Parallel magnetic fabrics in metamorphic, granitoid and sedimentary rocks of the Branisko and Čierna hora Mountains (E Slovakia) and their tectonometamorphic control

Physics of the Earth and Planetary Interiors, 51 (1988) 271—289 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands

271

Parallel magnetic fabrics in metamorphic, granitoid and sedimentary rocks of the Branisko and Cierna hora Mountains (E Slovakia) and their tectonometamorphic control Franti~ekHrouda 1, Stanislav Jacko 2 and Jaromir Hanák

1

Geofyzika, National Corporation, Brno (Czechoslovakia) 2

Department of Geology and Mineralogy, Mining Faculty, Technical University, Koiice (Czechoslovakia) (Received January 12, 1986; revision accepted January 30, 1987)

Hrouda, F., Jacko, S. and Hanák, J., 1988. Parallel magnetic fabrics in metamorphic, granitoid and sedimentary rocks of the Branisko and Cierna hora Mountains (E Slovakia) and their tectonometamorphic control. Phys. Earth Planet. Inter., 51: 271—289. The magnetic anisotropy of rocks from the Branisko and t~ierna hora Mountains (West Carpathians) was investigated, and the ferromagnetic and paramagnetic components were separated and found to be roughly coaxial. The magnetic fabrics in metamorphic, granitoid, and sedimentary rocks display very similar patterns of principal susceptibilities. Metamorphites underwent a polyphase prograde and retrograde metamorphism, granitoids were metamorphosed retrogressively, and sedimentary rocks suffered a very weak prograde metamorphism. Retrogressive and progressive Alpine metamorphism and the formation of magnetic anisotropy took place simultaneously, before, during, and after the emplacement of the Central West Carpathian nappes.

1. Introduction During investigation of the magnetic fabric in the rocks of the Cierna hora and Branisko Mountains, an unusual phenomenon has been found, The magnetic fabric patterns in metamorphic, granitoid and sedimentary rocks are very similar to one another, even though the rocks are obviously different both in age and in origin. The purpose of the present paper is to find the reason for this phenomenon, its relation to a polyphase development of the present structural pattern and to check the possible usefulness of such a test in solving some structural problems of the Central West Carpathians.

2. Geological setting and problems The Branisko and Cierna hora Mountains form the most eastern part of the West Carpathian 0031-9201/88/$03.50

© 1988 Elsevier Science Publishers B.V.

Internides. They are traditionally grouped with their northern morphostructural unit, i.e., the Core Mountains range. This opinion is based on a typical Core Mountains structural feature, i.e., the successive positions of a crystalline complex (cornposed of granitoids and metamorphites), overlain by an Upper Paleozoic and Mesozoic sedimentary cover sequence and by the Inner Carpathian nappes. This vertical succession is present in both regions. The Branisko and Cierna hora pre-Tertiary complexes are surrounded by the Intracarpathian Paleogene and Neogene sediments in the west and north and in the east, respectively. A very cornmon horst-type uplift of these complexes between Tertiary basins is controlled by Late Tertiary NE—SW, N—S and E—W fault sets. The southwestem border towards the Gemeric unit is overprinted by the Margecany thrust zone. A peculiar feature of the Branisko and Cierna hora Mountains within the Central West

272

Carpathians results from their wider crystalline basement relationships. The Branisko crystalline suite is correlated with the Tatric while the Cierna hora suite is correlated with the Veporic principal tectonic zone of the Inner Carpathians (Andrusov et al., 1973). Considering the direct contact of the Qerna hora formations with the Gemeric unit, and a very probable continuation of its southwestem crystalline unit (the Bujanova complex) into the Králóvá holá unit of the Central Veporides (Jacko, 1975, 1978) on the one side, and a parallelism between the Branisko basement and the bumbier crystalline complex (Rosing, 1947; Polák and Vozár, 1985) on the other, a remarkable shortening (at least four times) of the Cierna hora Veporic-type basement results. According to current knowledge this extensive space reduction is mainly the product of the Alpine tectonometamorphic events. Other very closely related tectonic problems are connected with the variable degree of Alpine metamorphism of the Upper Paleozoic sequences of the Branisko and Cierna hora suites, leading to their fundamentally different paleogeographical and paleotectonic parallelism. Thus the Upper Carboniferous sediments in the central part of the Cierna hora Mountains are regarded as the cover formations (Fusán, 1960; Jacko, 1975), while in the western part they are regarded as the Gemeric nappe (Rosing, 1947; Mahel’, 1953). The same complex paleotectonic relationship also concerns the tuffaceous Permian molasse sediments which are considered to be an integral part of a cover sequence (Fusán, 1960; Jacko, 1975) or to be partly a member of the supposed Kri~nánappe (Rosing, 1947). All the problems mentioned do not only concern the understanding of the structure and tectonometamorphic history of the Branisko and Cierna hora regions. They also directly influence a precise interpretation of the Inner West Carpathian region as a whole.

