Special Paper 423: The Evolution of the Rheic Ocean: From Avalonian-Cadomian Active Margin to Alleghenian-Variscan Collision Volume 423 || Structural evolution of the Prague synform

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doi: 10.1130/2007.2423(11) 2007;423;249-265Geological Society of America Special Papers

Tahar Aïfa, Petr Pruner, Martin Chadima and Petr Storch Skalou dikes. Consequences for the nappes emplacementtimes: An AMS, rock magnetism, and paleomagnetic study of the Svatý Jan pod Structural evolution of the Prague synform (Czech Republic) during Silurian

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249

Geological Society of AmericaSpecial Paper 423

2007

Structural evolution of the Prague synform (Czech Republic) during Silurian times: An AMS, rock magnetism, and paleomagnetic study of the Svatý Jan pod Skalou dikes. Consequences for the

nappes emplacement

Tahar Aïfa*Géosciences-Rennes, CNRS UMR6118, Université de Rennes 1, Bat.15, Campus de Beaulieu, 35042 Rennes Cédex, France

Petr PrunerMartin Chadima

Petr ŠtorchInstitute of Geology, Academy of Sciences of the Czech Republic, Rozvojova 135, 165 02, Prague 6, Czech Republic

ABSTRACT

Silurian effusive basalts and volcaniclastics compose the Svatý Jan volcanic center, which is located in the northwestern limb of the Prague synform, where three major volcanic phases have been recognized: the fi rst one of early to mid-Wenlock and the last of mid-Ludlow age. Two alkaline basalt dikes of late Wenlock to mid-Ludlow age, respectively tilted to the west and to the northeast, as observed in a 100-m-thick tuff sequence, which represents the second volcanic phase, have been extensively sampled. An anisotropy of the magnetic susceptibility (AMS) study of seventy-nine specimens taken from a 5-m-thick dike (dike1) and thirty-two specimens cored in a 3.5-m-thick dike (dike2) shows two different fabrics, carried mainly by Ti-magnetite and/or mag-netite, which are considered to be related to the transtensional opening phase of the dikes. Four components of magnetization, attributed to Middle-Late Silurian (C1), Middle-Late Carboniferous (C2), Cretaceous (B), and Paleocene (D), in agreement with already-published directions for the Bohemian Massif, have been isolated. They are carried by Ti-magnetite for components C1 and C2, hematite and goethite for components B and D. The opening mode, which controlled both dikes, corresponds to a dextral transtensional regime, as deduced from the AMS K1 axis. They may have been opened during several magmatic stages related to different injections during late Wenlock to mid-Ludlow times. The fi rst stage is dominant and controlled by the primary fabric, which is mainly oblate. With a NNW-SSE strike, perpendicular to the shortening direction, this fabric is in agreement with the direction of emplacement of the nappes during the Late Devonian. At that time the nappes emplacement that

*E-mail: [email protected].

Aïfa, T., Pruner, P., Chadima, M., and Štorch, P., 2007, Structural evolution of the Prague synform (Czech Republic) during Silurian times: An AMS, rock mag-netism, and paleomagnetic study of the Svatý Jan pod Skalou dikes. Consequences for the nappes emplacement, in Linnemann, U., Nance, R.D., Kraft, P., and Zulauf, G., eds., The evolution of the Rheic Ocean: From Avalonian-Cadomian active margin to Alleghenian-Variscan collision: Geological Society of America Special Paper 423, p. 249–265, doi: 10.1130/2007.2423(11). For permission to copy, contact [email protected]. ©2007 Geological Society of America. All rights reserved.



250 Aïfa et al.

INTRODUCTION

The Paleozoic evolution of the Prague basin (Czech Repub-lic) before its synformal deformation has been a new topic of interest for several years (e.g., Melichar, 2004). Many results previously obtained thanks to tectonics studies, sedimentology, and paleontology are actually in contradiction with the most recent data. It is now considered that the evolution of the Prague synform has been mainly controlled by allochtonous units. The previous results were mainly based on the sedimentary and vol-canic history of the present synform, but few data were published regarding its structural evolution (Matte et al., 1990; Matte, 1991, 2001; Havlícek, 1998; Kachlík and Patocka, 1998; Žák et al., 2005). Recent work on zircon dating using U/Pb sensitive high-resolution ion microprobe (SHRIMP) and Nd isotopic analysis has been published by Linnemann et al. (2004) in which they propose a geodynamical model suggesting a southwest transport direction of the ophiolitic complexes between the Moldanubian and Saxo-Thuringian zones from the Late Devonian until the Lower Carboniferous, that is, during the ultimate stage of clo-sure of the Rheic Ocean (Kröner and Hahn, 2003). The purpose of this article is to shed some light on this problem. The Rheic Ocean, the existence of which has been demonstrated in eastern North America (Keppie et al., 2000; McKerrow et al., 2000; Mur-phy and Nance, 2003), is still widely debated in western Europe, mainly because its width cannot be estimated using paleomag-netic data because the orientation is northwest-southeast. The use of the anisotropy of the magnetic susceptibility (AMS) to constrain the mode of opening of the dikes combined with the paleomagnetic technique, which can be used for dating the fab-rics, represents a useful tool to check the direction of the stress existing at the time of the opening. Consequently, the direction of the displacement of the nappe is possibly related to the closure of the Rheic Ocean, if the latter really existed. Because dikes are good stress indicators, we fi rst check these two techniques on our two dikes, which are both supposed to be Silurian in age (Štorch, 1987). We then examine the magnetic mineralogy of the main carriers for better constraints on the magnetic components and their magnetic ages.

GEOLOGICAL BACKGROUND

The Prague synform, which is preserved in the central part of the Barrandian area (Bohemian Massif) comprises a pile of Ordo-vician, Silurian (Fig. 1), and Devonian rocks more than 2.5 km thick. Unmetamorphosed sediments, moderately deformed by the Variscan orogeny and famous for their fossils and their detailed

stratigraphy, outcrop in the Prague synform. The sedimentation was associated and temporarily disturbed by rather intensive and largely submarine basaltic volcanism.

