR II), 40 km to the southeast of the main profile.Analysis of simultaneous recordings by the EISCAT magnetometer chain, and thin-sheet modelling of the effect of the Barents Sea, indicate that neither the source field effects nor the presence of the ocean are significant at periods below 200 s in the measurement area.The magnetotelluric data have been modelled with two-dimensional models representing the regional structure along the profiles. In addition to the regional structure, a thin inhomogeneous surface layer is included in the models in order to explain some local features of the measured response functions. Although details of the surface electrical structures are poorly resolved, the gross features of the geoelectrical cross section are considered to be reliable. The results divide the POLAR Profile into three different blocks. The better conducting Karasjok-Kittilä Greenstone Belt in the south has an average resistivity of less than 10 Ωm. The more resistant Lapland Granulite Belt, with a resistivity between 100 and 200 Ωm, is underlain by conductive (< 5 Ωm), N-dipping layers. The depth for the uppermost conductive layer varies from a few kilometres in the southwestern part of the granulite belt to 13 km in the northeastern part, from where it rises steeply towards the surface close to the boundary between the Lapland Granulite Belt and the Inari Terrain. These features appear to be continuous between the two parallel MT profiles. Within the Inari Terrain a conductive zone at an approximate depth of 10 km and with a resistivity of about 20 Ωm was identified in a resistive upper crust.The geoelectric cross section agrees, in gross detail, with the corresponding gravity, refraction seismic and reflection seismic cross sections of the POLAR Profile. All methods indicated a similar shape for the southwestern part of the Lapland Granulite Belt i.e., granulites have a gently NE-dipping boundary against the underlying Karelian Province. In the northeastern part of the granulite belt the geoelectric model and the gravimetric model show a rather steep S-dipping boundary against the Inari Terrain northeast of the granulite belt.
The effects of post-collisional deformation in Tunisia and the Pelagian block are shown to be pervasively distributed over wide areas (major structural provinces of the peri-Mediterranean mountain chains) or localized in narrow belts (major structural boundaries).A redefinition and reinterpretation of the Tell and Atlas provinces, the north-south axis and the Gafsa-Tozeur belts in Tunisia and the “rift zone” in the Strait of Sicily help to understand both the tectonic significance of distinctive structural associations which in the past were thought to be not compatible with the kinematics of the Europe-Africa system in Neogene time and their role as first-order kinematic elements during the post-collisional deformation history of the central Mediterranean region.
Gravity models illustrate changes in the degree of continental convergence in the Eastern Alpine-Western Carpathian region, and modifications to the lithosphere due to the plate convergence and subsequent Pannonian Basin extension. Analysis of the continental collision zone incorporates a kinematic model of ocean basin closure, whereby gravity anomalies and topography are viewed as part of a continuum of continental crustal shortening, erosion and isostatic rebound. Thick crust and high topography in the Eastern Alps, along with a broad Bouguer anomaly of −140 mGal amplitude, are consistent with about 175 km of crustal shortening, followed by 10 km of isostatic rebound. Eastward, crustal thicknesses and gravity anomaly widths and amplitudes are less, so that only about 50 km of continental crustal shortening and 4 km of rebound occurred in the Western Carpathians. Preservation of thick flysch deposits and small isostatic rebound are attributable to the high-density, shallow mantle of the intact continent-ocean transition zone.
The zone of transition from the African to the Eurasian plate extends into the Alpine area through the Adriatic promontory. Inverting simultaneously all available observations on the phase velocity dispersion of Rayleigh waves in that area it has been possible to construct several cross-sections of the lighosphere-asthenosphere system across the Eastern, Central and Western Alps including the zone of Ivrea. Severe deformations of the crust, of the lower lithosphere (th 'lid') and of the asthenosphere (the 'channel') must have occurred as a result of the plate collision. Thrusting with the resultant thickening and thinning as well as buckling of the various layers reaching to depths of about 250km are the dominant features found in all the sections. These heterogeneities in the upper mantle seem to extend to even greater depths thus confirming correlations with the long-wave-length variations in the gravitational potential as suggested earlier.-P.N.Chroston
The electrical structure of the crust and upper mantle beneath 3 regions of Scandinavia has been delineated by the magnetotelluric and the horizontal spatial gradient techniques. The analyses were applied to data recorded by the Scandinavian IMS magnetometer array complimented by telluric observations. Models compatible with the response functions observed in N Sweden and NE Norway/N Finland are distinctive by exhibiting: 1) a negligibly small resistivity contrast across the seismic Moho; and 2) the unequivocal existence of an electrical asthenosphere beneath both regions. In definite contrast, the response function observed in S Finland demands a highly conducting layer in the lower crust, and an order of magnitude increase in resistivity on entering the mantle. This increase is at a depth compatible with the known seismic Moho for the region. A qualitative measure indicates that the asthenosphere depth increases with increasing distance towards the centre of the N European craton.-from Author
Seismic refraction data for profiles crossing the Tornquist-Teisseyre line in Poland are given. The Precambrian crust of the Eastern European platform, 42-47 km in thickness, is separated from the Cadomian-Palaeozoic crust of Central Europe, 30-35 km in thickness by a 50-90 km-wide section of crust, 50-55 km in thickness. The lithosphere beneath the Precambrian platform is 110-135 km thick, possibly slightly thinner than beneath the Baltic Shield. -J.M.H.
