Laale1 of the Plate Teetonie Evolution of GEOLOGIAN TUTKIMUSKESKUS GEOLOGICAL SURVEY of FINLAND

TUTKIMUSRAPORTTI REPORT OF INVESTIGA TION 133

laale1 of the plate teetonie evolution of GEOLOGIAN TUTKIMUSKESKUS GEOLOGICAL SURVEY OF FINLAND

Tutkimusraportti 133 Report of Investigation 133

Tapio Ruotoistenmäki

A SCHEMA TIC MODEL OF THE PLA TE TECTONIC EVOLUTION OF FINNISH BEDROCK

Geologian tutkimuskeskus

Espoo 1996 Ruotoistenmäki, Tapio 1996. A schematic model of the plate tectonic evolution ofFinnish bedrock. Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey 0/ Finland, Report o/Investigation 133. 23 pages and 13 figures. The evolution of Finnish bedrock du ring the time interval from about 2.9 - 1.86 Ga is divided into three main stages: I) Kuhmoan (- Belomorian) orogeny (ca. 2.8 - 2.6 Ga), 2) Svecobaltian orogeny (ca. 2.2 - 2. 1 - 2.0 Ga) and 3) Kainuan-Svecofennian orogeny (ca. 1.97 - 1.86 - ... Ga). The beginning and closing stages of these orogenies are recorded by increases in intracratonic mafic magmatism that are attributed to increased thermal activity beneath the crust. Culminations in orogenie felsic magmatism and tectonic activity are evident as sharp statistical modal peaks in the age distribution of Fennoscandian detrital zircons. By combining the various episodic culminations in mafic and felsic activity it has been possible to propose aseries of plate tectonic models to explain the main evolutionary trends in the development of the Finnish bedrock in both time and space. The main emphasis of this paper has been the evolution of central and southern Finland. Therefore, the applicability ofthese models becomes less certain further to the north - northeast, where it seems that simultaneous but separate large scale processes were operating giving rise to the Belomorian and Kola orogens.

Key words: (GeoRef Thesaurus, AGI): crust, bedrock, evolution, plate tectonics, models, orogeny, igneous activity, Precambrian, Proterozoic, Finland

Tapio Ruotoistenmäki Geological Survey 0/ Finland Betonimiehenkuja 4 FIN-02150 ESPOO, FINLAND

ISBN 95 1-690-621-4 rSSN 0781-4240 Vammalan Kirjapaino Oy 1996 Ruotoistenmäki, Tapio 1996. A schematic model of the plate tectonic evolution of Finnish bedrock [Suomen kallioperän laattatektoninen kehitysmalli]. Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey 0/ Finland, Report o/lnves tigation 133. 23 sivua ja 13 kuvaa. Suomen kallioperän kehitys aikavälillä 2.9 - 1.86 Ga on jaettu kolmeen päävaiheeseen: 1) kuhmolainen (-belomorinen) orogenia (aikavälillä n. 2.8 - 2.6 Ga), 2) svekobalttilainen orogenia (n. 2.2. - 2.1 - 2.0 Ga) ja 3) kainuulainen-svekofennialainen orogenia (n. 1.97 - 1.86 - ... Ga). Näiden orogenioiden alku- ja loppuvaihetta kuvastaa mantereellisen emäksisen magmatismin lisääntyminen,joka voidaan yhdistää kuorenalaiseen lämpötilan kasvuun. Orogenian kulminaatio vaiheeseen liittyy tektoninen aktiivisuus ja hapan magmatismi,joka kuvastuu maksimeina sedimenttisten zirkoni en ikäjakaumassa. Happamen ja emäksisen magmatismin ikäjakaumista voidaan siten päätellä Suomen kallioperän kehityksen päävaiheet. Pääpaino tässä työssä on Etelä- ja Keski-Suomen kallioperän kehityksessä. Mallien sovellettavuus pohjoiseenja itään päin mentäessä on epävarmempaa, sillä niillä alueilla on ilmeisesti ollut samanaikaisia, mutta osittain erillisiä tektonisia prosesseja Kuolan ja belomoridien alueilla.

Asiasanat (Fingeo-sanasto,GTK): maankuori, kallioperä, kehitys, laattatektoniikka, mallit, orogenia, magmaattinen toiminta, prekambri, proterotsooinen, Suomi

Tapio Ruotoistenmäki Geologian tutkimuskeskus Betonimiehenkuja 4 02150 ESPOO

CONTENTS

Introduction ...... 7 Closing stage of the Archaean sea (ca. 2.97 Ga) ...... 9 Kuhmoan (-Be1omorian) orogeny: opening stage ofthe Karelian sea (ca. 2.8 Ga) ...... 9 'Svecobaltian' orogeny (ca. 2.2-... Ga) ...... 11 Svecofennian orogeny « 2 Ga) ...... 13 Conclusions ...... 20 Acknowledgements ...... 20 References ...... 21 Abbreviations:

AS: Archaean sea BMS: Belomorian sea BS: Baltian sea CG+SS: Central Finland granite island arc and Savo schist belt back-are basin DB: Diabase dykes EC: Eastern continent Gr: Granitoids GSF: Geological Survey of Finland H: Haveri seamount I: Iisalmi block IG: Greenstone volcanism 1+0: Jormua and Outokumpu ophiolites K+O: Kainuan-Outokumpu intracontinental sea KC: Karelian continent KG: Kuhmo greens tones KS: Karelian sea LBZ: Ladoga-Bothnian Bay fracture zone Lgb: Lapinlahti gabbro Mf: Mafic magmatism MORB: Mid-ocean ridge basalts NA: Näränkävaara-Auho fracture zone P: Pielavesi block Rgr: Rautavaara granulite SBC: Svecobaltian continent Sf: Svecofennian processes in Sweden and Estonia SfS: Svecofennian sea SG+TS: Southern Finland island arc and Vaasa-Tampere back arc basin SL: Karjalitic sills TH: Tervo-Haaparanta fracture zone To: Tonalitic magmatism Vgr: Varpaisjärvi granulite Vt: Vaasa overthrust

H ", I have just one more question .. , H (Columbo) Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey 0/ Finland, Report o/Investigatioll 133, 1996 A schematic model of the plate tectonic evolution of Finnish bedrock

