Precambrian Research, 64 (1993) 207-223 207 Elsevier Science Publishers B.V., Amsterdam

Major Palaeoproterozoic shear zones of the central Fennoscandian Shield

A. Karki*, K. Laajoki and J. Luukas Department of Geology, University of Oulu, Linnanmaa, 90570 Oulu, Finland Received March 28, 1991; revised version accepted September 22, 1992

ABSTRACT

Employing two recently studied crustal-scale shear zones as type examples, this paper summarizes the major Palaeopro- terozoic (Svecokarelian) shear tectonics of the central Fennoscandian Shield and demonstrates that this part of the Shield was not as stable during the Svecokarelian Orogeny as commonly assumed. The collision of the Svecofennian island arc with the Karelian Continent first created numerous NW-SE trending folds and thrusts of stages D] and D2, which were then modified by successive shearing during stages D3 and D4. Stage D3 built up a_ system of N-S trending shear zones, here named the Savolappi Shear System, the type example of which is the Hir- vaskoski Shear Zone. This is a dextral strike-slip shear zone at least 150 km long and 10-30 km wide, characterized by blastomylonitic fault rocks and various structures such as hook folds, Z-folds and sheath folds associated with the principal displacement zone, synthetic Riedel shears, and pinnate shears. The traces of the axial planes of F3 en-echelon folds deviate 15°-30° anticlockwise from the plane of the principal displacement zone. Other members of the Savolappi Shear System are the Pajala Shear Zone in northern Sweden and the Russian North Karelia Shear Zone in the east. Stage D4 created a conjugate shear system called the Finlandia Shear System, the type example of which is the Oulujarvi Shear Zone. This is a NE-SW trending sinistral strike-slip shear zone more than 250 km long and 20-30 km wide across its southwestern end. It is composed of a NE-SW trending principal displacement zone, synthetic Riedel shears, and pinnate shears with antithetic Riedel shears in a NW-SE direction. Typical fault rocks within these shears are S-C mylon- ites. The axial-plane traces of F4 folds of all scales diverge by 20°-40° clockwise from the plane of the principal displace- ment zone. The Kuopio Shear Zone is a conjugate NW-SE trending counterpart of the Oulujarvi Shear Zone. As a whole, the Finlandia Shear System forms a conjugate network of NW-SE and NE-SW trending shear zones which occupies most of the northern and central Fennoscandian Shield.

1. Introduction doga zone (e.g. Gadl and Gorbatschev, 1987). The early works on the Kainuu Schist Belt, A commonly overlooked feature of the Pa- which is situated well inside the "Archaean do- laeoproterozoic geology of the Fennoscandian main", nevertheless proved this to be one of Shield is the tectonic evolution of the Karelian the major Proterozoic tectonic zones in the area between the so-called Raahe-Ladoga Zone Shield (Wegmann, 1928, 1929; Vayrynen, (cf. Kahma, 1978) and the Lapland Granulite 1933), a fact verified by numerous later stud- Belt (Fig. 1). Most commonly, this area is ies (Laajoki 1973,1991; Kontinen, 1987,1992; considered an Archaean domain covered by Laajoki and Tuisku, 1990). Vayrynen (1954) anorogenic, intracratonic sedimentary and emphasized the intense folding of the Protero- volcanic rocks, collectively known as the Ka- zoic schists of Finnish Lapland and even con- relian formations. The present boundary of sidered them a separate orogenic belt which he Archaean crust trends along the Raahe-La- named the Lappides. The extensive area of granitoids in central Lapland provides more 'Corresponding author. evidence of the orogenic nature of the "Ar-

0301-9268/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved. SSDI0301 -9268 (93 )EO057-J 208 A.KARKIETAL.

•60 60°-

200km У Kola Province Svecofennian Province Archaean rocks Karelian Q Caledonian orogenic belt -.-.I Palaeoproterozoic cover*) Province I intrusive rocks p Phanerozoic cover Lapland Granulite Belt y. Shear zone s^ Thrust zone jS*' National boundary Fig. 1. Simplified geological map of the Baltic Shield showing the major shear zones and geological units: /=Kuhmo Complex; //=Iisalmi Complex; /7/=Pudasjarvi Complex; / = North Karelia Schist Belt; 2=Savo Schist Belt; i = Kainuu Schist Belt; 4=Northern Pohjanmaa Schist Belt; 5 = Kuusamo Schist Belt; 6 = Perapohja Schist Belt; 7=Kemijarvi Com- plex; 5=Kittila Greenstone Belt; HSZ=Hirvaskoski Shear Zone (see Fig. 2); ArSZ=Kuopio Shear Zone; OSZ= Oulujarvi Shear Zone (see Fig. 4); A^=Russian North Karelia Shear Zone; P5Z=Pajala Shear Zone. chaean domain", which was intensely de- 1929), and was developed further by Vayry- formed and reworked by Palaeoproterozoic nen (e.g. 1939), recalled by Gaal (1964) and orogenic processes both in Finland and in recently exemplified by Koistinen (1981) and northern Sweden (e.g. Witschard, 1984). Park and Bowes (1983). However, little re- The theory of thrust tectonics was applied to search has been devoted to the post-thrusting the Finnish part of the Fennoscandian Shield crustal-scale shear zones in Finland and neigh- as early as in the 1920's by Wegmann (1928, bouring areas, and most of what was done only MAJOR PALAEOPROTEROZOIC SHEAR ZONES OF THE CENTRAL FENNOSCANDIAN SHIELD 209

