Strain Discontinuities Within the Seve-K61i Nappe Complex, Scandinavian Caledonides

Journal of Structural (;eologv. Vol. 6. No 1/2. pp. I111 to 110. 1984 Ill t;1_8141/84 ${13.01~+ 0.00 Printed in Great Britain I'crgamon Press Ltd Strain discontinuities within the Seve-K61i Nappe Complex, Scandinavian Caledonides RICHARD J. LISLE Department of Structural Geology, Institute of Earth Sciences, Utrecht, Netherlands (Accepted m revised form 29 gun e 1983 ) Abstract--This paper utilizes the results of a regional survey of the state of finite strain carried out in an area built up of stacked thrust sheets to draw conclusions about the relative timing of penetrative deformation and thrust motion, and to place constraints on the deformation models devised to explain the geometry of the individual thrust sheets. A strain survey within the Seve-K61i Nappe Complex reveals abrupt changes which represent discontinuities in the strain pattern. One of these discontinuities coincides with the boundary between the Sere and K61i Nappes and separates intense transversal stretching strain within the Seve Nappe from the longitudinal strains of lower intensity within the K61i Nappe. The weight of geological evidence favours the interpretation of this strain discontinuity as a unconformity with significant deformation of the Sere Nappe occurring before rocks of the overlying K61i Nappe were deposited (? Middle Ordovician). A second discontinuity marks the junction between the Krutfjell Nappe and the underlying lower grade K61i rocks. The strain distribution is incompatible with a previous interpretation of the discontinuous Krutfjell Nappe as a string of mega-boudins--an interpretation which implies considerable post-thrust flattening of the nappe pile. The strain results and other evidence favour earlier deformation of the Krutfjell Nappe before these rocks were thrust over the underlying tectonic units. The lens-shaped outcrop configuration of the Krutqell Nappe is explicable in some cases if the geometry of this nappe is a westerly thinning wedge. Within the low grade K61i Nappe rocks north of the Grong Culmination the longitudinal stretching is difficult to explain in terms of easterly directed nappe movements whereas the clear transverse strains in other tectonic units appear to predate the main Silurian thrusting event. INTRODUCTION allochthonous unit belonging to the upper part of the exposed nappe pile and comprises rocks of variable THE MOST obvious and impressive result of the Caledo- metamorphic character. The lower part of the complex, nian orogeny in Scandinavia is the development of large- the Seve Nappe, is largely made up of amphibolite facies scale thrust faults. These thrusts, which throw meta-sediments and basic meta-igneous rocks with a allochthonous units eastwards over the Baltic Shield, core of gneisses and migmatites locally at granulite dominate the large-scale tectonic pattern and form the facies. Recent dating work suggests the presence of bounding surfaces between adjacent units of distinct Precambrian rocks within the nappe (Reymer et al. lithological and metamorphic characteristics. The latter 1980, Koark et al. 1980, Claesson 1982). The K61i are the consequences of the fact that the development of Nappe, itself a composite of thrusted slices, contains the thrusts postdates the establishment of the regional Ordovician and Silurian rocks and is generally of lower metamorphic sequences in the rocks. It is the amount of grade (greenschist facies). For details of the Seve and disruption of the metamorphic pattern which has K61i thrust units the reader is referred to Zwart (1974) allowed minimum distances of displacement along the and Zachrisson (1969), respectively. thrusts to be estimated. Such estimates suggest that the A pronounced feature of the Seve-K61i Nappe is its summed displacement for visible thrusts in the Scandina- wedge-like geometry (Zachrisson 1969). The basal Sere vian Caledonides could exceed 500 km (Gee 1975). thrust with its gentle westerly dip produces a large In contrast to the sedimentary and metamorphic pat- number of windows where lower nappes or basement terns, the regional pattern of deformation and its become exposed (Fig. 1). The upper contact of the relationship to thrusting is less clear and is the subject of Seve-K61i Nappe complex formed bv the basal thrust of this paper. To analyse the deformation pattern it was the ROdingsfjell Nappe crops out with a straighter course decided not to concentrate on the episodic nature of the suggesting a steeper angle of dip. This explains why no deformation, which has been demonstrated in many klippes of the R6dingsfjell nappe were produced. The areas, but instead to assess the total deformation suf- resulting wedge-shaped Seve-K61i Complex thins west- fered by the rocks. This approach, which involves a wards (Fig. 7). regional analysis of the finite strain, was attempted in a As I intend below to compare fabric patterns to the part of the Seve-K61i Nappe Complex where sufficient thrust geometry it is desirable to determine the direction strain markers are available to give a reasonable picture of thrusting from evidence other than that derived from of the finite strain distribution. fabric data. The wedge geometry can be used for the purpose if we assume that the symmetry of the wedge THE SEVE-KOLI NAPPE COMPLEX structures is related to the symmetry of the thrusting motion (Clough's method using thrust imbrications, see The Seve-K61i Nappe Complex forms a major compo- Ramsay 1969, p. 74). Lines of outwedging, or branch- nent of the Caledonides in Central Scandinavia. It is an lines (Boyer & Elliot 1982) corresponding to the lines of 101 102 R.J. LISLE 11 ° 120 130 140 -~-~ 15: 660 j Krutfjell area w~th transverse stretching hneahon W of Seve-Koh boundary Napte GC ,m,oa,,ooO e Borgefiell Window Pebble eqongatlon > Related hneahon ~ Koli Nappe ~-J@ L meat~on [rend .