Northern continuation of Caledonian high-pressure metamorphic rocks in central-western Spitsbergen Y. OHTA, A. A . KRASIL'SCIKOV, C. LEPVRIER and A . M. TEBEN'KOV Ohta. Y., Krasil'SEikov. A. A., Lepvrier, C. & Teben'kov, A. M. 1995: Northern continuation of Caledonian high-pressure metamorphic rocks in central-western Spitsbergen. Polar Research 14(3), 303- 315. Caledonian low-temperature, high-pressure metamorphic rocks, as first recognised at Motalafjella. central- western Spitsbergen, include characteristic brown dolostone and serpentinite. Similar rocks are scattered in the strandflats, along the eastern marginal fault of the Tertiary Forlandsundet Graben, and along thrust faults to the east on both sides of St. Jonsfjorden. The mineral chemistries of the constituent carbonates and oxides are diagnostic of high-pressure metamorphic rocks, containing high contents of MgO in the carbonates and C r 2 0 3 in the oxides. Based on a surface magnetic-anomaly survey and fault-plane obser- vations, some of these rocks are considered to have been pressed up along faults produced by strike-slip faulting during a n early stage of the Forlandsundet Graben formation. The distribution of these rocks indicates that the high-pressure metamorphic rocks extend as much as 50 km to the NNW from Motalafjella to Sarsoyra. Y . Ohta, Norsk Polarinstitun, P . O . Box 5072-Majorstua, N-0301 Oslo, Norway; A . A . Krasil'SEikou and A . M . Teben'kov, Polar Marine Geological Expedition, ul. Pobeda 24. Lomonosou, 189510 St. Petersburg. Russia; C . Lepurier, U R A CNRS 1759, Dkpartemenr de Gkotectonique, Uniuersitk Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Ceder 05, France. Introduction A Caledonian low-temperature, high-pressure metamorphic complex of subduction-zone origin, the Vestgotabreen Complex, occurs south of St. Jonsfjorden, central western Spitsbergen (Hors- field 1972; Ohta 1979; Ohta et al. 1983; Ohta et al. 1986; Hirajima et al. 1984; Hirajima et al. 1988). These rocks occur in a thrust complex and are divided into two units based on their metamorphic grade within the typical exposures in Motalafjella (Fig. 1B). The high-grade division has chloritoid-garnet- mica schists with eclogite inclusions, glauco- phane-garnet-mica schist, eclogitic metagabbros, omphacite quartzite (Kanat 1984; Hirajima et al. 1988), and micaceous, schistose marble. The protoliths are thought to be a deep-sea igneous- sedimentary succession (Ohta et al. 1983). The age of the igneous protoliths for the eclogitic metagabbros is considered to be ca. 1,100 Ma, a Sm/Nd model age, which is possibly an age of mantle derivation (Bernard-Griffiths et al. 1993). The low-grade rocks are black phyllites with chlorite-sericite assemblages, green phyllites with local pillow and dyke swarm structures and laws- onite-pumpellyite assemblages, limestone and quartzite lenses, and green-brown dolostone with fuchsite. Small tectonic inclusions of serpentinite occur within the dolostone (Ohta 1979; Hirajima et al. 1984; Teben'kov & Korago 1992). The metamorphism is of early Caledonian age (Ohta et al. 1989, Dallmeyer et al. 1990). Although typical high-pressure metamorphic rocks are restricted to outcrop from Motalafjella to SW Holmesletfjella, green-brown dolostone and serpentinite similar to those in the low-grade unit of the high-pressure complex are found at isolated localities in the western part of Oscar I1 Land, north of St. Jonsfjorden. The green-brown dolostone with fuchsite is genetically related to the hydration of ultramafic rocks (Teben'kov & Korago 1992) and is easily recognisable in the field. This rock type and associated serpentinite occur at Ankerfjellet, the eastern and southern parts of Kaffieyra and as far north as Sarsoyra. The last locality is ca. 50 km NNW from Mot- alafjella. These distinctive rocks follow thrust faults in mountainous areas to the east of the Forlandsundet Graben, but most of those in the strandflat areas within the graben are isolated due to extensive cover of Tertiary and Quaternary sediments. This paper presents mineralogical evidence to distinguish the green-brown dolostone of the 304 Y . Ohta et 01. & A 3 I b I Fig. /.Gicncralised geological map around Forlandsundet, from Sarsciyra to Eidembukta. with an index map A . Index map. western part of Spitsbergen. Explanation: Obliquely lined, Tertiary fold-thrust zone: horizontally lined, ba\ement rock\. Place names: K N . Kinnefjellet: PR, Protektorfjellet: O R . Orustdalen; RE. Recherchefjordcn; R A . Raudfjcllet: K V . Kvivrt)dden. T h r square indicates the area of the map at right B Gecilogical map. Explanation: 1, Tertiary: 2. Carboniferous. 