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The legacy of the European Geotraverse
Tectonophysics 314 (1999) 7–16 www.elsevier.com/locate/tecto
The legacy of the European Geotraverse D.J. Blundell * Geology Department, Royal Holloway, University of London, Egham Surrey TW20 0EX, UK Received 24 August 1998; received in revised form 10 March 1999; accepted for publication 26 April 1999
Abstract The European Geotraverse (EGT ) created a 4600-km profile across Europe from North Cape Norway to Tunisia. Not only did this produce the first comprehensive cross-section to a depth of 450 km of continental lithosphere covering an eighth of the Earth’s circumference, it covered the geological history of Europe from the Archaean to the present. EGT built up a detailed knowledge of the crust and upper mantle of Europe by integrating geological and geophysical information in a coherent way, continuously along a single profile. It illuminated the dramatic contrast between the thickness and complexity of the lithosphere of western Europe and that of Fennoscandia, which remain in isostatic equilibrium, and elucidated the multilayered elements of thickened Alpine crust and lithosphere and the dynamics of the western Mediterranean. Nine years on, the legacy of EGT is the platform it provided for major scientific advances that have stemmed from it, the greatest being through EUROPROBE projects. The PANCARDI project has discovered the subducted slab beneath the Carpathian Arc in the process of tearing apart from the lithosphere. URALIDES has discovered the Urals orogen is almost perfectly preserved since the Palaeozoic, and the TESZ project has elucidated the complex evolution of the transitional region between the Baltic Shield and the Caledonian and Variscan crustal terranes of western Europe. The scientific advances of EGT were matched by its achievement in mobilising an international workforce from every discipline of the Earth sciences. Friendships made in that endeavour are another legacy of EGT and many of those who started their research careers in EGT are now leading EUROPROBE projects and other collaborative ventures. We all owe a great debt to Professor Stephan Mueller who founded and led the European Geotraverse and was its enduring inspiration. © 1999 Elsevier Science B.V. All rights reserved. Keywords: continental tectonics; European Geotraverse; EUROPROBE; lithosphere–asthenosphere system; TESZ
1. The European Geotraverse Stephen Mueller was the architect and driving force of the European Geotraverse ( EGT ). It began in 1980 at the International Geological Congress in Paris when he discussed the prospect of a major lithospheric transect of Europe from north to south with Rudolph Tru¨mpy (at that * Fax: +44-17844-71780. E-mail address: [email protected] (D. Blundell )
time President of the International Union of Geosciences), Eugen Seibold (President of Deutsche Forschungsgemeinschaft) and Peter Fricker (Secretary General of the Swiss National Science Foundation). The proposed transect was on such a scale that its success was predicated upon organising a multinational, multidisciplinary research endeavour involving the collaboration of a large number of people, never before attempted in the Earth sciences. The mechanism for managing research on such a grand scale came from
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the European Science Foundation, based in Strasbourg and supported by the Science Research Councils of almost all European countries. The purpose of EGT was to generate a 4600-km transect from the North Cape of Norway south to central Tunisia, Fig. 1, to a depth of 450 km in order to encompass the whole lithosphere–asthenosphere system. Moreover, the transect traversed all the major orogenic belts and tectonic units of western Europe from the Archaean terrains in the far north, across the successively younger Proterozoic units of Fennoscandia, the hidden Caledonides beneath Denmark and northern Germany, the Variscides and the Alps to the Mediterranean area of currently active tectonics. Its main aim was ‘‘to develop a three-dimensional picture of the structure, properties and composition of the continental lithosphere of Europe as a basis for understanding its nature, evolution and dynamics’’ ( EGT, 1990). To this end, the transect covered a swathe of ground some 200–300 km in width. Led by a Scientific Steering Committee chaired by Stephen Mueller, the EGT was completed between 1983 and 1990. During this period, the wide range of experimental projects were coordinated by means of a succession of study centres and workshops from which ESF publications swiftly followed. Primary publication of the scientific results was in the normal refereed international journals, but particularly through a series of eight special EGT issues of Tectonophysics published between 1986 and 1991. A synthesis of the scientific outcome of the EGT and its bearing on continental lithosphere evolution and dynamics was published as a book with 25 accompanying maps and sections and a CD-ROM database (Blundell et al., 1992). The following discussion is drawn from the work described in this book. The backbone of EGT was a set of very long profiles to obtain wide-angle reflections and refractions of seismic waves from boundaries down to the base of the lithosphere (Ansorge et al., 1992). These produced the broadscale structural divisions of the crust and upper mantle along the transect. They were supplemented by seismic data from earthquakes in three significant ways. Regional analyses from the network of seismic observations across Europe produced topographic maps of the Moho and the lithosphere–
asthenosphere boundary. Surface wave data from a portable array of digitally recording broadband seismometers (NARS) produced the first coherent cross-section of the upper mantle S-wave properties to depths exceeding 500 km. The inversion of teleseismic data recorded at European observatories produced remarkably good tomographic images of the P-wave velocity structure of the crust and upper mantle along the EGT. The combination of all of these led to the composite lithospheric cross-section shown in Fig. 2. The seismic evidence was reinforced by other geophysical information such as gravity, magnetic and heat flow variations which had been compiled consistently along the whole transect. Temperature variations in the lithosphere modelled from heat flow data, for example, gave lithosphere thickness values consistent with those derived from seismic data, apart from areas such as the Sardinia Channel that are dominated by transient thermal effects due to current magmatic and tectonic activity (Della Vedova et al., 1995). Geodetic modelling of post-glacial uplift of Scandinavia indicated thin asthenosphere beneath thick lithosphere, consistent with the seismic data (Balling and Banda, 1992). Multichannel normal-incidence seismic reflection profiles provided higher-resolution images of crust and upper mantle structure in several key areas of EGT. In the Gulf of Bothnia, profiles from the BABEL survey (BABEL Working Group, 1993) across a major Early Proterozoic crustal boundary produced images remarkably similar to those observed across young mountain chains such as the Alps and Pyrenees where continental collision is evident as a consequence of plate tectonic movements. This finding gave strong support to the notion that plate tectonic processes similar to those directly observed as having been active during the past 200 Ma were taking place as long ago as 2000 Ma, and enabled the Proterozoic history of Fennoscandia to be interpreted with confidence in terms of plate tectonic processes ( Windley, 1992) in which a succession of island arc terranes were built up by progressively southward (as at the present day) migration of subduction. The seismic velocity layering of the upper mantle beneath the crust of Scandinavia picks out
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Fig. 1. Map of main tectonic elements of western Europe showing the location of the EGT in the swathe between the two broad lines (after Blundell et al., 1992).