INTRODUCTION

Since the pioneering paper of Anna Hietanen main basis for this work is in Fig. 1, which shows (1975) the plate tectonic theory has been widely the distributions of the ages of Fennoscandian invoked and applied in explaining the evolution of detrital zircons from central and southern Sweden the Proterozoic S vecofennian crust. Piirainen (1975) and Finland (Claesson et al., 1993), and zircon ages interpreted the Svecofennian metallogeny on basis of the principal rock components of the Belomo of plate tectonic theory. Gaal (1982, 1986) and rides (Bogdanova and Bibikova, 1993). The Fen GaaI and Gorbatschev (1987) applied the plate noscandian ages in the figure have been analyzed tectonic theory in explaining the evolution of late by SHRIMP ion microbrobe thus reducing the Archaean and Proterozoic crust in Finland and possibility of mixed ages. Because the sampIes are Russian Karelia. Ekdahl (1993) considered the from metagreywackes and quartzites from througout evolution of ore controlling Raahe-Ladoga zone central and southern parts of Finland and Sweden (Ladoga-Bothnian Bay zone, LBZ) in central Fin they can be taken as representative random sampies land using the plate tectonic theory. Lahtinen (1994) of felsic crustal blocks supplying material to Sve presents a comprehensive plate tectonic picture of cofennian sediments. Their age distribution is also the evolution of the Proterozoic bedrock of south wide, including all the known main periods of ern Finland. Korja (1995) examined the Svecofen evolution recorded in the Finnish bedrock with the nian crustal evolution using the large amount of exception of significant 2.1 - 2.0 age interval, deep seismic sounding data available from Fenno which indicates the existence of previously undoc scandian region. Park et al. (1984), Ward (1987) urnented, 'exotic' felsic crust close to the Fennos and Kohonen (1995) have considered the plate candian shield ca. 1.9 Ga ago (Huhma et al. , 1991 ; tectonic evolution in eastern Finland. Claesson et al. , 1993). The conventional multi In this paper I present a relatively simple series grain zircon ages given by Bogdanova and Bibi of plate tectonic models covering the evolution of kova (1993) represent the latest and, probably, the the Finnish bedrock from ca. 2.9 Ga to 1.86 Ga. The most reliable information of the evolution of Be-

90 25 80

70 20

10 I. 0 340033003200310030002900280027002600250024002300 2200 2100 2000 1900 1800

Age [Mal

_ Svecofennian c=::JBelomorian - . - Svecofennian - 0 - Belomorian

Fig. 1. V-Th-Pb isotopic age distribution for zircons from Svecofennian metasediments ana lyzed by SHRIMP ion microprobe by C laesson et al. ( 1993) and Belomorian intrusive ages (Bogdanova and Bibikova, 1993). Bars: frequencies, lines+circles: cumulative frequencies.

7 Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of In vestigation 133, 1996 Tapio Ruotoistenmäki

lomorian belt bordering the Archaean craton in the Fennoscandian orogenic processes appear to have east. Another important paper in this context is that of had both systematic and chaotic characteristics. Huhma (1986) which defined the location of the Although processes are considered here at a Archaean-Proterozoic boundary in Finland using Sm relatively local scale, i.e. from the viewpoint of the Nd, U-Pb and Pb-Pb isotopic analyses. Besides the Finnish region, one must keep in mind that in most above age data, the large number of multi-grain cases these processes have been related to global zircon ages produced by the Geological Survey of scale tectonics, even in Archaean time, as pointed Finland and those published in various papers have out by e.g. Gorbatchev and Bogdanova (1993) and been utilized in this work. Vuollo (1994). This is also apparent from the time The 2.1 - 2.0 age group mentioned above, which span represented by ocean opening and spreading. If can be assumed to represent ages of large scale the opening stage had las ted 40 Ma with a spreading crustal formation, can be correlated in time with the rate of ca. 10 cmly the ocean would accordingly have basic igneous events from Vuollo (1994) and Luuk been 4000 km wide. This also indicates that - espe konen (1992), corresponding to increased thermal cially iftransform margins develop - it is quite possi activity and rifting in continental areas. Thus, large ble that continental fragments that become separated scale orogenic processes and crust formation may in ocean opening are no longer juxtaposed after ocean be recorded in stable cratonic areas by rift-related closure. Thus, exotic terranes can be amalgamated in magmatism. these processes, as is for the evolution of the west In the following the driving forces ofthe orogen coast of North America (e.g. Howell, 1985; Gore ic processes are explained by the relative position and Sugar, 1985; Howell, 1995). of the (center of) mantle convection cells to the The models used in this paper are quite schemat continents; i.e. either convection currents and/or ic; for instance the various subduction zone proc continents can move. The relative movement ofthe esses, such as arcs, back-arc basins and magmatic center of convection currents is demonstrated e.g. processes have been represented in simplified form: in the present Pacific Ocean by the southeast drift as sedimentary wedges, subducting plates and mag of the East Pacific Rise as indicated by the vector ma 'bubbles'. Strike-slip movements associated to sum of movements of the hot spots in the maps oblique thrust directions have not been included in given by Grosvenor et al. (1995). In California all models. Moreover, special tectonic processes underthrusting of the spreading center under the (faulting, dykes etc.) related to certain tectonic North American continent closed down the sub events have not been repeated in later figures. The duction and initiated intracontinental transform scales of the figures are also arbitrary and the and extensional faulting as recorded by the San continental and oceanic blocks can be understood to Andreas fault zone (Grosvenor et al., 1995). consist of several sub-blocks which mayaiso have Compared to present day plate tectonic processes been separated and reunited during a single orogenie the heat flow from the Earth's mantle and core was cycle. The evolution of the sea is divided roughly in higher in Archaean and early Proterozoic eons, the two stages: 1) Opening stage due to rifting and sea convective flows were faster and the plates were floor spreading. 2) Closing stage initiated by sub generally sm aller and moving more rapidly (e.g. ducting of the cool seafloor continuing to collision Bickle, 1978 and Nisbet, 1987). Tomographic seis of continents after termination of rifting processes. mic modelling shows that down to a depth of ca. 300- General descriptions of Fennoscandian bedrock 400 km the convection currents even today are rela are available from e.g. Gaal and Gorbatschev (1987), tively vigorous, temperature anomalies are large and Gorbatchev and Bogdanova (1993) and also from convection is turbulent and may be chaotic (Montag many papers cited below. Therefore, the reader is ner, 1994). The chaotic ('interna!') behaviour of referred to those papers for further information. convection currents is reflected e.g. in the episodic This paper is a 'by-product' of my work in magmatic evolution of the Afro-Arabian rift system geophysics sub-groups in the international Global in the African plate described by e.g. Wilson (1993). Geoscience Transects (GGT; Korja and Korsman, However, the 'external' behaviour of convection 1993) and Mid-Norden projects (e.g. Ruotoisten currents is generally very systematic, as indicated mäki, 1993) intended as a ' working hypotheses' to by the linear continuity of mid-ocean ridges in be tested with constantly increasing geological and Atlantic and Pacific oceans. In the following it will geophysical information. be seen that the convection currents involved in the

8 Geologian tutkimuskeskus, Tutkimusraportti - Geological Su rvey 01 Finland, Report ollnvestigation 133, 1996 A schemati c model of the plate tectonic evolution of Finnish bedrock

