GEOLOGICA CARPATHICA, 51, 3, BRATISLAVA, JUNE 2000
133144
TECTONIC AND STRUCTURAL IMPLICATIONS
OF PALEOMAGNETIC AND AMS STUDY
OF HIGHLY METAMORPHOSED PALEOZOIC ROCKS FROM THE
GEMERIC SUPERUNIT, SLOVAKIA
JADWIGA KRUCZYK
1
, MAGDALENA K¥DZIA£KO-HOFMOKL
1
, MARIA JELEÑSKA
1
,
IGOR TÚNYI
2
, PAVOL GRECULA
3
and DANIEL NÁVESÒÁK
4
1
Institute of Geophysics of the Polish Academy of Sciences, Ks. Janusza 64, 01-452 Warsaw, Poland
2
Geophysical Institute of the Slovak Academy of Sciences, Dúbravská cesta 9, 842 28 Bratislava, Slovak Republic
3
Geological Survey of the Slovak Republic, Mlynská Dolina 1, 817 04 Bratislava, Slovak Republic
4
Geological Survey of the Slovak Republic, Werferova 1, 040 11 Koice, Slovak Republic
(Manuscript received March 1, 1999; accepted in revised form May 16, 2000)
Abstract: Paleozoic highly metamorphosed rocks were sampled in 4 localities situated along the Koice-Margecany
shearing zone (East-Carpathian dextral system), on two sites situated close to the Dobiná shearing zone (West-
Carpathian sinistral system), and on one site situated within the Gemeric Superunit. The West-Carpathian and East-
Carpathian shearing systems resulted in the division of Gemeric Superunit into a mosaic of small tectonic blocks.
Each site sampled for this study represents one such block. Our paleomagnetic study revealed that the rocks became
remagnetized in the Middle Miocene after a regional CCW rotation, probably during the period between anomaly 6
(20 Ma) and anomaly 5 (10 Ma). After the remagnetization episodes the blocks underwent rotations (around their
vertical axes) associated with activity of respective dextral or sinistral shearing zones. The results of the AMS study
suggest a correlation between the magnetic fabric and the Alpine tectonic deformation episodes.
Key words: Gemeric Superunit, Paleozoic, paleomagnetism, AMS, tectonic deformations.
Introduction
The Gemeric Superunit, which belongs to the Alpine-Car-
pathian-Pannonian (ALCAPA) block is situated at the south-
ern edge of the Central Western Carpathians, Figs. 1a, 1b. It
has a distinct belt structure, often with narrow lithological
strips of the length up to several tens of kilometers. The Ge-
meric Superunit is built by the Early and Late Paleozoic and
Triassic rocks. Paleogene and Neogene sediments cover its
marginal parts with the exception of the eastern margin, which
is in tectonic contact with the crystalline complex of Veporic
Superunit (Èierna Hora Mts). The superunit is cut into numer-
ous blocks by two systems of shearing zones trending NW-SE
and NE-SW. Rocks building the Gemeric Superunit underwent
polyphased deformation and metamorphism that took place
during the Variscan movements as well as during the Alpine
orogenies. It is supposed, that one deformational episode took
place in the Gemeric Superunit during the Variscan Orogeny
and that formation of nappe structures marked the end of
Variscan events. The present geological frame of the Gemeric
Superunit is due to Alpine tectonics (Grecula 1997; Plaienka
et al. 1997).
The purpose of this study is twofold:
●
to reveal this part of geodynamic history of the Gemeric
Superunit that became preserved by paleomagnetic charac-
teristics of its highly metamorphozed Paleozoic rocks;
●
to find correlations between the anisotropy of magnetic
susceptibility (AMS) and the shearing zones.
Geological setting and sampling
General remarks concerning the geodynamic features
During the Upper Mesozoic and the Tertiary the ALCAPA
area was subject to significant tectonic processing. The con-
vergence of the Afroarabian and the Euroasian litospheric
plates led to final collision of the Bohemian Massif with the
Apulia. This collision resulted in a formation of Alpine
nappes and northeastern shifting (escape) of fragments of the
Alpine and the Dinaric units, grouped in the Inner Car-
pathian, Tissia and Dacides terranes. Csontos et al. (1992)
and Márton et al. (1995) argue, that the escape was driven
from behind by pushing forces caused by the Bohemian Mas-
sif- Apulia collision, but influence of pulling forces caused
by subduction of the Euroasian Plate under the Inner Car-
pathians also contributed to this process. The escape move-
ments started after the Cretaceous folding in the Alps; these
were most intensive after the pre-Oligocene folding and ex-
tinguished during the Early Miocene. The escaping frag-
ments approached European Plate in different periods; for
Gemeric Superunit it happened during the Early Eocene
(Márton et al. 1995).
According to Peresson & Decker (1996) the Early and Mid-
dle Miocene tectonics of the ALCAPA region was dominated
by the N-S directed compression and E-W directed extension.
During the Late Miocene the direction of the compression
changed to an E-W trend. This, and the final oblique collision
134 KRUCZYK et al.
of the escaping Inner Carpathian units with the North Europe-
an Platform, caused their counterclockwise rotation and uplift
of the rigid basement rocks (Plaienka et al. 1997; Peresson &
Decker 1996). The paleomagnetic study by Márton et al.