3. Upper Ordovician-Middle Silurian: 4. Vendian tilloids; 3. Late Proterozoic successions. excluding the Vendian tilloids: 6 and 7. St. Jonsfjorden Group: 6, Calc-argillaceous phyllites (Alkhornet Formation). 7. Quartzite and phyllites (Lsvliebreen Formation). 8. garnet-biotite whist5 (the Mullernesset Formation i n S W S I . Jonsfiorden and the Pinkie Formation at Bourrefjellet in Prins Karls Forland); 9 , high-pressure metamorphic complex around Motalafjella: 10, schists and gneisses in Breggerhalwya: 11. lithological boundary; 12. fault and thrust The white areas are c o w r e d bv glacier,. B = Bulltinden. <: = Copper Camp. T = Thorkelsenfjellet. Northern continuation of Caledonian high-pressure metamorphic rocks 305 high-pressure metamorphic complex from other carbonate rocks. Their distribution can be cor- related with high magnetic-anomaly zones along the eastern margin of the Forlandsundet Graben; these zones reflect the Tertiary fault pattern, which is examined kinematically with respect to the fault-plane observations. Distribution of the green-brown dolostone and serpentinite The high-grade unit of high-pressure meta- morphic rocks in Motalafjella occurs as a thrust sheet above the low-grade unit, and the latter is unconformably covered by the Late Ordovician- Middle Silurian flyschoids. The low-grade unit and the flyschoids together form an upper limb of an overturned syncline, therefore, the low-grade unit structurally overlies the flyschoid strata (Ohta et al. 1983). A green-brown dolostone in Motalafjella occurs in the lowest structural position of the low- grade unit, with a thickness of 50-80 m (Hirajima et al. 1984; Teben’kov & Korago 1992), directly in contact with structurally underlying, but younger basal conglomerate, 0-5 m in thickness, of the flyschoid sediments, which contains subangular clasts of the green-brown dolostone and Ordo- vician gastropods in the matrix (Ohta et al. 1983). A grey limestone occurs structurally below (strati- graphically above) the basal conglomerate, and a thick conglomerate even below this limestone contains boulders of the green-brown dolostone. Some grey limestones form olistostromes in this conglomerate and include Late Ordovician to Middle Silurian corals and gastropods (Scrutton et al. 1976) and conodonts (Armstrong et al. 1986). These rocks and the stratigraphically over- lying sandstone-shale succession (Table 1) have been termed the Bulltinden Group and are of flysch origin (Harland et al. 1979). The high- pressure metamorphism has been dated ca. 475 Ma by the 40Ar/39Ar and Rb/Sr mica- whole rock methods (Dallmeyer et al. 1990) and is older than the unconformity. This geological relationship is exposed in the two ridges to the north of Motalafjella and southernmost part of Bulltinden (Fig. 1B). The unconformity has locally been disturbed by thrusts. The overturned syncline of the Late Ordovician to Middle Silurian Bulltinden Group, which occurs to the east of the high-pressure meta- morphic rocks, has a moderately west-dipping axial surface. Along the lower limb of this syncline, a thin green-brown dolostone and some black phyllites occur below the grey limestone of the Bulltinden Group on the south-eastern ridge of Holmesletfjella and crop out discontinuously along a thrust on the cliffs from middle-north to north-western Holmesletfjella to Copper Camp (Fig. 1B). The dolostone is thrust over the Alk- hornet Formation (areno-argillaceous phyllites and marble layers) of the St. Jonsfjorden Group (Harland et al. 1979; Hjelle et al. 1979; Berg et al. 1993) of probably Middle Proterozoic age. The thrust surfaces have moderate westerly dips at eastern Holmesletfjella and Motalafjella. Similarly discontinuous outcrops of the green- brown dolostone are found from the northern foothill of Bulltinden to the flat area between Thorkelsenfjellet and Bulltinden. ca. 200 m from the southern shore of St. Jonsfjorden, and their cleavages are almost horizontal (Fig. 1B). On the middle slopes of Ankerfjella on the northern side of St. Jonsfjorden, thin and discontinuous lenses of green-brown dolostone occur along roughly horizontal thrusts on both the southern and north- ern sides of the mountain, bounding the Vendian tilloids and the St. Jonsfjoden Group below and the conglomerate and shale-sandstone suc- cessions of the Bulltinden Group above. Table 1. Lithostratigraphy in Oscar I1 Land (modified from Berg et al. 1993). Paleogene Middle Carboniferous Early Devonian Late Ordvician-Middle Silurian Vendian Late Proterozoic Middle Proterozoic sandstones, conglomerates sandstones. carbonates conglomerates Bulltinden Group sandstone-shales