Eos, Vol. 82, No. 50, December 11,2001

EOS,TRANSACTIONS, AMERICAN GEOPHYSICAL U N I O N

V O L U M E 82 N U M B E R 50 DECEMBER 1 1 , 2001 PAGES 621-636

Seismic Probing of Fennoscandian Lithosphere

[Gregersen and TOR Working Group, 1999]; while the Eifel Plume project's primary objective is to unravel plume signatures [Ritter et ai, 1998]. Data combined from all of these deployments will provide an unprecedented opportunity

PAGES 621,628-629

The Svecofennian-Karelian-Lapland-Kola Transect (SVEKALAPKO) project is one of the five multidisciplinary key projects of Europrobe, a scientific program of the European Science Foundation (ESF) that studies the tectonic evolution of European continental lithosphere [Gee andZeyen, 1996].The SVEKALAPKO project [Hjeltand Daly, 1996] has adopted a multidisciplinary approach that uses geo­logical, penological, and geophysical methods to unravel the evolution of the crust and lower lithosphere in three major crustal segments of the Fennoscandian Shield: the Proterozoic Svecofennian and Lapland-Kola orogens and the intervening Archaean Karelia craton. Improved knowledge of the structure and evolu­tion of the Fennoscandian Shield should lead to a better understanding of plate-tectonic processes in the early history of the Earth.

Geophysical investigations within the SVEKALAPKO project include near-vertical and wide-angle seismic reflection and refrac­tion work, large-scale magnetotelluric studies, geothermal surveys, and an intensive study of teleseismic data with a network of 144 tem­porary and permanent seismic stations. The temporary seismic station array was operated between August 1998 and May 1999 covering the Proterozoic and Archaean crust in Finland and Russian Karelia (Figure l) .The two prime objectives of the SVEKALAPKO deep seismic project are to understand the evolution of the lithosphere-asthenosphere system beneath the Fennoscandian Shield, and to define crustal evolution in the major crustal segments of Proterozoic and Archaean age.To achieve these aims, a present-day structural image of the subcrustal lithosphere and asthenosphere has to be assessed.

The SVEKALAPKO tomography project included the third large deployment of tem­porary seismograph networks in Europe since 1996 (Figure 1, inset map). All of these projects were designed to derive an improved image of lithosphere and asthenosphere structure in various tectonic provinces of Europe to depths of 400 km by using high-resolution teleseismic travel time and surface wave tomography, receiver function analysis, and study of seismic anisotropy.The Teleseismic Tomography

Tornquist (TOR) project was aimed at a detailed investigation of the Trans-European Suture Zone (TESZ) between Phanerozoic and Proterozoic Europe (STZ on Figure 1 inset)

24*e 28°E

to establish a cross-section of lithosphere-asthenosphere structure to 400-km depth extending from central Europe across the TESZ to Proterozoic and Archaean Fennoscandia. This will complement earlier studies within

60°N

30£

Fig. 1. This location map shows temporary seismograph stations in the SVEKALAPKO area. Super­imposed on the station map are isolines in 2-km intervals depicting Moho depth variations, after Luosto [1997] and Korsman et al. [1999]. The dashed line indicates the approximate location of the boundary between the Proterozoic Svecofennian and Archaean at depth. The inset map shows the locations of temporary seismographs in other recent European experiments, the TOR and Eifel-Plume deployments /Gregersen et al., 1999; Ritter et al., 1998]. The dashed line in the inset map depicts the extent of the Sorgenfrei-Tornquist (STZ) and Tornquist-Teysseire zones (TTZ) that mark the northern margin of the Trans-European Suture Zone (TESZ). Original color image appears at the back of this volume.

Eos, Vol. 82, No. 50, December 11, 2001

Exposed Archaean

Concealed Archaean

Svecofennian Meso- and Neoproterozoic sediments Caledonian

Paiaeoproterozoic rift basins

Lapland Granulite Belt

Rapaktvi granitoids

Sveconorwegian

Phanerozoic

Paiaeoproterozoic ophiolite

Boundaries of major crystal segments

Fig. 2. Simplified geological map /Hjelt and Daly, 1996] of the Fennoscandian Shield. The inset map depicts the subdivision of the East European craton. Original color image appears at the back of this volume.

the European Geotraverse EGT [Blundell et al, 1992] to greater depths and with higher reso­lution. Preliminary results indicate deep-seated differences in mantle structure between Precambrian and Palaeozoic provinces in Europe to at least 400-km depth.