concerned southeastern Finland (e.g. Gaal, ern and central Fennoscandian Shield into 1972; Halden, 1982; Ward, 1987). Berthelsen three major units: and Marker (1986b) described an important (1) The Kola Province, which occupies the shear zone which they called the Baltic-Both- area northeast of the Lapland Granulite Belt nia Megashear, but their paper is based mostly and can be subdivided further into subunits on interpretations of aeromagnetic maps with- (cf. Gaal etal., 1989). out actual fieldwork. The shear tectonics of the (2) The Karelian Province, or the Karelides, Sveconorwegian part of the Fennoscandian comprising the area between the above unit and Shield have been a target of intense study, the Svecofennides (see below). This unit con- however (cf. papers in Tobi and Touret, 1985 sists of late Archaean crust and its Palaeopro- and in Gower et al., 1990; Park et al., 1991), terozoic cover (Laajoki, 1990) known collec- and the number of reports emphasizing shear tively as the Karelian formations (the term is zones as province boundaries and essential used here in the sense defined by Laajoki, tectonic elements even in the oldest parts of 1986). The Karelian Province can be divided cratons is increasing rapidly (e.g. van Biljon into numerous subunits, of which this paper andLegg, 1983; Daly, 1986; Hoffman, 1987). discusses the Archaean Kuhmo, Iisalmi and The present paper establishes the major Pudasjarvi Complexes, and the intervening shear systems and summarizes the Palaeopro- Karelian schist areas (Figs. 1, 2 and 4). This terozoic (Svecokareiian) tectonic evolution of province also contains the Kemijarvi Complex the part of the Fennoscandian Shield that oc- (this denomination replaces the informal name cupies central, eastern and northern Finland, "Central Lapland granite complex"), which is and continues into the neighbouring countries. a unit made up in part of granitized and mig- One of its aims is to stress the Palaeoprotero- matized Karelian formations and in part of zoic tectonics of the Archaean domain of the granites intruding the Karelian formations and central Fennoscandian Shield. The paper is their basement (Lauerma, 1982). Its origin, as based on detailed fieldwork carried out in cen- discussed in this paper, is closely related to Pa- tral and northeastern Finland (Laajoki and laeoproterozoic shear tectonics. Luukas, 1988; Karki and Laajoki, 1990; Laa- (3) The Svecofennian Province, or the Sve- joki and Tuisku, 1990; Karki, 1991; Laajoki, cofennides, is a unit southwest of the Karelian 1991; Luukas, 1991) and the reader is referred Province composed solely of Palaeoprotero- to these papers for the original field data. zoic crust. Large parts are interpreted as island arc complexes that collided with the Karelides. About half of the surface is made up of grani- 2. Geological subdivisions of the central toids. The boundary between units (2) and (3) Fennoscandian Shield has not yet been located precisely and its na- ture is disputed. Usually, it is placed at what is The Fennoscandian Shield has previously loosely called the Raahe-Ladoga Zone and is been subdivided into various units with differ- regarded as a palaeosuture (e.g. Hietanen, ent names (cf. Gaal, 1990 and references 1975) or a faulted block contact (e.g. Kors- therein). Some of these classifications are un- manetal., 1984). satisfactory in the present context because they treat the Karelian area (Fig. 1) as a single unit 3. General deformation history of the central and do not emphasize that at least its major, Fennoscandian Shield northeastern part participated actively in the Svecokareiian orogeny. The Karelian Province has been affected by In the present context, we divide the north- Svecokareiian progressive deformation which 210 A. KARKIETAL.

27'30 28°00'

66°00 ee°oo-

Y

Legend trend o( So/1 main lit ho logic boundary — •— strike slip fault and shear sense —*— D-|-D2 thrust fault

I* + intrusive госкз

j [ j ] layered intrusions

yS Archaean basement 65°00

10km

26°30 27 00 27°30' 28°00'

Fig. 2. Simplified geological map of northeastern Finland showing the N-S trending dextral Hirvaskoski Shear Zone which separates the Archaean Pudasjarvi and Kuhmo Complexes. The western margin of the shear zone is defined by the

Kalhama Fault. The area of Fig. 3 is framed. MAJOR PAIAEOPROTEROZOIC SHEAR ZONES OF THE CENTRAL FENNOSCANDIAN SHIELD 211

created its major structural features in four 4. The Hirvaskoski Shear Zone, a type

main stages. The deformational evolution can example of major D3 structures in the be divided into an early phase which involved Savolappi Shear System the translation of thrust nappes towards the craton and the creation of the related folds of Definition. Some time ago we recognized a

stages D{ and D2, and a younger phase of shear N-S trending shear zone of Palaeoproterozoic tectonism which produced the major shear paragneisses that is ~ 150 km long and 10-30 zones of the central Fennoscandian Shield. km wide (Fig. 2). It separates the Archaean The earliest compressional structures are Pudasjarvi and Kuhmo Complexes (Karki and identified by isoclinal F! folds (Koistinen, Laajoki, 1990; Karki, 1991). This D3 tectonic 1981; Ward, 1987) which are related on the zone was named the Hirvaskoski Shear Zone macroscopic scale to the first nappe emplace- (HSZ) after a locality in its central part. Lith- ments and recumbent folds (e.g. Gaal, 1990; ologically, its southern end has been mapped Luukas, 1991). After the D! deformation, the as the Kalhamajarvi Complex (Karki and Laa- earliest structures were refolded in an identical joki, 1990; Laajoki, 1991), and a fault which compressional field producing more open, defines its sharp western boundary with the Pudasjarvi Complex is named the Kalhama mostly upright F2 folds. These are the most Fault (Fig. 2). The eastern margin of the HSZ conspicuous mesoscopic and macroscopic is delineated by a shear zone which intersects structures in the Palaeoproterozoic formations the western margins of the northernmost tip of of the Finnish part of the central Fennoscan- the Kainuu Schist Belt, the Kuhmo Complex dian Shield. The D2 structures are approxi- and the Kuusamo Schist Belt (Fig. 2.). The mately in their original position (NW-SE) in southern end of the HSZ is cut by the Oulu- the Savo Schist Belt (Laajoki and Luukas, jarvi Shear Zone, whereas the northern end 1988) and the Northern Pohjanmaa Schist opens into a large horsetail structure and then Belt, but have been reoriented into a N-S di- tapers out in the gneisses and granitoids of the rection by later deformation in the Kainuu Kemijarvi Complex. Schist Belt (Laajoki and Tuisku, 1990; Karki, 1991; Gehor and Laajoki, 1992). The HSZ forms a dextral ductile shear zone (for terminology see Sylvester, 1988; Harding, After stage D , deformation changed from 2 1990) with a subvertical, N-S trending prin- folding to the ductile shear tectonism of stages cipal displacement zone (PDZ3). The whole D3 and D4 (Karki, 1991; Luukas, 1991) which shear zone is composed of miscellaneous faults created the shear systems described in this pa- and fault rocks related to diverse fold struc- per. After these ductile phases the style of de- tures located between the subzones of most in- formation changed again, because the now ex- tense shearing. The extent of shear strain var- posed sections were brought to a higher crustal ies greatly between the faults, which is verified level and were subjected to semi-brittle and by the diverse characters of the mesoscopic brittle faulting of long duration. Some of the structures and fault rocks (Bell and Etheridge, D3 and D4 shear zones were reactivated and, 1973; Wise et al., 1984; Xu et al., 1986). in consequence, show both initial ductile and Macroscopic structure. On the macroscale, younger brittle features, the latter being veri- the HSZ is build up of ductile strike-slip faults fied by anomalous electric conductivity (Korja or shear zones and less common folds identi- et al., 1991). In fact, brittle deformation seems fiable from regional mapping and aeromag- to be active even today, as demonstrated by netic greytone maps. Following the simple minor earthquakes in areas of shear zones shear model (Riedel, 1929; Aydin and Page, (Talvitie, 1971; Mustila and Korhonen, 1991). 1984), the faults can be grouped into three cat- 212 A. KARKIETAL.