grey limestones conglomerates Comfortlessbreen Group diamictites (tilloids) Daudmannsodden Group pelitic phyllites carbonates Vestgotabreen high-pressure metamorphic complex St. Jonsfjorden Group Alkhornet Formation (carbonate-phyllite succ.) L0vliebreen Formation (quartzite-phyllite succ.) 306 Y . Ohta et al. _ _ _ ~ I , / Andrcnsbrcen % Firnisundct Fig. 2 . High magnetic-anomaly segments and zones (simplified from Krasil'stikov et al. 1995). A. S a r s ~ y r a , with the profile A- B (lower figure). showing calculated magnetic-causative rock bodieb: black = serpentinites, dotted = metahasic rock. S and D , refer to Fig. 5. A t Snippen, on the southern coast of K a f f i ~ y r a (Fig. 2B ), narrow serpentinite lenses are interbed- ded with green and purple phyllites within a shea- red zone of ca. 100 m width, which bounds the Tertiary sediment to t h e west. A similar ser- pentinite and green phyllite assemblage occurs discontinuously along t h e northeastern foothills of K a f f i ~ y r a . Scattered outcrops of the basement rocks within t h e Forlandsundet G r a b e n in eastern K a f f i ~ y r a have dips of more than 30" to the east or Northern continuation of Caledonian high-pressure metamorphic rocks 307 west with NNW strikes. This structure is distinctly different from complex thrust duplexes which lie to the east of the graben margin fault. A ca. 1-3 m thick, strongly sheared serpentinite with talc-rich'slip planes occurs with a 3 m wide sheared conglomerate adjacent to the east along a river within the northern moraine of Aavats- markbreen at the foothills of southeastern Sarsoyra (Fig. 2A). The conglomerate has undeformed quartzite clasts in a greenish brown, weakly phyllitic sandy matrix and is similar to the conglomerates of the Bulltinden Group. To the east of the conglomerate are green phyllites, poss- ibly a member of the Vendian tilloid succession, bounded by a shear zone. To the west of the serpentine are black and green, shaly-sandy phyl- lites with thin intraformational conglomerate, quartzite and grey limestone layers, the Aavats- markbreen Formation of Waddams (1983) and Harland et al. (1993). They considered this for- mation the uppermost formation of the Vendian tilloid succession, but it is better correlated with the sandstone-shale succession of the upper part of the Bulltinden Group, based on Paleozoic microfossils recently found in massive limestones near northernmost Sarsoyra (Makarjev, pers. comm. 1992). Scrutton et al. (1976) also reported Paleozoic fossils from southern Sarsoyra. The cleavages in these metasediments are steep, 40- 70" to the east and west, and many tight folds are expected. Similar occurrences of serpentinite associated with brown-weathered dolostone lenses have been found along the foothills in eastern SarsByra. In addition, seven localities of brown dolostone with green fuchsite clots occur in the middle part of Sarsoyra (Fig. 2A). These green-brown dolo- stones and serpentinites are considered to be members of the low-grade, high-pressure meta- morphic complex, as discussed later. The thrust sheets comprising the high-pressure metamorphic rocks have moderate west dips in the eastern localities at southeastern Holmeslet- fjella and Motalafjella, but northwest of Bulltin- den and around Ankerfjella they are almost horizontal with gentle open synform structures. These structural features and scattered occur- rences of serpentinite and green-brown dolostone in the strandflats suggest that the thrust sheets, including the high-pressure metamorphic rocks, occur in the subsurface of Kaffioyra and Sarsoyra to the north. The moderate-steep structures of the basement rocks near the foothills were formed by later Tertiary deformations. Diagnostic minerals of the green- brown dolostone The green-brown dolostone at Motalafjella is white to grey on fresh surfaces, and is mostly massive with many local thin quartz veins in an irregular network; it has mm-to-cm scale irregular green clots with diffuse margins and scattered opaque grains. Schistose structure, marked by fine-grained layers, is present locally and the cleavages are partly marked by green colour. Local sulphide and ankerite-siderite mineral- isations are also seen. Serpentinite bodies of metres t o tens of metres size are included in the phyllites and dolostone, commonly enclosed by conformable slip-cleavages. The dolostone has 10-23 wt% CaO only within or near sheared zones; massive parts have less than 5 wt% CaO (commonly 1-2 wt%) and 23- 36 wt% of MgO (Table 2). Thus, this carbonate rock is primarily magnesite-stone that has been modified along the sheared zones into dolostone. Total