Tarfala Research Station Annual Report 2003/2004 On the Bedrock Geology of the Tarfala Valley: Preliminary Results of 2003 and 2004 Fieldwork

Graham B. Baird

Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, Minnesota 55455-0219, USA [email protected]

Abstract. The bedrock of the Tarfala Valley has been mapped as part of geologic investigations into the tectonics of the area and the significance of meso-scale ductile shear zones in the rocks found there. Two summers of fieldwork showed that eight major units could be distinguished and that each unit is probably an individual thrust slice. Most units possess good foliations and lineations, indicating significant deformation. Throughout the valley, fabrics and lithologies are quite planar (though local folding does occur) with SW strikes and gentle to moderate dips to the NW, typically with a down plunge lineation. This work is ongoing and modifications to the following in future publications should be expected as data analysis continues.

Introduction and Upper Allochthon, are present (Figure 1; Figure 2; Table 1) based on the descriptions in e.g. Stephens During August of both 2003 and 2004, field investiga- et al. (1985), Silvennoinen (1987), and Page (1993). tions were conducted within the Tarfala Valley to map Bedrock Geology the bedrock lithologies and the associated geologic structure of the valley. Motivating this mapping was: The structurally lower most rocks are Precambrian 1) Though bedrock geological maps of the Tarfala Granitoids of the Baltic shield. Apart from evidence Valley have been published (e.g. Bhattacharyya, 2000; of rare brittle Caledonian deformation, these display Andréasson and Gee, 1989), the detail is limited; 2) Ex- excellent primary igneous textures. The most typical tensive glacio-logical and geomorphological research unit is a pink syenitic pegmatite. The upper surface of is conducted and published on the Tarfala Valley each the granitoids is a major décollement marked by black year, so an accurate account of the type, distribution, “cherty” ultramylonite, that appears to locally contain and structure of the bedrock units is important as the pseudotachylite. The ultramylonite is never more than valley’s geomorphology and glacial geology is influ- a few tens of meters thick but it is also found as frac- enced by the bedrock geology; 3) Distinctive meso- ture fill immediately below the décollement. Along scale ductile shear zones are found within the valley this décollement, at two localities (extreme southern (see Bhattacharyya and Hudle-ston, 2001; Srivastava portion of the valley), a pure quartz quartzite/quartz et al., 1995); these warrant further study and need to be pebble conglomerate can be found associated with the understood in the regional tectonic context. For these ultramylonite. reasons it is important that the bedrock geology of the A black shale unit is present on the southeastern Tarfala Valley be understood. wall of the Tarfala Valley (SH, Figure 1). This is weakly deformed but clearly rests in tectonic contact with the Tectonic Framework underlying granitoid. Associated with and above the The Scandinavian Caledonides are often described as black shale is a gray slate unit that that forms a large having five major tectonostratigraphic units: 1) The fold approximately 500 m. east of Rännan (SL, Fig- Autochthon-Parautochthon; 2) the Lower Allochthon; ure1; Figure 3). The shale, slate, and quartzite/quartz 3) the Middle Allochthon; 4) the Upper Allochthon; pebble conglomerate probably represent the remains and 5) the Uppermost Allochthon. Descriptions of of a sedimentary sequence marginal to Baltica prior to these units are summarized well in e.g. Stephens et al. Caledonian deformation. (1985) and Page (1993). Within the Tarfala Valley eight The first continuous thrust sheet found in the valley major lithologic units, which are thought represent is the Jåkk Mylonite Gneiss (JMG, Figure 1), named for the Autochthon-Parautochthon, Middle Allochthon, its excellent exposure along the “Jåkk” (Tarfalajåkk)

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Meters 0 1000

Kaskasatjå Symbols contour interval 100 meters Sydöstra

hkagla Contacts; closely located,

ciären inferred 27 1500 Foliation and lineation measurement, KDC foliation dip given

N Tarfalajaure 31 35 KA 16 20 Kebnepakteglaciären 27 SMG40 35 QPS 21 18 Isfallsglaciären 30 28 Tarfalastationen TA 16 25 29 25 1500 10

KA 32 20 0 200 22 29 Nordtoppen 19 QPS 11 Kebnekaise Storglaciären 19

10 26 13 11 27 33 21 27 27 21 14 Rännan SL 69 21 19 4 KDC 74

30 SMG 7 SH 24 TA Ta 14 22 rfalajåk Units 23 5 k KDC - Kebne Dyke Complex

KA - Kebne Amphibolite JMG SMG - Storglaciären Mylonite Gneiss 9 TA - Tarfala Amphibolite 5 10 QPS - Quartzo-feldspathic gneiss and pelitic schist PC 1000 1000 JMG - Jåkk Mylonite Gneiss 29 17 SL - Slate SH - Shale PC - Precambrian Granitoids

Figure 1. Bedrock geology of the Tarfala Valley. See text for unit descriptions. Basemap, Holmlund and Schytt (1987).