egories according to their mutual orientations. faults are most frequent near the Kalhama

The N-S trending faults are interpreted as Fault, whereas F3 en-echelon folds are more

principal displacement zones (PDZ3), whereas common in the eastern part of the shear zone.

synthetic Riedel shears (R3), diverging 10-20° The total displacement caused by the D3 shear-

clockwise from the plane of the PDZ3, and pin- ing is estimated to be about 70-90 km. This is

nate shears (P3), diverging 10-20° anticlock- recognizable most clearly on the basis of the wise, represent the other fault components of break-up and displacement of the layered mafic the shear zone (Fig. 2). intrusions in northern Finland (Alapieti et al., The Kalhama Fault is one of the subvertical 1990) and the fold structures in Lapland.

faults following the direction of the PDZ3. D3 In Posio (Fig. 2), the eastern margin of the

Structural elements

So inclined/vertical /£» ft So with top direction S, or So Inclined / S3 inclined, vertical /f0 / S4 Inclined, vertical Jr S4 and siniatral fault 'unclassified schistosity axial trace of D1-D2

K.Laalohl -92 28°15'

Fig. 3. Simplified structural map of the surroundings of the village of Posio (Fig. 2). The quartzites of the central area (Kuoppavaara) are separated from the western, highly sheared mica schists and metabasites by the Nilojarvi fault and from the Kuusamo Schist Belt in the east by the Kuoppavaara fault. MAJOR PALAEOPROTEROZOIC SHEAR ZONES OF THE CENTRAL FENNOSCANDIAN SHIELD 213

Fig. 4. (a) Dextral semiductile fault in amphibole-rich paragneiss, showing a 10 cm wide ductile zone between the N-S trending blastomylonitic zones, (b) Sheath fold bordered by migmatitic, N-S trending blastomylonite zone in the west, (c) J-shaped D3 hook fold associated with a N-S trending fault, (d) Multiply deformed Palaeoproterozoic paragneiss showing a dextral N-S trending mylonite zone and associated D3 en-echelon folds and Z-folds, the axial plane trace of which diverges about 20° anticlockwise from the plane of the mylonite zone.

HSZ turns NNE and is marked by a sheared, stress field created the N-S striking dextral mylonitic zone of metapelites (Fig. 3) inter- strike-slip shear zones. secting the "Lappidic" trend of the Kuusamo Mesoscopic structures. The fault rocks asso-

Schist Belt. ciated with diverse D3 faults are mostly sheared

Dynamically, the stress field during stage D3 gneisses with metamorphic foliation of a sym-

differed from that of stage D2 in that the low- metric, recovered character. Sometimes the est, vertical principal stress and the interme- foliation is integrated with intrafolial, isoclinal diate, horizontal and NE-SW trending one fold structures. Blastomylonitic zones and changed places. The maximum principal stress asymmetric structures such as foliation fishes,

(ox) remained in a NE-SW direction. This indicators of dextral slip within the shear zone, 214 A. KARKIETAL.

have been detected in zones of maximum mation caused by the OSZ hampers mapping ' shearing. Sigma structures and other micro- of its southern extension. It is, nevertheless, scopic shear-sense indicators are also visible in probable that the sheared Vayrylankyla Nappe the mica-rich fault rocks. The dextral semiduc- (Laajoki, 1991) and the sheared core of the tile faults in the HZS consist of narrow blasto- southern end of the Kainuu Schist Belt (Ha- mylonitic zones bordered by wider zones of vola, 1981) represent the same tectonic zone. ductile deformation (Fig. 4a). If so, the original length of the HSZ was at least Asymmetric folds with axial-plane traces di- 300 km. The maps of Paavola (1980, 1984) verging 15° to 30° anticlockwise from the show clearly that a N-S trending shear zone

plane of the PDZ3 constitute en-echelon folds occurs at the southeastern corner of the Iisalmi

