Magnetic modelling in the French Cadomian belt (northern Armorican Massif)

Tectonophysics 331 (2001) 123±144

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Magnetic modelling in the French Cadomian belt (northern Armorican Massif) C. Cauvin-Cayet a,*, A. Galdeano b,1, E. Egal c,2, J.P. Pozzi a,3, C. Truffert c,4 a

Laboratoire de GeÂologie, Ecole Normale SupeÂrieure, 24 rue Lhomond, 75231 Paris cedex 05, France Laboratoire de GeÂomagneÂtisme, Institut de Physique de Globe, 4 place Jussieu, 75005 Paris, France c BRGM, B.P. 6009, 45060 OrleÂans cedex 02, France

b

Received 1 February 1998; accepted 1 February 2000

Abstract The French Cadomian belt (northern Armorican Massif) constitutes an excellent area for studying Panafrican orogenic processes in Europe for which geophysical methods are useful for obtaining a realistic interpretation of the geological structures at depth. Magnetic modelling of the area to the west of Saint-Brieuc Bay was carried out to determine the geometry of the Cadomian geological units. Modelling was computed along pro®les extracted from the regional aeromagnetic map. The shape of the magnetic bodies was determined with the help of interpretative geological sections and the total magnetization of the magnetic bodies was determined from a data base of about 350 rock sample measurements. The measured susceptibility (x ) ranges from 0:15 £ 1023 SI to 221:5 £ 1023 SI (the induced magnetization IM ˆ 40 A=M £ x:) The natural remanent magnetization (NRM) is weak and shows varied directions; it is therefore negligible at the formation scale, and only the magnetic susceptibility was taken into account for modelling. A computed mean IM was attributed to each magnetic body in the model. Measurements of the magnetic properties and thin-section observations of ®eld samples show that magnetite is the main carrier of the magnetization. The strong susceptibilities are consistent with ferromagnetic behaviour of multidomain grains, whereas the weak susceptibilities accord with dominantly paramagnetic behaviour. Sample measurements show that the SaintQuay intrusion, the acid part of the Lanvollon Formation and one of the three lenses of Squif®ec metagabbro are the most highly magnetic formations. Magnetic modelling made it possible to estimate the long-wavelength fold shape of the Binic formation. The basin, within this formation, reaches about 2.0±2.5 km depth. The modelling also indicates that the Saint-Quay intrusion cuts the Binic basin at its centre with vertical contacts. Modelling shows that the intrusion is apparently composed of two imbricated magnetic bodies in its eastern part, and of homogeneous magnetic bodies in its western part, which is consistent with the geological observations. The Lanvollon Formation exhibits an heterogeneous magnetic behaviour consistent with its lithology of intercalated acid and basic metavolcanic bodies. Magnetic modelling indicates a difference in thickness of the acid part of the Lanvollon Formation * Corresponding author. Laboratoire de GeÂologie, Universite du Maine, Av. Olivier Messiean, 72085 Le Mans Cedex 09, France. Fax: 133-144-32-22-00. E-mail addresses: [email protected] (C. Cauvin-Cayet), [email protected] (A. Galdeano), [email protected] (E. Egal), [email protected] (J.P. Pozzi), [email protected] (C. Truffert). 1 Fax: 133-1-44-27-33-73. 2 Fax: 133-2-38-64-36-85. 3 Fax: 133-1-44-32-20-00. 4 Fax: 133-2-38-64-33-34. 0040-1951/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0040-195 1(00)00239-0

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between the pro®les of the Binic basin area and those of the Squif®ec±Plouha area; this we interpret as re¯ecting an initial variation in the thickness of the acid volcanic bodies. The Binic and the Squif®ec±Plouha pro®les show that the southern boundary of the basic part of the Lanvollon Formation is probably vertical or steeply dipping to the north. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Armorican Massif; Cadomian orogeny; aeromagnetic survey; magnetic properties; magnetic pro®les

1. Introduction

2. Geological setting

The Cadomian belt of northern Brittany and Normandy is a Panafrican segment included in the Hercynian Armorican Massif (Fig. 1). However, this part of the Panafrican belt escaped the Hercynian deformation and constitutes an excellent area for studying Panafrican orogenic processes in Europe (Brun and BaleÂ, 1990; Chantraine et al., 2001). Since the region shows little relief, the orogenic structures cannot be observed directly in three dimensions (3D), which means that interpretation of the regional structures and their representation at depth are highly interpretative. Geophysical methods are therefore particularly useful for obtaining a realistic model of the deep tectonic structures there. The present paper is a contribution in a wider project including geophysical and geological surveys: the ARMOR project. The aim of the present study was to provide additional constraints for magnetic modelling, mainly relative to the magnetic properties of the different formations. Rather than undertaking a classical palaeomagnetic study, i.e. determining the original directions of magnetization, the adopted approach in terms of rock magnetization consisted in measuring the overall magnetic characteristics (natural remanent magnetization (NRM) and susceptibility) of the relevant formations, and integrating these measurements into the magnetic modelling. This involved problems of scale change between the sample measurements and their integration into the model, which are highlighted by this study. The work concentrates on the western part of Saint-Brieuc Bay where the relationships between the various geological formations are clearer than to the east of the bay. Our work led to a 2D, then 3D, representation from a set of sections across the geological units, and provided additional informations enabling geologists to re®ne the structural interpretation of the volcanic domains of the western Saint-Brieuc Bay.

