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GEOLOGICA CARPATHICA, FEBRUARY 2008, 59, 1, 19—30

www.geologicacarpathica.sk

1. Introduction

The Western Carpathians are the northernmost, W-trend-
ing branch of the Alpine orogenic belt, linked to the East-
ern Alps in the west and to the Eastern Carpathians in the
east. They are part of a complexly curved orogenic belt,
which originated from the collisional tectonics and escape
of fragments of the Adriatic (Apulian) plate with the Euro-
pean foreland during Oligocene to Miocene times (e.g.
Mahe  1986; Ratschbacher et al. 1991a,b). They are di-
vided into two principal tectonic domains [Outer (OWC)
and Inner Western Carpathians (IWC)] separated by the
Pieniny Klippen Belt (PKB; Fig. 1A; e.g. Biely 1989). The
IWC are formed by pre-Tertiary formations (Variscan
crystalline basement with Upper Paleozoic—Mesozoic
cover, overlain by Mesozoic nappes), exposed beneath
the post-tectonic Paleogene and Neogene sedimentary
and Miocene to Quaternary volcano-sedimentary covers.
Crystalline basement is exposed in several mountain
ranges that form three structural zones (from external to in-
ternal regions: the Tatric, Veporic and Gemeric belts), ar-
ranged roughly parallel to the PKB (Plašienka et al. 1997).

The geodynamic evolution of the IWC during Paleo-

gene times is not sufficiently understood. Basically, two
contrasting models are proposed: Kováč et al. (1994) ar-
gued from stratigraphic, fission track and paleomagnetic
data that during the Eocene some of the Tatric crystalline

Thermal evolution of the Žiar Mountains basement (Inner

Western Carpathians, Slovakia) constrained by

fission track data

MARTIN DANIŠÍK

1,

, MILAN KOHÚT

2

, ISTVÁN DUNKL

3

 and WOLFGANG FRISCH

1

1

University of Tübingen, Institute of Geosciences, Sigwartstrasse 10, D-72076 Tübingen, Germany;   martin.danisik@uni-tuebingen.de

2

Dionýz Štúr State Institute of Geology, Mlynská dolina 1, 817 04 Bratislava, Slovak Republic;  milan.kohut@geology.sk

3

Geoscience Center Göttingen, Sedimentology and Environmental Geology, Goldschmidtstrasse 3, D-37077 Göttingen, Germany

(Manuscript received February 13, 2007; accepted in revised form June 13, 2007)

Abstract:  Thermal  evolution of the Žiar Mts was studied using zircon and apatite fission track (ZFT and AFT)
thermochronology applied to basement and sedimentary rocks. Basement samples from the Tatric Unit of the Inner
Western Carpathians (IWC) yielded cooling ZFT ages in the range from 109 ± 5.9 to 92.1 ± 5.3 Ma and apparent AFT ages
between 33.7 ± 2.7 and 24.4 ± 1.7 Ma. Paleogene sediments of the Central Carpathian Paleogene Basin (CCPB) yielded
apparent AFT ages of 73.2 ± 2.7 and 49.6 ± 1.8 Ma. ZFT data show that during early Late Cretaceous thrusting the
crystalline basement was buried by a nappe pile to crustal depths with temperatures between 210 and 310 

°C, where the

crystalline core was metamorphosed at anchizonal conditions. AFT data revealed a complex thermal evolution, implying
that after nappe stacking and metamorphism the basement was exhumed to the surface and that it was buried beneath a
thick pile of sediments of the CCPB. The CCPB was inverted and the basement finally exhumed in the Late Oligocene to
Early Miocene. The AFT system in the basement and CCPB samples record also a distinct, Middle Miocene thermal event,
related to the increased heat flow induced by upwelling of hot mantle material and related volcanic activity. Our data
support the hypothesis that the study area was part of a large CCPB basin and contradict the widely accepted assumption
that the Variscan crystalline core of the Žiar Mts lacks an Alpine metamorphic overprint.

Key words: Cretaceous and Tertiary, Western Carpathians, Žiar Mts, burial and exhumation, thermochronology,
thermal modeling, fission track dating.

bodies of the IWC were uplifted in the course of collision
between the Adriatic and the European plate. In contrast,
based on the sedimentological, fission track and thermal
modeling data, Kázmér et al. (2003) and Danišík et al.
(2004) pointed out that the Paleogene forearc basin (Cen-
tral Carpathian Paleogene Basin – CCPB) played a more
important role in the geodynamic evolution of the IWC
than previously thought. These authors proposed that dur-
ing the Eocene period the pre-Tertiary formations of the
IWC underwent maximum burial when they were overbur-
dened by a thick pile of CCPB sediments.

In order to address this issue, we used zircon and apatite

fission track (ZFT and AFT) thermochronology, because
this technique can be efficiently used to decipher the ther-
mal evolution of rocks in the uppermost crust (e.g. Wagner
et al. 1977; Gleadow & Fitzgerald 1987) and to estimate
the amount of overburden (e.g. Gleadow et al. 1983;
Naeser et al. 1989). We targeted the small mountain range
of the Žiar Mts in the IWC (Fig. 1A) due to a number of
advantageous features provided by this area: (i) as com-
pared to the other core mountains of the IWC, the geologi-
cal structure of the Žiar Mts is simple – pre-Tertiary
formations are formed mainly by a deeply eroded Variscan
granitic pluton exposed in the central part of the range,
whereas the Mesozoic sequences are sparsely preserved in
the northern and southern marginal parts; (ii) remnants of
the CCPB sediments occur in close vicinity to the

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20

DANIŠÍK, KOHÚT, DUNKL and FRISCH

crystalline basement and provide an excellent opportunity
to constrain the original thickness of the eroded cover;
(iii) geological records of erosion and uplift events are
relatively well preserved; (iv) the crystalline basement of
the Žiar Mts is traditionally regarded to be exhumed and
uplifted during the Eocene (e.g. Kováč et al. 1994; Hók et
al. 1995, 1998), which would make it one of the earliest
emerged crystalline complexes of the Tatric belt; (v) ow-
ing to the “inherited information” of its assumed
geodynamic stability, the crystalline basement of the Žiar
Mts was considered to be promising from the viewpoint of
finding a geological structure for hosting radioactive
waste in a deep repository (e.g. Kováčiková et al. 1995).

