Tectonostratigraphy in the Velfjord-Tosen region, southwestern part of the Helgeland Nappe Complex, Central Norwegian Caledonides TERJE THORSNES & HELGE UZlSETH Thorsnes, T. & Leseth , H. 1991: Tectonostrat igraphy in the Velfjord-Tos en region, southwestern part of the Heigeland Nappe Compiex, Central Norwegian Caledonides. Nor. geol. unders. Bull . 421, 1-18. In the southwestern part s of the Helgeland Nappe Compl ex, dismembered ophiolites and their cover sequences are imbricated together with nappes consisting dominantly of continentally deri­ ved rock units. A local nappe sequence comprises three nappes, informally termed the Lower , Middle and Upper nappes . The Lower and Upper nappes con sist of partly migmatitic mica gneis­ ses and orthogn eisses associated with marbles and calc-silicate gneisses. Between these nap­ pes, the Middle nappe includes an igneous complex of mafic and ultramafi c rocks of cphiolitic affinity , unconformably overlain by a cover sequence with material derived partiy from the ophio­ lite substrate. Plutonic rocks , ranging from hornblende gabbros to diorites and granites, were emplaced at seve­ ral stages during the tectonometamorphic evolution of the region . Based on comparison with nearby areas, most of the regional metamorphism and deform ation would seem to have occurred in the Ordovician , with a later phase of thrusting during the Mid Silurian to Early Devonian Scandi­ an orogeny. Terje Tttorsnes , Norges geologiske underseketse, Postboks 3006, 7002 Trondhe im, Norway. Helge t.esett; Institutt for Kontinentetsokketunderseketser, 7034 Trondheim , Norway. Introduction been mapped between Grong and Bindal (Ro­ During the past decade, a number of detailed berts et al. 1983, Nordgulen & Bering 1987, studies of the southwestern parts of the Helge­ Husmo & Nordgulen 1988), where the Late land Nappe Complex (HNC) have been made, Ordovician /Early Silurian Heilhornet Pluton in order to elucidate the complex tectonic, (Nordgulen & Schouenborg 1990) intrudes metamorphic and intrusive history of this part both the underlying medium-grade supracrus­ of the Uppermost Allochthon. The present tal rocks (Kollung 1967, Schouenborg 1988) paper focuses on the tectonostratigraphical and the basal parts of the HNC (Fig. 1). North­ development of the Velfjord-Tosen region (Fig. west of the Heilhornet Pluton, the continuation 1), situated in the southwestern part of the of the base of the HNC has not been identifi­ HNC, based mainly on work forming parts of ed with certainty (Nordgulen & MitcheIl1988). cand.scient. theses (University of Bergen), Several authors (Prestvik 1972, Gustavson concentrating on the Velfjord (Leseth 1985) 1975, 1981, 1988, Nordgulen 1984, Bang 1985, and Sausvatn-Tosen areas (Thorsnes 1985). Leseth 1985, Sturt et al. 1985, Thorsnes 1985, 1987, Terudbakken & Mickelson 1986, Husmo & Nordgulen 1988) have shown that the consti­ Regional context tuent nappes of the HNC can be divided litholo­ The HNC forms the southern most part of the gically into two main groups: (1)nappes compo­ Uppermost Allochthon of the Scandinavian sed largely or entirely of high-grade gneisses, Caledonides in Central Norway (Gee et al. partly migmatitic, associated with calcareous 1985). As the name suggests, it is composed metasedimentary rocks ; (2) nappes characteri­ of a series of nappes recording a complex sed mainly by mafic and ultramafic igneous tectonic evolution (Fig. 1). Towards the east rocks of ophiolite /island arc affinity, uncon­ and southeast , the HNC lies structurally above formably overlain by a sequence of polymict low grade rocks of the Kbli Nappes (Foslie & conglomerates, schists , psammites and mar­ Strand 1956, Ramberg 1967, Gustavson 1973, bles. The latter rocks are found in a belt along 1981, 1988, Lutro 1979, Dallmann 1986). In the the coast (Leka - Sauren - R0d0Y - Skalveer), southwestern part, the base of the HNC has in the Tosen - Velfjord area, and in the MOSj0- 2 Terje Thorsnes & Helge Lese tii NGU - BULl. 421. 1991 Fig. 1: Simplified geological map of the Helgeland area. Central Norwegian Caledonides, show ­ ing the main Iithotecton ic units and intrusions (compiled from Gustavson 1981 . Norc -qulan ;: 1984. Bang 1985. Lesetn 1985. SKAL\ '/E R Sturt et al. 1985, Thorsnes 1985. Teruobakken & Mickelson 1986. and Nordgulen & Bering 1987). L - Lower nappe, M - Middle nappe, U • Upper nappe. H ­ Helthornet, T - TerrAk. N 1 1 4,0 HORTAVIER ~ I'ORI'H YRIC GRAMTE AND c::::::J ~II GM A TI TI C G:-iEISS. GRANODIORITE :\IARBL E. SC HIST ~ MONZONITE. MO NZODIOR ITE ~ SC HIST. I'SAMI\ IIT E. :\IARBL E. CO:-iGLO:\IE RATE ~ GRANO DIO RITE . GRAl'o'lTE _ VLTRAM AFIC ROCKS. GABllRO. TON ALIT E, GRANODIORITE D AMI' HIIlOLITE. GREE:-iSTO:-iE ~ GABllRO. DIORITE t:zJ ROCKS BELOW T HE H:-iC en area (Fig. 1). The rocks found in the S0r­ metamorphic evolut ion (Kollung 1967, Nordgu­ fjord area possibly belong to the coastal belt. len 1984, Nordgulen & Schouenborg 1990). In On Leka, in the extreme