145

Crustal structure and tectonic evolution of the Tornquist Fanregion as revealed by geophysical methods

HANS THYBO

Thybo, H. 2000–02–10: Crustal structure and tectonic evolution of the TornquistFan region as revealed by geophysical methods. Bulletin of the Geological Soci-ety of Denmark, Vol. 46, pp. 145–160. Copenhagen.

Crustal structure derived primarily from geophysical investigations reveals fea-tures that may be related to the complex tectonic evolution of the Tornquist Fanregion. This northwestwards widening splay of Late Carboniferous – Early Per-mian fault zones in the Danish region emanates from the Teisseyre-TornquistZone in northern Poland. Seismic reflections and velocity anomalies imagecollisional fault zones that formed during the Proterozoic and Palaeozoic amal-gamation of the crust. Re-equilibration of Moho appears to have taken placebefore late Palaeozoic rifting and magmatism initiated the main phase of basinformation that continued into the Mesozoic. The resulting, strong Moho topog-raphy, with variation between depths of 26 and 48 km, has been practically “fro-zen in” since then, although the late Cretaceous – early Cenozoic inversion tec-tonics may have formed a crustal keel underneath part of the Sorgenfrei-Torn-quist Zone which cuts across the Proterozoic crust of the Tornquist Fan region.

Key words: Crust, Mantle, Geophysics, Seismic velocity, Tornquist Fan

Hans Thybo, Geological Institute, University of Copenhagen, Øster Voldgade10, DK-1350 Copenhagen K, Denmark. 3 May 1999.

Crustal structure of the Tornquist Fan region aroundDenmark has been intensively studied by geophysi-cal investigations since the initiation of the EuropeanGeoTraverse project in the early eighties. Primarilyseismic projects have contributed substantially to theknowledge of the structure of the deep crust and up-permost mantle in the region. Interpretation of theseismic data has benefited from the extensive and welldeveloped databases of other geophysical data, suchas gravity and magnetic data. The detailed knowledgeof the sedimentary successions in the region from re-flection seismic data acquired for hydrocarbon explo-ration, provides a wealth of information regarding themain tectonic phases and are invaluable for calibra-tion of the deep seismic data.

The Tornquist Fan region occupies the northwest-ern part of the Trans European Suture Zone (TESZ),which is the area of Palaeozoic amalgamation of thecrust and lithosphere of Central Europe onto the Prot-erozoic Baltic Shield and East European Platform(Pharaoh & TESZ Colleagues 1996, Thybo, Pharaoh& Guterch 1999). The Tornquist Fan proper comprisesthe area between the Fennoscandian Border Zone andthe Trans European Fault and incorporates other fault

zones in between (Fig. 1). The Tornquist Fan is a post-collisional feature, which formed as a northwestwardswidening splay of faults during late Palaeozoic riftingof the region which also initiated wide-spread basinformation and general subsidence at the southwesterncorner of the former Baltica plate (Thybo & Berthel-sen 1991, Berthelsen 1992, Thybo 1997, Fig. 1). Thearea is characterised by pronounced positive Bouguergravity anomalies in spite of the occurrence of verydeep sedimentary successions of more than 10 kmthickness (Thybo 1990, Berthelsen 1992, Zhou &Thybo, 1997).

The main geophysical projects in the region of theTornquist Fan include the deep seismic projects (Fig.2): FENNOLORA (e.g. Guggisberg, Kaminski, &Prodehl 1991), EUGENO-S (EUGENO-S WorkingGroup 1988), EUGEMI (Aichroth, Prodehl & Thybo1992), MOBIL Search (Lie & Husebye 1994), BA-BEL (BABEL Working Group 1993), MONA LISA(MONA LISA Working Group 1997a,b), and Baltic’96(DEKORP – BASIN Research Group 1998). Theseprojects have provided models of the distribution ofseismic velocities and reflective structures in the crustand upper mantle of the region.

Thybo: The Tornquist Fan

DGF Bulletin 46-2.p65 17-01-00, 13:25145

146

This papers reviews some of the main findings fromthe projects with the primary aim of illustrating thegeneral tectonic evolution of the region. Such analy-sis of deep structure holds the potential of unravellingprimary geodynamic problems.

Main tectonic phasesThe Tornquist Fan developed in an area of the south-western part of the former Baltica plate that was amal-gamated mainly during the Proterozoic (Table 1).

Radiometric age determinations and stratigraphic re-lations indicate that the crust belongs to the Sveco-norwegian orogen (1050–950 Ma). This orogen con-sists of largely reworked parts of former units of theBaltic Shield: (1) the Svecofennian orogen (2100–1750 Ma) in the eastern part, (2) the Trans-Scandina-vian Igneous Belt (1840–1650 Ma), and (3) the Go-thian orogen (1700–1600 Ma) which today is onlypresent in the Bleking-Bornholm Block (Fig. 1;EUGENO-S Working Group 1988, Gorbatchev &Bogdanova 1993). The NS-striking SveconorwegianDeformation Front is exposed in southern Sweden,

Fig. 1. Map of main tectonic and geological features of the Tornquist Fan region (based on Ziegler 1990 and Berthelsenet al. 1992, after Thybo 1997). The same Lambert projection has been used for all maps in this article; for conformity thesame projection as used by the EGT project (Blundell et al. 1992) has been selected. The Tornquist Fan proper comprisesthe area between the FBZ and the TEF, incorporating other fault zones such as BF, FF, VFZ and RFZ. Abbreviations:BBB – Blekinge Bornholm Block, BF – Børglum Fault, BG – Brande Graben, BMF – Bamble Fault, CDF – CaledonianDeformation Front, EL – Elbe Lineament, FBZ – Fennoscandian Border Zone, FF – Fjerritslev Fault, GA – GrimmenAxis, GT – Glückstadt Trough, HG – Horn Graben, MH – Møn High, MNS – Mid North Sea High, MZ – Mylonite Zone,OG – Oslo Graben, RFH – Ringkøbing-Fyn High, RFZ – Rømø Fault Zone, RG – Rønne Graben, SG – SkagerrakGraben, SNF – Sveconorwegian Front, STZ – Sorgenfrei-Tornquist Zone, TEF – Trans-European Fault, TIB – Trans-Scandinavian Igneous Belt, TTZ – Teisseyre-Tornquist Zone, VFZ – Vinding Fault Zone.

Bulletin of the Geological Society of Denmark

DGF Bulletin 46-2.p65 17-01-00, 13:25146

147

but its southward continuation is hidden below thesedimentary basins. The southern Baltic Shield prob-ably was covered by sediments during the Precam-brian to Early Palaeozoic (e.g. Berthelsen et al. 1992).

The Phanerozoic geological evolution of the regionaround the Tornquist Fan has been governed by fourmajor tectonic events: (1) Caledonian collision tec-tonics, (2) the distant Variscan orogeny, (3) Mesozoicrifting and graben formation, and (4) Late Cretaceousto Early Cenozoic inversion (Ziegler 1990).

