The Global Geoscience Transects Project in Finland

THE GLOBAL GEOSCIENCE TRANSECTS PROJECT IN FINLAND

K. KORSMÄN and T. KORJA

KORSMAN, K. and KORJA, T. 1999. The Global Geoscience Transects Project in Finland. Bulletin of the Geological Society of Finland 71, Part 1, 5-12. K. Korsmän and T. Korja: Geological Survey of Finland, P.O. Box 96, FIN- 02151 Espoo, Finland. E-mail: [email protected]

An international Global Geoscience Transects subsurface crustal sections include geologic and (GGT) Project was initiated in 1985 with the pri- tectonic interpretations with observed and calcu- mary goal of integrating surface geological data lated geophysical anomalies. Data such as geo- with subsurface geophysical data to enhance our chemical, geodetic, borehole, earthquakes, electri- understanding of crustal structure from a variety cal resistivity, heat flow, and seismic reflection or of active and ancient tectonic environments refraction are presented where available. The first (Goetze & Monger 1992). The international GGT transect compilations were published by the ILP Project, which was completed at the end of 1998, and American Geophysical Union (AGU) in Au- has been a collaborative venture of the Internation- gust, 1991. Since then the digitization guidelines al Lithosphere Program (ILP) established by the were developed (Goetze & Williams 1993) and in International Union of Geological Sciences 1995 ILP decided that future transects will be (IUGS) and the International Union of Geodesy available only as digital products (see GGT and Geophysics (IUGG). Its goal was to construct homepage at http://quake.wr.usgs.gov/GGT). lithospheric transects, mainly crustal cross sections The studies along the SVEKA profile form a through crucial geologic and tectonic features, Finnish contribution to the GGT Project (Korja & such as mountain belts, gråbens, sedimentary ba- Korsmän 1993). The GGT/SVEKA transect, sins, and hazardous regions prone to the disastrous which contains the most comprehensive geologi- activities of earthquakes and volcanoes, in order cal, geochemical and geophysical data available to allow direct comparisons. All of the available for Finland, is located in the central part of the data have been assembled at an equal scale of 1:1 Fennoscandian Shield. It crosses the main tectonic million in order to enable global comparisons of units in southern and central Finland and covers similar structures as well as to display the nature an area 160 km wide and 840 km long extending and evolution of the Earth's lithosphere. Transects from near the Finnish-Russian border in the north- typically consist of strip maps that portray geo- east to the Finnish-Swedish border within the logical and geophysical data covering a region at Aland archipelago in the southwest (Figs. 1 and 2). least 100 km in width and variable in length. The In the northeast, the transect traverses across 6 K. Korsman and T. Korja

Norwegian

Sea

Bothniai Bay r

NORWAY 200 km Bothnian, RNWAND Saa PHANEROZOIC

Plutonic meto / Oslo Graben Catadonldee Autochthonous cover PRECAMBRIAN ESTONIA Sveconorweglan Jotnian or Rlphean Subjotnian (rapaWvt) Svecofennian Orogen Karelian Province. Prolerozolc/Archaftan Lapland - Kola Orogen. Prolenuotc/Arctiaean

Fig. 1. Main geotectonic units in the Fennoscandian Shield. IVB - Imandra-Varzuga Belt, LGB - Lapland Gran- ulite Belt, PB - Pechenga Belt, SVB - Skellefteå Volcanic Belt, TESZ - Trans-European Suture Zone, and WR - Wiborg Rapakivi. The GGT/SVEKA transect is outlined by a polygon.

the western part of the Karelian Province, which and is rejuvenating gradually to the southwest, the includes e.g. the Archaean Kuhmo Greenstone youngest rocks being the 1.2 Ga old Subjotnian Belt and the Palaeoproterozoic Jormua Ophiolite diabase dykes as well as the minor occurrences of Complex, and crosses the boundary zone between Neoproterozoic sedimentary rocks. The transect is the Karelian Province and the Palaeoproterozoic therefore ideally situated for studying the Sve- Svecofennian Orogen. Further to the southwest, cofennian orogeny and its effects in the Archae- the transect traverses across the northern and cen- an Karelian crust as well as the later, mainly Sub- tral part of the Svecofennian Orogen including, jotnian, extension of the Svecofennian crust. from the northeast to the southwest, the Pyhäsal- The GGT/SVEKA program, following the ob- mi Primitive Island Arc, the Central Finland Con- jectives of the international GGT program, has tinental Arc, and the Southern Finland Sedimen- been aimed at constructing a tectono-evolutionary tary-Volcanic Complex. The transect ends in an model of the crust along a transect with special area of Mesoproterozoic rocks, which includes emphasis on the crustal thickness variations, the rapakivi granites, Subjotnian diabase dykes, and high metamorphic temperatures associated with Jotnian sandstones. The geological evolution start- the Svecofennian orogeny, the significance of deep ed 3.2 Ga ago in the eastern part of the transect crustal conductors, and the effects of the Sve- The Global Geoscience Transects Project in Finland 7

