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The Tectonostratigraphy and Structural Evolution of the Sitas Area, North Norway and Sweden (68 0 N)

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The Tectonostratigraphy and Structural Evolution of the Sitas Area, North Norway and Sweden (68 0 N) The Tectonostratigraphy and Structural Evolution of the Sitas Area, North Norway and Sweden (68 0 N) PETER D. CROWLEY Crowley, P.D. 1989: The tecton ostratigraphy and structural evolution of the Sitas Area, Nort h Norway and Sweden (68 0 N). Nor. geo/. unders . Bull. 416, 25-46. The Sitas area conta ins rock s derived from the pre-Caledonian Scandinavian crato n and from an accreted Caledonian fore -arc comple x. These rocks form the par-autochthonous(?) Rombak Comp ­ lex which can be tied stratigraphically to Baltica and three allochthons which from the base up­ wards are: the Storriten Complex, the Langvatn nappe and the Marko nappe . The Storr iten Comple x is a schuppen-zone composed of rock s derived from hinterland equivalents of the Rombak Comp ­ lex while the Langvatn and Marko nappes are part of an accreted fore-arc terrane which can be correlated with parts of the Middle Koli Nappe Comple x (MKNC). The Marko nappe contains mafic rocks whose geochemical affinities suggest derivation from both MORB-Iike and arc-like sources. The MKNC was deformed by a series of five superposed Caledonian deformations (0 ,-0 ,). Only the latest two deformations that affected the MKNC, 0 , & 0 .. are recogn ized in the Storriten and Rombak Complexes. For this reason, the MKNC cannot be tied directly to the underlying domains and therefore to Baltoscand ia until D. when it was thrust onto the Storriten and Rombak Complex­ es. The earliest three defor mations may have occurred outboard of the lower Paleozoic margin of Baltoscandia. The two earliest defo rmations (0, & 0,) occurred under moderate pressu re amphi­ bolite -facies conditions (5750 C, 800 MPa) while later deformat ions occurred under lower PoT conditions. By D. the MKNC had cooled to greenschist-facies conditions (T ( 450 0 C, P ( 500 MPa). Most of the strain in the Sitas region was accommoda ted by large-scale ESE-directe d thrusting during 0 , and D•. 0 , thrusting imbricated the rocks in the Marko nappe and emplaced the Marko nappe onto the Langvatn nappe. D. thrust ing placed the MKNC onto the Storr iten Complex and the Storriten Complex on the Rombak Compl ex. In addition, it detached the sedimentary cover of the Rombak Complex from its crystalline basement, imbricated the cover and eventually delamina­ ted the uppermost metres to hundreds of metres of the basement form ing the Storriten Comp­ lex. Below this level, the crysta lline basement was not penetratively deformed by Caledonian events . Peter D. Crow/ey, Department of Geology, Amherst College, Amherst. MA 01002, U.SA Introduction The Sitas area (Plate 1) is located on the oceanic in origin. The continental basement southern margin of the Rombak window , a of the Rombak window was at a mid-crustal large fenster of pre-Caledonian basement be­ depth when this metamorph ic allochthon was neath the Caledonian nappe pile that straddles accreted to it. The Sitas area thus provides the Norwegian-Swedish border between 68° an excellent opportunity to study, by direct and 68° 30' N (Fig. 1). Precambrian granitic observation, the geometry of structures that gneisses within the Rombak window are local­ form in response to the accretion of an ocea­ ly overlain by the Tornetrask Formation, a nic terrane at an intermediate crustal level. very thin Vendian to Lower Cambrian platform This paper summarizes the results of an sedimentary sequence (Thelander 1982) that integrated geological , petrological and structu­ correlates with the 'Hyolithus' zone sequence ral study that was based on field work carried (Kulling 1964) of the Baltic craton . For this out during the summers of 1981-1983 and reason, the Rombak window can be conside­ 1985. The study concentrated on: (1) defining red to represent an extension, although per­ a tectonic stratigraphy within the allochthonous haps a displaced one (Tilke 1986) of the pre­ Caledonian rocks , (2) determining the nature Caledonian Baltic craton beneath the Caledoni­ of the contact between the pre-Caledonian an nappe pile. The continental basement of basement and the Caledonian nappes, and (3) the Rombak window is structurally overlain determining the pressure-temperature condi­ by an allochthonous terrane of medium-grade tions at which the Caledonian nappes were metamorphic rocks that are, at least in part, accreted to Scandinavian continental base- 26 Peter D. Cro wley NGU · BULl. 416. 1989 ~ Upper xsu . Rod ing- ij:ill ~ ~ ~~I i dd k ~"Ii l (ult ramafic pod) ~ ~Low('rKolt :.:..: N El Sevc 1 .,':- Storriten and Akka jaure Comp lexes ~ pre-C.l1(·doni'ln basement km o 10 zo Fig. 1. Simplified tectonic map of the Sitas region . (Compiled from Foslie 1941. 1942. Kautsk y 1953. Kulling 1964. 