Closing stage of the Archaean sea (ca. 2.97 Ga)

The earliest preserved indications of possible plate the 2.97 Ga orogeny may be correlated with the 3.1 - tectonic process in the Finnish bedrock date from the 2.9 Ga Saamian orogeny described by Gaal and late Archaean eon about 2.97 Ga ago when the pro Gorbatschev (1987 and references therein). posed 'Archaean sea' (AS; Fig. 2) between two Ar The 2.97 Ga Svecofennian detrital zircon age chaeancontinents, the 'Svecobaltiancontinent' (SBC, (Claesson et al. , 1993) marks the beginning of a ca. 3.4 Ga) and the 'Karelian continent' (KC, ca. 3.1 prolonged and apparently multi-phase 2.97 - 2.8 - Ga) was closing. The center of convection currents is 2.53 Ga Archaean age cluster, as is clearly shown assumed to have been under the 'mobile' Karelian in Fig. 1. This age cluster includes the 2.9 - 2.6 Ga continent, moving to the east (using present day Lopian orogeny described by Gaal and Gorbat coordinates), while the Svecobaltian continent was schev (1987 and references therein). The 3.1 Ga 'fixed' (i.e. there were no active convection currents ages represent the oldest crustal ages measured in below or behind it). The intracontinental greenstone Finland (e.g. Kröner et al. , 1981 , Paavola, 1986, vo1canism (IG in Fig. 2), particularly the Fe-rich GaalandGorbatschev, 1987, Vaasjokietal., 1993, tholeitiic amphibolites described by Luukkonen (1992) Vaasjoki et al. , 1994 and references therein). The could have been associated with tensional forces 3.3 - 3.4 Ga ages represent the oldest detrital zircon leading to intracontinental rifting due to convection ages of Claesson et al. (1993) in Fig. 1. According currents under the Karelian continent. If subduction to Vaasjoki (1989) the modelling of galena leads did take place, it was more probably beneath the implies the existence of ca. 3.6 Ga crust which, 'passive' , immobile Svecobaltian continent, rather however, has not yet been found in Fennoscandia. than against the direction of the convection currents In this model the 3.6 Ga ages have been associated below the Karelian continent. This closing stage of with the 'exotic' 3.4 Ga Svecobaltian continent.

SBC KC 3.6(?) - AS 3.1 Ga 3.4 Ga 2.97 Ga

Fig. 2. Closing stage of the Archaean sea (AS ; ca. 2.97 Ga). KC = Kareli an continent and SB C = Svecobaltian continent. IG = intracontinental rifting and greenstone vol cani sm.

Kuhmoan (-Belomorian) orogeny: opening stage of the Kar elian sea (ca. 2.8 Ga)

The age distribution in Fig. 1 indicates that the zircon ages from Belomorian intrusives obtained 2.97 - 2.53 Ga Archaean evolution was gradual and by Bogdanova and Bibikova (1993; Fig. 1), vary multistage. According to Luukkonen (1992) the ing between 2.83 - 2.75 - 2.35 Ga, indicate that the time interval 2.85 - 2.83 Ga was characterized by Belomorian magmatic activity during the opening large scale anatexis, migmatization and intracrus stage was minor (' Atlantic type'), main stage being tal differentiation. I attribute these processes to after the formation of the Kuhmo greenstones. thermal currents close to the edge of the Karelian Thus the interpretation of Gaal and Gorbatschev continent causing compression from the east and (1987) that Kuhmo greenstones represent a back the rifting and opening of the Belomorian sea arc rift related to the Belomorian geosynclinal (BMS, Fig. 3) between the Karelian continent and domain seems inconsistent with the age data. the 'Eastern continent' (EC), which was possibly The Kuhmo greenstone magmatism (KG in Fig. connected with the Svecobaltian continent (see e.g. 3), ca. 2.79 Ga aga apparently has some ophiolitic Fig. 4 in Gorbatchev and Bogdanova, 1993). The characteristics and has been described by Liipo et

9 Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey 0/ Finland, Report o/ In vestigation J 33, 1996 Tapio Ruotoistenmäki

SBC KC 3,6(?) - 3,4 - 2.97 - 3.1 Ga BMS EC 2.97 Ga

Fig. 3. Initial stage of Kuhmoan orogeny. Rifting of Kareli an conti nent and intrusion ofKuhmo Green stones (KG ; ca. 2.79 Ga). The Be lomorian sea (BMS) has begun to ...... ,.. . . -- , c1ose, with convergence against ...... '.. -----_ J the 'Eastern continent' (EC). al. (1994), Piirainen (1988) and Luukkonen (1992). subduction direction was against the passive conti In this model the magmatism has been attributed to nent, the Belomorian subduction was to the east increased thermal activity beneath Karelian conti beneath the Eastern continent (Fig 3). nent when the center of convection currents contin The thermal activity below Kuhmo greenstones ued moving westward and initiated intracontinen waned and the extension was replaced by compres tal rifting and greens tone volcanism. The green sional stress, manifested as ca. 2.74 - 2.7 Ga old stone magmatism initiated aperiod of active crus thrust faults in the Kuhmo area, as described by tal formation processes shown as increase in the Luukkonen (1992). These faults can be seen as number of the analyzed ages in Fig. 1. Below the regions of steep gravity gradients with increasing Belomorian sea thermal activity became weaker, gravity anomaly levels on the eastern side of the the spreading of the sea ceased and the closing Kuhmo belt (Elo, 1984, 1989). The anomalies can stage was initiated (Fig. 3). be attributed to depression of the crust along a zone The direction of the relative movement of the ofweakness existing along the suture ofthe Kuhmo center of the convection currents was towards the greens tones which resulted in more felsic upper Svecobaltian continent. It thus opened a new sea, crust being exposed in the west due a to lower level the Karelian sea (KS , Fig. 4), which probably of erosion. The downwarping can be connected to opened at the weakest point of the continents; i.e. thrust and pul! by the subducting sea floor under in the ca. 2.97 Ga suture. During the opening stage the Karelian continent (Fig. 4). Evidence for sub both continents were mobile and moving avay from duction is inferred from the calc-alkaline gran i the mid-ocean ridge. Thus, simultaneously with the toids described by Luukkonen (1992 and referenc opening of the Karelian sea, the eastward drift of es therein) accompanying the later stages ofthrust the Karelian continent was closing the Belomorian ing. The deep seisrnic sounding profiles across the sea, as indicated by an increase in number of the Kuhmo greenstone belt indicate that there is a gener analyzed ages in Fig. 1. Assuming again that the al tendency of thinning of the crust from west to east

KC 2.97 - 3. 1 Ga

Fig. 4. Opening and subduction stage of the Karelian sea (KS), wi th compression and thrust faults in Karelian continent (ca. 2.74 - 2.7 Ga). The framed part of the figure has been adopted from Luukkonen (1992). The nearly simultaneous clos ing and collision stages of the Belomorian orogeny have been omitted from this fi gure.

