GEOLOGICA CARPATHICA, 49, 1, BRATISLAVA, FEBRUARY 1998
3343
PALEOMAGNETIC STUDY OF TRIASSIC SEDIMENTS FROM
THE SILICA NAPPE IN THE SLOVAK KARST, A NEW APPROACH
JADWIGA KRUCZYK
1
, MAGDALENA KADZIALKO-HOFMOKL
1
, IGOR TÚNYI
2
,
PAVOL PAGÁÈ
2
and JÁN MELLO
3
1
Institute of Geophysics, Polish Academy of Sciences, Ks. Janusza 64, 01-452 Warsaw, Poland
2
Institute of Geophysics, Slovak Academy of Sciences, Dúbravská 9, 842 28 Bratislava, Slovak Republic
3
Slovak Geological Survey, Mlynská dolina 1, 817 04, Bratislava, Slovak Republic
(Manuscript received March 18, 1997; accepted in revised form December 11, 1997)
Abstract: Intensive paleomagnetic and rock magnetic study were performed for Triassic limestones from the Silica
Nappe in the Slovak Karst. Five exposures situated on the eastern and western side of the títnik-Pleivec fault were
sampled for this study. In all exposures a secondary component of remanence of normal polarity (N), carried by
secondary PSD magnetite was found. In the Silická Brezová exposure (SB) apart from the N component, another
secondary component of reversed polarity (R), carried by hematite, was isolated. Both components were acquired
after folding. The R component was acquired during the Odra reversal event in the Oligocene (Birkenmajer et al.
1977). Comparison of its direction with the reference data let us conclude that the area belonged during this time to the
African affinity. The declination of the R component suggests that after this magnetization period the studied region
rotated anticlockwise by about 90
o
around an intraplate vertical axis together with the whole Pelso megaunit. Accord-
ing to Márton et al. (1995) and Márton & Fodor (1995) the rotation took place in two phases, the first one by about 50
o
took place in the Early Miocene, the second one, by about 30
o
in the Late Miocene. The N component, isolated by
us, seems to have been acquired during the Middle Miocene after the first and before the second rotational phases: its
declination agrees with a counterclockwise rotation of the Silica Nappe by about 3040
o
during the Late Miocene, as
postulated by the cited authors. The inclination of the N component is lower, than the expected for Miocene, but
agrees with the Miocene results for the Bükk region also belonging to the Pelso block, confirming the idea about the
Miocene southern escape of the Pelso block (Márton 1993). The final tectonic activity in the study area was con-
nected with formation of the títnik-Pleivec fault (Late Tertiary-Quaternary). Our results suggest, that the fault is of
rotational type and resulted in different tilting of beds situated on its eastern and western sides.
Key words: Silica Nappe, paleomagnetic directions, Triassic limestones.
Introduction
Paleomagnetic investigations of the Triassic limestones from
the Silica Nappe which forms part of the Pelso megaunit were
first performed by Márton E. et al. (1988) and Márton P. et al.
(1991). The first of the cited papers concerns exposures situ-
ated in the Aggtelek Mts. in Hungary, the second one ex-
posures situated in the Slovak Karst. Their study revealed the
presence of the secondary component of natural magnetic re-
manence (NRM) of normal polarity and some traces of a
component of reversed polarity. The latter was not discussed
in detail, but the authors hinted that it is a primary Triassic
one. The best grouping of normal component found for the
Slovak Karst (Márton et al. 1991) calculated after results ob-
tained for five exposures was obtained after 65% unfolding.
According to the cited authors the mean direction (D = 319.9,
I = 42.4, k = 168,
α
= 5.9) suggests that the rocks were re-
magnetized during the Late Cretaceous, and that at that time
the studied area belonged to the African affinity. The cited re-
sults, as well as other paleomagnetic and stress study per-
formed within the Pelso megaunit indicate, that the whole
unit underwent 80
o
counterclockwise rotation against the sta-
ble European plate and that the rotation took place in two epi-
sodes: the first one with rotation of about 50
o
took place at the
end of the Early Miocene, and the second one with rotation of
about 30
o
took place at the beginning of the Late Miocene
(Márton et al. 1995; Márton & Fodor 1995).