FeO averages 5 w t % , but locally in the sheared zones this increases to 7% (Teben'kov & Korago 1992). Table 2. Chemical compositions of rocks and constituent min- erals of the green-brown dolostones from various localities. ROCK MINERAL 1 2 3 4 5 6 7 S i 0 2 T i 0 2 A1203 FeO* MnO CaO N a 2 0 KzO Cr203 NiO Total MgO 12.15 32.80 0.06 - 2.51 1.48 4.34 6.08 0.14 0.17 14.86 36.36 26.33 3.16 0.72 0.18 0.25 0.11 _ - - _ 61.36 80.34 - 0.03 0.00 0.02 0.25 - 0.02 0.02 0.00 0.04 3.76 0.01 0.00 0.01 0.01 - 4.25 4.57 3.66 3.88 - 0.11 0.21 0.04 0.02 44.41 17.61 18.00 46.31 45.23 0.18 30.48 30.50 0.02 0.88 - 0.01 0.00 0.00 0.00 - 0.02 0.01 0.00 0.00 - 0.08 0.00 0.00 0.00 - 0.00 0.00 0.24 0.00 48.35 52.62 53.31 50.30 50.31 1 , 2 Representative dolostone and magnesite-stone, both from Motalafjella (Teben'kov & Korago 1992). 3-7 New mineral analyses by EMPA: 3. magnesite from Motalafjella; 4 and 5, dolomites from Ankerfjella; 6 and 7, magnesites from Sars~yra. 308 Y . Ohta et a l . T u b k 3 . Chemical composition of fuchsites. new data by EMPA. I 3 4 > h SlO: TiOl ' 4 1 2 0 , FeO' M n O C a O Na,O KzO Cr,O, N10 Total MgO 48.42 46 91 48.96 48.04 46.89 46.22 0 . 0 5 0 03 0.12 0.11 0.08 0.07 27.46 26.12 3 2 . 0 9 30.88 25.39 21.92 0.02 0 02 0 02 0.00 0.00 0.00 1.61 2 31 1 . 6 1 1 6 9 1 . 2 1 1.39 0.10 0 !I3 0.05 0.07 - - (I 18 IJ 22 0.21 0.23 0.39 0.32 7.94 8 . 0 3 9.27 9.65 8.58 9.20 9.39 9 (16 1.40 2 82 11.81 11.12 (1.15 I176 0.05 0.01 - - 95.68 93.Yl 95.31 91.03 94.53 93.51 0.46 o 41 (I 5 3 n.s3 0.15 0 27 1 dnd 2 , from Motalaflella. 3 and 4. frum Ankerfjella. 5 and 6. from S a r s q r a The analyses of carbonate minerals from mass- ive dolostone of Motalafjella also show pure magnesite with less than 0 . 2 w t % C a O and 44 wt% M g O (Table 2). The green mineral is the chromium-bearing. (ca 10 wt% C r , 0 3 ) white mica. fuchsite (Table 3). The chromium contents a r e higher than that of some metamorphic rocks. which have commonly 2-5 wt% of CrZO,, and this is indicative of a close genetic relationship of the fuchsite with an ultramafic rock. The opaque minerals are chromite. some with more than 4 8 w t % Cr203. and chromian spinel (Table 4 ) . T h e analysed chromites from Motala- fjella contain variable amounts of Z n O , 3- 18 wt%, especially around the margins of grains. As much as 3 wt% Z n O has been reported from sagvandite in northern Norway (Donath 1931), but 13-18 w t % in t h e present minerals a r e exceed- ingly high. T h e low A1203 contents in these min- erals show that the Z n is not in the form of spinel o r gahnite, but possibly in the form of oxides or wlphides (S has not been analysed). Later sulphide mineralisation in the sheared zones may have partly modified the marginal parts of the chromite. T h e chromian spinels from Sarsoyra also contain some amounts of Z n O (Table 4). Their host dolostones are from sheared parts with later sulphide mineralisation. Fuchsite and chromium-bearing minerals, together with magnesite, provide critical evidence for a n ultramafic affinity of the dolostone (O'Hara 1967). and they discriminate these rocks from other carbonate rocks of sedimentary origin present in the area. Carbonate minerals of simllar dolostones from Ankerfjella and S a r s ~ y r a have been analysed by an E M P A as shown in Tables 2 , 3 and 4 . They also have magnesite composition and are associ- ated with chromium-bearing minerals demon- strating that these rocks, scattered as far north as northern Sarsoyra, belong t o the high-presure metamorphic complex. The extent of the high- pressure metamorphic rocks is, therefore, about SO km in a NNW-SSE direction. ruble 4 C'hemical cornpusitions of chrornites and ~ h r o m i a n spinels. new data by EMPA SIOl Ti0: A1?0, FeO' M n O C a O Ua.0 K 2 0 C r 2 0 , VIO ZnO Total MgO 7 I 3 4 5 6 7 8 9 10 11 0 5 0 0.511 0.YJ 0 SO 0.08 0.08 0.59 0.79 0.00 0.03 - - ~ - - 0.03 0.04 0 2 1 0 . 1 8 0.04 0.04 - 8.06 10.58 9.44 9.06 31.21 79.58 3 . W 4.10 28.55 28.95 32.04 I4.8O 29.44 21.93 21.46 14.34 20.25 44.08 43.68 14.36 11.15 13.13 O.(X) 0.00 0.M) ( I 00 U 1 1 0 28 (1.21 0.14 0.12 0.13 - 0.58 1.72 0.87 0.80 14.60 8.11 0.30 0.30 14.84 15.10 15.81 .- - - - 0.01 (1.00 0.04 0.00 0.00 0 . 0 0 - 0.00 0.(MI 0.OC 0 MI 0.04 0 . 1 1 0.14 0.09 0.01 0.00 - 0 . I M J 0.Ml O.(XI 0.00 0.03 (1.04 0 0 2 0.02 0.00 - 5 3 201 47.96 48.89 51.00 36.99 3 4 7 3 44 63 42.35 4 1 18 41.54 38.87 0 1x1 0 ( X I 0.00 0.03 0.09 0 08 0 3.7 0 . 5 1 0 04 0 . 2 3 0.21 - 0.11 17.96 2 86 13.51 12.94 - - 94 M ) 92 56 94.64 95.29 97.53 93.33 Y3.64 Y 2 16 W 14 100.17 101.20 .. . - - - 12 13 14 - - - 28.38 30.94 30.17 28.03 19.92 20.62 2.19 9.60 8.19 - - - - - - - - - 34.88 36.25 36.20 0.19 0.14 0.00 6.03 3.55 4.59 99.70 100.40 99.77 1-4 Ankerfjella. 