magnetic fabric study of the se rhenohercynian zone (bohemian massif): implications for dynamics of...

(2006) 93–109www.elsevier.com/locate/tecto

Tectonophysics 418

Magnetic fabric study of the SE Rhenohercynian Zone(Bohemian Massif): Implications for dynamics of the

Paleozoic accretionary wedge

Martin Chadima a,⁎, František Hrouda b,c, Rostislav Melichar d

a Institute of Geology, Academy of Sciences, Rozvojová 135, CZ-16502 Prague, Czech Republicb AGICO Inc., Brno, Czech Republic

c Institute of Petrology and Structural Geology, Charles University, Prague, Czech Republicd Institute of Geological Sciences, Masaryk University, Brno, Czech Republic

Received 16 June 2005; accepted 5 December 2005Available online 28 February 2006

Abstract

The progressive deformation recorded in the magnetic fabric of sedimentary rocks was studied in the SE Rhenohercynian Zone(RHZ), eastern margin of the Bohemian Massif, Czech Republic. Almost 800 oriented samples of the Lower Carboniferousmudstones and graywackes were collected from the SSE part of the Czech RHZ, so-called the Drahany Upland. The anisotropy ofmagnetic susceptibility (AMS) is predominantly controlled by the preferred orientation of paramagnetic phyllosilicates, mainlyiron-bearing chlorites. A regional distribution of the magnetic fabric within the Drahany Upland revealed an increasing deformationfrom the SSE to the NNW. In the SE, the magnetic fabric is bedding-parallel with magnetic lineation scattered in the bedding planeor trending N–S to NNE–SSW. Further to the NW, the magnetic foliation rotates from the bedding-parallel orientation to theorientation parallel to the evolving cleavage. This rotation is accompanied by a decrease of the anisotropy degree and the prolatenature of the anisotropy ellipsoids. The magnetic lineation is parallel to the strike of the bedding, bedding/cleavage intersection,pencil structure or the fold axes on a regional scale. In the NW part of the Drahany Upland, the magnetic foliation becomes parallelto the cleavage accompanied by an increase of the anisotropy degree and the oblate nature of the anisotropy ellipsoids. Theincreasing trend of deformation corresponds to the SSE–NNW increase in the degree of anchimetamorphism; both trends beingoblique to the main lithostratigraphic formations as typically observed in the sedimentary rocks of the accretionary wedges. TheSSE–NNW increase in deformation and anchimetamorphism continues to the Nízký Jeseník Mts., representing the northern part ofthe same accretionary wedge. The kinematics of deformation could not be unambiguously assessed. The observed magnetic fabricmay reflect either lateral shortening or horizontal simple shear or a combination of both mechanisms. Regarding the subductionprocess, it seems that the sedimentary sequences of the Drahany Upland were subducted, partly offscraped and accreted frontally orpartly underplated as opposed to the Nízký Jeseník Mts. where some return flow must have occurred.© 2006 Elsevier B.V. All rights reserved.

Keywords: AMS; Magnetic fabric; Accretionary wedge; Rhenohercynian Zone

⁎ Corresponding author. Tel.: +420 272 690 115.E-mail address: [email protected] (M. Chadima).

0040-1951/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tecto.2005.12.015

1. Introduction

Sediments of accretionary wedges involved inprocesses of subduction at convergent margins may

94 M. Chadima et al. / Tectonophysics 418 (2006) 93–109

undergo variable deformation history. The sedimentsdeposited on the subducting plate may be offscraped andfrontally accreted, partially or entirely thrust beneath theoverlying plate, or transferred from the subducting plateto the bottom of the overlying plate via underplating(e.g. Housen and Kanamatsu, 2003). If the sedimentsupply exceeds the capacity of the subduction zone, partof the sediment doubles back and flows up thesubduction zone (e.g. Moores and Twiss, 1995, p. 188).

The deformation history of an accretionary wedge isusually hard to study due to the scarcity or absence of alarge set of strain markers (i.e. deformed fossils,reduction spots, conglomerate pebbles, ooides). Indirectmethods of strain analysis have to be applied includingstudies of the preferred orientation of rock-constituentminerals by means of e.g. U-stage, X-ray diffraction,electron backscatter diffraction or neutron diffractionmeasurements. Another highly effective method tostudy rock fabric is the anisotropy of magneticsusceptibility (AMS; e.g. Tarling and Hrouda, 1993;Martín-Hernández et al., 2004).

The employment of the AMS to investigate theductile deformation of sedimentary rocks of theaccretionary wedges dramatically expanded since1980s (Hrouda, 1982; Housen and van der Pluijm,1991; Robion et al., 1995; Housen et al., 1996; Bakhtariet al., 1998; Parés et al., 1999; Aubourg et al., 1999,2000; Kanamatsu et al., 2001; Aubourg and Robion,2002; Parés and van der Pluijm, 2002, 2003; Housenand Kanamatsu, 2003; Aubourg et al., 2004). In thesestudies, the AMS of sediments of both ancient andrecent accretionary wedges was investigated.

The Rhenohercynian Zone (RHZ) in the easternmostBohemian Massif (Czech Republic) is considered as anexternal part of the Paleozoic accretionary wedge of theVariscan orogenic belt. The AMS has been successfullyemployed to study the deformation history along theRHZ in various parts Europe: SW England (Singh et al.,1975; de Wall and Warr, 2004), the French-BelgianArdennes (Robion et al., 1995, 1999), and theeasternmost Bohemian Massif (Dvořák and Hrouda,1972; Hrouda, 1976, 1978, 1979, 1993; Hrouda andPřichystal, 1995; Hrouda and Ježek, 1999; Hrouda et al.,2000) where the RHZ is exposed in two main areas.Whereas the NNE Rhenohercynian outcrop was studiedquite thoroughly (see references above) the results fromthe SSW outcrop are scarce (Hrouda and Ježek, 1999;Chadima et al., 2004). The purpose of this paper is topresent a deformation history of the SSW part of theCzech RHZ based on a large set of newly acquired AMSdata (Chadima, 2004). Together with previous works,this paper completes the magnetic fabric study of the

easternmost RHZ in the framework of the Variscanaccretionary wedge.