The above mentioned suggestion about premordiality and
Triassic age of the reverse component of NRM found in one
of the Silica Nappe exposure does not agree with the private
communication of Mock & Channel (1993). They studied
samples collected along a profile situated between the top
and bottom parts of the Silická Brezová exposure. Accord-
ing to their results normally and reversely magnetized beds
appear alternatingly along the profile suggesting several
succesive reversals. This inconsistency in interpretation of
the reverse component of NRM encouraged the present au-
thors to repeat the paleomagnetic study of the Silica Nappe
Triassic limestones.
Our first attempt at interpretation was published in ab-
stract form in Kruczyk et al. 1996. There we have stated,
that in the SB exposure (one of the six sampled by us) apart
from the secondary normal component of remanence, ap-
pears a well grouped component of reversed polarity with
the direction after bedding correction being: D = 89, I = 54,
α
= 5, k = 12. We interpreted it as the primary one of Triassic
age. Four other exposures revealed the normal secondary
component similar to the one isolated in the SB (one expo-
sure gave no interpretable results). The best grouping of the
normal component was obtained for the 25% unfolding, D =
322, I = 50,
α
= 10, k = 66. The closeness of this mean to the
mean obtained by Márton et al. (1991) led us to interprete
our result in the same way as synfolding remagnetization
component acquired in the Late Cretaceous when the Silica
34 KRUCZYK, KADZIALKO-HOFMOKL, TÚNYI, PAGÁÈ and MELLO
Nappe belonged to the African affinity. We have also adopted
the idea of Márton et al. (1995) and Márton & Fodor (1995)
about the two episodes of the counterclockwise rotation of
the Silica Nappe.
Despite the apparent logic of the results presented so far,
we have decided to proceed with the interpretation of our
data and to look more closely at the possible influence of
the important tectonic fault títnik-Pleivec (S-P) on our re-
sults. This fault cuts the area of our study into western and
eastern parts leaving three of sampled exposures at its west-
ern side. We have decided to check, whether the faulting in-
fluenced beds on both sides of the fault in the same way.
This paper presents the new, revised approach to our data.
In our new analysis we assumed a rotational character of the
S-P fault (see Dadlez & Jaroszewski 1994).
Outline of geology and sampling
The Silica Nappe, situated in the area of the Slovak Karst,
belongs to the Inner Western Carpathians, as well as to the
Pelso megablock Fig. 1a. The nappe is an allochthonous
unit shifted to its present position from the south due to the
collision of two fragments of the African and European
plates Apulia and Bohemia, in the paleoalpine period. Apart
from northward shifting, the Pelso block underwent several
rotations and became cut by numerous faults. The tectonic
activity in the Silica Nappe is thought to have begun during
the Late Jurassic and lasted until the Late Tertiary-Quater-
nary. The temperatures in the region during tectonic activity
did not exceed 200300
o
C.
The region of our study lies within the Silica Nappe on two
sides of the títnik-Pleivec (S-P) tectonic fault dated as Late
Tertiary-Quaternary directed NNW and divided into two
branches in its southern segment, see Fig. 1b. Two of the six
sampling localities: Silická Brezová (SB) and Silica (S) are sit-
uated close to the eastern border of the fault, one Èoltovo
(C) lies between the two southern branches of the fault. The
other three: Drienèany (D), Hruov (H) and Budikovany (B)
are lying close to one another at about 15 km to the west from
the fault. The geological sketch map of the studied area with
sampling sites is presented in Fig. 1b. Table 1 presents the age
and nature of the sampled limestones together with the bedding
parameters and number of hand samples collected in the field.
Hand samples were cut in the laboratory into standard cylin-
ders for paleomagnetic and rock magnetic purposes.
Technics of experimental study
Standard paleomagnetic investigations of collected material
were performed independently in the three paleomagnetic lab-
Fig. 1a. The location of the Pelso megaunit (after Márton et al.
1995), the black square denotes the study area.