7 - 1 4 from Motalafjella 1 = rim of chromite: 2 = core of 1: 3 and 4 = homogeneous chromites: 5 and 6, chromian spinels from from S a r s q r a : 7 and 8 = chromites; 9. 10 and 11 = chromian spinels; 12. 13 and 14 = Zn-rich chromian spinels. Northern continuation of Caledonian high-pressitre metamorphic rocks 309 Table 5 . Chemical composition of chloritoids (new analyses by EMPA) from a Tertiary grltty sandstone on the river hed in the northern moraine of Aavatsmarkbreen. Mg/Fe: Mg/Fe* + Mg (Fe* = total Fe as Fe”). 1 2 3 4 5 6 7 8 9 10 S O z 24.32 24.57 24.52 24.58 24.43 24.66 24.50 24.68 24.38 24.71 AlzO, 39.09 38.78 38.46 39.94 39.93 40.57 40.32 40.23 39.99 40.21 FeO* 25.49 26.07 25.57 26.94 25.88 23.81 23.87 23.83 24.14 23.91 MnO 0.47 0.59 0.51 0.25 0.19 0.77 0.84 0.85 0.89 0.83 MgO 2.00 2.18 2.28 1.01 1.03 1.87 1.59 1.74 1.42 1.63 Total 91.37 92.19 91.34 92.72 91.46 91.68 91 12 91.33 90.82 91.35 Mg/Fe 12.30 14.60 13.70 6.40 6.60 12.30 10.60 11.50 9.50 10.80 Detrital chloritoid grains A Tertiary conglomeratic sandstone exposing on the river bed along the northern side of Aavats- markbreen moraine contains detrital chloritoid grains in its matrix, as much as 0.2 volume percent. The chloritoid grains are fresh and angu- lar in shape, indicating a short transport distance. It is assumed that the weathering was of short duration and that the grains were moved as stream bed load. This rock also contains cm-size clasts of limestone with fragments of Late Paleozoic fossils. Because Carboniferous rocks are assumed to occur widely in this area before the devel- opment of Tertiary graben (Gabrielsen et al. 1992), the sources of the sandstone materials are considered to be local. The chloritoid may have been supplied from the low-pressure meta- morphic rocks exposed together with the ser- pentinite in the foothills of southeastern Sars0yra. The Mg/Mg + Fe* ratios (Fe* = total Fe as Fe+*) of the detrital chloritoid grains (Table 5 ) are similar to those of the chloritoid rims in the chloritoid-bearing schists of the high-pressure metamorphic complex (Ohta 1979; Hirajima et al. 1988), but not to those occurring on Prins Karls Forland (Manby 1983) (Fig. 3). It is not likely that the chloritoid grains were transported from Prins Karls Forland across the graben. Given the angularity of the grains and their amount, and the lack of this mineral in the Tertiary sediments in Kaffioyra, it is unlikely that these chloritoid grains had been transported from the Motalafjella area for ca. 50 km along the eastern margin of the Forlandsundet Graben basin. Possible southern continuation of the high-pressure metamorphic rocks The extent of the high-pressure metamorphic rocks to the south of Motalafjella is not known. However, a metabasic rock lense on the north- western slope of Kinnefjellet, ca. 5 km south of Motalafjella (Fig. l A ) , has amphiboles with a winchite core and actinolite margin (Ohta et al. 1992); winchite is an indicator of high-pressure metamorphism of moderate temperature range (Hirajima et al. 1988). Other metabasic rocks on the northern slope of Protektorfjellet (Fig. l A ) , Fig. 3. Comparison of the Mg/Fe* + Mg ratios of the chloritoids. White, from Motalafjella (Ohta 1979 and Hirajima et al. 1988); dotted, from Prim Karls 0 ) Forland (Manby 1983a); z 15. C 5 10. black, detrial chloritoid F grains in a Tertiary 5 . analyses by EMPA). 60 50 40 30 20 10 conglomeratic sandstone, southern S a r s ~ y r a (new Fe’ =total Fe as Fe+*. MgIIWg + Fe* 310 Y . Ohta et al. ca. 2 0 k m SSE of Motalafjella, have a pumpellyite-prehnite-chlorite-epidote assem- blage (Ohta et al. 1992). A brown dolostone, with a small amount of fuchsite with 1 . 7 w t l c Cr,03 has been reported by Hjelle (1962) from N E of Liknausen in Orustdalen (Fig. 1A) in southern Nordenskiold L a n d , between Isfjorden and Bellsund, ca. 50 km south of Motalafjella. If these rocks represent signs of the high-pressure metamorphism, the distribution of high-pressure metamorphic rocks becomes roughly 100 km in length. Farther to the south at Asbestodden, west of Recherchefjorden (Fig. 1A) on the southern side of Bellsund, a serpentinite with asbestos veins occurs in a limestone-rich phyllite succession of Late Proterozoic age (Dallmann e t al. 1990). Sul- phide and ankerite-siderite mineralisations, characteristic along some thrust zones around Motalafjella (Teben'kov & Korago 1992), have also been described from Raudfjellet (Fig. 1 A ) in northwest Hornsund and Kviveodden (Fig. 1 A ) on the southern coast of Hornsund (Czerny et al. 1992a, b) with some