Tectonophysics_Zona de Sutura

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Potential field modelling of the BalticaAvalonia (ThorTornquist)suture beneath the southern North Sea

J.P. Williamson a , T.C. Pharaoh a, *, D. Banka b, H. Thybo c , M. Laigle d , M.K. Lee a

a British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK b FNAG, Naturwissenschaften e.V., Neuenahrer Strasse 20, D-53343, Wachtberg, Germany

c Geologisk Institut, Kbenhavens Universitet, ster Volgade 10, DK-1350, Copenhagen K, Denmark d IGPG, 4 place Jussieu, F-75252 Paris cedex 05, France

Received 18 June 2001; accepted 21 November 2001

Abstract

Magnetic anomaly maps of the Trans-European Suture Zone (TESZ) highlight the contrast between the highly magneticcrust of Baltica and the less magnetic terranes to the SW of the suture. Although the TESZ is imaged on gravity maps,anomalies related to postcollisional rifting and reactivated rift structures tend to dominate.

Seismic and potential field data have been used to construct 2{1\left/2}-D crustal models along three profiles crossing theBalticaAvalonia suture in the southern North Sea (SNS). The first of these models lies along a transect assembled from

reflection line GECO SNST 83-07 and refraction profile EUGENO-S 2; the other two models are coincident with MONA LISA profiles 1 and 2. Additional structural information and density information for the cover sequence is available from releasedwells, while magnetic susceptibility values are compatible with values measured from borehole core samples.

Magnetic anomalies related to the suture are interpreted as due to magnetic Baltican basement of the Ringkbing-Fyn Highdipping SW beneath nonmagnetic Avalonian basement underlying the western part of the SNS. Low-amplitude, long-wavelength magnetic anomalies occurring outboard of the suture are interpreted as due to a mid-crustal magnetic body, possiblya buried magmatic complex. This might represent the missing arc related to inferred southward subduction of the Tornquist Sea, or an exotic element emplaced during the collision between Avalonia and Baltica. The present model supports animbricated structure within Baltica as indicated by the latest reprocessing of the MONA LISA seismic data.D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Baltica Avalonia; Field modelling; North Sea

1. Introduction

Magnetic anomaly maps of the Trans-EuropeanSuture Zone (TESZ) area (Pharaoh, 1999) highlight

the contrast between the highly magnetic crust of theEast European Craton and the less magnetic terranesto the SW of the TESZ (Banka et al., 2002) . Incontrast, the pattern of gravity anomalies in the regionrelates more to structures associated with the exten-sive rifting that affected this area in the late Palaeozoicand Mesozoic, and the subsequent reactivation of many of these structures, rather than the juxtaposition

0040-1951/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.PI I : S0040-1951(02)00346-3

* Corresponding author. Tel.: +44-115-936-3152; fax: +44-115-936-3200.

E-mail address: [email protected] (T.C. Pharaoh).

www.elsevier.com/locate/tectoTectonophysics 360 (2002) 47 60


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of different upper crustal types. The NW pa rt of theBalticaAvalonia or ThorTornquist suture (Berthel-sen, 1998; Pharaoh, 1999) is in general deeply buried

beneath younger sedimentary rocks except in the mid- North Sea High (MNSH)/Ringkbing-Fyn High(RFH). Here it s position has been inferred fromseismic data (MONA LISA Working Group,1997a,b) , and from wells that penetrate basement (Frost et al., 1981; Vejbaek, 1997) . Thus, the influenceof the younger cover on the potential fields is reducedcompared to the adjacent southern North Sea (SNS) basin, making this a good location to investigatecrustal structure by modelling.

This paper presents the results of integrated gravityand magnetic forward modelling conducted alongintersecting profiles across the ThorTornquist suture

(Fig. 1) . The profiles were selected to be coincident with seismic reflection and/or refraction data and,taken together, allow a 3-D structural interpretation

of the suture to be constructed.

2. Data sources and physical properties

2.1. Gravity data

Data have been extracted for each of the modelled profiles from the comp ilation made for the TESZ area(Banka et al., 2002) , which consists of Bouguer anomaly data onshore and free-air anomaly dat a off-shore. In order to illustrate the data coverage, Fig. 2shows an enlargement of a subset of this compilation,

Fig. 1. Location of the three modelled profiles, showing regional structural elements. ADBAnglo-Dutch Basin; DSHFZDowsing-SouthHewett Fault Zone; CDFCaledonian deformation front; CGCentral Graben; HGHorn Graben; ISIapetus Suture; MCMidlandsmicrocraton; MNSHMid-North Sea High; NGBNorth German Basin; OGOslo Graben; RFHRingkbing-Fyn High; S-TZ SorgenfreiTornquist Zone; VFVariscan Front.