(1995) and Márton & Márton (1996) performed in Southern
Slovakia and North Hungarian Central Range suggest, that
this rotation of the whole region ended at the Early-Middle
Miocene boundary (Karpatian-Badenian). In the present study
we show that the Middle and Late Miocene kinematics caused
local rotations of blocks comprising the Gemeric Superunit.
Along with the processes leading to the northeastward es-
cape, the above mentioned compressional stresses were com-
Fig. 1b. Position of the Gemeric Superunit in the West-Carpathian mountain system. 1 Klippen belt, 2 Tatric Superunit, 3 Veporic
Superunit, 4 Gemeric Superunit. (From Grecula et al. 1995, reprinted with permission of Geocomplex Bratislava).
Fig. 1a. Position of the Gemeric Superunit in the frame of the Carpathian mountain system. BM Bohemian Massif, RP Russian Plat-
form, MP Moesian Plate, HCR Hungarian Central Range, GHP Great Hungarian Plane, T Tatric Superunit, V Veporic Supe-
runit, G Gemeric Superunit, B Bükkic units, Bu Bucovinian units, Z Zemplín units, SG Supragetic units, Gt Getic units,
D Danubian units, TR Transsylvanides, A Apuseni Mts., M Mecsek Mts. (From Grecula et al. 1995, reprinted with permission
of Geocomplex Bratislava).
TECTONIC AND STRUCTURAL IMPLICATIONS OF PALEOMAGNETIC AND AMS STUDY 135
pensated along the wedge system by paired shear zones, one
trending NW-SE (the Koice-Margecany shearing zone
(KMSZ), the dextral strike-slip fault, the East-Carpathian
system), and the second shear zone trending NE-SW (the
Transgemeric shear zone (TGSZ), the sinistral strike-slip
fault, the West-Carpathian system). These paired zones re-
sulted in creating an arc structure of the Gemeric Superunit
and its division into a mosaic of blocks that could have been
subjected to local rotations, Fig. 2 (Grecula et al. 1990). In
the megascale the shear zones are demonstrated by the
change of the course of rock complexes, by retraction, as
well as destruction of the Variscan veins and the stratiform
mineralization (Návesòák 1993). The deformation of the Pa-
leozoic rocks associated with these events is of a brittleduc-
tile character.
The age of the shear zones is estimated on the basis of geo-
logical features as well as the Rb/Sr isotopic dating. As this
research was not performed specifically for the dating of the
shear zones, we may only assume that movements on them
started before the Cretaceous and ended during the Styrian
phase (the Middle Miocene) Grecula et al. (1990),
Návesòák (1993).
Návesòák (1993) distinguishes within the Gemeric Paleo-
zoic rocks three systems of mylonitic schistosity (S2, S3, S4)
and two lineations (L2, L3) caused by the Alpine tectonics.
The S1 and L1 systems linked with the Variscan deformation
stage are visible only within large blocks that were not influ-
enced by the Alpine shearing. The E-W trend of lineation L1
indicates the direction of an old shear system, the foliation
S1 indicates surfaces of metamorphic schistosity correspond-
ing to L1. The most characteristic lineation L2 (azimuth of
230°240° or 60° with a low dip) associated with the TGSZ
and LSSZ, mylonitic foliation S2 (azimuth of about 165°, dip
of about 40° with scatter from 0° to 70°) and foliation S3 (azi-
muth of 293°335°, shallow dip) are due to a compression
linked with the TGSZ and LSSZ (the Lacemberská dolina Val-
ley-the Stará Voda shearing zone, parallel to the TGSZ zone)
shear zones. The L3 lineation trends to 290° or 100° with a
shallow dip. The S4 foliation (azimuth of 230°235°, dip of
about 45°) is associated with the KMSZ zone.
Metamorphism
The mineral association of the majority of rocks of the Ge-
meric Superunit corresponds to the metamorphic facies of
green shists. In several zones of the Gemeric Superunit, also
in the enclaves of high pressure and middle temperature,
metamorphites and rocks of amphibolite facies (Grecula
1997; Radvanec 1994, 1997) with important retrograde meta-
morphic reworking have been preserved.
These processes, as well as strong folding, granitization,
local granite intrusions and ore mineralizations begun during
the Variscan orogeny. During the Alpine orogeny metamor-
phic and tectonic processes continued. The Variscan and Al-
pine metamorphic events resulted in well-developed blaste-
sis of magnetite, hematite and chlorite. The presence of
martite within magnetite grains, as well as transformation of
sulphides into Fe-hydroxides indicate that blastesis took
place during a retrograde phase. The influence of tectonic
factors is reflected in the general presence of the mylonitic
foliation. Márton et al. (1995) argue, that the activation of as-
tenosphere associated with the tectonic escape caused reheat-
Fig. 2. Map of the shear zones of the Gemerides. RSZ the Rejdová shear zone, DSZ the Dobiná shear zone, KMSZ the Koice-
Margecany shear zone, TGSZ the Transgemeric shear zone, HSZ the Hodkovce shear zone. 1 course of lithological units, 2
Variscan nappes, 3 Alpine rejuvenated Variscan (?) shear zones, 4 principal shear zones (Alpine), 5 shear zones of lower order,
6 faults with a character of pure shear (the youngest ones), black circles sampling localities: È Èrme¾, KV Vyný Klátov, J
Jahodná, MA Margecany, DO Dobinská Priehrada, ML Mlynky, GP Gemerská Poloma. (From Grecula et al. 1990, reprinted
with permission).