3. Lithostructural outline of the Branisko and Oerna hora Mountains The lithostratigraphical complexes of the Branisko Mountains and the Cierna hora Moun-

tains have been transformed into E—W and NW—SE megastructural units, respectively, by the Alpine deformations. The antiformal units of both regions principally consist of Early Paleozoic metamorphic and granitoid rocks. The synformal units rimming the limbs of the antiforms are filled with Late Paleozoic and Mesozoic sedimentary formations (Fig. 1). The Branisko crystalline complex is characterized by a predominance of metamorphic rocks over granitoids and by a relatively low grade of Alpine dynamometamorphism. The metamorphic rocks, originally ranging from biotite paragneisses to garnet—biotite paragneisses, are sporadically (in the south) intercalated with amphibolites. The Sequence was strongly migmatitized during Variscan contact metamorphism. Gramtoids are represented by two principal varieties: coarse- to medium-grained biotite granodiorite and finegrained aplitic granite. The Cierna hora crystalline complex differs from the Branisko complex in the following three aspects: (1) in the considerably higher variability of Variscan metamorphic and plutonic products, (2) in extensive Alpine polyphase dynamometamorphism of regional, but selective, character, (3) by a penetrative (nearly complete) Alpine fold and ruptural overprinting of the original Variscan fabric (Fig. 2(a), (b)). Metamorphic rocks of the Cierna hora crystalline complex are represented by augen migmatite and stromatite—nebulite migmatites, biotite gneisses and biotite—garnet gneisses (sporadically intercalated with thin amphibolite layers), and garnet—mica schists and staurolite—garnet—mica schists. They originated during Variscan metamorphism, but also underwent regional retrogressive metamorphism and phyllonitization (in alternating NW—SE zones) during Alpine tectonometamorphic events (Fig. 2(a)). Granitoids are represented by medium-grained biotite granodiorite and fine-grained to medium-grained biotite granodiorite often affected by strong deformation. The Late Paleozoic sequence begins in the Branisko and Cierna hora Mountains with weakly dynamometamorphosed Upper Carboniferous psammites, pelites and conglomerates. The Permian sequence consists of greywackes, interbe-

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the better known ~ierna hora Mountains is the product of the variable overprinting of at least nine tectonometamorphic phases, seven of which were of a syntectonic nature (Jacko, 1975, 1978, 1979). Because three of the phases have produced fold structures (Fig. 2(a)) of regional extent and of variable orientation, it can be expected that some magnetic fabrics will preserve, at least to a certain degree, the relict fold pattern of the synmetamorphic fold set. This fold pattern may have been completely overprinted by the successively younger Alpine penetrative schistosity associated with the Margecany thrust zone. In any structural conclusions drawn from the magnetic fabric it is as-

sumed, for the reasons mentioned, that spatial orientation, distribution and the successive development of penetrative tectonometamorphic structures could influence, at least partially, the present space pattern of magnetic minerals in all formations of the regions. Our current knowledge on these problems is briefly summarized for the ~ierna hora region in Fig. 2(a). With respect to our preliminary results, the succession and general features of the Branisko tectonometamorphic development are essentially comparable with those of the ~ierna hora. The only exception is the predominance of an E—W post-nappe fabric in the northern part of the Branisko Mountains.

277

(c)

(d)

Fig. 2(c). Orientations of principal Alpine mesostructures in the central part of the ~ierna hora Mountains. Contours of 2%, 10%, 20%, 35% are constructed from 26 S 3 subdomarn maxima obtained by statistical evaluation of 753 individual S3 planes. The pole of the S3 great circle dispersion ($S3) coincides with local reverse-slip faulting in relatively young mesodislocations of the S3 set. F3—F5: general fold axis Orientations due to D3 —D5 deformation stages. (d) Distribution of S~planes in one domain of the slightly reworked Alpine subdomains of the metamorphic rocks of the Bujanova complex, indicating the presence of tight mesoscopic folds within the subdomain area. Contours at 1, 2, 5, 9, 12 per cent. 118 SI~planes. Dashed great circle: axial plane subdomain position. Equal-area projection on lower hemisphere.

