Correlation between the Palaeozoic structures from West ...w3.ualg.pt/~jluis/artigos/elsa.pdfCorrelation between the Palaeozoic structures from West Iberian and Grand Banks margins

Tectonophysics 321 (2000) 57–71www.elsevier.com/locate/tecto

Correlation between the Palaeozoic structures from WestIberian and Grand Banks margins using inversion

of magnetic anomalies

Elsa A. Silva a, J.M. Miranda a,*, J.F. Luis b, A. Galdeano ca CGUL, Rua da Escola Politecnica, 58, 1250 Lisbon, Portugal

b Universidade do Algarve, Campus de Gambelas, 8000 Faro, Portugalc IPGP, 4 Place Jussieu, 75004 Paris, France

Received 9 June 1998; accepted for publication 13 February 2000

Abstract

The Ibero-Armorican Arc (IAA) is a huge geological structure of Pre-Cambrian origin, tightened during hercyniantimes and deeply affected by the opening of the Atlantic Ocean and the Bay of Biscay. Its remnants now lie in Iberia,north-western France and the Canadian Grand Banks margins. The qualitative correlation between these three blockshas been attempted by several authors (e.g. Lefort, J.P., 1980. Un ‘Fit’ structural de l’Atlantique Nord: argumentsgeologiques pour correler les marqueurs geophysiques reconnus sur les deux marges. Mar. Geol. 37, 355–369; Lefort,J.P., 1983. A new geophysical criterion to correlate the Acadian and Hercynian orogenies of Western Europe andEastern America. Mem. Geol. Soc. Am. 158, 3–18; Galdeano, A., Miranda, J.M., Matte, P., Mouge, P., Rossignol,C., 1990. Aeromagnetic data: A tool for studying the Variscan arc of Western Europe and its correlation withtransatlantic structures. Tectonophysics 177, 293–305) using magnetic anomalies, mainly because they seem to preservethe hercynian zonation, in spite of the strong thermal and mechanical processes that took place during rifting andocean spreading.

In this paper, we present a new contribution to the study of the IAA structure based on the processing of acompilation of magnetic data from Iberia and Grand Banks margins. To interpret the magnetic signature, a Fourier-domain-based inversion technique was applied, considering a layer with a constant thickness of 10 km, and takinginto account only the induced field. The digital terrain model was derived from ETOPO5 (ETOPO5, 1986. Reliefmap of the earth’s surface. EOS 67, 121) and TerrainBase (TerrainBase, 1995. In: Row III, L.W., Hastings, D.A.,Dunbar, P.K. (Eds.), Worldwide Digital Terrain Data, Documentation Manual, CD-ROM Release 1.0. GEODAS-NGDC Key to Geophysical Records. Documentation N. 30, April ) databases. The pseudo-susceptibility distributionobtained was repositioned for the 156.5 Ma epoch, using the Srivastava and Verhoef [Srivastava, S.P., Verhoef, J.,1992. Evolution of Mesozoic sedimentary basins around the North Central Atlantic: a preliminary plate kinematicsolution. In: Parnell, J. (Ed.), Basins on the Atlantic Seaboard: Petroleum Geology Sedimentology and BasinEvolution, Geological Society Special Publication No. 62, pp. 397–420] pole.

Using this coherent magnetic framework, we can verify that the continuity between adjacent blocks is quite good,in terms of the amplitude, wavenumber and magnetic susceptibility pattern. If we accept that the magnetic propertiescan be taken as a marker of the hercynian zonation, as was verified in previous studies (Miranda, J.M., Galdeano,A., Rossignol, J.C., Mendes-Victor, L.A., 1989. Aeromagnetic anomalies in mainland Portugal and their tectonic

* Corresponding author.E-mail address: [email protected] (J.M. Miranda)

0040-1951/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.PII: S0040-1951 ( 00 ) 00080-9

58 E.A. Silva et al. / Tectonophysics 321 (2000) 57–71

implications. Earth Planet. Sci. Lett. 95, 161–177; Galdeano, A., Miranda, J.M., Matte, P., Mouge, P., Rossignol,C., 1990. Aeromagnetic data: A tool for studying the Variscan arc of Western Europe and its correlation withtransatlantic structures. Tectonophysics 177, 293–305; Socias, I., 1994. Estudios de los Elementos del CampoMagnetico en la Espana Peninsular a partir de Datos Aeromagmaneticos. Ph.D. thesis, University of Madrid), wecan conclude that (1) the characteristic magnetic signature of Ossa Morena Zone is absent on the Iberian Marginand west of it; (2) no eastward continuation of the Collector Anomaly is found in Iberia; (3) only the inner zones ofthe Variscan Belt can be followed towards NW France; (4) there is a major ( left lateral ?) strike-slip fault along thenorthern Portuguese shoreline that cuts the IAA and significantly displaces the once-contiguous variscan units. © 2000Elsevier Science B.V. All rights reserved.