J.P. Williamson et al. / Tectonophysics 360 (2002) 476048


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overlain by the three profiles that were used inmodelling. Although the marine data coverage isgenerally good in the SNS basin, the relatively sparsemarine data tracks in the eastern part of the basinintersect the profiles at wide intervals, therefore limit-ing the along-profile resolving power of the data.

2.2. Magnetic data

Magnetic data for each profile have been extractedfrom the compilation made for the TESZ area (Bankaet al., 2002) . For the western part of the SNS, the dataare derived from a grid assembled for the publishedBGS 1:1,500,000 magnetic map of the UK (BritishGeological Survey, 1998) . This grid was derived froman aeromagnetic survey of the North Sea flown at 305m altitude with EastWest flight lines spaced 3 kmapart. The eastern part of the SNS is covered by a grid

based on the compilation made by Wonik (1992) : asthis is derived from many disparate sources, it is not possible to characterise the data set with a singlespacing measure.

As part of the process of merging the different magnetic data compilations to form a single, seamlessdata set, the data were upward continued to a uniform

height of 3 km. This acts as a low-pass filter on thedata, and therefore, short-wavelength anomalies asso-ciated with shallower sources may not be well imaged by the magnetic data.

2.3. Physical properties

Density values that have been used in the model-ling are listed in Table 1 . Values for Mesozoic andCenozoic sediments in the SNS are derived from welldata. Late Palaeozoic densities are consistent with

Fig. 2. Map of gravity data locations, showing the location of the three modelled profiles. T1 is Tran sect 1, ML1 is MONA L ISA 1 and ML2 isMONA LISA 2. Stippled areas illustrate where data have been extracted from the compilation of Wybraniec et al. (1998) .

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those used in previou s modelling studies of the SNS(e.g. Lee et al., 1993 ).

Early Palaeozoic volcanic and sedimentary rocksexposed in the Lake District and other localities may be representative of the (Avalonian) basement beneath the SNS. A density of 2760 kg m

3 has been used in publishe d modelling studies of the LakeDistrict (Lee, 1986) , and our assumed basement

density value (2750 kg m

3) is consistent with this.The crystalline basement of the East European Cratonis assumed to largely comprise high-grade metamor- phic rocks, for which a density of 2850 kg m

3 isrepresentative.

Lower crust and mantle densities are obviously not known by direct sampling. The values that have beenused are typical of values that have been used in previous modelling work (e.g. Lee et al., 1993 ).

Table 2 presents the magnetic susceptibility valuesthat have been used in the models. These may be

compared with the values presented in Table 3 , whichsummarises the results of direct kappameter measure-ments made on basement core samples in the Geo-logical Survey of Denmark and Greenland (GEUS)core store. These measurements reveal high magneticsusceptibility values in borehole core from the base-ment of the East European Craton, with much intra- andinterborehole variability (Table 3) . The assumption of auniform magnetic susceptibility for the basement of theEast European Craton is undoubtedly a simplification, but the value that has been assumed is consistent with

the measured values. Magnetic susceptibility values arecited throughout as 10

3 SI units.

3. Methodology

Pot ential field mo delling was done using GRAV-MAG (Pedley, 1991) , a 2-D interactive potential fieldmodelling program. GRAVMAG computes, along a profile, the gravity and magnetic effects of polygonsattributed with density and/or magnetic susceptibilitycontrasts, where each polygon represents the intersec-tion of the vertical plane of section with a prismatic body extending perpendicular to the section.

A whole-crust modelling methodology was used tointerpret the potential field data along these three profiles, in which the absolute anomaly values aremodelled without subtraction of a regional field. The process of modelling the long-wavelength features that are usually removed during regionalresidual separa-tion gives an insight into possible large-scale structuresand/or property variations that may be required in order to account for these anomalies. All models extend to adepth of 40 km, where there is a flat base to the model, below which the modelling half-space is assumed to befilled with the background density. For gravity model-

ling, this background density is selected in order torepresent the average density taken to the model base ina stable area (this is the reference crust) and the background density remains constant across all of the profiles. In the three modelled profiles presented in this paper, a uniform background density of 3000 kg m

3

was used. For magnetic modelling, a zero backgroundsusceptibility was used.