Jotnian sandstone Palaeoproterozoic Kainuu Schist Belt

I Subjotnian rapakivi granite Jonnua Ophiolite Complex IPelitic migmatite belt, granitic neosome Archaean Greenstone Belt Fig. 2. Geotectonic units in south- IPsammitic migmatite belt, trondhjeinitic neosome Archaean TTG crust ern and central Finland. The GGT/ i Central Finland Granitoid SVEKA transect is outlined by a I Complex I Palaeoproterozoic cover on Archaean I Pyhäsalmi Primitive I Boundary zone between Archaean contineni polygon. Island Arc I and Palaeoproterozoic Svecofennian Orogen

cofennian orogeny in the Archaean Karelian Prov- acterizing the Svecofennian crust, its evolution, ince (Fig. 3). Much attention has been given to and its geophysical and geological properties: understand the temporal and causal relationships 1 - Seismic data indicate remarkable variations between deformation, metamorphism, and magma- from 27 km to 65 km in the crustal thickness of tism because this is a necessary step for under- the Precambrian crust in Fennoscandia but most standing the evolution of a tectonically thickened of the variations in Moho depth can be explained crust metamorphosed under high-T/low-P condi- by variations in the thickness of the high-veloci- tions. ty lower crust which ranges from 0 to 30 km. The Svecofennides have a complex evolution 2 - The crustal thickness variations, which have that varies in space and time in detail. Many coe- a bimodal distribution in Fennoscandia, are mostly val, overlapping and continuous processes have compensated within the crust by density varia- resulted in a unique crust rich in geophysical and tions. Thinner crust (dominant crustal thickness in geological details (see e.g. Korsman et al. 1999). average 45 km) is found in regions that have ex- The major findings of the GGT/SVEKA work, perienced one or several anorogenic extensional based on geophysical and geological observations, events (e.g. the Archaean Karelian Province and can be summarized by the following features char- the Subjotnian rapakivi areas), whereas large parts 8 K. Korsman and T. Korja

12° 15" 18° 21° 24° 27° 30" 33° ,36° 39° ,42°

Fig. 3. (a) Crustal thickness and crustal conductors in Fennoscan- dia. Contours represent depths to M oho (km) obtained from refraction seismic studies. Moho depths are interpolated from 2D velocity models at sites shown as black dots. Dark brown areas indicate exposed parts of crustal conductors revealed by magnetometer and airborne electromagnetic data from the central and northeastern parts of the shield; no data are available from the southwestern part of the shield. The GGT/SVE- KA transect is outlined as a polygon. (b) Thickness of the lower- most high-velocity crustal layer. Contours represent thickness (km) of the 7.0 - 7.7 km/s layer. Thick- ness values are interpolated from 2D velocity models at sites shown as black dots. Dark brown areas show rapakivi intrusions. The GGT/SVEKA transect is outlined by a polygon. Original references to the data used to compile the maps and to other details can be found in Korsman et al. (1999). The Global Geoscience Transects Project in Finland 9 of the Svecofennian Orogen have notably greater that the current Moho boundary beneath the Ar- thickness (dominant thickness in average 55 km) chaean crust is likely Palaeoproterozoic in age in indicating that the crust does not always attain a both regions. "normal" thickness of ca. 40 km but may remain Original results of the GGT/SVEKA work have much thicker. Orogenic collapse, as a mechanism been published in separate articles and in special for producing normal (thinned) crust, was appar- volumes associated with the GGT/SVEKA Project ently inhibited in these areas and isostatic balance (Appendix 1). This Bulletin, published by the was achieved by density variations within the crust. Geological Society of Finland, is just one exam- 3 - Crust was thickened technically and by ple of the special volumes. Major scientific results magmatic under- and intraplating. Tectonic thick- of the GGT/SVEKA work has been published by ening involved both under- and overthrusting. The Korsman et al. (1999). Final maps and an explan- presence of the thick high-velocity lower crustal atory text including a complete list of references layer indicates magmatic under- and intraplating. to original work and data sources can be found in 4 - The thick Svecofennian crust has been pre- Korsman et al. (in print). served, because its density was increased by mag- The GGT/SVEKA Project was participated by matic intra- and underplating. The entire Sve- nearly all geoscientific organizations in Finland: cofennian crust equilibrated soon after magmatic Geological Survey of Finland, Espoo and Kuopio; underplating. Department of Geology at the University of Hel- 5 - Mafic high-velocity deep crustal layer was sinki, Institute of Seismology at the University of formed at several phases including the Palaeopro- Helsinki, Institute of Geology and Mineralogy at terozoic pre-Svecofennian extension of the Ar- the University of Turku, Geological Department chaean crust, the formation of the Svecofennian at Åbo Akademi, and the Institute of Geoscienc- island arc systems in subductions, the Svecofen- es at the University of Oulu. Many researchers nian post-collisional underplating caused by man- outside from Finland have helped us in the course tle upwelling and melting of the lower crust, and of the GGT/SVEKA work. the Subjotnian extension associated with the rapakivi magmatism. ACKNOWLEDGEMENTS. Besides the participat- 6 - High metamorphic temperatures in the Sve- ing organizations, the GGT/SVEKA Project has cofennian Orogen were caused by magmatic un- been supported by the Academy of Finland, Finn- derplating between 1.885 and 1.800 Ga. Extension ish Cultural Foundation, and Outokumpu Compa- started soon after subduction and thickening of the ny. Maps and the compilation of data and mod- crust. This extensional phase, however, did not els of the GGT/SVEKA Project were evaluated by thin the Svecofennian crust because a continuous Dr. Angela Jayko (Chairman of the international push from southwest in ongoing accretionary proc- GGT Project, US Geological Survey) and by pro- esses prohibited a true collapse of the crust. fessors Hans-Jorgen Goetze (former chairman of 7 - In general, the large-scale architecture of the GGT Project, Free University of Berlin) and the crust as imaged today have resulted from sev- Carl Ehlers (Åbo Akademi). eral tectonic processes that affected the whole crust. In the Svecofennides, the thickest crust was formed during the Palaeoproterozoic Svecofenni- REFERENCES an orogeny whereas the regions of thinner crust Goetze, H.-J. & Monger, J.W.H. 1992. Global Geoscience were formed during the Subjotnian extension. Transects Project: Achievements and Future Goals. 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