For the Cierna hora region we should like to stress the following: (1) A zone of D2 structures at least 400 m deep (cf. Fig. 2a) in the basement metamorphites. (2) Nearly complete reorientation of pre-D3 fabric into the D3 structures. (3) The evidently younger age of the Margecany-type thrust zones compared with the Gemeric unit thrust onto various stratigraphic formations of the Cierna hora region (Jacko, 1979). The problem of the leading role played by the post-nappe D3 structures in the present fabric of the Cierna hora region should be considered with respect to the orientation and development of older representative structures. From this point of view a hereditary influence on the Variscan fabric of the oldest D2 Alpine structures is very probable. This influence is hard to explain in the sense of directional analogy only. The analogy seems to be the result of an adaption of the Variscan structures to a proper (i.e., nearly recumbent) orientation of the early Alpine structures. This structural interference was the main cause of a common, penetrative, vertical cleavage set (also present in the basement rocks) which developed during the

emplacement of the Inner Carpathian nappes. This new and regionally penetrative cleavage set of mm—cm scale was the first stage (and a catalyst as well), not only for the regional development of D3 structures, but also for their uniform style regardless of their formation in a compositionally heterogenous environment. Taking into consideration the high readaption ability of magnetic minerals in the new stress field, it is not expected to find anything other than the Alpine orientation in the strongly reworked basement rocks.

5. Data presentation Magnetic anisotropy was measured with the KLY-2 AC bridge developed by Jelinek (1980) and computed using the ANIS0 11 program (Jelinek, 1977). In order to obtain a statistical evaluation of the magnetic anisotropy at individual localities, recourse was taken to the ANS 21 computing program (Jelinek, 1978), which enables a complete statistical evaluation of a group of symmetric second rank tensors to be carried out.

278

The results of the low-field magnetic anisotropy measurements are summarized in Tables I—Ill and in Figs. 3—11. In the tables, the first column contains the locality numbers (as presented in Fig. 1), the second the arithmetical means of the mean magnetic susceptibility, Km (K1 + K2 + K3 )/3, =

where K3 ~ K2 ~ K3 are the principal susceptibilities; the Km values are given in the order of 10—6 (SI units). In the third and fourth columns there appear pairs of values of the degree of magnetic anisotropy, H 100(K1 K3)/Km, and of the shape factor, U (2K2 K1 K3)/(K1 K3) (see JelInek, 1981). If U= +1, the magnetic fabric is perfectly planar, if 0 < U < 1, it is planar, if U 0, the magnetic fabric is linear—planar, if —1< U<0, it is linear, and if U= —1, the magnetic fabric is perfectly linear. The values of the H and U parameters given in the upper row are the arithmetical means of the values for individual specimens, while those given in the lower row represent the parameters derived from the mean susceptibility tensor for each locality as a whole (determined by averaging out the individual cornponents of the specimen susceptibility tensors in the geographical coordinate system and calculated

TABLE II Parameters of magnetic anisotropy of granitoid rocks from the ~ierna hora and Branisko Mountains Location No.

Km

H

(106)

(%)

t~iernahora Mountains 18 185 19

362

20

166

21

223

26

85

28

199

29

242

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144

Branisko Mountains 3 50 5

201

15

100

U

Type

8.4 7.0

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13.8 12.5 9.0 7.3 4.7 2.7 2.1 1.7 6.9 4.2 10.5

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6.2 4.3 6.8 5.5

0.19 0.44 0.76 0.67

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0.36 —0.34

lIb IIIc lIb IlIb lla lIla

Ic llc IVc

TABLE I Parameters of magnetic anisotropy of metamorphic rocks from the áerna hora and Branisko Mountains

__________________________________________ Location No.

Km (106)

~ierna hora Mountains 10 146 11

113

12

797

13

24

16

156

17

252

Branisko Mountains 2 382 4

339

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lIla Ic

_______________________________________________

with the ANS 21 program). In order to distinguish in the text between the anisotropy parameters for individual specimens and those derived from the mean tensor for a locality as a whole, the former will henceforth be called the specimen parameters, while the latter the locality parameters. In the fifth column the anisotropy type is mdicated, as introduced by Kligfield et al. (1977) for a comprehensive characterization of magnetic anisotropy at a locality. The distribution pattern of principal susceptibilities is characterized by a Roman numeral (I: only K3 directions concentrated, II: all three principal directions concentrated, III: only K1 directions concentrated) and the shape of the magnetic fabric by a letter suffix ((a): almost all specimens with linear magnetic fabrics, (b): some with linear and some with planar fabrics, (c): almost all with planar magnetic fabric). The localities where the directions of the principal

279 TABLE III Parameters of magnetic anisotropy of sedimentary and volcanic rocks from the ãerna hora and Branisko Mountains Location No.