Keywords: geological correlations; inversion; magnetic anomalies; paleo-reconstruction

1. Introduction to Matte (1986b), the huge detachment movementsand lateral escape mechanisms that took placeafter the collision phase are responsible for theThe European Variscan Belt (Fig. 1) is a

Palaeozoic strongly arcuated fold belt, whose actual proximity of the sutures that cannot bedirectly interpreted as the width of the previousnorthern limit can be followed from the SW of

Ireland and England until Germany (Fig. 1). This continental blocks.The complex geological episodes recorded inlimit, called Northern Variscan Front (Fig. 1), is

characterized by a succession of flat-lying thrusts this chain are very difficult to study, due to thefact that only the external zones are somehowto the North, putting Lower Ordovicic–Carbonic

terranes over a less-modified Lower/Middle preserved. The ‘internal zones’ — where the com-plete history should be reconstructed — are nowCarboniferous molasse cover. North of this

front, the Pre-Cambrian/Caledonian basement largely cryptic, as the result of their erosion leveland the rework by later orogenisis.unaffected by the Variscan orogeny (Matte,

1986b, 1991). The Iberian Massif is a well-exposed section ofthe Variscan Basement that escaped significantThe South limit of the belt is apparently con-

tinuous from the SE of the Bohemian Massif Alpine rejuvenation. Lotze (1945), based on paleo-geographic zonations, tectonic style, magmatism(Germany) to West Iberia (Fig. 1), where it is still

possible to identify a flat-lying thrust system with and metamorphism, identified several homogen-eous zones from NE to SW that became knownSE vergent folds, lying on a Pre-Cambrian meta-

morphic basement (Matte, 1986b). as Cantabrian, Western Asturian-Leon, Galicia-Tras-os-Montes, Central Iberian, Ossa MorenaThe Variscan belt is currently interpreted as the

result of the collision between three major conti- and South Portuguese Zones (Fig. 1). Their pre-dominant trend is NW–SE. Towards the north,nental blocks, leading to the closure of two oceanic

basins, known as Rheic and Massif Central oceans they present an arcuate shape, suggesting a connec-tion with NW France that was recognized by(Matte, 1986b). However, the ophiolitic sequences

presently identified correspond to small pieces Argan (1924) as the Ibero Armorican Arc (IAA).The separation between both segments of the Arcalong two discontinuous chains (Matte, 1986b)

and are found within allochthonous massifs at was due to the opening of the Bay of Biscay(Carey, 1958).Lizard (SW Britain), NW Iberia and Massif

Central (Central France), all subjected to strong Between the internal and the external zones ofthe IAA, one can observe a progressive migrationmetamorphism (Matte, 1986b; Matte and Hirn,

1988). of the deformation. At the internal zones — west-ern Asturian-Leon, Galicia-Tras-os-Montes,This ‘duplex subduction model’ is still contro-

versial, and Behr et al. (1984) argued that the Centro Iberian and Ossa Morena — one can findthe oldest terranes (Pre-Cambrian and Lowerknown sutures are too close in some places for a

bilateral subduction model. However, according Paleozoic), with intense syn-orogenic metamor-

59E.A. Silva et al. / Tectonophysics 321 (2000) 57–71

Fig. 1. Sketch map of the Variscan Belt from Western Europe until Grand Banks Margin — reconstruction for M25 (adapted fromSrivastava and Verhoef, 1992). Structural lineations from authors mentioned in Section 1: IAA, Ibero-Armorican Arc: NFLD,Newfoundland; NVF, Northern Variscan Front; RO, Rheic Ocean; MCO, Massif Central Ocean; AM, Armorican Massif; PTF,Porto-Tomar Fault; Ga-ToM, Galicia-Tras-os-Montes Zone; WALZ, Western Asturian-Leonese Zone; CZ, Cantabrian Zone; CIZ,Central Iberian Zone; OMZ, Ossa Morena Zone; SPZ, South Portuguese Zone; CA, Collector Anomaly; GB, Galicia Bank; FC,Flemish Cap; CGFZ, Charlie-Gibbs Fracture Zone.

phism and deformation. At the external zones — that almost two-thirds of the actual curvature ofthe IAA is of secondary origin. The primaryCantabrian and South Portuguese — one can find

Devono-Carbonian formations relatively less curvature is attributed to collision between irregu-lar continental plates. As Lorenz and Nichollsrecrystalized by high-grade metamorphism.

