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Tectonics of the Oppdal area, central Norway

Tectonics of the Oppdal area, central Norway ALLAN G. KRILL
GFF Krill, A. G., 1980 1231: Tectonics of the Oppdal area, central Norway.
Geologiska Foreningens i Stockholm Förhandlingar, Vol. 102, Pt. 4, pp. 523-530. Stockholm.
ISSN 0016-786X.

·  In the Oppdal area, a sequence of Caledonian thrust nappes was metamorphosed, recumbently folded, and refolded during the Caledonian orogeny.
The well known Oppdal augen gneisses are Proterozoic rocks belonging to a single nappe unit in the tectonostratigraphy. Another distinctive nappe unit was formed from feldspathic sandstone, intruded in late Proterozoic time by pre-tectonic dolerite dikes. Despite the new thrust interpretations, the early concept of ductile infrastructural deformation below a more rigid superstructure may still be accurate here in one of Wegmann's type localities of the Stockwerk tectonic model. ( Areal geology, tectonics, lithostratigraphy thrust faults, nappe, recumbent folds, augen gneiss, dolerite, dykes, metamorphic rocks, Caledonian Orogeny. Trondheim Nappe Complex, Western Gneiss Region, Proterozoic Lower Paleozoic. Oppdal, southern Sør Trøndelag, south central Norway N6217 N6048 E0944 E0902.

Allan G. Krill, Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, USA. Present address: Mineralogisk-Geologisk Museum, Sarsgate 1, Oslo 5, Norway;15 May 1980.

The Oppdal area includes the eastern margin of the Western Gneiss Region and the western margin of the Trondheim Nappe Complex in Central Norway. Here the 1960 Geological Map of Norway shows a succession of Proterozoic gneiss, "Eocambrian" metasediment, augen gneiss, and Lower Paleozoic schist, all subject to complex Caledonian folding (Holtedahl & Dons 1960; cf. Holmsen 1955).
Detailed work by several authors in the past twenty years has significantly improved our knowledge of this area, but doubts have been raised about the Caledonian interpretation of the structure. Thus it has recently been suggested that the medium-grade metamorphism and associated structures of the Oppdal area are all Proterozoic, because the rocks appear to be structurally and metamorphically continuous with Proterozoic rocks to the west. This new interpretation was further supported by arguments that Proterozoic Rb-Sr whole-rock isochrons of gneisses in the Oppdal area record metamorphic ages (Solheim 1977; cf. Råheim 1977).
As the details of the stratigraphy, structure, and geochronology of the Oppdal area become better resolved, the Caledonian tectonic history is becoming more certain. The Oppdal area involves an autochthon and parautochthon of Proterozoic basement and younger sedimentary cover, overlain by a sequence of Caledonian nappes involving both Proterozoic and Lower Paleozoic rocks. The whole succession of rock units was metamorphosed and recumbently folded together in "Pennine" style during the Caledonian orogeny.

Tectonostratigraphy

Autochthonous and parautochthonous units

·  Lønset unit. - The Lønset basement gneiss complex consists of orthogneisses, mainly of granitic and granodioritic composition. The gneisses vary from homogeneous to compositionally layered and from weakly to strongly foliated. They are generally medium grained; small K-feldspar augen are present, but are only very rarely larger than a centimeter across. Younger mafic and felsic dikes and other small intrusive bodies are common. Generally these intrusions too are strongly deformed and are concordant with the surrounding gneissic foliation, but some of the intrusions cut earlier metamorphic foliations. In an attempt to date the basement gneiss, six Rb-Sr whole-rock analyses have been made (Solheim 1977), but the interpretation of the data is uncertain. The points are scattered and do not define a true isochron, but they seem to indicate a Proterozoic age with a low initial 87Sr/86Sr ratio.
Mineral separates from rocks in the Oppdal area consistently yield isochrons with Caledonian ages (Solheim 1977). The absence of acid intrusive rocks in the overlying metasedimentary cover suggests that the intrusive events were confined to the earlier Proterozoic, and that the foliations cut by the intrusions are relicts of Proterozoic structures.

