The , Rinna and Nappes of the - district, southwestern Region.

SIGBJ0RN KOLLUNG

Kollung, S. 1990: The Surna, Rinna and Orkla Nappes of the Surnadal-Orkdal district, south­ western . Nor. geol. unoers. Bull. 418. 9- 17.

In the district between Surnadal and Orkdal within the 'Gula', the rock sequence between the Late Precambrian metasandstones of the Nappe and assumed Lower Ordovician rnetavol­ canites of the Steren Nappe, three major nappes have been distinguished. These tectonic units are here termed the Surna Nappe, the Rinna Nappe and the Orkla Nappe and are correlated with the Essandsje (Seve), 0 yfjell (Koli) and Gula Nappes, respectively. Metapelites represent the most widely distributed lithology, with basic metavolcanites occurring in the Surna and Orkla Nappes. Metamorphi c grade generally decreases upwards: amphibolite facies in the Surna Nap­ pe, garnet grade in the Rinna Nappe, and garnet to biotite grade in the Orkla Nappe. The lithologies of the Orkla Nappe are divided into two formations; the Hoston Formation below, composed mainly of greenschists and phyllites and the overlying Formation which consists mainly of phyllites.

S. Kollung. Hertug Skules qt. 10. 0652 6

Introduction In the district between Surnadal and Orkda l, bohm (1 896) divided the western schists into in the western part of the Trondheim Region 3 units. The lowest unit is the Are schist, in­ (Fig.1, Plate 1), a sequence of metapelites and terpreted as crystal line 'sparagmite' , forming basic metavolcanites, the socalled 'Gula', has the Seve Nappe, with its type locality at Aresku­ been studied . The rocks are situated between tan, in Jarntland , . Howeve r, only the Late Precambrian metasandstones (sparag­ lowermost part of the Are schist, a pale mites) of the Tingvoll Nappe and Lower Ordov i­ quartz-m ica schist, is of 'sparagmitic' character . clan metavolcanites of the steren Nappe. Stu­ The remainde r of Tornebohrn 's Are schist, dies from the Surnadal synform, in the west­ including hornb lende schist and brown mica ern part of the district, have recently been gneiss, corresponds to the amph ibolite-fac ies presented by Kollung (1984) and Rickard rocks. The higher units are the R0roS and the (1985). From Surnadal, the rock sequence on Brek schists, which correspond with schists the northern side of the synform has been in the eastern part of th Trondheim Region , followed over 0vre to Orkdal , with the and in the western most portion of the central Hoston-Svorkmo area given special attention Gula Group , respectively. Carstens (1920, in the present study . This rock sequence , refer­ 1951) simplified this scheme drastically, in pla­ red to as the 'western schists ', has been sub­ cing the western schists in one large unit, the divided tectonostratigraphically, and its regio­ Reros Group , which he corr elated with the nal correla tion is discussed . The lower part Gula Group schist. Carstens recognised the consists of amphibolite-facies rocks , i.e. mica difference between the lower , brown , mica schists , gneisses and amphibolites, while the schists and the upper, grey-green schists , but upper parts are greenschist-facies lithologies, believed that the contact between them was i.e. chlorite -mica schist , phyllites and green­ of trans itional character. Rutter et al. (1967) schists. cons idered the phyllit ic rocks in the upper Diverging opinions have been presented part of the western schists to belong to the regarding the tectonostratigraphy of the Ork­ Hovin Group , the metased iments occurring dal area; some are shown in Table 1. Tome- above the steren volcanites. From studies in 10 Sigbjorn Kollung NGU -B ULL.41a. 1990

Fig. 1. Geological setting of the Surnadal - Orkdal district. 1-2. Autochthon . Rocks of mainly Precambrian age. 1. Basement. Gneisses and granite. 2. Cover. Metasa ndstone and quartz ite. 3-10 . Allochthon . Rocks of Precambrian to Silurian age overth rust during the Caledonian orogeny. 3. -Boa Nappe and equivalent nappes. Granitic to tonalitic gneiss; in the south including metasa ndstone and quartz ite. 4. Tinqvoj], Leksdal Nappe. Augen gneiss (below), metasandston e. 5. Surna, BIAho Nappe. Mica schist and amphibolite.In SE. including Kbli and Gula Nappes. with phyllite and greenschist. 6- 7. Gula Nappe. 6. Sinqsas Format ion. Calc-silicate schist and gneiss, with quartz ite and amphibolite. 7. Undal Formation. Phyllite. 8-9. Storen Nappe. 8. Store n Group. Greenstone and amphi­ bolite. 9. Lower Hovin, Upper Hovin and Groups. Phyllite, greywacke, conglomerate, limestone , rhyolite, greenstone . 10. Metabasalt , Smola. 11- 12. Caledonian intrusive rock s. 11. Diorite and gabbro . 12. Granite, granodiorite and quart z diori te. 13. Fosen Group. Rocks of Devonian age. Sandstone and conglomerate. Comp iled from Ramberg (1973), Wolff (1976), Sigmond et al. (1984), Askvik &Rokoengen (1985) and Gee et al. (1985), with revisions based on the mapping of the present author.