Km (106)

~ierna hora Mountains 24 65

0.29 0.12 0.18

lb

purpose, the dependence of the acquisition remanent magnetization (IRM) on the magnetizing field was investigated. Each specimen was magnetized in a field of 20, 40, 60, 80, 100, 200, 400, 600, 800 and 1000 mT and its remanent magnetization was measured. The results are given in Fig. 3(a)—(c),

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sedimentary rocks. From each locality, one specimen was selected; in Fig. 3 only the number of the

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82

27

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236

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272

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320

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178

U

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Branisko Mountains 1 96

represented solely by magnetite. For this reason, before using it, we have to identify the mineral representing the ferromagnetic fraction. For this

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locality is given for a curve. It can be seen in Fig. 3(a), (b) that in metamorphic and granitoid rocks the IRM curves rise very rapidly and in fields of 200 mT they are more or less saturated. This behaviour is typical of magnetite (Kligfield et al., 1977; Zapletal, 1983), and, consequently, a magnetite-type mineral may be regarded as the representative of the ferromagnetic fraction in most —

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susceptibilities are so scattered that they are virtually isotropic are denoted as IV.

6. Magnetic mineralogy The magnetic susceptibility of the rocks investigated in the Cierna hora and Branisko Mountains is relatively low, of the order of iO~ (see Tables I—Ill). In these rocks the magnetic anisotropy may be controlled not only by ferromagnetic minerals, but also by paramagnetic mafic silicates (biotite, hornblende) and phyllosilicates. The contribution of the ferromagnetic and paramagnetic fractions to the total anisotropy can be found through the measurement of the low-field and high-field anisotropy (Hrouda and JelInek, 1985). However, this method is confined only to the rocks in which the ferromagnetic fraction is

metamorphic and granitoid rocks of the Cierna hora and Branisko Mountains. However, in some specimens the curves do not show perfect saturation in fields of 200 mT and the presence of hematite and/or goethite can be suspected. The question is what would be the influence of these minerals on the component separation, if these minerals were present? This was investigated through the detailed study of the torque curves in the range 650—850 mT for selected specimens. In this range, the curves of the torque versus field squared are represented by straight lines suggesting that, if hematite and/or goethite are present, their amount is so small that they cannot influence the separation in a significant way. On the other hand, as can be seen in Fig. 3(c), the IRM curves for the majority of specimens of the sedimentary rocks investigated rise relatively slowly and do not seem to saturate even in the strongest field used, viz. 1000 mT. This type of curve is characteristic of hematite which can be considered as the representative of the ferromagnetic fraction for the majority of sedimentary rocks. Only at localities 6 and 23 are curves of the magnetite type present. In selected specimens of rocks with the ferromagnetic fraction represented by magnetite, the

280

resolution of the total anisotropy into the ferromagnetic and paramagnetic components was made, using the method of Hrouda and Jelinek (1985). This method consists of the measurement of the magnetic anisotropy in the low field and in two high fields higher than the saturation field of magnetite. The results are summarized in Table IV and Fig. 4. In the table, the specimen number is given in the first column and the mean susceptibility in the second. In the third to fifth columns, the values of the U parameter are given as the total anisotropy (UT), as the ferromagnetic fraction (UF) and of the paramagnetic fraction (Un). The sixth column contains the values of the ratio, p (K~1 =

K~3)/( KF1 KF3), where K~1and K~3are the maximum and minimum susceptibilities of the paramagnetic fraction, respectively, and KF1 and KF3 are the maximum and minimum susceptibili—

—

ties of the ferromagnetic fraction, respectively. It can be seen in Table IV that, in individual specimens of granitoid rock, the values of the UT, UF and U~parameters are similar, indicating similar shapes for the magnetic fabrics of the total specimen, the ferromagnetic fraction and the paramagnetic fraction. The ratios are mostly p> 1, which means that the paramagnetic fraction contributes more to the total anisotropy than the ferromagnetic fraction. In the specimens of metamorphic rock, the shapes of ferromagnetic and paramagnetic ellipsoids sometimes agree and sometimes differ. The ratios are again mostly positive, but sometimes also negative. The different ellipsoid shapes in some specimens can be accounted for by a different response of the original metamorphic ferromagnetic and paramagnetic fabrics to the later deformations which are prob-

Aim

(a)

101 /2

101

~

15

101

22

200

1,00

500

800

1000

rnT

Fig. 3. Dependence of the acquisition remanent magnetization on the field. (a) metamorphic rocks, (b) granitoid rocks, (c) sedimentary and volcanic rocks.

281

Aim 10i

(b)

100

21

1~1

200

L~O0

20

600

800

1000

mT

Fig. 3. (continued).

TABLE IV Results of the resolution of magnetic anisotropy into ferromagnetic and paramagnetic components Specimen No.

