Tectonic evolution of the Svecofennian crust in southern Finland – a basis for characterizing bedrock technical properties Edited by Matti Pajunen Geological Survey of Finland, Special Paper 47, 15–160, 2008. TECTONIC EVOLUTION OF THE SVECOFENNIAN CRUST IN SOUTHERN FINLAND by Matti Pajunen*, Meri-Liisa Airo, Tuija Elminen, Irmeli Mänttäri, Reijo Niemelä, Markus Vaarma, Pekka Wasenius and Marit Wennerström Pajunen, M., Airo, M.-L., Elminen, T., Mänttäri, I., Niemelä, R., Vaarma, M., Wasenius, P. & Wennerström, M. 2008. Tectonic evolution of the Svecofennian crust in southern Finland. Geological Survey of Finland, Special Paper 47, 15–160 , 53 figures, 1 table and 9 appendices. The present-day constitution of the c. 1.9–1.8 Ga old Svecofennian crust in south- ern Finland shows the result of a complex collision of arc terranes against the Ar- chaean continent. The purpose of this study is to describe structural, metamorphic and magmatic succession in the southernmost part of the Svecofennian domain in Finland, in the Helsinki area. Correlations are also carried out with the Pori reference area in southwestern Finland. A new tectonic model on the evolution of the southern Svecofennian domain is presented. The interpretation is based on field analyses of structural successions and magmatic and metamorphic events, and on interpretation of the kinematics of the structures. The structural successions are linked to new TIMS and SIMS analyses performed on eight samples and to published age data. The Svecofennian structural succession in the study area is divided into deforma- tion phases DA-DI; the post-Svecofennian events are combined as a DP phase. Kinemat- ics in the progressively-deforming transtensional or transpressional belts complicates analysis of the overprinting relations due to their simultaneous contractional and exten- sional structures. In this work the Svecofennian structural succession in the study area is related to major geotectonic Events 1–5, E1-E5, on the basis of their characteristic kinematics and tectono-metamorphic and magmatic features. Geotectonic event E1 includes the DA deformation (corresponding to the D1 in the northern Svecofennian units) that deformed the oldest supracrustal sequences, includ- ing volcanic Series I. The horizontal DA structures formed during c. N-S shortening. Event 1 is suggested as an early major thrusting deformation at c. 1.9–1.88 Ga ago. The crust thickened and crustal slices from different depths, and geotectonic and depo- sitional environments, were juxtaposed. Geotectonic event E2 includes the DB deformation (D2 in the north). It is related to a crustal extension that caused a strong vertical shortening and prograde low-pressure (andalusite-sillimanite type) metamorphism. Extension is related to an island arc col- lapse; this collapse did not occur in the Archaean domain. The younger volcanic Series II evolved during the collapse and is proposed to represent intra-arc volcanism. Large amounts of syn-/late DB intermediate igneous rocks were also emplaced. E2 shows diachronic evolution, getting younger towards south; the age of E2 in the south is c. 1.88–1.87 Ga. The E2 extension was rapidly followed by a N-S shortening event, E3, that formed the contractional DC structures (D3 in the north) in the south at c. 1.87 Ga; E3 was also a diachronic event. According to the structure and age correlations, the northern Svecofen- nian units already underwent E3 contraction during the southern E2 extensional stage. This supposes that the crust as a whole was under a compressional regime and that E2 was a southwards-migrating localized event. In the Archaean continent and close to its border zone, the escape of crustal blocks towards the NNW-NNE occurred; movements oc- curred along the old Archaean or pre-collisional Proterozoic N-S-trending shear zones. After E3 the Svecofennian and Archaean domains formed a coherent continent and, therefore, the later geotectonic events occurred in this new continental crust. 15 Geological Survey of Finland, Special Paper 47 Matti Pajunen, Meri-Liisa Airo, Tuija Elminen, Irmeli Mänttäri, Reijo Niemelä, Markus Vaarma, Pekka Wasenius and Marit Wennerström Geotectonic event E4 deformed the new continental crust and is in the south pre- dominantly connected to the evolution of the Southern Finland Granitoid Zone (SFGZ). The E3 N-S stress field rotated into a SW-NE orientation. The pre-SFGZ crust in the south is characterized by the DC+D dome-and-basin interferences formed at c. 1.87 Ga due to the SW-NE transpressional deformation DD (D4 in the north). At that time, in the northern units, the wide-scale oblique contractional folding, eastwards transport and clockwise rotation of crustal blocks occurred. The deformation pattern of crustal blocks, like the Pomarkku and Vaasa blocks during D4 (DD in the south) and the Cen- tral Finland block during DF at c. 1.86–1.85 Ga, as well as related shear zone patterns, are interpreted as a product of escape tectonics related to the SW-NE transpression. Simultaneously