The French Cadomian belt (northern Armorican Massif) mainly comprises Neoproterozoic rocks that were emplaced and deformed between ,620 and ,530 Ma, during the setting, tectonic shortening and dismembering of an active margin. A general description and geodynamic interpretation of this belt are given by Chantraine et al. (2001) (with references therein). The present study concentrates on the western side of Saint-Brieuc Bay where the coastal outcrops provided numerous structural data (Brun and BaleÂ, 1990; Rabu et al., 1983) and where detailed mapping at 1:50,000 scale was recently completed (Egal et al., 1996a) (Fig. 1). This area investigated, mainly comprises the lithotectonic Saint-Brieuc Unit and its southern boundary marked by the Plouagat-CoeÈtmieux Fault. The St Brieuc domain is mainly composed of metaigneous units (plutonic and volcanic). These are locally overlain by Brioverian clastic rocks interpreted to be turbidite deposits (Denis, 1988). The studied area is centred on the Brioverian Binic basin. The lithological formations that were magnetically modelled are described brie¯y hereafter. The fry-Quemper formation is mainly composed of massive acid tuff, rhyolitic lava, andesitic lava and cinerite, quartz microdiorite and breccia (Egal et al., 1996b). The Plourivo basin is composed of red beds and volcanites (Auvray et al., 1980). The La Roche-Derrien formation, which crops out around the Paleozoic Plourivo basin, is composed of clastic sediments (wacke and pelite) interpretated as turbidites (Denis, 1988). The Binic formation is composed of a folded sequence of interbedded sandstone and pelitic schist that stratigraphically overlies the metavolcanics of the Lanvollon Formation. Although large zones of predominantly acid or basic composition, respectively, leptynites and amphibolites, have been mapped within

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Fig. 1. Geological map (scale in Lambert II in km).

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the Lanvollon Formation, acid-basic alternations are observed locally at the outcrop scale, and intermediate compositions exist in both the acid and basic parts of the formation which was dated as a whole at 588 ^ 11 Ma (Egal et al., 1996a). The Lanvollon metavolcanics and Binic metasediments are intruded by the Saint-Quay gabbro-diorite intrusion that, according to FabrieÁs et al. (1984), exhibits a concentric lithological variation in its eastern part with a mainly gabbroic inner zone and a dioritic outer zone. Due to the westward increasing metamorphic recrystallization, variation in the igneous paragenesis was not identi®ed farther west (Egal et al., 1996a,b). The intrusion, for which a minimum age of 569:3 ^ 0:6 Ma was proposed based on the amphibole and muscovite cooling ages (Dallmeyer et al., 1991), generated a metamorphic aureole in the regionally metamorphosed (greenschist to amphibolite facies) sediments of the Binic Formation (FabrieÁs et al., 1985). To the west and southwest of the Binic basin, the Squif®ec metagabbro forms three lens-shape intrusions (one northern small and two southern large) along or near the southwestern boundary of the Saint-Brieuc Unit. The protolith of this metagabbro was dated at 581 ^ 12 Ma (Egal et al., 1996a,b). The Plouha intrusion, located north of the Saint Quay gabbro-diorite intrusion, is composed of biotite tonalite ranging very locally to diorite. Relationships to the surrounding rocks are unclear and Egal et al. (1996a) did not recognize any metamorphic aureole around the intrusion. The southern boundary of the Saint-Brieuc Unit is cut by the Saint-Brieuc granodioritic (granite to quartz-diorite) intrusion which was dated at 533 ^ 12 Ma (Hebert et al., 1993) and is associated with the Ploufragan anatexites. The Cadomian rocks of Brittany are frequently crosscut by Palaeozoic dolerite dykes, rare or absent in the study area. 3. The aeromagnetic map The aeromagnetic map used during the present study was compiled from two aeromagnetic surveys: the ®rst ¯own in 1975 for ELF Aquitaine (the data were obtained by digitizing the paper maps) and the

second ¯own in 1992 on behalf of INSU-BRGM at a constant ¯ight altitude of 350 m (spacing: 500 m between lines and 5 km between tie lines). Both surveys were processed in order to compute anomaly maps through the substraction of a regional ®eld ®tted by minimum last square minimization. The obtained anomaly map were upward continued to a constant 350 m ¯ight altitude and combined to yield a data grid with cells 0:25 £ 0:25 km: This provided detailed aeromagnetic data that enable shallow structures to be studied down to a depth of several kilometres (Galdeano et al., 2001). The pro®les used for the magnetic modelling were extracted directly from this compilation (not reduced-to-the-pole). 3.1. Automatic structural analysis The magnetic anomalies of the Armor aeromagnetic survey are described by Galdeano et al. (2001). For the western part of Saint-Brieuc Bay (western part of the aeromagnetic survey), the most important anomalies are associated with the Saint Quay intrusion, the small northern lens of Squif®ec metagabbro and the acid Lanvollon Formation, In detail, three anomalies are associated with the acid Lanvollon Formation: a western elongate anomaly (anomaly A), an anomaly south of the Binic basin (anomaly B) and an anomaly surrounding the Plouha tonalite (anomaly C), one anomaly is associated with the small northern lens of Squif®ec metagabbro (anomaly D) and one anomaly is associated with the eastern part of the St. Quay intrusion (anomaly E). To highlight magnetic contrasts on the aeromagnetic map, an automatic structural analysis was carried out using the gmipack software (TOTAL/ BRGM). The data were obtained from the reducedto-the-pole map (Fig. 2), extracted from the general one computed by Galdeano et al. (2001), on which the anomalies are located above their sources. In this analysis, the maximum value of the horizontal gradient is evaluated so as to outline the source limits and thus the magnetization contrasts (Castaing and Debeglia, 1992). These limits (magnetization contrasts) are represented onto the magnetic map by bars (shown in blue on Fig. 2), the thickness of which is proportional to the horizontal gradient value and the direction and location of which are those of the magnetic disconti-