The Baltic Shield consists of several distinct crustal age provinces: an Archaean nucleus is surrounded by successively younger rocks to the S and SW. The Proterozoic crust is interpreted as having formed from repeated subduction-accretion episodes, with the subduction stepping progressively S and SW with time. (Following abstracts)-R.A.H.
Crustal structure along the Fennolora profile is derived from travel-time inversions. Except in the southernmost part, the crustal structure of the shield is relatively homogeneous, with high average velocities, a distinct seismic discontinuity between the upper and lower crust, and a 5-10 km thick crust-mantle transition zone at a depth of 40-50 km.-J.M.H.
The upper mantle structure along the Fennalora profile is derived from travel-time inversion. There is a detailed structure, including numerous low-velocity layers with compressional wave velocity (Vp) variations of + or -0.7 km/s. The base of the lithosphere is at a depth of 130-140 km (Vp approx 8.7 km/s). The top of the mantle transition zone is at a depth of about 440 km (Vp approx 9.6 km/s). -J.M.H.
Now at 12,000 meters, a research well at Kola in the Soviet Arctic has revealed the cause of a seismic discontinuity and has pioneered drilling techniques for the deep exploration of the earth's crust.
The European Geotraverse (EGT) has been planned as a major geoscience project that will run for 5–7 years involving collaborative efforts of geophysicists, geologists, penologists, geodesists, and other geoscientists from different European countries. A first draft proposal was elaborated in 1981 by a Working Group of the European Science Research Councils (ESRC), a standing committee of the European Science Foundation (ESF); it was subsequently modified following the amendments requested by the ESRC and was finally approved by the General Assembly of the ESF on November 9, 1982.
The broad aim of the EGT is to secure an understanding of how the continental lithosphere formed and reacted to changing physical and geometric conditions through time. One of the best locations for such a study is Europe because it is made up of a number of tectonic provinces ranging in succession from the oldest Precambrian areas of Scandinavia to the currently active area of the Mediterranean. The concept of a geotraverse has been chosen in order to provide a continuous, integrated study of sufficient scale to cross this whole region like a swathe that will give new information about the dynamics and vertical and lateral variations of the lithosphere, both within and between adjacent provinces.
Newly digitized and amplitude controlled record sections from the 1977 Southern Alps refraction campaign permitted a reinterpretation of the crustal structure in the area between western Lombardy and the Giudicaria fault. The resulting model exhibits considerable lateral heterogeneity: in the west, below 7.5 km of sediments of the Lombardy Basin, the crust reaches a depth of only 31 km, whereas it thickens towards the more mountainous area in the east, reaching a depth of 46 km below the Adamello Massif. Although the signal character of the corresponding reflections is somewhat erratic, the data are satisfied best by models with a low-velocity zone in the upper crust. An additional small velocity discontinuity from 6.2 to 6.4 km/s was found in the middle crust at around 20 km. Earlier interpretations, based on travel-times alone, included a layer with a velocity of about 7 km/s at this depth. This high-velocity layer was then interpreted as lower-crustal material of the Adriatic — African plate, which had been overthrust onto the European plate during the Alpine orogeny, thus explaining the uplift of the Southern Alps. However, this model of crustal doubling is questionable, because such a mid-crustal high-velocity layer is not in agreement with the amplitude data. The relatively thin crystalline part of the crust under the Lombardy Basin is interpreted, in accordance with geological evidence, as a relic of a Late Hercynian rifting event.