Km

UT

UF

U~

p

Metamorphic rocks ~HB 11/4/1 ~HB 13/5/1 ~HB 12/3/1 ~HB 10/3/1 L~HB17/2/1 1.~HB 2/4/1 L~HB 4/4/1

150 11 213 187 343 345 350

—0.98 0.18 —0.42 0.24 0.45 —0.65 0.77

0.72 —0.14 —0.42 0.19 0.44 —0.30 0.57

—0.84 0.28 0.88 0.27 0.45 —0.69 0.72

1.8 3.3 0.5 1.7 2.5 0.9 3.1

Granitoid rocks ~HB 26/2/1 L~HB18/3/1 ~HB 29/3/1 ~HB 30/2/1 ~HB 19/3/1 t~HB20/3/1 t~HB 3/5/1 t~HB5/4/1 ~HB 15/4/1

64 271 248 196 348 222 78 186 120

0.36 —0.55 —0.28 —0.77 0.38 0.13 0.25 0.75 0.58

1.5 2.4 1.5 3.6 0.4 1.6 3.7 1.7 1.8

0.20

0.42

—0.55

—0.54

—0.30 —0.28 0.28 0.39 0.17 0.48 0.06

—0.27 —0.47 0.53 —0.01 0.12 0.84 0.79

282 Aim

(c)

101

:

~‘i~~i 200

400

600

800

1000

ml

Fig. 3. (continued).

ably associated with retrogressive metamorphism. Figure 4 shows the orientations of the principal susceptibilities of the ferromagnetic and paramagnetic fractions. It can be seen that the orientations do not differ excessively and the paramagnetic and ferromagnetic fabrics may therefore be regarded as approximately coaxial. One may conclude that the total magnetic anisotropy of metamorphic and granitoid rocks is due to both paramagnetic and ferromagnetic fractions oriented coaxially, though the paramagnetic fraction is stronger in most specimens. As the discussion of the total anisotropy measurements is mostly based on the principal directions, which are roughly coaxial in both fractions, it seems to us that it is not necessary to carry out the laborious component separation for each specimen.

7. Magnetic fabric in metamorphic rocks The results of magnetic anisotropy investigation are summarized in Table I and Figs. 5 and 6. As can be seen in the table, the degree of magnetic amsotropy is moderate, ranging from 5% to 16%. The type of magnetic anisotropy is very variable. However, in the majority of localities, the specimen magnetic fabric is either planar or mixed (planar in some specimens and linear in others). On a locality scale, both the magnetic foliation and the magnetic lineation are developed at some localities, while at other localities only the magnetic lineation is well oriented and the magnetic foliation poles tend to create a girdle. In general, the magnetic foliation is oriented similar to the metamorphic schistosity.

283 N

mostly oriented NW—SE though there are a small number of specimens showing the magnetic lineation oriented NE—SW (Fig. 5(b)).

~ /

/ // ~

/

\

I

~

~

schistosity In the Branisko poles andMountains, the magnetic thefoliation metamorphic poles also imperfect girdles; these are oriented N—S create to NNW-SSE and the girdle of magnetic

/

S

/

foliation poles is better developed than that of metamorphic schistosity poles (Fig. 6(a)). The magnetic lineation is mostly oriented NE—SW, only in three specimens it is roughly perpendicular (Fig. 6(b)). The magnetic anisotropy patterns described above imply that the magnetic fabric was probably generated during the process of metamorphic schistosity formation. As the rocks investigated are metamorphosed retrogressively to various de-

A

/ //

Fig 4. Orientations of principal susceptibilities of paramagnetic (open symbols) and ferromagnetic (closed symbols) fractions in selected specimens of the crystalline rocks of the ~ierna hora Mountains. 0 maximum susceptibility, ~ intermediate susceptibility, 0 minimum susceptibility.

grees, and the metamorphic schistosity is mostly marked by new chlorite growth, it is probable that the magnetic fabric generated during ductile deformation is associated with retrogressive metamorphism.

In the ~ierna hora Mountains both the metamorphic schistosity poles and the magnetic foliation poles create well-developed girdles oriented NE—SW (Fig. 5(a)); the magnetic lineation is

8. Magnetic fabric in granitoid rocks

N

The results of magnetic anisotropy investigation are summarized in Table II and Figs. 7 and 8.

()

(b)

‘S

~

• 05S

—~

/

•s

S

_

N

KIN_ I \H •

N

F,

S

•

:

..

x

‘a

•

~

~

(

///~ ~•

•

•

.5

\~

•s

N

\ •s•

.•G

•

•

F,

~//

. 3 182 03 04

N~

5

•jV .5/

Fig. 5. Orientations of (a) magnetic foliation poles, and (b) magnetic lineation, in metamorphic rocks of the (~iernahora Mountains. Equal-area projection on lower hemisphere. Legend: (1) magnetic foliation pole (in (a)) or magnetic lineation (in (b)), (2) metamorphic or cataclastic schistosity pole, (3) bedding pole, (4) cleavage pole.