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Fig. 2. Composite cross-section of the lithosphere along the EGT: vertical exaggeration 4:1 (Blundell, 1992). M, Moho; TZ, Tornquist Zone; TEF, Trans-European Fault.
the relicts of the old northward-dipping subduction zones (see Fig. 2). Seismic profiles across the Caledonides and Variscides of Germany showed a marked contrast in crustal structure with that of Fennoscandia. Whereas the Scandinavian crust is ca. 45 km thick and laterally uniform in physical properties, the Caledonide/Variscide crust is uniformly 30 km thick and laterally highly variable. Most noticeably, no crustal or lithospheric roots remain to mark the Caledonian and Variscan orogens. Lithosphere thickness changes from >160 km beneath Fennoscandia to <90 km beneath the Variscides (see Fig. 2). Multichannel seismic reflection profiles were combined with wide-angle reflection data across the Alps to produce three-dimensional images of the deep crustal structure. They revealed a complex of crustal terranes in collision with one another, rotated in a variety of ways and split at midcrustal levels to form highly deformed cores and high level thrust and nappe complexes in the upper crust, leaving units of lower crust to be partly subducted along with upper mantle lithosphere. Crustal roots are asymmetric in form and litho-
spheric roots are laterally offset beneath them, extending to some 200 km depth. The amount of crustal shortening evident in the upper crust cannot be matched in any reconstruction by an equivalent volume of lower crust, which appears to have become detached and immersed in some way within the mantle. The geodynamic significance of the lateral variations in crustal and lithosphere structure illustrated in Fig. 2 may still not have been fully appreciated. Although it is generally accepted that edge forces such as ‘ridge push’ and ‘slab pull’ play a major role in plate tectonics, as first proposed by Orowan (1965) and Elsasser (1969), they are generally perceived as acting on lithosphere that is uniform except at plate boundaries. But it is evident from Fig. 2 that the continental lithosphere of Europe is far from uniform, varying laterally in thickness, composition, physical properties and rheological behaviour. Fleitout and Froidevaux (1982, 1983) recognised the significance of these heterogeneities and set up the theory for calculating stress fields for various density contrast configurations. Bott (1990) used this theory to calculate the stresses that can arise from the density contrasts at the
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Fig. 3. Stress distributions in models of crustal and lithospheric thickening with an elastic upper crust overlaying a visco-elastic substratum; simplified after Bott (1990): (a) tensional stresses due to a crustal root; (b) compressional stresses due to a lithospheric root; (c) stresses due to a lithospheric root directly below a crustal root; (d) stresses due to a lithospheric root laterally offset from a crustal root.
Moho and lithosphere/asthenosphere boundaries when roots develop beneath a mountain belt. Fig. 3 illustrates how the crustal root produces positive buoyancy leading to uplift and lateral extension whilst the lithospheric root (with opposite density contrast) produces negative buoyancy, subsidence and lateral compression. The presence of a lithospheric root (Fig. 3b) explains why a mountain chain is relatively narrow (e.g. Alps, Himalayas) since the stresses it creates laterally confine the zone of deformation. Importantly, if the lithospheric root is symmetrically disposed directly beneath the crustal root ( Fig. 3c) the forces due to one more or less cancel those due to the other. But if they are asymmetrically disposed and the crustal and lithospheric roots are laterally displaced
( Fig. 3d) — as is evident in Fig. 2 for the Alps — then stresses within the upper crust are not cancelled but extensional and compressional stresses are juxtaposed. This situation is not only found in the Alps but is also clearly seen around the western Mediterranean, both along the western boundary of the Apennines of Italy and around the Betics of southern Spain and the Alboran Sea. Indeed this juxtaposition of stresses exceeding 100 Pa (capable of generating earthquakes) is a common feature of the upper crust in areas of active tectonics. Of course, the two-dimensional limitation of the EGT precludes a full understanding of the force vectors generated by the three-dimensional heterogeneity of the lithosphere, but calculations to quantify uplift/subsidence rates and stress pat-
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terns across the Alps give a reasonable match with observations.
2. The legacy So much new information was gathered and compiled on an unprecedented scale during the 7-year time span of EGT that it is small wonder that the EGT Final Report ( EGT, 1990) concluded that ‘‘In many ways, EGT has only just begun… EGT has laid the foundations and will have a strong influence on all future ideas on the evolution and dynamics of continental lithosphere’’. Nine years on, it is now possible to appraise the veracity of this assertion. The ethos of multinational, multidisciplinary research effort has been continued in a variety of ways but most notably and successfully by EUROPROBE, a scientific programme supported by the European Science Foundation which began in 1992. Coordinated by a Scientific Steering Committee chaired by David Gee, EUROPROBE is organised through five key projects supported by three major projects and a further two developing projects to ‘‘improve our understanding of the tectonic evolution of the Earth’s crust and mantle, and the dynamic processes which controlled this evolution in time’’. (Gee and Zeyen, 1996). These projects are located in specific regions of Europe as shown in Fig. 4 in order to focus on particular scientific objectives. Perhaps the most spectacular results so far have come from the PANCARDI and URALIDES projects, and TESZ, investigating the nature of the boundary between the Phanerozoic crustal units of western Europe and the Precambrian of Fennoscandia has elucidated much of the deep structure of the ‘Trans-European Suture Zone ( TESZ )’ which derived from the Trans-European Fault defined by Berthelsen (see Fig. 1) during the course of EGT. Following directly from EGT, a second phase of BABEL (Meissner et al., 1996) focused on the southern Baltic Sea area in an attempt to unravel the complex transition zone between the old Proterozoic Baltic Shield continent, Baltica, and the continental terranes to the south and west, notably Eastern Avalonia, which came together in
the Caledonian Orogeny during the Lower Palaeozoic. Much of the evidence is now buried beneath the younger sediments of the North German basin and subsequent tectonic activity has superimposed younger structures upon the Caledonian and older features. A set of papers presented at a TESZ workshop, published in Geological Magazine, gave further enlightenment. Large-scale strike–slip faults with NW–SE trend were active across the area from late Carboniferous to early Permian ( Thybo, 1997), relating to Variscan movements to the south, associated with widespread magmatism. These may have created a thermo-mechanical destabilisation of the crust to initiate the North German basin. In 1996 a major seismic experiment was conducted by the DEKORP–BASIN Research Group (1999) which included a long, deep seismic reflection profile SW–NE across the North German basin together with a grid of profiles in the southern Baltic Sea to link with the southern end of the BABEL profile. This revealed a set of reflectors dipping NE below the Moho beneath the NE end of the North German basin, interpreted as the relict of multiphase Caledonian subduction of the Tornquist ocean as Eastern Avalonia closed in on Baltica. Within the overlying crust, the Caledonian suture is marked by a SW-dipping reflector from surface to 20–25 km depth beneath the northern third of the North German basin, interpreted as a north-vergent low-angle thrust. The DEKORP– BASIN Research Group have thus discovered that a wedge of Baltica crust extends much further south than had previously been thought, and this supported by new age determinations (Dallmeyer et al., 1999). Meanwhile, another network of deep seismic profiles across the North Sea west of Denmark was completed in the MONA LISA experiment (MONA LISA Working Group, 1997) to image the Caledonian suture. Here, southdipping reflectors in the crystalline basement, interpreted as north-vergent thrusts, could be linked directly with the crustal thrust beneath the North German basin and the emplacement of Eastern Avalonia against Baltica, but no evidence could be found of reflectors in the upper mantle equivalent to those beneath the southern Baltic Sea indicative of Caledonian subduction. Cenozoic
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Fig. 4. Locations of EUROPROBE projects (Gee and Zeyen, 1996).