1. Caledonian collision tectonics resulted in amalga-mation of a micro continent or a series of terranes ofAvalonian origin onto the Baltica plate. The suturebetween Baltica and Avalonia as well as the Caledo-nian Deformation Front in the study area have been

identified from borehole information between theNorth Sea (Frost, Fitch & Miller 1981) and the BalticSea (Katzung et al. 1993). Seismic interpretations (e.g.Thybo 1990, BABEL Working Group 1993, Guterchet al. 1994, MONA LISA Working Group 1997a,b)identify the Caledonian Deformation Front betweennorthwest Poland and the North Sea (Fig. 1).Undeformed lower Palaeozoic sedimentary rocks inparts of the deep basins indicate that a deep forelandbasin existed in front of the Polish-Danish-North Ger-man Caledonides, but there is only sparse evidencefor Silurian volcanism (Berthelsen 1992).

2. Stresses induced by the distant Variscan orogenyprobably caused dextral strike-slip movement duringthe Late Carboniferous and Early Permian. This acti-

Fig. 2. Coverage of the Tornquist Fan region by deep seismic profiles, including refraction/wide-angle reflection andnormal-incidence reflection profiles. A much denser network of commercial seismic profiles (not shown) images thesedimentary succession in parts of the area. For a list of deep surveys, see Thybo (1997). Additional lines are the MONALISA lines ML1-4 (MONA LISA Working Group, 1997a,b). The highlighted parts of the profiles are illustrated in Figs.3-6.

Thybo: The Tornquist Fan

DGF Bulletin 46-2.p65 17-01-00, 13:25147

148 Bulletin of the Geological Society of Denmark

Fig. 3. Part of BABEL profile A in the southern Baltic Sea (location is shown in Fig. 2; data after BABEL WorkingGroup, 1993). This profile crosses the Caledonian Deformation Front and the suture between Avalonia and Baltica.Notice the structural division between the upper sequence of Caledonian deformed Palaeozoic sediments (CD) and thelower sequence of Sveconorwegian deformed crystalline crust (SN). These two entities are separated by the characteris-tic double reflection from the O-horizon (O). Structures around the Moho indicate northward deformation, which isconsistent with “a crocodile type” Caledonian collision. A) Seismic normal-incidence reflection section, B) with seismicvelocity model superimposed, C) main reflections and a sketch tectonic interpretation of the collision structures.

DGF Bulletin 46-2.p65 17-01-00, 13:25148

149

vated the faults of the Tornquist Fan as links betweenNNE–SSW trending rift and graben structures, suchas the Oslo Rift, the Skagerrak, Rønne, Brande andHorn Grabens, and other possible deeply buriedgrabens. As such, the Tornquist Fan comprises the fol-lowing W- to NNW-trending fault zones (Fig. 1): theTrans-European Fault Zone (Berthelsen 1984), whichalternatively here will be defined as the southernmostextent of the Precambrian crust of the former Balticaplate (BABEL Working Group 1993); the Rømø andVinding Fracture zones (Cartwright 1990); the Fjer-ritslev and Børglum faults which were strongly reac-tivated during Mesozoic basin development (Surlyk1980, Michelsen & Nielsen 1993); and the Fennoscan-dian Border Zone in Kattegat which is the transitionfrom shield proper to basin areas (Liboriussen, Ashton& Thygesen 1987). The Ringkøbing-Fyn High (RFH)formed as a residual structural high between the Rømøand Vinding Fracture zones while a large area aroundthe Tornquist Fan was subject to extensive volcanismand magmatism in a probable transtensional environ-ment (Dixon, Fitton & Frost 1981, Ziegler 1990,Thybo & Schönharting 1991, Neu-mann et al. 1992).A dense swarm of magmatic dykes developed simul-

taneously in Scania (Bylund 1984). Subsequent cool-ing may have initiated the Permian and Triassic re-gional subsidence in the Danish and North GermanBasins (Sørensen 1986). Lately, it has been suggestedthat an east-dipping detachment into the upper man-tle has governed this late Palaeozoic evolution (Ber-thelsen 1998).

3. Triassic to Jurassic rifting of the Central, Vikingand Horn grabens and normal faulting in the DanishBasin caused by regional extension undoubtedly ac-celerated the subsidence (Nielsen & Balling 1990,Brink, Dürschner & Trappe 1992).

4. Inversion and transpressional deformation charac-terise the area during the Late Cretaceous and EarlyCenozoic, probably caused by Alpine compressionalstresses. The Sorgenfrei-Tornquist Zone (STZ) devel-oped as an inverted zone that cuts across other struc-tures of the Tornquist Fan region. In North Jutlandthe zone is bounded by the Fjerritslev and BørglumFaults of the Tornquist Fan, suggestive of reactivationof earlier fault zones during inversion. In Poland, theinversion zone developed in a Late Palaeozoic rift zone

Fig. 4. Line drawings of reflection sections along MONA LISA profiles 1 and 3 in a fence diagram. The two profiles, aswell as the other two MONA LISA profiles, show both NE and SW dipping reflections from the upper mantle. Thereflections cross in this non-migrated version, but the NE-dipping reflection stays above the SW-dipping reflection aftermigration, which indicate that the former is oldest. This is consistent with a model of Caledonian, NE-dipping collisionstructures in the upper mantle associated with the SW-dipping crustal suture. Abbreviation: CDF – Caledonian deforma-tion Front (the crustal suture). After Abramovitz and Thybo (1998).

Thybo: The Tornquist Fan

DGF Bulletin 46-2.p65 17-01-00, 13:25149

150

(Ziegler 1990) and possibly coincides in parts withthe palaeo-margin of the platform (Guterch et al.1986). The Grimmen Axis forms another short invertedzone (Fig. 1). The geological evolution since the EarlyCenozoic is characterised by regional subsidence inthe North Sea area and uplift of the Baltic Shield(Ziegler 1990).

Seismic images and modelsSveconorwegian and Caledonian collision,BABEL and MONA LISA profilesThe BABEL seismic reflection profiles image at leastthree Proterozoic and one Phanerozoic sutures in theBaltic and Bothnian Seas (BABEL Working Group1990 and 1993). Abramovitz, Berthelsen & Thybo(1997) interpreted a hidden terrane between theSvecofennian and the Gothnian orogens in the south-ern Baltic Sea, now concealed beneath a cover of sedi-ments. In the southern Baltic Sea, BABEL WorkingGroup (1993) interpreted south dipping reflectionsfrom the crust and north dipping reflections at the

Fig. 5. Crustal model across the Silkeborg Gravity High along EUGENO-S profile 2 (location is shown in Fig. 2) basedon integrated interpretation of seismic reflection and deep refraction, gravity and magnetic data (after Thybo &Schönharting, 1991).A) Model of seismic velocity with main tectonic and magmatic features indicated: interpreted Carboniferous-Permianvolcanic body (black) and deep intrusive body (crosses), Mesozoic and Palaeozoic sedimentary successions (verticalruling), upper mantle (horisontal ruling). Numbers indicate seismic velocity values in km/s. B) Gravity fit of the model;the full curve shows calculated gravity anomalies and the circles show measured values.