10 Geologian tu tkimuskeskus, Tutkimusraportti - Geological Survey 01 Finland, Report ollnvestigation 133, 1996 A schemati c model of the pl ate tectonic evolution of F innish bedrock

SBC KS 3,6(?) - 3.4 - 2.7 - KC 2.97 Ga 2.6Ga 2.97 - 3.1 Ga

Fig. 5. Closin g stage of the Karelian sea (ca. 2.67 - 2. 6 Ga). The thrust structures shown in Fig. 4 and structu res associated with the Belomorian orogeny are omitted from this and subsequent figures.

(Yliniemi, 1986, 1995, University ofOulu; personal estimate the culmination of the collision to have comm.) which is in agreement with the above model. started ca. 2.7 Ga aga (continuing toca. 2.6Ga?). We About 100 km E-SE of the Kuhmo greenstone thus have the formation of younger, ca. 2.7 - 2.6 Ga belt the nearly coeval, ca. 2.75-2.72 Ga Ilomantsi crustal material between the 2.97 - 3.1 - 3.4 - 3. 6(?) Kostamuksha felsic volcanism and related grani Ga continents. According to Ward (1995; Geol. toid intrusions have calc-alkaline affinities (Sorjo Surv. of Finland, spoken comm.) the prolonged nen-Ward,1993 andO'Brienetal., 1993) andcould heating could also be attributed to late orogenie also be interpreted as forming during tension due to extensional processes similar to those following the subducting slab, and subduction related H20- Svecofennian orogeny in the Uusimaa area ca. 1.84- rich magmatism. Nurmi (1993) has considered the 1.83 - Ga aga (see considerations later). Such proc genesis of the 2.74 - 2.7 Ga Hattu gold mineraliza esses can explain the (few) ages in the relative long tion in the Ilomantsi-Kostamuksha belt and con time span from 2.7 to ca 2.4 Ga in Fig. 1. cluded that they are related to hydrothermal proc According to Vaasjoki et al. (1994) many ofthe esses due to non-meteoric, slightly saline fluids. A Archaean gneis ses formed about 2.84 Ga ago, but source for such fluids could be saline sea water majority are ca. 2.75 - 2.67 Ga old; i. e. younger released from the subducting slab. than the Kuhmo greens tones, with ages ranging on When the center of the convection currents had both sides ofthe culmination of the collision in Fig. moved below (or behind) the Svecobaltian conti 1, which is in broad agreement with the above nent the movement ofthe Karelian continent ceased model. Bogdanova and Bibikova (1993 and refer and the Svecobaltian continent reversed its direc ences therein) report thorough structural rework tion of movement, resulting in closing of the Kare ing and thrust stacking on the Belomorian margin lian sea (Fig. 5). The 2.97 Ga plate beneath the ca. 2.65 - 2.6 Ga ago, which thus represent the latest Svecobaltian continent had started to erode due to stages of the Kuhmoan orogeny, when the Sveco increasing thermal flow. In the Ilomantsi-Kosta baltian-Karelian continent finally closed the Belo muksha belt Nurmi (1993) describes 2.7 - 2.68 Ga morian sea. The time span of the Kuhmoan-Belo old post-mineralization deformation and reheating morian orogeny agrees with that of the Lopian which he connects with possible intracontinental orogeny described by Gaal and Gorbatschev (1987). or continent-continent type collision. This is in However, their evolutionary model does not agree agreement with the ages in Fig. 1 where we can with that described above.

'Svecobaltian' orogeny (ca. 2.2-... Ga)

After the Kuhmoan orogeny the Karelian crust trusions, e.g. Näränkävaara (Alapieti, 1982), 2435 appears to have been relatively stable for a very Ma old Koitelainen (Pihlaja et al. 1986), and corre long time, as is evident from Fig. 1. About 150 Ma sponding mafie dyke swarms (Vuollo, 1994). Lay later, ca. 2.44 Ga ago a globai period of enhanced ered mafie massifs of similar age are also present in thermal activity produced large amounts of mafic the Kola Peninsula area (Gorbunov et al. , 1985). magmatism, recorded today by layered gabbro in- Dykes of same age group also oecur in Seotland,

11 Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Report of In vestigation J 33, 1996 Tapio Ruotoistenmäki

SBC KC 3.6(?) - 2.6 Ga BS 2.6 - 3.1 Ga

Fig. 6a. Opening stage of the Baltian sea (BS), intrusion of Karjalitic sills (SL), initiation of subduction (ca. 2.2 - 2.15 Ga). ca.2.15 Ga ca. 2.2 Ga

Fig. 6b. Plan view ofthe break up of th e Svecobaltian - Kare Fig. 6c. Plan li an continent (S BC and KC). view of th e Intrusion ofKarjalitic sill s (SL). ·I SBCI opening of The thick line refers to the boun the Baltian dary between convection cells. sea (BS).

Canada, Antarctica, Africa, Australia and possibly in tion existed, it was probably in the direction of the Greenland (Vuollo,1994 and references therein). Svecobaltian continent. At this stage, both continents Apparently no significant amounts of crustal addi can be considered as mobile (Fig. 6a and 6c). tions were generated in the Fennoscandian area (Fig. At about 2.1 Ga ago the evolution of the Sveco 1). However, Bogdanova and Bibikova (1993) have baltian orogeny culminated shown as increase in analyzed 2.43 - 2.4 Ga mafic and feTsic plutonics in age distribution in Fig. 1. The boundary between the Belomorides (Fig. 1). They also report sirnilarly convection cells shifted towards the Karelian con aged rocks and structures along the boundary be tinent (Fig. 7 a and 7b) resulting in the closure of the tween the Belomorian belt and Karelian domain which Baltian sea, the formation of 2.1 - 2.0 Ga felsic thus might indicate an orogeny of that age to the east crust and the intrusion of numerous Fe-tholeitic of present Karelian domain. From Fig. 1 we can see dykes within the Karelian continent (DB in Fig. 7a that the Belomorian age distribution has also peaks at and 7b; see Vuollo, 1994) due to increased heat 2.5 Ga and at 2.4 - 2.45 Ga. Bogdanova (1994) flow and an overall tensional stress regime. The connects the younger age group with extension of strong he at flow beneath the Karelian continent thickened crust due to collision at ca. 2.5 Ga. Howev mayaiso have caused thermal erosion beneath the er, these ages could also be attributed to late/post continent by remelting the lower parts of the crust orogenie extensional processes as suggested above. and thus obliterating the traces of earlier subduc In the following the orogenie processes are shown tion events. The increasing amount of detrital zir as ' block models' and as ' plan view' series. con ages of 2.1 - 2.0 Ga when progressing south Not until ca. 2.2 Ga aga convection cells again wards from Tampere to Orijärvi (Claesson et al. , exist beneath the Archaean Svecobaltian-Karelian 1993, Lahtinen, 1994) indicates that to the south of continent, causing it to rift along the 2.7 - 2.6 Ga present day Fennoscandia the Baltian sea was still boundary. This period ofthermal activity is seen as open (and perhaps did not totally close until the 2.2 Ga old Karjalitic sills described by Vuollo Svecofennian orogeny; see Fig. 7c). (1994 and references therein) (SL in Fig. 6a and 6b). Karhu (1993) reports highly 813C enriched 2.2- They formed in the dip-slip faults on the collapsing 2.1 Ga sedimentary carbonates from the Karelian edges of the evolving Baltian sea (BS). If subduc- craton and craton margin sequences referring to