Basaltic volcanics fi rst appeared during the late Early Ordo-vician and then formed the large Komárov complex in the south-western part of the newly originated basin. This volcanism cul-minated in the late Llanvirn Series and again, but less intensively, in the late Caradocian Series. It revived again in the Early Silu-rian but remained localized to the northern limb of the central and northeartern parts of the present Prague synform. The last isolated submarine eruptions of basaltic magma known from the late Emsian succession occurred in the central part of the Prague synform (Havlícek, 1987).

Extensive outcrops of Silurian effusive basalts and volcani-clastics (Patocka et al., 1993) belonging to the major Svatý Jan volcanic center are located between Beroun-Lištice, Svatý Jan pod Skalou, Záhrabská, and Lodenice. Three major volcanic phases have been recognized in the Svatý Jan center. The earliest phase started around the early mid-Wenlock (Chlupác et al., 1998), the second phase is of late Wenlock age, and the latest ceased in about the mid-Ludlow. Two dikes made of alkaline basalt showing well-developed feldspar phenocrysts have been found, cropping out in a small gorge associated with the steep slope of the left bank of the Kacák Creek, which is located between Sedlec and Svatý Jan pod Skalou (Fig. 2), 600 m northeast of the Svatý Jan Monastery (49.975°N, 14.136°E). These two dikes have been sampled in detail (Fig. 3A). They are situated in the lower part of a 100-m-thick tuff sequence (corresponding to the second volcanic phase) and represent either volcanic channels feeding the upper part of the volcaniclastic succession or, more probably, fi ssures that supplied the basaltic magma to the lava shield, characterizing the third (and last) volcanic period. A late Wenlock to mid- Ludlow age can thus be assumed for these dikes. During the Middle Devonian (Givetian), the fi rst Variscan oro-genic movements of the early Bretonian phase (Havlícek, 1963) terminated the sedimentation in the Prague synform and uplifted the whole Barrandian area (Kukal and Jäger, 1988; Havlícek, 1998). Post-Givetian folding and faulting affected the synform infi ll and closed the Barrandian marine sedimentary cycle. As a result of these movements, the Silurian sediments actually dip toward the southeast (between 14° and 35°) in the studied area (Fig. 2). Tuffs, which rest slightly unconformably on the shales, dip at ~18°–45° above the contact (Fig. 3A).

Dike1, which corresponds to the northwesternmost outcrop, is broadly north-south oriented and dips by 70° to the west. It is ~5 m in thickness and is characterized by wavy contacts on both sides. It was fi rst suggested (Aïfa et al., 2002) that this dike was

pre-dates this direction was probably associated with the sinistral closure of the Rheic Ocean, in agreement with post-Givetian folding and faulting, which deformed the synform infi ll and closed the Barrandian marine sedimentary cycle.

Keywords: Prague synform, dikes, stress, rheic, AMS, magnetization, Variscan orogen



Structural evolution of the Prague synform (Czech Republic) during Silurian times 251

TACHLOVICE

SV. JANBEROUN

P R A H A

VSERADICE

PANKRÁC

VELKÁ CHUCHLE

0 10 km

N

HYKOV

C Z E C H R E P U B L I C

PRAHA

Cretaceous and Tertiaryplatform cover

Silurian

studied outcrop1

KONKON?PRUSYPRUSYKONEPRUSY

CERNOSICE

REPORYJE

REPY

ZADNÍ TREBÁN

ZELKOVICE

1

Figure 1. Geological map of the Prague synform showing the main structural features; locations of the major mapped faults; and Cretaceous, Tertiary, and Silurian outcrops. The studied outcrop is indicated by the circled number 1: Svatý Jan pod Skalou.

dikes

dikesFigure 2. Detailed geological map with a zoom on the two sampled dikes (circled numbers 1 and 2).



252 Aïfa et al.

A

1 2 3 4 5

1

2

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E

K1Wa

K1Ca

K1Ea

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K1Ca

K1NEa

Center

East Border

Dike 2

West Border

Dike 1

n=7

n=13

n=12

n=17

n=32

n=30

B

Figure 3. (A) Block diagram representing both dike1 and dike2. 1—al-kaline basaltic dikes (dike1 may be composed of two successive in-trusions); 2—thin bedded limestone alternating with calcareous shale; 3—largely hyaloclastic alkaline-basaltic tuff with common volcanic bombs; 4—sampled sections across dike1 and dike2, respectively; 5—feldspar phenocrysts aligned along the contact rim of dike2. (B) Magnetic lineations (K1) from the borders and the center of the dikes showing the recorded magnetic fabric. All images are equal-area map-pings. Triangles—mean vector; squares—eigenvectors of the cylindri-cal best fi t; dashed lines—dike trace after unfolding; n—number of specimens used for the statistics. Contour intervals: 1 at a signifi cance level of ±1σ.




emplaced in an original fi ssure, which may have been opened repeatedly and used by subsequent intrusions. This scenario was suggested by some magnetic data and clear internal planar sur-faces at 1 m from the dike’s western contact and at 1.40 m from its eastern border. It is not necessarily always the case, however, because the difference in time between two successive intrusions can be very short and not discriminated by paleomagnetic data.

The wall surface at the southwest contact of the dike has a strike of 78° and a dip of 70° at the site of the sampled profi le. The same contact measured a few meters away gives values of 268°/78° and 262°/81°. The dip direction (strike) and magnitude (dip) of the northeast contact of the dike are 270° and 70°, respec-tively. Neighboring tuffs with volcanic bombs follow the bedding, although the bedding itself is diffi cult to measure. The dike steeply penetrates the tuffs. In the lower part of the outcrop the dike pen-etrates shales and thin-bedded limestones of mid-Wenlock age, dipping by 35° to the southeast (strike 120°). Two meters off the northeast contact there is a prominent strike-parallel tectonic plane within the dike. It may be considered as the boundary between two successive intrusions that used the same fracture.

Dike2, exposed on a steep slope above the creek, dips 38° to the southeast (strike 145°; Fig. 2) and shows moderately sinuous contacts with the host rock. This dike displays feldspar pheno-crysts, which are more or less parallel to its margins. This pattern can still be observed up to at least 10 cm from the dike margin. Neighboring tuffs close to the southwest contact exhibit a strike of 184° and a dip of 45°. Calcareous shales and laminated lime-stones of mid-Wenlock age cropping out below the tuffs are cut by the dike. Their topmost bedding plane, just below the overly-ing tuffs, dips by 30° to the south (strike 160°).

