www.geologicacarpathica.sk
GEOLOGICA CARPATHICA, FEBRUARY 2010, 61, 1, 19—27 doi: 10.2478/v10096-009-0041-0
Introduction
The Central Western Carpathians (CWC) is an interesting
and challenging area in which to study processes of exhuma-
tion. Occurrences of the Variscan crystalline basement are
common but, exposure can be poor and large portions of the
geological record are often missing. Crystalline complex
outcrops as isolated crustal blocks, lined-up in several oro-
gen-parallel belts (from the North to the South: the external
Tatric, the internal Tatric, the Veporic and the Gemeric belt;
Fig. 1A; modified after Andrusov 1968; Plašienka et al.
1997). This spatial arrangement resulted from forces induced
during the coupled tectonic processes of lateral extrusion
from the Eastern Alps toward the Carpathian region and sub-
duction roll-back beneath the Carpathian arc in the Miocene
(Royden et al. 1982; Ratschbacher et al. 1991a,b; Tari et al.
1992; Csontos 1995; Wortel & Spakman 2000; Frisch et al.
2000; Sperner et al. 2002).
Although the exhumation mechanisms are well described
(e.g. Ratschbacher et al. 1991a,b; Sperner et al. 2002), the tim-
ing of exhumation of individual crystalline complexes in the
CWC is still controversial due to a lack of reliable thermo-
chronological data (Kováč et al. 1994; Danišík et al. 2004,
2008a,b). This is somewhat surprising considering that crys-
talline complexes in the CWC were one of the first sites tar-
Thermal evolution of the Malá Fatra Mountains (Central
Western Carpathians): insights from zircon and apatite
fission track thermochronology
MARTIN DANIŠÍK
1
,2
, MILAN KOHÚT
3
, IGOR BROSKA
4
and WOLFGANG FRISCH
2
1
John de Laeter Centre of Mass Spectrometry, Applied Geology, Curtin University of Technology, GPO Box U1987, Perth WA 6845, Australia;
m.danisik@curtin.edu.au
2
Institute of Geosciences, University of Tübingen, Sigwartstraße 10, D-72076 Tübingen, Germany; frisch@uni-tuebingen.de
3
Dionýz Štúr State Institute of Geology, Mlynská dolina 1, 817 04 Bratislava, Slovak Republic; milan.kohut@geology.sk
4
Geological Institute, Slovak Academy of Sciences, Dúbravská cesta 9, P.O. Box 106, 840 05 Bratislava, Slovak Republic;
igor.broska@savba.sk
(Manuscript received May 12, 2009; accepted in revised form October 2, 2009)
Abstract: We apply zircon and apatite fission track thermochronology (ZFT and AFT, respectively) to the Variscan crystal-
line basement of the Malá Fatra Mts (Central Western Carpathians) in order to constrain the thermal history. The samples
yielded three Early Cretaceous ZFT ages (143.7±9.6, 143.7±8.3, 135.3±6.9 Ma) and one Eocene age (45.2±2.1 Ma),
proving that the basement was affected by a very low-grade Alpine metamorphic overprint. Although the precise timing
and mechanisms of the overprint cannot be unequivocally resolved, we propose and discuss three alternative explanations: (i)
a Jurassic/Cretaceous thermal event related to elevated heat flow associated with extensional tectonics, (ii) early Late Creta-
ceous thrusting and/or (iii) an Eocene orogeny. Thermal modelling of the AFT cooling ages (13.8±1.4 to 9.6±0.6 Ma)
revealed fast cooling through the apatite partial annealing zone. The cooling is interpreted in terms of exhumation of the
basement and creation of topographic relief, as corroborated by the sedimentary record in the surrounding Neogene depres-
sions. Our AFT results significantly refine a general exhumation pattern of basement complexes in the Central Western
Carpathians. A younging of AFT ages towards the orogenic front is evident, where all the external massifs located closest
to the orogenic front (including Malá Fatra Mts) were exhumed after ~ 13 Ma from temperatures above ~ 120 °C.