12 Geologian tutkimuskeskus. Tutkimusraportti - Geological Survey 0/ Finland. Reporl o/In vesligation 133. 1996 A schematic model of the plate tectonic evolution of Finnish bedrock

SBC BS KC 3.6(?) - 2.6 Ga 2.1- 2.6 - 3.1 Ga 2.0 Ga

Fig. 7a. Closing stage of the Baltian sea. intrusion of Fe-tholeitic dykes (DB . ca. 2.1 Ga) in the Karelian continent.

7b) ca.2.1 Ga 7c) ca. 2.05 Ga

Fig. 7b-7c. Plan view of the closing stage of the Baltian sea. Intrusion of Fe-tholeitic dykes (DB). Subduction under Svecobaltian continent. Con vection cell s move beneath and be hind the Karelian continent. increased burial rates of organic carbon, which Shield (Claesson et al., 1993, and references there might be connected with breakup of the Archaean in). The nearest rocks of that age to southern Fin continent and opening of the Svecobaltian sea. land could be buried below southern Baltian sedi The 2.1 - 2.0 Ga collision period thus accreted mentary rocks in southern Estonia. Both age groups sediments + subduction stage intrusives to the are also represented in Svecofennian detrital zircon Svecobaltian continent and Karelian continent re populations (Huhma et al., 1991; Claesson et al., sulting in a Paleoproterozoic continent whose 3.4 1993; Fig. 1) and in the Kola Peninsula rocks (- 3.6) Ga remnants are today visible in Ukrainan (Gorbunov et al., 1985 and references therein).

Svecofennian orogeny « 2 Ga)

About 2 Ga aga a new system of convection cells Fig. 8b) characterized at present by large number evolved (ar moved) below the Karelian continent of fractures and both, vertical and horizontal block opening the Kainuan-Outokumpu intracontinental movements (e.g. Kantinen, 1987 and Paavola, sea (K+0 in Fig. 8a and 8b). The opening started from 1991). south, in the Outokumpu area about 1.97 Ga aga Vuollo (1994) and Lahtinen (1994) suggest an (Koistinen, 1981), initiated intrusion of tholeitic dia island are or back are basin origin for the Outo base dykes (1.965 Ga; Vuollo et al. , 1992) and kumpu ophiolites. However, there is no major fel propagated to north to J ormua (1.96 Ga; Kantinen, sie magmatism wh ich could be linked with subduc 1987, Lahtinen, 1994). A similar type of model has tion at ca. 1.97 Ga aga, which is the age of Outo been presented by Park et al. (1984) and Kohonen kumpu ophiolites, as pointed out by Lahtinen (1995). The new sea cut offthe westernmost part of (1994). Moreover, Wang and Glower (1992) have the Karelian continent creating Iisalmi block (I in demonstrated that even relatively young (Juras sie)

13 Geologian lUlkimuskeskus, TUlkimusraporlli - Geological Survey of Finland, Report of Investiga tion 133, 1996 Tapio Ruotoistenmäki

SBC BS KC 3.6(?) - 2.6 Ga 2.1- 2.6 - 3.1 Ga 2.0 Ga

Fig. 8a. Opening of lhe Kainuan-Outokumpu sea and intrusion of tholeitic dykes (K+O, ca. 1.97 Ga).

ca. 2.0 - 1.96 Ga

Fig. 8b. Divergent convection cell s evolve or move beneath the Karelian continent. Intra continental rifting and opening of the Kai nu an-Outokumpu sea (K+O). Separation of Ii salmi block (I). Formation of the Haveri sea mount (H). continental rift basalts very often plot erratically in volcanics within the formation are characterized by MO RB and are basalt fields in tectonomagmatic MORB affinites in the lower sections and are mag geochemical discriminant diagrams. Thus, it is ma features in the upper parts. The Pb isotope quite possible that the geochemical signature ofthe whole rock age of ca. 1990±25 Ma for the lower much older Jormua and Outokumpu ophiolites could part ofthe formation (Vaasjoki et al. , 1987) is incon be misleading and they were connected with intra sistent with the other ages in the Tampere region, but continental oceanic rifting and obducted during does correlate with the Outokumpu-Jormua ages. later Svecofennian compression, as suggested by When comparing the 2.1 Ga and l.97 Ga ten Park et al. (1984) and as pointed out by Kontinen sional events in the Karelian continent it can be (1987), Ward (1987) and Ward et al. (1989). The seen that the former is characterized by a greater Peräpohja-Kuusamo intracontinental sea and the number of (known) dykes while the latter is char North-Lapland 'granulite basin' possibly formed acterized by few dykes and rapid opening of an simultaneously with the Kainuan-Outokumpu sea. intracontinental sea. This indicates that the 2.1 Ga During the opening stage of the Kainuan-Outo event was distinguished by weaker horizontal ten kumpu sea the proposed Haveri seamount (H in sion due to thermal uplift of crust causing vertical Fig. 8b) also formed on the edge of the mid-ocean (and horizontal strike slip-?) block movements ridge in the reopening Baltian sea. The volcanic (Fig. 9a) as described e.g. by Ward (1987). A sedimentary Haveri Au-Cu deposit has been de similar process along the west coast ofNorth Amer scribed by Kähkönen and Nironen (1994). The ica has been described by Wilson (1988). It is also