There is a clear difference between the dips and the strikes of the sediments and those of the volcaniclastics. Tuffs may have been deposited on the slope of a volcanic cone (although no slumps are observed), or the volcanites may have been deformed by some magma fl ow before the deposition of the volcaniclastics.

SAMPLING

All together, seventy-nine specimens were taken from dike1 following one transverse (perpendicular to the edges) section and two dike-margin parallel sections with a spacing (cracks and rock weathering permitting) of ~10 cm (Fig. 4). In the fi rst stage of our study, seven blocks were taken in order to cut pilot samples. They yielded fi fteen cubic specimens named “SV1” to “SV7.” In the second phase, a portable drilling machine was used and gave us thirty-three core samples, which provided forty-three cylindrical specimens. Twenty-fi ve core samples were also sampled in dike2 using the same device. They were completed later, with a total collection of thirty-two oriented specimens. Sampling was care-fully made, taking care of the distances between specimens, but also of fractures, fl ow lines, and chilled margins. Sampling paral-lel to the borders was made to characterize the mode of opening of the dike (Aïfa and Lefort, 2000), because such samples illus-trate the fi rst stages of the emplacement mechanism (Smith et

al., 1993; Lefort et al., 2006). If the opening of the dikes results from a transtensional opening, the contemporaneous stress direc-tions can be deduced from it and dated through the components of magnetization. AMS is used to determine the type of magnetic fabrics recorded in the dikes and thus their regional geotectonic environment. Sampling along the edges is useful to characterize the magma fl ow in a 3-D space and may help to discover possible remagnetizations associated with fl uid circulations or alterations.

ANISOTROPY OF THE MAGNETIC SUSCEPTIBILITY

In general, the opening of the doleritic dikes results from a vertical, oblique, or horizontal fl ow. In these conditions the regional stress is parallel to the trend of the dikes. This type of opening is responsible for a convergent lateral tiling of the feldspars (Blanchard et al., 1979; Moreira et al., 1999) and for a superimposed magnetic tiling (Aïfa and Lefort, 2000). In some rare cases the opening of the dikes is controlled by a transten-sional mechanism (Lefort et al., 2006). In these conditions the petrographic and magnetic tilings show an oblique and identi-cal trending of the tiling on both sides of the dike. This type of opening results from a regional stress oblique to the dike (Smith et al., 1993). The magnetic tiling (Aïfa and Lefort, 2000; Lefort et al., 2006) is usually associated with K1 or K2 (maximum and intermediate axes, respectively, of the AMS tensor). Thus it is important to check the K1 or K2 directions if we want to deduce the regional stress from the anisotropy tensors.

Using KLY-3S Kappabridge (Agico Brno), AMS was mea-sured for all samples. Anisotropy parameters, such as corrected anisotropy degree P′, shape parameter T, and direction of maxi-mum and minimum magnetic susceptibilities (K1, K3), were counted using the tensor notation of AMS (Jelínek, 1978).

Figure 3 shows the distribution of the various K1 values after the unfolding of each dike using the software of Pangaea Scientifi c (Stesky and Pearce, 1995) to draw the isocontours (Kamb 1959). For dike1 (Fig. 3B left), note that the eastern border shows a good cluster of K1 values with an oblique orientation with respect to the dike margin. The center of the dike shows a group of values that is clearly clustering along a great circle, oblique to the dike trend (but it also displays some mixture of the distribution along a great circle on the opposite side). The western border is nearly similar to the center, with two oblique distributions of K1, one being more clustered than the other. Therefore, we think that a postmagmatic emplacement fabric overprints the primary fabric.

Because K1 shows the same obliquity with respect to the borders of the dike, we assume that we are dealing with the sec-ond type of opening (see above). In these conditions the grains contributing to the AMS results were necessarily aligned perpen-dicularly to the stress direction before the cooling of the magma. We can thus assume that the opening of this dike resulted from a dextral transtensional regime (Aïfa and Lefort, 2000; Lefort et al., 2006). The same data also show the existence of a shallow inclination of maximum axis K1 (between 16° and 21° from west to east; Fig. 3B).



254 Aïfa et al.[E

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Dike2 (Fig. 3B right) has in its center a well-clustered K1 dis-tribution, which suggests an east-west fl ow oblique with respect to the edges of the dike. K1 distribution is also oblique to the dike walls on both margins, which again suggests a dextral transten-sional opening with a shallow dipping magma fl ow (between 28° and 3°; Fig. 3B). In the fi eld, some imbricated (Blanchard et al., 1979) phenocrysts have also been observed more or less parallel to the margins of the dike (which may extend up to 10 cm away from the margins). Their distribution may correspond to the ini-tial direction of the fl ow (Philpotts and Asher, 1994) before the transtentional opening.

ROCK MAGNETIC ANALYSIS

Temperature Variation of Magnetic Susceptibility

Pilot samples were used for a rock magnetic study. The tem-perature dependence of magnetic susceptibility measurements were carried out on several powder samples. Samples were heated to 700 °C and subsequently cooled down to room temperature in a CS-3 (Agico Brno) furnace (Parma and Zapletal, 1991). Mag-netic susceptibility was simultaneously measured using a KLY-3S Kappabridge (Jelínek and Pokorný, 1997).

Thermomagnetic curves of pilot samples from the borders and the center of dike1 show that no alteration occurred in the center (Fig. 4) where the presence of magnetite or Ti-magne-tite (Curie temperature ~580 °C) is dominant. This Ti-magne-tite sometimes co-exists with either a small amount of possible goethite or hematite, because we distinguish an increase in the heating part of the curve after 120 °C (Fig. 4C, E, F, G) or after 540 °C (Fig. 4A, B, D).

We are dealing here with two common “types” already mentioned by Hrouda et al. (2003): the most frequent of them (type I) shows a cooling curve with much higher susceptibility

values than the heating curve (Fig. 4A, B, D); the second (type II), less frequent, shows the heating curve presenting higher val-ues of susceptibility than for the cooling curve (Fig. 4C, E, F, G). Three specimens from this second type were heated and cooled twice, and one of them (Fig. 4C) was submitted to an Argon fl ow (reducing environment). We note that the heating curve of the second run is close to the cooling curve of the fi rst run. We also note that type I corresponds to the borders of the dike whereas type II corresponds to its center.