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Fig. 2. Close up view of the reduced-to-the-pole map showing geological boundaries (black lines), magnetic discontinuities (blue bars), sampling sites (dots) and the trace of the ®ve pro®les (P1, P2, P3, P4 and P5).

nuities. The results of the automatic structural analysis show are as follows: (1) Discontinuities very well de®ne the limit of the anomaly A of the acid Lanvollon Formation. Nevertheless, they are more scattered at the core of the anomaly where, in the southeastern part, they intersect the geological contact of the Saint-Quay intrusion. This distribution indicates the likely occurrence of Lanvollon Formation acid rocks beneath the SaintQuay intrusion and the shallow character of the intrusion in this area. (2) The anomaly B of the same formation is also well de®ned by the discontinuities which distribution reveals the complex structure of the formation in this part of the Saint-Brieuc Bay. A set of magnetization contrasts probably re¯ects mixing between the acid and basic Lanvollon Formation, as already observed from ®eld surveys (Egal et al., 1996a).

(3) The discontinuities are shifted to the south compared with the geological boundary (Egal et al., 1996b), outlining the northern part of Anomaly C (linked to the acid Lanvollon Formation). This probably indicates a southerly dip of the Lanvollon formation in this area if we consider that offshore geological boundaries are accurate. Conversely, the discontinuities follow the geological boundary in the southern part of the anomaly, indicating a probable vertical dip. (4) Discontinuities outline the northern lens of Squif®ec metagabbro, but they indicate a smaller feature than shown by the geological boundaries. The automatic structural analysis map may re¯ect a narrower magnetic structure of the lens at depth. (5) Different magnetic signatures on the reducedto-the-pole map occur for the eastern and western parts of the Saint-Quay intrusion, which is consistent with the geological data (FabrieÁs et al., 1984). The

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scattered distribution of discontinuities con®rms the structural complexity of the eastern part of SaintQuay intrusion, and also reveals the presence of likely different sources with different magnetizations. No such conclusion can be drawn for the western part of the intrusion. 4. Magnetic properties of the ®eld samples Magnetic modelling can be used to propose a geometry based on the distribution of total magnetization of the magnetic bodies. In the present study, so as to obtain realistic models, the total magnetization of the different bodies was computed from the measurement of numerous samples. The total magnetization of a magnetic body is the vectorial addition of the NRM (directly obtained by sample measurement) and the induced magnetization (IM, calculated from the measured susceptibility x ). As the geomagnetic ®eld intensity in the studied region is about 40 A/m, the IM intensity is represented by IM < 40 A/m £ x . The IM has the same direction as the ambient geomagnetic ®eld whereas the NRM directions are often different. The aeromagnetic map was therefore ®rst analysed in order to select areas where the magnetic signal is strong enough for collecting rock samples on the ®eld and extracting pro®les from the aeromagnetic map. In spite of the scarcity of outcrops in some areas, the sampling was as extensive as possible in order to obtain a good distribution of sample sites; thus about 200 samples were collected from almost 50 sampling sites in the western part of Saint-Brieuc Bay (Fig. 2). The measurements performed by Edel and AõÈfa (2001) at the Institut de Physique du Globe in Strasbourg and by Chauvin A. at the Geosciences laboratory in Rennes were added to our measurements to constitute a data base of about 350 samples. The x and NRM intensities were, respectively, measured with a Molyneux's susceptibility meter and a Molspin's spinner ¯uxgate magnetometer at the ENS laboratory in Paris. The susceptibility for the entire sampling ranges from 0:15 £ 1023 SI to 221:5 £ 1023 SI and the NRM from 10 23 to 1 A/m (Fig. 3). In spite of a good correlation between the outcropping formations and the anomalies at the map scale, the measurements

show that the susceptibility (x ) and NRM intensities are scattered within the same formation (Fig. 3) and even within the same outcrop. Scattered remanence can be due to lightning strikes (Clark, 1997). In this study, it is not the case because sampling sites are very scattered in a same formation. This hypothesis could explain one sample measurement but certainly not the entire set of sample measurements. As there is a predominance of the IM intensity over the NRM throughout the whole region and also in each formation (Fig. 3), the Koenigsberger factor Q (Q ˆ NRM/IM) being lower than 0.5 for most of the samples, and as the NRM presents dispersed directions within every formations, which has the effect of making the mean NRM negligible at formation scale, only the IM was taken into account for modelling. 4.1. Determination of a mean susceptibility for the different magnetic structures The results of the x measurements for the main formations of the studied area are given in Fig. 4. The mean x and the associated mean IM (noted between parenthesis after the susceptibility value) were calculated from the x measurements. The calculation of the mean IM for each formation illustrates the change of scale between the magnetization measured on the sample and the magnetization measured at the survey altitude. As the aeromagnetic survey smoothes the heterogeneities existing at sample and outcrop scale, we tried to estimate a representative mean magnetization for each formation, taking into account lithological variations; this mean value was then used in the modelling. Some experimental magnetic pro®les (Fig. 5) were carried out to model heterogeneous sources composed of many small bodies (prisms of 50 m £ 50 m) with different magnetizations. The chosen magnetizations were characterized by a mean IM and a standard deviation similar to that obtained from sample measurements. The result was then compared with that computed from a homogeneous source with a magnetization equal to the average magnetization of the heterogeneous source model. It appears that, from the altitude of the aeromagnetic survey, the two source types (heterogeneous and homogeneous) give anomalies of very similar intensity and shape. Consequently,

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Fig. 3. IM vs. NRM for the main geological formations in the area.

we considered that, in spite of the scattering of intensities measured on the natural samples, the mean magnetization calculated for each formation gives a fairly good representation of the magnetization detected at the aeromagnetic survey altitude.