14 Geologian tutkimuskeskus, Tutkimusraporui - Geological Survey 0/ Finland, Report o/ lnvestigation 133, 1996 A schematic model of the plate tectonic evolution of Finnish bedrock

a Kuhmoan sea considered above, as suggested by Liipo et al. (1994). After the Svecofennian sea floor had cooled sufficiently it started subducting under the Karelian continent about 1.93 - 1.92 Ga ago. In Fig. 1 this stage is shown by a sharp increase in the number of zircon ages. The earliest felsic magmas connected with this stage are minor but spatially widely distrib uted 1930-1910 Ma granitoids (Gr in Fig. 10c) described by Vaasjoki and Sakko (1988) and Lahti nen (1994). The main stage of subduction generat ed an island arc in the area now occupied by the central Finland granite complex and the associated Savo schist belt back-arc basin (CG+SS in Fig. IOa and IOd). In the Pyhäjärvi area the easternmost prim itive Savo schist belt volcanics described by Kousa et al. (1994) can be connected with the opening of the back-arc basin. The more evolved western se quence volcanics in the Pyhäjärvi area have a more mature arc affinity and are related to later 1890 - 1865 processes (Kousa et al., 1994). In the central Finland granite area the ca. 1897 Ma old Haukkamaa volca nogenic belt has arc affinities, with andesites being Fig. 9. Stretching of the crust due to underl yin g convection possibly in part subaerial and resembling those of currents. In (a) the stress is mainly vertical causing uplift of crust, vertical block movements, differential horizontal tension the evolved arcs (Tiainen et al., 1994). and intrusion of diabase dykes. In (b) the main tension is Somewhat after the formation of the central horizontal causing rapid rifting and opening of intraconti Finland granite island arc and Savo schist belt nental sea. back-arc basin, a new southern Finland island arc and Vaasa-Tampere back arc basin (SG+TS) possible that the convection cells migrated to the evolved on their southwest side (Fig. 10a and IOd), east (or weakened) before breaking the Karelian resulting in a double-arc system analogous to that continent completely. However, during the 1.97- suggested by Gaal (1986) and Ward (1987). The 1.96 Ga (and also 2.2 Ga) event, horizontal tension occurrence of 2.1 - 2.0 Ga old zircons in the Tam was dominant and much stronger, causing rapid pere sediments indicates that the Svecofennian sea breakup of the Karelian continent (Fig. 9b). was c10sing with the mid-ocean ridge vanishing The convection cells continued to migrate to beneath the Svecobaltian continent and the 2.1 - 2.0 wards the Svecobaltian continent and rifted the Ga edge of the Svecobaltian continent being rela Paleoproterozoic continent along the contact zone tively c10se to the Tampere area, as suggested by between 2.1 (- 2.0?) Ga Svecobaltian and 2.6 - 2.97 Huhma et al (1991). The amalgamation of the Ga Karelian continents (Fig. IOa and lOb) leaving Haveri seamount with Tampere arc and the initia only minor amounts of ca. 2.1 - 2.0 Ga rocks on the tion of its arc type magmatism described by Karelian side (Huhma, 1994; Geol. Surv. of Fin Kähkönen and Nironen (1994) can be associated land, spoken comm.). The maximum age for this with this stage (Fig. IOd). opening stage in this model is less than 1.96 Ga In the Evijärvi area, which forms part of the (age of Jormua ophiolites; youngest mafites in Vaasa arc, the ca. 1.88 - 1.9 Ga volcanites are Kainuan-Outokumpu sea). At about 1.94 - 1.92 Ga, interpreted by Vaarma et al. (1994) as having been during the opening stage of the Svecofennian sea erupted in a marginal basin environment. Kähkönen the tensional stress in the Karelian continent was (1994) interprets the mainly subaqueuous shosho replaced by compression from W-SW, initiating nitic volcanites of Tampere area to have been obduction of the Jormua and Outokumpu ophio extruded in evolved arc environment. However, lites (1+0 in Fig. 10a and lOb; e.g. Koistinen, 1981 Wilson (1993) states that shoshonites frequently and Kontinen, 1987). The obduction process was occur during or after continental collision follow probably due to the fact that the sea floor of the ing ocean basin c1osure. Moreover, the turbiditic Kainuan-Outokumpu sea was still relati vely hot metagraywackes-mudstones in the area are more ('Red Sea type') and thus not easily subducted. A typical of a basin type environment (Kähkönen and similar process has possibly happened also in the Leveinen, 1994). The evolved characteristics of

15 Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey 0/ Finland, Report o/Investigation 133, 1996 Tapio Ruotoistenmäki

SBC 2) ~G KC 3.6(?) - 3.4 - 2,97 - StS SG StS + 2.0(1) - 2.6 - 2.97 - 2.6(?) - 2.0 Ga + SS 3.1 Ga ~TS~=~~ ~~~~~~~~

Fig. 10a. Opening of the Sveeofennian sea (SfS), subduetion, evolution of eentral Finland island are + Savo back are basin (CG+SS) and of southern Finland island are + Vaasa-Tampere back are basin (SG+ TS). Closing stage ofthe Kainuan sea and obduetion oftheJormua and Outokumpu ophiolites (J+O; ca. 1.97 - .. . Ga). The grey 'double arrows' represent the southern eomponent of the eompression.

<1.96 Ga ca. 1.96 - 1.93 - .. . Ga

Fig. lOb. Plan view of rifting and opening Fig. 10e. Plan view of the 'mature stage' of the of the Sveeofennian sea (SfS) at the 2. 1 - Sveeofennian sea. Intrusion of the 1.93 - 1.91 Ga 2.0 Ga suture. Closing of the Kainuan granitoids (GR) (subduetion related?). Outokumpu sea and obduetion of the Jor mua-Outokumpu ophiolites (1+0).

ca, 1.9 Ga

Fig. IOd. Plan view of the evolution of the Sveeo fennian island are systems. To the north is the eentral Finland granite island are and Savo sehist bel! back-are basin (CG+SS), while to the south is the southern Finland island are and Vaasa-Tampere back are basin (SG+TS). Intrusion of the Suonenjo ki-Pyhäsalmi-Vihanti-Hailuoto mafie eomplexes (MF).