in the D3 shear zone. These are tight and usu- Complex (Fig. 1), and this Nilsia Shear Zone ally not harmonic, and are closely related to the can be correlated tentatively with the HSZ. blastomylonitic zones and other elements of the These zones appear to represent a large strike- shear zone (Fig. 4d). Hook- and Z-folds (Fig. slip fault zone which follows a Di-Di tectonic 4c) are detectable in mica-rich paragneisses zone (the Koillismaa-Kainuu-North Karelia and provide clear evidence of dextral slip dur- tectonic zone of Laajoki 1991) and subdivides ing shearing. The axial-plane traces are mostly the Archaean crust into the Kuhmo Complex parallel to the PDZ3 and their fold axes per- in the east and the Iisalmi and Pudasjarvi pendicular to the stretching Hneation. The hook Complexes in the west. folds are J-shaped folds with narrow faults par- Berthelsen and Marker (1986b) describe the allel to the axial-plane traces (e.g. Hudleston, Baltic-Bothnia Megashear and a megashear in 1989; cf. Fig. 4c), whereas the Z-folds are Russian North Karelia which trends in the products of entirely ductile deformation. same, approximately N-S direction. The Sheath folds (e.g. Schwerdtner, 1987) com- northern part of the former, which we here call prise the fourth class of folds within the shear the Pajala Shear Zone, is clearly analogous to zone (Fig. 4b). In two-dimensional sections the HSZ, and thus the central and northern these are seen as irregular basin and dome Fennoscandian Shield is traversed by at least structures or tube-shaped structures in the di- three rather closely spaced major N-S trend- rection of the stretching Hneation. The sheath ing shear zones: the Pajala Shear Zone, the folds are situated between blastomylonitic Hirvaskoski Shear Zone and the Russian North zones or zones of highest shear strain. Karelia Shear Zone. Together, these form a The Proterozoic paragneisses of the HSZ large system of shear zones that we call the Sa- have been intruded by Svecokarelian granites volappi Shear System (Fig. 7). and granitic pegmatites, amongst which the latter are genetically associated with the shear- 5. The Oulujarvi Shear Zone, type example of ing. The migmatization of the paragneisses and D4 shear zones of the Finlandia Shear System the amount of granitic material increase to- wards the north, and the gneisses of the north- Definition. The Oulujarvi Shear Zone (OSZ) ern part of the HZS are migmatites without is a NE-SW trending, subvertical shear zone primary structures. ~ 250 km in length which separates the Savo and Northern Pohjanmaa Schist Belts as well 4.1. Structural correlations and definition of as the Archaean Iisalmi and Pudasjarvi Com- the Savolappi Shear System plexes (Fig. 5). It transects the northern end of the Kainuu Schist Belt and continues as a The Kemijarvi Complex prevents the HSZ dying, semi-brittle fault system in the northern from being followed north, while the defor- part of the Archaean Kuhmo Complex. In its MAJOR PALAEOPROTEROZOIC SHEAR ZONES OF THE CENTRAL FENNOSCANDIAN SHIELD 215

PUDAS-y JARVI COM- PLEX

65 00- — 65 00

64 30-

trend of So/1 main lithologic boundary • ^*~- strike slip fault and shear sense D1-D2 thrust fault • D4-D2 thrust fault ophiolite complex 64 00 — \ +*\ intrusive rocks -64 00 IISALMI Archaean basement | Jotnian sandstone Y coM- NORWA25°30Y ' ' vPLEX N

10 km

Fig. 5. Simplified geological map of central Finland showing the major structures of the sinistral Oulujarvi Shear Zone. The NW margin of the shear zone is defined by the Auho Fault. 216 A.KARKIETAL.

southwestern and central parts, the OSZ trun- ones varying in length from 10 to 20 km and j cates and transposes all the earlier structures, being ~5 km wide (Laajoki and Tuisku, including the HSZ, in a ductile manner and in- 1990). The trends of the axial planes of the cludes a number of Palaeoproterozoic granites folds deviate less than 40° clockwise from the

(Laajoki, 1991) and migmatitic gneisses. The strike of the PDZ4. degree of migmatization and the amount of Major NE-SW trending sinistral strike-slip granitoids increase southwest wards. faults, classified as principal displacement

The OSZ has previously been called the zones (PDZ4), synthetic Riedel shears (R4) Auho Fault Zone (Laajoki, 1986) but is here diverging less than 20° anticlockwise from the

renamed on the basis of recent structural work. plane of PDZ4, and pinnate shears (P4) di- The name "Auho Fault" is nevertheless re- verging less than 20° clockwise from the plane

tained for a fault parallel to the principal dis- of PDZ4, have been identified all over the OSZ.

placement zone of the OSZ (PDZ4). Both the Dextral, NW-SE trending antithetic Riedel

OSZ and the Auho Fault are seen very clearly shears (R'4) have also been observed. The on aeromagnetic greytone maps, the former largest faults, the Auho Fault and the Pyhanta

forming a sheared and folded zone 30-40 km Fault, are associated with the PDZ4. wide at its southwestern end and narrowing to- The total sinistral displacement caused by wards the northeast, and the latter situated at the shear zone in the northern Kainuu Schist the north western border of the OSZ. Beit has been estimated to be ~ 40 km, the dis- The evolution of the OSZ follows the general placements along individual faults varying evolutionary model of strike-slip zones as de- from a few tens of metres to several kilometres. scribed by Harding and Lowell (1979), and Mesoscopic structures. The OSZ features Sylvester (1988). Applying Harding and Low- progressive formation of a strike-slip shear ell's (1979) model, the ductilely folded south- zone with mesoscopic structures largely simi-

eastern part of the OSZ represents the area of lar to those of the HSZ. The D4 deformation earliest en-echelon folding, continued defor- first produced ductile folds, whereas the later mation causing initiation of the sinistral strike- phase is characterized by ductile-semiductile slip faults. faulting. Many faults were also affected by Macroscopic structure. The macroscopic brittle deformation during younger structures identified by mapping (Karki and reactivation.

Laajoki, 1990; Laajoki and Tuisku 1990; Laa- The mesoscopic F4 folds are usually en-ech- joki, 1991; Luukas, 1991) and from the elon folds, their axial-plane traces typically di- aeromagnetic greytone maps consist of large en- verging 20° to 40° clockwise from the strike of

echelon folds and major faults with connecting the PDZ4. The characteristic folds within the minor faults and horsetail systems (Fig. 5). OSZ are asymmetrical S-folds which are more The most conspicuous macroscopic struc- open than the corresponding Z-folds of the

tures of the OSZ are large NE-SW trending en- HSZ (Figs. 6a and 6c). The penetrative S4 ax-

echelon folds, which represent interference ial-plane schistosity is well developed in the F4

patterns caused by the superimposition of D4 folds of the Puolankajarvi Formation (Laajoki

and D2, and arranged in the en-echelon man- and Tuisku, 1990). In many cases, one or both ner typical of large strike-slip shear zones (e.g. limbs have been sheared intensively and occa- Sylvester, 1988). The largest interference folds, sionally intruded by granitic material. reaching lengths of about 30 km and widths of Mesoscopic faults are common in exposures 8 km, are situated at the southwestern end of and mostly represent either NE-SW trending, the shear zone, whereas similar folds are some- sinistral R4 shears or NW-SE trending, dextral what smaller in the northeast, the principal R'4 shears. The banding in the highly meta- MAJOR PALAEOPROTEROZOIC SHEAR ZONES OF THE CENTRAL FENNOSCANDIAN SHIELD 217