Bulletin of the Geological Society of Denmark

DGF Bulletin 46-2.p65 17-01-00, 13:25150

151

crust-mantle boundary as images of the Caledoniansuture between Baltica and Avalonia (Fig. 3). The im-age indicates a “crocodile type” of suture where theuppermost crust of Avalonia obducted onto the Balticacraton while the lower crust and mantle subductedbelow Baltica. Thereby, BABEL Working Group re-located the Caledonian Deformation Front at least 50km further to the north in the southern Baltic Sea thanpreviously believed.

The collisional structures are conveniently locatedbeneath the Møn Block, which is the eastward exten-sion of the Ringkøbing Fyn High into the Baltic Seaarea. Because of only a thin Mesozoic cover, crustalreflectors are well resolved in reflection seismic pro-files, even with relatively small airgun arrays. Thisallows interpretation of crustal structures from com-mercial seismic profiles, originally acquired for oiland gas exploration. Lassen (1998) and Lassen, Thybo,& Berthelsen (submitted) present interpretations ofsuch data that clearly identify two sets of dipping re-flections, one south dipping set from the Palaeozoicsedimentary sequence and another, primarily west dip-ping from the crystalline crust. The two sets of dip-ping reflections are divided by a strong regionalmarker, the O-reflection, from the reflector betweenOrdovician shales and Cambrian quartzites. The di-rection of the BABEL profile is at an angle of ~45° tothe two sets of dipping reflections, such that they bothappear to be southwest dipping in the direction of theseismic profile. Therefore, only identification of theO-reflection and the apparent change in dip across thisreflector may reveal the structural division of the twosets of reflections in the BABEL profile.

The original interpretation by BABEL WorkingGroup (1993) must therefore be refined, such that onlythe upper set of dipping reflections is of Caledonianorigin, and the lower set is from the crystalline crust,most likely from thrusts and shear zones of the Sve-conorwegian orogen, which thereby has been tracedfurther south than the Sorgenfrei-Tornquist Zone (Fig.3). This observation indicates that the Caledoniancrustal suture at depth extends further south than ear-lier interpreted by BABEL Working Group (1993), atlower crustal depths perhaps as far as to the Elbe Line-ament. The northeast dipping reflections from depthsaround the Moho were interpreted by BABEL Work-ing Group (1993) as images of subduction structuresof the Caledonian orogeny.

This interpretation is also consistent with later datafrom the North Sea (MONA LISA Working Group1997 a,b, Abramovitz, Thybo & MONA LISA Work-ing Group 1998), where similar structural relationshave been imaged, although both northward and south-ward dipping reflections from the uppermost mantleare observed (Fig. 4). The two events may be corre-lated with westward resp. eastward dipping events inthe EW striking profiles, indicative of NE- and SW-dipping mantle reflectors. The crustal suture of Cal-edonian age has been interpreted from a crust cutting,

SW-dipping reflection (Abramovitz and Thybo 1999).Migration of the mantle events shows that the NE-dipping reflector is confined to the zone above theSW-dipping mantle reflector. The former reflectormust therefore be oldest, which indicate that it repre-sent a Caledonian structure such that the SW-dippingevent should be related to a later tectonic event, suchas late Palaeozoic extension.

A series of profiles of the Baltic’ 96 experiment onlyshows north dipping reflections from Moho level inthe Baltic Sea area. Hence, northward subduction orshearing in the uppermost mantle is indicated by theavailable information between the North Sea and theBaltic Sea. It may appear enigmatic that magmaticrocks, which may be related to Caledonian subduc-tion, generally are not identified in the Baltic Shield.However, this may be explained by either a polarityflip in subduction direction during the late phases ofcollision, cf. results of numerical modelling of colli-sion tectonics (Beaumont, Fullsack & Hamilton 1994),or it might be because the subduction zone had a veryshallow dip, such that no or only small amounts ofmelts were produced from the subducting slab.

Late Carboniferous – Early Permiantranstension, EUGENO-S profile 2One of the major features of the gravity field in theDanish area is the Silkeborg Gravity High, a positive

Table 1. Main tectonic events in the region of the Torn-quist Fan.

Event Age Effect

Svecofennian orogeny ~1.9 Ga Bulk part of Balticacrust forms

Gothian orogeny ~1.6 Ga Plate accretion(only marginal partsremain undisturbed)

Jothnian ~1.3 Ga Basin formationSveconorwegian orogeny ~900 Ga Formation and

deformation of thebasement of most ofthe Tornquist Fanarea

Vendian Rifting ~750 Ma Rifting and basinformation

Caledonian orogeny ~450 Ma Formation ofsouthern basementand foreland basin

Late Variscan orogeny ~250 Ma Rifting andvolcanism, forma-tion of the faults ofthe Tornquist Fan

Mesozoic subsidence

152 Bulletin of the Geological Society of Denmark

DGF Bulletin 46-2.p65 17-01-00, 13:25152

153

Bouguer anomaly of 40-50 mGal compared to thesurrounding gravity level (cf. Fig. 8). Thybo &Schönharting (1991) present an integrated interpreta-tion of gravity and magnetic data with seismic datafrom EUGENO-S profile 2, which crosses the gravityhigh at an oblique angle (Fig. 5). The integrated modelexplains the gravity high by a batholith in the crustbelow ca. 11 km depth. The body is characterised byhigh seismic velocity and density. It probably extendsto the crust-mantle boundary. Moho is slightly elevatedbeneath the body, which further contributes to the grav-ity anomaly. A thin layer of volcanic or subvolcanicorigin may explain the main magnetic anomalies andresidual gravity anomalies in the area. It also explainssome weak, but clear, first seismic arrivals from a shotpoint at the northern end of the profile. These arrivalsare up-to .5 s earlier than the main seismic phases fromthe crystalline crust.

The structural trend of the feature has been estimatedby stripping off the gravity effects of the Mesozoicsediments from the Bouguer anomalies in the Danisharea. The processed gravity anomalies indicate thatthe body extends along the northern edge of the east-ern Ringkøbing-Fyn High with a northward bend to-ward the Skagerrak from mid-western Jutland (Thybo& Schönharting 1991, Zhou & Thybo 1997). Thistrend is also supported by a contoured map of depthto Moho (Fig. 7a). The southern side of the body co-incides with the Vinding Fault zone, as inferred frommagnetic anomalies and seismic interpretation (Dik-kers 1977, Cartwright 1990). Magnetic modellingshows that the interpreted shallow volcanic sequencehas reverse polarity, which may be correlated with along polarity reversal in the Early Permian. The SSE-NNW elongated feature is, therefore, interpreted as abatholith that intruded into the crust during the lateCarboniferous – early Permian because of transten-sional, strike-slip movement on the Vinding Fault zoneof the Tornquist Fan.