2. Geological setting

The Paleozoic sedimentary units of the easternmostBohemian Massif are regarded to be equivalents to theRhenohercynian and Subvariscan Zones (Fig. 1a) of theVariscan orogenic belt (Dvořák and Paproth, 1969),even though they sometimes have local names (Dvořák,1995). In the eastern margin of the Bohemian Massif,the Variscan orogen evolved by the oblique collisionbetween the Moldanubian/Lugian terrane and the Pan-African Brunovistulian terrane, the outer part ofLaurussia (Matte et al., 1990; Kalvoda, 1995; Schul-mann and Gayer, 2000). During the progressivecollision, the Devonian to the Lower Carboniferoussedimentary basins were incorporated into the Variscanaccretionary wedge (Kumpera and Martinec, 1995).Unlike the general east–west trend of the EuropeanRhenohercynian Zone, the Czech section of the RHZtrends NNE–SSW. The geotectonic interpretation ofthat sharp bend is widely discussed (e.g. Grygar andVavro, 1995; Hladil et al., 1999).

The tectonic evolution of the Czech RHZ wasdescribed using the geosyncline model (Dvořák, 1973;Dvořák and Paproth, 1988). Later, based on geophysicaland structural data, accretionary wedge model with flatnappes, and thin-skinned structures was proposed(Čížek and Tomek, 1991).

In the eastern part of the Bohemian Massif the RHZis exposed in two main areas separated by the Haná faultzone. These areas are geographically termed as theNízký Jeseník Mts., and the Drahany Upland in the NE,and SW, respectively (Fig. 1b). The rocks of the RHZpredominantly consist of the Lower Carboniferousflysch sediments with subordinate occurrences of theDevonian sedimentary and volcanic rocks. The LowerCarboniferous sediments, i.e. mudstones, siltstones,graywackes and conglomerates, were sub-divided intoseveral formations based on the lithostratigraphic andbiostratigraphic studies (Figs. 1b, 2; Dvořák, 1973;Hartley and Otava, 2001). From west to east thesuccession comprises: the Protivanov, Rozstání andMyslejovice formations in the Drahany Upland; theAndělská Hora, Horní Benešov, Moravice, and Hradec–Kyjovice formations in the Nízký Jeseník Mts. (Fig. 1b).Based on the heavy mineral assemblages, the litholog-ical horizons between two considered areas of the RHZwere correlated (Hartley and Otava, 2001). Thepaleocurrent data indicate a predominant S/SW to N/NE transport direction (Hartley and Otava, 2001).

Fig. 1. (a) Position of the Rhenohercynian Zone within the European Variscan orogenic belt. (b) Two outcrops of the Czech Rhenohercynian Zone inthe east margin of the Bohemian Massif with the main lithostratigraphic formations.

95M. Chadima et al. / Tectonophysics 418 (2006) 93–109

The sedimentary rocks of the RHZ were progres-sively deformed in the compressional regime combinedwith horizontal NNE–SSW simple shear during theVariscan Orogeny (Melichar, 1996; Havíř, 1998). TheLower Carboniferous thrusting-connected deformationis responsible for the evolution of the NNE–SSWtrending folds with axial cleavage. The cleavageevolved preferentially in the incompetent mudrocksand is less evident in the competent graywackes. Basedon the occurrence of fracture and slaty cleavage, theintensity of ductile deformation gradually increasesfrom the SE to the NWof the Drahany Upland (Dvořák,1973). Correspondingly, the degree of anchimetamorph-ism increases from the SE to the NW as deduced fromthe illite crystallinity and vitrinite reflectance data(Dvořák and Wolf, 1979; Franců et al., 1999). The SEpart of the Drahany Upland has undergone a latediagenetic phase (estimated paleotemperature of 130–170°C), the central part (southern part of the Protivanovand Rozstání formations) falls into diagenetic zoneconditions (170–200°C), and the northern part of the

Protivanov Fm. can be regarded as the low-anchizone(240–300°C).

3. Sampling, laboratory techniques and datapresentation

Oriented cylindrical samples of 2.5cm in diameterwere collected using a gasoline powered portabledrill. Depending on the complexity of the mesosco-pically observed structures, an average number offive to ten samples were taken from each site.Sample selection reflects the predominant rock typeat sampling site, i.e. mudrock or graywacke. Whenalternation of mudrocks and graywackes was present,both lithologies were collected from one site. Theorientation of the mesoscopic planar and linear fabricelements, i.e. bedding, cleavage, fold axis, bedding/cleavage intersection, was measured in the closevicinity of each sample. In the laboratory, thesamples were cut into specimens of 2.2cm in heightusing a diamond-coated wheel saw. Each sample

Fig. 2. A simplified geological map of the Drahany Upland based on Dvořák (1965, unpublished) with the locations of the sampling sites andprevailing sampling lithology.


yielded one to three specimens. Overall, 67 distinctsites of the Lower Carboniferous rocks were sampledyielding 795 specimens (Fig. 2, Table 1); AMS

results of a few of them were published earlier inmethodological studies (Hrouda and Ježek, 1999;Chadima et al., 2004).

Table 1Parameters of AMS of individual sampling sites

Site Longitude Latitude N Lithology Type km[10−6]

σ(km)

P σ(P)

T σ(T)

l σ(l)

f σ(f)

