Museum "Villa Heidelberg"

The Scandinavian Caledonides:

Geology in the north-central Swedish Caledonides around Villa Heidelberg
Klimpfjäll (Vilhelmina, SW Västerbotten, S Lapland, Sweden)

C. Structure

1. Tectonic evolution
1.1 Finnmarkian
1.2 Scandian
2. Regional structure
2.1 Lower Allochthon
2.1.1 Blaik Nappe Complex
2.1.2 Börgefjell Duplex
2.2 Middle Allochthon
2.3 Upper Allochthon
2.4 Uppermost Allochthon
3 Cross sections

1. STRUCTURAL AND TECTONIC EVOLUTION

The five-tier terminology is based on the superpositon of different structural units that has been observed in the field and that nobody can deny. However, for the structural evolution the traditional view of deformation phases and stacking sequence is amended, in order to include recent radiometric and palaeomagnetic data, the consequences of which for the regional structure and tectonic evolution have not yet been comprehensively discussed.

The Seve Unit of the Upper Allochthon and the Middle Allochthon were deformed and metamorphosed in a Late Cambrian-Early Ordovician, "Finnmarkian" event, whereas the other units acquired their metamorphism and structure during the Middle Silurian to Early Devonian "Scandian" phase. During this latter event and caused by the stacking of thrust units in the Lower Allochthon, the regional Caledonian NE-SW trending folds originated. As a consequence, the lower thrust units occupy the antiformal areas and the eastern Caledonian margin, whereas the higher units are exposed in major synforms and towards west.

The Scandian orogenic evolution can be divided into several structural stages (Stephens 1977,1982), which are most clearly developed in the Köli units of the Upper Allochthon:

STRUCTURAL EVOLUTION

Stage I: Development of a pile of recumbent folds which face eastwards and trend parallel to the orogenic belt; a constrictional strain situation with extension on the limbs of the folds at a high angle to the present trend of the orogenic belt.
Stage II: Collapse of the tectonic pile leading to considerable modification of the shape of the earlier formed folds and the development of the regional foliation and lineation fabric; a flattening or plane strain situation due to progressive collapse of the tectonic pile, and with the principal elongation direction remaining at a high angle to the trend of the orogenic belt.
Stage III:Development of mylonite zones and major thrusts at a late stage in the collapse of the tectonic pile associated with continued, easterly overturned folding and the development of locally intense crenulation cleavage at higher structural levels. This is the stage of substantial lateral transport involving emplacement of the tectonic pile over the "granitic" basement of the Baltoscandian platform as well as overturned folding and thrusting of basement and cover together. The strain during this stage involved a large simple shear component oriented at high angles to the trend of the orogenic belt. The shear strain was inhomogeneously distributed throughout the tectonic pile, the maximum shear strain being presumably located in the more important mylonite and thrust zones (see also Zwart 1974).
Stage IV: Development of late antiforms and synforms with steep axial surfaces and locally strong crenulation cleavage, possibly related to gravitational disturbances due to the tectonic superimpositions of higher density cover on top of lower density, "granitic" basement.
Stage V:Gravitational collapse of the steepened limb of the major synforms at higher structural levels, producing kink folds with flat-lying axial surfaces and largely down-dip sense of overturn.
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2. REGIONAL STRUCTURE

2.1. Lower Allochton

The two major units of the Lower Allochthon, the Blaik Nappe Complex (BNC) and the Börgefjell Duplex are also distinct structurally, as a consequence of their relative position within the orogenic wedge. Whereas the BNC at the eastern Caledonian margin is relatively little deformed and metamorphosed, the Börgefjell Duplex due to its more internal position shows a stronger orogenic overprint. In general, strain and metamorphism increase westwards, and the Lower Allochthon units are treated here from east to west in the order of the increasing deformation.