easte rn parts of the Trondheim Region, Wolff following Tornebohrn (1896), extended the (1967) opposed Carste ns' view of correlating Seve Nappe into southwestern parts of the the Reros and the Gula schists , and the west­ Trondheim Region; however, he included the ern schists were renamed the Gula Group. entire sequence of western schists in the Chaloupsky (1977) included both mica schists Seve. Prestvik (1983) recognised a thrust con­ and phyllites in this unit; he also considered tact between mica schists and phyllites. He basic volcanites occurring in both types of corr elated the mica schists with the Gula Group schists to represent infolded parts of the St0­ of the central part of the Trondh eim Region ren Group. In a regional study, Gee (1978), NGU - BULL. 418, 1990 The Surna, Rinna andOrkla Nappes 11

Table 1, Tectonostratigraphy of the Hoston-Svorkmo area compared with earlier interpretations from neighbouring areas.

Ttirnebohm 1896 Carstens 1920, 1951 Prestvik 1983 Present paper steren Steren Group steren Group Greenstone Complex Nappe Greenstone, tutfite

Gula Brek schist Orkla Svorkmo Formation Nappe Phyllite and metasandstone Grey-green =Gula Hoston Formation mica schist Phyllite Complex Ul Greenschist and phyllite U; :E Rinna R0ros schist 0 Ul Nappe Chlorite mica schist R0ros E = Kti/i l! Ul

Tectonostratigraphy From studies in eastern areas it has become A simplified geological map of the Surnadal­ clear that the 'Upper schist' is situated tecto­ Orkdal district is shown in Plate 1. In the nostratigraphically between the greenschist Surnadal synform (Kollung 1984), five main (unit 3) and the greenstone (unit 5), and that it rock units were distinguished: is separated from the greenstone by a thrust. 5. Greenstone, metalava Also, an additional tectonic boundary has been 4. 'Upper schist'. Phyllite/mica schist and lime­ recognised, between the Rinna schist and the stone greenschist. 3. Greenschist, metatuff The distribution of rock units in the Hoston­ 2. Rinna schist. Chlorite-mica schist and phyll­ Svorkmo area is shown in Plate 2, which inclu­ ite des a cross-section from Haukaskarnrnen to 1. Surna Group. Mica schist and amphibolite. Vasslivatnet. The tectonostratigraphy of the area is shown in Table 1 and compared with The idea that the amphibolite-facies Surna the interpretations of Tornebohrn (1896), Cars­ Group is separated from the lower-grade rocks tens (1920, 1951) and Prestvik (1983) from above by a major thrust (Krill 1980,1985) has neighbouring areas. The proposed correlations been accepted. (Krill's Surna unit comprises with the Gula, Koli and Seve Nappes are dis­ only the upper part of the Surna Nappe of cussed in a later section. The upper phyllite this paper). Also, large discordances are pre­ unit, including metasandstone, is referred to sent between the greenschist unit and undoub­ as the Svorkmo Formation and corresponds ted steren greenstone in the eastern continua­ to the 'Upper schist' in Surnadal. It lies with tion of the Surnadal synform, in the northern tectonic contact beneath the Steren volcanites limb at Trensdal (RAheim 1979) and in the in an open E-W synform between Ringevatnet southern limb in the Romundstad area. The and Svorkmo, here called the Ringevatnet tectonostratigraphic position of the 'Upper synform. The underlying greenschist unit, ter­ schist' could not be conclusively determined. med the Hoston Formation, includes phyllite, Due to early isoclinal folding, this schist is which is different from the corresponding for­ situated within greenschist in the western mation in Surnadal. The Svorkmo and the part of the Surnadal synform and within green­ Hoston Formations together form the Orkla stone farther to the east. Nappe. The Rinna Nappe is represented by a 12 Sigbjorn Kol/ung NGU· BULL. 418,1990