136 KRUCZYK et al.
ing of crust. This assumption is supported by Peresson &
Decker (1996) who show the uplift of the astenosphere under
the Pannonian Basin caused by extension tectonics during
the Middle Miocene. We believe that this process induced
physico-chemical conditions suitable for the Middle Miocen
remagnetizations of older rocks.
Sampling
62 hand samples were collected for the present study. The
sampling was carried out in the following areas: along the
KMSZ shear zone forming the eastern border of the Gemeric
Superunit (exposures KV, È, J, MA), in the vicinity of the
Dobiná shear zone (DSZ one of shear zones belonging to
TGSZ system) in the western part of the Gemeric Superunit
close to its northern border (exposures DO, ML) and one ex-
posure (GP) lying in the middle part of the unit, see Fig. 2.
The rocks represent strongly metamorphosed and my-
lonitized sericitic schists (È, J), cataclasites (MA, KV), phyl-
lites and schists rich in carbonates (ML, DO, GP). Oriented
hand samples taken in the field were drilled into standard pa-
leomagnetic specimens in the laboratory.
Experimental procedure
All experimental work was performed in the Paleomagnetic
Laboratory of the Institute of Geophysics of the Polish Acade-
my of Sciences in Warsaw. Natural remanent magnetization
(NRM) was measured with a cryogenic magnetometer of 2G,
specimens were demagnetized thermally with the Magnetic
Measurements non-magnetic furnace or with a 2G alternating
field device. All apparatuses are installed within the Magnetic
Measurements compensating cage. The magnetic susceptibili-
ty and its anisotropy was measured with the KLY-2 Geofyzika
Brno bridge before the demagnetization procedure. The ob-
tained parameters (Tarling & Hrouda 1993) that will be dis-
cussed later comprise:
mean low field magnetic susceptibility
)
(
3
1
min
int
max
K
K
K
K
m
+
+
=
where
min
int
max
,
,
K
K
K
the maximum, intermediate and mini-
mum susceptibilities, respectively
anisotropy parameter
)
)
(
)
(
)
((
2
exp
2
3
2
2
2
1
m
m
m
P
η
η
η
η
η
η
−
+
−
+
−
=
′
, where
)
ln(
max
1
K
=
η
,
)
ln(
int
2
K
=
η
,
)
ln(
min
3
K
=
η
,
3
2
1
η
η
η
η =
m
shape parameter
)
(
)
2
(
3
1
3
1
2
D
D
D
D
D
-
-
-
=
T
; the
anisotropy ellipsoid is prolate (prevailing lineation) if T < 0, the
anisotropy ellipsoid is oblate (prevaling foliation) if T > 0
directions of K
max
and K
min
axes of the susceptibility
elipsoid.
Mean magnetic susceptibility was measured also after con-
secutive heating steps in order to monitor mineralogical
changes caused by heating. Results of paleomagnetic experi-
ments were analyzed with the program package PDA of Le-
wandowski et al. (1997), results of the AMS study with
the ANISO 11 program of Jelínek (1977) and Spheristat 2
program of Pangea Scientific.
The identification of magnetic minerals was done through
a microscopic examination of polished sections (performed
by Siemiatkowski from the State Geological Institute, Wro-
claw), a thermomagnetic analysis, and a study of hysteresis
parameters. The thermomagnetic analysis consisted of study-
ing the thermal decay of isothermal remanence Ir acquired in
1 T field during the heating to 700 °C in the air with the use
of the TUS Warsaw device. The blocking temperatures Tb
obtained here indicate that magnetic minerals are present in
the rock. The hysteresis parameters were measured with the
vibrating magnetometer VSM of Molspin with the highest
available field of 1 T.
General characteristics of sampled rocks
and their magnetic mineralogy
Sites situated along the KMSZ shear zone:
Èerme¾ (È) schistosity (azimuth/dip) 245/50, mylonitic
schists, 8 hand-samples. Paleontologic age is supposed to be
the TournaisianViséan; according to Grecula et al. (1990)
they became modified due to the Late Variscan shearing, but
mylonitisation originated during the Alpine tectonics. The
microscopic study reveals parallel texture manifested by my-
lonitic smears, calcite veins cutting the rock were also visi-
ble. Fe-hydroxides, pyrrhotite and chalcopyrite in clasts of
0.1 mm represent opaque minerals. Rocks are magnetically
weak isothermal remanence Ir of fresh specimens was too
low for a thermomagnetic analysis. Heating to 600 °C result-
ed in increase of Ir by about 30 times due to formation of
new magnetite. The hysteresis parameters (see Table 1) indi-
cate the presence of magnetic material with low and interme-
diate coercive force. The mean magnetic susceptibility Km
ranged between 400 and 800
×
10
6
SI and increased after
heating to 500 °C.