·  Åmotsdal unit. - Metamorphosed feldspathic sandstone, arkose, and quartzite form a psammitic cover on the basement gneiss, and are here named the Åmotsdal unit (after the seter Åmotsdal). These psammites are similar to the Norwegian 'sparagmites' farther southeast in the Mjøsa area (Bjørlykke et al. 1976) and are presumably also of late Proterozoic age. Conglomerates are not found upwards within the psammites, but basal conglomerates containing cobbles of quartzite and basement gneiss are recognized in several places. Cross beds preserved in three areas face upwards, away from the basement. No tillite has been recognized in the Åmotsdal unit, but south of Gjevilvatnet a few large boulders of gneiss in well sorted fine-grained sandstone may have been rafted by ice.
North of Gjevilvatnet the Åmotsdal unit also includes marble and kyanite-bearing graphitic schist (Midtre Kam Formation, Hansen 1971). These distinctive rocks may be metamorphic equivalents of Cambro-Ordovician limestone and alum shale along the Caledonian front in Sweden and in the Oslo region (Gee l979).
The Åmotsdal psammites are generally very strongly foliated, but these flagstones are not quarried commercially. The contact with the basement rocks is generally deformed into complete structural concordance and is difficult to establish within a few meters. In several areas the basement-cover sequence is repeated, or even repeated several times. Major repetitions of the parautochthonous Lønset gneiss appear as slices within the Åmotsdal unit. In the valley Åmotsdalen itself, map pattens and stratigraphic symmetry indicate that some repetition is due to folding, but in other parts of the area it is probably due to imbricate thrusting or a combination of folding and thrusting.

Allochthonous units

·  Risberget unit. -- The Risberget unit is characterized by coarse-grained augen gneiss. Common subordinate rock types are felsic gneiss, gabbro or metagabbro (amphibolite), anorthosite-gabbro and anorthosite. Massive unfoliated rapakivi granite with oligoclase rims on large K-feldspar megacrysts is preserved in several places (cf. Carstens 1924). Foliated rapakivi augen gneiss is also common, and locally contains anorthosite xenoliths.
The nature of the Oppdal augen gneiss has been much discussed. Carstens (1925) advocated an originally magmatic origin for the rapakivi gneiss. Rosenqvist (1943) postulated granitization, and most of the recent authors and mappers in the Oppdal area (eg. Holmsen 1955; Hansen, 1971) have accepted modifications of this interpretation, at least for the common augen gneiss without rapakivi rims. According to these interpretations, both igneous and sedimentary rocks were transformed to coarse augen gneiss by massive migration of K, Si, Fe, Mg, and other elements. Similar augen gneisses elsewhere have been interpreted as strongly deformed Proterozoic orthogneisses (Point 1975; Röshoff 1978).
The map pattern in the Oppdal area suggests that both the rapakivi and non-rapakivi augen gneiss are confined to a single tectonostratigraphic unit. Petrologic, textural, chemical, and isotopic results indicate that both types of augen gneiss are strictly meta-igneous rocks, and that the K-feldspar augen have not grown since the origina1 magmatic event (Krill 1980).
"Grada-tional" contacts between the augen gneiss and other rocks were produced by tectonic cataclasis, and repeated contacts are presumably caused by faulting or folding, like the repetitions of other rock units in the area. Recent Rb-Sr dating of whole-rock augen-gneiss specimens and of whole-rock specimens consisting of single large augen yielded errochron ages in the range 1450-1750 Ma (87Rb ( = 1.42 x 10 -11 a -1; Solheim 1977; Krill 1980).
Low initial ratios suggest that the isotopic values are magmatic. Detailed Rb-Sr study of rapakivi granite and gneiss shows that the whole-rock isotopic systems are easily disturbed, but that the ages are not easily reset even by strong metamorphism and deformation ( Krill 1980).
Similarly, pre-tectonic igneous ages are common from orthogneisses in the Pennine Zone of the Alps (e.g. Jäger 1970). Relicts of Proterozoic metamorphic foliation are rarely identified by the presence of crosscutting granitic rocks. A possible Proterozoic high-grade metamorphism is suggested by a small amphibolite body with plagioclase-garnet- hornblende corona texture, which is converted to presumably Caledonian, biotite-calcite-epidote schist where it is strongly foliated. The present position of this Proterozoic orthogneiss complex above the presumably younger Åmotsdal metasedimentary unit is best explained by thrust transport, and the entire Risberget unit is considered allochthonous. The Risberget unit can be closely correlated to the Tännäs Augen Gneiss Nappe occurring in the same tectonic position in Sweden (Strömberg 1961; Gee 197 5).