Fig. 6. (a) Chevron-type folds in the crest zone of a macroscopic D3 en-echelon fold, (b) Dextral semiductile fault in the direction of a D4 antithetic Riedel shear showing a 30 cm wide ductile zone cut by a NW-SE trending 1 cm wide mylonite zone. (с) S-shaped D4 fold in Palaeoproterozoic paragneiss; the axial-plane trace is in the direction of the PDZ4. (d) NE- SW trending mylonite zone which intersects Palaeoproterozoic mica schist. The sinistral shear sense is shown by the sigma structures in the mylonite zone. morphosed gneisses is usually parallel to the similar to those of the HSZ, there are also some shear surface. The semiductile faults are a few fundamental differences. The faults of the HSZ decimetres wide and consist of a narrow S-C feature blastomylonitic characteristics, whereas mylonitic zone rimmed by wider zones of duc- the corresponding structures in the OSZ have tile deformation (Fig. 6b). The S-C mylonites S-C mylonitic and more brittle characteristics. indicate sinistral shearing within R4 and the The angles between R3 and P3 appear to be

PDZ4. In addition, the sense of the shearing can smaller than those between R4 and P4, or else be determined microscopically from rolled the angle of internal friction upon deformation porphyroclasts and sigma structures (Fig. 6d). of the HSZ is smaller than that in the OSZ. Although the OSZ features shear structures These differences suggest that the present sec- 218 A. KARKIETAL.

tion of the HSZ represents a deeper crustal sec- the Gulf of Bothnia to Lake Ladoga, and Gadl tion and a higher grade of metamorphism than and Gorbatschev (1987) later called it the La- that of the OSZ. doga-Bothnian Bay Tectonic Zone. On the other hand, the term "Raahe/Bothnian Sea- Ladoga" is often associated with the "main 5.1. Structural correlations and definition of sulphide ore belt of Finland" as described by the Finlandia Shear System Kahma (1978). Korsman (1988), for exam- ple, states that the Raahe-Lagoda zone is a Laajoki (1986) suggested that the poorly boundary between the Archaean rocks in defined Raahe-Lagoda Zone may form a con- northern and eastern Finland and the Proter- jugate system with the OSZ (at that time ozoic ones, and hosts most of the major sul- termed the Auho Fault Zone), but doubted the phide ore deposits in Finland. In view of these justification of extrapolating the former zone multiple significance associations we are reluc- as a zone of intense shearing northwestward tant to continue the use of the "Raahe-Ladoga past the crossing point with the OSZ (Fig. 1). nomenclature" and prefer to apply the term Our subsequent detailed analysis of the geo- "Kuopio Shear Zone" to the part of the origi- physical data and the mapping results showed nal Raahe-Ladoga Zone of Gaal (1972) situ- that this suspicion was justified. ated southwest from the point where that zone In addition, we find that the concept of the crosses the OSZ. The Kuopio Shear Zone in- Raahe-Lagoda Zone is poorly defined and cludes the Suvasvesi, Haukivesi and Pihlaja- understood. Gaal (1972) considers this to be vesi Faults. Its structure has been documented a deep-seated wrench fault zone trending from by Gaal and Rauhamaki (1971), Halden

Default zone D4-fold 2.45 Ga mafic intrusion D -thrust basement D-fold 4 Default zone (°*P?) Anorogentc granite

Fig. 7. Tectonic evolution of the central Baltic Shield between 1.93 and 1.80 Ga ago: thrusting and folding during stage D2; development of the Savolappi Shear System during stage D3; structural features produced by N-S compression during stage D4. MAJOR PALAEOPROTEROZOIC SHEAR ZONES OF THE CENTRAL FENNOSCANDIAN SHIELD 219

(1982), and various authors in Korsman sian North Karelia Megashear define the west- (1988). •;.,.-. ern and the eastern members of this system, In addition to the major structures exempli- while the Hirvaskoski Shear Zone and its fied by the OSZ and the Kuopio Shear Zone, southern extension represent a third major N-

minor D4 structures are quite common in east- S striking shear zone between and parallel to ern Finland (Koistinen, 1981; Ward, 1987), the first-named pair. The creation of this shear the southern part of the Kainuu Schist Belt system marks a change from the folding and (Gehor and Laajoki, unpublished data) and thrusting of the early stages to the ductile

Lapland (Fig. 7c; Marker, 1988; Ward et al., shearing of stage D3. The main reason for this

1988; Gaal et al., 1989). These D4 structures, change appears to be the thickening of the Pa- which cover a major part of the central Fen- laeoproterozoic crust, which caused a change

noscandian Shield, form a network of sinistral of the stress field, and the D2 peak of the re- NE-SW trending and dextral NW-SE trend- gional metamorphism (Tuisku and Laajoki, ing shear zones which we call the Finlandia 1991). Shear System. The Perapohja Schist Belt between the Pa- jala and Hirvaskoski Shear Zones represents an 6. Discussion area in which a local N-S compression caused by northward displacement of the Pudasjarvi

The effects of the prolonged co'mpressional Complex during D3 faulting formed the E-W evolution of the Karelian and Svecofennian trending folds described by Salonsaari (1991). Provinces during the Svecokarelian Orogeny These form a counterpart of Vayrynen's have been described by many authors (cf. Gaal, (1954) "Lappidic folding". 1990, and references therein). The earliest re- Since the amount of granitoids increases to- cumbent folding and NE-directed thrusting to- wards the north within the HSZ and since the wards the craton can be identified as a belt of Pajala Shear Zone is also characterized by external thrusting and folding in the Karelian abundant synkinematic granitoids, we suggest formations. This fold and fault belt was formed that the formation of the Kemijarvi Complex by a NE-SW compression which caused accre- may have been associated with the left-step- tion of the northeastern Svecofennian island ping Hirvaskoski-Pajala pair of shear zones.