The Sorgenfrei-Tornquist Zone,BABEL profile AThe Sorgenfrei-Tornquist Zone is defined as the geo-logical inversion zone that extends in the TornquistFan region between Bornholm in the southeast andthe North Sea in the northwest (EUGENO-S WorkingGroup 1988). The Tornquist Zone was originally iden-tified from a magnetic lineament which appears as thenorthwestern continuation into Scania of an anomalyin the area of Poland, although it cannot be continuedfurther into Kattegat. Modelling indicates that themagnetic anomaly is caused by the edge of the Prot-erozoic platform in Poland (Krolikowski & Petecki1998), whereas it is most likely caused by the Per-mian volcanic dykes in Scania. Hence, the magneticanomalies in Scania probably are not generically con-nected to neither the anomalies in Poland nor the in-version zone.

BABEL profile A crosses the Sorgenfrei-TornquistZone northwest of Bornholm (Fig. 2, BABEL Work-ing Group 1991 and 1993, Thybo et al. 1994), wherethe inversion is strongly developed (Fig. 6). The twomain features of the reflection seismic profile are theinverted block of crystalline rocks near the surfaceand the crustal keel, imaged as a subdued Moho. Theinterpretation is based on integrated interpretation ofreflection and refraction data, such that both reflec-tion structures and seismic velocities are known alongthe profile. Therefore, the 7–10 km deep crustal keelhas been reliably constrained by the data.

The reflection seismic profile images the Sorgen-frei-Tornquist Zone as a “pop-up” structure of crys-talline basement around Bornholm between two sedi-mentary basins, the Hanö and Skurup basins of possi-ble Carboniferous-Permian or older age (Fig. 6). Bothbasins thickens towards the inversion zone, giving theimpression that they once formed a single wide basinthat was split into two by the inversion tectonic event.A pronounced intra-crustal reflection dips to the north-east from the southwestern boundary fault of the “pop-up” structure and sole as a listric fault in the top of thereflective lower crust. This has been interpreted as amain transpressional fault that accommodated themain compressional displacement that caused the in-version (Thybo et al. 1994). During basin formationit may have been a normal fault, which subsequentlywas reversely reactivated by the inversion tectonics.The reflective lower crust may have been a ductilelayer during inversion such that it could act as a de-collement for the displacement.

The crustal keel beneath the inversion zone extendsslightly to the northeast of the zone, perpendicular tothe strike direction. The reflective lower crust extendsacross the inversion zone with relatively constantthickness, whereas the keel appears non-reflective.BABEL Working Group (1993) interpreted the keelas a subversion zone, i.e. as the lower counterpart ofthe inversion zone, such that the compression at lower

Thybo: The Tornquist Fan

Figure 6. Part of BABEL profile A across the Sorgenfrei-Tornquist Zone due north and west of Bornholm in the Bal-tic Sea Seismic normal-incidence reflection section (A),with seismic velocity model superimposed (B), and with asketch tectonic interpretation of the inversion and subver-sion structures (C). The inversion feature near the surfaceis interpreted as caused by transpressional displacementon a listric fault which terminates downward in the top ofthe reflective lower crust. Simultaneously, the lower crustwas subdued by “subversion” such that a deep crustal keeldeveloped. This keel may balance the weight of the surficialinversion structure, such that isostatic balance is main-tained. However, the possibility cannot be completely ruledout that the lower crustal keel was caused by “underplat-ing”, i.e. by magmatic crystallization at the Moho duringCarboniferous-Permian volcanic activity.

←

DGF Bulletin 46-2.p65 17-01-00, 13:25153

154

Fig. 7. Maps of seismic features of the crust in the Tornquist Fan region. (A) Depth to the Moho discontinuity in km;(B) Contours of Moho depth superimposed onto the map of main tectonic features; (C) Map of thickness in km of thecrystalline and metamorphic crust, i.e. the consolidated crust without sediments; (D) Average velocity through thecrystalline and metamorphic crust in km/s.

Bulletin of the Geological Society of Denmark

DGF Bulletin 46-2.p65 17-01-00, 13:25154

155

crustal level was accommodated by formation of thisdeep crustal depression. The keel of low velocity anddensity may balance the weight of the crustal columnbeneath the elevated basement, such that isostatic bal-ance is retained. This interpretation is in agreementwith Thybo et al. (1994) who, however, also point outthat the keel may instead originate from “underplat-ing”, i.e. from magma that solidified at the base of thecrust during the Late Carboniferous to Early Permianmagmatic event. The keel coincides laterally equallywell as the inversion zone with the suite of volcanicdykes in the area and a magmatic origin cannot beruled out (Fig. 6). However, the lower part of the keelappears reflection free, which cannot easily be ex-plained by magmatic sills or a magma chamber thathas solidified at the base of the crust. A similar keelalso has been interpreted further to the northwest(Thybo 1990) and in Poland (Guterch et al. 1986,Guterch et al. 1999), where the Carboniferous-Per-mian volcanism was wide-spread and no localizeddyke swarm has been identified above the crustal keel.

Unless such a swarm be identified, the crustal keelmust most likely be regarded as associated with in-version-subversion tectonics.

Other geophysical data and evidenceMoho depth and average seismic velocity of thecrystalline crustMoho depth in the Tornquist Fan region shows strongundulation which correlates with main tectonic fea-tures of the area (Fig. 7a-b). The Moho is defined asthe depth at which the seismic velocity exceeds 7.8km/s, although originally defined from strong reflec-tions in earthquake seismograms. This level is oftenassumed to correspond to the crust-mantle boundary,but recent research has shown that this is not alwaysthe case (Griffin & O’Reilly 1987, Mengel & Kern1992). The sub-Moho velocity is generally 8.0 km/s inthe basin areas and significantly higher (>8.2 km/s)

Fig. 8. Shaded relief,Bouguer gravity anomalymap. The gravity topogra-phy has been illuminatedfrom the north to enhancelineaments in the data; thisalso identifies gravity highsand lows. Contour intervalis 10 mGal (data fromWybranic et al., 1998).Letters refer to anomaliesdiscussed further in the text:a-d: positive gravity at theOslo Graben, the Scaniandyke swarm, the Ringkø-bing-Fyn High, and theSilkeborg gravity High; e-f:negative gravity along theaxis of the Danish Basinand in the North GermanBasin; s – lineament alongthe Sveconorwegian Front; t– positive gravity at ahidden Svecofennianterrane and the Bleking-Bornholm Block.

Thybo: The Tornquist Fan

DGF Bulletin 46-2.p65 17-01-00, 13:25155

156

beneath the Baltic Shield and the Ringkøbing-FynHigh. Abramovitz et al. (1998) interpreted occurrencesof crustal type rocks in eclogite facies from relativelysmall sub-Moho velocities due north of the north-dip-ping mantle feature at the Caledonian suture in theNorth Sea, similar to velocities to the north of theCaledonian suture in the BABEL profile (Fig. 3).