1982. Gustavson 1974, & Hocqe s 1985). NGU • BULL. 416. 1989 Tectonostratigraphy and Structural Evolution 27 ment. This report emphas izes tne stratigrap­ Previous work hic and structural aspects of this study; the The Norweg ian portion of the study area was metamorphic petrology of the rocks in the initially mapped by Foslie (1941) and published study area has been discussed elsewhere at a scale of 1:100,000. This was included in (Crowley & Spear 1987, Crowley 1988). Gustavson's (1974) 1:250,000 map of the Nar­ The Sitas region can be divided into 3 tecto­ vik region. As part of a reconnaissance study nic domains separated by two major low­ of northern Norrbotten county, Kulling (1964) angle post-metamorphic thrust faults , the Fros­ produced the first map of the Swedish portion tisen and Mereftasfjell thrusts (Hodges 1985). of the area at a scale of 1:400,000. These Within each of the three tectonic domains the­ maps were valuable guides for this study ; re is a distinct tectonic stratigraphy. Beneath however, signif icant differences exist between the Frostisen thrust is the structurally lowest my map (Plate 1) and theirs , particularly in domain, the Rombak Comp lex composed of areas of difficult access . the Rombak window granitic gneisses and This study continues a northwest-southeast isolated outliers of the Tornetrask Formation transect across the Scandinavian Caledonides that are in depositional contact with the gneis­ from the Lofoten Islands to the Caledo nian ses. Between the Frostisen and Mereftasfjell foreland that was started by Tull (1978), Hakk i­ thrusts is the Storriten Complex (Hodges 1985) nen (1977), Bartley (1984) and Hodges (1985). a sliver zone of phyllonites and mylonites The 20 km transect from the Efjord culminati­ derived from lithologies similar to the underly­ on to the Rombak window , mapped at 1:50,000 ing Rombak Complex. Above the Mereftas­ by Hodges (1985) is cont iguous with the Sitas fjell thrust, a structurally complex sequence area. The transect has been continued to the of metasedimentary and meta-igneous rocks southeast of the Sitas area by Tilke (1986). crop out. This uppermost domain contains Part of the Rombak window to the north of rocks that I corre late with the Middle Koli this study has been mapped at 1:100,000 (Bir­ Nappe Complex (MKNC) of Stephens (1980) keland 1976). South of the Sitas area, Bjork­ and Stephens et al. (1985) exposed in the lund (1985) has mapped and described a thrust southern Norrbotten mountains. Within the complex known locally as the Akkajaure Comp­ Sitas area, the MKNC is composed of two lex. thrust nappes , a lower Langvatn nappe and an upper Marko nappe . To the east of the Sitas area, both the Seve and the Lower Koli nappes appear as eastward-thickening wedges Tectonic stratigraphy between the Storriten Complex and MKNC. Within the Sitas region , 29 mappable lithologi­ The Sitas area is located 15 km west of the cal units were recognized (Plate 1). All of the­ westernmost exposures of Lower Koli rocks se units have been multiply deformed and and 10 km west of the westernmost exposu­ metamorphosed. Contacts between many of res of Seve rocks . the lithological units are clearly tectonic, where­ All of the rocks in the Sitas area were regio­ as other contacts are gradational and are nally metamorphosed during Caledonian defor­ assumed to be depos itional. Still other con­ mation. The metamorphic grade and history tacts are of uncerta in origin ; these are sharp of each structural domain are different. The contacts, but do not appear to be zones of metamorphic grade increases structurally up­ concentrated strain and may be either deposi­ wards. Caledonian metamorph ic cond itions in tional or tecton ic. Although many contacts are the Rombak Complex never exceeded the tectonic, the lithological units appear in a regu­ middle greenschist facies (biotite grade). Storri­ lar vertical order and form a local tectonic ten Complex metamorphism reached the up­ stratigraphy. per greenschist facies (garnet grade) and the The stratigraphy of this region was first MKNC was metamorphosed to the amphibo­ described by Foslie (1941). Foslie did not re­ lite facies. cognize the prese nce of tectonic contacts within the Caledonian nappe pile and conside­ red the Rombak window basement granitoids to be Caledonian age intrusive rocks. Kulling (1964) provided the first detailed descriptions of the Vendian to Upper Cambr ian strat igraphy 28 Peter D. Crowley GU- BULl. 416. 1989 within the Rombak and Storriten Complexes . llodgev ( 1985) Th is study In his discuss ions of the stratigraphy within Isoclinal x .w, Section the MKNC, KUlling relied heavily on the work fold S.E. Section interpretatio n S.\V. limb of IS.E. limb of of Foslie (1941, 1942) but did recogn ize the aba ndoned Haugefje ll amiform Baugefjell an tiform tecton ic nature of the cont act between the basement granitoids and the Caledonian meta­ I sediments and of some of the contacts within I the nappe pile.
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