chlorite-mica schist, the lower of its two units fine-grained hornblende-porphyroblast schist, in Surnadal. The Surna Nappe is composed calcareous hornblende-porphyroblast schist, of mica schists, mica gneisses and amphiboli­ greenschist and amphibolite. The upper unit, tes. Lower units, the Tingvoll Nappe and Base­ which is mostly lacking in the north, consists ment, are shown undifferentiated in Plates 1 of a green to grey, fine-grained phyllite with & 2, but are not described in the text. A NNE­ biotite porphyroblasts. In the western part of SSW striking fault in the western part of Hosto­ Surnadal the phyllite grades into mica schist vatnet and Ringevatnet displaces most of the due to increasing metamorphism. In Surna­ rock units. It is referred to below as the Hos­ dal, within the Rinna schist, there is a zone ton fault. of dark greenschist, which is most continuous in the south. It may, however, represent an The Suma Nappe infolded part of the Orkla Nappe. In Surnadal and in the area east of the Hos­ ton fault, only the uppermost part of the Sur­ na Nappe is indicated on the maps (Plates 1 & 2). In the intervening area the unit attains The Orkla Nappe apparent thicknesses up to 2-3000 metres. The Hoston Formation. Basic metavolcanites The nappe is composed of mica schists and of the Hoston Formation occur extensively amphibolites, the former grading into gneisses throughout the district; east of the Hoston in the east. The thickest amphlibolites occur fault the formation includes phyllite. A lower in lower parts of the nappe, while micaceous phyllite at the base of the formation is up to rocks dominate in the upper part. 50 m thick, grey coloured, and has calcareous The mica schists and gneisses have a brow­ bands and thin intercalated beds of graphite nish colour due to abundant biotite. Garnet is schist. Near Hoston a ea. 10 m-thick limestone commonly present, in some areas together occurs at the top of the unit. with hornblende. Between the Hoston fault and The main metavolcanic unit has a maximum Gasvatn, a 10-50 m thick, white to brownish, thickness of about 500 m. It is dominated by pure limestone is interbedded with mica schist a dark, fine-grained, amphibolitic greenchist in the upper part of the nappe. The amphiboli­ with a finely banded structure which indicates tes, believed to be metavolcanites, are gree­ a tuffaceous origin. In thin-section the main nish-black in colour, commonly with abun­ minerals are plagioclase and green to brown­ dant, paler, keratophyric bands. Garnet is ish-green amphibole, with epidote and some common. Syn- to post-tectonic minor intru­ quartz and calcite. In the Romundstad area a sions of pegmatitic trondhjemite invade these paler green, tuffaceous schist constitutes the schists and amphibolites. A large trondhjemite upper and greater part of the unit. It contains body is situated south of Romundstad. Such plagioclase, quartz, chlorite, epidote, muscovi­ intrusions are absent in the overlying rocks. te, biotite ± calcite. In Surnadal there are, in some areas, more massive and paler coloured bands, thought to be rnetalava. The Rinna Nappe In a poorly exposed area east of Hoston the The Rinna Nappe has a maximum tectonic uppermost unit of the Hoston Formation con­ thickness of 300 m, and may be as thin as 20 sists of interfolded greenschist and grey to m on the northern side of the synform. It con­ green, partly calcareous phyllite. sists of two principal units. The main rock-type The Svorkmo Formation. This unit is up to in the lower unit is a rather coarse, quartz-rich about 500 m thick in the Hoston-Svorkmo chlorite-mica schist. This shows a grey, gree­ area and northwest of Rindal. On the south­ nish or brownish colour depending on the ern side of the synform the unit disappears 2 variable content of muscovite, chlorite and km east of Rindal. The main rock is a grey biotite. The schist is rich in garnet, locally also to green phyllite with quartz, muscovite and in hornblende porphyroblasts. Abundant thin chlorite as the principal minerals. Grey phyllite quartz veins and intense small-scale folding in the Hoston-Svorkmo area locally grades are characteristic features. In the east, the into black, graphitic phyllite. In the greater following additional rock-types are represen­ part of the district, biotite-rich schist is subordi­ ted: fine-grained, brown, biotite schist, quartz­ nate and garnet is scarce or absent. In Surna­ rich biotite-porphyroblast schist, quartzite, dal, the phyllite is commonly rich in biotite NGU-BULL.418.1990 TheSurne, Rinnaand Orkla Nappes 13 porphyroblasts, and westwards from Krokvat­ vatnet synform and in a narrow zone between net the biotite content in the groundmass in­ Valstad and Ringevatnet to the south of the creases, so that the unit grades into mica synform. In places, the tuffites alternate with schist. Garnet is fairly abundant here. Thin greenstone or occur as thicker units in which calcareous metapelites, phyllites and horn­ strongly schistose, phyllitic bands alternate blende-porphyroblast schist occur locally in with more massive, sandy layers with a granu­ and to the east of the Gasvatn area. lar texture. A granular, homogeneous tuffite Grey to white banded limestones appear is also present locally. The main minerals of between t.akseyan and the Gasvatn area at the tuffites are plagioclase, quartz, chlorite, two different levels, in the lower part of the epidote and muscovite. Even the greenstones formation and at the contact to the Steren may contain abundant quartz and muscovite. volcanites. In Surnadal, the upper limestone Gradational contacts between tuffite and gre­ occurs on the southern side of the synform, enstone are common. at two different structural levels; the lower one In the southernmost part of the map area within phyllite and the upper one within steren (Plate 2), greenschists, interpreted as meta­ greenstone or Hoston greenschist (Kollung tuffs, constitute an essential part of the vol­ 1984, fig.1). canite sequence. Interbeds of tuffite and gre­ Grey to grey-green metasandstones occur enstone are common. At Malisretra, the thick­ on the southern side of the Ringevatnet syn­ ness of this formation, which has a sharp form, at two levels. The upper sandstone, boundary with pillowed greenstone above, is which is up to 300 m thick, occurs at the about 400 metres. Both to the east and to the contact to the Steren volcanites. In addition west of Malisretra, the amount of pyroclastic to plagioclase, quartz, biotite and muscovite, material decreases, and greenstone becomes the sandstones have a rather high content of the dominant rock-type. epidote and chlorite, suggesting that basic Acidic volcanites are uncommon. In the rock debris formed a substantial part of the northwest, a grey to brown, fine-grained, schis­ original sediment. A schistose quartzite, 10 ­ tose quartz keratophyre with a faintly porphyri­ 50 m thick, occurs from Lakseyan to west of tic texture occurs in the Ringevatnet synform, R0rvatnet, close to a tight fold core containing at the base of the nappe. A few bodies of Steren greenstone. Quartzite bands are local­ hornblende gabbro intrude the lower volcani­ ly common in Surnadal, on the northern side tes. One large body, 4 x 2 km in area, occupi­ of the synform, whereas volcanites are scar­ es most of the western part of the Ringevat­ ce; a thin greenschist appears east of Laksey­ net synform. Except for foliated outer parts it an. Also east of t.akseyan, a 2000 x 800 m is mediumgrained and massive. Another gab­ lensoid body of pale green porphyritic trend­ bro located at the base of the nappe in the hjemite occurs within the Svorkmo Formation. south does not exceed 50 m in thickness, but is continuous from Valstad to near Malisretra. A large sheet-like gabbro, 6 km x 500 m, intru­ The Steren Nappe des lavas on both sides of Lomunda. In most areas, only the lower part of the St0­ The upper part of the Steren Group, occur­ ren volcanite unit is represented. Greenstone ring between 0vre Rindal and Romundstad, (metalava) is the most widespread lithology is composed of lavas with subordinate fine­ and is variably deformed, in part very strong­ grained basic intrusions and a large body of ly, and does not show any distinct pillow struc­ medium-grained hornblende gabbro west of tures. With increasing metamorphism west of Grenlivatnet. These lavas are also strongly Lomunda, and throughout Surnadal, dark deformed. They occur in a stacked pile of amphibolitic bands appear which commonly about 10 inverted thrust slices (Kollung 1984, show sharp contacts with the paler greensto­ fig. 1), and the thrusts can be followed over ne. The greenstone disappears west of Svor­ long distances towards the east. There, the ka, near the western limit of the map area, lavas are less deformed, with well-preserved due to the presence of an easterly plunging pillow structures. The inversion is indicated fold. by the pillow form and by an upward increase In the Hoston-Svorkmo area, this lower in the number of intrusions in each slice. unit also includes large amounts of clastic Both fine-grained gabbro and dykes are abun­ rocks. Tuffites are most abundant in the Ringe- dant. 14 Sigbjorn Kollung NGU· BULL. 418. 1990