Cogne (1977) studied in detail the correlation (1984) suggested for western Europe, the mainchanges in the trend of the Variscan structures canbetween the Iberian and the Armorican branches

(NW France) of the IAA. Despite the good general be related to the irregularities in the Laurentia/Baltica plate boundary. A model has been proposedagreement between both branches, there is no

perfect cylindrical symmetry: for instance, the Ossa by Dias and Ribeiro (1995) for the generation ofthe IAA, based on the action of a CantabrianMorena Zone does not have any analogous zone

on the Armorican branch (Fig. 1). indentor, whose sinistral rotation was active duringmost of the Carboniferous.Is has been established from paleomagnetic data

(Ries et al., 1980, Perroud and Bonhommet, 1981) Iberia and Grand Banks are conjugate margins,


having typical characteristics of non-volcanic pas- To overcome these difficulties, we must comparenot the magnetic anomalies themselves but somesive margins (Whitmarsh et al., 1990; Sibuet,

1992), with few syn-orogenic Mesozoic magmatic quantity that must reflect the magnetic propertiesof the crust, free of the influence of its presentshows. However, there are important asymmetric

characteristics between them, namely the greater geomagnetic co-ordinates.In this work, we present a pseudo-susceptibilitylithosphere thickness of the Newfoundland

Mesozoic basins compared with the West Iberian map that can be used to reconstruct the pastpositions of the Iberian and Grand Bank marginsMargin (Pinheiro, 1994). On the Canadian side,

the Grand Banks on Newfoundland correspond to prior to the opening of the North Atlantic Ocean.We used the pole parameters for the Kimmeridgiana large continental platform, where several exten-

sion basins from Upper Triassic-Lower Jurassic times given by Srivastava and Verhoef (1992):12.93°W, 66.90°N, rotation angle of −60.45°. Wehave been identified. Masson and Miles (1986)

studied their distribution both on the eastern use the Kimmeridgian reconstruction to study thecontinuity between the different geological unitsCanadian and on the western Europe platforms.

They conclude that both represent dispersed frag- on the conjugate margins.ments of a NE–SW rifting system from LowerMesozoic.

The most striking structures on the Canadianplateau correspond to the arcuate features that are 2. Data compilationlimited to the south by the Collector Anomaly(Fig. 1) and had already been interpreted by sev- The computation of a homogeneous magnetic

map for the West Iberian margin started with foureral authors (Lefort, 1979; Matte, 1986a,b;Galdeano et al., 1990; Srivastava et al., 1990) as different datasets. The area covered by the compi-

lation goes from 15 to 6°W and from 36 to 43.5°N.Upper Proterozoic structures, obliquely cut by theDover–Hermitage fault (Fig. 1), whose age is pre- From the ‘Compilation of Magnetic Data in

the Arctic and North Atlantic Oceans’ (GEODAS-sumably late Pre-Cambrian (Lefort, 1979).North of the Dover–Hermitage fault and lim- NGDC, 1993), all relevant ship marine magnetic

profiles were extracted. Several spurious dataited to the north by the Iapetus northern suture(Lefort, 1983; Lorenz and Nicholls, 1984; Ziegler, points, probably due to large location errors, were

discarded. The secular variation was computed1984), another region can be found (Fig. 1).According to Lefort (1979), the Dover–Hermitage with the DGRF-IGRF models for the time span

1965–1990.fault represents on land the southern limit of thePre-Cambrian basement of the Avalon Peninsula, Data for the ‘Aeromagnetic Survey Offshore

Portugal’ (Fairey Surveys Ltd., 1969a,b) were col-and its westward continuation is the Charlie–Gibbs Fracture Zone (Fig. 1). lected in 1969 for oil prospecting. The survey was

flown at 600 m average height covering the westernThe correlation between the main geologicalfeatures, now located in the Iberian and Canadian and southern Portuguese shelfs. A total of 7720

points were obtained from seven 1:200 000 scalemargins, using geological ‘markers’, has beenattempted by several authors (Haworth and maps by manually picking the intersections

between total field contours and flight lines. ThisLefort, 1979; Lefort 1980, 1983; Galdeano et al.,1990). The magnetic anomalies have played a survey was already reduced to the IGRF65.

