The extension of the Tornquist Zone in the Norwegian North Sea

RICHARD MICHAEL PEGRUM

Pegrum, R. M.: The extension of the Tornquist Zone in the Norwegian North Sea. Norsk Geologisk Tidsskrift, Vol. 64, pp. 39-68. Oslo 1984. ISSN 0029-196X. The Tornquist Zone is a fundamental structural lineation representing the southwestern margin of the East European/Fennoscandian Precambrian basement platform. It is characterised by complex, often rejuvenated, dip-slip and strike-slip faulting and frequently by tectonic inversion. The zone extends across Poland, beneath the southern Baltic into Skåne and northem Denmark. Evidence obtained from offshore oil and gas exploration suggests that the Tornquist Zone extends beneath the southern Norwegian North Sea at least as far as the line of the Viking Graben, some 500 km beyond its commonly assumed termination at the line of the Oslo Graben. R. M. Pegrum, Exploration Advisor, Statoil, Postboks 300, N-4001 Stavanger, Norway.

The term 'Tomquist Line' is now deeply en­ km. Superficially the Tornquist Zone is marked trenched in the literature as being synonymous by the tectonically inverted Polish Trough and with the southwestem border of the East Euro­ fundamentally by the contact between 'old' Pre­ pean Platform. However, as pointed out by Nor­ cambrian basement rocks of the East European ling (1981), if strict priority were to be applied in Platform and 'young' Precambrian basement the designation of geological features, 'Tom­ rocks lying to the southwest (Pozaryski 1977, quist' would be among the names to be invalidat­ Pozaryski & Brochwicz-Lewinski 1978). The ed. The term 'Zone' has been used in the present East European Platform is an ancient and com­ paper, as the edge of the Fennoscandian - East plex geological entity comprising ovoid Ar­ European Platform is marked by a linear belt of chaean (older than 2500 Ma) complexes of gran­ structural complexity varying in width from 20 ites, gneisses and metamorphic rocks, welded km to almost 150 km, for which 'line' is consid­ together by a network of early Proterozoic (older ered to be inappropriate (Fig. 1). Salient struc­ than 1600 Ma) tectonic beits (Watson 1976). Lo­ turallstratigraphic features associated with the cally the basement was strongly effected by the Tomquist Zone are reviewed, for convenience, Gothian orogenic phase (older than 1200 Ma). under four subheadings, namely the Polish, Bal­ The younger basement lying southwest of the tie, Danish and Norwegian Sectors. The persis­ Tornquist Zone was consolidated in the later tence of the Tornquist Zone through the first Proterozoic Dalslandian (Grenvillian, 1200-850 three areas is widely accepted. Its extension Ma) phase or in the early Baikalian (850-650 Ma) northwestwards across the Norwegian North Sea phase. In Poland the contact between the two is based in part upon new, and in part upon differing basement types is clearly marked by a recently published data. In the final section of zone of magnetic anomalies. The line of contact this paper the origin and development of the is apparently sinuous, being controlled by alter­ TornquistLine and its possible extension beyond nating NW and WNW trending segments (Fig. 2). the Viking Graben are briefly discussed. The basement rocks in Poland are buried beneath a variably thick sedimentary cover and are known only from deep boreholes (Sokolowski 1970). Boreholes drilled in northern Poland, north­ east of the Tornquist Zone, have established that The Polish Sector the Precambrian basement is overlain by a rela­ The Tornquist zone can be traced across Poland tively thin sequence of Lower Palaeozoic shales from the Baltic coast of Pomerania in the north­ and carbonates in a typical platform facies. The west to where it plunges beneath the Carpathian sediments are flatlying and undisturbed. Further foredeep in the southeast, a distance of some 800 to the west, within the Tornquist Zone (Pomorze 40 R. M. Pegrum NORSK GEOLOGISK TIDSSKRIFf l (1984)

Fig. l. The structural framework of the Tornquist Zone in Europe. Diagonal cross symbol: Precambrian basement at or near ' surface. Vertical cross symbol: Precambrian basement beneath platform cover of Palaeozoic and/or Mesozoic. Close vertical line: Caledonian metamorphic orogen. Wide vertical line: Caledonian folded Lower Palaeozoic sediments. Dark stipple: Alpine-Carpathian orogen. Medium stipple: Carboniferous Variscan foreland sediments. Light stipple: Variscan massifs at surface.

region), several wells have penetrated Silurian land. Immediately to the south, flat-lying Devon­ and Ordovician shales and siltstones of basinal ian and Carboniferous rocks have been encoun­ aspect. The sediments are not significantly meta­ tered. On the basis of the borehole evidence morphosed but are tectonically disturbed, dip­ several authors, following Buch et al. (1974) and ping at angles of 5° to 80°. Even further west, Glushko et al. (1975), have postulated a continu­ drilling on the northern peninsula of Riigen Is­ ous zone of Caledonian deformation, fringing the land, lying off the Baltic coast of the GDR, has East European Platform, extending from the proved the existence of steeply dipping (20°-70°) Baltic to the southwestern Ukraine, a distance of sequences of basinal Ordovician sediments, with some 900 km. Additional deep drilling in Poland thicknesses in excess of 3000 m, beneath flat­ has shown, however, that the Riigen-Pomorze lying Permo-Triassic rocks. The Ordovician sedi­ Lower Palaeozoic basin is separated from a ments are well compacted, quartz veined, some­ southern Lower Palaeozoic basin, in the vicinity times brecciated and have abundant slickensides. of the Holy Cross Mountains, by areas in which Metamorphism is, however, only incipient and the Permian is underlain by more or less flat­ the shales have a sparse but well preserved grap­ lying Silurian sediments (Pozaryski & Brochwicz­ tolite fauna. Late Ordovician and Silurian sedi­ Lewinski 1978). Furthermore the southern ba­ ments are apparently absent beneath Riigen is- sin, containing sediments ranging in age from NORSK GEOLOGISK TIDSSKRIIT l (1984) The Tornquist zone in the North Sea 41

++++-t++++++++++++ ++++++++++++++++++ . Gothian Basement + [J(2000-1200 Ma)

Folded Cambro-Silurian Trough

Early Baikalian Basement (850-650 Ma)

VARISCAN Folded Variscan Troughs INTERNIDES

Southwest boundary of ;:::::::::===:. Gothian basement based o km 100 --=-- on magnetic data CARPATHIANS

Fig. 2. The structural framework of the Tornquist Zone in Poland (after Pozaryski & Brochwicz-Lewinski 1978).

Cambrian to Carboniferous, was folded primar­ beneath the Carpathian orogen and does not ily during the Variscan orogenic phase. Broch­ impinge upon the East European Platform (1 & wicz-Lewinski et al. (1981) suggest that the con­ 2). The Variscan foredeep of northwestern Eu­ tact between the thick, folded Lower Palaeozoic rope does not extend across Poland in front of sequences of the Riigen-Pomorze region and the the Sudetan Massif, but dies out towards the thin platform sequences is a tectonic one, the east. Northwards Carboniferous sediments are juxtaposition resulting from major strike-slip limited by the Ringkobing - Fyn - Rugen high. faulting along the Tornquist Zone in the early Folded Carboniferous sediments extend across Devonian. the Tornquist Zone into the Podlasie Graben Devonian sediments of Emsian and younger and in the vicinity of the Holy Cross Mountains. age are found on either side of the Tornquist They may have originally linked eastward$ with Zone in north Poland in similar facies, and also the Pripat - Dnieper - Donbass Graben system extend far to the east across the East European of southwestern U.S.S.R. Platform. Carboniferous sediments have a limit­ From the Late Permian to the end of the ed distribution northeast of the Tornquist Zone Mesozoic the Tornquist Zone was primarily one but they may have suffered widespread erosion of active subsidence, leading to the development following elevation of the Platform during and of the Polish Trough (Pozaryski 1977, Pozaryski after the Variscan orogenic phase. The present & Brochwicz-Lewinski 1978). The zone of subsi­ expression of the Tornquist Zone owes much to dence was segmented by cross-trending faults its reactivation and modification during Variscan which probably were established during the Var­ orogenesis and fault deformation which extended iscan phase. A northern (Pomeranian), central into the early Permian. The importance of late (Kujawy) and southern (Holy Cross) sector can Variscan movement in triggering subsidence be distinguished. along the edge of the East European Platform Subsidence was greatest in the Kujawy sector has been discussed in several publications (Nor­ with original depositional thicknesses exceeding ting & Skoglund 1977, Noldeke & Schwab 1977, 10 km. Pozaryski & Brochwicz-Lewinski (1978) Schwab et al. 1979, Pozaryski 1977, Ziegler distinguish two major phases of subsidence, a 1981). 'Graben stage' lasting from the late-Permian to The Variscan orogen is exposed at the surface the early-Cretaceous and a 'Downwarp Stage' in the Sudetic Mountains of southern Poland, but which persisted until the end of the Cretaceous, swings southward in the subsurface to plunge when it was terminated by Laramide inversion. 42 R. M. Pegrum NORSK GEOLOGISK TIDSSKRIFT l (1984)

EAST EUROPEAN PLATFORM

= Southwestern margin of East European Platfonn.