with the termination of the D4 deformation in the north at 1868 ± 3 Ma, evolution of the Southern Finland Granitoid Zone (SFGZ) was initiated by the early DE tonalites in the south. The SFGZ shows a very complex pattern of DE, DF , DG and DH structures that formed in kinematics representing different local stress fields. Intense intermediate to felsic DE magmatism occurred in the oblique SE-extensional zones between the active N-trending deformation zones, the Baltic Sea-Bothnian Bay Zone (BBZ), Riga Bay Karelia Zone (RKZ) and the Transscandinavian Igneous Belt (TIB). In the erosion level, progressive extensional evolution led to the development of the pull-part basins with supracrustal sequences exemplified by the Jokela supracrustal association. The peak of DE extensional evolution is represented by gabbro intrusions at c. 1.84 Ga; this mafic and intense intermediate magmatism may be related to the volcanism in the pull-apart basins. The Porkkala-Mäntsälä shear/fault zone and the wide-scale FG folding are results of a short-lasting N-S shortening DG at c. 1.85 Ga. In southern Finland the late E4 evolution, from c. 1.84 to 1.80 Ga, is characterized by SE- NW transpression during DH. Sinistral movements in the N-trending shear zones, like the Vuosaari-Korso shear/fault zone, oblique dextral movements in the E-W-trending Hyvinkää and Southern Finland shear/fault zones, oblique contractional DH folds and the oblique extensional or spot-like granites and diabase dykes resulted. The localized DE SE-NW extension in the SFGZ turned to SE-NW transpression due to rotation of the zone under an overall SW-NE transpression between the major N-trending de- formation zones, the Baltic Sea-Bothnian Bay Zone (BBZ), Riga Bay-Karelia Zone (RKZ) and Transscandinavian Igneous Belt (TIB). The evolution of the major N-trending DI deformation zones, the BBZ, RKZ and TIB, already started during the DD deformation, but their latest phase, related here to the E5, deformed the Southern Finland Granitoid Zone (SFGZ) and the E-W-trending shear zones. The DG+H+I dome-and-basin interferences determining the present exposi- tion of the geotectonic and metamorphic units were formed. The youngest dated Sve- cofennian rock in the study area is a <1.8 Ga old amphibolite facies diabase dyke, which confines the fade of the Svecofennian orogeny in southernmost Finland. The post-Svecofennian deformations and magmatic events, included here as a combined DP , are strongly controlled by the older Svecofennian structures. In the study area the c. 1.65 Ga Bodom and Obbnäs rapakivi granite magmas were channelled by the intersections of the major Svecofennian crustal shear structures, like the southern Finland, Porkkala-Mäntsälä and the DI shear zones/faults, and the low-angle structures like the SE/H foliation planes controlled their emplacement. On a wider scale, rapakivis are located in close association with the major Svecofennian DI deformation zones such as the Baltic Sea-Bothnian Bay Zone (BBZ) and Riga Bay-Karelia Zone (RKZ). Tectonic and kinematic analysis combined with the tectonically-fixed age data on magmatic events establishes that the Svecofennian orogeny was a continuous dia- chronic event getting younger towards the south/southwest. The Svecofennian island arc system collided obliquely against the Archaean continent simultaneously with the growth of an island arc. The Svecofennian arc components were accreted to the Ar- chaean continent to form a new continental crust. The continent was deformed during the continued transpressional orogen into major crustal blocks that transported and rotated with simultaneously formed extensional zones – e.g. the pull-apart basins and granitoid zones – and strike slip terranes. The tectonic pattern is characteristic for ter- ranes formed by escape tectonics. In summary, the tectonic evolution of the Svecofennian orogeny is related to one oblique collision of the active, growing Svecofennian island arc system against the Archaean continent, and to subsequent transpressional continental deformation char- acterized by escape tectonics. 16 Geological Survey of Finland, Special Paper 47 Tectonic evolution of the Svecofennian crust in southern Finland Key words (GeoRef Thesaurus, AGI): crust, tectonics, structural geology, structural analysis, deformation, folds, shear zones, Svecofennian Domain, Proterozoic, Paleo- proterozoic, Uusimaa, Southern Finland * Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland * E-mail: [email protected] 17 Geological Survey of Finland, Special Paper 47 Matti Pajunen, Meri-Liisa Airo, Tuija Elminen, Irmeli Mänttäri, Reijo Niemelä, Markus Vaarma, Pekka Wasenius and Marit Wennerström INTRODUCTION At the very beginning of last century, J. J. Seder- tures in these regions. While ductile deformation holm (1907, 1923, 1926, compiled works in 1967), occurred in the south, the northern units behaved in the architect of modern geological research and a more rigid and brittle way (Pajunen et al. 2001b). mapping in Finland (e.g.