The Binic and La Roche-Derrien formations: the Binic Formation yielded homogeneous x measurements giving a mean susceptibility of 0:27 £ 1023 SI (IM , 0.011 A/m) and an associated standard deviation of 0:04 £ 1023 SI (IM , 0.002 A/m). This result

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Fig. 4. Susceptibilities of the main geological formations.

suggests a negligible susceptibility for the Binic basin, which can thus be considered as a non-magnetic formation. Only a few relatively scattered x measurements are available from the La Roche-Derrien formation. Their mean susceptibility is again negligible, and so they can also be considered as nonmagnetic. Acid Lanvollon Formation: the x measurements for this unit fall into two groups (Fig. 4a): strong values between 10 £ 1023 SI …IM ˆ 0:4 A=m† and 221:5 £ 1023 SI …IM ˆ 8:84 A=m†; and weak values between 0:18 £ 1023 SI …IM ˆ 0:0072 A=m† and 1:5 £ 1023 SI …IM ˆ 0:06 A=m†: According to the control measurements (Kappameter) made in the ®eld, the weak susceptibility values given by samples from the lenses of metasediments included in the

acid Lanvollon formation, and sometimes from the acid to intermediate rocks represent a maximum of 10% (volume) of the measurements, whereas the strong values represent 90% of the measurements. Taking these observations into account, the mean susceptibility was calculated at 63:7 £ 1023 SI …IM ˆ 2:55 A=m†: Basic Lanvollon Formation: The x measurements of the basic unit fall into two groups (Fig. 4b): one, representing about 91% of the measured susceptibilities, ranges from 0:32 £ 1023 SI …IM ˆ 0:013 A=m† to 10 £ 1023 SI …IM ˆ 0:4 A=m†; the other, representing about 9% of the susceptibilities, ranges from 31:6 £ 1023 SI …IM ˆ 1:26 A=m† to 50:1 £ 1023 SI …IM ˆ 3:18 A=m†: All the samples are characteristic of the formation and so were all taken into account for

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Fig. 5. Contribution test of a homogeneous body and non-homogeneous body.

calculating the mean susceptibility of 5:6 £ 1023 SI …IM ˆ 0:2 A=m†: Saint-Quay intrusion: in this unit, 96% of the x measurements range between 1:6 £ 1023 SI …IM ˆ 0:064 A=m† and 63:1 £ 1023 SI …IM ˆ 2:52 A=m†

(Fig. 4c) and fall into two groups: one, with about 45% of the samples, ranges from 1:6 £ 1023 SI …IM ˆ 0:064 A=m† to 12:6 £ 1023 SI …IM ˆ 0:504 A=m†; the other, with about 50% of the samples, ranges from 15:8 £ 1023 SI …IM ˆ 0:632 A=m† to 63:1 £ 1023 SI

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…IM ˆ 2:52 A=m†: Although FabrieÁs et al. (1984) distinguished gabbroic and dioritic facies in the Saint-Quay intrusion, these do not correspond to the two groups. In fact, owing to the heterogeneous metamorphism, no correlation could be made between susceptibility and petrological composition, and so the calculated mean susceptibility of about 17:7 £ 1023 SI (approximated to an IM of 0.7 A/m) is for the entire x measurements. Squif®ec metagabbro: the x values of the large southern lens of Squif®ec metagabbro (Fig. 4d) show a wide spread. Nevertheless, about 60% of the values range from 0:4 £ 1023 SI …IM ˆ 0:016 A=m† to 1 £ 1023 SI …IM ˆ 0:04 A=m†; 20% from 6:3 £ 1023 SI …IM ˆ 0:252 A=m† to 15:8 £ 1023 SI …IM ˆ 0:632 A=m† and 15% from 31:6 £ 1023 SI …IM ˆ 1:264 A=m† to 79:4 £ 1023 SI …IM ˆ 3:176 A=m†: The corresponding mean susceptibility is 10:3 £ 1023 SI (approximated to 0.4 A/m). The small northern lens of Squif®ec metagabbro is associated with a strong magnetic anomaly and samples from this lens show a high susceptibility. These values were not taken into account when calculating the mean susceptibility for the large southern lens because the two lenses have different petrological compositions. The mean calculated IM of the northern lens, based on ®ve samples only, is 3.5 A/m; this gives an indication for the modelling because, due to the lack of samples, the magnetization is probably overestimated. The acid Lanvollon Formation, the eastern part of the Saint-Quay intrusion and the northern small lens of Squif®ec metagabbro are associated with the strongest magnetic anomalies of the aeromagnetic map (Galdeano et al., 2001). The above calculations show that these formations also have the highest mean IM. Thus the pattern of susceptibility values presented above is consistent with the pattern of the magnetic anomalies intensity on the aeromagnetic map. 4.2. Magnetic mineralogy The magnetic mineralogy of the lithological formations was identi®ed through: (a) an optical determination of the opaque minerals on polished thin sections at BRGM; and (b) a determination of the magnetic carriers and their grain size(s) from hysteresis loops