reactivation of the Permo-Carboniferous dextral wrench faulting has provided yet further complexity to the region. This is probably best seen south of Bornholm where an earlier pull-apart basin developed on a releasing bend of the Tornquist fault zone has been inverted by dextral transpression between the late Cretaceous and Early Tertiary ( Erlstro¨m et al., 1997) Studies of the deep structure of the Late Palaeozoic Uralide orogen (Berzin et al., 1996; Carbonell et al., 1998) have revealed that this continental collision zone has remained more or less intact to the present day and retains both crustal and lithospheric roots. Nett compression from structural analysis was only some 17 km (Brown et al., 1997), far less than the Alps, for example, and no detachment of lower lithosphere has occurred. Because the lithospheric root remains, it balances the crustal root to preserve the mountain chain and only minor post-orogenic collapse (and associated extensional basins) appears to have occurred (Brown et al., 1998). It therefore provides an excellent laboratory in which to study the anatomy of an orogen. In contrast,
the PANCARDI project has made major advances in understanding the tectonic evolution of the Pannonian Basin and Carpathian Arc, with its links to the Alps and Dinarides. Of particular relevance to EGT, the deep structure of the Alps has been further illuminated subsequently in a major synthesis (Pfiffner et al., 1997) in which indentation and the delamination of slabs of lower crust and mantle lithosphere is shown to have featured on more than one occasion during the evolution of the orogen. Even more apposite is the finding emanating from PANCARDI that the lower crust and mantle lithosphere are currently in the process of tearing apart from the Carpathian Arc and sinking into the asthenosphere, the tear progressing clockwise around the arc from the NW to a point where it is only attached in the SE, Fig. 5 (Gee and Zeyen, 1996; Sperner et al., 1998). It has been possible to follow the progression of the tear from the subsidence/uplift history of each element of the arc — whilst the lower lithosphere is attached, its weight produces subsidence, but once torn away, the buoyancy of the crustal upper lithosphere results in uplift and extension (as
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Fig. 5. Model cross-section of the lithosphere through the eastern Carpathian Arc (Gee and Zeyen, 1996) showing detachment of a lithospheric slab. M, Moho.
shown in Fig. 3a). Accompanying the experimental observations of the dynamic behaviour of the Carpathian Arc are theoretical studies which use finite element schemes to model numerically the thermo-tectonic evolution of the delamination process ( Wong A Ton and Wortel, 1997; Wortel and Spakman, 1998) and its consequences for magmatism and mineralisation (de Boorder et al., 1998) which can be tested against observation. The thermal consequences of continental collision have yet to be fully explored but two-dimensional models using finite difference methods (Midgley and Blundell, 1997) are able to predict the thermal history of any particular point within the orogen and thus create P–T–t pathways for comparison with geochemical analyses of metamorphic rocks. One of their models, shown in Fig. 6, simulates the thermo-mechanical behaviour of a lower lithosphere detachment which leads to uplift of the upper lithosphere and a major thermal pulse as asthenosphere wells up beneath thinned lithosphere. The effect of this is a collapse of the orogen with extension and a rapidly thinning crust in which the geothermal gradient is significantly greater than in the classic rifting process. Metamorphic core complexes develop in such circumstances and, with the brittle/ductile transition
at a high level within the crust, extension in the lower crust is accomplished by means of anatomising ductile shear zones, quite possibly exploited by magmatic intrusion. Support for this scenario comes from the presence of highly reflective lower crust on deep seismic reflection profiles in regions where Caledonian and Variscan orogens have collapsed and their crustal roots have been lost during post-orogenic extension, such as is observed on BIRPS profiles ( Klemperer and Hobbs, 1991) in the Caledonides north of Scotland and in the Variscides of the Celtic Sea and English Channel. The high reflectivity of the lower crust in these areas contrasts markedly with the lack of reflectivity on the BIRPS SWAT profile across the continental margin SW of Ireland formed from rifting of the North Atlantic Ocean. Scientific advances that can be traced from EGT origins have mainly come about from the collaborative efforts of people who first worked together in EGT and their research students. The human capital built by EGT and the collaborative environment that it engendered have burgeoned in EUROPROBE and various other research networks across Europe. Nine years on, EGT remains the only transect across a continent on a lithospheric scale to have been completed. In Canada,
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Fig. 6. Model cross-section across a continental collision zone to illustrate the late-stage thermo-mechanical evolution of an orogen following delamination of the lower lithosphere, based on the models of Midgley and Blundell (1997). (a) Initial model of uniform 30 km thick crust (stippled), with the base of the lithosphere defined at 1300°C. A crustal scale thrust fault is initiated to accommodate compression, beneath which ductile deformation is assumed. Isotherms are shown at 200°C intervals. (b) Model after 5 mma−1 lateral compression for 16 Ma, showing the formation of crustal and lithospheric roots. Flexural isostasy is maintained. (c) Model as in (b) at 16 Ma with lithosphere root delaminated. (d ) Model at 32 Ma after 5 mma−1 lateral extension for 16 Ma.
however, the Lithoprobe programme, now entering its fifth and final phase (Clowes, 1997), will undoubtedly surpass the EGT in scale of operation, eventually completing an E–W transect across Canada. Its results complement those of EGT in traversing a region of terrane accretion across the Canadian Cordillera and the modern low-angle subduction of the Juan de Fuca plate from the western seaboard: a region in which crust and lithosphere thickening is spread out and no roots
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have developed as in the Alps. In consequence the zone of deformation is measured in thousands, rather than hundreds of kilometres. These studies have firmly established the vital role of the whole lithosphere/asthenosphere system in controlling the tectonic evolution of continental crust. The thermal history, in particular, is the controlling factor in metamorphic and magmatic processes and the generation of fluids within the lithosphere. Building upon the information and ideas generated by EGT and EUROPROBE it is now possible to examine those geodynamic processes which lead to the concentration of major ore deposits. Following a precedence set by EGT, a discussion with Rudolf Tru¨mpy and Peter Fricker led to the idea of setting up a new European Science Foundation scientific programme on geodynamics and ore deposit evolution (GEODE ), which began in April 1998. As important as the scientific foundation laid by EGT has thus been the creation of a scientific community stretching across Europe in which collaborative research flourishes and friendships are sealed. This is probably the most enduring legacy of EGT, for which we have much to thank the vision and leadership of Stephan Mueller.