In the Romundstad area, within the greensto­ be in contact with basement rocks (Kollung ne there are two tight folds containing pelites 1983). Within the present map area, Tingvoll of the Lower Hovin Group. The Hovin schist Nappe flagstone is up to 200 m thick near the is distinguished from the Svorkmo schist by Hoston fault, but pinches out at Kulia 6 km its typically low-grade metamorphic character; farther southwest, where granitic augen gneiss l.e., it is fissile and very fine-grained. underlies the rocks of the Surna Nappe. One kilometre southwest of Kulia, the lowest mica schist in the Surna Nappe wedges out, with Structures an angle of discordance of about 20°. The boundary between the biotite-rich Folds schists of the Surna Nappe and the chlorite­ Early isoclinal folds, F1, are common both in rich schists of the overlying Rinna Nappe is the Surna Nappe and in the overlying rocks. very pronounced. Also, the abrupt disappear­ This F1 folding led to a marked thinning of ance of the trondhjemitic pegmatites at the all rock units. In the GAsvatn and Ringevatnet contact to the Rinna Nappe is thought to be areas, steren greenstone and Svorkmo sedi­ a major argument for a thrust contact at this ments have been tightly folded in the F1 pha­ level. The strong tectonization along the con­ se, along ENE-WSW axes. tact varies from mylonitic, as seen in the river The above-mentioned two tight folds with BrandA west of Rl2ld, to a more brittle tectoni­ Hovin schist in the steren greenstone near zation distributed over a wide zone. The latter Romundstad are also considered to belong to type is most pronounced in the westernmost an early phase of deformation. In the same part of Surnadal, and south of Rinna, and is area, Rinna schist and Hoston greenschist are assumed to relate to movements later than the tightly folded. main thrusting. Large discordances were seen Folds of a later generation, F2, are coaxial at two localities. West of t.akseyan, an arnphi­ with F1, trending about E-W. The main Surna­ bolite in the Surna Nappe is cut out at an dal synform is ascribed to this phase (see angle of about 25° beneath the Rinna Nappe. also Rickard 1985). In the west, the southern Near the farm Heggem, Romundstad, where limb of the synform is inverted, but it reverts to the contact is exposed, the foliation in the a normal attitude about 4 km west of Rindal. Surna schist is oriented at 335/45°, and in the The western closure of a second-order F2 the Rinna schist at 380/45°. fold is found west of Qlvre Rindal. This fold Strong tectonization is also present at the is the main structure in an area to the south base of the Orkla Nappe, as seen on Hosto­ of the present one. It verges to the north and kammen, at Berge and at Qlstre Tokstad. On is inverted (Prestvik 1983). The central part the southern side of the synform mylonitic of the Hoston-Svorkmo area has been strong­ contacts are found in the Kjerkholten-Stomp­ ly influenced by F2 folding, the open Ringevat­ red area, 2 - 4 km east of Romundstad and net synform belonging to this phase. Minor outside the map area. Both the Rinna schist folds vary in style from tight to open and show and the overlying Hoston greenschist are variable axial surface dips. strongly deformed and extremely fissile near Late F3 folding along NE-SW axes has defor­ the contact. med the western most part of the Ringevatnet Indications of thrusting below the greensto­ synform. Furthermore, open to gentle, N-S F3 ne of the Steren Nappe are abundant. In the folding farther north has affected the Rinna western parts of the Ringevatnet synform and and Orkla thrusts. in the zone Ringevatnet Valstad, the Svorkmo phyllite is strongly crushed and the greenstone converted into a phyllonite. Likewise, phylloniti­ Thrust faults zed greenstone occurs west of GAsvatn and Thrust contacts occur below each of the four in some areas in Surnadal. Tectonic discordan­ large rock units described. Mapping west of ces are common between greeenstone and the present area has revealed several examp­ rocks of the Svorkmo Formation in Surnadal, les of discordant relationships between the and are also present at GAsvatn. At Trensdal Surna Nappe and the underlying rocks. The northeast of Rindal, a discordance is present rocks of the Surna Nappe overlie different between greenstone and Hoston greenschist members of the Tingvoll Nappe, or may even (RAheim 1979). In the Romundstad area, a NGU· BULL. 418. 1990 The Sums, Rinna and Orkla Nappes 15 major discordance between greenstone and Group by Peacey (1963). It consists of horn­ greenschist can be observed over a distance blende-mica schists and amphibolites. and is of 4 km, and the greenstone directly above underlain by psammites and semi-pelites of the thrust is strongly deformed. A clear con­ the Songsj0 Group, corresponding to the Ting­ tact relationship can be seen above Heggem, voll Nappe. A thrust fault marks the contact with the greenschist oriented at 320/20° and between these two rock units. the greenstone at 355/35°. Mica schists and amphibolites of the Surna The basal thrust to the steren Nappe is Nappe also occur in two large synformal struc­ clearly a very early deformation structure, as tures farther to the west, of which the south­ it is deformed by the F1 folds. If the interfol­ western one is known to the present author ding of the Rinna schist and the Hoston green­ (Kollung 1983). Peacey (1963) erroneously inc­ schist at Hornundstad is of the same age, luded the Surna rocks here in the Songsj0 then the same holds for the Orkla Nappe. Group; Johnsen (1979) termed them the Sjura­ Quite possibly, the thrusting may be almost sen Group, and on account of lithological diffe­ coeval with the folding. The age relationship rences he separated this group from the Gan­ between F1 folding and thrusting of the Rinna gasvann Group in the southeast. In both are­ and Surna Nappes has not been determined, as, however, Johnsen's Sjurasen and Ganga­ but the thrusts are deformed by the F2 folds. svann Group occupy the same tectonostratig­ The imbricate thrusts within the upper gre­ raphic position, that of the Surna Nappe. In a enstone in the area between 0vre Rindal and geological map compiled by Ramberg (1973), Romundstad (Kollung 1984, fig.1) are not affec­ based purely on lithological criteria, the meta­ ted by the main F2 fold and are accordingly of pelites of the Surna Nappe in Orkdal were a later age. They are, however, deformed by designated 'biotite schist'. This rock-type oc­ gentle, transverse F3 folds (but not within the curs extensively within Ramberg's (1973) map map area). The greenstone close to these area. thrusts is generally mylonitic, and the thrust character is also revealed by the abrupt tran­ The - Dombas district section of the fine-grained intrusions below the A tectonic boundary corresponding to that thrust planes. Chloritization is a characteristic above the Surna Nappe in the Surnadal­ feature, and the rock also contains dissemina­ Orkdal district has earlier been recognised ted calcite. farther south on the western side of the Trond­ heim region. High-angle faults In the Oppdal area, a major tectonic discor­ These late structures are found in areas east dance has been recognised between a lower of Rindal. The tauits strike N-S to NNE-SSW. complex including flagstone, mica schist and Along the Hoston fault, the eastern block has amphibolite in the west, and an upper comp­ subsided, with an apparent displacement up lex metamorphosed under low-grade conditi­ to 5 km. This fault continues over a long dis­ ons, in the east, with greenstones, green tance to the north, but dies out rapidly to the metasandstones and schists (Holmsen 1955). south. Displacements of up to 1500 m occur Farther south, in the Dornbas-Lesja area, along a fault in the Lomunda valley, and along Guezou (1978) defined the western boundary a fault west of Trensdal, The fault at Romund­ of the Trondheim Nappe Complex by the same stad has an apparent displacement of about tectonic contact, Le. between the Andberqshel 1 km. Complex, including amphibolites and mica schists of the Bottheim Group in the upper part, and the Stakah0i Group. While the latter is an equivalent to the Gula Group, the Bott­ heim Group was compared with the Seve Correlations unit. In the Oppdal-Surnadal area, Krill (1980, 1985) separated two nappes of mica schists Northwestern Orkdal and amphibolites, the lower Blah0 and the Outside the map area, the Surna Nappe east upper Surna Nappe, the latter distinguished of the Hoston faUlt is of great thickness, and by its abundant bodies of trondhjemite. In an continues northeastward to Orkdalsfjorden uppermost tectonic unit, the Tronqet-Steren (Fig. 1). This unit was termed the Ganqasvann Nappe, there are phyllites and basic volcanites. 16 Sigbjorn Kollung NGU-BULL.418,1990