Moho depth is as large as 48 km in parts of the Bal-tic Shield and up-to 36 km beneath the Ringkøbing-Fyn High, whereas it is as small as 24–26 km beneaththe basin areas where the Mesozoic sedimentary suc-cessions are thickest (Fig. 7b). The thickest crust ap-pears to be related to the Svecofennian part of theshield, although the crust is also thick along the trendof the Sorgenfrei-Tornquist Zone and the Permiandykes in Scania.

There is a pronounced southward trend of thin crustfrom the Oslo Rift – Skagerrak Graben to central Jut-land, from where a southeastward trend parallels theVinding Fracture Zone at the northern edge of theRingkøbing – Fyn High. Very thin crust is also asso-ciated with the Glückstadt Graben in North Germanyof Mesozoic age but only slight crustal thinning isassociated with the graben structures that dissect theRingkøbing-Fyn High. However, removing also thethickness of the sedimentary successions from thecrustal thicknesses, the map of thickness of the crys-talline and metamorphic basement also shows pro-nounced thinning beneath the grabens (Fig. 7c). Thecrystalline and metamorphic crust is in places as thinas 16 km, such that its thickness varies by a factor ofthree in the south Scandinavian area.

Thybo, Perchuc & Gregersen (1998) analyse theseismic reflectivity from Moho in the Tornquist Fanregion. They find a pronounced difference betweenareas with thick basins and areas of the shield and thebasement high. Beneath the Norwegian-Danish basin,the reflections from Moho are distinctly “ringing”, i.e.showing long reverberative reflectivity immediatelyafter the onset of the interpreted reflection from Moho.This is modelled by a series of strongly reflective lay-ers which is interpreted as mafic layering at the baseof the crust. The authors argue that underplating asso-ciated with the formation of the basins is indicated.Contrary, the waveform of Moho reflections from theBaltic Shield and the Ringkøbing-Fyn High has veryshort duration, indicative of a sharp Moho transitionof less than 2 km thickness.

The map of average seismic velocity through thecrystalline crust (Fig. 7d) shows two trends which aresimilar to the Moho map: One trend coincides withthe shallow Moho at the Silkeborg gravity anomalyand another trend coincides with the thick crust alongthe volcanic dyke swarm and the Sorgenfrei-TornquistZone in Scania. Both trends are also associated withpositive gravity anomalies (Fig. 8), which shows thatrocks of high density must be present in both places.These anomalies are most likely caused by crustal,mafic plutonic bodies of Carboniferous-Permian age,

as also supported by gravity and magnetic modelling.These features and their structural trends have been

integrated into a model of the Carboniferous-Permiantectono-magmatic evolution of the Tornquist Fan re-gion, which originally formed a marginal part of thepalaeo Baltica plate (Thybo 1997). Because the areawas situated at the northwestern end of the long Cen-tral European Tornquist Zone between present-dayBlack Sea and North Sea, Variscan tectonics inducedright lateral displacement that caused distributed lat-eral displacement across the faults of the TornquistFan. The NS-trending grabens across the Ringkøbing-Fyn High, the Skagerrak Graben and the Oslo Riftmay be understood as extensional grabens and riftstructures that formed because of the displacement.Only a fraction of the total displacement on the Cen-tral European Tornquist Zone was released across eachof the several individual fault zones. Strong influenceof a mantle plume in the area cannot be ruled out, butmay not be necessary to explain the geophysical ob-servations in the area. Based on the available geophysi-cal-geological data and interpretations between south-ern Poland and the North Sea, Berthelsen (1998) pro-poses a new tectonic model of the Carboniferous-Per-mian volcanism and rifting of the area. The modelinvolves lithospheric buckling because of compres-sion from the Variscan orogens, and east-dipping listricfaults at the main basin boundaries which extends intothe mantle. Future research will focus on the natureof the faults of the Tornquist Fan.

Gravity anomaliesThe map of gravity anomalies (Fig. 8) is based on anew database, which recently was constructed by aEuropean working group as part of the EUROPROBEproject on studies of the Trans European Suture Zone(Wybraniec et al. 1998). It shows the Bouguer anomalyonshore, and the free-air anomaly offshore. The mapdemonstrates that gravity anomalies are large in theTornquist Fan region and smaller in the shield area.Mantle densities are expected to be comparable toother areas because of “normal” sub-Moho seismicvelocities. Therefore, an explanation of the high grav-ity level requires high densities within the crust, atleast locally, which further support the interpretationof extensive mafic intrusions. The structural trends ofthe high gravity anomalies coincide with the trendsderived from seismic information along the interpretedmafic bodies and the Ringkøbing-Fyn High. The threelargest anomalies in the area are at the Oslo Rift –Skagerrak Graben, the volcanic dyke swarm in Scania,and the Silkeborg Gravity High, even though some ofthese anomalies are in areas of thick Mesozoic sedi-mentary cover.

Bulletin of the Geological Society of Denmark

DGF Bulletin 46-2.p65 17-01-00, 13:25156

157

Palaeozoic sediments in the North SeaThe extensive acquisition of geophysical data in theNorth Sea area has also facilitated studies of deep sedi-ment occurrences in more detail than is usually possi-ble for geophysical studies. Such studies have beenconcentrated around the Ringkøbing-Fyn High whichwas previously understood as a structural basementhigh where crystalline rocks were assumed to beclosely underlying a thin Mesozoic sequence (EUGE-NO-S Working Group 1988). Also basin areas showoccurrences of pre-Zechstein sedimentary rocks of up-to 5 km thickness, although details cannot be resolvedbecause of decreasing resolution with depth (Thybo1990). Most commercial reflection seismic sectionsshow coherent seismic energy below the “acousticbasement” which is often considered as the top pre-Zechstein stratigraphical level. Below this level, it isdifficult to distinguish the useful reflections from theambient noise, because they are weak compared towaves that were multiply reflected in the shallow se-quences and to the general background noise level.

Geophysical information about these intervals aremainly from refraction seismic, magnetic and gravitydata. In an interpretation of the gravity and magneticfields integrated with commercial reflection seismicprofiles, Zhou & Thybo (1997) concluded that thereare substantial occurrences of 2-6 km thickness of pre-Zechstein sedimentary rocks throughout the DanishNorth Sea area. The study involved stripping off thegravity effect of the Mesozoic sedimentary sequencesfrom the Bouguer anomaly in order to focus on thedeep sequences. Depth determination based on spec-tral analyses of the magnetic field was also applied.Abramovitz & Thybo (1998) and Nielsen, Klinkby &Balling (1998) interpret similar occurrences of 2-4 kmof Palaeozoic sediments below some of the MONALISA seismic lines in the North Sea, even beneath theCentral and Horn Grabens. Some of these occurrencesmay have been deposited in the Caledonian foredeepbasin. However, they occur on both sides of the Cal-edonian Deformation Front (CDF) (e.g. Vejbæk 1989and 1997), which indicates later subsidence duringthe Palaeozoic. If these pre-Zechstein sediments weredeposited exclusively in the foredeep basin, Caledo-nian thrusting should have developed far into theforedeep of the orogen itself without affecting thedeeper crystalline basement. The MONA LISA deepseismic reflection profiles (Fig. 5) all show crust cut-ting dipping reflections which are interpreted as im-ages of the Avalonia-Baltica suture (MONA LISAWorking Group 1997a,b).