284

S

Fig. 6. Orientations of (a) magnetic foliation poles, and (b) magnetic lineation, in metamorphic rocks of the Branisko Mountains. For legend see Fig. 5.

The degree of magnetic anisotropy is moderate, ranging from 2% to 14%. The type of magnetic anisotropy is even more variable than that in metamorphic rocks. The specimen magnetic fabric is mostly planar, but it is also linear at some localities and mixed at other localities. At some

_

_ •

5

F,.

•5 .5 •

‘.

/1~__

F, •

/~5 ,S

••

•

•

~ ••

I

-~i-:~ ~

S

-

•

•~•

localities, both the magnetic foliation and the magnetic lineation are well developed on a locality scale. However, there are also localities where only the magnetic lineation is well oriented while the magnetic foliation poles tend to create a girdle. In the localities showing mesoscopic foliation (catac-

/

/ /

•

\•

•/

S

:

~

5•

/

Fig. 7. Orientations of (a) magnetic foliation poles, and (b) magnetic lineation, in granitoid rocks of the c~iernahora Mountains. For legend see Fig. 5.

285

5

.

S

.•

//

S

Fig. 8. Orientations of (a) magnetic foliation poles, and (b) magnetic lineation, legend see Fig. 5.

lastic to metamorphic schistosity), the magnetic foliation is often near this mesoscopic foliation, In the ~iema hora Mountains the magnetic foliation poles create a relatively well-developed girdle oriented NE—SW, while the mesoscopic foliation poles create a large cluster and not a girdle (Fig. 7(a)). The magnetic lineation is oriented NW—SE in the majority of specimens, though it is even perpendicular in some rare specimens (Fig. 7(b)). In the Bramsko Mountains, both the magnetic foliation poles and the mesoscopic foliation poles tend to create a very imperfect girdle oriented N—S (Fig. 8(a)). The magnetic lineation also creates a girdle, oriented roughly E—W (Fig. 8(b)). The fact that the degree of anisotropy in the granitoid rocks is comparable with that in metamorphic rocks, the girdle pattern in magnetic foliation poles and the approximate parallelism of the magnetic to the mesoscopic foliation, all testify against the flow (intrusive) origin of the magnetic fabric in granitoid rocks of the Cierna hora and Branisko Mountains. On the contrary, the girdle pattern in magnetic foliation poles, the conformity of the magnetic fabric in granitoid rocks to that in metamorphic rocks, and the existence of the deformation indicators imply that it was probably

in

•

•

S

• •

5/;!!!

grarntoid rocks of the Branisko Mountains. For

ductile deformation that formed the magnetic fabric in granitoid rocks.

9. Magnetic fabric in sedimentary and volcanic rocks The results of the investigation of magnetic anisotropy are summarized in Table III and Figs. 9—11. The degree of magnetic anisotropy in sedimentary rocks, as presented in Table IV, is low; only at one locality in the NE Gemeric unit is it very high. The type of magnetic anisotropy does not vary much; mostly it is represented by a planar specimen magnetic fabric and well-developed magnetic foliation and lineation on a locality scale. In volcanic rocks investigated in the NE Gemeric unit, the degree of magnetic anisotropy is moderate, the specimen magnetic fabric is linear to mixed, and both the magnetic foliation and the magnetic lineation are well developed on a locality scale. In sedimentary rocks of the Cierna hora Mountans, the magnetic foliation poles create an imperfect girdle oriented NE—SW connecting the cluster of bedding poles with that of cleavage poles (Fig. 9(a)). The magnetic lineation is subhorizontal and

286 N

Ib)

Fig. 9. Orientations of (a) magnetic foliation poles, and (b) magnetic lineation in sedimentary rocks of the ~iema hora Mountains. For legend see Fig. 5.

mostly oriented NW—SE, though in subordinate specimens it is also perpendicular (Fig 9(b)). In the Branisko Mountains, the magnetic foliation poles in red sedimentary rocks create an imperfect and wide girdle oriented E—W, mostly deviating substantially from the cluster of the

bedding poles (Fig. 10(a)), while in Triassic quartzite they create two groups oriented N—S (Fig. 10(a)). The magnetic lineation in red sediments creates a large cluster oriented N—S and in quartzite it creates two small clusters, one oriented N—S and the other E—W (Fig. 10(b)).

Fig. 10. Orientations of (a) magnetic foliation poles, and (b) magnetic lineation, in Carboniferous sediments (circles) and Triassic quartzite (triangles) of the Branisko Mountains. For legend see Fig. 5.