Vyný Klátov (KV) no tectonic parameters, amphibo-
lite schists (cataclasites) belonging to the gneissic amphibo-
lite Klátov complex, 12 hand samples. The tectonic position
is ambiguous, the complex may be treated either as a small
nappe, or as a highly metamorphosed part of the Rakovec
nappe. The radiometric age (K-Ar and Ar-Ar) spans the time
from 448 to 383 Ma (the Upper Ordovician, Lower Devo-
nian). The metamorphic processes perhaps underwent here in
the amphibolite facies conditions which are indicated by the
presence of chloritized amphiboles with magnetites and post-
plagioclase smears composed of epidotealbitecalcite. The
microscopic study reveals the presence of automorphic mag-
netite with martite lamellae of 0.030.3 mm. Hematite forms
smears of 0.1 mm, tablets of 0.0010.1 mm and flakes. Sin-
gle sulphides are present within the hematite grains as well
as clusters of Fe-hydroxides with relicts of pyrites. Automor-
phic and framboidal pyrites are also visible. The thermomag-
netic analysis shows the presence of magnetite with Tb 560
575 °C, accompanied in some specimens with phase with Tb
of about 200 °C (goethite?) and hematite with Tb of about
650 °C (Fig. 3a,b). The hysteresis parameters (see Table 1),
TECTONIC AND STRUCTURAL IMPLICATIONS OF PALEOMAGNETIC AND AMS STUDY 137
despite the presence of hematite and perhaps goethite visible
in polished sections, are characteristic for multidomain mag-
netite (Day et al. 1977). The magnetic susceptibility Km is
high and ranges from 3200 to 22000
×
10
6
SI, it increases af-
ter heating to temperatures exceeding 600 °C in all speci-
mens.
Jahodná (J) schistosity 220/55, very fine-grained
sericitic schists (mylonites) with quarz lens, 5 hand samples.
Table 1: Hysteresis parameters measured for particular specimens.
Locality
Specimen Ms mA.m
2
Mr mA.m
2
Hc mT
Hcr mT
È
105 1
0.6
0.3
19
43
107 1
0.7
0.1
4
47
KV
134 1
443
25
7.5
35
135 1
272
11
5
30
138 2
239
9.5
5
30
J
113
2.7
0.8
44
310
114
3.9
0.7
29
280
MA schist
124 1
260
17
7
27
sandstone
132 1
0.2
0.06
24
62
sandstone
133 1
0.2
0.08
20
70
DO
174 1
0.5
0.2
28
160
174 2
1.23
0.24
41
65
175
0.3
0.09
8
48
176 1
0.5
0.17
12
30
176 2
0.35
0.09
3
35
ML
157 1
15
13
312
330
160 -1
?
0.7
175
330
161 1
?
0.3
127
170
162 1
2.1
1.9
237
250
163
6.4
5.0
108
115
167
?
0.4
52
230
GP
179 1
0.35
0.27
29
65
181
0.21
0.06
13
65
182
0.24
0.06
22
60
185 2
0.21
0.06
16
55
Ms saturation magnetization, Mr saturation remanence, Hc coercivity, Hcr
remanence coercivity
The paleontologic age of the rocks is supposed to be the
Lower Permian, whereas the radiometric K-Ar dating gives
126 Ma (the Lower Cretaceous) as the age of the metamor-
phic changes. The microscopic analysis reveals the presence
of automorphic hematites with sulphide grains, spherical ag-
glomerates of hematite grains probably of a post-pyrite ori-
gin, and single hematite tablets. Ilmeno-magnetite pseudo-
morphs were also visible. The thermomagnetic analysis
indicates hematite as the only magnetic mineral, the curves
of Ir vs. T are very similar to those shown in Fig. 3c. This con-
clusion is supported by the hysteresis parameters (cf. Table 1)
revealing high values of coercivity, and by the saturation field
exceeding the maximum field of the experiment. The Km val-
ues were low and ranged from 200 to 250
×
10
6
SI. Heating to
600 °C resulted in a significant increase of the Km.
Margecany (MA) schistosity 190/40, chlorite-sericite
schists, and polymict sandstones, both highly mylonitized.
Some 10 hand samples were taken from 3 sites, but only 8
samples from 2 sites (one site containing schists and the sec-
ond sandstones) were suitable for measurements. This lo-
cality is situated at the crossing of two shear zones: KMSZ
and TGSZ. The radiometric (Ar-Ar) unpublished age is 329.6
Ma, placing the rocks in the Upper Carboniferous. The mi-
croscopic analysis was performed only for one specimen rep-
resenting polimict sandstones and did not reveal the presence
of magnetic minerals. The thermomagnetic analysis that was
performed on several specimens shows the presence of mag-
netite and goethite in schists and magnetite in sandstones
the Ir vs. T curves are similar to the one shown in Fig. 3a.
The study on hysteresis parameters suggests the dominance
of multidomain magnetite in sericitic schists and SD and
PSD magnetite in sandstones (Day et al. 1977) see Table
1. The obtained values of the Km for schists were very high
(650070000
×
10
6
SI) for sandstones low (190220
×
10
6
SI), after heating to 550600 °C the Km increases.