Key words: Cretaceous, Tertiary, Western Carpathians, Malá Fatra Mts, thermal overprint and exhumation,
thermochronology, thermal modelling, zircon and apatite fission track dating.
geted more than 30 years ago by (at that time) the newly de-
veloped fission track dating method (Burchart 1972; Krá
1977). After a promising start, the interest of geologists in the
low temperature thermochronology and rock exhumation in
this region declined. It was a study of Kováč et al. (1994),
which proposed a general exhumation model based on fission
track data collected over the years by J. Krá , combined with
geochronological, stratigraphic, and paleomagnetic data. Ac-
cording to this model, crystalline bodies were exhumed over
an ~ 80 Myr period, starting in the internal zones (Gemeric
and Veporic Units, respectively) during the Late Cretaceous—
Paleocene (90—55 Ma), and terminating in the external zones
(external Tatric belt) in the Miocene (20—10 Ma). Despite being
favoured by a major fraction of the Slovak geological commu-
nity, this exhumation model does not seem to be in agreement
with tectonic models proposed for the CWC (Ratschbacher et
al. 1991a,b; Sperner et al. 2002; Kázmér et al. 2003).
In this study, we apply apatite and zircon fission track (AFT
and ZFT, respectively) thermochronology in an attempt to bet-
ter constrain the low-temperature thermal evolution of the
Malá Fatra Mts (MF). We targeted this area because its spatial
position and assumed exhumation history does not fit into the
general pattern of the exhumation model proposed by Kováč
et al. (1994), namely that this mountain range belongs to the
external Tatric belt and was exhumed in the Oligocene—Mi-
20
DANIŠÍK, KOHÚT, BROSKA and FRISCH
ocene (Kováč et al. 1994). This is earlier than another crys-
talline complex – the High Tatra Mts, which belongs to the
same belt and was exhumed between 20 and 10 Ma (Krá
1977; Kováč et al. 1994).
Our AFT data allows us to present a new thermal evolu-
tion model for the MF and refine the low-temperature cool-
ing history. Moreover, we speculate that our ZFT data
revealed a metamorphic event of Eocene age in the MF,
which is so far the youngest record of metamorphism ever
reported from Variscan crystalline basement in the Western
Carpathians.
Geological setting and available thermochrono-
logical data
The MF are typical core mountains composed of Variscan
crystalline basement covered by Mesozoic units and two su-
perficial nappes, emplaced during Late Cretaceous (Cenoma-
nian—Turonian) nappe-stacking (Fig. 1B; Andrusov 1968;
Plašienka et al. 1997). This pre-Tertiary basement forms a horst
surrounded by Cenozoic depressions – Turiec Depression in
the SE and Rajec Depression in the NW (Fig. 1C).
The MF crystalline basement consists mostly of Variscan
granitoids (zircon U-Pb and WR Rb/Sr ages: 353+11/—5 Ma,
360±10 Ma; Shcherbak et al. 1990; Bagdasaryan et al. 1992)
with minor Variscan metamorphics (amphibolite-facies para-
gneisses, orthogneisses and amphibolites, and migmatites).
The crystalline basement was affected by an Alpine, very low-
grade metamorphic overprint (P-T conditions: ~ 0.3 GPa at
~300 °C), which resulted in deformation and formation of
very low-grade mineral assemblages such as pumpellyite, epi-
dote, chlorite, muscovite, albite and microcline (Faryad &
Dianiška 2003). Inferring from analogy with other CWC
units, these authors attribute the Alpine overprint to Creta-
ceous nappe tectonics and collisional processes.
The oldest post-intrusive cooling of the MF granitic pluton
is documented by muscovite Ar-Ar dating with a plateau age
of 344.8 ± 2.2 Ma (Hók et al. 2000). In the Late Permian, the
basement was exposed and the Lower Triassic quartzites were
deposited. Sedimentation of the cover unit in epicontinental
and marine milieu continued with occasional hiatus until the
Early Cretaceous (Albian) when sandstone and carbonate
claystone sediments were deposited. During Cenomanian to
Turonian times, the crystalline basement with its sedimenta-
ry cover was overthrust by two nappes (Krížna Nappe and
Fig. 1. A – Distribution of the pre-Alpine crystalline complexes in Slovakia. B – Geological sketch map of the MF (modified after Lexa et al.