Table 2. Tectonostratigraphic correlation between the Surna­ Comparing the rest of the Gula with the dal - Orkdal district and the central/eastern Trondheim Region. According to Roberts & Wolff (1981) and Nilsen Hoston Formation, there are significant diff­ (1988), the stereo and MerAker Nappes constitute the sa­ erences. The Hoston phyllite is more calcare­ me, uppermost unit. Nilsen (1988) has informally combined the Essandsje and l2lyfjell units into one - the Essandsje­ ous than the main Svorkmo phyllite, and may 0yfjell Nappe. be compared to the calcareous schists in the Compiles from Gee et al. (1985) middle Gula unit, the SingsAs Formation (Nil­ and Nilsson (1988) Present paper sen 1978). Also, basic volcanites, here conside­ Central and eastern red to be correlatable with the Hoston green­ Trondheim region Surnadal- Orkdal District schist, are present in the SingsAs Formation, steren Nappe steren Nappe although only as thin, discontinuous layers Gula Nappe Orkla Nappe (Nilsen 1978). There is thus a decreasing volu­ Undal Formation Svorkmo Formation me of volcanic rocks in the Hoston/SingsAs SingsAs Formation Hoston Formation Asli Formation Formations, from Surnadal and Rindal, where greenschist is the only rock, to Orkdal, where MerAker Nappe both pelites and volcanites are thick, to the l2lyfjellNappe Rinna Nappe central Trondheim region where the volcanites Essandsj" Nappe Surna Nappe form only a minor proportion of the sequence. Remsklepp Nappe TingvolI Nappe A further argument for correlating the Hoston and SingsAs Formations is that no tectonic Osen-Rea Nappe breaK has been observed between the SVOfK­ Parautochthon mo and Hoston Formations. The eastern Gula Autochthonous cover pelites, the Asli Formation (Nilsen 1978), may Basement Basement possibly be represented by the lowest phyllite of the Hoston Formation. As recognised by Kjerulf (1871) and Torne­ Central andeastern Trondheim region bohm (1896), there are lithologies within the Comparing the Surnadal-Orkdal district with western schists which are similar to the R0ros central and eastern parts of the Trondheim schist in eastern Trondelaq. This is a Koli region, there are many striking similarities in unit, forming the 0yfjell Nappe (Wolff 1979, rock succession; some units are SUbstantially Roberts & Wolff 1981). The R0ros unit is com­ thicker, and many units occur which are not posed mainly of garbenschist and biotite­ present in the west (Table 2). The pyroclastic porphyroblast schist, Le. the Stuedal schist rocks in the steren Group of the Hoston­ (Kjerulf 1871, Reusch 1890, Tornebohrn 1896, Svorkmo area are correlated with tuffites for­ Bryn 1959). The Rinna schist in the Hoston ming the eastern and possibly oldest part of Svorkmo area is, in places, rich in hornblende the Steren Group sensu stricto, the Elgsj0 porphyroblasts, while an upper unit of the Formation of Nilsen (1978,1983). These tuffites Rinna Nappe consists of biotite-porphyroblast form a continuous zone between Steren and phyllite. In the underlying Essandsj0 schist Hjerkinn, in direct contact with rocks of the (Bryn 1959) or Essandsj0 Nappe (Wolff 1979), Gula Nappe to the east. equivalent to the Seve Nappe, there are litholo­ It has been shown, in the present account, gies which are similar to those in the Surna that the 'western schists' can be subdivided Nappe, Le. amphibolite, mica schist and meta­ into the Surna, Rinna and Orkla Nappes. The sandstone. Gula Group of central Trendelaq is bounded by pelitic units, while the middle part consists Conclusions mainly of calc-pelitic to calc-psammitic schists The long neglected three-fold division of the and gneisses (Wolff 1973, Olesen et a1.1973, 'Gula' in the southwestern Trondheim Region, Nilsen 1978). The western, partly graphitic proposed by Tornebohrn (1896), has been pelites, the Brek schist or the Undal Formation shown to be basically correct, and should be (Nilsen 1978), were correlated by Tornebohrn revived. The rock sequence in this district (1896) with the upper unit of the western belongs to three nappes, termed the Surna, schists, that is the Svorkmo Formation. Consi­ Rinna and Orkla Nappes, and these are corre­ dering the similarity in lithology and metamor­ lated with the Essandsj0 (Seve), 0ytjell (Koli) phic grade between the two formations, this and Gula Nappes of the eastern and central seems to be a well founded correlation. Trondheim Region. NGU-BULL.418,1990 The Sume, Rinna andOrkla Nappes 17