southwestern part the southwestern parts of the HNC metamorp­ of the HNC, UlPb dating of zircon in quartz hic rocks of continental origin were intruded keratophyres of the Leka Ophiolite Comple x by tonal ites and granod iorites prior to c. 500 has yielded an age of 497 ± 2 Ma (Dunning Ma (Nissen 1986). These intrusions are consi­ & Pedersen 1988, Furnes et al. 1988) for an dered to have been emplaced in a foliation­ immature arc stage of the ophiolite. forming orogenic phase in Late Cambrian time A variety of Caledonian plutons collectively (Nissen 1986). Many of the large plutons collec­ termed the Bindal Batholith are found in the tively grouped as the Bindal Bathol ith appear HNC. The intrus ions range in composition from to be essentially of Late Ordov ician to Early gabbro to quartz diorite and granite , and were Silurian age (Nordgu len & Schouenborg 1990), emplaced at various stages during the tectono- and a Silurian mineral age has been recorded NGU - BULL. 421. 1991 Tecto nos tratigraphy in the Ve/fjord-Tosen region 3 VElFJ ORD I"All---.lGNEOUS_ R Q C~ S m HORNBLENDE OIORITE . GRANI TE. GRA NOD IORlTE UPPER N APPE IT] MICAG"N EiSS M!DOL E~A~E IT] ~~:~RL~~~TEM~ C H~;~ B l[ . GPEE N PSAMMITE , CD ~: ~g:~~ ~~s cg6~~~::J~:~;E POl YMICT CD ~~~~Al~~~iT. O~A~~~~~~~~R~~IST I b I ~~~~ I ~~ c~~~~ ~~ ~G : A6~'N~~V:~;~T E IT] ~g~~;i.E~~,.~tttD S;SH~~~ I~:~C~~~O~SA R B l E co ~~~~ Lg~~~~~~EAATE . PQlYMICT MAFIC o:J ~t~:D~ : ~ I~S~M61~'R ~; ~~~~~O~~A :~I~~S ~ IGN£OUS BASEMENT UNIT , METAGABBRQ QI] ~~~X~T~ASSACl ~~~g~~ ~~~~T~.C ~I;~ B ~~ T H e;:12J;1 ~~~~~~~G ~TAES~ ~~ ~~T EU N IT L9-'iLEA NA P P ~ ~ GRANIT IC DYK ES ~ M ICA GN EI SS . CALe -SI LI CATE GNEISS C!D MARBLE M AJ OR THRUST ....L---.... MI NOR THRUST __ FAU LT __ TECtONICAL LY MODI FIED UNCON FORMI TY Fig. 2: Synoptic block diagram. showing the main lithOtectonic and structural units in the Velfjord-Nordfjellmark area . from the Mosjeen Gabbro (Teru dbakken & while the final accretion of the HNC occurred Mickelson 1986). during the Late Silurian/Early Devonian Scandi­ Most of the deformation and thrusting of the an phase of the Caledonian Orogeny. HNC has traditionally been considered to have occurred during the Mid-Late Silurian to Early Devonian Scandian phase of the Caledonian Tectonostratigraphy: a summary Orogeny, though in several stages (Roberts In the Velfjord-Tosen region, the rocks can et al. 1983). In the Hattfjelldal area, along the be assigned to three nappes, informally named eastern margin of the HNC, Dallmann (1986, the Lower, Middle and Upper nappes (Fig. 2) 1987) considers that polyphase eastwards The Lower and Upper nappes compr ise litho­ thrusting of the HNC took place in Middle demic complexes of gneisses and calcareous Silurian times. This thrusting postdated thrust rocks . emplacement of the underlying Hattfjelldal The Middle nappe is the most extensively Nappe, forming part of the K61i Nappes (Dall­ studied nappe unit in the present investiga­ mann 1986). Radiometric datings by Nordgulen tion. It can be divided into two units forming & Schouenborg (1990), however, indicate that a basement-cover couplet. The basal unit con­ the juxtaposition of the HNC with the structu­ sists of variably altered ultramafic rocks and rally underlying medium-grade supracrustals metagabbros. In the Velfjord area, the Hegge­ in the Vestranden region along the south­ fjord ultramafite constitutes the major part of western margin of the HNC, and internal im­ the igneous basement unit. The cover unit brication of at least parts of the HNC, took consists of schists. metaconglomerates, psam­ place prior to or during the Late Ordovician, mites and marbles. 4 Terje Thorsnes & Helge Leseth GU · BULL. 421. 1991 The Heggefjord ultramafite shows evidence M ETAMORPH ICGRADE of several phases of deform ation which occur­ red prior to deposition of the overlying metase­ ' ~T IN G dimentary cover rock s. In the latter unit, 4 phases of deform ation have been established HIGH -- (0 1-04), with the peak of metamorph ism (medi­ um-grade) during 02. Structures assoc iated with 01 are few and usually obliterated by the M EDI UM - J~---- penetrative S2 foliation, formed as a result LOW -- / - __ of isoclinal folding and thru sting during 02 L----------II~ T I ME (Fig. 3). Later deformation (03 and 04 ) refol­ D, ded the structures formed dur ing 0 1-02 and produced the presen t attitude of the rocks, Fig. 3: Schematic presentation of timing of metamorphism dipping 40-60° towards the ENE (for a more and thrusting for the cover sequence within the Middle nappe. detailed descr iption of the deform ation and metamorph ism, see page 14). metamorph ism in the area, intrude the contact (0. Nordgulen pers. comm. 1989). However, the present authors consider a thrust most Lower nappe likely, since the Lower nappe appare ntly struc­ The Lower nappe comprises a heterogeneous turally overlies the less deform ed and meta­ assemblage of gneisses, schists, calc-silicate morpho sed Sauren unit.