Isothermal Magnetization to Saturation: Coercivity Spectra

Some pilot samples from both borders and the center of dike1 were submitted to isothermal remanent magnetization (IRM) to saturation to characterize their coercivity spectra. Sample SV2/1, located 5 cm from the eastern border of dike1, shows relatively low coercivity values (Fig. 5A), whereas sam-ple SV6/2, located 101 cm from the same border, shows higher coercivity values (Fig. 5B). Our interpretation is that during the magma injection and opening of the dike, cooling of the magma in contact with the host rock is very rapid, which implies small magnetic grains ( single to pseudo-single domain, SD). These grains grow and develop (multidomain size, MD) toward the center of the dike because cooling becomes slower toward the center. Hence, theoretically we may expect high susceptibil-ity values toward the center of the dike if cooling is homoge-neously axisymmetric (Moreira et al., 1999; Aïfa and Lefort, 2001; Lefort et al., 2006). The discrimination between the two values of coercivity could be related to the presence of different injection phases. One may expect that the high coercivity value is related to the SD size of magnetite or Ti-magnetite, whereas the lower coercivity value holds for the MD size of the same type of mineral, the normalized magnetic moment being within the same range.

1 10 100 10000.0

0.2

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9700.49 mA/m

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max

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0.2

0.4

0.6

0.8

1.0

452318.35 mA/m

SV6/2

Peak field [mT]

M/M

max

B

Figure 5. Normalized IRM acquisition curves of specimens (A) SV2/1 from the border and (B) SV6/2 from the center, showing low and high coercivity values, respectively.



256 Aïfa et al.

Variation of Magnetic Properties within the Dikes

To study the mineralogical changes within the dikes, we used 3-D block diagrams to better illustrate the distribution of the bulk magnetic susceptibility K, the corrected anisotropy degree P′, the intensity of the natural remanent magnetization (NRM) M, and the shape parameter T as functions of distance, instead of plotting the scalar parameters along cross-sections. One purpose of this technique was to check the possible existence of abrupt changes of these parameters and thus to detect fractures or a pos-sible magmatic “zoning.” To this end we measured within each dike the various magnetic characteristics with a (0,0) reference at the lower left corner of the diagram. Gridding using the krigging technique was applied and after interpolation, four diagrams were obtained for each dike; they are presented in Figure 6. For dike 1, they show a large area of high K (Fig. 6A) on the right side of the dikes, this high value also affects the other parameters of the diagrams—M (Fig. 6B), P′ (Fig. 6C), and T (Fig. 6D). Note that M and K exhibit a positive linear correlation.

Most of the high values are concentrated along the eastern borders of both dikes. A perpendicular section crosscutting dike1 displays high values for NRM (NRMmax = 1.4 A/m) and the bulk susceptibility (Kmean = 0.06 SI). A profi le parallel to the eastern border also shows higher values for both K and M (Fig. 6A, B).

An east-west section across dike1 (Fig. 6C) shows evidence of two major P′ peaks, whereas a single value can be seen on dike2. The peak is located 1 m from the eastern edge of dike1 and reaches a value of 1.03. The other peak, which reaches 1.06, is located ~1.40 m from the dike’s western edge. Both peaks cor-respond on the fi eld to the two fractures that may have resulted from the rejuvenation of previous north-south cooling cracks. We speculate that this possible rejuvenation mainly affected the east-ernmost linear crack zone, because this structure shows a P′ value higher than 1.05 (Puranen et al., 1992). If we accept this criterion, this rock disruption could be considered as a fracture. The linear structure located in the west, which attains a value of only 1.03, could also be considered as a possible fracture.

COMPONENTS OF MAGNETIZATION

The processing of these dolerites also included a progres-sive thermal demagnetization using the MAVACS (Magnetic Vacuum Control System [Geofyzika Brno]; Príhoda et al., 1989) equipment at temperatures ranging between 80 and 680 °C with step intervals of 60–30 °C. Demagnetization using alternating fi eld (AF) technique has been applied using an LDA-3 apparatus (Agico Brno) until 100 mT, with steps every 5–20 mT. Separa-tion of the remanent magnetization components was carried out with the help of multicomponent analysis (Kirschvink, 1980; Man, 2003).

At each step of the thermal demagnetizations, the bulk magnetic susceptibility was measured to track any mineralogi-cal changes. In fact, in an oxidizing environment, magnetic sus-ceptibility versus temperature shows dramatically increasing

values (Fig. 7A4, B4). This increase is in agreement with a high (Fig. 7A3) or low (Fig. 7B3) unblocking temperature probably of Ti-magnetite (goethite; Fig. 7A4, B4), which transforms above 450 °C (120 °C) to magnetite (hematite), as shown by the nor-malized M/Mmax intensity values.

For thermal demagnetization we used twelve specimens; twenty-six specimens have been subjected to AF demagne-tizations. At least two components were extracted from each specimen, leading to the following components (Figs. 7 and 8; Table 1):

• Component B is of low fi eld and low blocking temperature. The computed mean direction seems to be slightly older than the present-day fi eld.

• Component C1 lies in a temperature range between 200 and 540 °C, and its AF demagnetization fi eld is between 10–20 and 40–65 mT, refl ecting probably the presence of magnetite or Ti-magnetite, with a component of magneti-zation (D = 204.3°, I = –15.2°, α95 = 7.9°). Two prelimi-nary conclusions can be drawn: (1) the data fi t the Middle to Late Silurian directions if we compare with the results obtained for black shales from the Kosov Quarry near Karlštejn, Bohemian Massif (D = 205°, I = –28°), with a paleorotation of 175–185° (Patocka et al., 2003); and (2) the magnetization measured in Silurian dikes is likely to be early Permian to late Carboniferous overprint.