arc to the Karelian province during T)x (Fig. 7, The space problem may have been resolved by left). Some thrusting and folding marking the tectonic underplating, which could, in turn, final closure of the oceanic basin between the have given rise to granitic diapirs and caused Svecofennian island arc and the Karelides was the migmatization of Archaean and Karelian

also associated with stage D2 of the progressive formations in the area of the Kemijarvi Com- Svecokarelian deformation (Luukas, 1991). plex. The ages of the Central Lapland grani- The culmination of the regional metamor- toids range from 1843 to 1770 Ma (Lauerma, phism took place during stage D2 (Holtta, 1982; Huhma, 1986) and support this 1988; Tuisku and Laajoki, 1990) and accre- interpretation. tion of the Svecofennian island arc occurred Approximately 1.85 Ga ago a new phase of between 1.93 and 1.85 Ga ago (Vaasjoki and compression started in a N-S direction, possi- Sakko, 1988; Gaal, 1990). bly because of the functioning of a new sub- The following phase of deformation created duction system in the south (cf. Edelman and the major N-S striking, vertical D3 shear zones Janus-Jarkkala, 1983; Vaasjoki and Sakko, of the Savolappi Shear System in the central 1988). This was relaxed by various D4 struc- and northern Fennoscandian Shield (Fig. 7, tures that formed in different places and at dif- middle). The Pajala Shear Zone and the Rus- ferent times (Fig. 7). The first to be produced 220 A. KARKIETAL. were the E-W striking folds (e.g. in the Tamp- sumably deeper sections of the Precambrian ere area; Campbell, 1980; Nironen, 1989) and crust than the northeastern and southeastern the 1.83-1.81 Ga old migmatization (Vaas- parts. joki and Sakko, 1988) in southern Finland. As a result of powerful N-S directed D4 Similar folds formed by a pure shear mecha- compression, the Iisalmi Complex was faulted nism have been identified in the northern part to a conjugate strike-slip system, the individ- of the Savo Schist Belt, where the E-W striking ual fault directions of which coincide with

D4 folds gradually swing into a NE-SW direc- those of the major shear zones described above. tion to form the en-echelon folds of the Oulu- There may also have been some thrusting or jarvi Shear Zone. This folding preceded the reverse faulting related to N-S compression in major sinistral shearing of the Oulujarvi Shear the northern part of this complex (Fig. 7c). In Zone (Luukas, 1991). Simultaneously there Lapland, the effects of N-S compression are may have been some reactivation and rotation represented by E-W striking folds in central of the D,-D2 thrust zones on the northern side Lapland and by the latest thrusting of the Lap- of the Iisalmi Complex (Figs. 1 and 7c). The land Granulite Complex (Berthelsen and Saariaho granite (1.91 Ga; Laajoki, 1991) Marker, 1986a; Marker, 1988). clearly predates the D4 deformation, whereas As a whole, the tectonic consequences of de- the intrusion of the Nattanen-type granitoids formation stages D3 and D4 appear to have following a NE-SW trend in the northern Fen- been much more important and complicated noscandian Shield (1.80 Ga; Front et al., 1989) than has been realized previously. In conse- and the synkinematic or postkinematic grani- quence, the portion of the Fennoscandian toids of the OSZ (e.g. the Takiankangas gran- Shield affected by them cannot be considered ite, 1.80 Ga, and the Avainlampi diorite, 1.80 an anorogenic cratonic area but rather an es- Ga, Laajoki, 1991) indicate the age of shear- sential part of the Svecokarelian Orogeny. The ing by stage D4. precise determination of the role of this realm During the generation of the Oulujarvi Shear in the Svecokarelian Orogeny must, however, Zone, its southeastern conjugate, the NW-SE await the completion of a tectonic synthesis for trending dextral Kuopio Shear Zone (Fig. 7c) the whole Shield, including also the Russian developed simultaneously with associated part of Karelia. granites of 1.78 Ga age (Halden, 1982). This shear zone has been considered a major tec- Acknowledgements tonic feature in Finland and has been included in the Raahe-Ladoga Zone (Gaal, 1972). We This paper is a contribution to the research are, nevertheless, inclined to regard the Ku- project "Metamorphism and Deformation of opio Shear Zone as part of the Finlandia Shear the Crust" sponsored by the Academy of Fin- System and do not support the idea that major land, and IGCP-project 275 "Deep Geology of strike-slip faulting occurred along its north- the Baltic Shield". The figures were drafted by western continuation past the point where it Mrs. Kristiina Karjalainen and the English of meets the OSZ. We rather suggest that in the the manuscript was improved by Mr. Malcolm latter area deformation was released by fold- Hicks. The manuscript benefitted from many ing. The distribution of the granitoids, the fruitful comments by Roland Gorbatschev, variations in deformation style and the grades Peter Ward and an anonymous referee. of metamorphism within the Oulujarvi and Kuopio Shear Zones suggest that their south- References western and northwestern parts, respectively, Alapieti, Т., Filen, В., Lahtinen, J., Lavrov. M., Smolkin, represent more ductile deformation of pre- V. and Voitsekhovsky, S., 1990. Early Proterozoic lay- MAJOR PALAEOPROTEROZOIC SHEAR ZONES OF THE CENTRAL FENNOSCANDIAN SHIELD 221