Concluding remarksThe deep structure of the crust and uppermost mantlein the Tornquist Fan region has been the subject ofintensive geophysical-tectonic interpretation over thelast 15 years. New data acquisition has mainly applied

seismic methods, both refraction and deep seismicreflection methods. The two methods have been inte-grated wherever practically and economically possi-ble. This new data has added substantially to ourknowledge of the deep tectonic structure of the re-gion and of the types of rocks that are present. Inter-pretation of the data has proven particularly success-ful where the seismic data has been integrated withother types of geophysical data, such as gravity andmagnetic data.

Interpretation of the new data has made it possibleto identify structures that were caused by the maintectonic events in the area. Nevertheless, several prob-lems remain for future research on the tectonic evolu-tion of the crust and mantle in the area. At the presentstage of interpretation we may make the followingconclusions which also lead to several new questionsto be addressed by future research projects:

1. A crustal keel has been identified around the Torn-quist Zone between southern Poland and Kattegat?Is it a feature related to subversion, underplatingor localized metamorphism?

2. Widespread tectono-magmatic events took place inthe Carboniferous-Permian. The rifting processesand subsequent cooling may have initiated theMesozoic subsidence of the basin areas. What isthe relative importance of Variscan induced defor-mation versus a possible mantle plume in the re-gion? Are the bounding faults of the basins listricand do they merge in a major mantle reaching east-dipping fault? To which degree may thermal sub-sidence after the magmatic heating explain the sub-sequent subsidence?

3. Geophysical research has identified significant oc-currences of Palaeozoic sedimentary sequences ofup-to 5–6 km thickness over much of the TornquistFan region. Is it possible to distinguish sedimen-tary rocks deposited in the Caledonian foredeepfrom those deposited during later subsidence? Howfar north and east may Caledonian deformation betraced in the sedimentary sequences? May the Pal-aeozoic strata be hydrocarbon bearing?

4. The Caledonian collision structures indicate crust-penetrating, relatively steep sutures near to the de-formation front; but may there be other similarfaults further south, or does the main boundary faultextend far southward to e.g. the Elbe lineament suchthat Baltica crust today forms the lower crust un-derneath northern Germany? Did Avalonia dockonto Baltica as a single micro continent or as sepa-rate terranes? Were the collisional faults reactivatedduring subsequent basin formation?

5. The crust still shows structure that may be relatedto the Proterozoic orogens of more than 900 Maage. Have such structures been reactivated in latercollisional and extensional events? How may suchstructure have survived later tectonic events, thatapparently have affected most of the area?

Thybo: The Tornquist Fan

DGF Bulletin 46-2.p65 17-01-00, 13:25157

158

AcknowledgmentsThis article is an extract of a talk presented at the Sym-posium, that was held at the Geological Institute,University of Copenhagen on 2 October 1998, in hon-our of A. Berthelsen on the occasion of his 70th anni-versary in April 1998. Many of the studies reviewedin this paper were carried out in close collaborationwith A. Berthelsen, who also introduced me to thewonders of tectonic interpretation of deep structure.His support and encouragement, which usually in-cluded frank and constructive criticism, is greatly ap-preciated. It is always a pleasure to collaborate anddiscuss with “Berthel”. Collaboration with many otherscientists on various aspects of this paper is also ap-preciated, in particular with T. Abramovitz, A. Lassenand S. Zhou and the many colleagues in EGT andEUROPROBE projects such as BABEL, MONALISA, POLONAISE and PACE. Reviews by O. RønøClausen (Aarhus) and L. Nørgård Jensen (Stavanger)are appreciated. This research was supported by theDanish Natural Science Research Council, theCarlsberg Foundation and the EU through the PACEprogramme.

This is EUROPROBE Contribution No. 295.

Dansk sammendragI de seneste 15 år har der været intensiv aktivitet om-kring geofysisk-tektonisk tolkning af strukturer i skor-pen og den øverste kappe i det danske område. Enrække tektoniske begivenheder har præget områdetsgeologiske udvikling: (1) Den svekonorvegiske oro-genese, hvorunder store dele af det krystalline grund-fjeld blev foldet, (2) Den kaledoniske orogenese, hvor-under de sydlige skorpeområder (omfatter bl.a. detsydlige Danmark og det nordlige Tyskland) blev sam-let med de øvrige områder, (3) Dannelse af rift-strukturer og intensiv magmatisk aktivitet i et stortområde omkring Danmark i sen Karbon til tidlig Perm,(4) Generel indsynkning gennem det meste af Meso-zoikum, med enkelte strækningsbegivenheder, somaccelererede den generelle indsynkning, og (5) Densen kretassiske til cenozoiske inversion i Sorgenfrei-Tornquist Zonen.

Med navnet “Tornquist Fan” menes en vifteformetdel af det danske område, som er gennemsat af et sætaf sen karbone til tidlig permiske forkastningszoner ien vifteform med udspring i det polske område. Derargumenteres for, at horisontale bevægelser hen overdisse forkastningszoner har haft afgørende betydningfor dannelsen af sen-palæozoiske grabenstrukturer iområdet og for den samtidige, kraftige magmatisk-vulkanske aktivitet.

Det analyserede datamateriale omfatter primært seis-miske data, der er blevet indsamlet i forbindelse meden række af dybseismiske projekter i området. Derhar været intensiv dansk deltagelse i de fleste af

projekterne: FENNOLORA projektet langs en profil-linie fra Nordkap til Polen i 1979, EUGENO-S pro-jektet hen over det danske område i 1984, det centraleEGT profil fra Nordtyskland til Italien i 1986, MO-BIL Search projektet i Skagerrak i 1997, BABEL pro-jektet i Østersøen i 1989, LT-7 profilet i Polen i 1990,MONA LISA projektet i den sydøstlige Nordsø i 1993og 1995, og Basin’96 i Østersøen 1996. Projekternehar så vidt teknisk og økonomisk muligt været gen-nemført som integrerede refleksions- og refraktions-/vid-vinkel refleksions-seismiske projekter, således atvi i dag har betydelig viden om dybe strukturer ogfordelingen af seismiske hastigheder til 80 km dybdei det behandlede område.

De seismiske profiler viser strukturer, der kan rela-teres til alle de ovennævnte tektoniske begivenheder.Der præsenteres eksempler, som omfatter dykkendeseismiske refleksioner, der kan relateres til svekonor-vegisk og kaledonisk deformation i grundfjeld og se-dimenter samt antyder nordhældende deformations-strukturer ned i øvre kappe af kaledonisk alder;magmatisk intrusion i skorpen af sandsynlig Karbon-Perm alder som kan relateres til dybe forkastninger iTornquist Fan systemet; og dybe strukturer omkringSorgenfrei-Tornquist inversionszonen, som antyder atdeformationen skete over listriske forkastninger medet decollement i nedre skorpe.