DV01 16° 45′ 25.0″ E 49° 24′ 42.6″ N 9 g IIIb 202.7 12.1 1.032 0.006 0.181 0.246 21 5 50 24DV02 16° 50′ 11.6″ E 49° 22′ 1.7″ N 10 g IIb 187.5 13.2 1.034 0.005 0.107 0.225 29 11 15 6DV03 16° 48′ 50.2″ E 49° 24′ 45.7″ N 10 g IIb 210.0 23.0 1.049 0.003 0.129 0.232 12 3 74 10DV04 16° 45′ 54.0″ E 49° 20′ 48.8″ N 10 g IIb 192.6 29.0 1.041 0.007 0.054 0.156 9 6 16 5DV05 16° 44′ 32.1″ E 49° 18′ 25.3″ N 10 g IIb 195.1 13.8 1.036 0.004 −0.076 0.340 17 6 21 7DV06 16° 51′ 7.0″ E 49° 31′ 28.1″ N 10 g IIb 181.7 9.7 1.042 0.005 0.244 0.303 n/a n/a n/a n/aDV07 16° 55′ 40.7″ E 49° 32′ 54.1″ N 10 g IIc 257.7 33.6 1.046 0.004 0.322 0.181 45 10 78 6DV08 16° 56′ 29.2″ E 49° 35′ 27.4″ N 10 g IIb 199.7 61.2 1.130 0.072 0.186 0.177 40 3 27 4DV09 17° 3′ 12.1″ E 49° 35′ 55.0″ N 10 m IIb 384.5 37.6 1.109 0.014 −0.073 0.095 9 6 11 5DV10 17° 4′ 15.4″ E 49° 31′ 55.1″ N 10 g Ic 162.8 10.0 1.029 0.004 0.746 0.117 60 28 10 7DV11 16° 59′ 58.8″ E 49° 27′ 47.5″ N 11 g IIc 337.1 76.4 1.112 0.014 0.825 0.070 21 9 9 7DV12 17° 0′ 24.4″ E 49° 25′ 6.4″ N 10 g IIc 169.2 32.5 1.038 0.005 0.521 0.155 36 16 10 6DV13 17° 7′ 25.9″ E 49° 22′ 2.9″ N 10 g Ic 186.6 12.2 1.036 0.009 0.477 0.209 23 7 19 9DV14 16° 55′ 24.0″ E 49° 15′ 32.6″ N 10 g Ic 136.1 8.3 1.031 0.003 0.686 0.183 42 20 16 5DV15 16° 53′ 20.5″ E 49° 16′ 29.8″ N 10 g Ic 138.0 14.8 1.054 0.008 0.682 0.121 39 29 18 8DV16 16° 42′ 56.6″ E 49° 13′ 20.5″ N 14 g IIb 239.4 36.3 1.015 0.005 0.206 0.363 46 25 38 26DV17 17° 1′ 32.0″ E 49° 40′ 57.3″ N 14 g Ic 252.8 66.0 1.077 0.021 0.316 0.238 24 17 31 26DV18 16° 53′ 19.9″ E 49° 28′ 33.1″ N 10 m IIc 369.4 24.0 1.132 0.013 0.320 0.084 56 35 71 8DV19 16° 54′ 42.3″ E 49° 29′ 33.2″ N 6 m IIIb 388.3 36.7 1.110 0.017 0.148 0.567 n/a n/a n/a n/aDV20 16° 55′ 46.1″ E 49° 15′ 58.9″ N 4 g IIc 163.6 12.1 1.044 0.005 0.518 0.105 45 3 13 6DV21 16° 44′ 57.8″ E 49° 32′ 40.0″ N 11 g IIb 168.4 32.0 1.062 0.020 0.151 0.339 n/a n/a n/a n/aDV22 16° 53′ 50.3″ E 49° 28′ 36.2″ N 11 g/m IIIa 298.3 69.4 1.080 0.011 −0.285 0.323 19 26 31 29DV23 16° 54′ 7.0″ E 49° 28′ 41.8″ N 12 g/m IIIb 305.6 28.4 1.077 0.017 −0.084 0.315 19 6 23 18DV24 16° 54′ 8.9″ E 49° 28′ 42.6″ N 9 m IIb 370.9 30.4 1.099 0.026 0.024 0.257 40 8 21 6DV25 16° 44′ 55.3″ E 49° 20′ 6.3″ N 12 m IIb 334.2 82.1 1.090 0.016 0.200 0.258 16 7 15 15DV26 16° 45′ 15.4″ E 49° 20′ 12.5″ N 15 g IIb 201.0 26.0 1.042 0.012 −0.233 0.227 26 7 25 14DV27 16° 46′ 26.6″ E 49° 22′ 12.8″ N 14 g IIIa 169.2 16.1 1.030 0.003 −0.638 0.297 n/a n/a n/a n/aDV28 16° 45′ 25.0″ E 49° 24′ 42.6″ N 24 g/m IIIb 242.6 44.2 1.033 0.007 −0.176 0.302 46 27 33 28DV29 16° 45′ 28.5″ E 49° 26′ 21.3″ N 12 g IIIb 90.4 15.3 1.035 0.011 −0.179 0.374 20 8 56 24DV30 16° 48′ 51.3″ E 49° 24′ 50.8″ N 26 g/m IIIb 236.2 53.9 1.049 0.015 0.009 0.462 60 22 53 24DV31 16° 45′ 30.8″ E 49° 11′ 11.6″ N 11 g Ic 214.8 8.3 1.021 0.007 0.552 0.323 39 24 13 6DV32 16° 52′ 6.1″ E 49° 14′ 29.4″ N 10 g Ic 163.9 25.5 1.047 0.009 0.568 0.247 51 21 10 8DV33 16° 56′ 44.2″ E 49° 18′ 4.9″ N 11 g Ib 183.7 36.1 1.040 0.014 0.207 0.515 47 18 21 12DV34 16° 53′ 22.8″ E 49° 24′ 12.4″ N 10 g Ib 234.5 44.9 1.042 0.014 0.028 0.343 20 11 10 8DV35 16° 55′ 36.5″ E 49° 26′ 11.6″ N 12 g Ic 190.9 14.8 1.031 0.006 0.287 0.247 31 16 27 16DV36 16° 46′ 12.7″ E 49° 13′ 54.5″ N 13 g/m IIc 280.7 73.3 1.058 0.022 0.210 0.175 27 16 34 27DV37 16° 52′ 10.4″ E 49° 17′ 27.4″ N 11 g Ic 176.9 8.3 1.034 0.008 0.605 0.309 53 23 17 12DV38 16° 51′ 9.1″ E 49° 23′ 40.6″ N 18 g/m IIIb 287.5 102.9 1.057 0.032 0.118 0.223 29 23 43 30DV39 16° 51′ 54.2″ E 49° 24′ 44.0″ N 11 g IIIa 261.8 89.4 1.050 0.014 −0.318 0.275 19 5 38 29DV40 16° 51′ 33.2″ E 49° 26′ 13.3″ N 13 g/m IIb 297.6 93.3 1.068 0.017 0.060 0.276 30 20 40 29DV41 16° 43′ 14.5″ E 49° 29′ 40.8″ N 10 g IIb 234.7 15.8 1.069 0.009 0.173 0.186 n/a n/a n/a n/aDV42 16° 45′ 25.5″ E 49° 32′ 33.6″ N 11 g IIb 215.0 18.3 1.078 0.019 −0.042 0.264 18 6 28 3DV43 16° 50′ 4.9″ E 49° 30′ 14.9″ N 12 g/m IIIa 239.8 39.4 1.053 0.006 −0.426 0.212 26 16 57 23DV44 16° 46′ 29.7″ E 49° 21′ 20.0″ N 10 m IIc 356.3 22.6 1.068 0.012 0.305 0.088 22 5 8 4DV45 16° 47′ 48.9″ E 49° 25′ 35.1″ N 10 g IIIb 200.9 17.2 1.039 0.007 −0.175 0.253 23 16 47 25DV46 16° 46′ 49.9″ E 49° 24′ 15.4″ N 8 g IIIb 130.8 19.3 1.015 0.008 −0.114 0.394 n/a n/a n/a n/aDV47 16° 48′ 14.9″ E 49° 26′ 21.3″ N 12 g IIb 174.5 15.8 1.030 0.006 0.041 0.247 n/a n/a n/a n/aDV48 16° 45′ 51.5″ E 49° 30′ 33.9″ N 12 g IIc 171.1 12.5 1.066 0.011 0.581 0.125 n/a n/a n/a n/aDV49 16° 55′ 24.6″ E 49° 15′ 18.3″ N 20 g Ib 123.9 6.2 1.023 0.005 0.280 0.357 26 18 18 10DV50 16° 53′ 20.1″ E 49° 16′ 29.9″ N 19 g Ic 166.3 12.5 1.054 0.013 0.720 0.239 41 21 12 5DV51 16° 53′ 5.5″ E 49° 19′ 37.7″ N 16 g IIb 232.9 13.3 1.034 0.006 0.211 0.357 23 8 21 12DV52 16° 49′ 9.4″ E 49° 18′ 26.4″ N 20 g Ic 209.1 8.3 1.032 0.007 0.499 0.434 39 22 25 12DV53 16° 48′ 12.6″ E 49° 14′ 26.6″ N 13 g Ic 153.4 25.8 1.022 0.003 0.290 0.228 63 16 30 12DV54 16° 43′ 50.5″ E 49° 27′ 49.3″ N 12 g IIb 233.3 21.9 1.074 0.010 −0.359 0.269 11 4 81 8DV55 16° 46′ 44.5″ E 49° 15′ 36.0″ N 11 g Ic 142.0 11.6 1.024 0.007 0.573 0.253 31 26 14 7DV56 16° 52′ 43.9″ E 49° 29′ 56.5″ N 9 g IIb 188.8 12.0 1.043 0.006 −0.237 0.288 n/a n/a n/a n/a