In this study, we aim to investigate the thermal history

of the samples taken from the crystalline basement and
also from sediments of the CCPB in order to better under-
stand the final cooling evolution of the Žiar Mts. Our FT
data enable us to present a new interpretation of the ther-

mal history of the Žiar Mts and will have implications for
the geodynamic development of the IWC.

For this purpose we first briefly review the geological

setting of the study area and the geochronological data
and briefly explain the potential of the FT method. Then
we present and interpret our data and discuss their infer-
ences with respect to the thermotectonic evolution of the
Žiar Mts and to the geodynamic evolution of the IWC as a
whole.

2. Geological setting

The Žiar Mts is one of the smallest core mountains in

the Tatric Unit of the IWC situated in Central Slovakia
(Fig. 1B). The mountains form an asymmetric, NW-trend-
ing and NE dipping horst structure, flanked by two Neo-
gene grabens (Fig. 1C).

Fig. 1. A – Simplified geological map of the Western Carpathians with location of the study area (black rectangle): OWC – Outer Western
Carpathians (vertical hatching); IWC – Inner Western Carpathians (pre-Tertiary formations are depicted in light grey); PKB – Pieniny Klip-
pen Belt (black); CCPB – Central Carpathian Paleogene Basin (dark grey); Neogene volcanic formations are depicted by horizontal hatching;
Neogene and Quaternary sedimentary formations are depicted in white. B  – Geological sketch map of the Žiar Mts and surrounding areas
(modified after Kohút 1999; Lexa et al. 2000) with location of the samples (black dots) and fission track data. The text beside the dots shows,
from top to bottom, the sample code, the central apatite FT age with 1  error in Ma (bold) and the central zircon FT age with 1  error (italic
in brackets). Measured track length distributions are presented in the bottom right. The numbers on histograms give, from top to bottom, the
sample code, the mean track length and the standard deviation of the track length distribution. C – Shaded DEM of the Žiar Mts.

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THERMAL EVOLUTION OF THE ŽIAR MOUNTAINS CONSTRAINED BY FISSION TRACK DATA (SLOVAKIA)

The pre-Tertiary formations of the mountains were ar-

ranged during Late Cretaceous (Turonian) nappe stacking
(Plašienka 1997) and consist of three tectonic units, from
bottom to top: Tatricum—Variscan crystalline basement
with sedimentary cover and two Mesozoic cover units –
Fatricum and Hronicum (e.g. Mahe  1986; Plašienka et al.
1997). The crystalline basement of the Tatricum is built
up of various unmetamorphosed granitoids (e.g. Plašienka
et al. 1997), related to the Variscan orogeny (Krá  &
Štarková 1995). The Mesozoic cover units consist pre-
dominantly of carbonates, their occurrences are restricted
to the northwestern and southeastern margin of the horst
mountains. The relics of a kaolin weathering crust of Oli-
gocene?—Middle Miocene age preserved in situ on the top
of the granitoids in the northern part of the mountains
(Fig. 1B) indicate that the basement must have experi-
enced a period of tectonic quiescence and formed a pene-
plain favourable for supergene kaolinization (Kraus
1989). The minimum age of the kaolinization is not ex-
actly known, since it was indirectly assessed from avail-
able AFT data (unpublished data of J. Krá , see Table 26 in
Kraus 1989). The horst mountains are cut by SW-NE ori-
ented faults into several blocks without significant verti-
cal displacement (e.g. Nemčok & Lexa 1990).

The post-tectonic cover is represented by sediments of

the CCPB and by sedimentary and volcanic rocks of Neo-
gene age. Sediments of the CCPB are exposed only along
the southern margin of the horst and have a maximum
thickness of a few hundred meters (Gross et al. 1984).
Boreholes, however, proved the presence of an up to
1000 m thick column beneath the Neogene deposits in
both adjacent intramontane depressions (Fendek et al.
1990). The sequence transgressively overlies the pre-Ter-
tiary formations and consists of: (i) Lutetian basal con-
glomerates and breccias, overlain by limestones and marls
(Borové Formation), pointing to syn-sedimentary tectonic
activity followed by subsidence; (ii) Priabonian to
Rupelian flysch – shales, siltstones and sandstones (Huty
and Zuberec Formations), indicating continued subsid-
ence; (iii) the sequence ends with locally preserved
Rupelian to Aquitanian sandstones and shales (Biely
Potok Formation), which unconformably overlie the Huty
Formation. Due to severe erosion and tectonic activity
during the Neogene, the original thickness of the Paleo-
gene sequence is not exactly known.

Neogene sedimentary formations are exposed in two

fault-bounded intramontane depressions (Turiec Depression
in the east, Horná Nitra Depression in the west). They
transgressively overlie all pre-Neogene formations and
reach a maximum thickness of more than 1000 m (Fendek
et al. 1990; Hók et al. 1995, 1998). In general, they consist
of Eggenburgian to Quaternary shales, sandstones, con-
glomerates and breccias, often interbedded by volcanic and
volcaniclastic rocks (for details see Hók et al. 1995, 1998).

The Neogene volcanic formations (lava flows, tuffs, py-

roclastic and volcanic breccias) are subduction-related
calc-alkaline rocks of Langhian to Serravalian age (ca.
16—12 Ma; e.g. Lexa & Konečný 1998 and references
therein). They form the remnants of stratovolcanoes in the

southeastern part of the study area and, as mentioned above,
can also be found in the sequences of neighbouring
Neogene depressions.