Earlier interpretations of Pn travel-times from the extensive quarry-blast observation scheme in western Germany - now supplemented by explosion data from the 1972 Rhinegraben experiment - have been checked and enhanced using the new MOZAIC time-term method. The large data set (762 travel times) continues to require a considerable anisotropy of upper-mantle P velocity. The resulting estimates of the overall velocity variation - probably 0.50–0.60 km/s about a mean value of 8.05 km/s, that is, 6 to 7 per cent anisotropy - and of the direction of the maximum velocity (close to 20° E of north) are reasonably reliable. However, the detailed form of the anisotropy is obscured by various limitations of the data.
These results allow a realistic assessment of the resolving power of refraction-based studies of velocity anisotropy in the lithosphere. It is concluded that though such studies are probably adequate if the measurement of in situ anisotropy is required within the context of a generalized discussion of lithospheric dynamics they are not appropriate if a detailed specification of the anisotropy is desired.
As preparation for the deep-seismic and other geophysical experiments along the Polar Profile, which transects the Granulite belt and the Kola collision suture, structural field work has been performed in northernmost Finland and Norway, and published geological information including data from the neighbouring Soviet territory of the Kola Peninsula, have been compiled and reinterpreted.Based on these studies and a classification according to crustal and structural ages, the northeastern region of the Baltic Shield is divided into six major tectonic units. These units are separated and outlined by important low-angle, ductile shear or thrust zones of Late Archaean to Early Proterozoic age. The lateral extension of these units into Soviet territory and their involvement in large-scale crustal deformation structures, are described. Using the “view down the plunge” method, a generalised tectonic cross-section that predicts the crustal structures along the Polar Profile is compiled, and the structures around the Kola deep drill-hole are reinterpreted.The Kola suture belt, through parts of which the Kola deep bore-hole has been drilled, is considered to represent a ca. 1900 Ma old arc-continent and continent-continent collision suture. It divides the northeastern Shield region into two major crustal compartments: a Northern compartment (comprising the Murmansk and Sörvaranger units) and a Southern compartment (including the Inari unit, the Granulite belt and the Tanaelv belt, as well as the more southernly situated South Lapland-Karelia “craton” of the Karelian province of the Svecokarelian fold belt).The Kola suture belt is outlined by a 2–40 km wide and ca. 500 km long crustal belt composed of 1.(1) Early Proterozoic (ca. 2400-2000 Ma old) metavolcanic and metasedimentary sequences which originally formed part of the attenuated margin of the Northern Archaean compartment, and2.(2) the remains of a ca. 2000-1900 Ma old, predominantly andesitic island-arc terrain.This island-arc terrain was built up above a SW-plunging subduction zone, initiated ca. 2000 Ma ago in the southern part of a newly formed oceanic domain, the Kola ocean. Due to continued subduction and complete consumption of this ocean, the northern passive margin deposits and the island-arc terrain were brought into tectonic juxtaposition, and during the final arc-continent and continent-continent collision, they were overthrusted onto the northern Archaean continent.Along its southern boundary, the Kola suture belt is tectonically overlain by the Archaean rocks of the Inari unit. This unit was derived from a microcontinent split from the Southern compartment, the depositional basin of the protoliths of the Granulite belt being formed to the south of the microcontinent. The Inari microcontinent appears to have wedged out towards the southeast, as the continuation of the Granulite belt north of the White Sea is in direct tectonic contact with the Kola suture belt.The Granulite belt is composed of high-grade paragneisses and minor amounts of meta-igneous rocks. The paragneisses formed from thick turbidite and mass flow deposits lain down in a back-arc basin south of the Inari microcontinent. A thermal anomaly beneath the partly oceanic basement of the back-arc basin is believed to have contributed to the ca. 2000-1900 Ma old granulite facies metamorphism of the granulite assemblages. Granulite facies conditions still prevailed when the Inari microcontinent overrode the granulites and when the Granulite belt as such was formed and was overthrusted (for at least 100 km) towards the southwest. In conjunction with the latter event, the rocks of the basement of the basin also became involved in thrust movements. These now form the Tanaelv belt, which shows gradational tectonic contacts towards underlying cover and basement rocks of the South Lapland-Karelia craton. Although not all parts of this craton were affected by the Svecokarelian deformation, it is considered to belong to the Karelian province of the Svecokarelian fold belt.A ca. 1900-1800 Ma old episode of wrench faulting and the intrusion of 1790-1770 Ma old post-kinematic granites concluded the Svecokarelian evolution of the northeastern Shield region.
The Variscan Belt of Europe originated from the confrontation of northern Europe and Gondwana, with intervening pre-Variscan blocks (Cadomian and older: Armorica, Tepla-Barrandean, Moravo-Silesian). Though a strike-slip component cannot be excluded, geological evidence suggests a subduction/collision process. Newly discovered features include basaltic rocks with MORB affinities, high-pressure metamorphic rocks, trench deposits, and large-scale allochthonism, which also affects the pre-Variscan basement, and has often led to an inversion of the metamorphic isogrades.