sulphides. These min- eralisation zones could be an extension of the thrust-dominated zone (senso lato) involving sheets of the high-pressure metamorphic rocks, although these rocks a r e not exposed on the present surface. Reference to the surface magnetic anomaly patterns T h e scattered distribution of very small, green- brown dolostone and serpentinite outcrops in Sarsoyra were considered by some geologists to possibly be Quaternary glacial erratics. However. a recent surface magnetic-anomaly survey over the area (Krasil'Sfikov et al. 1995) shows that distinctly high, positive-anomaly zones coincide exactly with the localities of these rocks (Fig. 2A). Examinations of magnetic susceptibility o n various rock samples showed that the green- brown dolomites, serpentinites, some magnetite- than a few hundred metres wide, and they extend roughly in a N-S direction for approximately 1- 4 km in Sarscbyra (Fig. 2A). A distinctive easternmost high-anomaly zone along the foothills of Sarscbyra, which coincides with t h e eastern marginal fault of the Tertiary Forlandsundet G r a b e n , has a N N W trend and consists locally of a left-stepping en echelon arrangement of the high-anomaly segments within the zone (Figs. 2A and 4). Another high-anomaly zone in the western part also has similar trend and en echelon arrangement of the high-anomaly segments. while NNE-SSW trending discon- tinuous zones in the middle part of S a r s ~ y r a shows a right-stepping en echelon segments in its north- ern part. These three high-anomaly zones sep- arate S a r s ~ y r a into two subareas: N W and SE (Fig. 4). T h e NW subarea has an acute edge to the south a n d the SE subarea shows pointing edge t o the north. This subarea pattern infers a sinistral strike-slip or transpressional movement which formed these subareas near t h e eastern margin of the Forlandsundet Graben. If this is the case, t h e N N W trending high-anomaly zones to the east and west of Sarscbyra consisted of left-stepping segments, can be explained as dextral strike-slip or transpressional fault zones with antithetic faults (Lowell 1972; Wilcox e t al. 1973). T h e small angles between t h e high-anomaly segments and the zones can be interpreted by a flattening of Kapp Graarud - bearing green phyllites and Mesozoic dolerites in t h e strandflat are the causative rocks Of these high. positive-anomalies. This Correlation Fig. 4. Schematic interpretation of the fault movemcnts along the high magnetic-anomaly zones in Sarseyra. Explanation of the arofile: Dotted area. Tertiarv sediments: broken hatched ~~ indicates that the small scattered outcrops are bedrock exposures. The results of the magnetic survey show t h a t distinctive high-anomaly segments are concen- trated in narrow zones. T h e segments are less area, Late Ordovician-Middle Silurian; white, Precambrian: black. the green-brown dolostone and serpentinite; broken line. f a u l t . Surface of the profile: solid lines indicate boundarles of high-anomaly zones; open arrows, inferred movement sense. Displacement by supposed strike-slip movements are sche- matically shown by strain ellipsoids. Northern continuation of Caledonian high-pressure metamorphic rocks 31 1 the zones to rotate the segments by later near orthogonal compression, as mentioned later. These explanations of the sense of fault move- ment can be debated; however, the present data do not allow any more solid conclusion, due to very poor exposures around the high-anomaly segments. It will be safe to state that the en echelon arrangements of the segments within the high-anomaly zones indicate a strike-slip move- ment along the eastern marginal fault of the For- landsundet Graben. The occurrences of serpentinite and green- brown dolostone coincide with the high-anomaly segments. The green-brown dolostone is very solid in thick layers, but it is strongly brecciated and sheared together with phyllites at all occur- rences in the strandflats. Thus, this rock is sup- posed to be pressed up, together with incompetent serpentinite, along the antithetic faults. The high magnetic anomalies can be modelled to be narrow, steeply west-dipping plates reaching as deep as ca. 5 0 m . (Krasil’SEikov et al. 1995) (Fig. 2A, lower figure, and Fig. 4). It seems that the steep inclinations of the causative rocks result in the high, positive-anomaly segments. But in other places the same rocks have an almost hori- zontal, thin sheet structure at ca. 50 or more metres depth; there they do not show any dis- tinctive high-anomaly, but apparently smooth, near normal magnetic fields, due to their small vertical thicknesses. A weak, but similar, pattern