·  Sætra unit. - The Sætra unit consists of strongly foliated metasandstone with thin biotite-epidote layers and thin sheet-like bodies of amphibolite. The Sætra unit lies above the Risberget unit in the tectonostratigraphy of the Oppdal area. All the flagstone quarries in this area, and probably most of those elsewhere in this part of Norway, exploit Sætra rocks. The primary character of the rocks is recognized in the least deformed areas, as locally among the flagstone quarries near "Sætra" farm, south of Oppdal. The least foliated feldspathic sandstones contain sedimentary cross beds, consistently facing away from the Risberget complex. Where best preserved, these cross beds and related sedimentary features strongly suggest fluvial deposition, and the generally high quartz content also suggests either a non-marine or a very shallow marine environment on a continental platform. Where the amphibolite sheets are least deformed, their origin as dolerite dikes is apparent.
Three unfoliated amphibolite dikes preserve relict igneous minerals, textures, and low-potas- sium tholeiitic composition. The three dikes together give an eleven point Rb-Sr whole-rock igneous age of 745 ( 37 Ma (87Sr/86Sr0 = 0.7046), and separately, two of the dikes each give comparable ages ( Krill 1980). Cross beds and sedimentary layering are locally preserved in low-strain areas near more rigid amphibolites, demonstrating that the sandstones were not penetratively deformed before the dikes were intruded in late Proterozoic time.
Hundreds of thin amphibolite sheets and biotite-epidote layers derived from them are found in the Sætra flagstones throughout the Oppdal area. In the Risberget complex, on the other hand, some of the amphibolites are clearly metamorphosed gabbro bodies, and none appear to be related to the dolerite dikes in the Sætra. No amphibolite or related rocks have been recognized in the Åmotsdal unit, demonstrating that the Sætra dikes were not intruded through the underlying basement and psammitic cover, and that the Sætra unit must be entirely allochthonous within the Oppdal area. All aspects of the Sætra unit - the type of psammite, tectono- stratigraphic position, dike chemistry, and Rb-Sr geochronology - correlate remarkably well with the Särv nappe of Sweden (Strömberg 1961; Solyom et al. 1979; Claesson 1976).
The amphibolites of the Sætra unit were recognized as dolerite dikes by Bjørlykke (l905) and by most subsequent workers in the Oppdal area. They were generally considered to be syn- tectonic intrusions and their abundance and re-stricted tectonostratigraphic position was not recognized. Closer study of the Sætra dikes was encouraged by recent emphasis of the tectonic significance of the Ottfjället dolerite dikes of the Särv Nappe (Gee 1975).