(continued on next page)


Table 1 (continued)

Site Longitude Latitude N Lithology Type km[10−6]

σ(km)

P σ(P)

T σ(T)

l σ(l)

f σ(f)

DV57 16° 55′ 43.3″ E 49° 29′ 43.6″ N 10 m IIIa 318.3 23.7 1.060 0.008 −0.412 0.225 16 5 48 18DV58 16° 56′ 38.2″ E 49° 30′ 49.3″ N 9 g IIc 220.4 26.8 1.060 0.008 0.234 0.194 37 9 20 16DV59 16° 53′ 0.9″ E 49° 42′ 30.2″ N 6 g IIc 241.9 12.4 1.094 0.030 0.587 0.077 64 13 11 7DV60 16° 49′ 56.9″ E 49° 41′ 56.9″ N 8 g IIb 150.3 27.8 1.073 0.029 0.242 0.219 9 6 45 6DV61 16° 50′ 24.3″ E 49° 39′ 22.4″ N 9 g Ic 250.3 43.4 1.083 0.013 0.678 0.051 77 14 8 5DV62 16° 56′ 4.1″ E 49° 28′ 6.9″ N 11 g IIb 206.7 31.4 1.050 0.008 0.446 0.366 16 8 10 5DV63 16° 57′ 18.9″ E 49° 28′ 26.1″ N 15 g/m IIIb 382.3 52.2 1.129 0.031 0.179 0.274 13 4 13 13DV67 16° 43′ 50.9″ E 49° 29′ 30.5″ N 11 m IIa 285.5 36.0 1.094 0.005 −0.249 0.134 17 11 80 8DV68 16° 46′ 41.0″ E 49° 34′ 35.5″ N 13 g IIb 189.0 14.6 1.063 0.003 0.026 0.164 n/a n/a n/a n/aDV69 16° 48′ 42.2″ E 49° 37′ 34.9″ N 12 m IIb 279.1 44.3 1.071 0.018 0.355 0.409 79 8 16 6DV70 16° 50′ 28.6″ E 49° 36′ 7.4″ N 22 g/m IIb 278.5 46.2 1.053 0.016 0.124 0.428 20 10 29 15

The site name; longitude, latitude of the site; numbers of specimens measured (N); lithology (g— sites with graywacke, m— sites with mudstones, g/m — sites with both graywackes and mudstones); the anisotropy type characteristics (Type): I — only k3 directions concentrated, II — all threeprincipal directions concentrated, III— only k1 directions concentrated, a— almost all specimens with linear magnetic fabric, b— some with linearand some with planar magnetic fabric, c — almost all specimens with planar magnetic fabric; the arithmetical means and standard deviations (ó) ofkm=(k1+k2+k3) /3, P=k1 /k3, T=2 ln(k2 /k3) / ln(k1 /k3)−1, l — angle between the magnetic lineation and the bedding strike direction (possiblybedding/cleavage intersection or fold axis), f— angle between the magnetic foliation and bedding (n/a— not applicable for the sites of massive rockswhere no bedding plane was observed mesoscopically).


The anisotropy of the low-field magnetic suscepti-bility (AMS) was measured with a KLY-3S SpinnerKappabridge (Jelínek and Pokorný, 1997) in the rockmagnetic laboratory in Agico, Inc., Brno, CzechRepublic. The measuring field intensity was 300A/mat operating frequency of 875Hz.