upstream from Rännan. The Jåkk Mylonite Gneiss is a unit and likely represents the undeformed protolith for quartz and feldspar rich ultramylonite to augen mylo- most of the Jåkk Mylonite Gneiss. A quartzo-feldspatic nite with strong and pervasive foliation and lineation gneiss and pelitic schist (QPS, Figure 1) occurs locally defined by flattened and stretched mineral aggregates. in the center of the valley between the Jåkk Mylonite Large pods of nearly undeformed medium grained Gneiss and the higher units. Most lithologies within syenite (tens of meters thick and extending possibly the quartzo-feldspatic gneiss and pelitic schist are hundreds of meters laterally) can be found within this mica rich and may be metamorphosed and deformed

04B1–2 Tarfala Research Station Annual Report 2003/2004 sediments of similar origin to the slate, shale and quartzite/quartz pebble conglomerate described above. On the ridge east of the valley there is a small area of outcrops with lithologies largely consistent with the Kebne quartzo-feldspatic gneiss and pelitic schist within the Dyke valley and is though to be the same unit. Due to lack Complex of exposure along portions of the eastern ridge, some unit contacts may actually be different than currently mapped. Shear Zones Folding

The Tarfala Amphibolite (TA, Figure 1) and Kebne (Upper Allochthon

Amphibolite (KA, Figure 1) outcrop over most of the Seve Nappe valley and are only distinguished from each other by Kebne their tectonostratigraphic position (Figure 2). Both are Amphibolite a medium grained hornblende rich amphibolite with lesser to equal portions of plagioclase. Garnet can be seen in most outcrops but is locally absent. Gneissic Storglaciären ) layering is common and is defined by changes in the Mylonite hornblende to plagioclase proportions. Foliation and Gneiss lineation are moderate to weak and are defined by the alignment of hornblende crystals. Complex outcrop- Quartzo- scale folding of the gneissic layering can be found most Tarfala feldspathic Amphibolite commonly in the uppermost portions of the Kebne gneiss and Amphibolite. pelitic schist Separating the two amphibolite units (Tarfala Amphibolite and Kebne Amphibolite) is the Storgla- Locally slate, Allochthon ciären Mylonite Gneiss (SMG, Figure 1). Typically the Middle shale, and Jåkk Ultra- Storglaciären Mylonite Gneiss is a quartz-feldspar rich quarztite Mylonite mylonite gneiss with lesser amounts of garnet and mica. At most Gneiss localities it is augeniferous and almost always pos- Parautochthon sesses strong foliation and lineation defined by aligned Autochthon mica, feldspar augen, and stretched mineral aggregates. Precambrian The Storglaciären Mylonite Gneiss probably represents Granitoids highly deformed meta-sediments. The Kebne Dyke Complex (KDC, Figure 1), structurally the highest unit Figure 2. Schematic tectonostratigraphy of the Tarfala Valley with in the valley, is starkly different from the lower in the context of the regional tectonostratigraphy framework, units of the valley as it largely lacks deformation. see Stephens et al. (1985). Original igneous textures are common (see Andréas- son and Gee, 1989) including the chilled margins of similar to what is characteristic of the two amphibolite the individual dolerite dyke sheets that comprise the units lower in the valley, is observed. unit. Though lacking deformation, there is significant In the Tarfala Valley the Seve Nappe, one of evidence, at the microscopic scale, of metamorphism a number of thrust sheets that make up the Upper and recrystallization. The contact between the Kebne Allochthon, is made up of the two amphibolite units, Amphibolite and the Kebne Dyke Complex is a décol- the quartzo-feldspathic gneiss and pelitic schist, the lement, indicated by the high degree of deformation Storglaciären Mylonite Gneiss, and the Kebne Dyke manifested in meso-scale ductile shear zones and the Complex . The Jåkk Mylonite Gneiss and slate(?) com- complex folding of the Kebne Amphibolite. prise the Middle Allochthon. The shale and quartzite/ Both the Tarfala Amphibolite and Kebne Amphi- quartz pebble conglomerate are part of the Autohthon- bolite are likely to be deformed and metamorphosed Parautohthon, while the Precambrian Granitoids are equivalents of the dolerite dykes (KDC), and their of course part of the Autochthon (Figure 2; Table 1). tectonostratigraphy may well be the result of thrust Structural Geology duplication. On the north side of Storglaciären, near the equilibrium line, a transition from undeformed Significant shear has occurred in all units from the dolerite dykes to foliated and lineated amphibolite, Jåkk Mylonite Gneiss to the bottom of the Kebne Dyke

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Table 1. Regional tectonostratigraphic unit correlation in Swedish Caledonides. Sarek Mountains, research area of An- dréasson et al. (1992) is located 50km southwest of the Tarfala Valley. Singis-Tjuoltajaure area (Page, 1993) is immediately south of the Tarfala Valley. The Indre-Troms area, studied by Stølen (1997), is 100 km northeast of the Tarfala Valley. Cor- relations are based on unit descriptions and structural level. Tarfala Valley nomenclature developed during the 2003-20041 field mapping except italic terms which were originally derived by Andréasson and Gee (1989). (?) indicates more tenuous correlations. Major Sarek Mountains Singis-Tjuoltajaure Tarfala Valley Indre-Troms Tectonostratigraphic Unit (Andréasson et al., 1992) (Page, 1993) (this report) (Stølen, 1997; abbreviated) (see Stephens et al., 1985) Salka Group Patta Quartzite Not Described Rusjka Graphitic Schist Not Present in Psammitic Schist