The Fennoscandian Shield

The Fennoscandian Shield is the major exposure of Precambrian rocks in Europe. Its northeastern region is characterized by marked contrasts in lithospheric evolutionary history. The field area of the deep seismic experiment extends from the Proterozoic Svecofennian in the south and southwest to the Archaean Karelian Province (Figures 1 and 2).The Karelian craton is a late Archaean granite-greenstone terrane.At a few localities, ages of 3.1 Ga were measured, while much of the terrane formed at about 2.8 Ga [Gadl and Gor-batschev, 1987] .The southwestern boundary of the Karelian province with the Svecofennian is a well-defined, northwest-southeast-oriented collisional zone.The Svecofennian province consists of juvenile Paleoproterozoic crust, less than 2.0 Ga old.The island arc complex collided at about 1.9 Ga with the Karelian craton and it was subjected to low pressure metamorphism. After this time, the Karelian province experienced Proterozoic thermal and mechanical disturbance at its margins; the major part of the terrane remained stable in contrast to the Archaean Lapland-Kola orogen to the north, which was variably reworked in the Paleoproterozoic.

The Archaean-Proterozoic boundary (Figure 1, dashed line in the station map) marks the southwestern-most extent of where traces of Archaean rocks have been found in Svecofenn­ian granites [Korsman et al, 1999] .This line marks at depth the boundary between ancient plates, the Archaean craton to the east, and the Svecofennian to the west. It coincides only partly with the boundary between outcropping Archaean rocks and Svecofennian, which is depicted in the simplified geological map of Fig­ure 2.The largest crustal thickness, reaching 60 km, is observed in the boundary zone between Svecofennian and Archaean (Figure 1). This very thick crust is not reflected in surface topography It is explained mainly by variations in the thickness of an anomalously high-velocity high-density lower crust [Luosto, 1997; Korsman etal, 1999].

Fieldwork

First station installations were made in August 1998, but the full extent of the network was only reached in November 1998. By this time, participants from 10 institutions in eight countries had brought into operation 139 temporary seismographs, in addition to the existing permanent seismographs. Of the temporary seismographs, 123 were set up in Finland and 16 in Russian Karelia. All stations were operated with three-component seismometers; of these,

46 stations were equipped with broad-band sensors and the others with short-period seismometers. Data were continuously recorded with sample frequencies, depend­ing on station type, varying from 20 Hz to 100 Hz, and with 150 Hz at some of the Russian stations. All stations were set up inside buildings as protection against the harsh seasonal conditions.

After the fieldwork, event-based data were extracted from the huge data base of continuous recordings. A total of 1356 events was finally selected for distribution and further analysis among the participating institutions. Of the 1356 events, 442 are teleseismic with distances A > 30° (magnitude > 5.0) and 914 local and regional events including quarry blasts at A < 30°.This enormous task has just been com­pleted. For the first 3 years, the data set will be used exclusively by the participating institu­

tions. The data will then become available to other investigators.

The final aim of the SVEKALAPKO project is the integrative interpretation of all available data; that is, seismic and magnetotelluric models, potential field, heat flow, and petrophysi-cal data—to derive a petrophysical model and the three-dimensional temperature field in the upper mantle beneath the craton. Among the various methods to be applied are teleseismic travel-time tomography to obtain a tomographic image of the lithosphere and asthenosphere to 400 km depth; surface wave tomography to find out whether a litho­sphere- asthenosphere boundary can be traced; the receiver function method to inves­tigate seismic discontinuities in the upper mantle and mantle transition zone; as well as shear wave splitting and P-wave polarization and residual analysis to study seismic anisotropy

Eos, Vol. 82, No. 50, December 11, 2001

Moho? 410-km 660-km

Moho 410-km 660-km

Fig. 3. Receiver function image derived from the TOR data set and a subset of 19 stations from the SVEKALAPKO experiment. Traces have been sorted from south (bottom) to north (top). Thick horizontal bar indicates a break in the profile and change of distance scale. A total of 1699 traces was used to derive the bottom part of the section, and a total of 522 traces was used for the top part. Dark streaks represent P-S waves that are converted at a boundary where seismic velocity increases with depth. The signal at 18-s delay time in the northern part of the section represents crustal multiples.

The data will also be used for a local and regional travel-time tomography by means of quarry blast and earthquake recordings.