2.1 Older geochronological and thermochronological
data

There is general lack of geochronological data

available from the Žiar Mts, with the exception of Ar/Ar
ages on muscovite and biotite from the granitic rocks,
yielding  338.1 ± 1.7 and 287 ± 1.3 Ma, respectively (Krá
& Štarková 1995). These ages are interpreted as cooling
ages of the granitic massif after emplacement during the
Variscan orogeny.

Actually the only thermochronological data can be

found in the work of Kováč et al. (1994), who reported
four apatite fission track ages from the granitic pluton,
ranging from 52 ± 7 to 46 ± 5 Ma. However, these data lack
important information such as exact sample location and,
even more important, track length data, which are critical
for proper interpretation of the ages (see section 3). The
authors interpret their ages in terms of the closure tempera-
ture concept, correlating the age to the passage of the
~ 110 °C isotherm (Dodson 1973; Wagner & Van den
haute 1992). In conclusion they found evidence for
Eocene uplift and denudation of the crystalline basement
and related it to the Eocene compressional event.

3. Fission track dating method

Fission track analysis is a standard thermochronological

technique based on the spontaneous nuclear fission of

238

U, which produces linear trails of radiation damage (fis-

sion tracks) in the crystal lattice of U-bearing minerals.
The principle of fission track dating relies on the constant
production rate of fission tracks through time (Price &
Walker 1963). The fission track age is determined from the
ratio of the parent isotope (

238

U measured via fission

tracks induced by thermal neutron irradiation procedure)
and daughter products (spontaneous fission tracks
counted per unit area observed on the polished surface of
a mineral). The technique can be applied to a variety of
U-bearing minerals, apatite and zircon being the best
studied and most common. Owing to the phenomenon of
fission track annealing, measured fission track ages need
not necessarily be related to the age of rock/mineral
formation or a distinct geological event.

Fission tracks are stable over geological time, but only

at relatively low temperatures (

~ 60 °C in apatite,

~ 210 °C in zircons). At elevated temperatures in the so-

called partial annealing zone (PAZ), the fission tracks are
gradually repaired, or “annealed”. At temperatures above
~ 120 °C in apatite and ~290 °C in zircon they are totally
annealed in a short time (e.g. Zaun & Wagner 1985;
Hurford 1986; Wagner & Van den haute 1992).

In case of apatite the fission tracks are approximately

16  m long and, depending on the time spent in the PAZ,
they are progressively shortened to different extents. The

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22

DANIŠÍK, KOHÚT, DUNKL and FRISCH

resulting length distribution of spontaneous tracks is thus
a direct record of the thermal history experienced by the
sample. Age and length distribution therefore enable a
quantitative reconstruction of the thermal history of a
sample (Gleadow et al. 1986a,b) by means of sophisticated
numerical annealing models (e.g. Laslett et al. 1987; Green
et al. 1989; Crowley et al. 1991; Ketcham et al. 1999).

As mentioned above, the measured FT age alone di-

rectly refers to a geological event only in special cases.
Depending on the nature of the thermal history experi-
enced, there are essentially three different kinds of “ages”
recognized in the thermochronological system: formation
ages, cooling ages and apparent ages. The meaning of
these can be straightforwardly resolved from the track
length distributions (see Fig. 2).

(i) “Formation” (or “event”) ages are typical of volcanic

rocks, when they rapidly cooled through the PAZ
(Fig. 2A). The resulting track length distribution is
unimodal and narrow (as shown by standard deviations –
SD – of  < 1.5  m) with long mean track length
(MTL > 14  m).

(ii) If a sample undergoes slow and monotonous cooling

through the PAZ (Fig. 2B), the track length distribution is

unimodal and broader (SD > 1.5  m) and the MTL shorter
( < 14  m). The age is described as a “cooling” age. This
kind of cooling is common in basement rocks with a
simple cooling history. An important point here is that the
observed age is not related to a distinct geological event
but the cumulative effect of the slow passage of the rock
through the PAZ.

(iii) The third case shows a complex thermal history

with a period of reheating into the PAZ (Fig. 2C), which is
a common case in rocks buried by sediments. The track
length distribution in a such case is frequently bimodal
and broad (SD > 2.5  m) with a short MTL ( < 13  m). The
age is a combination of two components and described as
an “apparent” (or “mixed”) age. Like in the previous case,
the age has no direct significance in terms of the timing of
a geological event. However, even in this case it is pos-
sible to reveal the magnitude and timing of cooling and
reheating periods by thermal modeling.

In conclusion, without information on track length dis-

tribution the true meaning of the AFT ages cannot be un-
raveled. In order to provide a clear picture of the meaning
of AFT data, the data are commonly presented as a combi-
nation of measured ages, track length distributions (histo-
grams with MTL and SD) and modeled time-temperature
paths or envelopes of possible solutions.

The precise usage of terminology is essential in under-

standing FT data. In the past thermochronological data
were often incorrectly interpreted in terms of “uplift” or
“uplift rates”. Therefore we begin with definitions of terms
used throughout this paper. We follow the works of En-
gland & Molnar (1990), Ring et al. (1999), Stüwe & Barr
(1998), and Stüwe (2002), and use the following termi-
nology:

The terms uplift (motion upwards) and subsidence (mo-

tion downwards) describe vertical motion relative to a
fixed reference level (e.g. relative to the geoid, sea level,
or other reference points). Two kinds of uplift are distin-
guished: (i) surface uplift – vertical motion the Earth’s
surface; and (ii) rock uplift – vertical motion of rock.
Both terms should always be used together with specifica-
tion of the reference level. Surface uplift can be directly
determined for instance from paleobotany, paleoaltimetry,
paleoclimatology or GPS data. Both types of uplift can be
indirectly determined from sediments in the surrounding
basins. However, neither of these motions can be inter-
preted from thermochronology. In this study, due to lack
of data, the term uplift is only used for rock uplift, with the
erosion base level as a reference level.