ered intrusions in the northeastern part of the Fenno- Gehor, S. and Laajoki, K., 1992. Deformation of the scandian Shield. Miner. Petrol., 42: 1-22. Nuasjarvi Basin of the early Proterozoic Kainuu Schist Aydin, A. and Page, В., 1984. Diverse Pliocene-Quater- Belt, Finland. Res Terrae, Ser. A, 5:18 (abstr.). nary tectonics in a transform environment, San Fran- Gower, C.F., Rivers, T. and Ryan, B. (Editors), 1990. cisco Bay region, California. Geol. Soc. Am. Bull., 95: Mid-Proterozoic Laurentia-Baltica. Geol. Assoc. Can. 1303-1317. Spec. Pap., 38. Bell, E. and Etheridge, M., 1973. Microstructure of my- Halden, N.M., 1982. Structural, metamorphic and ig- lonites and their descriptive terminology. Lithos, 6: neous history of migmatites in the deep levels of a 337-348. wrench fault regime, Savonranta, eastern Finland. Berthelsen, A. and Marker, M., 1986a. Tectonics of the Trans. R. Soc. Edinburgh, Earth Sci., 73: 17-30. Kola collision suture and adjacent Archean and Early Harding, T.P., 1990. Identification of wrench faults using Proterozoic terrains in the northeastern region of the subsurface data: criteria and pitfalls. Am. Assoc. Pet. Baltic Shield. Tectonophysics, 126: 31-55. Geol. Bull., 74: 1590-1609. Berthelsen, A. and Marker, M., 1986b. 1.9-1.8 Ga old Harding, T. and Lowell, J., 1979. Structural styles, their strike slip megashears in the Baltic Shield, and their plate-tectonic habitats, and hydrocarbon traps in pe- plate tectonic implications. Tectonophysics, 128:163- troleum provinces. Am. Assoc. Pet. Geol. Bull., 63: 181. 1016-1058. Campbell, D.S., 1980. Structural and metamorphic de- Havola, M., 1981. Pre-Quaternary rocks. Sheet 3433, Sot- velopment of migmatites in the Svecokarelides, near kamo. Geological map of Finland 1:100000. Tampere, Finland. Trans. R. Soc. Edinburgh, Earth Hietanen, A., 1975. Generation of potassium-poor mag- Sci., 71: 185-200. mas in the northern Sierra Nevada and the Svecofen- Daly, M.C., 1986. Crustal shear zones and thrust belts: nian in Finland. J. Res. U.S. Geol. Surv., 3: 631-645. their geometry and continuity in Central Africa. Phi- Hoffman, P.F., 1987. Continental transform tectonics: los. Trans. R. Soc. London, A 317: 111-128. Great Slave Lake shear zone (ca. 1.9 Ga), northwest Edelman, N. and Jaanus-Jarkkala, M., 1983. A plate tec- Canada. Geology, 15: 785-788. tonic interpretation of the Precambrian of the archi- Holtta, P., 1988. Metamorphic zones and the evolution of pelago of southwestern Finland. Geol. Surv. Finl., Bull., granulite metamorphism in the early Proterozoic Pie- 325,33 pp. lavesi area, central Finland. Geol. Surv. Finl., Bull., Front, K., Vaarma, M., Rantala, E. and Luukkonen, A., 344, 50 pp. 1989. Keski-Lapin varhaisproterotsooiset Nattas- Hudleston, E., 1989. The association of folds and veins in tyypin graniittikompleksi, niiden kivilajit, geokemia ja shear zones. J. Struct. Geol., 11: 949-957. mineralisaatiot. Summary: Early Proterozoic Natta- Huhma, H., 1986. Sm-Nd, U-Pb and Pb-Pb isotopic nen-type granite complexes in central Finnish Lap- evidence for the origin of the Early Proterozoic Sve- land: rock types, geochemistry and mineralization. cokarelian crust in Finland. Geol. Surv. Finl., Bull., Geol. Surv. Finl., Rep. Inv., 85, 77 pp. 337, 48 pp. Gaal, G., 1964. Jatul und Karelische Molasse im S-Koli- gebiet in Nordkarelien und ihre Beziehungen zum Ge- Kahma, A., 1978. The main sulphide ore belt of Finland birgsbau des Prakambrischen Orogens. Bull. Comm. between Lake Ladoga and the Bothnian Bay. Bull. Geol. Geol. Finl., 213, 45 pp. Soc. Finl., 50: 39-43. Gaal, G., 1972. Tectonic control of some Ni-Cu deposits Karki, A., 1991. Puolankajarven kompleksin ja Lansi- in Finland. 24th Int. Geol. Congr., Montreal 1972, 4: Puolangan gneissikompleksin arkeeinen ja proterot- 215-224. sooinen rakennekehitys. Unpublished Phil. Lie. The- Gaal, G., 1990. Tectonic styles of Early Proterozoic ore sis. University of Oulu. 117 pp. deposition in the Fennoscandian Shield. Precambrian Karki, A. and Laajoki, K., 1990. Pre-Quaternary rocks. Res., 46:83-114. Sheet 3442, Puolanka. Geological Map of Finland Gaal, G. and Gorbatschev, R., 1987. An outline of the 1:100 000. Precambrian evolution of the Baltic Shield. Precam- Koistinen, T.J., 1981. Structural evolution of an early brian Res., 35: 15-52. Proterozoic strata-bound Cu-Co-Zn deposit, Outok- Gaal, G. and Rauhamaki, E., 1971. Petrological and umpu, Finland. Trans. R. Soc. Edinburgh, Earth Sci., structural analysis of the Haukivesi area between Var- 72:115-158. kaus and Savonlinna, Finland. Bull. Geol. Soc. Finl., Kontinen, A., 1987. An early Proterozoic ophiolite—the 43:265-337. Jormua mafic-ultramafic complex, northeastern Fin- Gaal, G., Berhelsen, A., Gorbatschev, R., Kesola, R., Leh- land. Precambrian Res., 35: 313-341. tonen, M.I., Marker, M. and Raase, P., 1989. Structure Kontinen, A., 1992. Southern part of the Kainuu Schist and composition of the Precambrian crust along the Belt—a brief introduction to the general geological set- POLAR Profile in the northern Baltic Shield. Tecto- ting and excursion stops. Res. Terrae, Ser. A, 7: 23-27. nophysics, 162: 1-25. Korja, Т., Pernu, T. and Heikkinen, E., 1991. Geoelectric 222 : A.KARKIETAL.