Fig. 1b. Geological map of the studied part of the Silica Nappe. Bold line títnik-Pleivec fault, 1 Gemeric Unit, 2 Bôrka Nappe,
3 Meliata Unit, 4 Turòa Unit, 5 Silica Unit, 6 Tertiary cover. Sampling places are denoted by filled circles: SB Silická Bre-
zová, S Silica, C Èoltovo, H Hruov, D Drienèany, B Budikovany.
PALEOMAGNETIC STUDY OF TRIASSIC SEDIMENTS FROM THE SILICA NAPPE 35
Fig. 2. Typical thermomagnetic curves obtained for Silická Bre-
zová (a), Hruov (b), Drienèany (c). Ir isothermal remanence
acquired in the field of 1T; 1 curve of the first heating, 2
curve of the second heating.
Locality
Bedding,
A z/tilt
Paleonto-
logical age
Lithgology N um ber of
sam ples
Silická Brezová
SB
258/18 N orianH allstadt
lim estones
31
Silica S
298/53 Low er
Ladinian
Reiphlin
lim estones
9
È oltovo C
35/65
U . A nisian-
L. Ladinian
Reiphlin
lim estones
8
H ruov H
120/50 Ladinian
Reiphlin
lim estones
4
D rienèany D
120/35 Ladinian
Reiphlin
lim estones
10
Budikovany B
120/35 N orianH allstadt
lim estones
8
oratories: Warsaw (Kruczyk, Kadzialko-Hofmokl), Bratislava
(Túnyi, Pagáè) and Barcelona (Túnyi). They comprised ther-
mal demagnetization with a non-magnetic furnace (Magnetic
Measurements in Warsaw, MAVACS Geofyzika Brno in Bra-
tislava and Schoenstedt in Barcelona) and alternating field de-
magnetization (2G device in Warsaw). Natural remanent mag-
netization was measured with the 2G kriomagnetometer in
Warsaw, JR5 spinner magnetometer (Geofyzika Brno) in Brati-
slava, and SQUID in Barcelona. The demagnetization results
were analysed in Warsaw with the use of a special program
package.
Rock magnetic study comprising identification of mag-
netic minerals were performed in Warsaw and Barcelona.
Magnetic minerals were identified with optical microscopy,
by means of thermomagnetic analysis, magnetic hysteresis
measurements and IRM acquisition curves. Thermomagnet-
ic analysis consisted of thermal decay in a non-magnetic
space of the isothermal remanence Ir acquired in a 1T field
in (non-heated) fresh specimens and in the same specimens
annealed to 600
o
C. The results reveal blocking tempera-
tures of magnetic minerals present in the rock before and af-
ter heating. Hysteresis curves were measured with the VSM
apparatus of Molspin Ltd with the highest available field of
1T. The obtained parameters of saturation magnetization Is,
saturation remanence Irs, coercive force Hc and coercivity
Hcr bring information about the kind and grain size of mag-
netic minerals present in the rock. Measurements of IRM
acquisition curves performed with Molspin in Barcelona for
fresh and heated specimens also help in identification of
magnetic minerals.
Magnetic susceptibility was measured before heating and
after each heating step in order to monitor mineral changes
that could influence natural remanence. Anisotropy of sus-
ceptibility before heating and after the final heating step
was also measured. The KLY2 bridge of Geofyzika Brno
was used for this purpose, analysis of measurements of sus-
ceptibility anisotropy was performed with the ANISO11
program of Jelinek (1977) and the Spheristat Programme.
Magnetic mineralogy
Study of magnetic minerals carriers of NRM show,
that the amount of magnetic minerals in the studied lime-
stones is very low resulting in very low values of natural
remanence, saturation magnetization and saturation rema-
nence. Thermomagnetic curves (Fig. 2a,b) suggest, that
magnetic minerals comprise mostly fine-grained magnetite
accompanied by hematite in the SB and sometime with a
small amount of iron hydroxides (as in D Fig. 2c).
Heating in air to 600
o
C results in production of new mag-
netite from nonmagnetic minerals curves 2 in Fig. 2.
Isothermal remanence Ir increased due to heating from
several to several tens of times. Most extensive study was
performed for the SB limestones, because in this exposure
the normal and reversed components were found. Analy-
sis with an optical microscope shows, that the SB lime-
stones are very fine-grained with some pigment of proba-
bly magnetite origin visible between the grains. In some
Table 1: Lithostratigraphic characteristics of sampled Triassic
limestones.