2000) with location of the samples (black dots) and fission track ages in Ma. C – Shaded digital elevation model of the MF with local names.
21
THERMAL EVOLUTION OF THE MALÁ FATRA MOUNTAINS (CENTRAL WESTERN CARPATHIANS)
Choč Nappe, respectively) consisting mainly of Mesozoic
carbonates (Plašienka et al. 1997). The estimated thickness
of the overburden after thrusting is up to 3000 m, including
internal imbrications within the nappe units (Mahe 1986;
Hók et al. 2000).
There are no post-tectonic sediments preserved on the horst
of the MF, but post-tectonic evolution can be traced in the sed-
imentary record of the surrounding depressions. So-called
“Gosau deposits” (Late Cretaceous post-tectonic formations;
e.g. Michalík & Činčura 1992) have not been found in the
Rajec and Turiec Depressions. The first post-tectonic record is
represented by deposits of the Central Carpathian Paleogene
Basin (CCPB; Gross et al. 1984). The sedimentation in the
Rajec and Turiec Depressions began with basal carbonatic
conglomerates derived from the Mesozoic nappes (Borové
Formation – Lutetian—Bartonian), reaching an average thick-
ness of ~ 150—200 m (boreholes RK-22 in Rajec Depression
(Šalaga et al. 1976) and NE part of the Turiec Depression
(Hók et al. 1998)). It is important to note that there is no evi-
dence of erosion of the MF crystalline basement prior to the
CCPB transgression. The Paleogene sedimentary sequence
continues with flysch of the Huty and Zuberec Formation
(Bartonian—Priabonian), which reaches up to 1400 m in both
depressions and indicates rapid subsidence (Gross et al. 1984).
There are no Neogene sediments preserved in the Rajec De-
pression. In contrast, in the Turiec Depression the sequence
continues with up to ~ 1000 m thick column of Neogene
(Middle Miocene to Quaternary) sediments (Fendek et al.
1990; Hók et al. 1998).
Exhumation of the MF massif was first investigated by Krá
(1977) who presented one AFT age (25±18 Ma) measured on
granite and argued for Neogene uplift. This interpretation and
age was later adopted by Kováč et al. (1994) in a general ex-
humation model for the CWC. More thermochronological
data was presented by Hók et al. (2000), who reported the
age of 72 ± 3 Ma (Ar-Ar dating of sericite from an ultramylo-
nite) and argued for Alpine mylonitization of granitoid rocks.
The calculated average exhumation rates for mylonites are
500 m/Myr (Hók et al. 2000).
Samples and methods
For this study, seven samples of granite were collected
from surface outcrops (see Fig. 1B for sample location). The
investigated granites are predominantly hypidiomorphic,
medium-grained, without visible metamorphic foliation.
They were affected by fluid alteration in the post-magmatic
phase and during later low-grade metamorphic overprint.
This is shown by the crystallization of secondary mineral as-
sociation (sericite, saussurite, chlorite), giving the rocks a
slightly greenish colour.
Sample preparation and fission track analysis followed the
procedure outlined by Danišík et al. (2007a). The external de-
tector method (Gleadow 1981) was applied with the etching
protocols of Donelick et al. (1999) for apatite (5.5 M HNO
3
for
20 seconds at 21 °C) and Zaun & Wagner (1985) for zircon
(eutectic mixture of KOH and NaOH at 215 °C for 7 hours).
The zeta calibration approach (Hurford & Green 1983) was
adopted to determine the age. Samples were analysed with a
Zeiss Axioskop 2 microscope equipped with a digitizing tab-
let and drawing tube, and controlled by the computer program
FT Stage 3.11 (Dumitru 1993). Tracks in apatites and mica
detectors were counted with 1250
× magnification using a dry
objective while tracks in zircons were counted under the same
conditions but using an oil objective (Cargille oil type B,
n = 1.515). FT ages were calculated using TrackKey 4.2g
(Dunkl 2002). The annealing properties of apatite grains were
assessed by measurement of D
par
values (D
par
– the mean
etch pit diameter of fission tracks on prismatic surfaces of apa-
tite; e.g. Burtner et al. 1994). The low-temperature thermal
history based on AFT data (age, track length and D
par
data)
was modelled using the HeFTy modelling program (Ketcham
2005), operated with the multi-kinetic annealing model of
Ketcham et al. (1999).