• Component C2 lies in the temperature range between 200 and 540 °C and its AF demagnetization fi eld is between 10(20) and 40(65) mT, refl ecting probably magnetite or Ti-magnetite. Its component of magnetization fi ts the Carboniferous direction for Bohemian Massif (Krs et al., 2001; Edel et al., 2003; Patocka et al., 2003). Tilt-corrected mean direction of remanent magnetization (D = 179.72°, I = 11.8°, α95 = 13.3°) corresponds to the Middle or Late Carboniferous direction for the Bohemian Massif (with no signifi cant rotation).

• Component D shows thermal demagnetizations (only samples SV5–SV7 were processed) with a temperature ranging between 580 °C (in some rare cases, 620 °C) and 680 °C and carried mainly by hematite, the AF demagne-tization fi elds are between 20(40) and 80(100) mT, similar to the B component.

For dike2 only seventeen specimens out of thirty-two were subjected to AF demagnetizations. The C1 component of mag-netization is not recorded in this dike. The other components of magnetization (B, C2, and D) are isolated on the borders as well as in the center of the dike (Figs. 7 and 8; Table 1). As an example specimen SV2–26 from dike2 recorded B and C2 components that show anti-parallel directions in the orthogonal diagram (Fig. 7D1). In this case great circle analysis has been carried out to isolate the best-fi tting component.

It can be observed on a declination versus inclination plot of the distribution of C1 and C2 components (Fig. 8E) that C1 and C2 directions are never superimposed. In addition, the mean value of C1 inclinations is –15.2° with good cluster (α95 = 7.9°), C2




B

C

D

A

Dike1 Dike2

Oblate

Prolate

OblateProlate

100 100150 150200 200

500

250300350400450500

50

250 300 350 400 450

0

Y (cm) X (cm)

T0.50.0

100 100150 150200 200

500

250300350400450500

50

250 300 350 400 450

0

Y (cm) X (cm)

P'1.05

100 100150 150200 200

500

250300350400450

50

250 300 350 400 450

0

Y (cm) X (cm)

1.0

0.50.0

M (A/m)

100 100150 150200 200

500

250300350400450500

50

250 300 350 400 450

0

Y (cm) X (cm)

K (SI)0.040.020.00

100 100150 150200 200

50 0

250300350400450

50

250 300 350

0

Y (cm) X (cm)

T0.0

100100150

150200200

500

250300350400450

50

250 300 350

0

Y (cm) X (cm)

P'1.02

1.04

100 100150

150200200

500

250300350400450

50

250 300 350

0

Y (cm) X (cm)

M (A/m)

0.4

0.8

100100

150150

200200

500

250300350400450

50

250 300 350

0

Y (cm) X (cm)

K (SI)

0.00

0.02

N

N

N

N

N

N

N

N

Figure 6. Three-dimensional block diagrams for both dike1 (left column) and dike2 (right column) representing (A) the values of the bulk mag-netic susceptibility, (B) the intensity of NRM, (C) the corrected degree of anisotropy, and (D) the shape parameter. Magnetic north is indicated by an arrow.



258 Aïfa et al.

B

C1

B

C2

B

C2

D B

AB D

C

1

3

2

4

1

3

21

3

2

1

3

2

4




Component B

Component C1

Component C2

B

N

E

S

DownUp

A

N

E

S

WComponent D

N

E

S

SE

S

D

C

S

S

S

N

BC2D

Dike 2

B

C2D

Dike 1C1

E F

D (˚)

I (˚)

B

W

W

W

Figure 7. Examples of in situ stereographic projections of iso-lated components of magnetization B, C1, C2, and D for (panels A, B) dike1 and (panels C, D) dike2. Full circles: lower hemi-sphere, open circles: upper hemisphere. Orthogonal projections of thermal (in °C) (panels A2, B2 for dike1) or alternating fi eld (in mT) (panels C2, D2 for dike2) demagnetizations. Open (solid) circles indicate projection onto the vertical (horizontal) plane. Normalized intensity of magnetization vs. temperature (panels A3, B3) and vs. demagnetizing fi eld (panels C3, D3). Magnetic susceptibility vs. temperature showing mineralogical changes of probably hydroxides above 400 °C (panel A4) and 200 °C (panel B4).

Figure 8. Stereographic projections of isolated components of magnetization (panel A) B, (panel B) D, (panel C) C1, and (panel D) C2 found in dike1 with their mean 95% confi dence levels before (left) and after (right) unfolding. Same notations as in Figure 7. Declination vs. inclination diagrams for all com-ponents from both (panel E) dike1 and (panel F) dike2 after unfolding.



260 Aïfa et al.

TA

BLE

1. C

OM

PO

NE

NT

S O

F M

AG

NE

TIZ

AT

ION

ISO

LAT

ED

IN T

HE

ST

UD

IED

SIT

ES

(49

.975

°N, 1

4.13

6°E

)

P

GV

PG

V

Dik

eno

.P

ositi

on

Cp

N

D

(geo

)I

(geo

)α9

5(g

eo)

D (tilt

) I

(tilt

) α9

5(t

ilt)

Lat

(geo

)Lo

ng(g

eo)

Pal

eola

t(g

eo)

dp(g

eo)

dm (geo

)La

t(t

ilt)

Long (tilt

) P

aleo

lat

(tilt

) dp (tilt

) dm (tilt

) 1

All

B

35

357.

8 83

.2

5.7

109.

158

.2

5.7

63.3

7 13

76

.59

11

11.2

18

.4

65

38.8

6.

2 8.

5 1

All

C1

13

196.

6 –9

.3

7.9

204.

3–1

5.2

7.9

42.5

9 17

1.38

–4

.68

4 8

43.2

16

0.1

–7.8

4.

2 8.

1 1

All

C2

11

191.

2 26

.8

13

179.

711

.8

13.3

–2

5.1

2.13

14

.17

7.8

14.4

–3

4.1

14.5

6

6.9

13.5

1

All

D

29

2.3

85.9

6.

4 11

3.3

56.7

6.

4 58

.13

14.7

5 81

.84

12.6

12

.7

15.2

63

.3

37.3

6.

7 9.

3 1

Eas

t B

19

35

5.4

81.5

7.

4 96

.567

.6

10.9

66

.53

10.8

3 73

.36

14.9

15

.4

33

63

50.5

15

.2

18.2

1

Eas

t C

1 9

197.