36 KRUCZYK, KADZIALKO-HOFMOKL, TÚNYI, PAGÁÈ and MELLO
specimens cherry-red irregular clusters, probably of he-
matite origin, were observed. IRM acquisition curves
performed for SB material show presence of low and
high coercive mineral phases magnetite and hematite,
respectively Fig. 3a. After heating to 600
o
C magnetite
decidedly prevails Fig. 3b.
Study of hysteresis parameters were performed for 7
specimens from SB and 2 from S before heating, after heat-
ing to 300
o
C and to 600
o
C. Fig. 4a,b presents an example
of typical hysteresis curves obtained before heating. Pres-
ence of diamagnetic minerals is proved by the slope of the
curve, Fig. 4a, the same curve after slope correction in
shown in Fig. 4b. Slope corrected values of hysteresis pa-
rameters are presented in Table 2. Heating to 300
o
C does
not change them much, Is and Irs increase only after heating
to 600
o
C. Values of Hc and Hcr obtained for S specimens
decrease due to heating, probably as a result of increased a
magnetite/hematite ratio. Fig. 5 presents parameters Irs/Is
versus Hcr/Hc. According to Day et al. (1977) and Channel
& Mc Cabe (1994) the results lie inside the pseudosingle
domain (PSD) area of the plot characteristic for magnetite,
both before and after heating. This result suggests, that the
magnetite grains prevailing in fresh specimens are PSD
grains of secondary origin, similar to the new magnetite
grains formed due to heating.
Mean magnetic susceptibility (Kmean) of all the studied
limestones is very weak and due mainly to paramagnetic
and diamagnetic minerals; its values range from 10 to
Fig. 4. Typical hysteresis curve showing (a) diamagnetic slope and (b) the same curve after slope correction.
Fig. 3. IRM acquisition curves for specimens from Silická Brezová, (a) before heating and (b) after heating to 600
o
C.
PALEOMAGNETIC STUDY OF TRIASSIC SEDIMENTS FROM THE SILICA NAPPE 37
10
×
10
6
SI. It increases after heating to 450550
o
C due to
formation of new magnetite Fig. 6. Directions of Kmax
(maximum susceptibility) axes obtained for the SB speci-
mens form a semi-regular pattern Fig. 7. They are distrib-
uted along a weakly pronounced girdle roughly perpendicular
to the NE-SW direction of tectonic tension suggested by Már-
ton & Fodor (1995) for the Middle Miocene.
Paleomagnetic results
Demagnetization of pilot specimens with thermal and al-
ternating field methods showed, that they respond much
better to the temperature, than to the field method. There-
fore most of the material was demagnetized thermally. The
following figures (Fig. 8ae) present the typical demagneti-
zation results obtained for material from each exposure. The
most numerous group of collected and demagnetized sam-
ples comes from the SB exposure. Here, as is seen in
Fig. 8a, the NRM is composed of two components a nor-
mal one demagnetized in the temperature range of 300350
o
C, and a reversed one isolated at a high temperature of
about 600
o
C and posessing much lower intensity. Fig. 9a
presents stereographic distribution of both normal and re-
versed components from this exposure showing, that the
grouping of the normal one is better than the reversed. In the
S limestones the well pronounced normal component with
unblocking temperatures in the range 200400
o
C and direc-
tion similar to the direction of N component found in the SB
is present. At temperatures higher than about 400
o
C, the in-
Fig. 6. Mean susceptibility after consecutive heating steps mea-
sured for specimens from Silická Brezová (SB), Hruov (H),
Budikovany (B) and Drienèany (D).
Fig. 5. Hysteresis parameters for Silická Brezová and Silica lime-
stones. Irs saturation remanence, Is saturation magnetization,
Hcr remanent coercivity, Hc coercive force. Single domain
(SD), pseudo-single domain (PSD) and multidomain (MD) fields af-
ter Day et al. (1997). Open circles SB before heating, full cir-
cles SB after heating to 300
o
C for 30 min, open triangles SB
after heating to 600
o
C for 30 min, full triangles S before heating,
open squares S after heating to 300
o
C for 30 min, crosses S
after heating to 600
o
C for 30 min.