Data for the ‘Aeromagnetic Survey of Portugal’major role in the identification of continuitiesbetween geological units. However, the amplitude (Miranda et al., 1989) were collected in the early

1980s, at an average height of 3000 m (Mirandaand shape of the magnetic anomalies are a functionof the geomagnetic latitude, survey height, and et al., 1987a,b). The IGRF80 model had already

been extracted.processing techniques, making it difficult to achievequantitative comparisons and leading easily to The ‘Aeromagnetic Survey of Spain’ (Socias,

1994) covers the Spanish mainland and has a meansignificant mistakes.


Fig. 2. Differences between (a) the GEODAS-NGDC and the Aeromagnetic Survey Offshore Portugal datasets and (b) the 600 and3000 m compilations. These differences were calculated between the gridded values corresponding to the overlap areas.

height of 3000 m. The magnetic anomalies were 3000 m level by analytical upward continuation,considering that its mean level was 1000 m.already referred to the IGRF85.

Two single grids were computed. The first gridfor the 600 m level, merging only the marine dataand the ‘Aeromagnetic Survey Offshore Portugal’, 3. Three-dimensional inversionwas used for a detailed study of the transitionbetween onshore and offshore structures in western Using a homogeneous magnetic total field map

and a good description of the topography, oneIberia. The second grid was computed for the3000 m level merging all data and was used for can calculate a horizontal magnetisation distribu-

tion M(x, y) using several basic assumptions onall comparative studies between the conjugatemargins. The individual surveys were firstly pre- the thickness and vertical homogeneity of the

magnetic layer. This inversion procedure elimi-processed, gridded with Smith and Wessel’s (1990)algorithm, continued upward to 600 or 3000 m nates the dependency of the shape and amplitude

of the magnetic anomalies with the distanceand merged with a weighted distance method.Details are given in Silva (1995). between the survey altitude and the seafloor, rather

different when we compare the water depth for theThe overlap areas were used to estimate theaccuracy of the final maps, and the results are Iberian and the Grand Banks platforms.

The magnetization distribution, calculated aspresented in Fig. 2a (overlap between theGEODAS-NGDC and the Offshore Portugal data above, can still be considered as the cumulative

effect of both the remnant and the induced compo-sets) and Fig. 2b (comparison between the 600 mcompilation and the Portuguese 3000 m survey). nents. However, as we are interested only in conti-

nental lithosphere, we can consider that theThe final compilation for the 3000 m grid is shownin Fig. 3. magnetization is essentially of an induced origin.

Thus, we can calculate pseudo-susceptibilities,The magnetic data for the Grand Banks marginwere made available by the ‘Geological Survey of using a global model, such as IGRF, to describe

the inducing field. This quantity will be indepen-Canada’ (Verhoef et al., 1996) as a 5 km grid(GAMMAA5). A sub-region was extracted, cover- dent of the present geomagnetic co-ordinates and

will reflect only the physical properties of theing the Grand Banks area between 65–40°W and40–54°N. This dataset has been referred to the underlying crust.


Fig. 3. Magnetic anomaly map of the West Iberian Margin at 3000 m, with contours every 20 nT. Ga-ToM, Galicia-Tras-os-MontesZone; CIZn, Centro Iberic Zone (northern region); CIZs, Centro Iberic Zone (southern region); OMZ, Ossa Morena Zone; SPZ,South Portuguese Zone; CCSZ, Coimbra-Caceres Shear Zone; FFT, Ferreira-Ficalho Fault; F1, Regua-Verin Fault; F2, VilarcaFault; NF, Nazare Fault; Figueira F.Z., Figueira Fracture Zone; Aveiro F., Aveiro Fault (Murillas et al., 1990); GIB, Galicia InteriorBasin (Murillas et al., 1990); IAP, Iberia Abyssal Plain; TAP, Tagus Abyssal Plain; Back-dashed lines near IAP and TAP, probablelocation of Ocean–Continental Transition (Whitmarsh et al., 1990, 1993; Pinheiro et al., 1992; Masson et al., 1994; Pinheiro, 1994);


The prefix ‘pseudo’ is used to emphasize that main field, which has different magnitudes inCanada and Iberia.the obtained values are not the real magnetic

susceptibilities of the different geological units but, The pseudo-susceptibility map for the WestIberian Margin is shown in Fig. 4. A similar strat-instead, its vertical average. As we are only inter-

ested in the study of the lateral heterogeneity, this egy was followed for the Grand Banks magneticgrid.quantity can be considered as meaningful.