__,._ Approximate boundary of Mesozoic marginal depression. 7J77 Zone of positive inversion. Marginal inversion troughs.

Pre - Mesozoic rocks of Holy Cross Mountains. Miocene of the pre - Carpathian foredeep. Carpathian thrust - front. CARPATHIANS r· o 50 IOOkm ..... j : '·.., ._.. .J. . ..,_

Fig. 3. The relationship between the Polish Trough and the boundary of the East European Platform (after Pozaryski 1977, Pozaryski & Brochwicz-Lewinski 1978).

During the Graben Stage the depositional area dence was renewed in the late Triassic and con­ was delimited by synsedimentary faults and flex­ tinued through the Jurassic into the early Creta­ ures commonly arranged in en echelon zones. ceous. Mobility of the floor of the trough is The graben system varied from 80 km to almost reflected by frequent changes in the location and 150 km in width. The late Permian (Zechstein) directional trend of successive depocenters. Rap­ was characterised by thick evaporites in the Ku­ id local thickness and facies variations, non­ jawy and Pomeranian sectors, but by thin mar­ sequences and erosional gaps resulted from the ginal sulphates and carbonates towards the rotation and collapse of fault blocks and from southeast and also extending eastward onto the movement of the Zechstein evaporites which East European Platform. Differential subsidence commenced in the late Triassic and continued dominated the early Triassic, with basinal argilla­ intermittently throughout the Mesozoic. A clear ceous and calcareous sequences showing a 300 % relationship is often noted between the location increase in thickness compared to the marginal of salt structures and dislocations in the basin arenaceous sequences. A period of stability suc­ floor (Pozaryski 1977, Pozaryski & Brochwicz­ ceeded in the Middle Triassic following the es­ Lewinski 1978). The phase of increased instabil­ tablishment of a seaway to the Tethyan Ocean in ity which lasted from the late Triassic to the end the south, and carbonate deposition was wide­ of the Jurassic corresponds to the phase of re­ spread (Sokolowski 1976). Differential subsi- gional crustal extension effecting much of north- NORSK GEOLOGISK TIDSSKRIFT l (1984) The Tornquist zone in the North Sea 43

15"1' BALTIC SEA

' \

l l l l l l l. ) \_ 53"N l l

R /l ,.. "'�· o"/.,. q ,.,. ��-

l / ! 1.. ., ; \ ..... ' 51"N ... r' \ \ ' i /

15"E l. zo•E

Fig. 4. Isopach map (in km) of the late-Permian to Mesozoic sediments of the Polish Trough (after Pozaryski 1977). Diagonal shading: Pre-Zechstein rocks at surface; vertical shading: zone of positive tectonic inversion.

western Europe and the North Sea. mian to Mesozoic sediments in the Polish Trough During the Cretaceous, the zone of subsidence (Pozaryski 1977). In the southeastern axial zone widened into a linear downwarp, not delineated of the trough the inversion is complete, the pre­ by steep faultsand flexures. The late Cretaceous Permian floor being exposed in the Holy Cross sedimentary fil! in the Downwarp Stage reached Mountains. Further north, in the Kujawy sector, thicknesses of the order of 2000 m near the basin sediment thicknesses in the order of 7 km are still axis. Tectonic inversion of the basin began dur­ preserved, and in Pomerania thicknesses are in ing the late Cretaceous and was largely con­ the order of 4 to 5 km above the base of the trolled by reactivation of the same faults which Zechstein. Schematic cross-sections of the pre­ controlled the grabenal collapse. The faults com­ inversion and post-inversion disposition in the monly show a re versal in their direction of throw. southem, central and northem sectors of the The relationship between the edge of the East trough are illustrated in Fig. 5. European Platform and the marginal depression The inversion structures in the axial zone of is shown in Fig. 3. The axial zone of positive the Polish Trough are associated with reverse inversion corresponds more or less closely with faults, overtumed flexures and small scale the zone of earlier grabenal subsidence and is overthrusts related to compressional strike-slip flanked by secondary negative inversion troughs. or oblique-slip fault movements in the basement. Fig. 4 shows the present thickness of Late Per- Pozaryski & Brochwicz-Lewinski (1978) relate 44 R. M. Pegrum NORSK GEOLOGISK TIDSSKRIFT l (1984)

LINE C-C' (Post-lnversion) SW NE

LINE C - C' - POMERANIA

LINE C-C' (Pre-inversion) NE SW

6:�: " t6

LINE B-B' (Post-inversion)

LINE B-B' (Pre-inversion) LINE B - B' - KUJA WY

10 10

LINE A-A' (Post-inversion) SW NE o o

:] ·�, ��-,. ·�� ·r: LINE A - A' - NORTHERN LINE A-A' (Pre--inversion) SW NE HOL Y CROSS MOUNTAINS O ry

Horizontal ac•le ------Ver1iul scale in kilometers

Fig. 5. Diagrammatic cross-sections, Polish Trough, before and after structural inversion. For location of sections, see Fig. 4. Pz = Zechstein, Tr = Triassic, J = Jurassic, Cr = Cretaceous. (After Pozaryski 1977, Pozaryski & Brochwicz-Lewinski 1978.) NORSK GEOLOGISK TIDSSKRIFT l (1984) The Tornquist zone in the North Sea 45 the compressive stresses to the northward impact beneath the Pomorze region of Poland, south­ of the Carpathian (Alpine) orogen which tended west of the Darlowo Block, has been briefly to move the block west of the Tornquist Zone discussed in the preceding section. towards the north-west relative to the stable East The WNW trending fault system extending European Platform, setting up a right-lateral along the southern flank of the Ringkøbing- Fyn compressive couple. In the southeastern part of High extends into this area south of the Moen the trough, where Zechstein evaporites are ab­ and Arkona horst blocks. This fault system forms sent and the pre-Permian graben floor is ex­ the present northeast limit of Devonian and Car­ posed, a clear relationship is revealed between boniferous sediments. Structural and stratigra­ the deformation of the cover rocks and basement phic evidence suggests that the present limit has faults (Pozaryski 1977). Further north in the Ku­ been controlled by late- or post-Variscan uplift, jawy and Pomerania sectors, the presence of faulting and erosion. Dadlez (1974) postulates Zechstein evaporites has commonly resulted in a that Carboniferous sediments may have original­ zone of decollement and structural disharmony ly bad a more widespread distribution and a between the post Zechstein cover rocks and the connection may have existed eastward to the basin floor. The same inversive mechanism is, now isolated occurrence in the axis of the Peri­ however, clearly operative. There is a notable baltic Depression in Lithuania/Latvia. A connec­ progressive northwestward decrease in the inten­ tion may also have existed northwestwards along sity of the inversion tectonics away from the the Tornquist Zone to the Oslo region, where Carpathian front. marine Carboniferous sediments have been re­ ported by Olaussen (1981). The WNW trending faults on the south flank of the Moen and Ar­ kona blocks also control the northeast limit of The Baltic Sector early Permian clastics and volcanics. The occur­ The structural framework of the Tornquist Zone rence of thick volcanic sequences (Rost & Schi­ in the Baltic Sector is shown in Fig. 6. The manski 1967) is indicative of post-Variscan ex­ faulted edge of the East European Platform can tensional faulting along this zone. be traced from Poland northwestwards into the Zechstein sediments extend across the north­ Fennoscandian Border Zone of southwestern em GDR into.Pomerania, a fairly thick and com­ · Sweden (Skåne). The Fennoscandian Border plete evaporite section being developed. The ba­ Zone is a structurally complex area of block sinal evaporites pass northwards in central Rii­ faulted Precambrian basement rocks, early Pa­ gen into a thin sulphate - carbonate sequence laeozoic sediments and late Triassic to late Cre­ and finally, in north Riigen, into thin marginal taceous sediments, which separates the Fennos­ clastics of uncertain age (Miinzberger .et al. candian Platform from the Danish Trough (Nor­ 1966). Marginal sulphate - carbonate deposits ling & Skoglund (1977). Precambrian basement are also developed on the Darlowo Block, and rocks also outcrop on Bornholm and on the sea seismic evidence (Dadlez 1974) suggests that floor at the crest of the Cristianso Horst, forming they thin out completely northwards on the part of a major southeast trending gravity anom­ Bornholm - Darlowo high. aly (Andersen et al. 1975). Lower Palaeozoic The seismic evidence also indicates that a rocks of the platform cover outcrop in Skåne and Zechstein interval 400-500 m thick is preserved on Bornholm, and also extensively on the floor over the downfaulted Gryfice block, here re­ of the Baltic Sea (Floden 1977). The Lower Pa­ ferred to the Gryfice Graben, and a connection laeozoic cover thickens progressively to the south may have existed northward into the Danish and southeast, the sub-Cambrian peneplain Zechstein basin. reaching a depth of more than 2000 m southeast An intermittently open connection appears to of the island of Gotland. Gently dipping Lower have been maintained between the Polish Palaeozoic shelf sediments have been drilled on Trough and Danish Trough, across the southern the Darlowo Block (Leba Uplift) near the Baltic Baltic, throughout the Triassic, Jurassic and Cre­ coast of Poland at depths of 800-1000 m (Dadlez taceous (Dadlez 1967, Baartman & Christensen 1967, Sokolowski 1976). The occurrence of 1975, Pozaryski & Brochwicz-Lewinski 1978). strongly folded and weakly metamorphosed The axis of this connection appears to run north­ Lower Palaeozoic basinal sediments on the Ar­ westwards through the Gryfice Graben and to kona Block, at the north tip of Riigen Island, and the southwest of Bornholm, where gravity data 46 R. M. Pegrum NORSK GEOLOGISK TIDSSKRIFT l (1984)

