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.