Transect 1 (Fig. 3) was assembled from interpre-tations of reflection seismic line SNST83-07 (VanHoorn, 1987; Van Wijhe, 1987; Klemperer and

Hobbs, 1991) and refraction profile EUGENO-S 2(EUGENO-S Working Group, 1988; Thybo and

Table 2Magnetic susceptibility values used in the models

Mid-crustal body (Avalonia) Blue 50Upper crust (Baltica) Pink 30 60Middle crust (Baltica) Deep pink 20 130Lower crust (Baltica) Magenta 50 60Modelling background 0

All values are given in 10 3 SI units.

Table 1Density values used in the models

Sea water Pale blue 1030Tertiary Cenozoic Yellow 2000U Cretaceous Green 2250L Cretaceous Grey-blue 2300Jurassic Triassic Pink-orange 2450 2550Zechstein (salt) Cyan 2220Rotliegend Orange 2550Carboniferous Dark red 2600Lower Palaeozoic Dark blue 2600Upper crust (Avalonia) Green 2750Upper crust (Baltica) Pink 2780Middle crust (Baltica) Deep pink 2850 2880Lower crust (Avalonia) Dark green 2900Lower crust (Baltica) Magenta 2880Mantle Deep blue 3270

Modelling background 3000All values are given in kg m

3 .



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Schonharting, 1991) , which together form a near-continuous profile across the SNS, from near-eastern

England onto the East European Craton. The startingGRAVMAG model was constructed from digitiseddepth interpretations of the two individual profiles,with the sedimentary section being constrained bywell data in the gap between the NE end of SNST83-07 and the SW end of EUGENO-S 2.

Starting models for the two MONA LISA profiles(Figs. 4 and 5) were based on digitised versions of the published velocity models (Abramovitz et al., 1998;Abramovitz and Thybo, 2000) . Initial density valueswere assigned to polygons based on conversion of

velocity to density using the mean curve published byBarton (1986) , although these were then adjusted

where necessary during the modelling in order toconstruct geologically plausible models that honour the observed gravity anomaly data.

4. Late Palaeozoic, Mesozoic and Cenozoicsedimentary basins

Due to the effects of Zechstein salt, reflectionseismic data provide only patchy information about the pre-Permian sedimentary succession in the SNS.

Table 3The results of direct magnetic susceptibility measurements made on core samples in the GEUS core store using an Exploranium kappameter (models KT-5 and KT-9)

Well Depth range (m) Lithology v min( 10 3 SI)

v max( 10 3 SI)

v mean( 10 3 SI)

BORG-1 3078 3080 Greywackes (age unknown) 80 330 160BRONS-1 2528 2535 Quartz wackes (Ordovician) 0 160 2PER-1 2780 Granite/Granodiorite,

cataclased (age unknown)20 40 30

P-1X 3491 3494 Amphibolitic gneiss, pegm.,mylonitic overprint

180 7900 1756

LOGUMKLOSTER-1 27112724 Greywackes, cleaved(Caledonian)

140 650 310

UGLE-1 3052 3057 Phyllite, folded and veined,PC clasts (age unknown)

280 1490 740

VARNAES-1 2170 2188 Phyllite/phyllonite, sheared,in part conglomeratic

0 590 70

2188 2190 Phyllite/phyllonite 20 280 1402224 2233 Phyllite (age unknown) 120 550 270

ABENRA-1 2329 2346 Metapelite (age unknown) 0 120 50 NOVLING-1 3759 3762 Ronde Formation (Silurian) 120 490 250PERNILLE-1 3615 3623 Greywackes (Silurian) 60 430 180SLAGELSE-1 2641 2856 Rastrites Shale (Silurian) 60 1570 240

29192946 Alum Shale (L Ordovician) ND ND ND29322973 Slagelse Quartzite

(Lower Cambrian)0 180 39

TERNE-1 3356 3358 Hardeberga Sandstone(Lower Cambrian)

0 120 50

ARNUM-1 1833 1837 Conglomerate, clasts of PCschist and gneiss (unknown)

100 7110 2480

18431845 Conglomerate, clasts of PCschist and gneiss (unknown)

360 860 570

GLAMSBERG-1 910 912 Amphibolitic gneiss(Sveconorwegian)

110 210 150

GRINDSTED-1 1622 Biotite gneiss, pegmatitic(Sveconorwegian)

130 480 260

IBENHOLT-1 8525 8528 Granitic gneiss(Sveconorwegian)

710 730 720

Values are given in 10 3 SI units.