The bulk susceptibility is presented in terms of meansusceptibility, km (Nagata, 1961), the eccentricity andshape of the anisotropy ellipsoid are expressed as degreeof anisotropy, P (Nagata, 1961), and the shapeparameter, T (Jelínek, 1981), respectively. These para-meters are defined as follows:

km ¼ ðk1 þ k2 þ k3Þ=3P ¼ k1=k3

T ¼ 2lnðk2=k3Þ=lnðk1=k3Þ−1

where k1≥k2≥k3 are the principal susceptibilities.The P parameter indicates the intensity of the

preferred orientation of magnetic minerals in a rockand the T parameter indicates the shape of the anisotropyellipsoids; it varies from −1 (perfectly linear magneticfabric) through 0 (transition between linear and planarmagnetic fabric) to +1 (perfectly planar magneticfabric). In addition, an angle between the magneticfoliation and bedding, denoted as angle f, and an anglebetween the magnetic lineation and the bedding strikedirection (possibly bedding/cleavage intersection or foldaxis), denoted as angle l, are calculated.

The arithmetical means and standard deviations (σ)of the above parameters in individual sites are

presented in Table 1. In addition, this table containsthe numbers of specimens measured (N), longitude,latitude of the site, lithology, and the anisotropy typecharacteristics (Type) as introduced by Kligfield et al.(1977). The distribution pattern of the principal suscep-tibilities is characterized by a Roman numeral (I: onlyk3 directions concentrated, II: all three principaldirections concentrated, III: only k1 directions concen-trated) and the shape of the magnetic fabric ischaracterized by a letter suffix (a: almost all specimenswith linear magnetic fabric, b: some with linear andsome with planar magnetic fabric, c: almost allspecimens with planar magnetic fabric).

4. AMS in deformed sedimentary rocks — generaloverview

The magnetic fabric of sedimentary rocks providesinformation on the deposition and compaction pro-cesses. In addition, in sedimentary rocks that under-went ductile deformation, which is the frequent case ofthe accretionary wedges, it can serve as a sensitiveindicator of the progressive ductile deformation. Thefirst model of the development of the AMS duringprogressive deformation of a sedimentary rock waspresented by Graham (1966), mostly confirmed andvery subordinately modified by Hrouda (1982) andKligfield et al. (1983), among others. The models ofthe AMS development in progressively strainedsedimentary rocks of the accretionary wedges weredeveloped by Hrouda (1982), Housen and van derPluijm (1991), Parés et al. (1999), and Aubourg et al.


(2004). Among them, that by Parés et al. (1999)appears to be the most illustrative, following the six-stage model of rock fabric arising from the ductiledeformation of compacted argillaceous sedimentelaborated by Ramsay and Huber (1983). For the pur-pose of our study, the deformation types are describedas follows.

Type 1: In sedimentary rocks unaffected by tectonicductile deformation, the magnetic foliation is alwaysoriented more or less parallel to the bedding, whilethe magnetic lineation is mostly roughly parallel tothe near-bottom water current directions determinedusing sedimentological techniques. Less frequently,the magnetic lineation may be perpendicular to thecurrent direction, which is typical of the flyschsediments of the lowermost A member of the Boumasequence. The degree of AMS is relatively low andthe AMS ellipsoid is in general oblate (e.g. Hamiltonand Rees, 1970).Type 2: During the process of diagenesis, theoriginally sedimentary magnetic fabric may beslightly modified due to the earliest deformationaccompanying this process. If the ductile deforma-tion is represented by vertical shortening due to theloading by the weight of overlying strata, the degreeof AMS and the oblateness of the AMS ellipsoidincrease, while the magnetic foliation and lineationretain their orientations (e.g. Lowrie and Hirt, 1987;Hrouda and Ježek, 1999). If the ductile deformationis represented by the bedding parallel shortening orby the bedding parallel simple shear or by both, thedegree of AMS decreases and the magnetic fabric,initially planar, becomes more triaxial or even linear.The magnetic lineation deviates gradually from thedirection of flow towards that of maximum strain,often creating a bimodal pattern, the magneticfoliation remaining near the bedding (e.g. Hroudaand Ježek, 1999; Parés et al., 1999; Aubourg et al.,2004).Type 3: The magnetic fabric of rocks that underwentslightly stronger deformation of the pencil structurestage is characterized by neutral to prolate AMSellipsoids, by the magnetic lineations groupedparallel to the pencil structure directions and by themagnetic foliation poles being either virtuallyperpendicular to the bedding or creating a minorgirdle perpendicular to the magnetic lineations. Thismagnetic fabric represents a combination of sedi-mentary and tectonic contributions.Type 4: The weak cleavage stage is characterizedby clearly visible cleavage in outcrops, by moderate

or even strong girdles in magnetic foliation poles,and magnetic lineation parallel to the bedding/cleavage intersection lines. The degree of AMS ismoderate and AMS ellipsoids are still prolate orneutral.Type 5: The magnetic fabric of rocks that undergonestrong deformation of the strong cleavage stage,typically developed in rocks with slaty cleavage, ischaracterized by relatively high degree of AMS,strongly oblate AMS ellipsoids, magnetic foliationswell parallel to the bedding and magnetic lineationsgrouped parallel to bedding/cleavage intersectionlines.Type 6: The stage of strong cleavage with stretchinglineation is characterized by parallelism of magneticfoliation to the slaty cleavage, and the magneticlineations perpendicular to the bedding. The degreeof AMS is strong and AMS ellipsoids are moderatelyoblate.

5. Results

5.1. Magnetic mineralogy

The knowledge of minerals controlling the magneticanisotropy is the essential part for the interpretation ofthe AMS of any rock. Various magnetic minerals maybuild different magnetic fabrics and respond differentlyto the progressive deformation. In rocks characterizedby low susceptibility, the AMS is predominantlycontrolled by the preferred orientation of paramagneticminerals, most frequently phyllosilicates (Borradaile etal., 1986; Rochette, 1987; Hrouda and Jelínek, 1990).As the magnetic susceptibility of the studied sedimen-tary rock does not exceed 460×10−6 SI (Table 1, Fig.3a), the dominance of paramagnetic minerals in thecontrol of the magnetic fabric is highly probable. Thesusceptibility distribution is bimodal having maximumfrequencies around 200×10−6 and 350×10−6 SI (Fig.3a). After a survey of individual specimens, the formermaximum is attributed to graywackes, while the later isascribed to mudrocks.