Tolkede dybder til Moho viser en kraftigt ondule-rende topografi i dybder mellem 26 og 48 km. Mohoer defineret som en seismisk laggrænse, som ofte an-tages at befinde sig nær overgangen mellem skorpeog kappe. Topografien kan ikke umiddelbart relaterestil de orogene strukturer, hvilket antyder at der er sketen udjævning af skorpe-kappe overgangen inden desen-palæozoiske tektoniske begivenheder, hvorunderstrækning og “underplating processer” synes at haveforårsaget hovedparten af den observede topografi.Den kretasisk-cenozoiske inversionstektonik kan dogmuligvis have forårsaget skorpefortykkelse omkringinversionszonen. I et bælte omkring inversionszonenmellem Kattegat og det sydlige Polen finder man endyb “Moho-køl” til 44–55 km dybde. Denne kraftigefortykkelse af skorpen kan derfor tilskrives “subver-sionprocesser”, hvorunder den nedre skorpe er blevetpresset ned i kappen i forbindelse med, at den øvreskorpe er blevet skubbet opad langs de listriske for-kastninger. En anden mulighed er dog, at skorpe-kø-len skyldes magmatisk “underplating”, hvorvedmagmatiske smelter er størknet ved bunden af skor-pen. Den sidste mulighed synes sandsynlig i områdetomkring Bornholm og Skåne, hvor der findes ensværm af vulkanske gange, hvorimod noget tilsvarendehidtil ikke er observeret under det tykke sedimentæredække i Polen.

Tolkningen af de seismiske data viser strukturer idet danske område, der kan relateres til de kendtetektono-magmatiske begivenheder, der er foregået si-den den Svekonorvegiske Orogenese for næsten 1000millioner år siden. Det er stadig en gåde hvordan så-

Bulletin of the Geological Society of Denmark

DGF Bulletin 46-2.p65 17-01-00, 13:25158

159

danne strukturer har kunnet overleve de kraftige om-dannelser, der er sket siden de oprindeligt blev dan-nede.

ReferencesAbramovitz, T., Berthelsen, A. & Thybo, H. 1997. Prot-

erozoic sutures and terranes in the southeastern BalticSea interpreted from BABEL deep seismic data. Tec-tonophysics 270, 259–277.

Abramovitz, T., Thybo, H. & MONA LISA Working Group1998. Seismic structure across the Caledonian Deforma-tion Front along MONA LISA profile 1 in the southeast-ern North Sea. Tectonophysics 288, 153–176.

Abramovitz, T. & Thybo, H. 1998. Pre-Zechstein struc-tures around the MONA LISA deep seismic lines in thesoutheastern North Sea. Bulletin of the Geological So-ciety of Denmark 45, 99–116.

Aichroth, B., Prodehl, C., & Thybo, H. 1992. Crustal struc-ture along the Central Segment of the EGT from seis-mic-refraction studies. Tectonophysics 207 43–64.

BABEL Working Group (Baltic and Bothnian Echoes fromthe Lithosphere) 1990. Early Proterozoic plate tecton-ics, evidence from seismic reflection profiles in the Bal-tic Shield. Nature 348, 34–38.

BABEL Working Group 1991. Deep seismic survey im-ages the structure of the Tornquist Zone beneath theSouthern Baltic Sea. Geophysical Research Letters 18,1091–1094.

BABEL Working Group 1993. Deep seismic reflection/re-fraction interpretation of critical structure along BABELprofiles A and B in the southern Baltic Sea. GeophysicalJournal International 112, 325–343.

Beaumont, C., Fullsack, P. & Hamilton, J. 1994. Styles ofcrustal deformation in compressional rogens caused bysubduction of the underlying lithosphere. Tectonophys-ics 232, 119–132.

Berthelsen, A. 1984. The tectonic division of the BalticShield. In Galson, D.A. & Mueller, St. (eds.) Proceed-ings of the first workshop on the European Geotraverse,the Northern Segment, 125–142. ESF, Strasbourg,

Berthelsen, A. 1992. Mobile Europe. In Blundell, D.J.,Mueller, St. & Freeman, R. (eds.) A continent revealed;The European Geotraverse project, 11–32. CambridgeUniversity Press,.

Berthelsen, A. 1998, The Tornquist Zone morthwest of theCarpathians: An intraplate pseudosuture. GeologiskaFöreningens i Stockholm Forhandlingar 120, 223–230.

Berthelsen, A., Burollet, P., Dial Piaz, G.V., Franke, W., &Trümpy, R. 1992. Tectonics. Atlas of compiled data. InFreeman, R. & Mueller, St. (eds.) Atlas of compiled data;A continent revealed; The European Geotraverse, 3–5.Cambridge University Press.

Brink, H.J., Dürschner, H. & Trappe, H. 1992. Some as-pects of the late and post-Variscan development of theNorthwestern German Basin. Tectonophysics 207, 65–95.

Bylund, G. 1984. Palaeomagnetic aspects of the TornquistLine. Geologiska Föreningens i Stockholm Förhandlin-gar 106, 384–386.

Cartwright, J. C. 1990. The structural evolution of the Ring-købing-Fyn High. In Blundell, D.J. & Gibbs, A.D (eds.),

Tectonic evolution of the North Sea Rifts, 200–216.Clarendon Press, Oxford.

DEKORP-BASIN Research Group 1998. Survey providesseismic insights into an old suture zone. EOS 79(12),151,159.

Dikkers, A.J. 1977. Sketch of a possible liniament patternin Northwest Europe. Geologie en Mijnbouw 56, 275–285.

Dixon, J.E., Fitton, J.G. & Frost , R.T.C. 1981. The tec-tonic significance of post-Carboniferous igneous activ-ity in the North Sea Basin. In Illing, L.V. & Hobson,G.C. (eds.), Petroleum geology of the continental shelfof North-West Europe, 65–65. Heyden, London.

EUGENO-S Working Group 1988. Crustal structure andtectonic evolution of the transition between the BalticShield and the North German Caledonides (the EUGE-NO-S Project). Tectonophysics 150, 253–348.

Freeman, R. & Mueller, St. (eds.) 1992. Atlas of compileddata; A continent revealed; The European Geotraverse,73 pp. and 25 maps. Cambridge University Press.

Frost, R.T.C., Fitch, F.J. & Miller, J.A. 1981. The age andnature of the crystalline basement of the North Sea Ba-sin. In Illing, L.V. & Hobson, G.C. (eds.), Petroleumgeology of the continental shelf of North-West Europe,43–57. Heyden, London.

Gorbatchev, R. & Bogdanova, S. 1993. Frontiers in theBaltic Shield. Precambrian Research 64, 3–21.