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Fig. 3. Transect 1 model, extending from near-eastern England to SW Sweden. The following features are indicated: Dowsing-South Hewett Fault Z(CG), Horn Graben (HG), Ringkbing-Fyn High (RFH), North Danish Basin (NDB) SorgenfreiTornquist Zone (STZ). The ThorTornquist suture excrust from km 500 to km 350.


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Although Carboniferous rocks have been proved beneath the SNS, the thickness of these sediments isnot well known. Stripped gravity anomalies suggest locations where there may be significant accumula-tions of late Palaeozoic sedimentary rocks, but thereare few constraints on their thickness or density. All

three models show Carboniferous basins overlying the basement, but the geometry of these basins is not wellconstrained by independent evidence.

Fig. 4. Gravitymagnetic model based on profile MONA LISA 1.The eastern Mid-North Sea High (MNSH) and the Horn Graben(HG) are indicated. The ThorTornquist suture extends downwardsthrough the crust from km 120 to km 250.

Fig. 5. Gravitymagnetic model based on profile MONA LISA 2.The Ringkbing-Fyn High (RFH), the Horn Graben (HG) and the North German Basin (NGB) are indicated. The Thor Tornquist suture extends downwards through the crust from km 100 to km 280.



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The Mesozoic basins of the SNS are well imaged byreflection seismic data. Transect 1 crosses four major sedimentary basins: the Sole Pit Basin, the Central

Graben, Horn Graben and the Norwegian DanishBasin. It is important to emphasise that these individual basins did not develop simultaneously and that they arenot genetically related (Vejbaek, 1990) . The reasons for this are beyond the scope of this paper.

Although the Sole Pit Basin contains a large thick-ness of Mesozoic sedimentary rocks, it is marked by a positive gravity anomaly due to the effects of lateCretaceous and Alpine basin inversion, which has brought once deeply buried strata closer to the sur-face. The positive anomaly is largely accounted for bythe relative structural elevation of strata within the basin, together with the postulated basement uplift beneath the basin.

Transect 1 crosses the Central Graben at an obliqueangle. On this profile, the margin of the graben ismarked by a positive gravity anomaly and a coinci-dent magnetic anomaly: as noted in Section 6, theseanomalies are interpreted as due to a postulated basicintrusion located offline to the southeast. Seismicreflection data indicate that this basin has also beenstrongly inverted, in pre-Cenozoic time, and has beenseverely affected by salt diapirism.

The transect continues to NE, crossing the HornGraben obliquely at its southern end in Germanwaters, where it is up to 5 km deep. The pre-Zechsteinsequence includes Devonian, Carboniferous and Rot-liegendes strata. Rifting here began in Carboniferous Permian time, with the main phase occurring in theearly Triassic and only negligible subsidence in Juras-sic time. Significant uplift occurred in mid to lateJurassic time (Ziegler, 1990) . Although a geneticrelationship with the Oslo Graben has been suggested(e.g. Ziegler, 1978 ), dissimilarities in the evolution of

these two basins appear to rule such a link out (Vejbaek, 1990) . The graben was not significantlyinverted by Alpine events, and post-rift thermal sub-sidence in Cenozoic time was also rather limited(Vejbaek, 1990) . Halokinesis is concentrated at themargin of the graben.

The NorwegianDanish Basin is crossed in Danishwaters, to the south of the deeper Fjerritselv Trough.Here the Mesozoic basin is 57 km deep and comprises a complete succession of Zechstein andyounger strata. The most rapid tectonic subsidence

was during Zechstein time, whereas Triassic subsi-dence is thought to be lar gely due to phase trans-formations (Vejbaek, 1990) .