Locality Temperature
o
C
Is
mA/m
2
Irs
mA/m
2
Hc
mT
Hcr
mT
SB
20
300
600
0.2-0.9
0.3-1.1
0.5-1.2
0.1-0.08
0.08-0.4
0.1-0.4
25-35
26-35
28-45
45-80
60-80
60-105
S
20
300
600
0.28, 0.31
0.26, 0.36
0.75, 1.00
0.07, 0.12
0.07, 0.09
0.23, 0.34
31, 59
36, 37
20, 22
130, 150
50, 80
70, 65
Is - saturation magnetization, Irs - saturation remanence,
Hc - coercive force, Hcr - coercivity of remanence
Table 2: Ranges of hysteresis parameters obtained for the SB speci-
mens and values of hysteresis parameters obtained for the S specimens.
Fig. 7. Distribution of Kmax directions for Silická Brezová.
38 KRUCZYK, KADZIALKO-HOFMOKL, TÚNYI, PAGÁÈ and MELLO
Fig. 8. Typical demagnetization results obtained (a) for Silická
Brezová, (b) for Silica, (c) for Èoltovo, (d) for Hruov,
(e) for Drienèany. Left up stereographic projections, left
down decay of intensity of remanence: Irm/Inrm intensity of
remanence after consecutive heating steps/intensity of natural rema-
nence before heating, right Zijderveld orthogonal plots.
tensity of remanence increases and its directions become er-
ratic, but have reversed polarity Fig. 8b and Fig. 9b. A
similar situation is encountered in the C rocks (Fig. 8c) the
normal component of NRM with declinations smeared within
the fourth quadrant and inclinations ranging between 30
o
and
65
o
is isolated in the 200400
o
C temperature range. At high-
er temperatures, the intensity of remanence increases (after
500
o
C very rapidly). This component has reversed polarity
and directions scattered throuout the whole stereonet
Fig. 8c and Fig. 9c. In the H and D rocks only the normal
component of NRM was found. In the H specimens this com-
ponent, isolated at temperatures higher than 400
o
C, has well
grouped directions Fig. 8d and Fig. 9d. In the D rocks de-
magnetization temperatures ranged from 200 to 400
o
C and
the isolated directions form a rather scattered group Fig.
8e and Fig. 9e. From the B exposure no interpretable results
were obtained. For each of the presented exposure the mean
directions of obtained normal components in situ (bbc) and
after correction for the bedding (abc) were calculated. For the
SB limestones mean direction was also calculated for the re-
versed component. The results, together with parameters of
Fisher statistics are presented in Table 3a and Fig. 10a,b.
PALEOMAGNETIC STUDY OF TRIASSIC SEDIMENTS FROM THE SILICA NAPPE 39
Discussion of the results
The data presented in Table 3a and Fig. 10a,b indicate that
the full bedding correction considerably increases the scatter
of mean directions of N component in comparison with the in
situ distribution. But in the in situ coordinate system they do
not form a very tight cluster and their distribution has a more
ellipsoidal, than circular shape. Therefore we have decided to
see how incremental unfolding of our exposures influences
the scatter of the mean directions. We have calculated appro-
priate means assuming the 15, 25, 50, 75 % of unfolding (f).
As is shown in Fig. 11a the result obtained with 25 % of un-
folding of all five exposures gave the best grouping. The dis-
tribution of means for this case is shown in Fig. 10c and the
obtained mean direction is presented in Table 3b. This result
was interpreted as Late Cretaceous remagnetization in our
previous work (Kruczyk et al. 1996). The obtained declina-
tion suggested CCW rotation of the Nappe by about 25
o
around the intraplate vertical axis.
The reversed component was isolated in SB and some trac-
es of it were found in the S and C exposures, but the scatter of
its directions in the two latter places is too large for calcula-
tion by reasonable means. Only the SB results form a cluster
tight enough for calculation of the mean (Table 3a, Fig. 9a).