The terms exhumation (motion towards the surface)

and  burial (motion away from the surface) describe
vertical displacement of rocks relative to the Earth’s
surface. They can be indirectly determined for instance
from geobarometry, geochronology, or thermochronology,
when the thermal structure of the crust as defined by iso-
therms can be reconstructed with sufficient accuracy.
Thermochronological data in fact do record the motion of
rock relative to the isotherms, which form the reference
frame. Therefore they can be interpreted in terms of exhu-
mation or burial, but cannot tell anything about uplift

Fig. 2. Three different cooling paths of hypothetical samples (upper
panel:  A – rapid cooling resulting in formation age; B – slow
steady cooling resulting in cooling age; C – complex cooling with
thermal overprint resulting in apparent age) and corresponding track
length distributions (lower panels). The fission track results are
calculated using the model of Ketcham et al. (1999) with the initial
track length of 16.1  m. The numbers on the track length histograms
give, from top to bottom, the calculated fission track age, the mean
track length and the standard deviation of the track length
distribution. Note that inspite of markedly different cooling styles,
the resulting age is the same (60 Ma) in all three cases.

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THERMAL EVOLUTION OF THE ŽIAR MOUNTAINS CONSTRAINED BY FISSION TRACK DATA (SLOVAKIA)

movements with the exception of the special case when
surface uplift equals zero.

4. Sampling and analytical procedure

The sampling campaign focused primarily on the crys-

talline basement and sediments of the CCPB (for locations
and coordinates see Fig. 1B and Table 1). All but one of
the samples were taken from surface outcrops; sample DA-6
was collected in the drill core from the exploratory bore-
hole RAO-4 in the central part of the mountains (Kohút et
al. 2004), from the depth of 43 m below surface. The base-
ment samples were collected along a NNE-trending
transect crossing the central part of the crystalline core.
Two samples of the CCPB sediments were collected from
the southern foothill of the mountains. Sample DA-1 is a
sandstone of the Biely Potok Formation (Šimon et al.
1997), which was dated as Chattian—Aquitanian by
palynomorphs (

~26—20 Ma; Šimon et al. 1994). Sample

DA-2 is a sandstone attributed to the Huty-Zuberec Forma-
tions; the age is indirectly determined by the overlapping
rocks as Upper Oligocene (Rupelian?) (

~33—26 Ma;

Šimon et al. 1997).

The analytical procedure used for FT analysis can be

found in the Appendix.

5. Results

AFT ages were successfully determined on 5, ZFT ages

on 3 samples (Tables 1 and 2, Fig. 1B). All samples passed
the chi-square test at the 95 % confidence interval. The FT
ages are reported as central ages with 1  errors. ZFT
ages were determined on basement samples only; they
are 109.1 ± 5.9 (DA-3), 92.1 ± 4.3 (DA-6), and 92.1 ± 5.3
(DA-5) Ma. AFT ages are in the range from 73.2 ± 2.7 (DA-1)
to 24.4 ± 1.7 Ma (DA-5), no age-elevation correlation was
observed. Track length distributions were measured on 3
apatite samples (DA-1, DA-2, DA-3; Table 1, Fig. 1B). All
samples are characterized by unimodal, fairly broad track
length distributions [SD between 1.4 (DA-1) and 1.9  m
(DA-2)] with short MTL [MTL between 12.0  m (DA-2)
and 13.1  m (DA-1)]. In all samples Dpar values (Dpar –
etch pit diameter of fission tracks parallel to the crystallo-
graphic c-axis at the polished, etched, and analysed apa-
tite surface; Crowley et al. 1991; Naeser 1992; Burtner et
al. 1994) range from 1.5 to 1.7  m (Table 1), pointing to a
homogeneous composition of the apatites close to the
fluorapatite end-member (Ketcham et al. 1999).

6. Interpretation and discussion

6.1 ZFT data

ZFT data from the basement show similar central ages in

the range of 

~110—90 Ma. From the AFT data (see below)

and the zircon single grain age distribution [P(

2

) > 95 %]

Table 1: 

Apatite fission track data

a

.

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24

DANIŠÍK, KOHÚT, DUNKL and FRISCH

we consider the ages not to be rejuvenated during the Ter-
tiary. The ages can be interpreted in two ways: (i) as cool-
ing ages – recording the cooling of the basement
following the thermal peak of metamorphism; or (ii) they
can directly document the thermal peak of metamorphism.
During metamorphism, the basement reached temperature
conditions sufficient to fully reset the ZFT system
( > 210 

°C). But, the maximum temperature must have

been less than 

~ 310 °C, so the Ar/Ar system in micas was

not fully reset, as indicated by Ar/Ar ages of 338.1 ± 1.7
and 287 ± 1.3 Ma (Krá  & Štarková 1995; closure
temperature of Ar/Ar system in muscovite is 

~350 °C and

in biotite 

~310 °C; Harrison et al. 1985; McDougall &

Harrison 1988).

ZFT ages indicate that the basement experienced a ther-

mal maximum during Albian to Turonian times. This coin-
cides with the well-known period of thrusting in the
Western Carpathians (Plašienka 1997). Thus we interpret
the ZFT ages as related to the tectonic burial of the Tatric
crystalline basement due to nappe stacking of the overly-
ing Mesozoic nappes. The crystalline complex was buried
to depths of 

~8—10 km in order to explain the required

temperatures using a gradient of 

~30 °C/km. However, be-

cause it is likely that the burial occurred in the
geodynamic context of an accretionary prism (see, e.g.
Plašienka et al. 1997), a lower geothermal gradient may be
expected and the basement might have reached greater
depths.