features and their tectonic implications of the Kainuu ships during polyphase deformation in the Svecofen- Schist Belt. Res Terrae, Ser. A, 5: 34 (abstr.). nides of the Kaavi district, eastern Finland. Trans. R. Korsman, K. (Editor), 1988. Tectono-metamorphic evo- Soc. Edinburgh, 74:95-118. lution of the Raahe-Ladoga zone. Geol. Surv. Finl., Park, R.G., Ahall, K. and Boland, M., 1991. The Svecon- Bull., 343, 96 pp. orwegian shear-zone network of SW Sweden in rela- Korsman, K., Niemela, R., Hautala, T. and Wasenius, P., tion to mid-Proterozoic plate movements. Precam- 1984. Metamorphism as an indicator of evolution and brian Res., 49:245-260. structure of the crust in eastern Finland. Bui. Geol. Riedel, W., 1929. Zur Mekanik geologischer Brucher- Surv. Finl., 328, 40 pp. scheinungen. Zentralbl. Mineral., Geol. Palaeontol., Laajoki, K., 1973. On the geology of the South Puolanka Abh., B: 354-368. area, Finland. Geol. Surv. Finl., Bull., 263, 54 pp. Salonsaari, P., 1991. Vanttauskosken alueen kallioperan Laajoki, K., 1986. The Precambrian supracrustal rocks of deformaatio. Abstract: Deformation of the bedrock of Finland and their tectono-exogenic evolution. Pre- the Vanttauskoski area, Northern Finland. Res Terrae, cambrian Res., 33: 67-85. Ser. B, 15, 61pp. Laajoki, K., 1990. Early Proterozoic tectofacies in eastern Schwerdtner, W.M., 1987. Interplay between folding and and northern Finland. In: S. Nagvi (Editor) Precam- ductile shearing in the Proterozoic crust of the Mus- brian Continental Crust and Its Economic Resources. koka-Parry Sound region, central Ontario. Can. J. Developments in Precambrian Geology 6, Elsevier, Earth Sci., 24: 1507-1525. Amsterdam, pp. 437-452. Sylvester, A.G., 1988. Strike-slip faults. Geol. Soc. Am. Laajoki, K., 1991. Stratigraphy of the northern end of the Bull., 100: 1066-1703. early Proterozoic (Karelian) Kainuu Schist Belt and Talvitie, J., 1971. Seismotectonics of the Kuopio Region, associated gneiss complexes, Finland. Geol. Surv. Finl., Finland. Bull. Comm. Gdol. Finl., 248, 41 pp. Bull., 358, 115 pp. Tobi, A.C. and Touret, L.R. (Editors), 1985. The deep Laajoki, K. and Luukas, J., 1988. Early Proterozoic stra- Proterozoic Crust in the North Atlantic Province. tigraphy of the Salahmi-Pyhanta area, central Fin- NATO ASI Series C, Math. Phys. Sci., Vol. 158. land, with emphasis on applying the principles of lith- Tuisku, P. and Laajoki, K., 1990. Metamorphic and odemic stratigraphy to a complexly deformed and structural evolution of the early Proterozoic Puolan- metamorphosed bedrock. Bull. Geol. Soc. Finl., 60: 79- kajarvi Formation, Finland, Part II. The pressure- 106. temperature-deformation-composition path. J. Meta- Laajoki, K. and Tuisku, P., 1990. Metamorphic and morph. Geol., 8:357-391. structural evolution of the early Proterozoic Puolan- Vaasjoki, M. and Sakko, M., 1988. The evolution of the kajarvi Formation, Finland, Part I. Structural and tex- Raahe-Ladoga zone in Finland: isotopic constraints. tural relations. J. Metamorph. Geol., 8: 357-374. Geol. Surv Finl., Bull., 343: 7-32. Lauerma, R., 1982. On ages of some granitoid complexes van Biljon, W.J. and Legg, J.H. (Editors), 1983. Special in northern Finland. Bull. Geol. Soc. Finl., 54: 85-100. Publication No. 8 of the Geological Society of South Luukas, J., 1991. Salahmin-Pyhannan alueen stratigrafia Africa, Johannesburg, 203 pp. ja rakennegeologia. Abstract: The stratigraphy and Vayrynen, H., 1933. Uber die Stratigraphie der karc- structure of the Salahmi-Pyhanta area, central Fin- lischen Formationen. C.R. Soc. Geol. Finl., 6: 54-78. land. Res Terrae, Ser. В., 16, 131 pp. Vayrynen, H., 1939. On the tectonics of the Karelian zone. Marker, M., 1988. Early Proterozoic thrusting of the Lap- 17th Int. Geol. Congr., Moscow, Compt. Rend., pp. 57- land Granulite Belt and its geotectonic evolution, 70. northern Baltic Shield. Geol. Foren. Stockholm Forh., Vayrynen, H., 1954. Suomen kalliopera, sen synty ja geo- 110:405-410. loginen kehitys. Otava, Helsinki, 260 pp. Mustila, L. andKorhonen, H., 1991. Macroseismic obser- Ward, P., 1987. Early Proterozoic Deposition and defor- vations in Finland 1989-1990. Rep. S-26, Inst. Seis- mation at the Karelian Craton Margin in Southeastern mol., Univ. Helsinki, 12 pp. Finland. Precambrian Res., 35: 71-93. Nironen, M., 1989. The Tampere Schist Belt: structural Ward, P., Harkonen, I., Nurmi, P.A. and Pankka, H., 1988. style within an early Proterozoic volcanic arc system Structural studies in the Lapland greenstone belt, in Southern Finland. Precambrian Res., 43: 23-40. northern Finland and their application to gold miner- Paavola, J., 1980. Рге-Quaternary rocks. Sheet 3334, Nil- alization. Geol. Surv. Finl., Spec. Pap., 10: 71-77. sia. Geological Map of Finland 1:100 000. Wegmann, C.E., 1928. Uber die Tectonic der jiinger Fal- Paavola, J., 1984. Kallioperakarttojen selitykset, lehti 3334 tung in Ostfinnland. Fennia, 50, 16. Nilsia. English summary: Pre-Quaternary rocks of the Wegmann, C.E., 1929. Beispiele tectonischer Analysen des Nilsia map-sheet area. Geological Map of Finland Grundgebirges in Finnland. Bull. Comm. Geol. Finl., 1:100 000. 87:98-127. Park, A. and Bowes, D., 1983. Basement-cover relation- Wise, D.U., Dunn, D.E., Engelder, J.T., Geiser, P.A., MAJOR PALAEOPROTEROZOIC SHEAR ZONES OF THE CENTRAL FENNOSCANDIAN SHIELD 223

Hatcher, R.D., Kish, S.A., Odom, A.L. and Schamel, Xu, J., Wang, P, Ching, R. and Ye, Z., 1986. Ductile de- S., 1984. Fault-related rocks: suggestions for terminol- formation and regional strain field in the southern seg- ogy. Geology, 12: 391-394. ment of the Tancheng-Lujiang Fault Zone, Eastern Witschard, F., 1984. The geological and tectonic evolu- China. In: C.Y. Wang (Editor), Internal Structure of tion of the Precambrian of northern Sweden, a case for Fault Zones. Pure Appl. Geophys., 124: 337-364. basement reactivation? Precambrian Res., 23:273-315.