References Ansorge, J., Blundell, D., Mueller, St., 1992. Europe’s lithosphere — seismic properties. In: Blundell, D., Freeman, R., Mueller, St. (Eds.), A Continent Revealed: the European Geotraverse. Cambridge University Press, Cambridge, pp. 33–65. BABEL Working Group, 1993. Integrated seismic studies of the Baltic Shield using data in the Gulf of Bothnia region. Geophys. J. Int. 112, 305–324. Balling, N., Banda, E., 1992. Fennoscandian uplift. In: Blundell, D., Freeman, R., Mueller, St. (Eds.), A Continent Revealed: the European Geotraverse. Cambridge University Press, Cambridge, pp. 127–131. Berzin, R., Oncken, O., Knapp, J.H., Perez-Estaun, A., Hismatulin, T., Yunusov, N., Lipilin, A., 1996. Orogenic evolution of the Ural Mountains: results from an integrated seismic experiment. Science 274, 220–221. Blundell, D.J., 1992. Integrated lithospheric cross section. In: Blundell, D., Freeman, R., Mueller, St. ( Eds.), A Continent Revealed: the European Geotraverse. Cambridge University Press, Cambridge, pp. 96–102. Blundell, D., Freeman, R., Mueller, St. ( Eds.), A Continent
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Revealed: the European Geotraverse 1992. Cambridge University Press, Cambridge. de Boorder, H., Spakman, W., White, S.H., Wortel, M.J.R., 1998. Late Cenozoic mineralization, orogenic collapse and slab detachment in the European Alpine Belt. Earth Planet. Sci. Lett. 164, 569–575. Bott, M.H.P., 1990. Stress distribution and plate boundary force associated with collision mountain ranges. Tectonophysics 182, 193–209. Brown, D., Alvarez-Marron, J., Pe´rez-Estau´n, A., Gorozhanina, Y., Baryshev, V., Puchkov, V., 1997. Geometric and kinematic evolution of the foreland thrust and fold belt in the southern Urals. Tectonics 16, 551–562. Brown, D., Juhlin, C., Alvarez-Marron, J., Pe´rez-Estau´n, A., Oslianski, A., 1998. Crustal-scale structure and evolution of an arc–continent collision zone in the southern Urals, Russia. Tectonics 17, 158–171. Carbonell, R., Lecerf, D., Itzin, M., Gallart, J., Brown, D., 1998. Mapping the Moho Beneath the Southern Urals. Geophys. Res. Lett. 25, 4229–4233. Clowes, R.M. (Ed.), LITHOPROBE Phase V Proposal — Evolution of a Continent Revealed 1997. Lithoprobe Secretariat, University of British Colombia, Vancouver, BC. Dallmeyer, R.D., Giese, U., Glasmacher, U., Pickel, W., 1999. First 40Ar/39Ar age constraints for the Caledonian evolution of the Trans-European Suture Zone in NE Germany. J. Geol. Soc., London 156, 279–290. DEKORP–BASIN Research Group, 1999. Deep crustal structure of the North German basin: new DEKORP–BASIN’96 deep-profiling results. Geology 27, 55–58. Della Vedova, B., Lucazeau, F., Pasquale, V., Pellis, G., Verdoya, M., 1995. Heat-flow in the tectonic provinces crossed by the southern segment of the European Geotraverse. Tectonophysics 244, 57–74. EGT, 1990. European Geotraverse Project (1983–1990) Final Report. European Science Foundation, Strasbourg. Elsasser, W.M., 1969. Convection and stress propagation in the upper mantle. In: Runcorn, S.K. ( Ed.), The Application of Modern Physics to the Earth and Planetary Interiors. WileyInterscience, London, pp. 223–246. Erlstro¨m, M., Thomas, S.A., Deeks, N., Sivhead, U., 1997. Structure and tectonic evolution of the Tornquist Zone and
adjacent sedimentary basins in Scania and the southern Baltic Sea area. Tectonophysics 271, 191–215. Fleitout, L., Froidevaux, C., 1982. Tectonics and topography for a lithosphere containing density heterogeneities. Tectonics 1, 21–56. Fleitout, L., Froidevaux, C., 1983. Tectonic stresses in the lithosphere. Tectonics 2, 315–324. Gee, D.G., Zeyen, H.J. ( Eds.), EUROPROBE, 1996. Lithosphere Dynamics: Origin and Evolution of Continents. EUROPROBE Secretariat, Uppsala University, Uppsala, Sweden. Klemperer, S., Hobbs, R.W., 1991. The BIRPS Atlas. Cambridge University Press, Cambridge. Meissner, R., Blundell, D., Snyder, D., McBride, J. ( Eds.), The BABEL Project: Final Status Report 1996. European Commission, Brussels. Midgley, J., Blundell, D., 1997. Deep seismic structure and thermo-mechanical modelling of continental collision zones. Tectonophysics 273, 155–167. MONA LISA Working Group, 1997. Constraints from MONA LISA deep seismic reflection data. Geology 25, 1071–1074. Orowan, E., 1965. Convection in a non-Newtonian mantle, continental drift and mountain building. Philos. Trans. R. Soc. London 258A, 284–313. Pfiffner, O.A., et al., 1997. Deep structure of the Swiss Alps. Birkhau¨ser Verlag, Basel. Sperner, B., Zweigel, P., Moser, F., Girbacea, R., Lorenz, P., 1998. Plate-tectonics of the Carpathian Arc. Ann. Geophys. 16 Suppl. 1, C15. Thybo, H., 1997. Geophysical characteristics of the Tornquist Fan area, northwest Trans-European Suture Zone: indication of late Carboniferous to early Permian dextral transtension. Geol. Mag. 134, 597–606. Windley, B., 1992. Precambrian Europe. In: Blundell, D., Freeman, R., Mueller, St. (Eds.), A Continent Revealed: the European Geotraverse. Cambridge University Press, Cambridge, pp. 139–152. Wong A Ton, S.Y.M., Wortel, M.J.R., 1997. Slab detachment in continental collision zones: an analysis of controlling parameters. Geophys. Res. Lett. 24, 2095–2098. Wortel, M.J.R., Spakman, W., 1998. The evolution of the Alpine–Mediterranean region: from structure and kinematics to dynamics. Ann. Geophys. 16, Suppl. 1, C12.