It is widely accepted that the crystalline basement of the

Žiar Mts has not experienced any metamorphism since
Variscan times (e.g. Plašienka et al. 1997). However, our
ZFT data show that the basement must have undergone at
least very low-grade metamorphism. This conclusion is
open to be corroborated by other kinds of data, such as il-
lite crystallinity, maturation of organic material, fluid in-
clusions, stable isotope analysis, or higher-temperature
geochronometry.

From the perspective of regional geology it is important

to note that similar evolution was documented in the East-
ern Alps, where the Austroalpine basement experienced
metamorphism related to the Eo-Alpine orogeny (e.g.
Frisch & Gawlick 2003) and some units were buried and
metamorphosed even at HP/LT conditions. The peak of
the metamorphism occurred at 

~100± 10 Ma (e.g. Thöni

& Jagoutz 1992; Thöni & Miller 1996), which exactly
fits our data.

6.2 AFT data

6.2.1 Basement samples

All samples from the granitic basement yielded Oli-

gocene AFT ages from 33.7 ± 2.7 (DA-6) to 24.4 ± 1.7 Ma
(DA-5) and are thus younger than the Eocene AFT ages re-
ported by Kováč et al. (1994; see section 2.1). The reason
of this discrepancy lies, according to our opinion, in the
dating technique used. Our data were measured by the ex-
ternal detector method (Gleadow 1981) using zeta calibra-
tion (Hurford & Green 1983), which became a standard
technique in the early nineties. In contrast, the data re-
ported by Kováč et al. (1994) were obtained by using the
population dating technique (POP), which ruled in the
seventies and eighties but since then has been abandoned.
One of the advantages of the external detector technique
is that it overcomes the problem of uncertainty about the
value of the fission decay constant (

F

), and it does not re-

quire knowledge of the absolute thermal neutron fluence
(see Wagner & Van den haute 1992). In contrast, the POP
technique requires precise knowledge of the absolute ther-
mal neutron fluence, which is measured by gamma activ-
ity of metal detectors and is difficult to determine, and
considers the decay constant of natural fission, which is
not exactly known (the most frequently published values
for 

F

 range from 6.85 to 8.46 10

—17

 year

—1

; Wagner &

Van den haute 1992). Both parameters can, depending on
the lambda value and metal detector used, introduce an
uncertainty of up to 35 % into the age calculation (see e.g.
Bigazzi 1981; Dunkl et al. 2003). The phenomenon of ex-
cessively old FT ages obtained by the POP method was
also observed by other authors (e.g. Andriesen 1990;
Dunkl et al. 2003), and there were attempts to recalculate
the POP AFT ages by using different values of the decay
constant. However, since the value of the decay constant
used for calculation was not published by Kováč et al.
(1994), the recalculation could not be done. We therefore
do not consider the ages of these authors further on.

Track lengths were measured only in the sample DA-3.

The sample yielded a unimodal, fairly broad (SD = 1.5  m)
distribution with short MTL (12.9  m), typical of rocks,
which experienced a complex thermal history and a long
or repetitive stay within the apatite PAZ (Gleadow et al.
1986a,b). Therefore the AFT ages from the basement are
interpreted as apparent ages without direct geological

Table 2: Zircon fission track data

a

.

a  

N – number of dated zircon crystals; 

s

  (

i

)  – spontaneous (induced) track densities ( 10

5

 tracks/cm

2

); N

s

 (N

i

) – number of counted

spontaneous (induced) tracks; 

d

  – dosimeter track density ( 10

5

 tracks /cm

2

); N

d

 – number of tracks counted on dosimeter; P(

2

probability obtaining Chi-square value (

2

) for n degree of freedom (where n = No. of crystals— 1); Age ± 1  – central age  ± 1 standard

error (Galbraith & Laslett 1993). Ages were calculated using zeta calibration method (Hurford & Green 1983), glass dosimeter CN-2, and
zeta value of 123.6 ± 2.1 year /cm

2

.

background image

25

THERMAL EVOLUTION OF THE ŽIAR MOUNTAINS CONSTRAINED BY FISSION TRACK DATA (SLOVAKIA)

meaning. The true meaning of the data can only be unrav-
eled by thermal modeling (section 7).

6.2.2 Sedimentary samples

Samples from the Lower Miocene (Chattian—Aquitanian)

and the Oligocene (Rupellian—Chattian) sandstones of the
CCPB yielded AFT ages of 73.2 ± 2.7 Ma (DA-1) and
49.6 ± 1.8 Ma (DA-2), respectively. Similarly to the
basement sample, track length distributions of both
samples are unimodal and broad (SD = 1.4—1.9  m) with
short MTL (12 and 13.1  m), indicating a complex ther-
mal history (Gleadow et al. 1986a,b). Again, the AFT ages
are apparent ages without direct geological meaning.

The AFT ages of both samples are clearly older than

their stratigraphic age (

~ 33—20 Ma). This means that the

samples have not undergone thermal overprint after depo-
sition, so the AFT thermochronometer has not been fully
reset and possibly still records the information on the ther-
mal history of the source area. There seems to be no princi-
pal difference in the single grain AFT ages: both samples
passed the chi-square test (P(

2

) > 90 %) and show very low

dispersion (see Fig. 3), indicating that the dated crystals
form single age populations. This can be interpreted in two
ways: (i) either the apatite grains have derived from source
areas with cooling ages of 

~73 and ~50 Ma, or (ii) all apa-

tite crystals, coming originally from different sources and
having diverse AFT ages, were partially reset after deposi-
tion and the AFT thermochronometer was set to a uniform
value. We prefer the second option, because there are sev-
eral sources of evidence indicating that the samples must
have been thermally overprinted during the post-deposi-
tional period. We will develop this idea in the next section.

paths (or envelopes). It should be kept in mind that the
thermal models are supposed to serve as a sort of test that
can justify or dismiss presumptive hypotheses, so the
modeling results should not be overestimated. Further, the
modeling results are often strongly dependent on the
skills of the modeler and the unavailability of other con-
straints on the thermal history results in too many accept-
able solutions. Therefore the models should be constrained
by as much information as possible.