oo...<==--==�.;.,50km P OLA ND

Fig. 6. Structural framework of the Southern Baltic (after Dadlez 1974, Baartman & Christensen 1975, Baartman 1976, Wannas 1979 and Kumpas 1980). Light stipple: Pre-Mesozoic Rocks at surface; medium stipple: positive structural blocks in the subsurface; dark stipple: zone of positive structural inversion of the Polish Trough.

suggest that 4--6 km of sediments may be pre­ maximum thickness of the order of 2000 m. The served in a narrow graben 15-20 km wide (An­ Tertiary cover is thin and discontinuous in the dersen et al. 1975). Baartman (in Baartman & Baltic Sector, in part due to regional uplift at the Christensen 1975) records more than 4 km of end of the Cretaceous. sediment in a downfaulted trough at the south The effects of end Cretaceous (Laramide) end of Øresund, the narrow seaway between structural inversion can be traced northwards Skåne and the Danish island of Sjælland, on a from Pomerania into the southern Baltic (Dadlez northwestward continuation of this trend. The 1974, Pozaryski & Brochwicz-Lewinski 1978). Arkona Block, in contrast to the Gryfice Gra­ The structural inversion has uplifted the axial ben, has only a thin Mesozoic cover, the Triassic zone and the northeastern flank resulting in sedi­ being much reduced and the Jurassic and Lower ments as old as the Triassic subcropping the thin Cretaceous being very thin or absent. Similarly a Tertiary cover. The zone of structural inversion reduced and incomplete sequence is present on can be followed northward to Skåne, where the the Darlowo Block and on Bornholm (Gravesen Lower Palaeozoic and Precambrian floor of the et al. 1982). The Gryfice Graben thus provides a northeast flank of the basin has been differential­ direct northwestward continuation of the axial ly uplifted and subsequently exposed by erosion. graben zone found beneath the Polish Trough, Fig. 6 shows that the structural framework in discussed in the previous section, and is consid­ the Baltic sector is controlled by a number of ered here to reflect the fundamental course of differentfault trends, especially WNW, NW, N-S the Tornquist Zone. and NE. Munter (1973) recognised all these In the Upper Cretaceous the Southern Baltic trends in the Precambrian basement of Born­ was an area of more general subsidence, again in holm and noted the predominance of strike-slip direct continuity with the Polish Trough, and offsets in the order of 500 m to 2500 m. Many of marls and limestones were deposited with a the fault trends have been repeatedly rejuvenat- NORSK GEOLOGISK TIDSSKRIFT l (1984) The Tornquist zone in the North Sea 47 ed, often with later movements obscuring the tectonic zone (Stromberg 1976; Roshoff & La­ effects of earlier movements and rendering de­ gerlund 1977). tailed reconstruction difficult or impossible. The Baltic sector of the Tornquist Zone is thus The WNW trend, dominant in the Moen and typified by multidirectional faulting which has Arkona Blocks and in a broad zone extending suffered repeated reactivation. Faults with a Pre­ into northem Poland, was strongly reactivated at cambrian origin have been moved during multi­ the end of the Variscan orogenic phase, and its ple phases of Palaeozoic, Mesozoic and probably role in limiting Devonian, Carboniferous and early Tertiary compression and dilation. The early Permian sediments has been mentioned Baltic sector is especially complicated by the ob­ earlier. This trend is also associated with early lique intersection of the Ringkøbing - Fyn - Permian volcanicity in northern GDR. Moen- Rugen High trend with the northwester­ Fundamental NW trending lineaments are pre­ ly trending edge of the East European - Fennos­ sent in the basement rocks of the Fennoscandian candian Platform. Drilling data in the southem and East European Platforms (Watson 1976, Baltic are sparse and many details of the struc­ Stromberg 1976) and are reflected in the regional tural and stratigraphic development have yet to trend of the Tornquist Zone. Lindstrom (1960, be clarified. The late Permian - Mesozoic cover 1967) recognises small-scale late Palaeozoic has, however, been investigated by geophysical compressional tectonics along NW trends result­ surveys which confirm a general pattern of fault­ ing in the folding and thrusting of Cambro-Siluri­ controlled subsidence through to the Lower Cre­ an sediments in Skåne and also the formation of taceous being succeeded by a more general 'funne! grabens' by Cambrian - earl y Ordovician downwarp in the late Cretaceous and finally by extensional faulting and possibly wrench-faulting an end Cretaceous inversion stage. This develop­ along the same trends. Bolau (1951, 1959, 1972, ment sequence closely matches that of the Polish 1973) has carefully documented repeated move­ sector and can be related to the physical continu­ ments along WNW and NW trending faults in ity of the Tornquist Zone. Skåne. The faults outline polygonal blocks which have moved independently of each other, result­ ing in abrupt facies and thickness changes, espe­ cially in the Jurassic. Phases of movement in the The Danish Sector Variscan, in the late Triassic, in the late Jurassic The major tectonic elements in the Danish Sec­ and in the Cretaceous have all been documented. tor of the Tornquist Zone are shown in Fig. 7. Kumpas (1980) traces important NW trending Dominant features are the NW trending Fennos­ faults from Skåne into Hano Bay, northeast of candian Border Zone in the northeast and the Bornholm, where they again show important WNW trending chain of high structural blocks in phases of movement in the late Jurassic and late the southwest which form part of the Ringkøbing Cretaceous. - Fyn High. The Fennoscandian Border Zone Faults trending N-S, NNE and NE (Rhenian can be traced from Bornholm, through Skåne, trend) are locally important in southern Skåne across the Kattegat, beneath the northern part of and especially in the Bornholm Gat, the seaway the Danish mainland (Jutland) and further to the between Bornholm and the Swedish mainland. northwest associated with the Fjerritslev Fault Wannas (1979) suggests that Jurassic age strike­ system. The Ringkøbing - Fyn High beneath slip faulting along these trends may be responsi­ Denmark is subdivided into three discrete fault ble for segmenting and offsetting the Christianso blocks, the Grinstead Block, the Glamsbjerg Horst. Similar age strike-slip faulting has been Block and the Moen Block, by cross-cutting gra­ reported from central Skåne by Hjelmquist bens. The multiple fault blocks of the Ringkøb­ (1975) where vertical Permo-Carboniferous do­ ing-Fyn High are arranged in an overall en lerite dikes have been offset several hundred echelon pattern (Sorgenfrei 1969) which may re­ metres along NNE trending faults. An associ­ sult from a basic wrench fault framework (Baart­ ation between late Liassic- early Dogger volcan­ man 1973). Between the shallow basement of the ic plugs and the NNE fault trend is also noted. Fennoscandian Border Zone and the Ringkøbing That the trend is much older in origin is indicated - Fyn High there is the major downwarp of the by the occurrence of a number of Precambrian Danish Subbasin (Rasmussen 1978) which con­ dikes with the same direction, in northeast tains a post-Rotliegend sedimentary fill exceed­ Skåne, and parallelism with the great Vattern ing 5 km in the axial zone (Fig. 8, cross-section 48 R. M. Pegrum NORSK GEOLOGISK TIDSSKRIFT l (1984)

\ \ Kilometres \ \ �----50 \ \ \ i \ 1 i l l ; l SWEDEN l S \ � l N l l � l J l l l l l l {

Fig. 7. Structural framework of the Danish sector of the Tornquist Zone (after Baartman & Christensen 1975, Baartman 1976, Wannas 1979 and Kumpas 1980). Light stipple: Pre-Mesozoic rocks at the surface; dark stipple: positive structural blocks in the subsurface.