16 Geologian tutkimuskesku s, Tutkimusraportti - Geological Survey 0/ Finland, Report o/ lnvestigation J 33, 1996 A schematic model of the plate tectonic evolution of Finnish bedrock the sediments described by Kähkönen and Levei gabro belonging to Vihanti area. However, it re nen (1994) can be attributed to the more evolved mains uncertain, if this age is representative for the sources of the southern Finland island arc. This is age of the main peak of mafic magmatism in the area. also supported by the observations by Hakkarainen These gravity maxima appear to control most ofthe (1994) who examined the Hämeenlinna-Somero ore showings in the area, raising the possibility that volcanics in the southern Finland island arc area, a the large amount of mafic magma in the upper crust region, which is also characterized by numerous has provided the heat required for forming ore Svecofennian mafic and felsic intrusives. He con bodies from earlier (ca. 1920 Ma old; Kousa et al. , c1uded that they represent partly subaerial mature 1994) mineralizations generated originally by hy arc volcanics, which have been extensively eroded, drothermal volcanic-exhalative activity. producing large amounts of reworked material. The main c10sing stage of the Svecofennian Subduction related magmatism within the interi orogeny at 1.9 - 1.86 Ga is shown in Fig. lla and or of the Karelian continent has not been identified 11 b. The central Finland island arc + Savo back arc (Kontinen, 1987, Ward, 1987). However, the ca. basin sediments were thrusted against the Karelian 1908± 16 Ma granodiorites described by Paavola continent. The southern Finland island arc + Vaa (1988) could be connected with magmatism acti sa-Tampere back arc basin sediments then collided vated by volatiles from the subducting slab and the against these. The Haveri volcanics were deformed 1895±15 Ma Lapinlahti gabbro (Paavola, 1988) and emplaced between the central Finland granites with tensional fracturing above the slab. The rela and Tampere back arc (Fig. llb). tively small amount of subduction related magma During the main stage of the collision the 'more tism in the continent area can be explained in two plastic' Tampere and Savo back-arc basins acted as ways: 1) The magmatism was concentrated mainly 'buffer zones' by thickening rapidly and absorbing in the arc and back-arc basin (CG+SS), as in Japan collision energy. At present these zones are character today. 2) The central Finland granite arc and Savo ized by strongly folded, ca. 1885 Ma old tonalitic schist belt basin sediments are 'exotic'; i.e. they migmatite zones (To in Fig. 11 b; e.g. Korja et al., have been emplaced into their present position 1994 and references therein). Both areas are also along the Ladoga-Bothnian Bay Zone as suggested distinquished by exceptionally thick crust (ca. 55-60 by Park et al. (1984) and Ward (1987). Possibly km; Grad and Luosto, 1987. Yliniemi, 1995, Uni both alternatives have been involved in the proc versity of Oulu; personal comm., Heikkinen et al. , esses as will be discussed later. 1995). In the Pielavesi area the ca. 1890 Magranulite In Savo the culmination of the extensional evo metamorphism culminated at apressure of about 5.5 lution of the back-arc basin was associated with a kb and at temperatures of 800-880° (Hölttä, 1988). large amount of mafic magmatism (Mf in Fig. IOd), In the Tampere area the direction of the 1885 Ma shown today as regional gravity maxima from Suo compression was from south (Kilpeläinen et al. , nenjoki to Pyhäsalmi, Vihanti and Hailuoto (Elo et 1994), which indicates that the originally SW al. , 1978; Ruotoistenmäki, 1993). Vaasjoki and direction of Svecofennian thrusting had rotated Sakko (1988) report an age 1901±12 from Alpua antic10ckwise to south (the grey 'double arrows' in

SBC KC 3,6(?) - 3,4 - 2.97 - SG CG 2.0(?) - 2.6 - 2.97 - 2.6(?) - 2.0 Ga TS SS 3.1 Ga

Fig. lla. Culmination of the Svecofennian orogeny: closing of the Svecofennian sea and collision (1.9 - 1.86 -". Ga).

17 Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey 01 Finland, Report ollnvestigatiol1 133, 1996 Tapio Ruotoistenmäki ca. 1.885 Mo

Fig. Ilb. Plan view of lhe collision stage of lhe (CG+SS) and (SG+TS) island arc systems with Haveri volcanics in between. Overthrusting ofthe Vaasa block (Vt) and upthrusting ofthe Varpaisjärvi and Rautavaara granulites (Vgr and Rgr). Intrusion of tonalites (To). Evolution of the Ladoga-Bothnian Bay zone (LBZ) dextral faulting and its conjugate Näränkävaara-Auho faults (NA). Tervo-Haaparanta fracture zone (TH) represents the NE branch of LBZ. Later Svecofennian processes in Sweden and Estonia are denoted by (Sf).

Fig. 12. Collision of the Pielavesi block (P) and lisalmi block (I). To = tonalitic magmatism, Vgr = Varpaisjärvi granulites, Rgr = Rautavaara granulites Lgb = Lapinlahti gabbro and E = erosion level.

18 Geologian tutkimuskesku s, Tutkimusraportti - Geological Survey oi Finland, Report oi In vestigation 133, 1996 A schemati c model of the plate tectonic evolution of Finnish bedrock

Fig. 10a and lla). The last remains of these sub to be related to Proterozoic thrusting. The hydra duction-collision processes are still visible as thick tion ofthe mylonites described by Paavola (1991 b) ened crust and as possible remains of ancient sub can be connected with waters released from the ducting plates under Bothnian Bay in two Babel subducted slab and the underthrusted sedimentary deep seismic reflection profiles (Babel Working wedge (Hölttä, GSF, 1995; spoken comm.). The Group, 1993 and references therein). mixed age tonalitic dykes (1880 - 1924 Ma) de The nearly simultaneous SW-S thrusts 'rolled' scribed by Paavola (1988) may correspond to the the Vaasa-Tampere arc around the central Finland 1885 Ma tonalitic collision related magmatism. arc and thrusted the Vaasa area over the central This is also in agreement with Lahtinen (1994), who Finland granite (Vt in Fig. 11 b) as suggested by the relates them with magmatic underplating in the mar gently dipping structures described by Nykänen ginal part of the Archaean craton. Korsman et al. (1960) and Petri Virransalo (1995, GSF; spoken (1984) reported that in the Pielavesi area the erosion comm.). level decreases from west to east along the boundary The rotation of the convergence direction from between Archaean and Proterozoic blocks. This is W -SW to south resulted in oblique collision, form also in agreement with the geometry in Fig. 12. ing the large dextral Ladoga-Bothnian Bay shear The collision and underthrusting caused thick system described by e.g. Ekdahl (1993) and during ening of the crust that is still evident in the deep later stages their conjugate sinistral Näränkävaara• seismic Sveka refraction profile crossing the area Auho fracture zone (LBZ and NA in Fig. Ilb). (Grad and Luosto, 1987. Yliniemi, 1995, Universi Figure 12 depicts in more detail the collision ty of Oulu; personal comm.). The granulite facies event between the Svecofennian Pielavesi block rocks are also visible as high velocity layers in the (P) in the Savo schist belt and the Karelian-Archae upper crust. The crust has remained thick after an Iisalmi block (I). During the oblique collision the erosion because the upper crust granulites and Pielavesi back-arc basin thrusted beneath the Iisalmi mafic rocks in the Pielavesi and Iisalmi blocks have block, causing reverse and dextral strike slip faulting increased the average density. Moreover, in the Sve in the Iisalmi block. The uplifted Archaean blocks ka profile, the P-wave velocities of the lower crust and exhumed Varpaisjärvi granulite facies blocks and upper mantle appear to decrease to the NE beneath the Jatulian (2.3 - 2.1 Ga) diabases cutting them (Vgr in Archaean crust representing the mixing of more felsic Fig. 12 and llb), as described by Paavola (1991b). material with lower parts of the Archaean craton. A The geometry of Fig. 12 also explains the faster similar, exceptionally thick crust (ca. 55-60 km) has cooling of the Rautavaara rocks (Rgr) situated east of also been observed in the Fennia deep seisrnic refrac the Varpaisjärvi granulites (Hölttä, 1995; Geol. Surv. tion profile beneath the southern Finland island arc + ofFinland, spoken comm.). The Varpaisjärvi granu Tampere back arc basin areas, as mentioned above. lites were at deeper crustal levels and they were In this area too the upper crust appears to be reheated with proterozoic magmatism which is not exceptionally dense (Ruotoistenmäki, 1992). met in the Rautavaara block. In Northern Finland the compression caused Thrusting possibly took place along reactivated thrusting in the Peräpohja-Kuusamo sea and cen extensional dip-slip faults generated during the open tral Lapland greenstone basin (regional synclines), ing stage of the Svecofennian sea. The above model resulting in intensive folding and uplift of central is also supported by the fact that no isotopic indica Lapland granite, which can be considered as a tions of Archaean rocks have been found on the regional anticline between Peräpohja-Kuusamo and western side of the Tervo Haaparanta fault zone (TH central Lapland synclines. The passive northern in Fig. 11 b), which defines the collisional suture most margin of Svecofennian thrusting was possi between the western Proterozoic Pielavesi-Vihanti bly behind the North-Lapland granulites which Hailuoto block and eastern Archaean Iisalmi block may have been thrusted southwards during the (Vaasjoki, 1995; GSF, spoken comm.). Therefore, it Svecofennian collision (e.g. Ruotoistenmäki, 1977). seems improbable that there is an Archaean wedge Svecofennian compression was followed by ex beneath the Pielavesi block, or that the Svecofennian tension in southern Finland about 1.84 - 1.83 Ga had been overthrusted significantly to the east. ago. This final stage of the Svecofennian orogeny According to Paavola (1991 b) the number of J atu is manifested as ca. 100 km wide and 500 km long lian diabase dykes in the Varpaisjärvi area increases potassium granite belt described by Ehlers et al. at deeper erosion levels. This also suggests that (1993). The granite magmatism is apparently due to diabases have been uplifted with the granulites rapid pressure release caused by extensional collapse constraining the maximum age of the uplifting as and crustal thinning along listric faults, as suggest J atulian. Paavola (1991 b) also considers the very ed by Väisänen et al. (1994) and Korja (1995). intensively tectonized blastomylonites in the area The models starting the Svecofennian evolution