2 –8

.3

11

202

–12.

7 9.

9 41

.95

170.

77

–4.1

7 5.

3 10

.6

42.7

16

3.7

–6.5

5.

1 10

.1

1 E

ast

C2

6 19

6.2

28.9

17

18

4.8

24.5

20

.2

–23.

05

–2.8

6 15

.43

12.8

23

.3

–27

8.9

12.9

11

.6

21.6

1

Eas

t D

11

21

1.4

88.4

8.

5 12

0.6

60

9.9

47.2

2 11

.68

86.8

17

17

14

.7

56.4

40

.9

11.3

14

.9

1 C

ente

r B

12

33

2.5

85.5

9.

8 11

5.4

58.7

9.

8 57

.69

6.42

81

.05

19.3

19

.5

15.8

60

.6

39.4

10

.8

14.6

1

Cen

ter

C1

2 20

0.4

–14.

1 15

21

0.4

–17

43

.91

165.

45

–7.1

6

41

.6

152.

1 –8

.7

1 C

ente

r C

2 4

191.

4 24

.1

25

181.

39.

7 24

.9

–26.

62

1.68

12

.61

14.2

26

.6

–35.

1 12

.6

4.9

12.7

25

.2

1 C

ente

r D

14

16

.6

82.6

9.

9 10

7.1

56

9.9

63.6

5 23

.45

75.4

4 18

.8

19.3

17

.8

67.8

36

.6

10.2

14

.2

1 W

est

B

4 46

.5

79.9

28

10

4.5

51.1

27

.5

60.4

5 43

.71

70.3

9 50

.3

52.6

15

.4

72.8

31

.8

25.1

37

.2

1 W

est

C1

2 19

0.2

–8.7

72

19

8.6

–18.

4

43.5

9 –1

80

–4.3

8

46

.6

166.

9 –9

.4

1 W

est

C2

1 16

3.8

21.9

160.

1–4

.6

–2

7.04

32

.02

11.3

6

39

.42

–140

–2

.3

1 W

est

D

4 33

8.1

79.7

32

10

6.2

62.5

32

67

.45

–5.2

7 70

.03

58.4

61

.1

23.6

63

.3

43.8

39

49

.9

2 A

ll B

9

15.5

75

.7

12

123

59.4

12

.3

74.5

1 41

.17

62.9

9 20

.7

22.6

13

.1

55.3

40

.2

13.8

18

.4

2 A

ll C

2 6

210.

7 42

11

19

1.1

19.8

11

.3

–10.

95

–14.

17

24.2

4 8.

5 13

.8

–29

1.7

10.2

6.

2 11

.8

2 A

ll D

8

144.

8 86

.6

16

145

48.6

15

.6

44.3

19

.59

83.2

2 30

.8

30.9

–4

.6

44.2

29

.5

13.4

20

.5

2 N

orth

east

B

6

15.1

74

.7

20

121.

359

.8

19.6

75

.89

44.9

9 61

.32

32.4

35

.6

14.2

56

.1

40.7

22

.3

29.5

2

Nor

thea

st

C2

3 20

4.5

38.9

8.

9 18

8.9

14.6

8.

9 –1

4.84

–9

.31

21.9

7 6.

3 10

.6

–32.

1 3.

7 7.

4 4.

7 9.

1 2

Nor

thea

st

D

5 92

.7

85.2

24

13

9.2

48.9

23

.5

48.6

1 28

.63

80.4

7 46

.1

46.6

–2

.4

48.7

29

.8

20.5

31

2

Sou

thw

est

B

3 16

.3

77.5

16

12

6.1

58.5

16

.1

71.8

5 35

.55

66.0

9 28

.2

30.1

11

53

.8

39.2

17

.7

23.9

2

Sou

thw

est

C2

3 21

7.7

44.9

28

19

3.4

25.2

28

–6

.54

–19.

3 26

.49

22.4

35

.4

–25.

7 –0

.4

13.3

16

.2

30.2

2

Sou

thw

est

D

3 20

0.9

82.5

34

15

4.2

47.4

33

.8

35.9

8 7.

69

75.2

5 64

.3

65.9

–8

.2

36.9

28

.6

28.5

43

.8

N

ote:

Cp—

nam

e of

the

mag

netic

com

pone

nt m

easu

red.

D, I

—de

clin

atio

n an

d in

clin

atio

n, r

espe

ctiv

ely,

bef

ore

(geo

) an

d af

ter

(tilt

) be

ddin

g co

rrec

tion

of th

e ho

st r

ock

for

spec

imen

s fr

om b

oth

bord

ers

(eas

t, w

est)

and

for

the

cent

er o

f the

dik

e; α

95—

Fis

her

stat

istic

par

amet

er; d

p, d

m—

sem

iaxe

s of

the

oval

of 9

5% c

onfid

ence

abo

ut th

e m

ean

pole

; N

—nu

mbe

r of

spe

cim

ens

used

to c

ompu

te th

e m

ean

valu

es; P

aleo

lat—

pale

olat

itude

(°N

); V

GP

—vi

rtua

l geo

mag

netic

pol

e (la

t [°N

], lo

ng

[°E

]).




being characterized by mean inclinations of 11.8° (α95 = 13.3°) for dike1 and 19.8° (α95 = 11.3°) for dike2. Thus, they are very different. Nevertheless, these components (C1 and C2) are asso-ciated to the “same” magnetic minerals (Ti-magnetite and/or magnetite) and correspond both to temperatures ranging between 200 and 540 °C (Fig. 7A) or to AC fi elds ranging between 20 and 65 mT (Fig. 7C). Taking into account of all these observations we can, however, suggest that C1 and C2 are different: C1 is prob-ably primary and possibly overprinted by C2.

It has been shown by numerous fi ndings that many of the pre-Variscan rock formations of the Bohemian Massif were partly or totally remagnetized during the Variscan orogeny, most probably during the Carboniferous to the Early Permian (Krs et al., 2001; Edel et al., 2003).