288

phic rocks formation phism, the generated

probably originated during ductile deassociated with polyphase metamormagnetic fabric in granitoid rocks was during deformation associated with

(2) The magnetic fabric is of a composite type. In Late Paleozoic and Mesozoic formations it approximates to a great circle pattern of the postnappe (i.e., D3) fold and thrust fabric (Fig. 2) in

cataclasis and partial recrystallization, and the magnetic fabric in sedimentary rocks was formed during deformation associated with very weak progressive metamorphism. Consequently, the magnetic fabrics in all the rock types are deformational in origin and consistent features and their similar orientation imply that the deformation of these rocks took place probably at the same time, during the same geological processes. The age of the crystalline complexes of the ~ierna hora and Branisko Mountains is Early Paleozoic (Jacko, 1979), except for the Alpine retrogressive metamorphism. The granitoid magmatism is considered to be Late Variscan. The majority of sedimentary rocks investigated in the present paper (showing deformational magnetic fabric) are Permian in age (Vozár et al., 1978). In addition, Triassic quartzite from one locality is also included. From the common deformation of metamorphic, granitoid and sedimentary rocks it can be concluded that this deformation has to be Alpine in age. The retrogressive metamorphism of metamorphic rocks and the weak progressive metamorphism of sedimentary rocks took place at a depth where recrystallization of crystalline rocks and transformation of clay minerals into phyllosilicates in sedimentary rocks were possible owing to elevated P— T conditions. Such conditions may have occurred in the Central West Carpathian area in the following three tectonometamorphic phases of the Alpine orogenic cycle: (1) during the emplacement of the Central West Carpathian nappes, (2) during the post-nappe formation of the dominant fold and thrust fabric with a NE—SW and NW—SE orientation, and (3) during the development of the thrust zones of the Certovica and Margecany type (Jacko, 1983). From the relationship between the magnetic fabric and the mesoscopic rock fabric in the regions investigated the following conclusions can be drawn: (1) The magnetic fabric is in good agreement, above all, with the main representative fabric elements of the Alpine mesostructural pattern.

the Branisko and Cierna hora Mountains. In the Gemeric unit it approximates to the orientation of the monocinal S3 (schistosity originating in the final tectonometamorphic phase associated with movement on the Margecany thrust zone (Fig. 2(b) and Fig. 11)). (3) The embryonic girdle pattern of the magnetic lineation is compatible with both the orientation of the relatively older D3 fold fabric (F3 axes in Figs. 2(a), (b)) and relatively younger F4 and F5 fold fabrics which create buckle or conjugate fold sets of the D4 or D5 deformational stages, respectively. (4) In sediments of the Branisko Mountains the magnetic fabric pattern is different in the Lower Triassic quartzite and in the Permian sediments. The magnetic fabric in the Lower Triassic quartzite taken from the NE margin of the region sensitively reflects both types of regional structures in the area: (a) a well pronounced ESE—WNW megasyncline, and (b) significant ESE—WNW thrusts and NE—SW and N—S faults (cf., Fig. 10(a), (b)). Permian sediments have been sampled on the opposite, i.e., western, part of the Branisko horst where distinctive N—S and NW—SE faults cut the original E—W fabric which is, moreover, clearly overprinted by NW—SE fold structures. All these tectonor~etamorphic redistributions are partly preserved in the magnetic fabric pattern (Fig. 10(a), (b)). The most pronounced imprint is logically made by the youngest and structurally penetrative NW—SE and N—S cleavage sets joining similarly oriented faults. (5) The magnetic fabric pattern in the Cierna hora metamorphites is more complex (Fig. 5(a), (b)). Poles of both the magnetic and mesoscopic foliations clearly outline both the Variscan (i.e., E—W) and the Alpine (i.e., NW—SE) fold orientation. Moreover, magnetic lineations have a steeply inclined SW orientation which could be of either rotational or later kink fold origin. The latter case is obviously the shearing produced in the NE—SW cleavage (fault zones of the D5 deformation stage (Jacko, 1979)).

289

(6) The magnetic fabric in the ~ierna hora Mountain granitoids (Fig. 7(a), (b)) reflects either the effect of the deformation represented by the flattening due to homogeneous shearing along two sets of shearing planes, finally giving rise to mylonites and ultramylomtes (the phenomenon impressively developed on a mesoscopic scale) or some trace of the inherited Variscan great circle orientation (the feature typical in this region for marginal zones of intrusive bodies). (7) Evident N—S great circle arrangements of magnetic foliation poles and of schistosity poles, and their common steep dips in metamorphic rocks of the Branisko Mountains coincide well with their upright fold fabric. More pronounced magnetic foliation dips to the north in some specimens (Fig. 6(a)) probably reflect Late Tertiary thrust reactivation. Magnetic lineation distribution (Fig. 6(b)) preserves either the original fold fabric orientation or directional magnetic mineral redistribution due to intense cleavage shearing along the NW—SE and NE—SW fault sets. (8) The magnetic foliation in the Branisko Mountain granitoids (Fig. 8(a)) can be supposed to reflect an interference of original E—W planar structures and the Alpine reactivation of this Variscan planar set. At first sight, isotropic distribution of magnetic lineation (Fig. 8(b)) could also be explained as resulting from the interference patterns discussed above.