5. Crust of Baltica East European Craton

5.1. Early Palaeozoic sedimentary strata

The early Palaeozoic sedimentary cover of Balticacomprises the strata of two megasequences. A rela-tively thin, tectonically undisturbed sequence of Cambro-Ordovician age was deposited on the pas-sive margin of Baltica following rifting of a con- jugate margin away from the East European Craton,during break-up of the Rodinia Pannotia supe rcon-tinent in late Proterozoic time (Dalziel, 1997) . Theoffshore sequence is comparable to that exposed inthe Oslo Graben, mostly comprising shales andlimestones, which attains a thickness of 376 m inthe Terne-1 borehole. The carbonaceous Alum Shaleof latest Cambrian to early Ordovician age is of critical importance. Its high content of graphite leadsto its distinctive conductivity signature. Magneto-telluric experiments identify this O-horizon far tothe south into the northern part of the German Basin

(ERCEUGT Group, 1992) . The high graphite content also means that this is an important detachment horizon (decollement) in the Scandinavian Caledo-nides, an d similar behaviour is inferred here in theoffshore (Lassen et al., submitted for publication) .The second megasequence comprises shales andgreywackes of Silurian age, deposited in a rapidlysubsiding marine foredeep (Michelsen and Nielsen,1993) marginal to the Thor suture, following closureof the Thor Tornquist Sea between Avalonia andBaltica (Cocks and Fortey, 1990) . These strata may

be 7 km thick in Poland (Dadlez, 1982) . Involve-ment in subsequent foreland-directed thrusting mayhave resulted in considerable tectonic thickening of this megasequence. An overall density of 2600 kg m

3

has been assigned to both megasequences, which arenot differentiated in the models for reasons of clarity.

5.2. Middle crust

The exposed Fennoscandian Shield on the main-land of Sweden, at the NE end of Transect 1,



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comprises hornblende- and biotite-bearing gneissesand schists of the Gothian and Sveconorwegian prov-inces. The offshore boreholes Arnum-1, Frederick-

shavn-1, Glamsbjerg-1, Grin sted-1, Ibenholt -1 andJelling-1 prove similar rocks (Vejbaek, 1997) . Someof these have given Ar Ar ages in the range 880 700Ma, demonstrating the offshore extension of theSveconorwegian province. In the onshore, the meta-morphic rocks are intruded by variable amounts of granite. While these are also likely to be present offshore, bodies of batholithic size do not appear tomake a significant contribution to the gravity field.Instead, the middle crust is satisfactorily modelled byan upper layer with a density of 2780 kg m

3 andvariable magnetic susceptibility in the range 5060;and by a lower layer with density in the range 2850 2880 kg m

3 with a more variable susceptibility (upto 130, see next section). This interpretation is con-sistent with the three layered crystalline crust inferredfrom the seismic refraction modelling (EUGENO-SWorking Group, 1988) and subse quently confirmed by wide-angle reflection profiling (BABEL WorkingGroup, 1993) . Velocity layers of 6.16.4, c. 6.6 and6.97.2 km s

1 inferred from the seismic modellingcorrespond to densities of 2780, 2850 and 2880 kgm

3 in our models.

5.3. Intrusive complexes

Transect 1 crosses a strongly positive ( + 30 mGal)Bougu er gravity anomaly known as the SilkeborgHigh (Fig. 3, km 660). Further interpretation of theEUGENO-S 2 transect by Thybo and Schonharting(1991) led them to propose that the anomaly wascaused by a dense magnetic body of possible Permo-Carboniferous age, located in the middle crust of theso-called Tornquist Fan (Thybo, 1997, 2000) , in the

transition between the thick lithosphere of the East European Craton and the much thinner lithosphere below Palaeozoic Europe at the northwest end of theTESZ. An alternative hypothesis is that the bodyrepresents an intrusive complex of Sveconorwegianage, comparable to that exposed on the Norwegianmainland (Olesen et al., in preparation). In the model(Fig. 3) , the Silkeborg body is modelled by a polygonwith density 3000 kg m

3 and magnetic susceptibilityof 130, similar in size and shape to that inferred byThybo and Schonharting (1991) . A steeply inclined,

sheet-like body up to 10 km wide is the inferred causeof a prominent magnetic low on the transects MONALISA 1 (Fig. 4, km 25) and 2 (Fig. 5, km 45). This

might also be interpreted as a steep shear zone inwhich magnetic susceptibility has been reduced byshearing of a more magnetic protolith.

5.4. Lower crust

A density of 2900 kg m 3 has been assigned to

the lower crust, which is presumed to comprisemetamorphic and intrusive rocks in granulite facies.Magnetic susceptibility in the range 50 60 isassumed for these rocks, which are inferred to lienear the Curie point.