The mineralogical control of the magneticsusceptibility and its anisotropy was further studiedon representative specimens employing the magneticand non-magnetic analyses (Chadima et al., 2004).A combined approach showed that the paramagneticminerals are the dominant carriers of magneticsusceptibility and the AMS is predominantlycontrolled by the preferred orientation of chloritebasal planes (Chadima et al., 2004; Hansen et al.,2004).

Fig. 3. The distribution of (a) mean susceptibility, (b) anisotropy degree, and (c) the shape parameter for the entire set of the studied mudstones andgraywackes.


5.2. Magnetic fabric

The distribution of the anisotropy degree ranges fromalmost isotropic to the anisotropy degree as high as 28%(P=1.28), while the most frequent degree of anisotropyis only about 3% (Fig. 3b). The shape of the anisotropyellipsoid is widely variable. Generally, there are morespecimens possessing oblate anisotropy than prolate(Fig. 3c).

An indistinct positive correlation can be observedbetween bulk susceptibility and degree of anisotropy (Fig.4a). When the shape of anisotropy ellipsoid is plottedagainst the anisotropy degree (P–T plot), a ‘triangular’pattern is clearly seen: the ‘low-anisotropy’ specimens

Fig. 4. (a) Correlation between the mean susceptibility, km, and the degree of ashape of anisotropy ellipsoid, T, for the entire set of the studied mudstones a

possess both oblate and prolate shapes whereas the ‘high-anisotropy’ specimens are always oblate (Fig. 4b). Such adistribution of anisotropy degree and shape was alreadyobserved in the large sets of AMS data in low-deformedterrains (e.g. Hrouda, 1982). The anisotropy parametersof individual specimens reflect different states of thetransition from bedding-parallel to cleavage-parallelmagnetic fabric accompanied by an initial decrease ofanisotropy degree (e.g. Graham, 1966; Kligfield et al.,1983; Parés and van der Pluijm, 2003).

Regional variations of P, T, f and l for each site areplotted in the map of the studied area. In order to assessthe variation of AMS parameters within lithostrati-graphic formations in N–S direction, twelve different

nisotropy, P, (b) correlation between the degree of anisotropy, P, and thend graywackes.


regional groups were appointed. The individual siteswere grouped according the prevailing type of themagnetic fabric (see Chapter 4). The groups subdividethe N–S trending lithostratigraphic formations intoseveral sections (Fig. 5, Table 2).

Fig. 5. Magnetic fabric for the groups of the sampling sites (see Table 2). Thbedding coordinate system. The black squares, gray triangles, and open cdirections, respectively.

The average magnetic susceptibility varies within thestudied area (Fig. 6a). As the susceptibility is dependenton the lithology, the presented values partly reflect thedistribution of the major bodies of particular rock types(Fig. 2). In general, the magnetic susceptibility

e stereoplots are in the equal-area, lower-hemisphere projection in theircles represent maximum, intermediate, and minimum susceptibility

Table 2Groups of sites, number of specimens (N), location, sites included, and prevailing type of magnetic fabric as described in Chapter 4

Name N Location Sites Type

PRF2 75 Protivanov Fm. (South part) DV01, 28, 29, 45, 46, 47 4PRF3 144 Protivanov Fm. (Central part) DV06, 21, 41, 42, 43, 48, 54, 56, 67, 68, 69, 70 4–5PRF4 23 Protivanov Fm. (Northwest part) DV59, 60, 61 5ROF1 71 Rozstání Fm. (South part) DV04, 05, 25, 26, 27, 44 3ROF2E 42 Rozstání Fm. (East Central part) DV38, 39, 40 4ROF2W 36 Rozstání Fm. (West Central part) DV03, 30 3–4ROF3 67 Rozstání Fm. (North Central part) DV18, 19, 22, 23, 24, 57, 58 3–4ROF4 34 Rozstání Fm. (Northernmost part) DV07, 08, 17 4–5MYF1 139 Myslejovice Fm. (Southeast part) DV14, 15, 20, 31, 32, 37, 49, 50, 52, 53, 55 1–2MYF2E 47 Myslejovice Fm. (East Central part) DV12, 13, 33, 51 2–3MYF2W 43 Horizon between Rozstání and Myslejovice Fms. DV02, 34, 35, 62 2–3MYF3 46 Myslejovice Fm. (North part) DV09, 10, 11, 63 3Anomalous fabric 27 Myslejovice Fm. (Southernmost part) DV16, 36 –


indistinctly increases in the S–N direction withinindividual lithostratigraphic formations (Fig. 6a).

The anisotropy degree, P, is relatively high in thenorthern part of the Drahany Upland and decreases tothe south (Fig. 7a). Within the individual lithostrati-graphic formations the distinct increase of P values canbe observed from S to N (Fig. 6b). The anisotropy in theSE of the Drahany Upland is significantly low,possessing oblate shapes (Fig. 6b, c). On the otherhand, there are highly anisotropic mudrocks in thenorthern part of Myslejovice Fm. possessing distinctlyoblate anisotropy (Fig. 6b, c). The shape of theanisotropy ellipsoids of the specimens from the southernand central part of both the Protivanov and RozstáníFms. are neutral, ranging from a prolate to an oblatefield (Fig. 6c). The magnetic anisotropy of the speci-mens from the northernmost extents of both formationsis high and oblate in shape (Fig. 6b, c).

The regional distribution of the shape parameter T isnot that variable. Three indistinct belts are seenaccording to the predominant oblate or prolate anisot-ropy (Fig. 7b). The belts are running slightly obliquelyto the limits of the formations in the NNE–SSWdirection. The belt of predominantly oblate fabricspecimens covers the northwest and central part of theProtivanov Fm. and continues to the northern part of theRozstání Fm. The belt of predominantly prolate speci-mens starts in the southern part of the Protivanov andRozstání Fms., and continues through the Rozstání Fm.to the northern parts of the Myslejovice Fm. A third beltof predominantly oblate specimens covers almost all theMyslejovice Fm.

The angle f gradually increases from SSE to theNNW (Fig. 8a). The highest values are reached for thesites where the magnetic lineation is parallel to thecleavage plane (e.g. DV07, DV43, DV54, DV67).

The low values at the two sites of the NW part of theProtivanov Fm. (DV59 and DV61) can be explained bythe bedding/cleavage parallelism. Although the defor-mation is apparently high, the magnetic foliation doesnot deflect from the bedding plane.