In the following, we briefly review existing geological

and thermochronological data that can help us to con-
strain the thermal evolution of the dated structural block.

The thermal model of the sedimentary samples DA-1

and DA-2 was constrained by their depositional age
(T = 10—20 

°C at ~ 33—26 and ~26—20 Ma, respectively)

and that of the basement sample DA-3 by the ZFT age
(T 210 

°C at  ~110—90 Ma)  and by the age of exposure

of the basement to the erosion level (T = 10—20 

°C at

~ 20—15 Ma). The latter was inferred from the first
occurrences of according detrital material in the Early? to
Middle Miocene sequences in the neighbouring depres-
sions (Gašparik et al. 1995; Hók et al. 1995, 1998).

Modeling was performed in the unsupervised search

style. For all samples it revealed fairly similar tT paths
(Fig. 4) with one conspicuous feature: all samples exhibit
a distinct thermal event during the Middle Miocene, when
they were suddenly reheated to temperature levels be-
tween 

~60 °C and ~120 °C and afterwards cooled down to

near-surface conditions. This thermal event coincides with
the Middle Miocene volcanic activity in the region (

~15—

12 Ma) and affected the entire study area. This inference is
also corroborated by the vitrinite reflectance data (R

O

)

from the Serravalian coal formation in the Handlová mine
to the southeast of the Žiar Mts (for location see Fig. 1B).
Petrík & Verbich (1995) reported R

O

 values from 0.28 up

to 0.46 %, corresponding to maximum paleotemperatures
of 

~90—100 °C (Sweeney & Burnham 1990). The most ob-

vious reasons for temperature increase were (i) mantle up-
welling and/or (ii) magmatic activity producing large
stratovolcanoes in the vicinity, as inferred from the mul-
tiple occurrences of volcanic material in the Neogene
sedimentary record. The paleo-geothermal gradient in the
Middle Miocene cannot be quantified, because there exist
no AFT or vitrinite reflectance data from vertical profiles.

Although the AFT memory in the sedimentary samples

was partially erased during the Miocene, it is still possible
to get some information on the provenance of the samples.
The thermal modeling revealed a fairly broad range of tT
paths characterized by cooling between 

~85 and ~50 Ma.

Thus it can be concluded that the samples originated from
a source area that cooled down through the apatite PAZ
during Late Cretaceous to early Paleogene times. This in-
terpretation is limited insofar the starting point in the ther-
mal model could not be constrained by ZFT data.

The modeled cooling path of the basement sample sug-

gests that, before reaching surface conditions in the Late
Oligocene to Early Miocene, the basement cooled rapidly
through the apatite PAZ between 

~ 35 and ~25 Ma. Prior

to this period of fast cooling, the basement was residing in

Fig. 3. Radial plots of the samples representing CCPB sediments.

7. Discussion

7.1 Thermal history modeling

As already advised in sections 6.2.1 and 6.2.2, the mea-

sured AFT ages are in fact geologically meaningless. But,
it is possible to reveal their meaning by thermal modeling
and to express the results in terms of time-temperature

background image

26

DANIŠÍK, KOHÚT, DUNKL and FRISCH

temperature conditions above 

~ 120 °C so that the AFT

system was totally reset. Therefore the cooling evolution
of the basement in the period between the onset of fast
cooling and the preceding mid-Cretaceous thermal event
as fixed by the ZFT age cannot be reconstructed by ther-
mal modeling alone (see the field marked with question
mark in Fig. 4C). The simplest answer to the question what
happened to the basement between 90 and 35 Ma was pro-
posed by Kováč et al. (1994): after mid-Cretaceous thrust-
ing and burial the basement cooled down in a moderate
fashion from temperatures above 

~ 210 °C to ~120 °C and

remained there buried over a period of 60 Ma. There is,
however, some evidence indicating that the basement
must have been exposed to erosion after the thrusting and
before the Paleogene transgression. For instance, Činčura

& Köhler (1995) report Paleoalpine karstification of the
Mesozoic carbonates during Late Cretaceous to early Pa-
leogene times, when extensive parts of the IWC were ex-
posed to subaerial weathering. Furthermore, there are
abundant occurrences of Mesozoic rocks in the basal se-
quence of the Borové Formation of the CCPB (

~50—39 Ma,

Lutetian—Bartonian) showing that the Mesozoic nappes
were eroded during that time. Most importantly, rare
occurrences of granitic pebbles are reported from the
Borové Formation near Ráztočno close to the Žiar Mts
(Šimon et al. 1997). This signifies that even the granitoids
of the basement were eroded prior to the transgression of the
CCPB sediments.