E-E'). The Danish Subbasin is the eastern end of and gravity faulting, varying from 10 to 30 km in a regional feature, the Norwegian-Danish Basin, width. In the south the Vattern Zone is truncated which extends westwards to the central North by the Tornquist Zone in Skåne. Brochwicz­ Sea. Lewinski et al. (1981) recognise a similar contact Three dominant fault trends can be recog­ between two differing basement types in the Do­ nised, running NW, WNW and NNE (referred to brogea Massif near the Black Sea coast of Rou­ as 'Dinaric', 'Hercynian' and 'Rhenian' in Baart­ mania and speculate on the possibility of a 1500 man & Christensen 1975). The major structural km sinistral strike-slip displacement of Caledon­ elements are known from geophysical investiga­ ian age along the margin of the East European tions and from deep boreholes, mainly connected Platform (Teisseyre- Tornquist Line). with the search for hydrocarbons. The general Northwards the Våttern Zone terminates correctness of the basin framework is supported against a NW lineament (parallel to the Tom­ by gravity, magnetic, LANDSAT and bathymet­ quist Zone) referred to by Stromberg (1976) as ric studies (Frost 1977) and by geothermal stud­ the Gudbrandsdal- Vårmland- Våstervik Zone. ies (Madsen 1974). The latter is an ancient zone of supposed strike­ The Precambrian basement outcrops exten­ slip displacement. The Oslo Graben system is sively in southern Sweden. A major NNE trend­ most probably also associated with a Precam.­ ing tectonic zone (Vattern Zone of Stromberg brian fracture zone (parallel to the Vattern 1976, Roshoff & Lagerlund 1977) crosses the Zone) which is similarly truncated to the north area, separating an eastem, older, Svecokarelian by the Gudbrandsdal - Viirmland - Våstervik (1600 Ma and older) basement to the east from a Zone. younger, Sveconorwegian - Grenvillian (1000 West of the intersection of the Våttern Zone Ma) basement to the west. The Viittern Zone is and the Fennoscandian Border Zone in Skåne, marked by a belt of steep schistosity, shearing similar age Precambrian basement (Sveconorwe- NORSK GEOLOGISK TIDSSKRIFT l (1984) The Tornquist zone in the North Sea 49

H H' SSE NNW 0 0

2 STAVANGER 3 PLATFORM 3

FARSUND BASIN 4 o km 30 4

G � NNE� SSW 0 0

2

3 3

4 O km 5 Fjerritslev Fault 4 Zone F F' s N

E E' s NJSW NE

Fierrltslev Fault Zone

Vertical se ale: Two-way Time in seconds

Fig. 8. Diagrammatic cross-sections, Danish and Norwegian sectors. For lines of section see Figs. 7 & 9. Te= Tertiary; Cu Upper Cretaceous; Cl= Lower Cretaceous; J = Jurassic; TR=Triassic; Pz=Permian Zechstein. D-D' and E-E' after Baartman & Christensen; F-F' after Smidt 1982; G-G' after Hamar et al. in press; H-H' after Skjerven et al. in press.

4 - Geologisk Tidsskr. 1/84 50 R. M. Pegrum NORSK GEOLOGISK TIDSSKRIFT l (1984) gian - Grenvillian) is present either side of the of deep boreholes (Christensen 1971, 1973, Poul­ Tomquist Zone and as a result the strong gravity sen 1974). Baartman & Christensen (1975) sug­ anomalies noted in the Polish Sector, arising gest that some 2000 m of Silurian sediments, fromthe juxtaposition of two different basement similar to those of Pomerania, may be present types, die out. The Tomquist Zone does, howev­ southwest of the Fennoscandian Border Zone. er, retain a relative ly weak gra vi ty and magnetic The nature of any direct connection between expression in the Danish Sector (Frost 1977) and Lower Palaeozoic rocks beneath the Danish Sub­ also northwestwards from northem Jutland to­ basin and those preserved within the Oslo Gra­ wards the Norwegian Coast (Holtedahl & Selle­ ben is unknown, but Floden (1973) suggests such voll 1971). a connection can only be very limited. Carbonif­ The extension of the Fennoscandian Border erous and Devonian rocks are apparently limited Zone from Skåne across the Kattegat to northern northwards by the Ringkøbing - Fyn - Rugen Jutland has been described in some detail by Highs. The present limit is, however, most prob­ Baartman & Christensen (1975). A 50 km wide ably erosional following late- or post-Variscan complex of NW and WNW trending horsts and uplift, and limited occurences of Carboniferous grabens has been mapped, with marine seismic and Devonian sediments beneath Denmark reflection data, as a direct continuation of the north of the Ringkøbing - Fyn system are quite similar zone crossing Skåne. The predominant possible, especially in the light of Olaussen's NW and WNW trends are cut and offset by NNE (1981) identification of marine Carboniferous and NE trending faults resulting in a mosaic of sediments in the Oslo Graben. Volcanic rocks of fault blocks. The presence of Precambrian base­ late Carboniferous to early Permian age have ment at relatively shallow depth beneath the been encountered beneath Denmark, and their northern tip of Jutland has been confirmed by extension northward into the Oslo Graben is well drilling. A borehole at Fredrikshavn on the documented. Ziegler (1981) suggests that the co­ northwest coast reached basement gneiss at 1274 pious occurrence of volcanic rocks in the Oslo m below sea level (Sorgenfrei & Buch 1964). The Gra ben is the result of a weak crustal point at the basement surface dips gently up towards the intersection of two fundamental fracture zones, north and outcrops extensively on the sea floor the NNE one controlling the Oslo Graben and of the eastern Skagerrak off the west coast of the NW trending Tomquist Line. Sweden (Floden 1973). This area represents an Zechstein sediments are well developed in the extension of the Fennoscandian Platform in Danish Subbasin, and thick evaporites are pre­ which NW and WNW trending faults are of little sent beneath west central Jutland where they importance. Floden (1973) recognised three ma­ form numerous piercement structures. The jor NNE trending fractures, the easternmost northeast limit of Zechstein sediments approxi­ (Hvaler Deep fracture system) constituting a mates to the southern margin of the Fennoscan­ southerly branch of the Great Permian Fault dian Border Zone between northern Jutland and which forms the east boundary of the Oslo Gra­ northern Sjælland (Baartman & Christensen ben. Basement gneisses outcrop on either side of 1975). Toward the southeast along the axis of the the Hvaler Deep fracture and Palaeozoic sedi­ Danish Subbasin evidence of salt deformation ments, if present at all, would seem to be re­ dies out and it is supposed that the Zechstein is stricted to a 20--30 km strip beneath Permian in a marginal anhydrite/dolomite facies. Zech­ volcanics extending southwards from outer Oslo stein sediments have not been positively identi­ Fjord. The NNE trending fracture zones are con­ fiedeast of the Stevns and Moen (Møn) basement sidered to be of Precambrian age, although a blocks, and the direct connection between the major rejuvenation occurred associated with the Danish and Polish Zechstein basins via the Gryfice formation of the Oslo Graben in the Permian, Graben, proposed by Dadlez (1974), has yet to be and minor faults extending up into the Mesozoic confirmed. cover further south indicate late Mesozoic and Triassic sediments are extensively developed possibly early Tertiary readjustments. beneath Denmark. Within the Danish Subbasin The distribution of Lower Palaeozoic rocks they rest conformably on Zechstein sediments beneath Denmark is imperfectly known, but but marginally they overstep onto Palaeozoic Cambrian, Ordovican and Silurian rocks, similar rocks or the Precambrian crystalline basement. to those of the platform cover northeast of the The isopach map published by Berthelsen (1980, Tomquist Zone, have been reached in a number Fig. 3) shows that in north central Jutland, below NORSK GEOLOGISK TIDSSKRIFT l (1984) The Tornquist zone in the North Sea 51 the axis of the Danish Subbasin, the Triassic may Below north central Jutland, Zechstein eva­ locally exceed 5000 m in thickness, in part due to porites have acted as a zone of decollement, and penecontemporaneous Zechstein salt movement. the association between basement faults and the A second depocenter with more than 4000 m of post-Zechstein cover rocks is less clear (Fig. 8, Triassic sediments has been mapped beneath the Cross-section E-E'). Further to the northwest, southem Kattegat in an area where Zechstein however, the Zechstein evaporites are only thin­ evaporites are thin or absent. Additional depo­ ly developed and the relationship between base­ centers with thicknesses in excess of 4000 m are ment faults and the Mesozoic cover is again evi­ associated with the Fjerritslev Fault system dent. Smidt (1982) has published a seismic line which may have been active during the Triassic. across the Fjerritslev Fault (redrawn in Fig. 8, Within the Fennoscandian Border Zone from Cross-section F-F') which shows that a down­ northem Jutland to Skåne only a reduced section throw to the north, with a displacement in the of late Triassic age is generally present. The order of 3000 m, has been partially inverted by a marginal Triassic sediments thin progressively to­ late Cretaceous reversal of throw. A marked wards the northeast. Baartman & Christensen increase in thickness of the Jurassic-Lower Cre­ (1975) observe that the Triassic in this zone is taceous interval occurs northeastwards across the frequently cut by NW trending faults. In some Fjerritslev Fault, resulting in a NW trending cases major offsets within the Trias die out or are trough (Fjerritslev Trough of Frost 1977, Fig. 1). represented by only minor offsets in the Jurassic, The Fjerritslev Fault system and its associated indicating a phase of late Triassic (Early Kim­ thickened Jurassic - Lower Cretaceous interval merian) faulting. can be traced directly into the Norwegian sector, Jurassic and Lower Cretaceous sediments were where it is termed the Farsund Basin (Fig. 8, deposited extensively in the Danish Subbasin. Cross-section G-G'). Jurassic deposits reach a maximum thickness of The chain of Mesozoic Troughs extending more than 1200 min northwest Jutland, associat­ from Skåne (Malmo- Y stad Trough) across the ed with the Fjerritslev Fault system (see Michel­ Kattegat (Ålborg Trough), and across northem sen 1978, Fig. 2). Jurrasic and Lower Cretaceous Jutland into the Fjerritslev- Farsund Trough, all sediments are significantly thicker in the Katte­ show a similar history of Jurassic- Lower Creta­ gat in a NW trending trough running along the ceous fault controlled subsidence ( ascribed here edge of the Fennoscandian Border Zone (Fig. 8, to transtension) and a phase of late Cretaceous Cross-section D-D'). This is the Ålborg Trough of early Tertiary inversion (ascribed to transpres­ Baartman's 1976 map, published in Rasmussen sion). This zone of tectonic mobility is believed 1978. The trough is bordered to the south by a zone to be the superficial expression of the buried of faulting and flexuring which can be traced margin of the Fennoscandian Platform, the Torn­ across the Kattegat from eastern Jutland to nor­ quist Zone. This zone has an over-all northwest thern Sjælland and across the Øresund to Skåne trend which diverges somewhat from the WNW (the Helsingør- Helsingborg Flexure of Larsen trend of the Ringkøbing - Fyn High and the et al. 1968). The Ålborg Trough has been partially complementary Norwegian - Danish Basin. The inverted, with the result that in the neighbourhood intersection of the Danish Subbasin and the of the island of Anholt pre-Cretaceous rocks sub­ Tomquist Zone beneath Sjælland and the south­ crop beneath the Pleistocene drift. A deep secon­ em Baltic may be responsible for the consider­ dary inversion trough is present south of the able structural complexities in the latter areas. flexure zone in which up to 2200 m of Upper Cretaceous sediments are downfaulted and pre­ served. Early Upper Cretaceous sediments are folded and cut by dextral strike-slip faults dose to The Norwegian Sector the flexure zone. Baartman & Christensen (1975) conclude that the NW trending fault zones in the The main structural features recognised in the Kattegat have responded to north or northeasterly southem Norwegian North Sea, with the excep­ directed compression by wrench-faulting. The la­ tion of the Central Graben Complex, are sum­ test movements were probably of late Cretaceous marised in Fig. 9. The main elements have been to early Tertiary (Lararnide) age. This corres­ described by Rønnevik et al. (1975), Hamar et ponds well with the Alpine transpression which al. (1980) and Hamar (1982). The faults depicted inverted the Polish Trough system. in Fig. 9 are mapped as cutting the base of the