19 Geologian tutkimu skesku s, Tutkimusraportti - Geologica/ Survey 0/ Finland, Report o/ ln vestiga tion 133, / 996 Tapio Ruotoistenmäki

100 ing 150 Ma are very rare (Howell, 1995; Fig 13). However, there still exists a problem with the 80 ' symmetrie' , relatively short-lived Svecofennian sea: if the sea had been open for say 80 Ma, with a cft. half spreading rate of ca. 5 cm/a, the Svecofennian 60 Q) .ro2: sea would have been ca. 4000 km wide at its 'S E maximum. If we assume that the Svecofennian 40 ü'" orogeny has created a zone of new crust roughly 400 km wide and 50 km thick, this would require 20 that the average thickness of the (mainly) sedimen tary cover of the Svecofennian sea would have been much more than 5 km; i.e. the sedimentation 180 120 60 o rate would have been over 125m/Ma (closer to Age (Ma) 200m/Ma). This is a relatively high rate (see e.g. Fi g. 13. Age di stribution of oceanic cru st at the Earth 's Heezen and MacGregor, 1973), which could ne ver surface. Redrawn from Howell ( 1995). theless be realistic given the probably high topo graphie relief after the 2.1 - 2.0 Ga collision. Thus, a more plausible explanation is the one from rifting and opening of new sea ca. 2.1 - 2.05 Ga suggested by Park et al. (1984) and Ward (1987) ago(e.g.Parketal., 1984,Gaal, 1986, Lahtinen, 1994 that Finnish S vecofennian area represents a collision and Kohonen, 1995) are problematic because of the zone where more or less exotic island arc systems strong asymmetry in ages. In those models the open have accumulated, as demonstrated in figures IOd and ing stage of the Svecofennian sea would have lasted llb. This is supported by the nonlinearity of Sve at least 150 Ma while the closing stage is generally cofennian 'belts', especially in central Finland and accepted to have lasted ca. 40 - 50 Ma. However, if the also by the existence of apparently two thrusting Svecofennian opening started about 1.96 Ga aga as in directions 1) W -SW and 2) S along the Ladoga the model described above, the opening and closing Bothnian Bay zone (e.g. Kilpeläinen et al. , 1994). periods are relatively symmetrie. Moreover, these During the latest stages of Svecofennian evolu models fail to explain the large number of 2.1 - 2.0 tion the bedrock of southern Finland was still ages in Fig. 1 which apparently refer to orogeny very modified by late- and post-tectonic processes, such close to Karelian continent and thus to the closing as the intrusion of potassium granites mentioned stage of existing sea. Moreover, the average age of above and rapakivi intrusions (see e.g. Vaasjoki et recycling of present sea floor, whose spreading al., 1988) causing further regional metamorphism, rates are probably slower than those during Proter magmatism and migmatization. These processes, ozoie (Nisbet, 1987), is ca. 110 Ma, and ages exceed- however, are not considered here.

CONCLUSIONS

The main stages of the evolution of Finnish duction and continental collision, creating new bedrock between. 2.9 - 1.86 Ga can be modelIed felsic crust. The main plate tectonic driving force is with relatively simple plate tee tonic processes. The assumed to be convection cells whose location Kuhmoan (- Belomorian) orogeny (ca. 2.8 - 2.6 relative to the continents varies. Ga), the Svecobaltian orogeny (ca. 2.2 - 2.1 - 2.0 The main emphasis of this paper has been the Ga) and the Svecofennian orogeny (ca. 1.97 - 1.86 evolution of central and southern Finland bedrock. - ... Ga) are all explained with models starting from The applicability of these models becomes more continental rifting characterized by mafic magma uncertain further to the north and northeast, where tism, followed by opening of intracontinental ocean. it seems that simultaneus, but separate large scale Subsequent ocean closure is associated with sub- processes were operating.

ACKNOWLEDGEMENTS

In this context I want to thank my fellow geologists tolerance. Discussions with and comments by Pent in the GSF for their patience for answering to my 'just ti Hölttä, Heikki Lukkarinen, Matti Pajunen, Matti one more question'. In particular Peter Sorjonen Vaasjoki, Hannu Huhma, Raimo Lahtinen and Ka Ward and Tapio Koistinen have shown admirable levi Korsman have been inspiring and valuable.

20 Geologian tutkimuskeskus, Tutkimusraportti - Geological Survey of Finland, Reporl of lnvestigation 133, 1996 A schematic model of the plate tectonic evolution of Finnish bedrock

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