and Curie temperatures at the Saint-Maur Geomagnetism laboratory. Magnetite is observed on all the polished sections of rocks with a strong susceptibility. It is generally associated with other mineral species such as ilmenite or, more rarely, hematite and pyrite, but pyrrhotite has nowhere been recognized. Hagstrum et al. (1980) already have observed big grains of magnetite on polished sections from St Quay intrusion samples. During the Curie temperature measurements, susceptibilities were determined by a Kappabridge KLY2 while temperature increase was controlled by a CS2-furnace. The Curie temperature of samples with a strong susceptibility reveal the occurrence of magnetite (Fig. 6a). In addition, the Curie temperature measurements of samples from the acid Lanvollon metavolcanics with a strong susceptibility show a peak at 300±3508C: such a peak either corresponds to the Curie temperature of pyrrhotite, or to the mineralogical transformation of part of the magnetite into maghemite. However, as no pyrrhotite was observed on the corresponding polished sections and as a short drop of susceptibility after 3508C showed a non-reversible curve (revealing a mineralogical transformation), the second hypothesis is favoured. The Curie temperature measurements also indicate the occurrence of hematite …Tc ˆ 6758C† in samples with a weak susceptibility (Fig. 6b). The hysteresis loop study was performed at room temperature with an automatic translation inductometer working within the poles of an electromagnet producing ®elds up to 1.6 T. The loops show that all the lithologies with a strong susceptibility have a ferromagnetic behaviour (Fig. 6c) associated with the occurrence of multidomain grains (e.g. Hcr/ Hc . 4 and Jrs/Js , 0.05; Day et al., 1977; Dunlop, 1973). Thus, Q , 1 related to strong susceptibilities is probably explained by the occurrence of multidomain magnetite grains (Parry, 1965; Stacey, 1963). The rocks with a weak susceptibility exhibit a predominantly paramagnetic behaviour and hysteresis parameters are not usable (Fig. 6d). In this case, the weakness of ferromagnetism explains why Q is ,1. Therefore, we consider that magnetite is the main magnetic carrier of the studied Cadomian rocks. The distribution of magnetite is heterogeneous even within a formation. This is the case, for example, for the basic Lanvollon Formation and for the Saint-Quay

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Fig. 6. Curie temperature: (a) sample of the acid Lanvollon formation with a strong susceptibility; (b) sample of the basic Lanvollon formation with a weak susceptibility) and Hysteresis loops; (c) sample of the acid Lanvollon formation with a strong susceptibility; (d) sample of the basic Lanvollon formation with a weak susceptibility).

intrusion which show variable magnetic susceptibility. Petrologic observation shows that magnetite is probably inherited from the magmatic evolution of these rocks. 5. Magnetic modelling The aim of this paper is to propose a subsurface geometry for the geological structures of the western part of St. Brieuc Bay through magnetic modelling. In the ®rst instance, the elongate shape of the anomalies on the aeromagnetic map indicates that locally the sources of the anomalies can be considered as cylindrical. It is thus possible to use a 2D modelling approach that is relatively easy to carry out and

which often constitutes the ®rst step for more sophisticated modelling. Five pro®les extracted from the aeromagnetic map (Fig. 2) were processed for magnetic modelling: three across the Binic basin in a sub N±S direction and two parallel sections from Squif®ec to Plouha, to the west of the Binic basin, along a NE±SW direction. The ®ve pro®les are roughly perpendicular to the geological structures and the magnetic anomalies. The limits on surface of the magnetic bodies used in the 2D modelling correspond to the lithological boundaries on the geological map. Interpretative geological sections enabled to have a preliminary idea of the shape of the magnetic bodies at depth. Each magnetic body corresponding to a geological structure is associated with a homogeneous IM

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determined as described above. In a few cases, however, the magnetic pro®les partially cut the anomalies near their rim, which means that the mean IM of the formation concerned is not really representative and was not always taken in this case into account. The modelling was computed using the hypermag program (Saltus and Blakely, 1993) which allows direct modelling with the help of an active graphic window. While it is easy to adjust the geometry of the different bodies so as to ®t the calculated pro®le to the measured pro®le with arbitrary accuracy, our goal was to respect the geology. Analysis of the aeromagnetic map (Galdeano et al., 2001) shows that the geology is best represented by the short-wavelength anomalies corresponding to subsurface sources. The best ®t is therefore obtained for ªmagnetic structuresº down to 5±6 km depth. As gravimetric modelling was also carried out for the same region (Truffert et al., 2001), it was possible to compare data from the two modellings to obtain better constraints. 6. Discussion After many trials, it was possible to construct a reasonable geometry of the magnetic bodies representing the geological formations along the ®ve studied pro®les. Fig. 7 represents one of these tests along Pro®le 1. All of the contacts between the magnetic bodies are vertical and no particular geometry is given to them. This trial shows that magnetic constraints exist, even if they are not always very strong. The modelled pro®les discussed below do not constitute unique solutions. However, the magnetization of the various bodies are relatively constrained, and therefore, the number of solutions for the geometry of the magnetic bodies is not in®nite. 6.1. Pro®les through the Binic basin area (Pro®les 1± 3) Pro®le 1 (Fig. 8a), Pro®le 2 (Fig. 8b) and Pro®le 3 (Fig. 8c) are located near the western coast of St. Brieuc Bay. Pro®le 1(Fig. 8a), which was also used for 2.5D gravimetric modelling (see Pro®le 4 in Truffert et al., 2001), runs from just east of north to just west of south and crosses the La Roche-Derrien Formation,