the 10th Carpatho-Balcaman Geological Association Congress, pp. 1—41. Ende, C., Bratislava, van den, 1977. Paleomagnetism of Permian Red Beds of the Dome de Barrot (S. France). Thesis, Univ. Utrecht. Fusán, 0., 1960. Contribution to the Mesozoic stratigraphy of the Branisko and f~iernahora Mts. Geol. Pr. Správy, 18: 31—37 (in Slovak). Hamilton, N. and Rees, Al., 1971. The anisotropy of magnetic susceptibility of the Fransciscan rocks of the Diablo Range, Central California. Geol. Rundsch., 60: 1103—1124. Hrouda, F., 1982. Magnetic anisotropy of rocks and its application in geology and geophysics. Geophys. Surv., 5: 37—82. Hrouda, F. and Jelinek, V., 1985. Resolution of ferromagnetic and paramagnetic components of magnetic anisotropy, using combined and high-field anisotropy measurements. IAGAlow-field NEWS, Session 1.14, Prague. Jacko, S., 1975. Lithological—structural development of the southern part of the Bujanova massif. Unpublished report, Faculty of Sciences, Comenius University, Bratislava, 304 PP (in Slovak). Jacko, S., 1978. Lithological—structural characteristics of the central region of the ~ierna hora Mts. zone. Zap. Karpaty, Sér. Geol., 3: 59—80 (in Slovak). Jacko, S., 1979. Geological section across the ~ierna hora Mts. and their boundary with the Gemeric. In: M. Mahel’ (Editor), Tectonic Sectionspp.of 185—192. the West Carpathians. Geol. Ust. D. Stüra, Bratislava, Jacko, S., 1983. Some problems in solving the pre-Alpine structure in the Tatroveporide crystalline complexes. In: International Geological Correlation Program project No. 5, Newsletter 5, pp. 51—53. Jelinek, V., 1977. The Statistical Theory of Measuring Anisotropy of Magnetic Susceptibility of Rocks and its Application. Geofyzika, Brno, 88 pp. JelInek, V., 1978. Statistical processing of anisotropy of magnetic susceptibility measured on groups of specimens. Stud. Geophys. Geod. 22: 50—62.

The methodological aspects of our magnetic and mesostructural fabric study can be SUffl~ marized as follows: (1) Magnetic fabric shows a high sensitivity to superimposed tectonometamorphic events, (2) Magnetic fabric pattern in all lithostructural units investigated is undoubtedly of complex (i.e., of interference) type. (3) Simultaneous analysis of magnetic and mesostructural fabrics can be a useful tool in resolving the tectonometamorphic history, even in complex regions of the Central West Carpathian type.

Jelinek, V., 1980. Kappabridge KLY-2. Leaflet Geofyzika, national corporating, Brno, 2 pp. Jeinek, V., 1981. Characterization of magnetic fabric of rocks. Tectonophysics, 79: 563—567. Kligfield, R., Lowrie, W. and Dalziel, I.W.D., 1977. Magnetic

References

Paleogeographic Development of the West Carpathians. GUDS, Bratislava, 346 pp. Zapletal, K., 1983. Some magnetic properties of magnetite and hematite. Unpublished report, Geofyzilca, Brno, 80 pp.

Andrusov, D., Bystrick~’,J. and Fusán, 0., 1973. Outline of the structure of the West Carpathians. Int. Exc. guidebook of

susceptibility as a strain indicator in the Sudbury Basin, Ontario. Tectonophysics, 40: 287—308. Mahel’, M., 1953. Some problems of the N Gemeric geosyndine. Geol. Zborn., 4: 221—254, (in Slovak). Polâk, M., 1985. Mesozoic of the N part of the Branisko Mts. Geol. Pr. Spravy, (in press). Polâk, M. and Vozár, J., 1985. Explanations to the geological map 2743-Lipany. In: M. Polák (Editor). Unpublished Report, F., Geofond, Bratislava, 28 pp. Verhhltnise des Branisko Rosing, 1947. Die geologischen Gebirges und der óerna hora Region (Karpathen). Z. Dtsch. Geol. Ges., 99: 123—136. VozIr, J., Marschalko, R., Milik, M. and Nêméok, J., 1978.