of the high mag- netic-anomaly segments and zones is recognisable in the eastern margins of Kaffi~yra (Fig. 2B). An indistinct left-stepping en echelon arrangement of high-anomaly segments can be seen in this zone, which coincides with the eastern marginal fault of Forlandsundet Graben. Scattered exposures of the green-brown dolostones, serpentinites and some other magnetite-bearing phyllites occur along this zone and a short, N-S trending zone around Snippen. In western Kaffi~yra, the magnetic high-anom- aly segments are not as distinctive as in the middle part of S a r s ~ y r a , possibly due to thicker Tertiary cover as in the westernmost part of Sarseyra. However, the general similarity of the high-anom- aly patterns in Kaffi~yra and Sarseyra implies similar underlying magnetic causative rocks, including the green-brown dolostone and ser- pentinite of the high-pressure metamorphic com- plex. Fault plane observations and Tertiary movements The eastern margin of Forlandsundet Tertiary Graben is bounded by steeply dipping, 320-360”- striking fault segments. The tectonic development of this basin comprises several episodes of post- depositional brittle deformation, also revealed by the paleostress state evolution in the Tertiary strata (Lepvrier & Geyssant 1985; Lepvrier 1990; Gabrielsen et al. 1992; Kleinspehn & Teyssier 1992). The graben-like aspect is due to a late phase of extension/transtension and has devel- oped in early Oligocene in relation with the plate- tectonic reorganisation which occurred at that time. Prior to this late event, the basin had suf- fered a phase of near-orthogonal compression during Eocene time (Lepvrier 1990; Gabrielsen et al. 1992), and the marginal faults are cross-cut and offset by a conjugate set of the WNW-ESE and WSW-ENE striking strike-slip faults (Fig. 1B). A still older strike-slip or transpressional phase can be proposed as suggested by the exist- ence of a first set of structures (Lepvrier 1990; Gabrielsen et al. 1992) and by the interpretation of the high magnetic-anomaly segments and zones. A similar three-stage structural evolution, including successively dextral transpression, con- traction and extension/transtension, is observed for the kinematics of Tertiary deformation in the West-Spitsbergen fold-and-thrust belt (Braathen & Bergh 1995). Based mainly on the regional plate-tectonic setting, it has been suggested that the For- landsundet Tertiary basin developed initially as a transtensional zone between right-stepping dex- tral wrench faults in a general regime of tran- spression (Lowell 1972; Steel et al. 1985; Ohta 1982, 1988; Lepvrier 1988; Gabrielsen et al. 1992). This pull-apart model for the Forland- sundet Graben is consistent with sedimentological observations (Rye-Larsen 1982) and the inter- pretation of the magnetic-anomaly patterns (Kra- sil’SEikov et al. 1995), but structural kinematic indicators for strike-slip movement are rare. Within the Tertiary deposits cropping out near the eastern margin of the basin, observed fault striae on small-scale fault planes are related only to the episodes of compression and final extension/transtension. These can be seen along the northwestern coast of S a r s ~ y r a , in the Pale- ogene Sarsbukta Formation, north of Aavats- markbreen (locality S in Fig. 2A) and around 312 Y . Uhta et al. A 6 2 1 8 0 " . 800 3 R = 0 8 8 6 6 3 300O. 5 O 6 R = 0.139 V 0 3 186'. 32O R = 0.602 u 3 . 340°, 1 0 0 N E N C u ? 3 1 " . 14O S R = 0 3 7 2 0 3 1 2 3 " . l o o u l : 1 8 8 O . 83O - 02: 540, 50 S R = 0.506 u 3 323". 5" Northern continuation of Caledonian high-pressure metamorphic rocks 313 Snippen in southern Kaffi~yra (Lepvrier 1990), where movements in relation to these two suc- cessive episodes of faulting can be seen on NE- SW and E-W to ESE-WNW striking sets of fault planes (Fig. 5D and E). The fault-planes have not been restored to their pre-folding/tilting orien- tations in Fig. 5D. However, the pitches of the striae are nearly similar to the dips of the beds, the poles of which are shown by small open circles i n Fig. 5D. These fault striae will be roughly horizontal, when the beds are rotated to the hori- zontal position. The slip-movements which formed these striae took place prior to the folding/ tilting during the earliest stage of the com- pressional event. Only the final extensional event (Fig. SE), in part observed on the same fault- planes and marked by dip to oblique slip move- ments, occurred after the folding/tilting of the strata, and its rotation is assumed to be very small. Instead of pure extension, the NW-SE direction of the minimum stress axis at that location, with respect