The angle l shows the inverse trend as opposed to thef angle (Fig. 8b). The incipient deformation stage isusually sensitively recorded in the angle between themagnetic lineation and paleocurrent direction (Hroudaand Stráník, 1985). Since the paleocurrents could not beusually measured at the individual sites, the above-described angle cannot be expressed. Instead, the anglebetween the magnetic lineation and the strike of bedding(or bedding/cleavage intersection) was measured (l),following the idea of Aubourg et al. (2004) that theshortening direction is perpendicular to the strike of thebedding. The angle l can be employed for assessment ofthe early stages of deformation only. At sites with strongdeformation, the magnetic lineation is no longer parallelto the bedding/cleavage intersection and high l anglemay give misleading results. The l is high for the SE partof the Myslejovice Fm. and decreases to the NNW. Itshows similar values for the sites of the central part ofthe Myslejovice and the entire Rozstání Fm. The siteDV10 is shown as clearly undeformed. The lowestl values are reached for the site from the central andnorthern part of the Protivanov Fm. The most deformedsites (DV59 and DV61) show very high l values becausethe magnetic lineation is parallel to the dip of thecleavage plane.

The type of the magnetic fabric is progressivelychanging not only among lithostratigraphic formationsof the Drahany Upland but also within the individualformations (Fig. 5, Table 2). In the direction from the SEto the NW, the magnetic lineation starts to be parallel tothe bedding strike or bedding/cleavage intersection.

Fig. 6. Box-and-whisker plots of the mean susceptibility, km, the degree of anisotropy, P, and the shape parameter, T, for the groups of the samplingsites. For abbreviation see Table 2. The mean value is represented by the median being the central line in the central box. The central box covers themiddle 50% of the data values, between the lower and upper quartiles. The “whiskers” extend out to extremes (minimum and maximum values), butonly to those points that are within 1.5 times the interquartile range.


Later, the magnetic foliation rotates from the bedding-parallel orientation to the orientation parallel to theevolving cleavage. This rotation is accompanied by theobserved decrease of the anisotropy degree and theprolate nature of the anisotropy ellipsoids. Whenmagnetic foliation becomes parallel to the cleavage,the degree of anisotropy again increases and theanisotropy ellipsoid becomes again oblate.

6. Discussion

In general the progressive transition from thebedding-parallel to the cleavage-parallel magnetic fabriccan be attributed to the mechanical reorientation orkinking of the large phyllosilicate grains (Rathore,1979) or to neo-crystallization (Housen and van derPluijm, 1991). Since the degree of deformation can beregionally correlated with the anchimetamorphic zones,

we may assume that the growth of new phyllosilicatestook place not only during the diagenesis but also duringthe subsequent deformation. This process furtherenhanced the deformational magnetic fabric. It wasdemonstrated that during the course of the progressivedeformation of argillaceous sediments, the mechanicalprocesses (grain kinking and rotation) are favoured inrelatively low-energy environments, whereas chemicalprocesses (grain dissolution and neocrystallization) arefavoured in relatively high-energy environments (vander Pluijm et al., 1998). As the grade of anchimeta-morphism of the Drahany Upland is very low, we mayassume that mechanical reorientation and kinking wasthe dominant process during the origin of thedeformational magnetic fabric (Fig. 3 in Chadima etal., 2004). Moreover, it appears that the phyllosilicatefabric in the graywackes reflects the cleavage orienta-tion faster than the phyllosilicate fabric in mudrocks.

Fig. 7. The regional variations of the arithmetical means of the AMS parameters expressed as grayscale symbols together with values for each site plotted in the map of the Drahany Upland: (a)anisotropy degree, P; (b) the shape parameter, T. The bold lines depict the outlines of the groups of the sampling sites (see Table 2).

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Fig. 8. The regional variations of the arithmetical means of angles between principal AMS directions and bedding plotted in the map of the Drahany Upland: (a) angle between the magnetic foliationand bedding, f angle; angle between the magnetic lineation and the bedding strike direction (possibly bedding/cleavage intersection or fold axis), l angle. Note that the above angles are not applicablefor the sites where no bedding plane was observed mesoscopically (see Table 1).

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As the graywackes have not undergone as muchvertical compaction as mudrocks, they easily reflecttectonic shortening in their fabric by means of grainrotation.

In the southern part of the Myslejovice Fm., almostno deformation or very weak bedding parallel shorten-ing in the NW–SE direction is observed. Thisdeformation is reflected only in some specimens bythe rotation of the magnetic lineations into the NE–SWdirection, i.e. perpendicular to the shortening (Fig. 5).

In the central part of the Myslejovice Fm., weobserve some shortening in the NW–SE direction.Although this deformation was probably accompaniedby some volume loss, the shortening had to becompensated by an extension in the NE-SW direction.

The situation observed in the northern part of theMyslejovice Fm. is more complex. The evolution of themagnetic fabric would have required a very high amountof vertical compaction. Moreover, the rocks from thisarea possess the highest bulk susceptibility values in theset of studied mudrocks and graywackes. The original‘uncompacted’ sediment in that area probably containeda high amount of clay minerals. During the process ofcompaction, these minerals were transformed intochlorites and micas leading to the high volume loss aswell as to the increase of the bulk susceptibility. Theresulting magnetic fabric is oblate, highly anisotropicand bedding-parallel. Such fabric was very difficult tobe overprinted by any subsequent deformation. Despitethat fact, the observed N–S lineation probably reflectsan incipient deformation represented by the W–Eshortening (Fig. 5).

The strain represented by bedding-parallel shorteningincreases from the south to the north in the rocks of theRozstání and Protivanov Fms. The W–E to NW–SEshortening was compensated by an extension either inthe N–S to NE–SW directions or in the verticaldirection. The assumed horizontal NNE–SSW simpleshear connected with a lateral shortening may have ledto a transpressional regime. Both horizontal (parallel tothe simple shear direction) and vertical magneticlineations may have evolved during a transpressionalregime (Ježek and Hrouda, 2002). The development ofthe magnetic fabric could not give supporting evidenceeither for the lateral shortening or to the dextralhorizontal simple shear. The assumed transpressionaldeformational regime is a combination of the WNW–ESE lateral shortening and the NNE–SSW horizontalsimple shear. The ellipsoids of the magnetic fabricevolved in both deformation regimes would have thesame orientation. The magnetic foliation would bevertical striking NNE–SSW. The magnetic lineation

would be sub-horizontal trending NNE–SSWor verticalif lateral shortening was predominant. Both deforma-tions, horizontal simple shear and lateral shortening,probably acted together in the studied area. If no volumeloss during the deformation was allowed for, the NNE–SSW simple shear deformation accommodated thenecessary extension generated by WNW–ESE lateralshortening.