~ ._~ ~ 2~ Seve Regional hneatlon Sere Nappe Nappe 650 Lake , ,\ '\ ( 640 - \ % ~,, '}":L-L; / o Fig. I. Pebble extension lineations in the Koli Nappe and stretching lineations in the Sere Nappe (see Appendix A). intersection of thrust planes which bound the wedges, 1974, Biermann 1979, Ghosh etal.1979, Van Roermund can be readily constructed in a number of places on the 1982) and the discovery of Precambrian age for these geological map, An example of the branch-line is rocks means that we can place only very broad limits on obtained by joining the points of disappearance of the the timing of deformation. In view of these complexities, Seve Nappe at the Grong Culmination and Akkajaura it is convenient to adopt the classification proposed by Culmination (Zachrisson 1973). The average direction Williams & Zwart (1977), which for this general area, of these lines is 030 ° N which is close to the average trend distinguishes a group of structures, overlapping in time of the Caledonian front ((121)° N). The deduced direction with the main metamorphism and which contributed to of thrusting is perpendicular to this direction (lI0- the production of the pronounced fabric in the rocks, 120°N). Hossack (1983) gives additional examples from from a group of structures which postdate the fabric and the Scandinavian Caledonides of the type of information deform it. The fabric produced bv the first group of to be gained by the construction of branch-lines. structures is of regional distribution and is of piano- linear (LS)type. The linear component of the fabric is parallel to mineral lineations, rodding and fold axes. STRAIN PATTERNS Williams & Zwart (1977) and Calon (1977, p. 82) list characteristics of this fabric which suggest that it is the TheSere Unit result of extremely intense deformation. The associated folds have highly flattened shapes. Also sheath folds, The polyphase nature of the deformation of the Seve assumed to have developed by extreme strain modifica- rocks has bcen repeatedly described (Trouw 1973, Zwart tion of non-cylindrical fold forms, are drawn out parallel Strain discontinuities in the Seve-K61i Nappe, Scandinavian Caledonides 1!t3 to the lineation. Strongly rotated garnets are common and large angles of rotation have been recorded (Schoneveld 1977). These angles are probably functions not only of the amount of strain but also of the vorticity (the degree of non-coaxiality) associated with the deformation history. However, if geological flow with high vorticity is exceptional, as Pfiffner & Ramsay (1982) argue, then these rotational angles must be the result of high strains. If simple shear is assumed, some garnet inclusion spirals indicate shortening in excess of 90°/,, (Williams & Zwart 1977). Calon (1979, p. 82) uses boudinage to estimate minimum strain of 80% shortening. Data on orientation of this lineation have been com- piled from student theses of the universities of Leiden and Utrecht and are shown in Fig. 1. The average plunge direction of this lineation is around 290 °, the direction estimated for motion of the thrusts from branch-line information. Similar lineation orientation has been observed in other areas in the Scandinavian Caledonides (e.g. Kvale 1953, Lindstr6m 1958, Hooper 1968), and suggests a straightforward genetic connection between the lineation and the thrusting. However, it appears that lineations of different ages share this direction. Calon (1979, p. 84) records lineations defined by kyanites and Ghosh et al. (1979) record high-grade minerals in the Fig. 2. Distribution of strain magnitudes (expressed in E, units) within necks of boudins indicating extension during a part of the Seve-K61i complex. E, is a function of the axial ratios of metamorphism whilst elsewhere strong mylonitic linea- the strain ellipsoid tions are developed along thrust planes which clearly l+el h - l+e_, ). post-date the metamorphism (Zwart 1974). I conclude a- l+e, l+e~ therefore that a transversal fabric in some of the Seve and is calculated from rocks existed before the thrusting. 1 [(in a)Z+(In b)-'+(In ab)2] I~ The KOli Unit A survey of the strain within the K61i Nappe is made J~imtland, just west of the Seve-K61i contact: (b) some possible by the widespread occurrence of deformed higher grade regions within the K61i distinguished as the conglomerates.

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