the Binic Formation, the acid Lanvollon Formation, the Binic Formation, the Saint-Quay intrusion, the Binic Formation, the Lanvollon Formation, the Saint-Brieuc diorite and the Ploufragan migmatite. The pro®le displays three sets of anomalies: ² A northern set (anomalies C) associated with a small outcrop of acid Lanvollon Formation. Its intensity (,100 nT) corresponds to the rim of the anomaly C observed on the aeromagnetic map. The magnetic constraints in this part of the pro®le are very weak and we did not use the mean IM calculated for the acid Lanvollon Formation because it was too strong to ®t the anomaly. Also, as the pro®le is offshore in this area, the surface geometry of the associated body is not suf®ciently constrained. ² The second set (,230 nT), to the south, is located above the Saint-Quay intrusion (anomaly E). ² The last set comprises three short-wavelength anomalies above the basic Lanvollon Formation (anomaly B). In this set of anomalies, the strongest one reaches about 500 nT and the two others reach ,200 and ,130 nT, respectively, from north to south. Pro®le 2 (Fig. 8b) is oriented exactly north±south and crosses the same units as Pro®le 1. All the data are offshore north of the Saint-Quay intrusion. Some uncertainty exists concerning the geological boundaries in this area. No equivalent gravimetric modelling exits, but as Pro®les 1 and 2 are close together, no real change in the geometry of the bodies exists. Pro®le 2 displays three sets of anomalies. ² The northern one is very weak (,30 nT) and, as for the previous pro®le, represents the rim of the anomaly C. Magnetic constraints are again weak in this area. ² The second set, associated with the Saint-Quay intrusion (anomaly E), presents two peaks suggesting that the source of this anomaly is heterogeneous. The strongest peak reaches ,300 nT and the second one reaches ,200 nT. ² The third set of anomalies (B), as in Pro®le 1, is composed of three peaks, the strongest one reaching ,300 nT and the two others reaching ,150 and ,200 nT, respectively, from north to south.

C. Cauvin-Cayet et al. / Tectonophysics 331 (2001) 123±144 Fig. 7. Test on Pro®le 1: 1. La Roche-Derrien Formation; 2. Binic Formation; 3. Basic Lanvollon Formation; 4. Acid Lanvollon Formation; 5. Saint-Quay gabbro-diorite intrusion; 6. Saint-Brieuc granodiorite; 7. Ploufragan anatexite and migmatite.

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136 C. Cauvin-Cayet et al. / Tectonophysics 331 (2001) 123±144 Fig. 8. Magnetic modelling in the Binic area. (a) Pro®le 1: 1. La Roche-Derrien Formation; 2. Binic Formation; 3. Basic Lanvollon Formation; 4. Acid Lanvollon Formation; 5. Acid Lanvollon Formation, when the magnetization is changed; 6. Saint-Quay gabbro-diorite intrusion Ð core; 7. Saint Quay gabbro-diorite intrusion Ð rim; 8. Saint-Brieuc granodiorite; 9. Ploufragan anatexite and migmatite; 10. Heterogeneity in Ploufragan anatexite and migmatite. (b) Pro®le 2: 1. La Roche-Derrien Formation; 2. Binic Formation; 3. Basic Lanvollon Formation; 4. Acid Lanvollon Formation; 5. Acid Lanvollon Formation, when the magnetization is changed; 6. Saint-Quay gabbro-diorite intrusion Ð core; 7. Saint-Quay gabbro-diorite intrusion Ð rim; 8. Squif®ec Metagabbro; 9. Saint-Brieuc granodiorite; 10. Ploufragan anatexite and migmatite. (c) Pro®le 3: 1. La Roche-Derrien Formation; 2. Binic Formation; 3. Basic Lanvollon Formation; 4. Acid Lanvollon Formation; 5. Acid Lanvollon Formation, when the magnetization is changed; 6. Saint-Quay gabbro-diorite intrusion, when the magnetization is changed; 7. Plouha tonalite; 8. Squif®ec Metagabbro; 9. Saint-Brieuc granodiorite; 10. Ploufragan anatexite and migmatite.

C. Cauvin-Cayet et al. / Tectonophysics 331 (2001) 123±144

Fig. 8. (continued)

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Fig. 8. (continued)

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Pro®le 3 (Fig. 8c) is located 3 km to the west of and parallel to Pro®le 2. From north to south, it crosses the La Roche-Derrien and Binic formations, the acid Lanvollon Formation intruded by the Plouha tonalite, the Binic basin intruded by the Saint-Quay diorite, the basic Lanvollon Formation, the Squif®ec metagabbro, the Saint-Brieuc diorite and the Ploufragan migmatite. It displays three sets of anomalies. ² The northern set contains two peaks (,200 and ,300 nT) associated with the acid Lanvollon Formation (anomaly C). ² The second set reaches ,370 nT and is associated with the Saint-Quay intrusion (anomaly E). ² The last set of anomalies (B) is the same as for Pro®les 1 and 2. From north to south, the anomalies reach ,300, ,370 and ,200 nT. North of the Saint-Quay intrusion the pro®le is offshore, and the geological boundaries are less well constrained. As seen earlier, the peaks associated with the rim of the anomaly C, in the north of the pro®les, provide no strong magnetic constraint. In the east of the SaintQuay intrusion (Pro®les 1 and 2), the shape of the anomaly indicates that the associated magnetic body is not simple. The Saint-Quay intrusion was therefore modelled as two imbricated bodies with a higher IM for the core of the intrusion. In addition, the magnetic modelling shows that the contacts of the eastern part of the Saint-Quay intrusion are subvertical. As, Pro®le 1 cuts the edge of the anomaly E, the magnetic constraints of the intrusion in this area are quite less strong. In the west (Pro®le 3), the depth of the SaintQuay intrusion was constrained by its modelled depth in Pro®le 2, by the depth indicated by the 2.5D gravimetric modelling (see Pro®les 4±6 in Truffert et al., 2001) and by geological assumptions. This led us to use a stronger magnetization than previously calculated (see section: determination of a mean susceptibility for the different magnetic structures). The modelling shows that the western part of the SaintQuay intrusion can be modelled by a single magnetic body and that, in spite of its funnel-shape at depth, the contacts are almost vertical. One result arising from this set of pro®les, therefore, is the westward disappearance of a heterogeneous magnetic source on behalf of a homogeneous magnetic source for the