Taking into consideration these arguments, we con-

clude that the basement was residing in near surface con-

Fig. 4. Thermal modeling results of AFT data of the samples (A, B – sediment samples DA-1 and DA-2; C – basement sample DA-3)
obtained with HeFTy program (Ketcham 2005). Results are displayed in time-temperature diagrams (left diagrams). Light grey
envelopes – good fit; thick black line – best fit; black boxes – fixed constraints defined according to independent geological data.
Right diagrams – frequency distribution of measured confined track length data overlain by a calculated probability density function
(best fit). Meas. and Mod. – measured and modelled age in Ma (first numbers) and MTL ± SD in  m (second and third numbers),
respectively; GOF – goodness of fit (statistical comparison of the measured input data and modeled output data, where a “good” result
corresponds to value 0.5 or higher, “the best” result corresponds to value 1). Note that modeled tT paths are valid only inside the
apatite PAZ, however outside this temperature range they must not necessarily represent the real thermal trajectory of a sample unless
constrained by other data. For explanation of the field marked with question mark in the diagram C, see the text.

background image

27

THERMAL EVOLUTION OF THE ŽIAR MOUNTAINS CONSTRAINED BY FISSION TRACK DATA (SLOVAKIA)

ditions prior to the Paleogene transgression. Thus, the
thermal model can be improved by an additional con-
straint (T = 10—30 

°C at ~ 50 Ma). The resulting cooling

path (Fig. 5)  shows that the thermal event at 

~110—90 Ma

recorded by ZFT data was followed by a period of cooling,
which might have occurred in a wide time span between
~ 100 and  ~60 Ma. We interpret this post-metamorphic
cooling event as a result of tectonic exhumation of the
basement due to gravitational collapse and lateral exten-
sion of the nappe stack (Ratschbacher et al. 1989).

After residence in the surface levels in Late Cretaceous

to early Paleogene times, the basement was reheated to
temperatures sufficient to fully reset the AFT system
(T 120 

°C). We suggest that the period of heating was as-

sociated with the sedimentation in the CCPB, which de-
veloped at that time as a forearc basin in the Eastern Alps
and IWC (Kázmér et al. 2003), and indicates that the base-
ment was buried beneath a thick pile of sediments. Since
the paleo-geothermal gradient in the Paleogene is not
known, the amount of overburden can be estimated only
indirectly: there is an indication that the CCPB formed in
a cold crustal environment with low heat flow, as inferred
from its fore-arc position (Kázmér et al. 2003) and from
paleotemperatures as deduced from fluid inclusions (Hurai
et al. 1995). Thus, we assume that the basement was buried
by a 

~8—12 km thick pile of CCPB sediments by using a

gradient of 

~10—15 °C/km.

Important implications from these conclusions are: (i)

inspite of only sparse remnants, the entire area of the west-
ern IWC was buried by CCPB sediments in the Paleogene;
(ii) during Paleogene, the geodynamic evolution of the
western IWC was identical with that of the eastern IWC,
where significant accumulations of CCPB sediments

reaching up to 6 km are still preserved, and additional col-
umn of 

~6 km has been eroded, as it is documented by

fluid inclusion data (Hurai et al. 2000).

Another consequence is that the minimum age of the ka-

olin weathering crust in the Žiar Mts can be constrained
more precisely. As mentioned in sections 2 and 2.1, it was
proposed that the granitic batholith of the Žiar Mts
emerged already in the Eocene – much earlier than the
other granitic bodies of the Tatric belt. Therefore weather-
ing of the granite commenced earlier and the kaolin
weathering crust could develop to a higher maturation
stage than anywhere else in the Tatric part of the IWC
(Kraus 1989). However, our data clearly show that the
batholith was still buried by sediments until middle Oli-
gocene times, and weathering could therefore have started
only after erosion of the CCPB sediments and exhumation
of the basement. This happened probably during the Late
Oligocene—earliest Miocene time as can be inferred from
(i) the stratigraphic record – the youngest CCPB member
preserved in the study area is of Chattian—Aquitanian age;
(ii) the thermal modeling results – suggesting cooling as-
sociated with exhumation of the basement after inversion
and erosion of the CCPB. From the Early to Middle Mio-
cene, the basement must have been residing in near-sur-
face conditions, forming a slightly undulating, low-relief
surface favourable for supergene kaolinization. The period
of kaolinization was terminated in the late Middle Mio-
cene by tectonic activity causing vertical movements of
tectonic blocks and creating the present horst-and-graben
structure of the study area (e.g. Nemčok & Lexa 1990).
The basement of the Žiar Mts was uplifted and largely
eroded, as documented by occurrences of coarse crystal-
line pebbles and redeposited remnants of the kaolin

Fig. 5.  Sumarizing time-temperature evolution of the crystalline basement drawn from regional geological considerations (main
geological events and stratigraphic record preserved in the study area are depicted at the bottom of the left diagram) and from thermal
modeling results of AFT data of the sample DA-3, constrained by an additional constraint (T = 10—30 

°C at  ~50 Ma, see text for expla-

nation). For explanation of the diagrams see the caption of the previous figure.

background image

28

DANIŠÍK, KOHÚT, DUNKL and FRISCH

weathering crust in the Serravalian to Tortonian sequence
of the Turiec Depression (Gašparik et al. 1974; Gašparik
1985; Kraus 1989).

8. Conclusions

The new fission track data combined with geological

evidence allow us to improve the thermal history of the
Žiar Mts and to draw the following conclusions:

– The Variscan crystalline basement of Žiar Mts expe-

rienced a complex thermal evolution characterized by
post-metamorphic cooling and two periods of reheating –
(i) during the Eocene and (ii) during the Middle Miocene.

– During the middle Cretaceous (

~110—90 Ma) com-

pressional tectonics, the crystalline basement of the Žiar
Mts was overridden by Mesozoic nappes, buried to a
depth with temperatures between 

~ 210 °C and ~310 °C,

and metamorphosed at least in anchizonal conditions, as
documented by ZFT and Ar/Ar data. The crystalline base-
ment thus records an Eo-Alpine overprint.

– The mid-Cretaceous thermal event was followed by a

period of fast cooling of the basement through the apatite
PAZ to near-surface conditions, as indicated by modeling
results; we relate this cooling process to tectonic exhuma-
tion of the basement due to gravitational collapse and lat-
eral extension of the nappe stack.