Localities situated at the DSZ shear zone:
Dobinská Priehrada (DO) bedding 180/34, chlorite
schists (mylonites) with calcite smeared veins, 11 hand sam-
ples. The assumed age is the Upper Carboniferous. Within the
veins plagioclases in clasts, epidotes, chlorites, albites are en-
countered. The age of metamorphism is uncertain, likely oc-
curring during the large time span from the Neovariscan to the
Jurassic or even the Cretaceous. The microscopic analysis re-
veals the presence of post-pyrite Fe-hydroxides with pyrite
relicts. The thermomagnetic study supports this result reveal-
ing various Ir vs. T curves showing the presence of phase with
Tb of 200 °C, hematite and low amounts of magnetite (the Ir-T
curves are similar to those shown in Figs. 3a and 3c). After the
heating new magnetite appears. The Km ranges from 400 to
750
×
10
6
SI and increases after heating to 600 °C. The values
of hysteresis parameters (Table 1) should be regarded only as
estimates because the rocks here are too weak for reliable
measurements with our VSM.
Mlynky (ML) bedding 320/65, 12 hand samples, strong-
ly carbonatized pyroclastic phyllites belonging to the diabase
series of the Rakovec Nappe. Their estimated age is the Upper
DevonianLower Carboniferous, they became metamor-
phosed during the Variscan orogeny in the greenschist facies
conditions, but the Alpine events erased the Variscan features.
The microscopic analysis revealed hematite in tablets of 0.01
0.03 mm and less than 0.001 mm as the main magnetic miner-
al. The thermomagnetic analysis supports this conclusion (Fig.
3c), the hysteresis parameters attain values characteristic for
hematite (Table 1) as well. The Km ranges from 190 to
750
×
10
6
SI and increases after heating.
Locality siutated outside the main shear zones:
Gemerská Poloma (GP) bedding 350/51, 9 hand sam-
ples, ankeritic carbonaceous beds cut by calcite veins, prod-
uct of regional metasomatosis. The assumed age of the rocks
is Upper SilurianLower Devonian, the age of metasomatic
alteration is supposed to be either Variscan, or Alpine. The
polished sections have intensive brownish colouring charac-
teristic for fine Fe-hydroxides. The thermomagnetic analysis
reveals the presence of goethite and, perhaps, a small amount
of magnetite. Pyrite is probably also present, because after
the heating pyrrhotite appears on the Ir vs. T curves. Heating
to 650 °C results in a thousandsfold increase of Ir due to the
appearance of pyrrhotite and magnetite. Here, as in DO, re-
sults of measurements of the hysteresis parameters are only
estimations. The Km ranges from 230 to 600
×
10
6
SI and in-
138 KRUCZYK et al.
Fig. 3a.
Fig. 3b.
TECTONIC AND STRUCTURAL IMPLICATIONS OF PALEOMAGNETIC AND AMS STUDY 139
creases considerably after heating to 450 °C; at this tempera-
ture pyrite transforms into pyrrhotite.
Paleomagnetic results and discussion
The pilot specimens were demagnetized thermally and
with an alternating field (AF). As the thermal demagnetiza-
tion was much more effective than the AF, the bulk of the
collection was demagnetized thermally (to temperatures 500
675 °C) and results of this procedure were taken for inter-
pretation. The isolation of the characteristic (CHRM) com-
ponents of the NRM was performed by the program package
of Lewandowski et al. (1997) based on the principal compo-
nent analysis of Kirschvink (1980). The previous section
shows that the studied rocks have various lithologies, are
highly metamorphosed and contain inhomogeneously distrib-
uted and varied magnetic minerals of secondary origin occur-
ring in various forms. Hence, the demagnetization curves are
complicated and difficult to resolve. Nevertheless, we were
able to isolate the CHRM in six localities in sufficient
amount of specimens for mean directions to be calculated for
each of them. The analysis of the demagetization results of
majority of specimens reveals the presence of more than one
component of the NRM. We tried to find among them the
components with relatively high unblocking temperatures
(Tub) appropriate to the magnetic mineralogy of specimens
ignoring the components appearing in temperatures lower
than 200 °C. The mean directions in situ and after tectonic
correction together with the parameters of the Fishers statis-
tics are summarized in Table 2.
Localities situated along the KMSZ shear zone:
Èrme¾ (È) intensity of the NRM ranges from 0.7 to
3.5 mA/m. There is no data for this locality because we were
not been able to isolate any characteristic component of the
NRM.
Vyný Klátov (KV) intensity of the NRM is in the
range 12300 mA/m. The CHRM found in 7 specimens from
6 hand samples in the Tub range of 475 to 575 °C is carried
by magnetite. An example of demagnetization results is
shown in Fig. 4a.
Jahodná (J) intensity of the NRM ranging from 0.1 to
0.6 mA/m is carried by hematite grains of various genera-
tions. The thermal treatment does not demagnetize it fully,
beacuse the remanence remaining after annealing in 550
600 °C becomes unstable it changes in intensity and di-
rection during the measurement procedure. Nevertheless we
Fig. 3. Results of the thermomagnetic analysis along with corresponding polished sections. a specimen from KV containing magnetite
×
500 and hematite lamelae and post-pyrite + hydro-Fe-oxydes
×
200, b specimen from KV containing goethite, c specimen from
Mlynky containing hematite
×
500. Irs isothermal remanence acquired in the field of 1 T.
Fig. 3c.