Because of the differences in their locations it is likely that the C1 direction corresponds to the direction of the original com-ponent, whereas C2 corresponds to either the rotation of the C1 component or to some remagnetization during the Hercynian orogen. The mean difference in orientation between C1 and C2 is signifi cant and large enough (δD = 24.6°, δI = 32°) to explain

a possible rotation of component C1 with respect to C2. Accord-ing to conclusion 1 above, the distribution of virtual geomagnetic pole (VGP) fi ts remarkably well with the apparent polar wander path (APWP) of the Bohemian Massif, and the poles are located very close to the Silurian pole of that massif. According to con-clusion 2, the distribution of VGP fi ts remarkably well with the APWP of the Bohemian Massif, and the poles are located very close to the Carboniferous poles of the massif. The distribution of VGPs after (tilt) bedding correction for dike1 and dike2 are documented in Figure 9.

A detailed examination of the data suggests that a small amount of rotation may have occurred preferentially near the eastern edge of the dike. Because the magnetic component C1 is probably of the same age (Wenlock–Ludlow) as the basalt and picritic basalt lava fl ows, C1 can be considered as associated with the dike emplacement. It is interesting to note that in the center of the dike, where no major disruptions are known, there are only a few C1 directions still preserved. It is important to note that all the components—B, D, C1, and C2—do not show any preferen-tial location in the dikes.

d2/C2

d1/C2d1/C1

d1/C1

36V

d1/C2

d2/C2

Figure 9. Virtual pole positions after (tilt) bedding correction for dike 1 (d1) and dike 2 (d2). Names of the component are listed in Table 1. The virtual pole position 36V is of the Barrandian, Karlštejn, Middle Silurian, contact aureole of basalt sill. Apparent polar wandering path, inferred from the East European craton for Early Devonian (D1) to Middle Triassic (T2) time span, is presented by a dashed line.



262 Aïfa et al.

RELATIONSHIP BETWEEN MAGNETIZATION AND CARRIERS

If we investigate the nature of the carriers we may discrimi-nate between those that carry multiple components and those with single components. As an example of multicomponent carriers, samples SV2/1 and SV6/2 carry three components each: B and D components in common and C1 and C2, respectively, which is in agreement with their coercivity spectra. C1 is mainly carried by MD magnetite whereas C2 is carried by SD magnetite.

In a fi rst interpretation, we demagnetized thermally and by AF twelve and twenty-four specimens, respectively, from which four components of magnetization have been separated (Fig. 8, Table 1).

Note that when P′ is high, C1 tends to disappear, but this is not a strict rule. As an example, in the center and the west side of dike1, no major shear zone has been observed. In samples with C1 components, lineations are still preserved. Regarding the component of magnetization C2, which is a Late Carboniferous remagnetization, obviously it is nearly missing in the western side of dike1, whereas it is well recorded in the dike’s eastern border, where P′ values are greater than 1.03. Signifi cantly, there is a positive correlation between M and K, which is one reason to suspect that fractures may favor fl uid circulation and overprint the primary component C1.

The fabric is mainly oblate but can also be prolate in the eastern sides of the dikes (Fig. 6D). This combination is probably related to secondary minerals, as shown in Figure 4. In fact in this fi gure, which represents the distribution of the magnetic minerals along a cross-section of the dike, the distribution of maghemite or Ti-magnetite is associated with either goethite or hematite.

We also notice in the thermomagnetic curves that type I (Fig. 4A, B, D) mentioned by Hrouda et al. (2003) is located mainly in the borders. If we take this criterion into account, we may defi ne the width of each border: for dike1 the eastern border is more fractured and records higher values of P′, and the width may reach 140 cm, whereas the western border is limited to a maximum width of 20 cm.

On the section along the x-axis it can be observed that the association of Ti-magnetite and hematite is usually located along the rims of the dike, whereas the samples characterized by Ti-magnetite only or Ti-magnetite and goethite are usually located in the center of the dike. This result suggests various interpretations:

• It may indicate that the initial magma was mainly char-acterized by Ti-magnetite and that some of the primary Ti-magnetites were transformed into magnetites, because the alkaline basaltic tuffs, which constitute the host rock, contain a large amount of titanium.

• It may be also associated with a late fl uid circulation along the borders of the dike, as already observed by Aïfa and Lefort (2000).

• It may at least represent a different magma injection, as suggested in our introduction. However, this interpretation

will not be favored here, because rare C1 components are still observed in the center of the dike.

STRUCTURAL IMPLICATIONS AND CONCLUSIONS

In a previous interpretation, the evolution of the present Prague synform during Silurian times was characterized by the move-ment of individual segments along deep synsedimentary faults (Kríž, 1998). The sedimentation and the widespread volcanism were considered to be controlled by three main faults (the Prague, Tachlovice, and Koda faults), which delineated three main stripes (the northern, central, and southern segments; Fig. 1). The latter two faults, however, have been interpreted by Melichar (2004) as planes of detachment (i.e., thrust faults separating different thrust units). The original orientation of these faults was not con-sidered as typical of the Paleozoic but was thought to refl ect the orientation of some deep Cadomian structures (Havlícek, 1963, 1998). This interpretation suggests that the predominant vertical movements recorded along the N65° faults (reaching 1000 m and even 2700 m between the Cambrian and the Lower Devonian; Fig. 1) did not result from a general extension of the lithosphere that controlled Ordovician-Devonian rock units of the Barrandian area but rather from a compressional regime.

It is along these faults and along some N10°W faults that the calc-alkaline and sub-alkaline Silurian volcanism was sup-posed to link (Štorch, 1998). On the contrary, the late shearing episode previously thought to have taken place along these faults, even if limited, now appears to be unlikely, based on the most recent structural data. The general picture that can be given of the Prague synform during Silurian times strongly suggests the existence of a generalized piano-touch tectonics generated in a northeast-southwest compressional regime and followed by a general thrust and nappe tectonics.

According to Melichar (2004), regarding the question of ver-gence in the Prague synform, fi eld evidence agrees with asym-metrical indicators of tectonic movement on fault planes or in a proximal zone of simple shear. If we adopt this way of think-ing, we can bring, with our AMS and paleomagnetic data, fresh information on the direction of displacement of the nappes in the Prague synform. This information fi ts our results of asymmetri-cal opening of the Svatý Jan pod Skalou dikes.