2.1.1 Blaik Nappe Complex

Structurally, the Blaik Nappe Complex of the Lower Allochthon can be divided from the east to the west, into one or several eastern, marginal thrust systems (as yet unnamed) and, towards west, the Lövnäs Duplex, the Njakafjäll Duplex and the Krutsjön Nappe.
Whereas the floor thrust of the Caledonian Nappes is well documented at the estern margin of the fold belt, a putative roof thrust at the marginal thrust units of the Lower Allochthons has been removed by erosion. Therefore, it is no longer possible to decide whether the thrust systems had a roof thrust or not, i. e. whether the structure was a duplex or an imbricate stack. However, farther towards west, the Lower Allochthon is overlain by the higher nappe units and there a roof thrust to the Lower Allochthon is preserved, showing unequivocally the thrust systems as duplexes. This is the case, for example in the Lövnäs Duplex to the east of the Middle Allochthon around the upper part of Lake Malgomaj or the Vojmsjön Duplex, around Daikanvik. To the west of this outlier of the Middle Allochthon, the Njakafjäll Duplex is composed of numerous horses exposed in a major regional antiform trending NNE-SSW, which was caused by the stacking of the Njakafjäll duplex (Gayer and Greiling 1989). The structural and metamorphic evolution involves a single pre-thrusting deformation phase which produced small-scale isoclinal or drag folds and a penetrative foliation in incompetent rocks, synchronous with illite recristallization (Greiling 1985). Ilite crystallinity data define a "metamorphic" grade between lower anchizone and epizone (Greiling 1985, Warr et al. 1996). Subsequent shearing and thrusting relates to nappe transport and stacking of horses, and the latter also led to folding in the overlying units. Stretching lineations and branch line geometry suggest a tectonic transport direction towards the ESE (ca. 110°), which is consistent for all the horses in the Lower Allochthon (Gayer and Greiling 1989). A late stage of compression is documented by out-of-sequence thrusts, which locally carried the Lower Allochthon over the Middle Allochthon (e.g. east of Grytsjö, L. Njakafjäll).

In the Njakafäll type area and towards southwest, the structure is a well-developed hinterland-dipping duplex with the strike generally NNE-SSW, normal to the transport direction.Towards north, however, some horses can be seen to terminate and others bend into an E-W-strike. North of Marsån and east of Blaikliden, the approximate general E-W strike and northern dip of thrusts imply a lateral termination of the Njakafjäll Duplex. As a consequence, the Njakafjäll duplex is overlain, towards the north, by the Krutsjön Nappe, which, in turn, is covered northwards by the overlying Middle Allochthon. The Krutsjön Nappe reappears farther north, and in the Krutsjön area (23 F NO), where it represents the only exposed unit of the Lower Allochthon.

2.1.2 Børgefjell Duplex

The rocks within the Børgefjell Window have traditionally been considered as autochtonous by most geologists (Kautsky 1948, Zachrisson 1969, 1986, Gustavson 1973, Greiling 1974). However, influenced by information from windows elsewhere in the Caledonides, including seismic reflection data, a new interpretation of the structural scenario has recently developed, involving considerable shortening and eastwards transportation (Hossack 1985, Morley 1986, Gayer and Greiling 1989, Hurich et al. 1989, Palm et al. 1991, Greiling et al. 1993). Therefore, the deeper structure can be assumed to be represented by a large-scale duplex with a high proportion of basement rocks involved.
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2.2 Middle Allochthon

The nappe units of the Middle Allochthon share a common Caledonian tectonic evolution (Greiling 1989). In pelitic rocks, mineral assemblages representing an early peak of metamorphism are characterized by coexisting almandine garnet, biotite and muscovite and are indicative of the transition between greenschist and amphibolite facies metamorphism with minimum P-T conditions of about 500°C and 4 kb. Subsequent mylonitization is associated with retrogression of garnet and brown biotite into coexisting muscovite, chlorite, green biotite and rare stilpnomelane in pelitic rocks, indicating lower greenschist facies metamorphic conditions. Less deformed domains still retain higher grade assemblages that remained apparently unaffected by retrogression.