Acknowledgements NIIsen, O. 1978: Caledonian sulphide deposits and minor I am grateful to Orkla Industrier A.s for financing the work, iron formations from the southern Trondheim region, and to G.Grammeltvedt for discussions and help. I am also . Nor.geol.unders. 340. 35-85. indebted to M.Stephens and A.Krill for critically reading an NIIsen, O. 1983: The nature and tectonic setting of melan­ early version of the manuscript and correcting the English. ge deposits in , near steren, Central Nor­ Three anonymous referees recommended several improve­ wegian Caledonides. Nor. geol. unders. 378. 65-81. ments to a later version. NIIsen, O. 1988: The tectonstratigraphic setting of strata­ bound sulphide deposits in the southern Trondheim region, Central Norwegian Caledonides. Nor.geol. unders. Bull. 412. 55-66. Olesen, N.O., Hansen, E.S., Kristensen, L.H & Thyrsted. T. 1973: A preliminary account on the geology of the - area, the Trondheim region, Central Nor­ References wegian Caledonides. Leidse Geol. Meded. 49. 259-276. Askvik, H. & Rokoengen K. 1985: Geologisk kart over Peacey, J.S. 1963: Deformation in the Ganga.svann area. Norge, berggrunnskart 1:250000. Nor. Nor.geol.unders. 223. 275-293. geol. unders. Prestvik. T. 1983: Rapport vsdrerende kartlegging i Dragset­ Bryn, K.0. 1959: Geologien pa. sendre del av Kartblad ornradet i , S0r-Tr0ndelag. Unpubl.report Geolo­ Essandsje. Nor. geol. unders. gisk tnst., Universitet i Trondheim. 205.5-16. Ramberg, H. 1973: Beskrivelse tll berggrunnsgeologisk Carstens, C.W. 1920: Oversigt over Trondjemsfeltets berg­ kart over streket - Hemnefjord, S0r-Tr0nde­ bygning. Kgl.Nor.Vidensk.Se/sk. Skr.1919. 1.152 pp. lag. Nor.geol.unders. 299. Carstens, C.W.1951: Lekksnteltets geologi. Nor.Geol. Reusch, H. 1890: Geologiske iaktagelser fra Trondhjems Tidsskr. 29. 9-25. Stift. Kgl. nor. Vidensk.Se/sk. Skr. 7. 1-60. Chaloupsky,J.1977: H010nda. Berggrunnskart 1521 11. Nor. Rickard, M.J. 1985: The Surnadal synform and basement geol.unders. gneisses in the Surnadal- district of Norway. Gee, D.G. 1978: Nappe displacement in the Scandinavian In Gee, D.G. & sturt, BA (eds.). The Ca/edonide oro­ Caledonides. Tectonophysics 47. 393-419. gen Scandinavia and related areas. John Wlley & Sons, Gee. D.G., Guezou. J.C.• Roberts, D & Wolff, F.C.1985: The Chichester, 485-498. central-southern part of the Scandinavian Caledonides. Roberts, D. & Wolff, F.C. 1981: Tectonostratigraphic deve­ In Gee, D.G. & Sturt, BA (eds.) The Ca/edonide Orogen lopment of the Trondheim region Caledonides, Central - Scandinavia and related areas. John WHey & Sons. Norway. J. Struct. Geol. 3. 487-494. Chichester. 109-134. Rutter. E.H.,Chaplow, R.E.& Matthews, J.E.1967: The geolo­ Guezou, J.C. 1978: Geology and structure of the Dombas­ gy of the Lekken area, S0r-Tr0ndelag. Nor.geol. Lesja area. southern Trondheim region. south central unders. 255. 21-36. Norway. Nor.geol.unders. 340. 1-34. Ra.heim, A. 1979: Structural and metamorphic break betwe­ Holmsen, P, 1955: Trekk av Oppdalsfeltets geologi. Nor. en the Trondheim basin and the Surnadal synform. Geol. Tidsskr. 35. 135-150. Nor. Geol. Tidsskr. 59. 195-198. Johnsen, S.0.1979: Geology of the area west and north­ Sigmond. E.M.O., Gustavson, M.& Roberts, D. 1984: Berg­ west of Orkdalsfjorden, ser-rrendetaq. Nor.geol. grunnskart over Norge 1:1 million. Nor. geol. unders. unders. 348. 33-46. Tornebohm, A.E.1896: Grunddragen af det centrala Skandi­ Kjerulf, Th. 1871: Om Trondhjems stifts geologi. Nyt Mag.f. naviens bergbygnad. Kgl. Sv. Vet.Akad.Handl. 28. 210 Naturv. 18. 1-79. pp. Kollung, S. 1983: Geologiske underseketser pa. kartblad Wolff, F.C. 1967: Geology of the Mera.ker area as a key to t.ekken. Unpubl.report Orkla Industrier A.s. the eastern part of the Trondheim region. Nor.geol. Kollung, S. 1984: The Surnadal syncline revisited. Nor.Geol. unders. 245. 123-146. Tidsskr.64, 257-262. Wolff, F.C. 1973: Mera.ker og Feeren. Beskrivelse tll de Krill, A.G.1980: Tectonics of the Oppdal area, central Nor­ berggrunnsgeologiske kart (AMS M711) 1721 I og 1722 way. Geol.Foren. Stockh. rem. 102. 523-530. 11 - 1: 50 000. Nor.geol.unders. 295, 42 pp. Krill, A.G.1985: Relationships between the Western Gneiss Wolff, F.C.1976: Geologisk kart over Norge, berggrunns­ Region and the Trondheim Region: Stockwerk tectonics kart Trondheim 1:250 000. Nor. geol. unders. reconsidered. In Gee, D.G. & Sturt BA (eds.): The Wolff, F.C.1979: Beskrivelse tll de berggrunnsgeologiske Caledonide Orogen - Scandinavia and related areas. kartbladene Trondheim og 0stersund 1: 250 000. Nor. John Wlley & Sons, Chichester. 475-484. geol. unders. 353. 77 pp.