For the computation of the magnetization distri-bution, we used the 3D-inversion technique ofMacdonald et al. (1980), that is an extension of 4. Reconstruction of the conjugate marginsthe Parker and Huestis (1974) method. The inver-sion procedure was carried out in the Fourier The age of the oldest spreading episode that is

recognized in the Iberian margin is Kimmeridgiandomain, which bestowed an enormous advantage,considering the amount of information. Two sepa- (isochron M25–156.5 Ma) according to Malod and

Mauffret (1990) and Srivastava and Verhoefrated grids are required in this technique: one withthe bathymetry and the other with the magnetic (1992). This was the epoch selected for the ‘recon-

struction’ of the past positions of the Iberian anddata. The bathymetric grids were obtained fromthe Global Ocean 5 min-ETOPO5 grid (ETOPO5, Canadian blocks.

This reconstruction involved a rotation of the1986) and TerrainBase dataset (TerrainBase,1995). pseudo-susceptibility distribution for the West

Iberia Margin grid, using the Srivastava andIn the inversion procedure, we considered amagnetic source layer with a constant thickness of Verhoef (1992) pole, and its merge with the

Canada pseudo-susceptibility grid, kept fixed in its10 km and with an upper layer that follows thebathymetry. The magnetization direction was con- present co-ordinates. The final map was reinterpo-

lated with a step of 0.064°, this being the resultsidered as collinear to the axial dipole.To avoid numerical instability at short wave- shown on Fig. 5.

One of the problems of this representationlengths, a bandpass filter with a cosine taper from8 to 16 km was applied after each iteration. concerns the limit chosen to represent the OCT

(Ocean Continent Transition). In this work, weAlthough the setting of a long-wavelength filter iscritical in the sense that it affects of the solution, used the 3000 m bathymetric contour to clip the

West Iberian Margin grid and as a reference foron a local scale, it does not significantly changethe form of the pseudo-susceptibility distribution. the Canadian grid. We are not implying that the

3000 m isobath corresponds to the OCT. We canThe chosen long-wavelength filter was a cosinetaper from 200 to 400 km to avoid the cut-off of see from Fig. 5 that the 3000 m contour does not

exactly match what seems to be the ‘magnetic’medium wavenumber anomalies that, though notsignificant in the oceanic domain, can be important limits of the Grand Banks block. However, it is

interesting to note the agreement between the SWin the interpretation of continental areas.The inversion procedure enables us to take into Iberian and the Grand Banks blocks, both charac-

terized by large-wavenumber, medium-amplitudeaccount the different topography and the varyingdistance between the magnetic layer and the survey anomalies, with a magnetic signature that can be

interpreted as continental, with no lineations orlevels. However, the magnetization is also a conse-quence of the thermal, mechanical and chemical similar oceanic spreading features.

The fit obtained is generally good (Fig. 5),history of the rock and of the amplitude of the

GB, Galicia Bank; VGS, Vasco da Gama Seamount; VS, Vigo Seamount; JS, Josephine Seamount; TS, Tore Seamount; TMR, ToreMadeira Ridge; J, J Anomaly; Sin, Sintra sub-volcanic Anomaly; Si, Sines sub-volcanic Anomaly; M, Monichique sub-volcanicAnomaly; Ca, Cadiz Golf sub-volcanic Anomaly.


Fig. 4. Pseudo-susceptibility map of the West Iberian Margin. Amplitudes have been multiplied by 100.


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leaving a small gap in the SW Portuguese margin. CA and south of CA region rather different, butThere is a significant overlap between the Flemish also their Kimmeridgian location does not seemCap Bank and the Galicia Bank (repositioned). to indicate any geological relationship.