There is no possibility for performing a fold test here, but
knowing that this component is carried by submicroscopic,
probably secondary hematite we suppose that it was acquired
after folding and its Triassic age is hardly possible. It should
be treated in the same way as the N component isolated in SB
and be calculated with the same unfolding parameter. In order
to resolve the question of the proper frame of coordinates for
both components we took into account the geological situa-
tion of sampled localities assuming, that the fault has a rota-
tional character. According to Dadlez & Jaroszewski (1994) a
rotational fault is characterized by a curved trajectory and
may change the prefaulting structural tilt. According to this
definition we have made several trials of calculation of mean
N direction assuming different unfolding for exposures situat-
ed at both sides of the S-P fault. The exposures SB and S are
the eastern ones, the exposure C situated between two
southern branches of the fault is also treated as an eastern
one because of similarity of the in situ direction of its N com-
ponent to N directions obtained for SB and S. The H and D
exposures form the western group. The results obtained for
numerous trials with various values of f show that the best
grouping is obtained if the eastern group is unfolded to 25 %
and the western one to 50 % see Fig. 11b presenting
changes of k for f = 0 and f = 25 for the eastern group and f
changing from 25 to 75 % for the western one. Mean direc-
tion is not influenced much with the changes of f, its declina-
tion ranges from 319
o
to 327
o
and inclination ranges from 54
o
to 64
o
. The final best result is presented in Table 4 and
Fig. 10d. It confirms our assumption concerning the rotation-
al character of the S-P fault. According to this result the nor-
mal component of NRM (N) was aquired after folding and
before faulting of the study area and the S-P fault changed the
bedding tilt on its eastern and western sides in a different
way. Faulting resulted in increasing the original tilt of the
eastern side by 25 %, and of the western by 50 % (values
Locality nDbbc Ibbc =
95
k Dabc Iabc =
95
k Polarity
SB
101 311
61
2 37 293
50
2 37
N
SB
69
98
-69 5 12
89
-54 5 12
R
S
23 329
63
6 30 312
13
6 30
N
C
17 303
58 11 12
1
22 11 12
N
H
18 323
25
4 80 358
65
4 80
N
D
19 322
42
9 14 357
71
9 14
N
Mean
normal
5
318
50 16 22 333
48 34 6
N
Component
D (25%)
I (25%)
=
95
k
R
95
-65
5
12
N
322
50
9
66
Age
African
Decli-
nation
African
Incli-
nation
European
Decli-
nation
European
Incli-
nation
References
Early Triassic
339.8
28.9
30.8
30.0
Westphal et
al. 1986
M -L. Triassic
342.7
44.6
38.8
36.3
Jurassic
336.8
36.2
21.7
58.6
Late Cretaceous
348.3
48.5
15.0
56.0
Late
Cretaceous-
Paleocene
357.2
52.0
13.1
56.4
Paleocene-
Eocene
0.4
54.0
11.1
56.6
Oligocene
4.2
62.0
20.7
57.7
Miocene
5.8
61.6
9.3
60.1
Silica Nappe
Triassic
sediments
magnetized in
Late Cretaceous
319.9
42.4
Márton et
al. 1991
Locality
% untilt
D
I
=
95
k
plat
25
307
59
2
59
S
25
320
51
6
30
C
25
327
55
11
12
H
50
332
47
4
80
D
50
331
58
9
14
N Final
Mean
SB,S,C 25
H,D 50
324
54
7
113
35
Table 3a: Mean paleomagnetic directions for studied localities
calculated in situ (bbc) and after full tectonic correction (abc).
Table 3b: Mean paleomagnetic directions of R and N components
after 25 % unfolding of all exposures.
n number of entries, Dbbc, Ibbc declination and inclination before bed-
ding correction, Dabc, Iabc declination and inclination after edding correc-
tion, D (25%), I (25%) declination and inclination after 25% unfolding,
α
95
, k
Fisherian parametrs
Table 4: Mean directions of normal component N calculated for
studied exposures with different stages of untilting.