The legacy of the European Geotraverse D.J. Blundell * Geology Department, Royal Holloway, University of London, Egham Surrey TW20 0EX, UK Received 24 August 1998; received in revised form 10 March 1999; accepted for publication 26 April 1999
Abstract The European Geotraverse (EGT ) created a 4600-km profile across Europe from North Cape Norway to Tunisia. Not only did this produce the first comprehensive cross-section to a depth of 450 km of continental lithosphere covering an eighth of the Earth’s circumference, it covered the geological history of Europe from the Archaean to the present. EGT built up a detailed knowledge of the crust and upper mantle of Europe by integrating geological and geophysical information in a coherent way, continuously along a single profile. It illuminated the dramatic contrast between the thickness and complexity of the lithosphere of western Europe and that of Fennoscandia, which remain in isostatic equilibrium, and elucidated the multilayered elements of thickened Alpine crust and lithosphere and the dynamics of the western Mediterranean. Nine years on, the legacy of EGT is the platform it provided for major scientific advances that have stemmed from it, the greatest being through EUROPROBE projects. The PANCARDI project has discovered the subducted slab beneath the Carpathian Arc in the process of tearing apart from the lithosphere. URALIDES has discovered the Urals orogen is almost perfectly preserved since the Palaeozoic, and the TESZ project has elucidated the complex evolution of the transitional region between the Baltic Shield and the Caledonian and Variscan crustal terranes of western Europe. The scientific advances of EGT were matched by its achievement in mobilising an international workforce from every discipline of the Earth sciences. Friendships made in that endeavour are another legacy of EGT and many of those who started their research careers in EGT are now leading EUROPROBE projects and other collaborative ventures. We all owe a great debt to Professor Stephan Mueller who founded and led the European Geotraverse and was its enduring inspiration. © 1999 Elsevier Science B.V. All rights reserved. Keywords: continental tectonics; European Geotraverse; EUROPROBE; lithosphere–asthenosphere system; TESZ
1. The European Geotraverse Stephen Mueller was the architect and driving force of the European Geotraverse ( EGT ). It began in 1980 at the International Geological Congress in Paris when he discussed the prospect of a major lithospheric transect of Europe from north to south with Rudolph Tru¨mpy (at that * Fax: +44-17844-71780. E-mail address: [email protected] (D. Blundell )
time President of the International Union of Geosciences), Eugen Seibold (President of Deutsche Forschungsgemeinschaft) and Peter Fricker (Secretary General of the Swiss National Science Foundation). The proposed transect was on such a scale that its success was predicated upon organising a multinational, multidisciplinary research endeavour involving the collaboration of a large number of people, never before attempted in the Earth sciences. The mechanism for managing research on such a grand scale came from
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the European Science Foundation, based in Strasbourg and supported by the Science Research Councils of almost all European countries. The purpose of EGT was to generate a 4600-km transect from the North Cape of Norway south to central Tunisia, Fig. 1, to a depth of 450 km in order to encompass the whole lithosphere–asthenosphere system. Moreover, the transect traversed all the major orogenic belts and tectonic units of western Europe from the Archaean terrains in the far north, across the successively younger Proterozoic units of Fennoscandia, the hidden Caledonides beneath Denmark and northern Germany, the Variscides and the Alps to the Mediterranean area of currently active tectonics. Its main aim was ‘‘to develop a three-dimensional picture of the structure, properties and composition of the continental lithosphere of Europe as a basis for understanding its nature, evolution and dynamics’’ ( EGT, 1990). To this end, the transect covered a swathe of ground some 200–300 km in width. Led by a Scientific Steering Committee chaired by Stephen Mueller, the EGT was completed between 1983 and 1990. During this period, the wide range of experimental projects were coordinated by means of a succession of study centres and workshops from which ESF publications swiftly followed. Primary publication of the scientific results was in the normal refereed international journals, but particularly through a series of eight special EGT issues of Tectonophysics published between 1986 and 1991. A synthesis of the scientific outcome of the EGT and its bearing on continental lithosphere evolution and dynamics was published as a book with 25 accompanying maps and sections and a CD-ROM database (Blundell et al., 1992). The following discussion is drawn from the work described in this book. The backbone of EGT was a set of very long profiles to obtain wide-angle reflections and refractions of seismic waves from boundaries down to the base of the lithosphere (Ansorge et al., 1992). These produced the broadscale structural divisions of the crust and upper mantle along the transect. They were supplemented by seismic data from earthquakes in three significant ways. Regional analyses from the network of seismic observations across Europe produced topographic maps of the Moho and the lithosphere–
asthenosphere boundary. Surface wave data from a portable array of digitally recording broadband seismometers (NARS) produced the first coherent cross-section of the upper mantle S-wave properties to depths exceeding 500 km. The inversion of teleseismic data recorded at European observatories produced remarkably good tomographic images of the P-wave velocity structure of the crust and upper mantle along the EGT. The combination of all of these led to the composite lithospheric cross-section shown in Fig. 2. The seismic evidence was reinforced by other geophysical information such as gravity, magnetic and heat flow variations which had been compiled consistently along the whole transect. Temperature variations in the lithosphere modelled from heat flow data, for example, gave lithosphere thickness values consistent with those derived from seismic data, apart from areas such as the Sardinia Channel that are dominated by transient thermal effects due to current magmatic and tectonic activity (Della Vedova et al., 1995). Geodetic modelling of post-glacial uplift of Scandinavia indicated thin asthenosphere beneath thick lithosphere, consistent with the seismic data (Balling and Banda, 1992). Multichannel normal-incidence seismic reflection profiles provided higher-resolution images of crust and upper mantle structure in several key areas of EGT. In the Gulf of Bothnia, profiles from the BABEL survey (BABEL Working Group, 1993) across a major Early Proterozoic crustal boundary produced images remarkably similar to those observed across young mountain chains such as the Alps and Pyrenees where continental collision is evident as a consequence of plate tectonic movements. This finding gave strong support to the notion that plate tectonic processes similar to those directly observed as having been active during the past 200 Ma were taking place as long ago as 2000 Ma, and enabled the Proterozoic history of Fennoscandia to be interpreted with confidence in terms of plate tectonic processes ( Windley, 1992) in which a succession of island arc terranes were built up by progressively southward (as at the present day) migration of subduction. The seismic velocity layering of the upper mantle beneath the crust of Scandinavia picks out
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Fig. 1. Map of main tectonic elements of western Europe showing the location of the EGT in the swathe between the two broad lines (after Blundell et al., 1992).