Concerning the hercynian sutures, they do notshow any clear continuous magnetic signaturealong the Ibero Armorican Arc: Rheic Oceansuture has no clear equivalent in NE Iberian

5. Discussion Margin, and the same situation exists concerningthe Massif Central suture.

The first comment that we can make on the Should we expect to find a specific magneticreconstruction concerns the magnitudes of the signature for the hercynian sutures? The answer ispseudo-susceptibility values in the neighbouring probably no: from the analysis of deep refractionareas of both grids, after rotation (Fig. 6). In fact, studies over Iberia, we concluded that the thicknessno significant contrast is observed, as can be seen, of the crust is approximately constant across thefor example, if we compare the SE Grand Banks variscan ‘zones’ (Mendes-Victor et al., 1993), andarea and the SW Portuguese area (Fig. 6b). Here, the ofiolitic remnants that have been identified bythe initial magnetic anomalies (Fig. 6a) have some- field geology are dismembered (Ribeiro andwhat different amplitudes, while the pseudo-sus- Fonseca, 1993), as a consequence of the magnitudeceptibility distributions, obtained after analytical of the mechanical and thermal processes that fol-continuation and inversion (Fig. 6b), are similar. lowed the closure of the hercynian oceans and theWe can conclude that the methodology followed tightening of the IAA. Therefore, when a magneticdoes provide a better way to establish a geometrical trend is followed as a marker of a suture, what wecorrelation based on magnetic parameters. are really following is a lateral magnetization

At the Canadian margin, north of the Dover– contrast between different blocks that was pre-Hermitage fault, we observe (Fig. 5) a N–S to

served during a long geological period. In thisNE–SW magnetic discontinuity that corresponds

sense, we can look for the continuities betweento the Iapetus suture (Lefort, 1983; Lorenz and‘magnetic similar’ terranes and, as a workingNicholls, 1984; Ziegler, 1984).hypothesis, interpret the existence or lack of conti-The arcuate magnetic features that characterizenuity as a marker of tectonic activity.the Grand Banks platform (Fig. 5) are limited to

If we consider the magnetic compilation ofthe south by the Collector Anomaly (CA). BetweenFig. 5 in more detail, we can emphasize two othersuccessive arcuate basement highs, we can identifyconclusions. The first conclusion regards the Ossalow pseudo-susceptibility areas that were interpre-Morena Zone: in the Portuguese margin, weted by Lefort (1979) as Palaeozoic and/or Meso-cannot identify any sign of its typical magneticCenozoic basins.pattern (small wave number and high amplitudeThe CA corresponds to a huge magnetic linea-anomalies). The second conclusion regards thetion with large amplitudes and large wavenumber,sharp discontinuity that follows the NWthat can be followed until 52°W.Portuguese coastline, reflected in the 3000 m totalOne of the conclusions that can be obtainedfield anomaly as a steep horizontal gradient of thefrom the inspection of Fig. 5 is that the easterntotal field anomalies that have not been yet iden-continuation of the CA cannot be detected in thetified (Miranda et al., 1989) due to the lack ofWest Iberian Margin. Thus, according to the pre-coverage of the Portuguese aeromagnetic survey.sent reconstruction, the eastern homologous of the

We can integrate these two observations if weCA should be located further south of the Iberianconsider that a major strike-slip fault trendingPeninsula, and the hypothesis raised by LefortN13W lies along the NW Portuguese coast. We(1983) of the similarity between the CA and thecan even try to evaluate the slip magnitude if weOssa Morena Zone (OMZ) seems very unlikely.

Not only are the magnetization patterns of OMZ, correlate the magnetic anomalies from both sides


Fig. 6. Comparison between histograms of magnetic anomalies and of pseudo-susceptibilities, within 1 square degree of area locatedin SE boundary of Grand Banks and SW Portuguese margin: (a) magnetic anomalies of the two conjugate areas at present locationand (b) pseudo-susceptibilities ofs the two conjugate areas after the Iberia rotation.

of the fault. In doing so, we will arrive at a value Another result concerns the identification of anEW accident north of the Grand Banks Ridge,of approximately 100 km. If we take this displace-

ment into account, we can verify that the trend of which has already been identified by Haworth andLefort (1979) (cf. Fig. 5) . This feature obliquelythe hercynian zonation is still preserved in the

western Portuguese margin at least concerning the cuts the hercynian trend west of the Portuguesemargin.Galicia–Tras-os-Montes and Centro Iberian zones.