5.5. SorgenfreiTornquist Zone

The Sorgenfrei Tornquist Zone (STZ) lies adja-cent to the Skaggerak Kattegat Platform. Its near-surface expression is as a plexus of major normalfaults associated with asymmetric graben structuresand frequently with reverse displacements, lying between the Fjerritselv Fault (in the SW) and theBorglum Fault (in the NW). These dis placements,which are schematically indicated in Fig. 3 (km

780), reflect transtensional and transpre ssional move-ments along the STZ in Mesozoic time (Surlyk, 1980;Michelsen and Nielsen, 1993) . The Fjerritslev Troughlocally contains 10 km of sedimentary fill. Alpinecompressional stress led to inversion and transpres-sion in late Cretaceous and early Cenozoic time.Variscan dextral transpression is also inferred to haveaffected the late Carboniferous early Permiansequences (Thybo, 2000) . Berthelsen (1998) has sug-gested that the STZ is a crust-penetrating featureextending into the upper mantle, possibly forming a

back-stop to Variscan deformation (and Alpine inver-sion). The long and complex history of movementswithin the STZ suggests that it is indeed a crust- penetrating feature. On the other hand, the Cambro-Ordovician sequence can be correlated with precision(and little variation in thickness) across the zone(Franke, 1993) suggesting minimal displacement at the early Palaeozoic level; the modelling presentedhere suggests that the zone lies entirely within arelatively homogeneous mid-crustal block (magneticsusceptibility 130 10

3 SI), so that the STZ does



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not juxtapose Precambrian crust with strongly con-trasting physical properties.

6. Crust of Avalonia

6.1. Upper crust

The crust underlying the Southern Permian GasBasin in the SNS, to the southwest of the mid-NorthSea High and Ringkbing-Fyn High is generallyassumed to be of Avalonian affinity, implying sim-ilarity to the outcrops of Early Palaeozoic basement rocks that occur both near the basin margins (Brabant Massif, Charnwood) and further afield (Lake District)in England. However, this basement is undrilled for over 250 km between the Dowsing-Sout h Hewett Fault Zone (DSHFZ) and the RFH, and Lee et al.(1993) , Pharaoh et al. (1995) and Pharaoh (1999)have suggested that it may be suspect with respect tothe rest of Eastern Avalo nia, forming a distinct South-ern North Sea Ter rane (BGS, 1996) or Far EasternAvalonia Terrane (Winchester et al., 2002) . In theabsence of empirical data, the crust in this region isassigned physical property values comparable to thoseof Avalonia, i.e. density 2750 kg m

3 , magnetic

susceptibility 0.At the extreme SW of Transect 1, the crust is

intruded by a g ranitic body, one of the postulatedWash granites (Allsop, 1983) , which may be relatedto SW subduction of Avalonian crust alon g theDowsing-South Hewett line in Ordovician time (Phar-aoh et al., 1993) . A polygon located on the model near km 250 lies close to the margin of the Central Graben:examination of gravity and magnetic anomaly maps(Banka et al., 2002) indicates that the causative bodylies somewhat offline to the SE and therefore makes a

poor 2-D modelling target. The body is associatedwith a positive gravity anomaly, as well as with amagnetic anomaly, and is interpreted as a basicintrusive body.

6.2. Concealed mid-crustal magmatic complex

Long-wavelength, low-amplitude magnetic anoma-lies are seen on all three profiles, occurring outboard of the ThorTornquist suture, i.e. to the SWon Transect 1 ( Fig. 3, km 250400), to the SSE on

ML1 (Fig. 4, km 215270) and to the SSW on ML2(Fig. 5, km 240). Both the areal extent and the spatialcoherence of these anomalies have influenced our

interpretation in favour of a single buried magnetic body (magnetic susceptibility 50.0 10 3 SI), with

an irregular upper surface at a mean depth of approx-imately 12 km, which gives rise to anomalies that match the observed data well. The relief on the upper surface of this body may be a primary irregular, e.g.intrusive contact, or more speculatively, due to post-emplacement thrust imbrication, as inferred for asimilar body located s outh of the DSHFZ, in theAnglo-Brabant Massif (Lee et al., 1993) .

A possible alternative source for the observedanomalies is magnetic volcanic rocks of Permo-Car- boniferous age. The seismic data give good geomet-rical constraints down to the base Permian, and theselimit the amount of volcanic material that can bemodelled. Consequently, a rather high susceptibilityis required to generate anomalies of sufficient magni-tude, although the possibility of strongly remanent magnetic lavas remains.