Early Caledonian deformation in the Middle Allochthon is documented by recumbent, isoclinal folds with a penetrative schistosity, deformed by generally open folds attributed to a second structural phase. These latter folds are associated with an axial planar crenulation cleavage that may be penetrative in incompetent rocks. Subsequent deformation led to intense shearing and development of a penetrative mylonitic fabric in some structural units. In other units, penetrative deformation was restricted to their margins or to distinct internal shear zones with only a weak overprinting by a fracture cleavage. Associated with the planar mylonitic fabric is a strong lineation enclosing both relic mineral grains and newly formed minerals, indicating a tectonic transport direction towards the SE-SSE (ca. 140°). The Middle Allochthon was subsequently affected by out-of-sequence thrusts, which locally transported the Lower Allochthon over the Stalon Nappe, or the Särv Nappe over the Seve. Later fault-bend folding occurred when the Middle Allochthon was transported as the "passive" roof of the Lower Allochthon.

The Fjällfjäll Antiform, which enters the Fatmomakke map sheet (23F, NV) from the west, forms part of a major structure which has its main extension within 23E Sipmeke and terminates just north of the map boundary, in 24F Tärna. The antiform is slightly asymmetric with a steep eastern limb which becomes overturned in the area north of Ransarån. Here, further complications occur, demonstrated by repetitions of the Fjällfjäll Arkose and Köli rocks. These disturbances probably resulted from early out-of-sequence thrusting (breaching thrusts) which transported the Middle Allochthon meta-arkose on top of the overlying Köli sequence. As a result, the Fjällfjäll Antiform was probably initiated during this phase of thrusting and further tightened during a later stage of duplex development, associated with the emplacement of the Lower Allochthon. This latter stage of deformation caused further updoming of the overlying thrust sequence and also deformed the regional low-angle thrusts and previous out-of-sequence thrusts, as demonstrated in the section (cf. also section of 23 E, SO).
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2.3 Upper Allochthon

2.3.1Seve Nappe

After the establishment of the internal Seve tectonostratigraphy, as described in section B.4.1, the Seve units were repeated, probably due to a subsequent phase of out-of-sequence thrusting. This pattern is evident in the southern part of 23 F SV and illustrated in the section of this map sheet (Zachrisson 1993). The structural evolution has been documented comprehensively by Trouw (193). For metamorphic conditions see section B.4.1.

2.3.2 Köli Nappe

According to Trouw (1973), the traces of four tectonic phases can be distinguished: F1 produced folds and a slaty cleavage, S1, F2 and F3, respectively. During F2 folds were formed with the most conspicuous regional cleavage plane, S2, as axial surface cleavage. S2 is predominantly a crenulation cleavage; it may be accompanied locally by a mineral lineation, L2min. S2 is folded in F3 folds, as, for instance, in the Ransaren Synform, F4 structures are of the same type as F3 ones, folding S2, with a divergent orientation. Locally, they are superimposed on F3 folds. These four phases, F1 - F4 correspond roughly to the four phases distinguished by Zachrisson (1969, 1971) as pre-F1, F1, F2 and F3, respectively.

F1 is a phase of strong deformation, responsible for deformed pebbles and large-scale, tight to isoclinal folding, with a penetrative slaty cleavage as axial surface cleavage.

Folds. About a dozen folds with the S2 crenulation cleavage cutting straight through both limbs were distinguished in the field. They are apparently older than F2. The crenulated slaty cleavage (S1) in the hinges of these folds forms an angle with the bedding of up to 90°. Although highly deformed, S1 appears to be subparallel to the axial surfaces of these early folds, which are therefore interpreted as F1 folds. The mapping demonstrated a repetition of certain units, probably due to the tight folds, which must predate F2 as well, since S2 again runs obliquely through them. Although the relationship between S0 and S1 in the hinges of these large-scale structures is not clear, they are also interpreted as F1 folds.

Cleavage. The slaty cleavage, S1, is in many thin sectins parallel or sub-parallel to S0. Although this might partly be due to later deformation, it is thought to illustrate the tightness of F1 folding. In the field S1 is rarely to be found as a measurable surface, distinct from S0.

Folds. Many meso-scale F2 folds, with S2 as axial surface, can be observed in the field. They are mainly close or open asymmetrical folds, which by their attitude indicate a large-scale F2 antiform in the western part of the area. Towards the east, in the steep limb of the large antiform, they are less abundant and close to the eastern boundary of the phyllite belt, they are very rare. F2 fold axes are generally parallel to d2; they plunge gently to the NNE.