6. Conclusions the main geological features of the North Atlanticconjugate margins. The possibility to do so relieson the fact that the hercynian terranes kept specificThe points discussed above are summarized in

a sketch presented in Fig. 7, where the different magnetic signatures that are easily recognized onthe continental areas (Miranda et al., 1989; Socias,blocks were relocated according to the same

Kimmeridgian reconstruction and where the major 1994), on the French (Galdeano et al., 1990) andon the Canadian shelves (Lefort, 1980).features identified in the Grand Banks margin

were added. Without taking into account the different alti-tude surveys, its acquisition characteristics and itsThe magnetic anomalies have been used for a

long time as a tracer for the correlation between geomagnetic co-ordinates, the comparison between

Fig. 7. Interpretative schema of the continuity between the different structures prior to the opening of the Atlantic Ocean. CGFZ,Charlie-Gibbs Fracture Zone; CA, Collector Anomaly; F-discontinuity related with a fault; GB, Galicia Bank; FC, Flemish Cap;Ga-ToM, Galicia-Tras-os-Montes Zone; CIZN, Central Iberian Zone (northern region); CIZs, Central Iberian Zone (southern region);OMZ, Ossa Morena Zone.


magnetic signatures from both sides of the Atlantic able all the data of the Aeromagnetic OffshorePortugal survey and Spain survey, also to Drremains rather subjective. Only the inversion of

the magnetic anomalies — taking into account the Macnab and Dr Verhoef of the Geological Surveyof Canada for making available the GAMMAA5topography — and the determination of a pseudo-

susceptibility distribution allow the computation grid.The authors also acknowledge the earnestof a pre-drift magnetic reconstruction.

The correlation between the hercynian zones co-operation and support from all who, in anyway, contributed to this work, in particularand their magnetic signatures leads to several

conclusions regarding the geometric relationships Professor Mendes Victor (CGUL) and ProfessorAntonio Ribeiro (DGFCUL). The participationbetween the once-contiguous continental blocks.

The first conclusion regards the OMZ. Its mag- of the first author in this work was funded by aJNICT grant. This work was partially funded bynetic signature is completely absent west of the

Portuguese mainland, either in the Iberian Margin the PRAXIS Project EPROMAP ref. n.2/2.1/CTA/374/94.or west of it. Also, the eastward prolongation of

the Colector Anomaly cannot be identified in theIberian Margin. The relationship betweenthe Colector Anomaly and the southern limit of

Referencesthe OMZ is unlikely, even concerning the pre-driftlocation.

Argand, E., 1924. A Tectonique de l’Asie. In: Proc. 13th Int.The second conclusion regards the possible exis-Geol. Congr. (Brussels), 171–372.

tence of an important left lateral strike-slip fault, Behr, H.J., Engel, W., Franke, W., Giese, P., Weber, K., 1984.along the present-day NW Portuguese coast-line. The Variscan Belt in Central Europe: main structures, geo-

dynamic implications, open questions. Tectonophysics 109,This accident deeply affects the post-hercynian15–40.evolution of the Portuguese margin, generating a

Carey, S., 1958. The tectonic approach to continental drift. In:large displacement on the Galicia–Tras-os-MontesContinental Drift Symp., University of Tasmania, Hobart,

and North Central Iberian zones, with respect to 177–354.their equivalents in the Portuguese continental Cogne, J., 1977. La chaıne hercynienne ouest-europeenne corre-

spond-elle a un orogene par collision? Propositions pourshelf. This large strike-slip displacement is proba-une interpretation geodynamique glogale, Ecologie et Geo-bly related to the absence of the Ossa Morena inlogie e l’Himalaya. Coll. Int. CNRS 268, 111–129.the SW Iberian shelf.

Dias, R., Ribeiro, A., 1995. The Ibero-Armorican Arc: a colli-The final conclusion regards the structure of sion effect against an irregular continent? Tectonophysics

the Ibero-Armorican Arc. The continuity of the 246, 113–128.ETOPO5, 1986. Relief map of the earth’s surface. EOS 67, 121.hercynian zonation between Iberia and NW FranceFairey Surveys Ltd., 1969a. Operational Report on an Aero-can only be traced for the inner zones. Therefore,

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Correlation between the Palaeozoic structures from West ...w3.ualg.pt/~jluis/artigos/elsa.pdfCorrelation between the Palaeozoic structures from West Iberian and Grand Banks margins