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Fig. 2. Composite cross-section of the lithosphere along the EGT: vertical exaggeration 4:1 (Blundell, 1992). M, Moho; TZ, Tornquist Zone; TEF, Trans-European Fault.
the relicts of the old northward-dipping subduction zones (see Fig. 2). Seismic profiles across the Caledonides and Variscides of Germany showed a marked contrast in crustal structure with that of Fennoscandia. Whereas the Scandinavian crust is ca. 45 km thick and laterally uniform in physical properties, the Caledonide/Variscide crust is uniformly 30 km thick and laterally highly variable. Most noticeably, no crustal or lithospheric roots remain to mark the Caledonian and Variscan orogens. Lithosphere thickness changes from >160 km beneath Fennoscandia to <90 km beneath the Variscides (see Fig. 2). Multichannel seismic reflection profiles were combined with wide-angle reflection data across the Alps to produce three-dimensional images of the deep crustal structure. They revealed a complex of crustal terranes in collision with one another, rotated in a variety of ways and split at midcrustal levels to form highly deformed cores and high level thrust and nappe complexes in the upper crust, leaving units of lower crust to be partly subducted along with upper mantle lithosphere. Crustal roots are asymmetric in form and litho-
spheric roots are laterally offset beneath them, extending to some 200 km depth. The amount of crustal shortening evident in the upper crust cannot be matched in any reconstruction by an equivalent volume of lower crust, which appears to have become detached and immersed in some way within the mantle. The geodynamic significance of the lateral variations in crustal and lithosphere structure illustrated in Fig. 2 may still not have been fully appreciated. Although it is generally accepted that edge forces such as ‘ridge push’ and ‘slab pull’ play a major role in plate tectonics, as first proposed by Orowan (1965) and Elsasser (1969), they are generally perceived as acting on lithosphere that is uniform except at plate boundaries. But it is evident from Fig. 2 that the continental lithosphere of Europe is far from uniform, varying laterally in thickness, composition, physical properties and rheological behaviour. Fleitout and Froidevaux (1982, 1983) recognised the significance of these heterogeneities and set up the theory for calculating stress fields for various density contrast configurations. Bott (1990) used this theory to calculate the stresses that can arise from the density contrasts at the
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Fig. 3. Stress distributions in models of crustal and lithospheric thickening with an elastic upper crust overlaying a visco-elastic substratum; simplified after Bott (1990): (a) tensional stresses due to a crustal root; (b) compressional stresses due to a lithospheric root; (c) stresses due to a lithospheric root directly below a crustal root; (d) stresses due to a lithospheric root laterally offset from a crustal root.
Moho and lithosphere/asthenosphere boundaries when roots develop beneath a mountain belt. Fig. 3 illustrates how the crustal root produces positive buoyancy leading to uplift and lateral extension whilst the lithospheric root (with opposite density contrast) produces negative buoyancy, subsidence and lateral compression. The presence of a lithospheric root (Fig. 3b) explains why a mountain chain is relatively narrow (e.g. Alps, Himalayas) since the stresses it creates laterally confine the zone of deformation. Importantly, if the lithospheric root is symmetrically disposed directly beneath the crustal root ( Fig. 3c) the forces due to one more or less cancel those due to the other. But if they are asymmetrically disposed and the crustal and lithospheric roots are laterally displaced
( Fig. 3d) — as is evident in Fig. 2 for the Alps — then stresses within the upper crust are not cancelled but extensional and compressional stresses are juxtaposed. This situation is not only found in the Alps but is also clearly seen around the western Mediterranean, both along the western boundary of the Apennines of Italy and around the Betics of southern Spain and the Alboran Sea. Indeed this juxtaposition of stresses exceeding 100 Pa (capable of generating earthquakes) is a common feature of the upper crust in areas of active tectonics. Of course, the two-dimensional limitation of the EGT precludes a full understanding of the force vectors generated by the three-dimensional heterogeneity of the lithosphere, but calculations to quantify uplift/subsidence rates and stress pat-
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terns across the Alps give a reasonable match with observations.
2. The legacy So much new information was gathered and compiled on an unprecedented scale during the 7-year time span of EGT that it is small wonder that the EGT Final Report ( EGT, 1990) concluded that ‘‘In many ways, EGT has only just begun… EGT has laid the foundations and will have a strong influence on all future ideas on the evolution and dynamics of continental lithosphere’’. Nine years on, it is now possible to appraise the veracity of this assertion. The ethos of multinational, multidisciplinary research effort has been continued in a variety of ways but most notably and successfully by EUROPROBE, a scientific programme supported by the European Science Foundation which began in 1992. Coordinated by a Scientific Steering Committee chaired by David Gee, EUROPROBE is organised through five key projects supported by three major projects and a further two developing projects to ‘‘improve our understanding of the tectonic evolution of the Earth’s crust and mantle, and the dynamic processes which controlled this evolution in time’’. (Gee and Zeyen, 1996). These projects are located in specific regions of Europe as shown in Fig. 4 in order to focus on particular scientific objectives. Perhaps the most spectacular results so far have come from the PANCARDI and URALIDES projects, and TESZ, investigating the nature of the boundary between the Phanerozoic crustal units of western Europe and the Precambrian of Fennoscandia has elucidated much of the deep structure of the ‘Trans-European Suture Zone ( TESZ )’ which derived from the Trans-European Fault defined by Berthelsen (see Fig. 1) during the course of EGT. Following directly from EGT, a second phase of BABEL (Meissner et al., 1996) focused on the southern Baltic Sea area in an attempt to unravel the complex transition zone between the old Proterozoic Baltic Shield continent, Baltica, and the continental terranes to the south and west, notably Eastern Avalonia, which came together in
the Caledonian Orogeny during the Lower Palaeozoic. Much of the evidence is now buried beneath the younger sediments of the North German basin and subsequent tectonic activity has superimposed younger structures upon the Caledonian and older features. A set of papers presented at a TESZ workshop, published in Geological Magazine, gave further enlightenment. Large-scale strike–slip faults with NW–SE trend were active across the area from late Carboniferous to early Permian ( Thybo, 1997), relating to Variscan movements to the south, associated with widespread magmatism. These may have created a thermo-mechanical destabilisation of the crust to initiate the North German basin. In 1996 a major seismic experiment was conducted by the DEKORP–BASIN Research Group (1999) which included a long, deep seismic reflection profile SW–NE across the North German basin together with a grid of profiles in the southern Baltic Sea to link with the southern end of the BABEL profile. This revealed a set of reflectors dipping NE below the Moho beneath the NE end of the North German basin, interpreted as the relict of multiphase Caledonian subduction of the Tornquist ocean as Eastern Avalonia closed in on Baltica. Within the overlying crust, the Caledonian suture is marked by a SW-dipping reflector from surface to 20–25 km depth beneath the northern third of the North German basin, interpreted as a north-vergent low-angle thrust. The DEKORP– BASIN Research Group have thus discovered that a wedge of Baltica crust extends much further south than had previously been thought, and this supported by new age determinations (Dallmeyer et al., 1999). Meanwhile, another network of deep seismic profiles across the North Sea west of Denmark was completed in the MONA LISA experiment (MONA LISA Working Group, 1997) to image the Caledonian suture. Here, southdipping reflectors in the crystalline basement, interpreted as north-vergent thrusts, could be linked directly with the crustal thrust beneath the North German basin and the emplacement of Eastern Avalonia against Baltica, but no evidence could be found of reflectors in the upper mantle equivalent to those beneath the southern Baltic Sea indicative of Caledonian subduction. Cenozoic
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Fig. 4. Locations of EUROPROBE projects (Gee and Zeyen, 1996).