Cleavage.Microscopic analysis demonstrated that S2 is a crenulation cleavage in the western and middle part of the phyllite belt. In the more psammitic beds the cleavage is sometimes of the fracture cleavage type and along the eastern border of the belt only a slaty cleavage has generally developed.

Lineations: Three types of lineations related to F2 are distinguished: intersection lineations S2 x S1, and S2 x S0, L2 and d2, respectively, and a mineral lineation L 2min defined by an elongted fabric of mica and quartz crystals. L2 could only be distinguished from d2, where S1 forms a clear angle with S0.

F3 is among other things responsible for the formation of the Ransaren Synform. This is a composite structure, with two hinges. In one of them, which appears on the map NW of Stornäs, S2 changes from a steep west-dipping attitude to a north-dipping one. In this hinge zone numerous micro- and meso-scale F3 folds have developed. They are open, close or tight folds, locally of the kink- or accordion fold-type. Their orientation is related to the major structure. They have steep south-west dipping axial surfaces and fold axes with a moderate plunge to the NW. The other hinge of the Ransaren Synform is situated NW from Klimpfjäll, where S2 curves around from a N-dipping to a N-S striking, almost vertical position. Here the axial surface, S2, dips NW and the fold axis plunges NE. Apart from the hinge zone near Stornäs, F3 folds are not abundant in the phyllite belt. Their orientation is rather variable. F3 microfolds, to be seen as crenulation lineations on S2 surfaces are common in the entire phyllite belt. Along the axial surface of F3 folds, a new crenulation cleavage has only rarely formed.

The general E-W shortening during F3, discussed by Zachrisson (1969) is probably the cause of the steep position of S2 in the eastern part of the phyllite belt. It makes sense that the incompetent phylllites, close to the contact with the more rigid Seve schists, were highly compressed. The same effect can be observed around the rigid ultramafic body of Aunere, where F3 folds are abundant and very tight.

In some places F4 folds or lineations are refolded by a later phase, which is labelled F4. In general the F4 structures can only be distinguished from F3 phenomena by their diverging orientation.
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2.4 Uppermost Allochthon

A polyphase structural and metamorphic evolution can be demonstated in the Uppermost Allochthon. Whereas this evolution begins already in pre-Caledonian times in parts of the unit, deformation continues throughout the Caledonian orogeny. A polyphase history can also be studied in the marginal shear zone between the Upper Allochthon (Gjersvik Nappe) and the Uppermost Allochthon (Helgeland Nappe Complex).

3. Cross Section

The sections are drawn assuming a sole thrust with a constant dip of 1.5 °(cf. Gee et al. 1978, Bierlein and Greiling 1993) from the eastern Caledonian margin towards the WNW. They trend at a low angle to the tectonic transport direction of the Lower Allochthon (110°). Previous sections across the Njakafjäll duplex have been published by Kulling (1972) and Gayer and Greiling (1989). The former paper covers the Stalon tunnel section and related drillholes, both of which are shown on the map (23 F, SE). The present section shows the Njakafjäll Duplex as a relatively simple hinterland dipping duplex. Horses in the east are composed exclusively of the Sjoutälven Group (c. 300 m thick), whereas horses further west also contain the stratigraphically lower Risbäck Group, which is primarily thickening westwards. The thickness of basement rocks in the horses can only be inferred from the sections.

An important feature are minor thrusts breaching the roof thrust of existing or developing duplexes, locally transporting a lower unit over an overlying major tectonic unit. Such out-of-sequence thrusts affect the contacts between the Särv and Seve Nappes and between the Lower and Middle Allochthons. Successively, they cut across previous thrusts, making them inactive and coupling underlying tectonic units to the overlying nappe. Therefore, they may represent an important displacement mechanism in the moving orogenic wedge.

Westwards, the sections form direct continuations of those constructed for the eastern map sheets (23 F SV).The section postulates a Lower Allochthon duplex beneath the Fjällfjäll antiformal structure, exposing in its core the Middle Allochthon beneath the Upper Allochthon. Further west, the Børgefjell antiformal structure is apparently also a Lower Allochthon duplex. However, only the eastern part of this duplex has yet been documented iin detail (Greiling 1988, see also Gustavson 1973, 1981, Dallmann 1984).
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