6.3. Lower crust

Beneath the SNS, B IRPS deep reflection profiles

(Blundell et al., 1991) show a reflective lower crust underlying a nonreflective upper crust. Our modelsincorporate a two-layer Avalonian crust, with alower crust of density 2900 kg m

3 and near-constant thickness of 10 km.

7. ThorTornquist suture zone

7.1. Geological and seismic evidence

The Thor suture (Berthelsen, 1998) marks theclosure of the Tornquist Sea, which separated Balticaand Avalonia, and was sufficiently wide to maintainthe faunal distinctiveness of these palaeocontinents for much of Ordovician time (Cocks and Fortey, 1982) .The role of subduction in the closure of this ocean basin, and its geometry, is debated, e.g. see Pharaoh(1999) for a review. The late stages of closure inAshgill time almost certainly involved oblique con-vergence, with counterclockwise rotation of Avaloniaand dextral strikeslip along the suture zone (Pharaoh



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et al., 1995) . Further deformation of the suture zone isknown to have occurred when the Silurian strata of the Baltica foredeep were incorporated in a foreland

thrust belt, culminating in the Scandian deformation phase.Recently acquired deep seismic reflection profiles

(MONA LISA, BASIN 96) support the existence of aSW-dipping crustal interface between Baltica andAvalonia, with high-velocity lower crust of Balticantype exte nding beneath Avalonia as far south asHamburg (Thybo et al., 1990; Rabbel et al., 1995;Bayer et al., 2002) . A prominent SW-dipping mid-crustal reflector on the MONA LISA data set wascorrelated with the ThorTornquist su ture by theMONA LISA Working Group (1997a,b) . Reprocess-ing and reinterpretation of these data, presently in progress (Laigle et al., in preparation) suggests that the situation is actually more complex, and that thisreflector may be correlated better with the O-horizondetachment surface, rather than the suture itself.Commercial seismic reflection data in the Baltic Sea

and on the RFH (Vejbaek, 1990, 1997; Lassen et al.,submitted for publcation) indicate that the early Palae-ozoic cover of the former Baltica margin is involved

in a foreland overthrust belt to the north of the suture.On the eastern flank of the Horn Graben, Transect 1 passes close to a commercial seismic reflection profile(Fig. 10 of Vejbaek, 1990) , which shows strong S-dipping reflectivity from listric surfaces within the basement. These may represent early Palaeozoicmetasediments deformed within the thrust belt, or the fabric of the ThorTornquist suture itself. In thelatter case, the internal structure of the suture zone isas complex as the history inferred above wouldindicate.

7.2. Geometry

Combined gravity magnetic modelling demon-strates the presence of a sharp, curviplanar interface between the strongly magnetic crust of the East European Craton and the less magnetic crust of

Fig. 6. (a) MONA LISA 1 magnetic model; (b) MONA LISA 2 magnetic model. Filled polygons are those which have been assigned a magneticsusceptibility. Unfilled polygons have no susceptibility. The intensity of shading reflects the susceptibility value.



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Avalonia, producing a strong m agnetic anomalyclose to the surface in the RFH (Fig. 4 , Banka et al., 2002) . This interface is here correlated with t he

ThorTornquist suture zone. Profile ML2 (Fig. 5) ismost orthogonal to the interface, and most closelyapproximates to a true dip section. The interfacedips from 2 km depth at km 150 to 27 km depth(Moho interface) at km 250, an average dip of about 14 j . The modelling suggests that the slope of theinterface is concave upwards (listric), being steeper in the upper crust (c. 40 j ), decreasing to 16 j in themid crust and abou t 7j in the lower crust. T he other models, for ML1 (Fig. 4) and Transect 1 (Fig. 3),confirm this geometry, but being orientated moreobliquely to the inferred suture, these models showreduced apparent dips. They nevertheless confirmthe rather sharp, apparently listric nature of theinterface, and its dip to SW. The solution is almost invariant.

To the north of the suture, the observed anomaly pattern is modelled by inferring thin scales of Balticancrust (associated with gravity and magnetic highs)imbricated with early Palaeozoic strata of the Balticamargin (associated with gravity and magnetic lows),across the northern part of the RFH. Below the DanishBasin, the early Palaeozoic cover is mainly thickened

in the southern part, suggesting that the front of Caledonian deformation may be located in the vicin-ity.