4 • - Geologisk Tidsskr. 1184 52 R. M. Pegrum NORSK GEOLOGISK TIDSSKRIFT l (1984)

Fig. 9. The structural framework of the southern Norwegian North Sea. Light stipple: Pre-Mesozoic rocks at surface; dark stipple: positive structural blocks in the subsurface.

Zechstein or, where the Zechstein is absent, the gest that it was initiated during the Permian and pre-Zechstein 'basement'. The fault map is com­ reactivated duringthe Kimmerian and Laramide. piled from regional Statoil maps and represents They also note tectonic inversion and wrench the work of several different interpreters and is faulting associated with this zone. based on seismic data of varying quality and grid The Stavanger Platform is truncated in the density. The general reliability can, however, be west by N-S to NE trending faults which throw evaluated by the dose agreement with the base down to the west into the Egersund Subbasin. Zechstein structure map independently compiled These faults are approximately on trend with a and recently presented by Skjerven et al. (1983). southwestward continuation of the Caledonian The Precambrian basement platform of south­ Front and may be a shallow, rejuvenated expres­ em Norway extends southwards into the off­ sion of this feature. This is supported by the very shore, as the Stavanger Platform, where it is limited published data from boreholes which covered by a progressively southward thickening have reached the crystalline basement (Frost et wedge of late Permian, Mesozoic and Tertiary al. 1981). Well 8/3-1, located on the Sele High, sediment. Ramberg et al. (1977) note that the was terminated in a biotite schist in greenschist dominant fracturetrend in the onshore basement facies which yielded a Caledonian age (418 ± 4 platform is NNE to NE, subparallel to the Oslo Ma). Well 18/11-1, located near the east margin Graben and the Caledonian front. Offshore only of the Egersund Subbasin, reached chlorite a few post-Zechstein faults show this trend. The schists of Caledonian aspect which were unfortu­ principal direction is the NW to WNW extension nately not suitable for radiometric dating. Well of the Fjerritslev Fault System which truncates 10/5-1, however, located dose to the Fjerritslev the Stavanger Platform in the south. Skjerven et Fault Zone and east of the Egersund Subbasin, al. (1983) recognise this fault zone as an exten­ drilled into severely altered granites yielding a sion of the Fennoscandian Border Zone and sug- Precambrian minimum age of 698 Ma. The gran- NORSK GEOLOGISK TIDSSKRIFT l (1984) The Tornquist zone in the North Sea 53

NORWAY

01-2 -2-3 Kilometres --­ - - -3-4 o 50

Fig. 10. Generalised structure map on the base of the Zechstein. Values two-way time in seconds.

ite is considered to be similar to the Dalslandian and Triassic are in direct continuity through the granites of the basement platform of southern Norwegian- Danish Basin, but the Jurrasic and Norway which were emplaced earlier than 870 Lower Cretaceous distribution is more fragmen­ Ma (Frost et al. 1981), but the lower age could tary as a result of tectonic and halokinetic move­ indicate reworking in the Cadomian orogeny to­ ments. The Jurassic and Cretaceous stratigraphy wards the end of the Precambrian (550-700 Ma). and structure of the south- southeastern Norwe­ Three further important tectonic elements are gian North Sea have recently been presented by recognised northwest of the Egersund Subbasin; Hamar et al. (1983) and by Hesjedal & Hamar these are the Sele High, the Utsira High and the (1983). Much of the southern Norwegian off­ Ling Graben. Both the Sele High and Utsira shore area is blanketed by Upper Cretaceous High are terminated to the south by NW to chalks and marls and by Tertiary clastics which WNW trending faults which are considered to be thicken southwestwards to a depocenter located a shallow expression of fundamental faulting re­ over the buried Central Graben near the U.K./ lated to the Tornquist Zone. These areas are Norwegian boundary. Fig. 10 is a regional struc­ discussed in greater detail below. ture map indicating the present depth to the base Between the Fjerritslev Fault Zone and the of the Zechstein expressed in two-way travel Ringkøbing- Fyn High system (represented on time. The Stavanger Platform, Sele High, Utsira Fig. 7 by the East North Sea Block and the High and Egersund Subbasin are clearly ex­ Holmsland Block) Iies the major downwarp of pressed. In Fig. 11 the Tertiary cover has been the Norwegian - Danish Basin, termed the Fis­ removed, emphasising the thin Mesozoic cover kebank Subbasin in its western extension. The and possible structural continuity of the Stavang­ Fiskebank Subbasin has a thick sedimentary fill er Platform, Sele High and Utsira High. of Zechstein to Tertiary rocks. The Zechstein Zechstein evaporites are approximately limit- 54 R. M. Pegrum NORSK GEOLOGISK TIDSSKRIFT l (1984)