·  BIåhø unit. - The Blåhø unit lies tectonically above the Sætra unit. It consists dominantly of garnet-mica schist and amphibolite; less common are potassium-deficient psammitic schist and gneiss, micaceous quartzite, marble and serpentinite. Trondhjemite has not been recognized here, but pre- or syn-metamorphic trondhjemite dikes are abundant in Blåhø equivalents in the Surnadal Synform, north of the Oppdal area (Lønset 1969. Råheim 1977). Ultramafic bodies are also abundant in Blåhø- type rocks south of the Oppdal area ( Scott 1967. Prost et al. 1977).
I have inferred that the Blåhø unit is thrust on the underlying Sætra on the basis of lithologic and stratigraphic considerations. The assemblage of rocks from which the Blåhø was formed (pelites, basic volcanics, ultramafics) strongly suggests an oceanic environment. It is considered unlikely that such rocks were deposited directly on the sandstones of the Sætra unit, or that the Blåhø ultramafics (now serpentinites) were intruded through the continental crust that presumably underlay the Sætra. Locally however, the contact between the Blåhø and Sætra appears "gradational", with psammitic schist and quartzite occurring low in the Blåhø section.
The Blåhø schists are the most useful rocks in the Oppdal area for determining metamorphic grade. Garnet-grade conditions were reached everywhere. South of Oppdal, in rocks of pelitic composition, staurolite and kyanite occur as minor metamorphic phases and large kyanite blades are found in late, cross-cutting quartz veins. Compositions of co-existing biotite and garnet of Blåhø rocks in the center of the Oppdal area suggest metamorphic temperatures in the range 500-550(C ( Eggen 1977).
Retrogressive effects are most easily seen in the Blåhø schists, and chlorite partially replacing biotite, hornblende, and garnet is common. Chloritized rocks occur in fault zones, especially in the eastern part of the Oppdal area, and such zones separate the eastern Blåhø belt from the underlying Sætra and Risberget rocks. The age of the Blåhø rocks is uncertain, but Rb-Sr whole-rock studies have yielded only scattered Caledonian-age patterns ("Røros Group", Råheim 1977. Solheim 1977; Krill 1980).
The Blåhø unit is correlated with the "Røros" schists of Surnadal ( Råheim 1977) and the Bottheim Group of the Andbergshøi Complex (Guezou 1978) in the Western Gneiss Region. It might also be correlated eastward with the Gula Group (Wolff 1976) or with the Seve Nappe (Guezou et al. 1972; Prost et al. 1977; Gee 1978), and may include parts of both units.

·  Tronget unit. - The overlying Tronget unit involves rocks of the Trondheim Nappe Complex in the eastern part of the Oppdal area. The range of rock compositions in the Tronget unit is similar to that in the Blåhø, but Tronget rocks are metamorphosed to a lower grade and less strongly deformed. Primary sedimentary and volcanic textures are commonly preserved. Phyllite, schist, metagreywacke, and volcani-clastic metasediments are common, and lime- stone, quartz keratophyre, greenstone, greenstone conglomerate, and pillow lava also occur (Holmsen 1955). There is a large body of trondhjemite north of Oppdal (Holmsen 1955), and west of Oppdal, near the Blåhø contact, is a serpentine conglomerate.
The Tronget unit is considered allochthonous above the Blåhø. Low- and high-angle faults generally separate the Tronget unit from the underlying rocks. These faults differ from the other faults in the area in being syn- or post- metamorphic; cataclastic textures with local pseudotachylite are found within the fault zones.
Tronget rocks mainly reached only the biotite zone of greenschist facies metamorphism, but Tronget rocks are locally garnet-bearing near the Blåhø contact. The westward increase from biotite to garnet zones shown by Goldschmidt (1915) is generally accurate, although most of his garnet-zone rocks are in the Blåhø unit below the later faults.