Table 5: Expected paleomagnetic field directions calculated for the
Silica Nappe (lat 48.5N, long 20.5E) after African and European ref-
erence data by Westphal et al. 1986, and direction of normal compo-
nent of NRM obtained for the Silica Nappe by Márton et al. (1991).
plat - paleolatitude in degrees, other symbols as in Table 2
M - L Triassic = Middle-Late Triassic
40 KRUCZYK, KADZIALKO-HOFMOKL, TÚNYI, PAGÁÈ and MELLO
of f expressed in relation to the present tilt). Taking all said above
into account we came to the conclusion, that the mean direction
of the normal cmponent of NRM presented in Table 4 presents
the direction of the magnetizing field. This component is a sec-
ondary one carried mainly by secondary PSD magnetite and is of
chemical origin. According to the above discussion we calculat-
ed the R component with 25 % of unfolding, the respective result
is shown in Table 3b and this result will be the subject of inter-
pretation as the final direction of this component.
In order to discuss the possible ages of the two obtained com-
ponents of natural remanence we have compared them with the
directions expected for the Silica Nappe under the assumption of
its African or European affinity, and with the data obtained for
the Silica Nappe by Márton et al. (1991), see Table 5.
PALEOMAGNETIC STUDY OF TRIASSIC SEDIMENTS FROM THE SILICA NAPPE 41
Fig. 9. Stereographic distributions of isolated components of NRM in situ: (a) Silická Brezová, normal and reversed components, (b)
Silica, normal and reversed components, (c) Èoltovo, normal and reversed components, (d) Hruov normal components, (e)
Drienèany normal components. Ranges of unblocking temperatures are shown above each plot with the exception of Silická Brezová.
Comparison of the results obtained here with the refer-
ence ones shows the following:
inclination of the R component is close to the expected
African inclination for the Oligocene, declination of this
component implies counterclockwise (CCW) rotation of the
study area around an intrablock vertical axis by about 90
o
during the times following the magnetization event. It was
probably acquired during the Odra reversal event deter-
mined during study of the Tertiary basaltic rocks in Lower
Silesia and dated at about 27Ma, Birkenmajer et al. (1977).
declination of the N component is very close to the
declination of the component obtained by Márton et al.
(1991), but its inclination is different.
declination of this component implies CCW rotation of
the study area around an intrablock vertical axis by about
3040
o
during times following the magnetization event.
42 KRUCZYK, KADZIALKO-HOFMOKL, TÚNYI, PAGÁÈ and MELLO
inclination of the N component is lower, than the one
expected for the African and European Miocene. The
paleolatitude calculated for the Silica Nappe from this incli-
nation is 35
o
and agrees with paleolatitude obtained for the
Miocene for the Gemer-Bükk region also belonging to the
Pelso block (Márton 1993). The author explains this result as
due to the southern escape of the Pelso block during the
Miocene. Our result confirms this idea.
Sense, angles and possible ages of rotations implied by
our results agree with conclusions of Márton et al. (1995)
and Márton & Fodor (1995) drawn for the whole Pelso
megaunit of which Silica Nappe is only a part. According to
these authors the Pelso megaunit underwent two phases of
CCW rotation: the first one, by about 50
o
, took place in the
Early Miocene and the second one, by about 30
o
, took place
in the Late Miocene. According to this timing the N compo-
nent obtained here was acquired between both rotational
phases and the R one during one of the Oligocene inver-
sion periods.
Conclusions
1. All investigated Triassic limestones became remagne-
tized during the Tertiary due to the tectonic activity (com-
pressions, extensions, rotations) that took place in the Pelso
megaunit.
2. Remagnetization took place after folding.
3. Remagnetization processes took place in two different
times.
Fig. 10. Mean directions of normal components obtained for all studied localities with the exception of Budikovany: (a) in situ (bbc),
(b)) after bedding correction (abc), (c)) after 25% of unfolding of all exposures, (d)) after 25% of unfolding of Silická Brezová,
Silica and Èoltovo and 50% of unfolding of Hruov and Drienèany.