140 KRUCZYK et al.
isolated the CHRM in 6 specimens from 3 hand samples in
the Tub range of 250450 °C. An example of demagnetiza-
tion result is shown in Fig. 4b.
Margecany (MA) intensity of the NRM in schists is
27-300 mA/m and about 0.2 mA/m in sandstones. The de-
magnetization results indicate that the NRM is carried main-
ly by magnetite, small components carried by goethite de-
magnetize very quickly. The directions of the CHRM
isolated in specimens from both lithologies have normal and
reversed directions and are highly scattered. Only in five
specimens (from 1 sandstone and 2 schists samples) we were
able to isolate the group of similar CHRM directions. Its Tub
did not exceed 400 °C. Fig. 4c presents an example of the de-
magnetization results.
Localities situated along the DSZ schear zone:
Dobiná (DO) intensities of the NRM range from 0.2 to
3 mA/m. The remanence is carrried mainly by magnetite and
hematite present in various ratios. Fig. 4d shows an example
of the demagnetizing curve for specimen with predominance
of magnetite, the NRM for specimens with hematite predom-
inance did not demagnetize during the thermal treatment but
became unstable after the heating to 600 °C. The CHRM was
isolated in 12 specimens from 9 hand samples in Tub of
575 °C (only in one specimen this component had Tub of
about 200 °C).
Mlynky (ML) intensity of the NRM ranges from 0.8 to
40 mA/m. The NRM is carried by magnetite and hematite of
various generations. In some specimens remanence, after
heating to 600 °C became unstable. The CHRM was isolated
in 8 specimens from 5 hand samples in Tub ranges 500
550 °C or 625675 °C. An example of the demagnetization
results for specimen with NRM carried by hematite is shown
in Fig. 4e.
Locality situated outside main shear zones:
Gemerská Poloma (GP) intensities of the NRM range
from 0.1 to 0.4 mA/m. The natural remanence demagnetized
completely or in great percent in the temperatures 300
350 °C (see Fig. 4f) indicating that it is carried mainly by
pyrrhotite. As this mineral was identified neither by the mi-
croscopic analysis nor by the thermomagnetic analysis we
suppose that it appears as very fine (submicroscopic) grains,
probably due to alterations of pyrite which is present here in
abundance. In temperatures exceeding 400 °C the remanence
increases very quickly. The CHRM was isolated in 8 speci-
mens from 5 samples in the Tub range of 300350 °C.
The plot of the mean directions obtained for investigated lo-
calities in situ are presented in Fig. 5a. This plot and the data
in Table 2 show that the in situ directions for KV, J and MA
differ in declinations, but their inclinations are similar to each
other. Lack of bedding parameters enables tectonic correction
of them, but the similarity of inclinations suggests the post-
folding origin of the CHRM and mutual rotations of the blocks
represented by the exposures. The mean directions obtained
for the three remaining localities differ in declinations and in-
clinations from each other both in situ and after full correction
and differ from the in situ directions of the KV, J and MA (Ta-
ble 2, Fig. 5b). In order to see whether part untilting of the DO,
ML and GP will move their inclinations closer to the other
three we performed the inclination-only fold test of Enkin
(1994), Enkin & Watson (1996). This test is used for finding
the degree of untilting that gives minimum dispersion of incli-
nations. This degree is indicated by the maximum of the plot
of the Fisher's precision parameter k versus degree of untilt-
ing. In this way we may answer the question of whether the re-
manence studied is pre-tilting, post-tilting or was aquired at
some intermediate stage. The result of this test performed for
the directions of all six localities shows that the best fit would
be obtained by 50% untilting of the DO, ML and GP, the value
of k attains here maximum of 221. Another trial performed for
45% untilting of DO nad ML and 65% of untilting of the GP
gave better estimate: the maximum of k increased to 572
Fig. 6. The results of this test for all six exposures combined
are presented in Fig. 5c. The mean directions calculated for the
Table 2: Directions of the characteristic CHRM component for the Gemerides. Geographic position: 20.5°E, 48°N. Reference data after
Besse & Courtillot (1991) for Middle Miocene D = 6, I = 62.
Loc.
N N/n
D/I
In situ
D/I
f.cor.
D/I
p.cor.
a
95
k
plat
pol
DD=DoDref
KMSZ
KV
6/7
313/-60
-
-
13
21
41
R
127 CW
J
3/6
190/-65
-
-
18
18
47
R
92 CCW
MA
3/5
275/65
-
-
20
16
47
N
4 CW
DSZ
DO
9/12
320/51
265/66
303/61
45%
13
11
42
N
63 CCW
ML
5/8
68/70
342/30
2/61
45%
9
42
42
N
4 CCW
Outside main shearing zones
GP
5/8
109/72
13/46
25/62
65%
6
98
43
N
17 CW
Loc. locality; N/n number of hand samples in which this CHRM was found/number of specimens taken for calculations; D/I in situ
declination/inclination before tectonic correction; D/I corrected declination/inclination after correction; f.cor. full correction; p.cor.
partial correction; 45% (65%) untilt. 45% (65%) untilting;
α
95
, k parameters of Fishers statistics; plat paleolatitude; pol polarity of
CHRM; CW clockwise; CCW counterclockwise; Do declination obtained in this study; Dref reference declination; R angle of local
rotation. KMSZ Koice-Margecany shearing zone; DSZ Dobiná shearing zone. The directions taken for interpretation are in bold letters.