Structure

Despite general geological evidence of early major thrusting, during which the units just described were stacked into tectonostratigraphic succession, structural and textural evidence of the thrust faults is sparse and inconsistent. Later metamorphism and deformation has apparently obliterated most of the early fault evidence.
Fluxion textures and cataclastic features are found in all the rock units, but they are not specifically located at unit boundaries and can all be attributed to syn-metamorphic strain.
After the early thrusting, the resulting tectonostratigraphic succession of rock units in the Oppdal area was metamorphosed and folded in a pair of major recumbent folds. The anticlinal fold hinge of this pair is found within the map area west of Gråhø. The anticlinal hinge is the intersection of the root zone of the recumbent fold nappe and the present-day land surface. To the south, the root zone is removed from the top of the Lønset dome by erosion, whereas to the north it is hidden by overlying rocks. Where it is seen near Gråhø, the Åmotsdal unit forms the core of the isoclinal anticline, and the Risberget, Sætra and Blåhø units occur successively outward. Numerous sedimentary crossbeds are well preserved in the center of the anticline, showing that the psammites had not been penetratively deformed before this folding. The easternmost exposed segment of the recumbent anticline, here synformal, is represented by the eastern strip of the Risberget unit, bordered symmetrically by the Sætra and Blåhø units both above and below.
The Sætra rocks of the well known flagstone quarries south of Oppdal are in the inverted limb of the recumbent anticline, and cross beds seen near the quarries are indeed upside down.
The complementary synclinal hinge of the fold pair contains Blåhø in the core, and lies to the east of and structurally below the anticlinal root zone. Thus, the recumbent fold pair verges eastward, toward the Caledonian foreland, in classic Pennine style. Repetition of Sætra (?) and Risberget (?) rocks along the far western edge of the map area may indicate additional fold sets, but the unit geometry here is not yet fully resolved. The recumbently folded pile is itself deformed by major upright folds with approximately east-west axes. These folds include the Åmotsdalen antiform, the Stor Hornet synform, and the Gjevilvatnet antiform. Typically they are tight structures to the west but open up eastward. The Åmotsdalen antiform weakly folds the rocks of the Tronget unit, but the present contact between the Blåhø and Tronget rocks is controlled by later high-angle faults and does not appear folded.
Finally, structural doming of the rocks of the Lønset dome and regional depression of the Trondheim Nappe Complex produced the easterly dip of the Tronget rocks and of the entire eastern side of the Oppdal area (Wegmann 1959).
The Oppdal area was a type locality of the Stockwerk tectonic model of Wegmann ( 1935; cf. 1959; Hansen 1971). The concept of a migmatitic front transforming Paleozoic sediments to granitic gneiss and augen gneiss is not accuraten but the structural aspects of the model may be correct. The Stockwerk tectonic model considers the problem of what happens to overlying rocks while rocks at depth are undergoing metamorphism and major recumbent folding. The model recognizes three distinct levels. The lower infrastructure, the seat of the main metamorphism and ductile deformation, is separated by the Abscherungszone of faulting and steep metamorphic gradient from the overlying superstructure, where the metamorphic grade is lower and deformation is less ductile. The contact between the Tronget rocks and the lower units is controlled by late faults that developed, or continued to develop, after the main metamorphism and deformation. Thus, the observed faults cannot be considered an Abscherungszone, but other structural and metamorphic features seem to favor the Stockwerk tectonic model.
The early recumbent folds do not involve the Tronget unit, unless parts of what is here mapped as Blåhø include higher grade Tronget rocks (cf. Nilsen 1978, Pl. 1). The later upright folds in the central and western part of the Oppdal area dramatically open up or terminate toward the east near the Tronget unit, as if they were influenced by a coldern less ductile superstructure above. If the Tronget unit did not form the superstructuren another rock unit must have occupied its present position, and this mysterious unit must have been removed by thrusting, as erosion could not have reached the same tectonostratigraphic level over such a large area. My present interpretation is that the Tronget unit was already in nearly its present position at the time of metamorphism and itself forms the superstructure. Folding of the Tronget rocks by the Åmotsdalen antiform suggests that the Tronget unit was present at least during the late folding of the infrastructure. A similar tectonic pattern is seen north of the Oppdal area, where the isoclinal Surnadal Synform opens up eastward but folds the Støren Nappe of the Trondheim Nappe Complex (cf. Råheim l979). Late faults of unknown displacement create some metamorphic and structural discordance (Råheim l979) but the direct extension of the Surnadal fold into the Støren Nappe and the westward increase in metamorphic grade of Støren rocks (Goldschmidt l9l5; Krill, unpublished data) toward the faults suggests that the Støren Nappe formed the superstructure during infrastructural isoclinal folding. The structural and petrologic evidence is difficult to evaluate, and the validity of the Stockwerk model remains an intriguing problem.

Acknowledgements -

John Rodgersn New Haven, and David Gee, Uppsala. provided constructive suggestions during all stages of this project. Discussions and field excursions with Jean-Claude Guezou, Orsay, David Roberts and Fredrik Chr. Wolff, Trondheim. Arne Råheim, Steinar Solheim, and Bjørn Tørudbakken. Oslo, and many others are also very much appreciated. Field work was partially supported by grants from the Geological Society of America. the U.S. National Science Foundation, the Sigma Xi Society, and the Geology Field Fund of Yale University.

A Swedish contribution to the International Geodynamics Project.