reactivation of the Permo-Carboniferous dextral wrench faulting has provided yet further complexity to the region. This is probably best seen south of Bornholm where an earlier pull-apart basin developed on a releasing bend of the Tornquist fault zone has been inverted by dextral transpression between the late Cretaceous and Early Tertiary ( Erlstro¨m et al., 1997) Studies of the deep structure of the Late Palaeozoic Uralide orogen (Berzin et al., 1996; Carbonell et al., 1998) have revealed that this continental collision zone has remained more or less intact to the present day and retains both crustal and lithospheric roots. Nett compression from structural analysis was only some 17 km (Brown et al., 1997), far less than the Alps, for example, and no detachment of lower lithosphere has occurred. Because the lithospheric root remains, it balances the crustal root to preserve the mountain chain and only minor post-orogenic collapse (and associated extensional basins) appears to have occurred (Brown et al., 1998). It therefore provides an excellent laboratory in which to study the anatomy of an orogen. In contrast,
the PANCARDI project has made major advances in understanding the tectonic evolution of the Pannonian Basin and Carpathian Arc, with its links to the Alps and Dinarides. Of particular relevance to EGT, the deep structure of the Alps has been further illuminated subsequently in a major synthesis (Pfiffner et al., 1997) in which indentation and the delamination of slabs of lower crust and mantle lithosphere is shown to have featured on more than one occasion during the evolution of the orogen. Even more apposite is the finding emanating from PANCARDI that the lower crust and mantle lithosphere are currently in the process of tearing apart from the Carpathian Arc and sinking into the asthenosphere, the tear progressing clockwise around the arc from the NW to a point where it is only attached in the SE, Fig. 5 (Gee and Zeyen, 1996; Sperner et al., 1998). It has been possible to follow the progression of the tear from the subsidence/uplift history of each element of the arc — whilst the lower lithosphere is attached, its weight produces subsidence, but once torn away, the buoyancy of the crustal upper lithosphere results in uplift and extension (as
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Fig. 5. Model cross-section of the lithosphere through the eastern Carpathian Arc (Gee and Zeyen, 1996) showing detachment of a lithospheric slab. M, Moho.
shown in Fig. 3a). Accompanying the experimental observations of the dynamic behaviour of the Carpathian Arc are theoretical studies which use finite element schemes to model numerically the thermo-tectonic evolution of the delamination process ( Wong A Ton and Wortel, 1997; Wortel and Spakman, 1998) and its consequences for magmatism and mineralisation (de Boorder et al., 1998) which can be tested against observation. The thermal consequences of continental collision have yet to be fully explored but two-dimensional models using finite difference methods (Midgley and Blundell, 1997) are able to predict the thermal history of any particular point within the orogen and thus create P–T–t pathways for comparison with geochemical analyses of metamorphic rocks. One of their models, shown in Fig. 6, simulates the thermo-mechanical behaviour of a lower lithosphere detachment which leads to uplift of the upper lithosphere and a major thermal pulse as asthenosphere wells up beneath thinned lithosphere. The effect of this is a collapse of the orogen with extension and a rapidly thinning crust in which the geothermal gradient is significantly greater than in the classic rifting process. Metamorphic core complexes develop in such circumstances and, with the brittle/ductile transition
at a high level within the crust, extension in the lower crust is accomplished by means of anatomising ductile shear zones, quite possibly exploited by magmatic intrusion. Support for this scenario comes from the presence of highly reflective lower crust on deep seismic reflection profiles in regions where Caledonian and Variscan orogens have collapsed and their crustal roots have been lost during post-orogenic extension, such as is observed on BIRPS profiles ( Klemperer and Hobbs, 1991) in the Caledonides north of Scotland and in the Variscides of the Celtic Sea and English Channel. The high reflectivity of the lower crust in these areas contrasts markedly with the lack of reflectivity on the BIRPS SWAT profile across the continental margin SW of Ireland formed from rifting of the North Atlantic Ocean. Scientific advances that can be traced from EGT origins have mainly come about from the collaborative efforts of people who first worked together in EGT and their research students. The human capital built by EGT and the collaborative environment that it engendered have burgeoned in EUROPROBE and various other research networks across Europe. Nine years on, EGT remains the only transect across a continent on a lithospheric scale to have been completed. In Canada,
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Fig. 6. Model cross-section across a continental collision zone to illustrate the late-stage thermo-mechanical evolution of an orogen following delamination of the lower lithosphere, based on the models of Midgley and Blundell (1997). (a) Initial model of uniform 30 km thick crust (stippled), with the base of the lithosphere defined at 1300°C. A crustal scale thrust fault is initiated to accommodate compression, beneath which ductile deformation is assumed. Isotherms are shown at 200°C intervals. (b) Model after 5 mma−1 lateral compression for 16 Ma, showing the formation of crustal and lithospheric roots. Flexural isostasy is maintained. (c) Model as in (b) at 16 Ma with lithosphere root delaminated. (d ) Model at 32 Ma after 5 mma−1 lateral extension for 16 Ma.
however, the Lithoprobe programme, now entering its fifth and final phase (Clowes, 1997), will undoubtedly surpass the EGT in scale of operation, eventually completing an E–W transect across Canada. Its results complement those of EGT in traversing a region of terrane accretion across the Canadian Cordillera and the modern low-angle subduction of the Juan de Fuca plate from the western seaboard: a region in which crust and lithosphere thickening is spread out and no roots
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have developed as in the Alps. In consequence the zone of deformation is measured in thousands, rather than hundreds of kilometres. These studies have firmly established the vital role of the whole lithosphere/asthenosphere system in controlling the tectonic evolution of continental crust. The thermal history, in particular, is the controlling factor in metamorphic and magmatic processes and the generation of fluids within the lithosphere. Building upon the information and ideas generated by EGT and EUROPROBE it is now possible to examine those geodynamic processes which lead to the concentration of major ore deposits. Following a precedence set by EGT, a discussion with Rudolf Tru¨mpy and Peter Fricker led to the idea of setting up a new European Science Foundation scientific programme on geodynamics and ore deposit evolution (GEODE ), which began in April 1998. As important as the scientific foundation laid by EGT has thus been the creation of a scientific community stretching across Europe in which collaborative research flourishes and friendships are sealed. This is probably the most enduring legacy of EGT, for which we have much to thank the vision and leadership of Stephan Mueller.