NORWAY

&•4]2-3 Kilometres ------3-4 o 50

Fig. 11. Generalised restored structure map on the base of the Zechstein after removal of post-Cretaceous cover. Values two-way time in seconds. ed northeastwards by the Fjerritslev Fault system ben and in the North Atlantic (Pegrum & Moun­ but further west extend into the southern part of teney 1978). Triassic filled grabens segmenting Egersund Subbasin, the Ling Graben and also the Ringkøbing - Fyn High, such as the Horns into the southern Viking Graben. On the Utsira Graben between the East North Sea Horst and High and probably on the southern flank of the the Holmsland Block, indicate that the Triassic Stavanger Platform the Zechstein is represented extensional phase was widespread and predomi­ by thin, marginal dolomite/anhydrite sequences. nantly followed N-S lines of weakness. Triassic sediments overstep the Zechstein and Linear troughs of thickened Jurassic and Low­ extend northeastward to the flanks of the Sta­ er Cretaceous sediments are found in association vanger Platform, Utsira High and Sele High and with the Fjerritslev Fault Zone. The Farsund much more thickly into the Egersund Subbasin, Subbasin, lying north of the Fjerritslev Fault where more than 1000 m of Triassic clastics are (Fig. 8, Cross-sections G-G' and H-H'), is an deposited against the east bounding fault of the extension of the Fjerritslev Trough discussed in Sele High (Skjerven et al. 1983). The faults the previous section. The continuity of the Fen­ bounding the Egersund Basin, especially on its noscandian Border Zone across the Skagerrak eastern flank, can be traced northwards through from the southern Norwegian offshore to north­ the Stord Basin and across the eastern Horda em Jutland is notable in that it would seem to Platform parallel to the west Norwegian coast preclude any direct post-Zechstein structural (Horda Fault System). This fault system may connection between the Oslo Graben/Bamble overlie a deeper fracture zone in the Caledonian Fault Zone and the Horns Graben - this has or Precambrian basement but was certainly acti­ often been assumed in structural compilations. vated by a period of Triassic rifting, penecontem­ The stratigraphy and structure of the Farsund poraneous with early rifting in the Viking Gra- Subbasin are discussed by Hamar et al. (1983). NORSK GEOLOGISK TIDSSKRIFT l (1984) The Tornquist zone in the North Sea 55

Subsidence, accompanied by synsedimentary fault movements, extended through the Triassic, Jurassic and Lower Cretaceous. Weak structural inversion in the Laramide phase led to Jocal re­ verse faulting, and uplift and removal of the Upper Cretacous cover from the axial zone. Skjerven et al. (1983) also note the effects of structural inversion and wrench faulting associat­ ed with the Fjerritslev Fault Zone. Fig. 12 shows the Jocation of another WNW trending sedimentary trough, with a markedly thickened Jurassic- Lower Cretaceous interval, at the southem end of the Egersund Subbasin. The trough Iies between the Stavanger Platform and Flekkefjord 'High' and extends north­ westward to the Sele High. The trough is partial­ ly structurally inverted, especially along its Fig. 12. Location map of the Outer Egersund Trough. Cross northeastem flank. The structure of the trough is symbol: structural highs; stipple: inversion structures (after illustrated in Fig. 13 by five parallel cross-sec­ Skjerven et al. in press). tions (1-1' to M-M') Jocated approximately 15 km apart. Cross-sections 1-1' and J-J' extend from the margin between the trough and the Stavanger Stavanger Platform, across the trough, to the Platform. Detailed mapping suggests that a num­ Flekkefjord 'High'. The Jatter feature is not a ber of anticlinal folds, forming an overall en basement high comparable to the Sele and Utsira echelon pattem, are associated with a zone of Highs but rather part of a zone on the south basement faulting which clearly shows inverted flank of the trough exhibiting relative structural displacement. Original faults down to the south­ stability during the Jurassic - Lower Cretaceous, west have been steepened, Jocally overtumed subsequently emphasised by post-Cretaceous and throw down to the northeast in the shallower (Laramide) fault movements. Both cross-sec­ horizons. The seismic data indicate that the tions show structural inversion associated with structural inversion occurred during the Upper the edge of the Stavanger Platform. Cretaceous. A structure map on the base of the Cross-section K-K' extends from the Stavanger Cretaceous over part of this inversion zone has Platform across the trough and over a structurai­ been published by Skjerven et al. (1983, Fig. 8). Jy Jow zone between the Flekkefjord 'High' and The structural inversion is considered to be a Sele High. Southwestward thinning of the Juras­ result of late Cretaceous transpressional move­ sic- Lower Cretaceous interval is apparent as is ment of basement faults of the Tornquist Zone the northem wedge out of the Zechstein se­ following a phase of late Jurassic- Lower Creta­ quence. Cross-sections L-L' and M-M' again ceous transtensional movement. It is notable that show thickening of the Jurassic- Lower Creta­ the inversion dies out northwards in the Eger­ ceous interval, inversion against the Stavanger sund Subbasin and is associated primarily with Platform and Sele High, and clearly show the the NW to WNW trending faults. Structural in­ disposition of the Triassic sediments in the version along the southwest margin of the trough downthrown fault block forming the Egersund is observed Jocally, but the increasing decolle­ Subbasin. Cross-section L-L' illustrates a pro­ ment effect of the Zechstein salt toward the nounced intrusive salt fea ture associated with the southwest largely obscures the basement/cover Sele High boundary fault. Elsewhere within this tectonic relationship. Hesjedal & Hamar (1983) trough a clear relationship has been noted be­ have identified a period of possible structural tween faults in the basement and Zechstein salt inversion affecting the Sele High in the mid­ structures, suggesting that salt movement was at Cenomanian. Present well control is too sparse Jeast in part triggered by sub-salt faulting and not to carry out detailed structural analyses, and the due entirely to halokinetic mechanisms. regional significance of this phase is uncertain. Fig. 14 shows in greater detail structures re­ Evidence of tectonic inversion and oblique-slip sulting from inverted fault movement along the faulting at the intersection of the extended Tom- 56 R. M. Pegrum NORSK GEOLOGISK TIDSSKRIFf l (1984)

LINE M-M' wsw ENE

2

LINE L-L' wsw ENE

2 3

LINE K- K' wsw ENE

LINE J- J' wsw ENE

LINE l -l' wsw ENE

Fig. 13. Diagrammatic 3 cross-sections, Outer Egersund Trough. For lines of section see Fig. 12. Stratigraphic symbols as in Fig. 8. o 10 20 30 40km. Numbers l to 5 identify VERTICAL SCALE : TWO-WA Y TIME IN SECONDS faults shown on Fig. 12. HORIZONTAL SCALE NORSK GEOLOGISK TIDSSKRIFT l (1984) The Tornquist zone in the North Sea 57

NE NE Sea bed

Te Te

...... · ·� • itm• � • ...... --="

Horizontal scale Vertical scale two-way time m sec.

Fig. 14. Details of invel"liion structures on northeast flank of Outer Egel"liund Trough.

quist Zone with the South Viking Graben in and overtuming of the western boundary fault by Norwegian Block 15/9 has been accepted for middle Cretaceous compression and subsequent publication elsewhere (Pegrum & Ljones in 'Mid-Tertiary' rejuvenation, deforming the Pa­ press) and is only briefly reviewed here. Fig. 15a laeocene reservoir sands. Fig. 17 is a similar shows a generalised structure map (in two-way reconstruction based on a north-south seismic time) of the eastem half of Block 15/9 on the line. The partial inversion and local overtuming base of the Cretaceous. A complex pattem of en of several faults should be noted. The interpreta­ echelon anticlinal and synclinal structures associ­ tion of Block 15/9 is based on a closely spaced ated with NW and NE trending faults, some of rectilinear grid (500 m or less) of good quality which have been overtumed, has been mapped. seismic data and on the stratigraphic results from Fig. 15b presents a simplified tectonic map indi­ 16 deep boreholes, two of which penetrate be­ cating several apparant left-Jateral strike-slip dis­ neath the Zechstein. Gas bearing intervals in placements along NW trending faults ascribed to Triassic, Middle Jurassic, Upper Jurassic and Pa­ a period of late Jurassic transtension. Towards laeocene sandstones have been found to date and the middle of the Cretaceous the stress field is the area is currently being actively explored. believed to have switched to a transpressional The NW trending Zone of structural complex­ one, steepening and partly overturning earlier ity in Block 15/9 is directly on trend with the normal faults. Transpressional movements were partially inverted Mesozoic trough at the south­ renewed in the 'Mid - Tertiary', deforming a em end of the Egersund Subbasin, and with the wedge of Palaeocene submarine fan sands which Farsund Subbasin, both of which show a very form the reservoir in the 15/9 Gamma Gasfield. similar developmental sequence. This zone also The outline of the Palaeocene gas accumulation corresponds with the southem termination of the is shown in Figure 15, and its control by the Utsira High, the Sele High and the Stavanger rejuvenated fault pattem is clear. Fig. 16 pre­ Platform, and lead to the conclusion (Pegrum & sents a developmental sequence based on an Ljones in press) that the Tomquist Zone east-west seismic line across the Gamma fault extends considerably further to the northwest block. Of special note is the apparent steepening than has been previously supposed. Fig. 18 sum- 58 R. M. Pegrum NORSK GEOLOGISK TIDSSKRIFT l (1984)