PALEOMAGNETIC STUDY OF TRIASSIC SEDIMENTS FROM THE SILICA NAPPE 43
Fig. 11. Scatter parameter k against the % of unfolding f of Silica Nappe exposures (a) the same f for all exposures (b) full circles:
exposures SB,S,C in situ, hollow triangles: exposures SB,S,C with f = 25 %.
the reversed component R isolated only in one exposure
and carried by secondary hematite was formed in the Oli-
gocene, most probably during the Odra reversal event
(Birkenmajer et al. 1977). Its declination shows that after its
acquisition the study area was rotated counterclockwise by
about 90
o
. It corresponds to the sum of angles of rotation of
both CCW Miocene rotational phases.
the normal component N found in all exposures and car-
ried by secondary magnetite was acquired after the first ro-
tational phase during the Middle Miocene. Its declination
suggests that after its acquisition the study area was rotated
counterclockwise by about 3040
o
.
4. The inclination of the R component agrees with the incli-
nation expected for the Silica Nappe under the assumption
that during the Oligocene it belonged to the African plate.
Birkenmajer K., Jeleñska M., Kadzia³ko-Hofmokl M. & Kruczyk
J., 1977: Age of deep seated fracture zones in Lower Silesia
(Poland), based on K-Ar dating and palaeomagnetic dating of
Tertiary basals. Ann. Soc. Geol. Pol., XLVII, 4, 545552.
Channel J.E.T. & Mc.Cabe C., 1994: Comparison of magnetic hys-
teresis parameters of unremagnetized and remagnetized lime-
stones. J. Geophys. Res., 99, No. B3, 46134623.
Day R., Fuller M. & Schmidt V.A., 1977: Hysteresis properties of
titanomagnetites: grain size and compositional dependence.
Phys. Earth. Planet. Int., 13, 260267.
Dadlez R. & Jaroszewski W., 1994: Tektonika. PWN, Warszawa, 1743.
Jelinek V., 1977: The statistical theory of measuring anisotropy of mag-
netic susceptibility and its application. Geofyzika, Brno, 588.
Kruczyk J., Kadzia³ko-Hofmokl M., Túnyi I., Pagáè P. & Mello J.,
1996: Paleomagnetism of the Triassic limestones from the
Silica Nappe, Slovak Karst-tectonic implications. Abstract to
the 5th Biennial Meeting New Trends In Geomagnetism.
Geol. Carpathica, 47, 3, 159160.
5. The paleolatitude of the Silica Nappe during N remag-
netization period agrees with paleolatitude of the Gemer-
Bükk region confirming the idea of the southern escapeof
the Pelso Unit.
6. The títnik-Pleivec fault that was formed after rota-
tions and remagnetizations changed tilting of the investigat-
ed beds lying on both its sides in a different way. It in-
creased the tilt of beds more on its eastern, than on its
western side.
Acknowledgement: The initial information on sampling lo-
calities by the lahe Dr. R. Mock is appreciated. The authors
are grateful to Dr. J. M. Parés, Director of Paleomagnetic
laboratory of IEC CSIC Barcelona, who kindly made it pos-
sible to perform measurements in the laboratory mentioned.
Márton E., 1993: The itinerary of the Transdanubian Central
Range: An assessment of relevant paleomagnetic observa-
tions. Acta Geol. Hung., 37, 12, 135151.
Márton E. & Fodor L., 1995: Combination of palaeomagnetic and stress data
a case study from North Hungary. Tectonophysics, 242, 99114.
Márton E., Márton P. & Less G., 1988: Paleomagnetic evidence of
tectonic rotations in the southern margin of the Inner West
Carpathians. Phys. Earth. Planet. Int., 52, 256266.
Márton P., Rozloník L. & Sasvári T., 1991: Implications of a
palaeomagnetic study of the Silica Nappe, Slovakia. Geophys.
J. Int., 107, 6775.
Márton E., Vass D. & Túnyi I., 1995: Late Tertiary rotations of the
Pelso megaunit and adjacent Central Western Carpathians.
Knihovnièka ZPN, 16, 97108 (in Slovak).
Westphal M., Bazhenov M.L., Lauer J.P., Pecherski D.M. & Sibuet
J.C., 1986: Paleomagnetic implications on the evolution of
the Tethys Belt from the Atlantic Ocean to the Pamirs since
the Triassic. Tectonophysics, 123, 3782.
References