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~ I I ,' d 1 I ' .-u I I ' 13 en ';:: I ' ;;; 0 1 I ~ Ci 0 I ' <;; ' 1 ' "0 ClJ ~ en 1' 0 1. ' I ( <;; "1::J '\. "0 1 E'" ClJ :::l - (f) 4- \ 1 ~ '\.1 -5" CL '5 c- E ''\.1 .- 11: E'" Vl 1 ~ '8' '0 Cl "0" " { Z .'~~"\ Ii.E Ui ,..: ~ 0: NGU Bulletin 418 - Kollung - PI. 2 Geological map of the Hoston­ Svorkmo area . . .:-...;... ~. . .1ggend o Quat ernary deposits Sh"ren Napp~ N r=:m Met agabbr o Basic metavolcanites Met at uf fit e ± basic / metavolcanites + + ~..... "' ':::..... + / Quartz keratophyre + + + + + l.6rrncY Orkla Napp~ + + + + + + Svorkmo For mati on ~ + + + + + + Trondhjemite Hostovatnet Cd + + + + ~ Quartz ite + + c::::J + + + () 1>: <1 Met asandst one + + + c=:===l ,--- ~ Gr eenish calcareous phyll ite Jakopsm ~ rd ~ / ~ ~ Greenschist / ~ ~ Metalimest one --- Grey t o black phyllite D------Host on Formati on -- -- ~ Met alimest one ~ ~ Greenschist Grey t o green, partly ~ calcar eous phylli te Rinna Napp~ Greenschist

Quartzite Chlor it e-mica schist, biotite schist, hornblende porphyroblast schist Surna NapPL Met alimest one Mica schist and gneiss / wit h t rondhj emit e int r usions

vvvvv + vvvv .v Amphibolit e / wit h trondhj emite v v v v. v, intrusions Tingvoll N a p p~ Und ifferentia ted gneisses and schist Basemen t Undif f er ent iat ed granitic gneiss and granite Schistosity Section Haukas kammen Vasshvatnet Lineati on Thrust-fnu lt

0 I -, Ha ukas-I ", I" -, High-angle f ault kammen 11'\'1 \ \ ll \ v, '\ \ J akO p5 m~ r a Vorma Lian A1-----1 B Section line A I \ ' \ , \ s:

unexposed area

Plate 2. Geological map of the Hoston - Svorkmo area.