Data from thirty-two Expanding-Spread Profiles carried out in the Gulf of Lions and Ligurian Sea in September 1981 were used to study the deep structure of Western Mediterranean Basin.P-refractions and precritical reflections were observed, down to the Mohorovicic discontinuity, despite the presence of a halokinetic Messinian salt layer, and they have been used in the x—t domain to derive the velocity structure of both sedimentary cover and crust.Three different geological domains are distinguished: a thick continental domain, with a velocity of 6.2 km s−1 and a crustal thickness of more than 20 km, that is continued seawards by a thinned continental domain with a crustal thickness of 5–7 km only, and then in the center of the basin, a double-layered crust with velocities of 5.8 and 6.9 km s−1 and total thickness of about 5 km, interpreted as an oceanic domain, with basement at about 10 km below sea level. The Gulf of Lions margin and the Ligurian Sea are two contrasting areas: the thinned continental domain is very wide in the Gulf of Lions (100 km) and considerably reduced in the Ligurian Sea. A paleo-oceanic ridge appears to be present in the Gulf of Lions. The two basins are separated by a major fracture zone. Ante-rift sediments are observed in the Gulf of Lions.
A seismic refraction profile was shot in the Ligurian Sea between Italy and Corsica in September 1983. The profile, which is part of the southern portion of the European Geotraverse, was shot using small dynamite charges, and the seismograms were recorded by ocean bottom seismometers and land stations, resulting in a number of partially overlapping reversed profiles.The refraction data were interpreted by means of the velocity-intercept and ray-tracing methods, using seismic reflection derived velocity-depth information for the sedimentary section. Refraction velocities of 4.8, 6.0 and 6.6 km s−1 were calculated for the base of the sedimentary section, crystalline basement and lower crust, respectively. An upper mantle velocity of 7.4 km s−1 was inferred using the refraction profiles recorded on Corsica.The interpretation of the data indicates a prominent high in the crystalline basement in the northern part of the profile, while the sedimentary section increases in thickness from about 1.5 km over the basement high to over 6 km between the basement high and Corsica, and includes a considerable thickness of pre-Miocene sediments.The crust-mantle boundary shows shoaling towards the centre of the profile, where a minimum crustal thickness of 16 km is reached. The model, therefore, is that of a stretched and thinned continental crust which was rifted in Oligo-Miocene times. The presence of a low-velocity upper mantle may be indicative of the present renewal of subcrustal activity under the Ligurian Sea.
A review of geothermal data from Fennoscandia (Northern Segment of the EGT)
- N Balling
- D A Galson
Balling, N. and Galson, D.A., in prep. A review of geothermal data from Fennoscandia (Northern Segment of the EGT).
Crustal structure under Sardinia
- Banda
The major events responsible for the geological structure of Sardinia
- Cherchi
Recommendations for an integrated programme of geothermal studies along the EGT
- Galson
Kolskaja Sverchglubokaja
- Kozlovsky
The tectonics of the western and southern Alps: correlation between surface observations and deep structure
- Laubscher
Structure and dynamics of the lithosphere-asthenosphere system beneath the Baltic Shield
- Panza
A lithospheric seismic experiment along the European Geotraverse through Central Europe in 1986
- Prodehl
The European Geotraverse
- Blundell
Crustal structure of Corsica and the lower lithosphere under the Corsica-Sardinia block
- Egger
Eine zweidimensionale refraktionsseismische Interpretation der Geschwindigkeits-Tiefen-Struktur des oberen Erdmantels unter dem Fennoskandischen Schild (Project FENNOLORA)
- Guggisberg
Deep structure and thermicity of the Provencal Basin
- Burrus
Reinterpretation of refraction data in the western Southern Alps
- Deichmann
Preliminary inversion of NARS data: a low-density channel beneath Western Europe
- Dost
Heat flow along the Southern Segment of the EGT
- Lucazeau
New evidence for a density anomaly in the upper mantle below the Southern Alps
- Schwendener
A seismic model of a refraction profile across the western Po Valley
- Stein
Deep structure of the Western Alps: new constraints from the EGT-S 1983 seismic experiment
- Thouvenot
The structural zonation of Tunisia
- Zargouni
EUGENO-S: fieldwork and first results
- Flueh
Lateral variations in the lithosphere in correspondence of the Southern Segment of the EGT
- Panza