Fig. 6a and b shows the magnetic models for thetwo MONA LISA profiles: here, increasing colour saturation corresponds with increasing susceptibility.These figures highlight the difference in magnetic properties of the crust between Baltica and Avalonia,and show how the sources of the magnetic anomalieshave been modelled as imbricated slivers of magnetic basement.

In all three profiles, there is a mass deficiency tothe south of the suture: this has been modelled by postulating a sliver of dense, nonmagnetic Avalonianlower crust that has been thrust over the magneticcrust of Baltican affinity. Other than this, the model-ling reveals little of the internal crustal structure of this easternmost, possibly exotic, part of Avalonia.More sophisticated modelling here must await improvements in knowledge of Carboniferous basingeometry, when better reflection seismic data becomeavailable.

8. Discussion

8.1. Geometrical effects

The model profiles were selected to be coincident with regional seismic reflection or refraction lines.Transect 1 crosses some major structures (notably theCentral Graben and the Horn Graben) at an obliqueangle: in these cases, because the modelling algo-rithms assume that bodies in the model are perpen-dicular to the plane of section, the computed anomalycannot match exactly the observed anomaly. Themagnitude of the mismatch increases as the angle of intersection between the profile and the body deviatesfrom a right angle.

A second geometrical effect present in Transect 1is the occurrence of an anomaly that is caused by anoffline body. This situation is a 3-D modelling prob-lem and, as such, it cannot be properly addressed in atwo-dimensional model. An example of this is seen inthe Transect 1 model at km 240 where the intrusive body shown in the model lies, in reality, to the SE of the profile.

8.2. Long-wavelength magnetic anomalies

The southwest of the southern North Sea is char-acterised magnetically by a long wavelength ( c 200km) negative anomaly of approximately 50 nT. Themismatch between the observed and calculated mag-netic anomalies SW of km 200 on Transect 1 is due tothis regional anomaly, which is difficult to account for in a crustal model. It is possible that this regionalanomaly arises as a consequence of removing themagnetic reference field from the observed data, or,speculatively, that the anomaly is due to magneticsources in the upper mantle.

8.3. The ThorTornquist suture

One of the main results of the modelling is theapparent sharpness of the Thor Tornquist suturezone, which belies the complex geometry and historyinferred from the geological evidence reviewed above.

The modelling also infers the presence of a mag-netic mass in Avalonian mid-crust just to the south of the Thor Tornquist suture, in order to account for long-wavelength, low-amplitude anomalies. In the



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absence of any firm petrological or isotopic evidence,the origin of this body is of course a matter of speculation. Its geometry, size, location and inferred

magnetic properties are consistent with formation of acalc-alkaline arc-related magmatic complex, associ-ated with subduction of oceanic lithosphere, beneaththe NE edge of Far Eastern Avalonia. In this context,the apparent similarities to the crustal structure of the NE edge of the Anglo-Brabant Massif, described byLee et al. (1993) , are rather interesting.

9. Conclusions

(1) Magnetic anomalies related to the ThorTorn-quist suture are interpreted as due to magnetic Balti-can basement of the Ringkbing-Fyn High dippingSW beneath nonmagnetic Avalonian basement under-lying the western part of the SNS.

(2) Our modelling suggests that the suture zone between Baltica and Avalonia type crust is relativelysharp, and provides a tighter constraint on its geom-etry than presently available from deep seismic data.

(3) Low-amplitude, long-wavelength magneticanomalies occurring outboard of the suture are interpreted as due to a mid-crustal magnetic body, possi-

bly a buried magmatic complex. This might represent the missing arc related to inferred southward sub-duction of the Tornquist Sea, or an exotic element emplaced during the collision between Avalonia andBaltica.

(4) The present models support an imbricatedcrustal structure at the margin of Baltica, as indicated by the latest reprocessing of the MONA LISA seismicdata.

Acknowledgements

This paper is a contribution to the EUROPROBETESZ project, by participants of the EU-funded PACEResearch Network (Contract No. ERBFMRXCT970136). Access to basement core samples in the archiveof GEUS (Geological Survey of Denmark andGreenland), Copenhagen, is gratefully acknowledged.

This paper is published with the permission of theDirector, British Geological Survey (NERC).

This is a EUROPROBE publication.

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