to the marginal faults, better corresponds to a regime of transtension. A variable direction of this stress axis during the latest kinematic epi- sode has been observed over the length of the basin (Lepvrier 1990). Near the eastern marginal fault in the basement rocks around Dahltoppen (locality D in Fig. 2A), fault activity was also multiphase, and super- imposed slip striae have been observed on several NNW-SSE trending fault-planes. Prior to the compressional (Fig. 5B) and extensional/transtensional events (Fig. 5C) simi- lar to Fig. 5D and E , an earlier striae-set has been recognised, and they show dextral strike-slip movements on the fault planes (Fig. SA), which is in agreement with the estimated dextral move- ment from the magnetic-anomaly patterns. These faults are cut by those on which the compressional striae are distinct. The NNE-SSW striking fault- set may correspond to the pseudo-conjugate sin- istral fault planes (Figs. 4 and 5A). It is natural that the basement has experienced the folding/ tilting that occurred in the adjacent Tertiary strata, which show less than 35" westerly dips in this part of the area. But this rotation did not modify the orientations of dextral strike-slip striae on the NNW-SSE fault-planes which would be only somewhat steeper in their pre-folding/tilting orientation. The basement under the folded Ter- tiary cover does not need to have been folded as block-faulting/tilting of the basement could flexure the cover sediments. Tight fold structures have been observed in the sliver of Carboniferous rocks in Svartfjella, south- west of St. Jonsfjoden (Ohta 1988). The folds have 45-65" west-plunging axes. These structures are cut by the younger extensional faults parallel to the eastern marginal fault of the graben, and could be drag folds related to the Tertiary tran- spression (Lepvrier 1988, 1990). Conclusions The green-brown dolostone, actually a magnesite- stone, with fuchsite and accessory amounts of chromite and chromian spinel, is an important component of the low-grade, high-pressure metamorphic complex of Motalafjella and i t is commonly accompanied by tectonic blocks of serpentinite. These two rock types can be used to trace the distribution of these high-pressure metamorphic rocks. Similar dolostone and serpentinite occur north of St. Jonsfjorden as far north as Sarsoyra. They follow along thrusts at Holmesletfjella and Ankerfjella, along steep faults near the eastern margin of the Tertiary Forlandsundet Graben, and as scattered outcrops on the strandflats. Mineralogical study on the diagnostic minerals in the green-brown dolo- stones from these areas suggests an affinity with the high-pressure metamorphic rocks and, accordingly, indicates that the complex extends ca. 50 km to the north-northwest from Motala- fjella. These rocks have high, positive magnetic sus- ceptibility and clearly mark high magnetic-anom- aly segments and zones on the strandflats. One of the high-anomaly belts coinciding with the eastern marginal fault of the Forlandsundet Graben con- sists of a left-stepping en echelon arrangement of Fig. 5. Successive fault movements observed on fault planes. All are lower hemisphere projections. A , B and C, Dahltoppen (locality D in Fig. 2A) within the basement of southern Sarsbyra; D and E, Tertiary rocks of the Sarsbukta Formation on the river bed in the moraine north of Aavatsmarkbreen (locality S in Fig. 2A). Note that all the projections have not been restored to their primary orientations. A . Phase I , dextral transpression. B and D , Phase 11, compression; C and E , Phase 111, extension/ transtension. Small open circles in D and E = poles of bedding plane, stars with 5 , 4 and 3 projections = maximum (ol), intermediate (02) and minimum (03) principal stress axes, respectively, and their positions are given in degrees at the lower-left corner. Azimuth and plunge value (R = 02 - 03/01 - 0 3 ) is shown at the lower-right corner. 314 Y . Ohta et al segments associated with the green-brown dolo- stone and serpentinite. This is considered t o sug- gest a strike-slip movement along the marginal fault. Successive fault movements have been rec- orded on the fault planes as superimposed striae. Acknowledgemenrs. - Constructive discussions were kindly given by W. K. Dallmann and A . Hjelle. Norsk Polarinstitutt. K. L Kleinspehn and C. Teyssier. University of Minnesota. U.S.A. We thank K. L. Kleinspehn for improving the English. The comments of the two referees are also deeply acknowl- edged References Armstrong. H. A.. Nakrem.H. A . & O h t a . Y . 1986: Ordovician conodont, from the Bulltinden Formation. MotalafjeUa. cen- tral-we