The Deep Seismic SVEKALAPKO Data Set

As an example of the potential of this data set in combination with a data set obtained in the TOR project (Figure 1), we show a receiver function cross-section of crust and upper mantle from the North German basin into the SVEKALAPKO area (Figure 3). Receiver functions are basically P-S converted waves recorded at teleseismic distances that arrive immediately after the P wave.The delay time of P-S converted phases relative to the first arriving P wave increases with the depth of conversion. Figure 3 shows the P-S converted wave field averaged over 100-km intervals of horizontal distance that were moved at 5-km intervals and plotted at the locations of the conversion points at 410 km depth. Geologically the section begins in Phanerozoic central Europe, crosses the TESZ into the Proterozoic Svecofenn­ian of Sweden and southwestern Finland, and ends in the Archaean Karelian craton.

We can spot several interesting features in this geotraverse.The crust-mantle boundary can be traced from the south at about 30-km depth (corresponding to 3 s delay time) across the TESZ into southern Sweden, where it deepens to about 50-km depth, an observa­tion that has been reported by Gossler et al. [1999]. No clear coherent Moho is visible in the SVEKALAPKO part of this display Because of the projection of the images onto one section, this feature reflects the fact that there are large variations in crustal thickness in central Fin­land. Mantle discontinuities are also visible in this image.The transition zone between 410 km (at about 43 s delay time) and 660 km (at about 68 s delay time) discontinuities thickens across the TESZ toward Scandinavia, indicat­ing lower mantle temperatures there. In the Fennoscandian shield, P-S converted phases from the 410-km and 660-km discontinuities arrive earlier than in the IASP91 reference model, which indicates a cooler mantle above 410 km depth.

To improve the resolution of the planned tomographic analysis of the lower lithosphere, the contribution of the crust and its lateral variation to the overall teleseismic travel time has to be determined independently. Therefore, the three-dimensional crustal struc­ture has been compiled from the abundant controlled source seismic surveys in Finland and the adjoining regions (Figure 4a).This perspective view clearly shows the presence of a high-velocity lower crust of variable thick­ness and the large variations of crustal thick­ness in the survey area, which is causing the diffuse image of the Moho in the receiver functions of Figure 3.The anomalous crustal feature is already part of the interpretation by

Eos, Vol. 82, No. 50, December 11,2001

V P (km/s) • 6.0 6.5 7.0 7.5 8.0

1

0 100 200 300 400 500 600 700 800 x ( k m )

Fig. 4. (a) This block diagram for the three-dimensional crustal P-waue velocity structure of the Fennoscandian Shield was compiled from controlled source seismic data. The layer with anomalously high velocity in the lower crust is clearly visible; the dashed line marks the top of the lower crust, (b) Travel time residuals of aP wave front are shown arriving from the north through the three-dimensional crustal model. Original color image appears at the back of this volume.

Korja et al. [1993] .The velocity increases from 7 km/s at its top to values of 7.85 km/s above the Moho with variations of thickness between 0 km and 24 km. Figure 4b shows a map view at the Earth's surface of travel time residuals relative to the IASP91 reference model for a wavefront originating from a point source at 33° distance, 0° back azimuth, and propagating through the three-dimensional crust in Figure 4a from a depth of 70 km. After correction for these laterally varying travel times, a reliable high-resolution picture of the lower lithosphere-asthenosphere system can be determined.

Acknowledgments

The following institutions participate in the EUROPROBE SVEKALAPKO seismic tomography experiment: University of Oulu, University of Helsinki, University of Uppsala, Kola Science Center, Institute of the Physics of the Earth-Moscow, ETH Zurich, GFZ Potsdam, Geophysical Institute of CAS-Prague, Spetzgeofisika MNR Moscow, University of Grenoble, University of Strasbourg, University of Stuttgart, and St. Petersburg University.The SVEKALAPKO field project is supported by the Academy of

Finland; national science funding agencies in France, Sweden, and Switzerland; the Institute of Geophysics of the Polish Academy of Sciences; GeoForschungsZentrum Potsdam (GFZ); the GFZ Geophysical Instrument Pool; and by a European Union Independent Inter­national Association (INTAS) grant.Thanks are due to the European Science Foundation for supporting several workshops of the SVEKALAPKO groups in St. Petersburg, Russia, and Lammi,Finland; and to J.Stephen Daly, Joachim Ritter, and John A. Goff for reviewing the manuscript.