Köli Nappe Nappe Köli Rusjka Calcareous Schist Tarfala Valley Låutak Formation Vidja Assemblage (?) Keddåive Nappe Rohkunborri Nappe Aurek Assemblage Kebne Dyke Complex Sarektjåkkå Nappe (dyke complex) Kebne Amphibolite

Upper Allochthon Allochthon Upper Storglaciären Mikka Nappe Augen gneiss Mylonite Gneiss Seve Nappe Nappe Seve Savotjåkka Assemblage Garnet Amphibolite Tarfala Amphibolite Amphibolites Quartzo-feldspatic gneiss Lower gneiss sheet and pelitic schist Skarja Nappe (?) Middle Allochthon Syenite Nappe Middle Allochthon Jåkk Mylonite Gneiss Mylonitic Basement Rocks Slate Parautochthonous/Allochthonous Autochthonous Sediments Shale Parautochthon/ Cover Rocks (Dividal Rocks) (Dividal Group) Parautochthon/Autochthon Autochthon Parautochthonous/Allochthonous Precambrian Granitoids Precambrian Basement Precambrian Basement

Jåkk Mylonite Gneiss

Folded Cleavag e

Slate ?

Shale

Precambrian Granitoids

Figure 3 – Photo showing the association of the Precambrian Granitoids, shale, slate, and Jåkk Myloinite Gneiss units along the stream west of Rännan (Figure 1). Here the slate is folded due to significant thrusting relative to the Precambrian Granitoids.

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A Acknowledgements I thank Peter Hudleston, my advisor, for providing this research project at such a great location. Field assistance by Josh Spindler (August 2003) and Keith Brugger (August 2004) was imperative and greatly ap- preciated. This work would not have been possible if it was not for the generous help of the Tarfala Research Station’s researchers and staff. In particular, I would like to thank Peter Jansson for providing stimulating conversation and some resources used in this research. Stereonets were produced by Estereografica–GR (v. 1.0) by Ernesto Cristallini. References Cited B Andréasson, P. G., and Gee, D. G., 1989. Bedrock geology and morphology of the Tarfala area, Kebnekaise Mts, Swedish Caledonides. Geogr. Ann. 71A(3–4): 235–239. Andréasson, P. G., Svenningsen, O., Johansson, I., Solyom, Z., and Xiaodan, T., 1992. Mafic dyke swarms of the Baltica-Iapetus transition, Seve Nappe Complex of the Sarek Mts., Swedish Caledonides. GFF, 114(1): 31–45. Bhattacharyya, P., 2000. Shear zones of northern Swedish Caledonides [Ph.D. Dissertation]. University of Min- nesota, Minneapolis, Minnesota, 165 pgs. Bhattacharyya, P., and Hudleston, P., 2001. Strain in duc- tile shear zones in the Caledonides of northern Swe- den: a three-dimensional puzzle. J. Struct. Geol., 23: 1549–1565. Holmlund, P., and Schytt, V., 1987. Glaciärerna i Tarfala, Kebnekaisemassivet. Geogr. Ann., 69A(3–4). Figure 4. A) Poles to foliation, the average of 206, 15 NW is Page, L. M., 1993. Tectonostratigraphy and Caledonian struc- marked by the square and great circle, n=193. B) Lineation ture of the Singis-Tjuoltajaure area, central Norrbotten measurements, average of 28–>294 marked by the square, Caledonides, Sweden. GFF, 115(2): 165–180. n=157. Silvennoinen, A. (subproject leader), 1987. Pre-quaternary rocks, Northern Fennoscandia. Geological Surveys of Finland, Norway and Sweden: Helsinki, scale Complex as indicated by the penetrative foliations and 1:1,000,000. lineations. All units are probably thrust-fault bounded. Srivastava, H. B., Hudleston, P., and Earley III, D., 1995. For the most part, the units are relatively planar and Strain and possible volume loss in a high-grade ductile folding only occurs as local structures that are more shear zone. J. Struct. Geol., 17: 1217–1231. Stephens M. B., Gustavson, M., Ramberg, I. B., and Za- in the nature of warps on the scale of the units (Figure chrisson, E., 1985. The Caledonides of central-north 4). Tight meter-scale folds are also found within in- Scandinavia – a tectonostratigraphic overview. in Gee, dividual units, especially the Kebne Amphibolite. All D. G., and Sturt, B. A. (eds.): The Caledonide Orogen – structures and kinematics indicators are consistent with Scandinavia and Related Areas, 135–162. tectonic transport towards the SE. This is particularly Stølen, L. K., 1997. Bedrock geology of the Altevatn- Måskanvarri area, Indre Troms, northern Scandi-navian well reflected in the fairly uniform trajectories of the Caledonides. Bulletin - Norges Geologiske Under- NW plunging transport parallel lineations (Figure 4). sokelse, 432: 5-23.

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