TECTONIC AND STRUCTURAL IMPLICATIONS OF PALEOMAGNETIC AND AMS STUDY 141
appropriate unfolding ratios are summarized in Table 2 and
shown in Fig. 5c together with the in situ directions of the
KV, J and MA. Their inclinations remain in the narrow range
of 6065° but declinations differ considerably. We believe
that each locality is situated within an individual small tec-
tonic block and explain the differences in declinations as re-
sult of local rotations of the blocks connected with the shear-
ing zones.
The inclinations obtained here are close to the inclinations
calculated for the Gemeric Superunit for the Middle Miocene
(1020 Ma) after the European reference data of Besse &
Fig. 4a-f. Examples of the thermal demagnetization experiments. (The names of localities in abbreviated form are assigned to respective
figures).
Courtillot (1991): D
ref
=6, I
ref
= 62. The observed differences
between the final declinations Do and the Dref reflect the
amount and sense of local rotations of blocks that took place
after their remagnetization. Angles and sense of rotations are
summarized in the respective column in Table 2 as
∆
D =
D
o
D
ref
.
The presented results show that according to paleomagnetic
data blocks situated along the dextral KMSZ zone (KV and J)
rotated clockwise, the block represented by MA situated at the
crossing of KMSZ (dextral) and TGSZ (sinistral) zones rotat-
ed counterclockwise as well as those situated along the sinis-
Sample GEMa 134
N
Sample GEMa 114
Sample GEMa 133
Sample GEMc 163
Sample GEMb 183
Sample GEMd 169
N
N
142 KRUCZYK et al.
tral DSZ (DO, ML) zone. The block represented by GP situat-
ed within the unit rotated clockwise. The senses of rotations
agree with character of respective shearing zones.
The first paleomagnetic research in the area of the Gemer-
ic Superunit was performed by Hanu & Krs (1963). They
studied problems of hydrothermal mineralization on metaso-
matic sideritic deposits in the region of Dobiná, Mlynky,
Roòava and others, and metasomatic magnesitic deposits in
Bankov nearby Koice. On the basis of the study performed on
undemagnetized rocks containing hematite they concluded,
that there were no rotations in the region of the post-dating
mineralization study. The interesting thing is, that their mean
paleomagnetic direction in situ of undemagnetized NRM spec-
imens sampled in the siderite vein-filling in the mine in
Mlynky (D = 77°, I = 56°,
α
95
= 13°) is very close to the
Mlynky in situ result of the present paper for which the CCW
rotation is very small. The same is true for their result from the
magnesite at Bankov near Koice (D = 198°, I = 70°,
α
95
= 4°40) and Jahodná of the present study both results
suggest the CW rotation (29° in Jahodná and 12° in Bankov).
The timing of events suggested by paleomagnetic study
seems to be as follows: the Middle Miocene remagnetization
of rocks postfolding along the KMSZ zone, synfolding
along the DSZ zone and in the GP locality. The magnetiza-
tion in GP seems to be acquired during a stage of folding lat-
er than that along the DSZ. The presence of both normal and
reversed polarity remanences suggests, that remagnetization
processes were not synchroneous, but took place in the Ge-
meric Superunit during respective periods of reversed and
normal polarity. The time span of remagnetization episodes
is limited by the tectonic events that took place in the Gemer-
ic Superunit to the period between the end of regional rota-
tion (KarpatianBadenian according to Márton et al. 1995
and Márton & Márton 1996) and extinction of activity along
the shear zones (the Styrian phase, end of the Middle Mi-
ocene). Taking these constraints into account we believe that
remagnetization processes took place during the time span
between anomalies 6 (20 Ma) and 5 (10 Ma), when, accord-
ing to the geomagnetic polarity time scale of Merril et al.
(1996), the polarity of the geomagnetic field changed many
times.
AMS study
The measurements of the anisotropy of magnetic suscepti-
bility show that in all studied localities the values of the
Fig. 5. Mean directions of the characteristic component of rema-
nence (CHRM) isolated in HT range in studied localities. a in
situ, b KV, J, MA in situ, DO, ML, GP after full tectonic
correction, c the best fit: KV, J, MA in situ , DO and ML
after 45% untilting, GP after 65% untilting. Diamond R denotes
the reference field.
Fig. 6. The results of the inclination-only test, Enkin (1994) for
KV, J, MA in situ , DO, ML, GP after 50% untilting (open sym-
bols) and KV, J, MA in situ, DO and MA after 45% untilting,
GP after 65% untilting (full symbols). k Fishers precision
parameter.