Griffin, W.L. & O’Reilly, S.Y. 1987. Is the continental Mohothe crust-mantle boundary? Geology 15, 241–244.

Guggisberg, B., Kaminski, W. & Prodehl, C. 1991. Crustalstructure of the Fennoscandian Shield: A traveltime in-terpretation of the long-range FENNOLORA seismicrefraction profile. Tectonophysics 195, 105–137.

Guterch, A., Grad, M., Janik, T., Materzok, R., Luosto, U.,Ylieniemi, J., Lück, E., Schulze, A. & Förste, K. 1994.Crustal structure of the transition zone between Precam-brian and Variscan Europe from new seismic data alongLT-7 profile (NW Poland and eastern Germany). Conseilsdes Researches, Academie des Sciences, Paris 319(II),1489–1496.

Guterch, A., Grad, M., Materzok, R. & Perchuc, E. 1986.Deep structure of the earth’s crust in the contact zone ofthe Palaeozoic and Precambrian platforms in Poland(Tornquist-Teisseyre Zone). Tectonophysics 128 251–279.

Guterch, A., Grad, M., Thybo, H., Keller, G.R., & POLO-NAISE Working Group 1999. POLONAISE’97 – Inter-national seismic experiment between Precambrian andVarscan Europe in Poland. Tectonophysics, in press.

Katzung, G., Giese, U., Walter, R., von Winterfelf, C. 1993.The Rügen Caledonides, northeast Germany. Geologi-cal Magazine 130, 725–730.

Krolikowski, C. & Petecki, Z. 1998. Magnetic database.Polish Geological Institute, Warszawa.

Lassen, A. 1998. En seismisk tektonisk tolkning af regionalestrukturer i et område af den sydvestlige Østersø. Cand.Scient. Thesis, Geological Institute, University of Co-penhagen.

Lassen, A., Thybo, H., & Berthelsen, A., submitted. Re-flection seismic mapping of orogenic fronts below plat-form cover. Geology.

Lie, J.E. & Husebye, E. 1994. Simple-shear deformationof the Skagerrak lithosphere during the formation of theOslo Rift. Tectonophysics 232, 133–141.

Mengel, K. & Kern, H. 1992. Evolution of the petrological

Thybo: The Tornquist Fan

DGF Bulletin 46-2.p65 17-01-00, 13:25159

160 Bulletin of the Geological Society of Denmark

and seismic Moho - implications for the continental crust-mantle boundary. Terra Nova 4, 109–116.

MONA LISA Working Group 1997a. MONA LISA – Deepseismic investigations of the lithosphere in the south-eastern North Sea. Tectonophysics 269, 1–19.

MONA LISA Working Group 1997b. Closure of the Torn-quist Sea: Constraints from MONA LISA deep seismicreflection data. Geology 25, 1071–1074.

Liboriussen, J., Ashton, P. & Thygesen, T. 1987. The evo-lution of the Fennoscandian border zone in Denmark.Tectonophysics 137, 21–29.

Michelsen, O. & Nielsen, L.H. 1993. Structural develop-ment of the Fennoscandian Border Zone, offshore Den-mark. Marine and Petroleum Geology 10, 124–134.

Neumann, E.-R., Olsen, K.H., Baldridge, W.S. & Sundvoll,B. 1992. The Oslo Rift: a review. Tectonophysics 208,1–18.

Nielsen, L., Klinkby, L. & Balling, N. 1998. Evidence fordeep sedimentary units in the Ringkøbing-Fyn High off-shore Denmark from seismic wide-angle and normal in-cidence observations. Bulletin of the Geological Soci-ety of Denmark 45, 1–10.

Nielsen, S.B. & Balling, N. 1990. Modelling subsidence,heat flow, and hydrocarbon generation in extensionalbasins. First Break 8, 25–31.

Pharaoh & TESZ Colleagues 1996. Trans European Su-ture Zone. In Gee, D.G. & Zeyen, H.J. (eds.), EURO-PROBE 1996 - Lithosphere Dynamics: Origin and Evo-lution of Continents. 41–54, EUROPROBE Secretariate,Uppsala University.

Sørensen, K. 1986. Danish Basin subsidence by Triassicrifting in a lithosphere cooling background. Nature 319,660–663.

Surlyk, F. 1980. Denmark, Finland, Sweden, Norway. In:Geology of the European countries, 1–50. Graham andTrotman, London.

Thybo, H. 1990. A seismic velocity model along the EGTprofile – from the North German Basin into the BalticShield. In: Freeman, R., Giese, P. & Mueller, St. (eds.),The European Geotraverse: Integrative studies, 99–108.European Science Foundation.

Thybo, H. 1997. Geophysical characteristics of the Torn-quist Fan area, northwest TESZ: Indication of Late Car-boniferous to Early Permian dextral transtension. Geo-logical Magazine 133, 597–606.

Thybo, H. & Berthelsen, A. 1991. The Tornquist Zone andthe Tornquist Fan. Annales Geophysicae 9 (suppl.), C21.

Thybo, H. & Schönharting, G. 1991. Geophysical evidencefor early Permian igneous activity in a transtensionalenvironment, Denmark. Tectonophysics 189, 193–208.

Thybo, H., Abramovitz, T., Lassen, A. & Schjøth, F. 1994.Deep structure of the Sorgenfrei-Tornquist Zone inter-preted from BABEL seismic data. Zeitschrift der Geolo-gischen. Wissenschaften 22, 3–17.

Thybo, H., Perchuc, E., & Gregersen, S. 1998. Interpreta-tion in Statu Nascendi of seismic wide-angle reflectionsbased on EUGENO-S data. Tectonophysics 289, 281–294.

Thybo, H., Pharaoh, T. & Guterch, A. (eds.) 1999. Geo-physical investigations of the Trans European SutureZone. Tectonophysics, in press.

Vejbæk, O. 1990. The Horn Graben, and its relationship tothe Oslo Graben and the Danish Basin. Tectonophysics178, 29–49.

Vejbæk, O., 1997. Dybe strukturer i danske sedimentærebassiner. Geologisk Tidsskrift 4, 1–31.

Wybraniec, S, Zhou, S, Thybo, H., Forsberg, R., Perchuc,E., Lee, M., Demianov, G.D., & Strakhov, V.N. 1998.New map compiled of Europe’s gravity field. EOS79(37), 437, 442.

Zhou, S. & Thybo, H. 1996. Calculation of residual grav-ity anomalies in northern Jutland, Denmark. First Break14, 129–134.

Zhou, S. & Thybo, H. 1997. Pre-Zechstein geology of thesoutheast North Sea, offshore Denmark – a geophysicalperspective, First Break 15, 387–395.

Ziegler, P.A. 1990. Geological Atlas of Western and Cen-tral Europe. 239 pp. The Hague: Shell InternationalePetroleum Maatschappij/Amsterdam: Elsevier.

DGF Bulletin 46-2.p65 17-01-00, 13:25160