O km

Fi . 15. Eastern half of Norwegian North Sea Block 15/9. A: generalised structure map in the base of the Cretaceous. Values are in _ tw�-way time in seconds. B: simplified tectonic analysis of 15A showing postulated left-lateral stnke·shp faults. (After Pegrum & Ljones, in press. )

marises the observed relationship between Lara­ The Development of the Tornquist mide inversion and the distribution of Zechstein Zone evaporites in a regional context. It is considered highly probable that the paucity of evidence of Pre-Caledonian structural inversion in the areas of thick Zech­ The Tornquist Zone is believed to belong to a set stein evaporite cover is due in large part to the of northwesterly tectonic zones traversing the decollement effect of the salt. The continuity of Fennoscandian - East European Precambrian the NW trending Tornquist Zone from central basement platform. These northwesterly linea­ Poland to the central North Sea is striking and ments are Precambrian in age, considered to be supports the conclusion that it is a fundamental at least 1000 Ma old by Watson (1976) and possi­ element in the structure of Europe. No less strik­ bly 2500 Ma old by Stromberg (1976). Stromberg ing is the evidence of tectonic inversion extend­ (1976) also recognised a weaker northeasterly set ing for more than 2000 km from the Alpine of lineaments of similar age and suggested that compressive front. The origin and genesis of the together they belong to a 'primary' crustal fracture Tornquist Zone is discussed in the final section. system. Ramberg et al. (1977) recognised a smaller scale pervasive NW and NE fracture sys­ tem throughout the basement platform of south- NORSK GEOLOGISK TIDSSKRIFf l (1984) The Tornquist zone in the North Sea 59

w E

LINE N-N'

A. JURASSIC EXTENSION

Cl

• •

B. LOWER - MIDDLE CRETACEOUS COMPRESSION

Tp

C u • •

C. LATE CRETACEOUS- PALEOGENE SUBSIDENCE

Te

Tp

•

o

Vertical scale in two- way Length of section : Approx 1 O km. time .. 1

Fig. 16. Diagrammatic east-west cross-section, eastern half of Block 15/9 showing the postulated develop­ mental sequence. For line of section see Fig. 15B. (After Pegrum & Ljones, in press.) 60 R. M. Pegrum NORSK GEOLOGISK TIDSSKRIFf l (1984)

s N LINE O- O'

J

• Tr

GAMMA FAUL T BLOCK Pz

A. JURASSIC EXTENSION

Cl

•

B. LOWER MIDDLE CRETACEOUS COMPRESSION

Tp C u • •

C. LATE CRETACEOUS­ PALEOGENE SUBSIDENCE

Te

Tp

•

D. MID- TERTIARY COMPRESSION o � l­ Length of section : Approx 15km U) Vertical scale " in two- way time c o (,) G) 1 fl)

Fig. 17. Diagrammatic north-south cross-section in the eastern half of Block 15/9 illustrating the postulated develop­ mental sequence. For line of section see Fig. 15B (After Pegrum & Ljones, in press.) NORSK GEOLOGISK TIDSSKRIFT l (1984) The Tornquist zone in the North Sea 61

rJ FENNOSCANDIAN

AREAS OF LARAMIDE INVERSION • EX TENT OF ZECHSTEIN SALT

Fig. 18. Relationship between tectonic inversion and distribution of Zechstein evaporites in northern Europe.

ern Norway and considered that it may represent elsewhere knowledge of the crystalline basement an ancient regmatic shear pattern initially formed is confined to limited outcrops in the U. K. and in response to planetary forces. Watson (1976) within the Variscan Massifs across Europe notes that the length, straightness and parallel (Zwart & Dornsiepen 1978, Stromberg 1981). alignments of the northwesterly Iineaments are Most of these occurrences carry evidence of Ca­ indicative of a tectonic regime acting on a very domian (Baikalian, Assyntian) deformation large scale. The lineaments on the East Europe­ which took place towards the end of the Precam­ an Platform fall dose to small circles about a pole brian (550-700 Ma). The Cadomian orogenic in the vicinity of the Sea of Okhotsk (55°N, phase has been related by Zwart & Domsiepen 1500E). Watson (1976) concludes that their pat­ (1978) to the initial opening of the Iapetus Ocean tern is consistent with an origin as transform between the North American and European dislocations which could have developed during crustal plates. The Jack of evidence of Cadomian bodily drift of the crustal plate. deformation northeast of the Tornquist Zone, Knowledge of the Precambrian basement where the basement is generally Grenvillian southwest of the Tornquist Zone is only frag­ (1000 Ma) and older, suggests that an important mentary. Pre-Caledonian rocks have been tectonic boundary was active along the Tomquist reached in a few deep boreholes in the North Sea Zone at this time. and northern Europe (Frost et al. 1981), and

5 - Geologisk Tidsskr. 1184 62 R. M. Pegrum NORSK GEOLOGISK TIDSSKRIFT 1 (1984)

Fig. 19. Development of the Tornquist Zone in northern Europe. A: cross symbol: Precambrian basement; dark stipple: Caledonian metamorphic orogen; diagonal lines: folded Lower Palaeozoic sediments; light stipple: weakly folded or unfolded Lower Palaeozoic sediments. B: cross symbol: pre-Upper Carboniferous; dark stipple: Variscan orogen. C: cross symbol: pre-Mesozoic; stipple: Mesozoic depositional troughs. D: cross symbol: pre-Mesozoic; light stipple: Tertiary sedimentary basins; dark stipple: Alpine-Carpathian orogen.