References Ansorge, J., Blundell, D., Mueller, St., 1992. Europe’s lithosphere — seismic properties. In: Blundell, D., Freeman, R., Mueller, St. (Eds.), A Continent Revealed: the European Geotraverse. Cambridge University Press, Cambridge, pp. 33–65. BABEL Working Group, 1993. Integrated seismic studies of the Baltic Shield using data in the Gulf of Bothnia region. Geophys. J. Int. 112, 305–324. Balling, N., Banda, E., 1992. Fennoscandian uplift. In: Blundell, D., Freeman, R., Mueller, St. (Eds.), A Continent Revealed: the European Geotraverse. Cambridge University Press, Cambridge, pp. 127–131. Berzin, R., Oncken, O., Knapp, J.H., Perez-Estaun, A., Hismatulin, T., Yunusov, N., Lipilin, A., 1996. Orogenic evolution of the Ural Mountains: results from an integrated seismic experiment. Science 274, 220–221. Blundell, D.J., 1992. Integrated lithospheric cross section. In: Blundell, D., Freeman, R., Mueller, St. ( Eds.), A Continent Revealed: the European Geotraverse. Cambridge University Press, Cambridge, pp. 96–102. Blundell, D., Freeman, R., Mueller, St. ( Eds.), A Continent
16
D.J. Blundell / Tectonophysics 314 (1999) 7–16
Revealed: the European Geotraverse 1992. Cambridge University Press, Cambridge. de Boorder, H., Spakman, W., White, S.H., Wortel, M.J.R., 1998. Late Cenozoic mineralization, orogenic collapse and slab detachment in the European Alpine Belt. Earth Planet. Sci. Lett. 164, 569–575. Bott, M.H.P., 1990. Stress distribution and plate boundary force associated with collision mountain ranges. Tectonophysics 182, 193–209. Brown, D., Alvarez-Marron, J., Pe´rez-Estau´n, A., Gorozhanina, Y., Baryshev, V., Puchkov, V., 1997. Geometric and kinematic evolution of the foreland thrust and fold belt in the southern Urals. Tectonics 16, 551–562. Brown, D., Juhlin, C., Alvarez-Marron, J., Pe´rez-Estau´n, A., Oslianski, A., 1998. Crustal-scale structure and evolution of an arc–continent collision zone in the southern Urals, Russia. Tectonics 17, 158–171. Carbonell, R., Lecerf, D., Itzin, M., Gallart, J., Brown, D., 1998. Mapping the Moho Beneath the Southern Urals. Geophys. Res. Lett. 25, 4229–4233. Clowes, R.M. (Ed.), LITHOPROBE Phase V Proposal — Evolution of a Continent Revealed 1997. Lithoprobe Secretariat, University of British Colombia, Vancouver, BC. Dallmeyer, R.D., Giese, U., Glasmacher, U., Pickel, W., 1999. First 40Ar/39Ar age constraints for the Caledonian evolution of the Trans-European Suture Zone in NE Germany. J. Geol. Soc., London 156, 279–290. DEKORP–BASIN Research Group, 1999. Deep crustal structure of the North German basin: new DEKORP–BASIN’96 deep-profiling results. Geology 27, 55–58. Della Vedova, B., Lucazeau, F., Pasquale, V., Pellis, G., Verdoya, M., 1995. Heat-flow in the tectonic provinces crossed by the southern segment of the European Geotraverse. Tectonophysics 244, 57–74. EGT, 1990. European Geotraverse Project (1983–1990) Final Report. European Science Foundation, Strasbourg. Elsasser, W.M., 1969. Convection and stress propagation in the upper mantle. In: Runcorn, S.K. ( Ed.), The Application of Modern Physics to the Earth and Planetary Interiors. WileyInterscience, London, pp. 223–246. Erlstro¨m, M., Thomas, S.A., Deeks, N., Sivhead, U., 1997. Structure and tectonic evolution of the Tornquist Zone and
adjacent sedimentary basins in Scania and the southern Baltic Sea area. Tectonophysics 271, 191–215. Fleitout, L., Froidevaux, C., 1982. Tectonics and topography for a lithosphere containing density heterogeneities. Tectonics 1, 21–56. Fleitout, L., Froidevaux, C., 1983. Tectonic stresses in the lithosphere. Tectonics 2, 315–324. Gee, D.G., Zeyen, H.J. ( Eds.), EUROPROBE, 1996. Lithosphere Dynamics: Origin and Evolution of Continents. EUROPROBE Secretariat, Uppsala University, Uppsala, Sweden. Klemperer, S., Hobbs, R.W., 1991. The BIRPS Atlas. Cambridge University Press, Cambridge. Meissner, R., Blundell, D., Snyder, D., McBride, J. ( Eds.), The BABEL Project: Final Status Report 1996. European Commission, Brussels. Midgley, J., Blundell, D., 1997. Deep seismic structure and thermo-mechanical modelling of continental collision zones. Tectonophysics 273, 155–167. MONA LISA Working Group, 1997. Constraints from MONA LISA deep seismic reflection data. Geology 25, 1071–1074. Orowan, E., 1965. Convection in a non-Newtonian mantle, continental drift and mountain building. Philos. Trans. R. Soc. London 258A, 284–313. Pfiffner, O.A., et al., 1997. Deep structure of the Swiss Alps. Birkhau¨ser Verlag, Basel. Sperner, B., Zweigel, P., Moser, F., Girbacea, R., Lorenz, P., 1998. Plate-tectonics of the Carpathian Arc. Ann. Geophys. 16 Suppl. 1, C15. Thybo, H., 1997. Geophysical characteristics of the Tornquist Fan area, northwest Trans-European Suture Zone: indication of late Carboniferous to early Permian dextral transtension. Geol. Mag. 134, 597–606. Windley, B., 1992. Precambrian Europe. In: Blundell, D., Freeman, R., Mueller, St. (Eds.), A Continent Revealed: the European Geotraverse. Cambridge University Press, Cambridge, pp. 139–152. Wong A Ton, S.Y.M., Wortel, M.J.R., 1997. Slab detachment in continental collision zones: an analysis of controlling parameters. Geophys. Res. Lett. 24, 2095–2098. Wortel, M.J.R., Spakman, W., 1998. The evolution of the Alpine–Mediterranean region: from structure and kinematics to dynamics. Ann. Geophys. 16, Suppl. 1, C12.