TECTONIC AND STRUCTURAL IMPLICATIONS OF PALEOMAGNETIC AND AMS STUDY 143
Fig. 7. Results of the AMS study. Plots presented for each locality
show mean direction of the magnetic lineation Kmax (solid
square), mean direction of Kmin (solid circle), magnetic foliation
plane (thin line), direction of tectonic lineation fitting best magnet-
ic lineation (cross), tectonic foliation plane fitting best magnetic
foliation (bold line). Appropriate labels denoting the tectonic lin-
eations and foliations systems are also included. Abbreviations de-
noting the localities are the same as in Fig. 3.
Table 3: The characteristics of the anisotropy of magnetic suscep-
tibility together with the corresponding directions of the tectonic
lineation L and the foliation planes S after Návesòák (1993).
Locality
Kmax in situ Kmin in situ
P
L
S
KMSZ
È
257/53
38/29
1.081.17
-
S4
KV
354/70
250/7
1.041.21
-
-
J
196/46
32/23
1.031.10
-
S4
MA
295/9
45/57
1.101.28
L3
S4
DSZ
DO
66/24
320/31
1.06 1.13
L2
S2
ML
258/22
135/51
1.03 1.6
L3
S3
Outside Main Shearing Zones
GP
scattered results
1.03 1.10 -
-
Fig. 8. Plot of the magnetic foliation planes obtained in this study
with the exception of KV together with the corresponding tectonic
foliation planes.
P anisotropy parameter, for formulas see text; L mineral lineation; S
mineral foliation; KMSZ Koice-Margecany shearing zone; DSZ Dobiná
shearing zone.
situated within an independent tectonic unit (see chapter Geo-
logical setting and sampling) the magnetic foliation does not
correspond to any tectonic one. The situation in MA suggests,
that magnetic fabric reflects both shearing systems: East-Car-
pathian through magnetic foliation and West-Carpathian
through magnetic lineation. In the DSZ zone the situation is
different: in the chlorite schists of DO magnetic foliation cor-
responds with the S2 and the magnetic lineation with the L2 ,
whereas in the phyllites of the ML with S3 and L3 systems,
respectively. Systems L2, S2 are related to the TGSZ zone,
systems S3, L3 to the LSSZ zone, both belonging to the
same West-Carpathian system. Figure 8 summarizes the mag-
netic foliation planes (with the exception of the result for KV)
and the tectonic foliation planes. The presented pattern shows
distinct relations between the magnetic foliation in localities
from the KMSZ zone and the tectonic foliation S4 associated
with this zone (East-Carpathian shearing system). Magnetic
foliations in localities from the DSZ zone are close to both tec-
tonic foliations linked with the West-Carpathian shearing sys-
tem proving that the magnetic anisotropy originated due to the
Alpine deformations. The rotations suggested by the paleo-
magnetic research (see chapter Paleomagnetic results and dis-
cussion) agree with the sense of shearing.
anisotropy parameter remain in a broad range (Table 3). The
directions of the maximum and minimum anisotropy ellipsoid
cluster reasonably well everywhere with the exception of the
GP. The mean directions of Kmax and Kmin axes in situ and
that of tectonic lineations L and foliations S (Návesòák 1993)
corresponding to the magnetic anisotropy directions in respec-
tive localities are summarized in Table 3. Fig. 7 presents mean
directions of Kmax and Kmin and the magnetic foliation
planes together with the corresponding tectonic lineations and
foliation planes. In the localities from the KMSZ zone there
are no correlations of the magnetic and tectonic lineations,
with the exception of MA where the Kmax direction is close to
the L3. The magnetic foliation (Kmin) corresponds to the tec-
tonic (mylonitic) foliation S4 in three localities from the
KMSZ zone (C, J, MA) whereas in the KV which is perhaps
144 KRUCZYK et al.
Conclusions
1 The NRM of the studied rocks is carried by secondary
magnetic minerals and represents secondary overprinting.
2 The presence of reversed polarity of CHRM in KV
and J and normal polarity in remaining localities indicates
that remagnetization was not synchronous.
3 In the localities situated along the KMSZ zone the re-
magnetization took place after folding, in the localities situat-
ed along the DSZ zone and within the unit during folding.
4 All the studied Paleozoic metamorphic rocks of the
Gemeric Superunit became remagnetized during the Middle
Miocene probably between anomaly 6 (20 Ma) and anomaly
5 (10 Ma).
5 After the remagnetization episodes the particular tec-
tonic blocks became rotated due to the activity of the shear-
ing systems, blocks lying close to the dextral KMSZ zone ro-
tated clockwise (with the exception of MA that was
influenced by both systems), blocks lying close to the sinis-
tral DSZ zone rotated counterclockwise.
6 The AMS originated due to the activity of the shear-
ing zones, the magnetic fabric agrees with the tectonic fab-
ric: S4 fits the magnetic foliation in the KMSZ localities, S2
and S3 fit the magnetic foliation in the DSZ localities.
Acknowledgments: The work was done in the frame of the
scientific cooperation between the Slovak Academy of Sci-
ences and the Polish Academy of Sciences with support of
the Institute of Geophysics, Polish Academy of Sciences,
Warsaw, Project 5/1998 and the Slovak Grant Agency
VEGA, Project 2/5136/98.
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