In terms of magnetic fabric types introduced forsediments of accretionary wedges by Parés et al. (1999),the rocks of the Myslejovice Fm. show the type 1possessing virtually undeformed sedimentary magneticfabric, type 2 indicating the earliest deformation, andrarely type 3 with pencil structure magnetic fabrics. Inthe Rozstání and Protivanov Fms the type 1 is present inturn only rarely, while the types 2 and 3 are quitefrequent. In some localities, even type 4 is presentindicating relatively strong deformations associatedwith formation of slaty cleavage.

The deformation undergone by the rocks of accre-tionary wedges during subduction process can beconcisely characterized in the way as described byMoores and Twiss (1995). The deformed rocks of theaccretionary wedges are mostly sediments derived fromeither overriding or downgoing plate. The mode of flowbetween these plates may depend largely on the relativerates of sediment supply and removal. If the supply ofsediment is less than or equal to the capacity of thesubduction zone, the sediment entering the subductionzone may be all subducted, partly subducted and partlyunderplated or partly offscraped and frontally accreted,partly subducted, and partly underplated. If the sedimentsupply exceeds the maximum capacity of some chokepoint along the subduction zone, part of the sedimentthat reaches the choke point must double back and flowup the subduction zone. This flow may reach the surfaceand expose high-pressure metamorphic rocks. Thepattern of sediment flow at the inlet to the subductionzone significantly affects the evolution of the accretion-ary wedge. As sediment is dragged toward thesubduction zone inlet, it encounters the resistance andshearing imposed by the overriding plate, and it shortensand thickens, largely by thrust faulting.

The magnetic fabric in rocks of the Drahany Uplandranges from almost purely sedimentary to partiallydeformational in origin, corresponding to that of theundeformed stage or the earliest deformation stage orthe pencil structure stage. In addition, there is relativelyinfrequent occurrence of the cleavage in the region andthe rocks show signs of only weak anchimetamorphicchanges. All these observations indicate that during thesubduction process there was virtually no or very


limited return flow. The capacity of the subduction zonewas evidently at least equal to or larger than the supplyof the sediment that was subducted, partly offscrapedand accreted frontally and may have been partlyunderplated. This observation is in contrast with thatin the Nízký Jeseník Mts. where also stronglyanchimetamorphosed rocks with strong slaty cleavagepassing into metamorphic schistosity are present(Hrouda, 1979). These rocks correspond to types 5and 6 and with no doubt indicate that a return flow hadoccurred.

In the fossil accretionary wedges the zones of similardeformation grade and anchimetamorphism are ingeneral oblique to the stratification (e.g. Merriman andFrey, 1999). This fact may explain the differencebetween the Nízký Jeseník Mts. and the DrahanyUpland where the equivalent lithostratigraphic forma-tions (c.f. Hartley and Otava, 2001) are relativelyweakly deformed in the Drahany Upland compared tothe higher deformation evidenced in the Nízký JeseníkMts.

7. Conclusions

The main carriers of the magnetic susceptibility inthe Lower Carboniferous rocks of the Czech RHZ areparamagnetic minerals, i.e. iron-bearing chlorite andmica. The magnetic anisotropy of the studied rocks ispredominantly controlled by the preferred orientation ofchlorite basal planes.

Based on the development of magnetic fabric thedegree of deformation increases from the SE to the NWof the Drahany Upland. This result corresponds to theSE–NW increasing degree of anchimetamorphismderived from the illite crystallinity and vitrinitereflectance data (Franců et al., 1999). While the SEpart of the Myslejovice Fm. is virtually undeformed,the magnetic fabric in the central part of theMyslejovice Fm. reflects some WNW–ESE beddingparallel shortening. The magnetic fabric in the northernpart of the Myslejovice Fm. is rather anomalous,strongly influenced by a high diagenetic compaction.Such compacted sediment is not suitable for reflectingthe initial stage of deformation. Despite this fact, a tightgrouping of magnetic lineations in the N–S directionwas observed reflecting some bedding parallel short-ening in the W–E direction. The magnetic fabric of theRozstání and Protivanov Fms. progressively developsfrom the south to the north. The magnetic foliationprogressively rotates from the bedding-parallel tocleavage-parallel. The N–S to NNE–SSW trendingmagnetic lineation is parallel to the bedding/cleavage

intersection, pencil structures or the fold axes on aregional scale.

The SSE–NNW increasing trend of deformationcontinues to the Lower Carboniferous flysch rocks ofthe Nízký Jeseník Mts., representing the northern partof the same accretionary wedge. This is in accor-dance with the oblique trends of the similardeformation grade and anchimetamorphism as gener-ally observed in sedimentary rocks of the accretion-ary wedges.

Regarding the mechanism of subduction it seems thatthe sedimentary sequences were subducted, partlyoffscraped and accreted frontally and may have beenpartly underplated in the case of the present dayDrahany Upland as opposed to the Nízký Jeseník Mts.where some return flow must have occurred.

Acknowledgement

The authors would like to express the specialthanks to the people who assisted during the fieldwork. The list is very long and cannot be, unfortu-nately, fully presented. The previous magnetic anisot-ropy data were collected and kindly provided by JiříKos. The field work, laboratory measurements andattendance at conferences was mainly financed byAgico, Inc., Brno, Czech Republic. The attendance atthe EGS 2003 Meeting was supported by the HlávkaFoundation, Prague. The comments and suggestions ofan anonymous reviewer considerably improved thepresent paper. The research was partly carried out inthe framework of the Research Plan of the Institute ofGeology, Academy of Sciences of the Czech Republic,#AV0Z30130516.

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magnetic fabric study of the se rhenohercynian zone (bohemian massif): implications for dynamics of...

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