Caledonian (Fig. 19A) It is tentatively suggested that the fundamental structural differences between the Scandinavian Major Caledonian structural features cannot be and Scottish Caledonides are due to their loca­ correlated from Scotland, beneath the North Sea tion on opposite sides of a major transform fault into Scandinavia. Northeast of the extended system, corresponding to the extended Tomquist Tomquist Zone the Scandinavian metamorphic Zone. The transform system is envisaged as be­ and basement nappes are thrust southeastwards ing operative during both Cadomian opening of onto the autochthonous basement platform. the Iapetus Ocean and during its Caledonian dos­ Southwest of the extended Tomquist Zone, in ing. The zone of transform movement may also Scotland, the metamorphic nappes are overth­ have involved the Ringkøbing- Fyn system, con­ rust northwestwards onto the Hebridean base­ trolled by WNW lineaments both on its north ment platform. The latter includes medium to flank (Vinding Line of Dikkers 1977) and on its high pressure granulites and layered anortho­ south flank. A branch of the transform system sites, typical of the North American - Greenland may have continued eastwards across the East Platform but not of the Fennoscandian - East European Platform through the Dnieper - Don­ European Platform (Watson 1976), and is clearly bass fracture system. a fragment of the North American Craton sepa­ The zone of Caledonian folding extending rated by Mesozoic and Tertiary opening of the along the southern boundary of the Ringkøbing­ North Atlantic. Fyn High into the Rugen- Pomorze region could NORSK GEOLOGISK TIDSSKRIFT l (1984) The Tornquist zone in the North Sea 63 be ascribed to transpressive deformation along . . . and may have determined the main axis of this postulated transform zone. Although the the Witch Ground Graben.' The Wyville­ Lower Palaeozoic sediments within this zone are Thompson Ridge connects the southem end of strongly folded, cleaved and faulted, they are the Faroe Bank to the U.K. Continental Shelf clearly not comparable to the NE trending high­ and is parallel to the Iceland - Faroe volcanic grade metamorphic beits of the Scottish and ridge. Scandinavian Caledonides. Their deformational Subsurface control for the Caledonian rocks style is much more comparable to that in the beneath the North Sea and northwest Europe is Cambro-Silurian rocks of central Wales. Watson extermely sparse (Frost et al. 1981) and still inad­ (1976) suggests that during Caledonian orogene­ equate for the proper reconstruction of major sis Europe southwest of the Tomquist Zone was Caledonian features. The northwestward exten­ already largely controlled by a complex block/ sion of the Tomquist Zone as a transform belt basin framework which originated during the Ca­ bisecting the Caledonide orogen is, however, domian or earlier, and Zwart & Domsiepen considered to be compatible with presently (1978) note that the more or less continuous known facts. Palaeozoic sequences within the Variscan Mas­ sifs indicate that there is very little stratigraphic Variscan (Figs. 19 Band 20) evidence of a Caledonian orogenic phase over much of Europe. The Variscan orogenic cycle covers a long time Fig. 19a indicates a relative right-lateral dis­ span from the early Devonian (Acadian Phase, placement along the proposed Caledonian trans­ 400-360 Ma) to the late Carboniferous and early form system based on the opposing vergences of Permian (Sudetan Phase, 320-300 Ma). In many the Scottish and Scandinavian Caledonides. The places the Variscan belt exhibits autochthonous relative movements could, however, be left-lat­ folding (Krebs 1976, Zwart & Domsiepen 1978) eral if a mechanism of basement underthrusting and does not involve extensive nappe structures. was operative. A left-lateral displacement in the Variscan massifs extend across middle Europe order of 1500 km has been proposed by Broch­ from southwest U. K. in the west to the Bohe­ wicz-Lewinski et al. (1981), separating the mian Massif in the east. The northem extemal Vattem Zone in Skåne from its possible continu­ zone has well developed folds and thrusts show­ ation in the Dobrogea Massif of the Romanian ing a northerly vergence. Late Variscan deforma­ Black Sea coast. tion spread onto the northem foreland where The extension of the Tomquist Zone north­ extensive wrench-faulting and volcanism persist­ westward from the Viking Graben is speculative. ed into the Permian. The widespread strike-slip A possible alignment is via the Witch Ground and oblique-slip faulting has been ascribed to a Graben and northeast of the Scottish mainland. major right-lateral shear zone extending from the Fundamental structural differences have been southem Urals to the northem Appalachians noted on either side of this zone by Johnson & (Arthaud & Matte 1977). Pre-existing zones of Dingwall (1981). North of the Moray Firth Ba­ weakness in the Variscan foreland with NW or sin, in the Fair Isle area, the peneplaned base­ WNW trends were reactivated with right-lateral ment reveals the presence of deeply truncated strike-slip offsets. NE or NNE trending faults west facing half grabens, whereas to the south, in were reactivated with left-lateral offsetsand have the Inner Moray Firth, the structural framework been considered as forming a conjugate pair with is considerably more complex and the half gra bens the NW trends by W. Ziegler (1975). Both the face east (Johnson & Dingwall 1981, Fig. 3). A NW and NE trending systems were already em­ similar relationship is noted further to the west, placed in the foreland by Caledonian, Cadomian where the West Shetland Basin and associated and earlier tectonism. W. Ziegler (1975) recog­ half grabens are west facing but further south, in nised the Tomquist Line and Elbe Line as impor­ the Hebrides and Minch Basins, the half-grabens tant zones of Variscan strike-slip faulting. The are east facing. It may be highly significant that Elbe Line was postulated as continuing north­ in the intervening area the dominant NE Cale­ westwards to link up with a persistent structural donian lineation is replaced by a NW trend which lineament extending from the Fladen Ground Johnson & Dingwall (1981) relate to the Wyville­ Spur to the Fair Isle area, bounding the Shetland Thompson Ridge, noting that it is a 'system of Platform to the south. An alternative interpreta­ deep-seated crustal weakness, later reactivated tion suggested here is that the latter lineament 64 R. M. Pegrum NORSK GEOLOGISK TIDSSKRIFT l (1984) can be considered as an extension of the Torn­ trending lineaments and initial fracturing in the quist Zone, offsetto the southwest along the line proto-Atlantic (Pegrum & Mounteney 1978). of the South Viking Graben. P. Ziegler (1981) Triassic rifting also affected the Northwest Eu­ also recognises Variscan right-lateral wrench ropean continental shelf. The Viking Graben and movements along the 'Tornquist-Teisseyre Line', Horda Fault Zones were reactivated, leading to but terminates this feature at its intersection with the uncoupling of the Shetland Platform and the postulated Oslo- Bamble- Horns rift zone. Horda Platform from the Fennoscandian Plat­ The Ringkøbing - Fyn High system came into form. The Ringkøbing - Fyn High was also seg­ prominence as a major positive feature during mented by N or NE trending faults forming gra­ the late Variscan (Ziegler 1981), possibly as a bens or half-grabens. The Tornquist Zone was continental flexure complementary to the Varis­ marked by transtensional collapse and in the can Foredeep, the latter resulting from crustal Scythian marine connections were established via loading by the Variscan orogen to the south ( see the Polish Trough to Tethys. This connection mechanism proposed by Cohen 1982, Fig. 7). remained intermittently open through the Jurassic The Ringkøbing - Fyn High system presently and early Cretaceous (Sokolowski 1976). terminates eastwards against the Tornquist Zone In the late Triassic and early Jurassic, regional in the Arkona Block of north Riigen (Fig, 6), tensional stresses continued and differential sub­ and similarly the Variscan Foredeep terminates sidence led to a through-going rift system linking eastward. A tentative reconstruction of the tec­ up the Viking Graben, Central Graben and tonic framework, after removing 300-350 km of North Netherlands Trough. The Tornquist Zone right lateral strike-slip along the Tornquist Zone, persisted as a belt of structural instability, pre­ is illustrated in Fig. 20B. This reconstruction is dominantly transtensional, marking the bounda­ based upon the speculative realignment of the ry between the stable shield and unstable Eu­ Ringkøbing- Fyn High (RFH) with the Mazury rope. - Byelorussian High (MHB) and the Variscan A particularly strong pulse of rifting in the foredeep (VF) with the Podlasie Graben (PG) middle Jurassic resulted in widespread volcanism and Dnieper- Donbass Graben (DDG) of the associated with the Tornquist Zone. Volcanic southwestern U.S.S.R. The reconstruction also centres are found in Skåne, in the Egersund fortuitously aligns other major structural fea­ Subbasin (Well 17/9-1) in the southern Viking tures, notably the Horns Graben (HG) with the Graben (Norwegian well 15/8-1, Pegrum & Vattern Tectonic Zone (VTZ), the Great Glen Ljones in prep.) and especially in the Forties Fault (GGF) with the Horda Fault system (HF) area of the outer Moray Firth. Whiteman et al. and the easterly facing Minch Fault (MF) with (1975) suggested that the Forties volcanic centre the easterly facing Shetland boundary fault along may be located at a plume generated triple junc­ the margin of the Viking Graben (VG). The tion at the Viking Graben - Central Graben - Midland Valley Graben, bounded by the High­ Outer Moray Firth Basin convergence. The land Boundary Fault (HBF) and southern Up­ northwestward extension of the Tornquist Line lands Fault (SUF), converges with the Central argued in the present paper suggests that these Graben (CG) and Oslo Graben (OG) in a trilete views should be modified and that the location of pattern. The occurrence of chemically similar the Forties volcanic centre was structurally pre­ late Carboniferous/early Permian volcanics in all determined by a 'weak spot' at the crossing of the three grabens supports a possible plume generat­ fundamental Tornquist Zone, with deep faults ed triple junction origin (Whiteman et al. 1975). controlling the Viking Graben and Central Gra­ The present day relationship between these ben. structural features is cartooned in Fig. 20A. In the middle and late Jurassic the Viking and Central Grabens formed the dominant rift sys­ tem (Ziegler 1981), but the continuation of trans­ Mesozoic extension (Fig. 19C) tension along the Tornquist Zone led to the During the late Permian and Triassic northwes­ development of a chain of linear troughs which tern Europe was progressively subjected to re­ include the Outer Egersund Trough, the Farsund gional extension (Ziegler 1981) and widespread - Fjerritslev Trough, the Ålborg Trough, the rifting began in which pre-existing lines of weak­ Malmo Trough and the Polish Trough System. In ness played an important role. Early rifting is Norwegian Block 15/9, in Skåne and in the especially associated with north or northeast southern Baltic, evidence of strike-slip faulting is NORSK GEOLOGISK TIDSSKRIFf l (1984) The Tornquist zone in the North Sea 65

A

B

Fig. 20A. Simplified Variscan tectonic framework of northern Europe.

Fig. 208. Simplified Variscan tectonic framework of northern Europe after removal of approximately 350 km of right-lateral displacement along the Tornquist Zone. MF=Minch Fault; GGF= Great Glen Fault; HBF = Highland Boundary Fault; SUF= Southern Up­ lands Fault; CG=Central Grabe0; HG= Horns Graben; VF= Variscan Foredeep; VG= Viking Graben; HF= Horda Fault System; OG= Oslo Graben; VTZ= Vattern Tec­ tonic Zone; MBH= Mazury-Byelorussia High; PG= Podlasie Graben; DOG=Dnieper­ Donbass Graben. 66 R. M. Pegrum NORSK GEOLOGISK TIDSSKRIFT l (1984) found and an overall left-lateral transtensional stimulating discussions. Thanks are also due to Siri Pegrum for stress system between the stable northeastern her work on the typescript and to the draughting department for the illustrations. platform and unstable Europe is proposed. Manuscript received January 1984 Alpine compression Convergence of the Afro-Arabian Plate and the European Plate began in the eastern Tethys in the late Jurassic and early Cretaceous. In the References Polish Trough, Jurassic and early Cretaceous left-lateral transtensional rifting was replaced in Andersen, O. B., Larsen B. & Platou, S. W. 1975: Gravity and geological structure of the Fennoscandian Border Zone in the later Cretaceous by a more general down­ the Southern Baltic Sea. Bull. Geo/. Soc. Denmark 24, 45- warp and subsequently, at the end of the Creta­ 53. ceous, by right-lateral transpression. The Arthaud, F. & Matte, P. 1977: Late Palaeozoic strike-slip compressional component arose from the north­ faulting in southern Europe and North Africa: result of a right-lateral shear zone between the Appalachians and ward-directed impact in the Carpathians. The Urals. Geo/. Soc. Am. Bull. 88, 1305-20. switch from transtension to transpression along Baartman, J. 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