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anomaly E associated with the Saint-Quay intrusion. This result is consistent with the geology where no lithological zonation (diorite and gabbro) is described in the west of the St Quay intrusion (FabrieÁs et al., 1984; Egal et al., 1996a). None of the three pro®les show an anomaly associated with the Binic basin and the detrital formations (Binic and La Roche-Derrien) which corroborates the sample magnetic measurements. All the three pro®les show a good lateral continuity in the modelled shape of the Binic basin. Its bottom is modelled at a depth of around 2.0±2.5 km, and it has a long-wavelength fold shape with the southern limit being vertical for about the ®rst 500 m before taking on a northward dip at depth. These results agree with gravimetric modelling (see Pro®les 4 and 5 in Truffert et al., 2001). The last set of short-wavelength anomalies (outcropping or shallow sources) are associated with the basic Lanvollon Formation (anomaly B). As these rocks have a weak IM, the anomalies are probably associated with small shallow bodies with a strong magnetization (2.55 A/m). A very good ®t is obtained if we include small imbricated bodies in the basic Lanvollon Formation using the magnetization calculated for the acid Lanvollon Formation. And, then, magnetic modelling reveals the occurrence of acid Lanvollon rocks under the Binic basin. Magnetic modelling shows a southern boundary of the basic Lanvollon Formation dipping to the north with the dip angle varying from one pro®le to another. Magnetic and gravimetric modelling agree well with this geometry (see Pro®les 4 and 5 in Truffert et al., 2001). The Plouha tonalite is modelled in Pro®le 3. Its magnetization (0.25 A/m) was determined from a dozen samples. As magnetic constraints are weak in this pro®le, the shape of the Plouha tonalite is not very precise. In the north of the pro®le, the data are offshore and uncertainties exist concerning the geological boundaries on the map. According to the magnetic modelling of Pro®le 3, the southern boundary of the northern Binic Formation should probably be shifted to the south and the northern boundary of the Binic sediments north of the Saint-Quay intrusion should be shifted to the north. No anomaly is associated with the Saint-Brieuc diorite, the Squif®ec metagabbro or the Ploufragan migmatite. The observed signal above these lithological units is negative and likely belongs to a positive

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anomaly located south of the pro®les. The mean magnetization calculated for the big lens of metagabbro is too strong to be used in the modelling. As a result, no valuable geometric information can be extracted from this part of the pro®le because the considered bodies must be non-magnetic. 6.2. The Squif®ec±Plouha pro®les (Pro®les 4 and 5) Pro®le 4 (Fig. 9a) and Pro®le 5 (Fig. 9b), located to the west of the Binic basin, are oriented NE±SW (i.e. perpendicular to the anomalies of this area). Lying about 1.5 km apart, the two pro®les cross, from northeast to southwest, the La Roche-Derrien Formation, the acid Lanvollon Formation intruded by the Plouha tonalite and two lenses of the Saint-Quay intrusion, and the basic Lanvollon Formation intruded by a small northern lens and a large southern lens of Squif®ec metagabbro. Pro®le 4 was also used for 2.5D gravimetric modelling (Truffert et al., 2001). This pro®le shows ®ve peaks. The four northern ones belong to a set of anomalies associated with the acid Lanvollon Formation (anomalies A and C) and, from northeast to southwest, respectively, reach ,200, ,350, ,100 and ,400 nT. The ®fth peak (,700 nT) is associated with the northern lens of Squif®ec metagabbro (anomaly D). Pro®le 5 contains three notable anomalies that, from northeast to southwest, reach ,230, ,600 and ,250 nT. The two northern anomalies are associated with the acid Lanvollon Formation (anomalies A and C) and the southern one with the northern lens of Squif®ec metagabbro (anomaly D). The depth of the La Roche-Derrien Formation in the north of the pro®les is ,2 km. The Saint-Quay intrusion was modelled as two imbricated magnetic bodies in the east (Pro®les 1 and 2) whereas about 3 km to the west (Pro®les 4 and 5) it can be modelled as a homogeneous magnetic body. The western part of the Saint-Quay intrusion (Pro®le 4) is here represented by two separated bodies which can be modelled by homogeneous lens-shaped magnetic bodies with the mean IM (0.7 A/m) calculated above. Their northerly dip is strongly constrained by the shape of the anomalies (A and C in this pro®le), but no real constraint exists concerning the connection of the two lenses at depth. On Pro®le 5, the Saint-

Quay intrusion is represented by a very shallow lens which may correspond to the end of the intrusion and is consistent with the automatic structural analysis map. In this case, the occurrence of the acid Lanvollon Formation beneath the Saint-Quay intrusion is con®rmed. To the west, according to the magnetic modelling, the Lanvollon Formation is represented by a mixing of acid and basic bodies as observed on the geological map. The pro®les of the Binic basin area (Pro®les 1, 2 and 3) and those of the Squif®ec±Plouha area (Pro®les 4 and 5) show a large difference in the thickness of the acid bodies: in the Binic area, these bodies are about 200 m to 1 km thick whereas along the Squif®ec± Plouha pro®les, they are about 2±4 km thick. This difference accords with the variability in the size of the acid bodies on the geological map. The Squif®ec± Plouha pro®les cross a very large zone of acid volcanic rocks whereas the Binic pro®les only cross small acid lenses. Magnetic modelling shows that this large variation in the size of the acid bodies represented on the geological map is also observed at depth. We thus have a distinction between major and local bodies of acid volcanics in 3D. There is no geological ®eld evidence to interpret the presence of distinct acid bodies of highly variable size (alternations have also been observed at outcrop scale) in terms of tectonic dismembering. It is therefore considered that this variation could be due to the presence of different (mainly separated) sources of acid volcanism. The southerly dip of the contact between the northern acid Lanvollon Formation and the Plouha tonalite was revealed by the automatic structural map analysis. This analysis also suggests a vertical dip of the acid Lanvollon Formation to the south of the Plouha tonalite. As in the pro®les of the Binic area, the southern boundary of the basic Lanvollon Formation is vertical to steeply dipping to the north. The Lanvollon Formation is intruded by the Plouha tonalite and by two lenses of Saint-Quay intrusion (see earlier). The Plouha tonalite is funnel shaped (determined by the negative anomaly located between the two northern positive peaks) and reaches a depth of 2.5 km. To the northwest, the Plouha tonalite has a smaller volume and it was necessary, for the model to ®t the anomaly, to add an underlying Lanvollon Formation. The anomaly D, associated with the small northern