Authors

G. Bock, U.Achauer,A.Alinaghi,J.Ansorge, M. Bruneton, W. Friederich, M. Grad,A. Guterch, S.-E. Hjelt, THyvonen, J.-PIkonen, E. Kissling, K. Komminaho,A. Korja, PHeikkinen, E. Kozlovskaya, M. VNevsky, N. Pavlenkova, H. Pedersen, J. Plomerovd, T.Raita, O. Riznichenko, R. G Roberts,S. Sandoval,LA. Sanina,N. Sharov, J. Tiikkainen, S. G Volosov, E. Wielandt, K.Wylegalla,J.Yliniemi,and Y.Yurov For additional information, contact Gunter Bock, GeoForschungsZentrum Potsdam,Telegrafenberg, 14473 Potsdam, Germany; E-mail: bock@gfz-potsdam.de

References

Blundell, D., R. Freeman, and St. Mueller (eds . ) ,v4 Continent Revealed: The European Geotraverse, 275 pp., Cambridge University Press, New York, 1992.

Gaal,G.,and R. Gorbatschev,An outline of the Pre­cambrian evolution of the Baltic Shield, Precam­brian Res., 35, \5-52, mi.

Gee,D.G.,and H.J.Zeyen (eds.),EUROPROBE 1996-Lithosphere Dynamics: Origin and Evolution of Continents, 138 pp., EUROPROBE Secretariate, Uppsala University, Uppsala, Sweden, 1996.

Gossler, J., R. Kind, S.V Sobolev, H. Kampf, K. Wylegalla, M. Stiller, and TOR Working Group, Major crustal features between the Harz mountains and the Baltic Shield derived from receiver functions, Tectonophys., 314,321-333,1999.

Gregersen, S., and TOR Working Group, Important findings expected from Europe's largest seismic array,£bs, Trans. AGU, 80,1,1999.

Hjelt,S. E., and J.S. Daly, SVEKALAPKO, Evolution of Paiaeoproterozoic and Archaean Lithosphere, in EUROPROBE 1996 - Lithospheric Dynamics: Origin and Evolution of Continents, edited by D. G. Gee and H.J.Zeyen, 138 pp.,EUROPROBE Secretariate, Uppsala University Uppsala, Sweden, 1996.

Korja,A.,T. Korja,U. Luosto,and PHeikkinen,Seismic and geoelectric evidence for collisional and extensional events in the Fennoscandian Shield— implications for Precambrian crustal evolution, Tectonophys., 219,129-152,1993.

Korsman, K.,T. Korja, M. Pajunen, PVirransalo, and GGT/SVEKA Working Group,The GGT/SVEKA transect: Structure and evolution of the continen­tal crust in the Paleoproterozoic Svecofennian Orogen in Finland,M Geol. Rev, 41,287-333,1999.

Luosto, U , Structure of the Earth's crust in Fennoscandia as revealed from refraction and wide-angle reflection studies, Geophysica, 33,3-16, 1997.

Ritter, J. R. R., U. R. Christensen, U. Achauer, K. Bahr, and M.H.Weber,Search for a mantle plume under central Europe,Eosjrans.AGU, 79,420,1998.

Eos,Vol. 82, No. 50, December 11, 2001

Fig. 1. This location map shows temporary seismograph stations in the SVEKALAPKO area. Super­imposed on the station map are isolines in 2-km intervals depicting Moho depth variations, after Luosto [1997] and Korsman et al. [1999]. The dashed line indicates the approximate location of the boundary between the Proterozoic Svecofennian and Archaean at depth. The inset map shows the locations of temporary seismographs in other recent European experiments, the TOR and Eifel-Plume deployments /Gregersen et al., 1999; Ritter et al., 1998]. The dashed line in the inset map depicts the extent of the Sorgenfrei-Tornquist (STZ) and Tornquist-Teysseire zones (TTZ) that mark the northern margin of the Trans-European Suture Zone (TESZ).

Eos,Vol. 82, No. 50, December 11, 2001

Fig. 2. Simplified geological map /Hjelt and Daly, 1996] of the Fennoscandian Shield. The inset map depicts the subdivision of the East European craton.

Eos, Vol. 82, No. 50, December 11, 2001

6.0 6.5 7.0 7.5 8.0

x (km)

Fig. 4. (a) This block diagram for the three-dimensional crustal P-wave velocity structure of the Fennoscandian Shield was compiled from controlled source seismic data. The layer with anomalously high velocity in the lower crust is clearly visible; the dashed line marks the top of the lower crust, (b) Travel time residuals of a P wavefront are shown arriving from the north through the three-dimensional crustal model.