– In Late Paleocene—Early Eocene times, the emerged

basement was progressively buried by sediments of the
CCPB. During maximum burial the basement was reheated
to temperatures above 

~120 °C, indicating that the origi-

nal thickness of Paleogene sedimentary column reached
several kilometers in the study area. Thus we conclude
that a thick pile of CCPB sediments covered the western
part of the IWC.

– In the Late Oligocene to Early Miocene, the base-

ment of the Žiar Mts was exhumed; the CCPB was disinte-
grated and inverted, most of the sediments were eroded.
The minimum age of the kaolin weathering crust in the
Žiar Mts must be Late Oligocene—Early Miocene, as in-
ferred from the stratigraphy and AFT data.

– During the Middle Miocene, the samples from the

basement and from the CCPB were reheated to tempera-
tures between 

~60 °C and ~ 120 °C, as recorded by the

AFT data. This thermal event is related to the increased
heat flow induced by upwelling of hot mantle material
and related volcanic activity.

Acknowledgments: The German Science Foundation
(DFG) funded this study. We would like to thank J.
Kuhlemann (Tübingen) for assistance in the field and I.
Kraus (Bratislava) for providing valuable information on
the kaolin weathering in the Western Carpathians. Con-
structive reviews of the manuscript by Harald Fritz (Graz),
Franz Neubauer (Salzburg) and Dušan Plašienka (Bra-
tislava) are gratefully acknowledged. Gerlinde Höckh,
Dorothea Mühlbayer-Renner and Dagmar Kost (Tübingen)
are appreciated for careful sample preparation.

Appendix

Analytical procedure

Apatite and zircon crystals were recovered from whole-rock

samples using standard magnetic and heavy liquid separation tech-
niques. Apatites were embedded in epoxy, zircons in PFA
Teflon

TM

. Prepared mounts with grains were polished to 4  geom-

etry. Spontaneous fission tracks in apatites were revealed by etching
with 5.5 M HNO

3

 solution for 20 seconds at 21 

°C (Carlson et al.

1999; Donelick et al. 1999). Zircons were etched in an eutectic
mixture of KOH and NaOH at 215 

°C for 20 to 80 hours (Zaun &

Wagner 1985).

Uranium contents of the samples were assessed by irradiating

with thermal neutrons, which induce fission in a proportion of 

235

U

sample atoms. The induced tracks were recorded in an external de-
tector of low-uranium muscovite sheets (Goodfellow mica

TM

), at-

tached to the sample during irradiation (Gleadow 1981). Samples
were irradiated in the thermal column of the TRIGA nuclear reactor
at Oregon State University, Oregon, USA, where the Cd ratio is 

~14

which is well-suited to FT use. In the thermal facility, the reference
value of neutron flux at the face (fully inserted) is 8 10

10 

n/cm

2

/

sec; the flux gradient is approximately 2 % per cm. Neutron fluence
was monitored using Corning glass dosimeters CN-2 (for zircons)
and CN-5 (for apatites), with a known uranium content of 37 ppm
and 12 ppm, respectively (Hurford & Green 1982). Requested neu-
tron flux for apatite samples was 4.5 10

15

 n/cm

2

 and 1.5 10

15 

n/

cm

2

 for zircon samples. The apatite and zircon samples were irradi-

ated for 

~ 16 and ~5 hours, respectively.

After irradiation, mica detectors were etched in 40% HF for 30

minutes at 21 

°C. Finally, the mounts with corresponding micas

were attached side by side on a glass slide. Spontaneous 

238

U and

induced 

235

U fission tracks were counted under the Zeiss Axioskop 2,

equipped with a digitizing tablet, red LED cursor, drawing tube at-
tachment, and controlled by the computer program FT Stage ver-
sion 3.11 (Dumitru 1993). Tracks in apatites and mica-detectors
were counted with 1250  magnification using dry objective, tracks
in zircons were counted under the same condition but using oil im-
mersion (Cargille oil type B, n = 1.515). Only crystals with well-pol-
ished surface parallel to the crystallographic c-axis and
homogeneous uranium distribution were analysed, regardless of
track density. The minimum of 25 grains from crystalline rocks and
50 grains from sediments were counted. Lengths of horizontal con-
fined tracks and Dpars in apatites were measured only on grains
oriented parallel to the crystallographic c-axis. A total of 100 hori-
zontal confined tracks per sample were measured in order to obtain
a representative and statistically robust distribution.

The IUGS-recommended zeta calibration approach was used to

determine the ages (Hurford & Green 1983). The zeta value of
322.7 ± 5.3 for dosimeter glass CN-5 and 123.6 ± 2.1 for dosimeter
glass CN-2 has been derived by analyst M. Danišík from 13 deter-
minations of apatites and 9 of zircons from the Fish Canyon Tuff,
Durango, and Tardree Rhyolite (Hurford 1998). FT ages were cal-
culated with program TrackKey version 4.2 (Dunkl 2002).

Thermal history modeling based on AFT data

Modeling of the low-temperature thermal history based on AFT

data was carried out using the HeFTy modeling program (Ketcham
2005). An inverse Monte Carlo algorithm with multi-kinetic an-
nealing model (Ketcham et al. 1999) was used to generate tT paths.
The annealing kinetics of apatite fission tracks was assessed by mea-
surement of Dpar values. The input parameters we used in this
study are the following: the central FT age with 1  error; track
length distribution (c-axes projected after Ketcham 2003); kinetic

background image

29

THERMAL EVOLUTION OF THE ŽIAR MOUNTAINS CONSTRAINED BY FISSION TRACK DATA (SLOVAKIA)

parameter: Dpar values; the initial mean track length was estimated
from Dpar and c-axis projected track lengths; the end of the tT path
was set to 10 

°C according to present-day mean surface tempera-

ture. Known geological information such as stratigraphic age of
sedimentary samples or ZFT data measured on the same samples
were converted into time-temperature constraints in the form of
boxes. The tT paths were modeled in unsupervised search style.

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