The C1 component corresponds with inclinations known at the end of Silurian times. Component C2 is compatible with paleomagnetic results already known for the end of the Carbon-iferous. Component D is very similar to a previously published paleolatitude for the Paleocene. Component B, which is close to the D component but older than it according to the published paleolatitude, could be by comparison Cretaceous in age.

We mainly concentrate here on C1 and C2 components for our interpretation. This restriction comes from the chrono-logical limitation of the companion articles, which are all devoted to the Paleozoic evolution of the Bohemian Massif.

The AMS results obtained on the two dikes of the Svatý Jan pod Skalou area show the existence of an asymmetrical




opening of both dikes. Study of the declination of the ASM K1 components shows that the two dikes were emplaced during a dextral transtensional opening. One of the most important results regarding these dikes is that they both opened by means of the same mechanism but show a difference in their remanent mag-netization, as dike2 is devoid of C1 magnetic component. This difference implies that dike2 may be younger than dike1 if the C2 component is primary.

Calculation of the difference between the directions of the two different stresses based on the AMS data show that the regional stress suffered a counterclockwise rotation of ~40° between the emplacement of dike1 and that of dike2. This result explains why these dikes display different inclinations.

Our data do not provide any evidence on whether the Rheic Ocean existed, but we observe that the counterclockwise rotation of the stress as a function of time was also probably responsible for a modifi cation of the direction of the displacement of the nappes. This counterclockwise rotation of the nappes emplace-ment strongly suggests that the Rheic Ocean (if its existence is really supported by other data) should have changed its azimuth of subduction between the emplacement of the two dikes or closed following a sinistral shearing.

After fi eld observations on dike1, two fracture zones devel-oped in a direction nearly parallel with the border of the dike. Because of the existence of very discrete slikensides showing evidence for horizontal displacement, there is no criterion for the sense of shearing. Consequently, we do not know whether

the slikensides are witness to a continuity of the stress direction (dextral) or not (sinistral). In any case, because the high P′ value (P′ = 1.06) is located on the eastern side of dike1, we can assume that the eastern disruption really corresponds with a fracture but we cannot say whether the western disruption corresponds with a fracture (P′ = 1.03) or with a cooling crack.

According to the literature (Von Raumer et al., 2003; Linnemann et al., 2004; Schulz et al., 2004), the initial opening of the Rheic Ocean would have occurred in a northwest-south-east direction. Jelenska et al. (2001) suggested, based on olis-toliths coming from the Bardo basin, that the nappes emplace-ment occurred in the Middle–Late Devonian. This observation suggests that, at that time, the Sudetes had already collided with Baltica and that the Saxo-Thuringian and the Teplá-Bar-randian plates were already welded. So far as the subduction of the Rheic Ocean is concerned, our AMS data suggest two solu-tions (Fig. 10): (1) possible modifi cation of the direction of this azimuth of subduction during the closure of the Rheic Ocean or (2) counterclockwise rotation of the Silurian shortening direc-tion during the collision. Because the transtensional opening of the two dikes remains dextral during Silurian and Carboniferous times, which implies a counterclockwise rotation of the shorten-ing direction and a late-stage sinistral transpressional collision (Fig. 10).

If we follow this interpretation, the Prague synform shows, in the Silurian, some affi nities with the convergence episode that affected Baltica, Avalonia, and Laurentia. These affi nities are not

K1

K1

W E

"Opening Mode"

K1

K1

NW

SE

B

Dike1 Dike2

K1K1

α

B

G

α1

2

Rheic

1a

2a

N

CA

Figure 10. Reconstitution of the tectonic evolution of the Svatý Jan pod Skalou dikes. The orientations of the AMS lineations (K1) close to the border of the dike are oblique; they show that the emplacement of the magma resulted from a dextral trans-tensional opening mode. Large open arrow—regional shortening direction; small open arrows—direction of extension; solid line arrows—sense of shearing; α counterclockwise rotation angle (~40°) between the (A) Middle to Late Silurian and (B) the Middle to Late Carboniferous, represented by the shortening directions. (C) Diagram showing the hypothetical evolution of the Rheic Ocean between the Middle to Upper Silurian and the Middle to Upper Carboniferous times. Two solutions are possible: modifi cation of the direction of the azimuth of the subduction plane during the closure of the Rheic Ocean, and counterclock-wise rotation of the shortening direction during collision. Middle-Late Silurian shortening direction (1), Middle-Late Carbon-iferous shortening direction (2), and their respective corresponding nappes emplacement (1a, 2a). B—Baltica; G—Gondwana. If the second solution is valid, it implies a counterclockwise transpression for the closure of the Rheic Ocean.



264 Aïfa et al.

consistent with the rifting that has been supposed to affect the Armorican-Bohemian plates at that time (Lewandowski, 1997, 1998, 1999; Marheine et al., 2000; Schätz et al., 2002; Robardet, 2003; Torsvik and Cocks, 2004). However, if we accept the gen-eral idea that the Bohemian and Armorican massifs correspond to pieces detached from Gondwanaland and thus were located south of the Rheic Ocean (and not north of it), we must admit that some tightening may have existed between some of these pieces when they were rifting away from Gondwanaland. This sugges-tion would reconcile the apparent compression we have evidence of, the slow sedimentation that existed during the Silurian in the synform (Kríž, 1998), and the Gondwana faunas that character-ize this area.

ACKNOWLEDGMENTS

We thank the Ministry of Foreign Affairs (Direction des relations et de la coopération internationales) for a two-year grant through programme Barrande 2001–2002 (grant 03229QA), the Cen-tre National de la Recherche Scientifi que through Geosciences-Rennes (UMR6118), and the Academy of Sciences of the Czech Republic (grant A 3013406). We are deeply indebted to Dr. J.-B. Edel and Professor D. Tarling for the careful comments and sug-gestions, which helped to improve the fi nal text. We also are grate-ful to Dr. Linnemann for giving us the opportunity to contribute to this special volume. This article is a contribution to the Interna-tional Geological Correlation Program Projects 485 and 497.

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Special Paper 423: The Evolution of the Rheic Ocean: From Avalonian-Cadomian Active Margin to Alleghenian-Variscan Collision Volume 423 || Structural evolution of the Prague synform