C. Cauvin-Cayet et al. / Tectonophysics 331 (2001) 123±144 Fig. 9. Magnetic modelling in the Squif®ec±Plouha area. (a) Pro®le 4: 1. La Roche-Derrien Formation; 2. Basic Lanvollon Formation; 3. Acid Lanvollon Formation; 4. Saint-Quay gabbro-diorite intrusion; 5. Plouha tonalite; 6. Squif®ec Metagabbro; 7. Small lens of Squif®ec Metagabbro; 8. Heterogeneity in the Squif®ec Metagabbro. (b) Pro®le 5: 1. La RocheDerrien Formation; 2. Basic Lanvollon Formation; 3. Acid Lanvollon Formation; 4. Saint-Quay gabbro-diorite intrusion; 5. Plouha tonalite; 6. Squif®ec Metagabbro; 7. Small lens of Squif®ec Metagabbro; 8. Heterogeneity in the Squif®ec Metagabbro.

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Fig. 9. (continued)

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lens of Squif®ec metagabbro, ®ts well with a magnetic body having the mean magnetization previously calculated (Pro®le 4). This body is 1.0±1.5 km deep, but this probably represents a minimum depth because the magnetization is probably overestimated (see above); in a test to compensate for a weaker magnetization, it was necessary to increase the volume and depth of the lens. The funnel-shape of the lens was predicted by the automatic structural analysis; this shape is maintained westward (Pro®le 5), but the magnetization used must be weaker to ®t the anomaly. Three hypotheses can explain this shape: (1) uncertainties concerning the map boundaries of the body (although, according to geological observations, the boundaries are relatively precise in this area); (2) erosion has almost exhumed the root of the intrusion; and (3) the lens has an aureole that is not magnetized and so cannot be modelled (in which case the lens does not have a true funnel shape). We have no evidence to chose between the last two hypotheses. No geometric information is given by the shape of the large southern lens of Squif®ec metagabbro because it is an almost non-magnetic body. To ®t the very weak anomaly associated with the big lens of Squif®ec metagabbro, it was necessary to model a small strongly magnetized body (2 A/m) imbricated within the lens. 7. Conclusion For the western part of St. Brieuc Bay, the ARMOR high-resolution aeromagnetic survey shows large magnitude anomalies varying from 500 to 700 nT. These are associated with the Lanvollon Formation, the Saint-Quay intrusion and a small lens of Squif®ec metagabbro. Modelling these large magnitude anomalies shows that the calculated sample magnetizations, which are based on IM, are suf®ciently accurate to make it possible to constrain the shape of the magnetic bodies at depth. Moreover, the consistency of magnetic pattern from one pro®le to another in a set of pro®les is striking. The overall results for the western part of St. Brieuc Bay can be summarized as follows. The Saint-Quay intrusion cuts the Binic basin at its centre with vertical contacts. The depth of this intrusion is modelled at 2±3 km. Its eastern part is

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composed of two imbricated magnetic bodies; one, which represents the core, is wrapped by the second, which has a lower susceptibility. These two bodies merge westward into a single homogeneous weakly magnetic body. The adopted magnetization of the western body corresponds to the calculated one whereas the magnetization of the eastern body is stronger than the calculated one. The internal magnetic structure of the eastern part of the Saint-Quay intrusion is consistent with geological observations. The general shape of the Binic formation is a longwavelength fold with a southern boundary that is vertical to a depth of about 500 m and then dips to the north. The depth of the Binic basin is modelled at 2.0±2.5 km, which is con®rmed by a gravimetric inversion and is consistent with the depth evaluated by geological studies. The mixing of the acid and basic Lanvollon Formation in the Binic basin area and along the Squif®ec±Plouha pro®les, as indicated by the magnetic modelling, is consistent with geological ®eld observations. This mixing occurs not only at the surface but also at depth. The difference in thickness between the bodies of acid Lanvollon Formation in the Binic basin area and those in the Squif®ec±Plouha area is correlated with the presence of acid volcanic bodies that were originally of very different sizes. The magnetic modelling shows that the southern contact of the basic Lanvollon Formation is vertical to steeply northward dipping. In conclusion, this set of modelled pro®les shows that the integration of magnetic measurements provides valuable constraints on the magnetic structures. In turn, particularly in this study, these data provide a good indication of the deep structure of the Cadomian orogeny in the western part of St. Brieuc Bay. Acknowledgements Field work was supported by the ªArmor projectGeoFrance 3D programº. High ®eld measurements were performed at the laboratoire de GeÂomagneÂtisme de Saint-Maur. We are indebted to G. Dubuisson for his participation in the sampling programme. We thank Sir Patrick Skipwith, Bt. for improvement of the English. We are also grateful to Dr J.P. Busby

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