Analytical results
The results of ZFT and AFT analyses are summarized in
Table 1 and shown in Figs. 1B and 2A. All samples passed
the chi-square test at the 95% confidence interval and thus
are considered to form one age population. All ages are re-
ported as central ages with 1 sigma errors.
Fig. 2. A – Track length distributions; explanation of histograms
(from top): sample code; mean track length ± standard deviation in
µm; number of measured tracks. B – Corresponding thermal mod-
elling results of AFT data displayed in time-temperature diagrams
modelled with HeFTy program (Ketcham 2005). Light grey enve-
lopes indicate good fit; solid black lines indicate the best fit. GOF is
goodness of fit (statistical comparison of the measured input data
and modelled output data, where a “good” result corresponds to val-
ue 0.5 or higher). The modelled cooling trajectories are valid only
within the partial annealing zone.
22
DANIŠÍK, KOHÚT, BROSKA and FRISCH
Three samples (MF-1, MF-3, MF-6) yielded Early
Cretaceous ZFT ages (143.7 ± 9.6, 143.7 ± 8.3,
135.3 ± 6.9 Ma) while one sample (MF-1) yielded an
Eocene age of 45.2 ± 2.1 Ma.
Six samples (MF-1, MF-3, MF-4, MF-5, MF-6, MF-7)
yielded a tight cluster of Middle—Late Miocene AFT
ages ranging from 13.8±1.4 to 9.6 ± 0.6 Ma. All sam-
ples are characterized by D
par
values of ~ 3 µm, which
indicates chlorine rich composition of apatites that are
typically more resistant to annealing than fluorine rich
apatites (Green et al. 1989; Carlson et al. 1999; Barba-
rand et al. 2003). Owing to low uranium content and
young AFT age, it was possible to measure track length
distributions (TLD) only in two samples (Table 1,
Fig. 2A). The TLD’s are unimodal, narrow (standard
deviations: 1.0 and 1.4 µm), negatively skewed, with
mean track lengths of 13.7 and 13.6
µm. Such TLD’s
are typical of moderate to fast cooling through the par-
tial annealing zone (PAZ) of apatites (Gleadow et al.
1986a,b). This was confirmed by thermal modelling re-
sults which revealed fairly similar time-temperature
paths for both samples, characterized by two stage
cooling history: a period of faster cooling through the
PAZ between ~ 13 and 9 Ma and a slower cooling last-
ing from ~ 9 Ma until the present (Fig. 2).
Interpretation and discussion
ZFT data
Since the track lengths in zircons were not measured
and thermal history could not be modelled, interpreta-
tion of ZFT ages is not straightforward and the meaning
of the data is less definitive. Therefore we discuss sev-
eral scenarios that can explain the observed age pattern
(see also Fig. 3).
Three samples from the western part of the range re-
vealed similar ZFT ages in the range of ~ 135—145 Ma.
All three samples passed the chi-square test and thus
represent single age populations. At first glance, the
ages can be interpreted as cooling ages, recording a
cooling of the basement in the Early Cretaceous. How-
ever, when plotted together (Fig. 3C), the spectrum of
single grain ages is fairly broad and ranges from ~185
to ~100 Ma. Although such a broad spectrum may be
representative of a distinct cooling event (see cooling
curve ‘a’ in Fig. 3B; arguments supporting this inter-
pretation are presented in paragraph 5 of this section), it
may also reflect a partial rejuvenation of the ZFT ther-
mochronometer and ZFT ages may thus be apparent.
In the following paragraph we present three argu-
ments supporting this interpretation (i.e. apparent ZFT
ages; cooling curves ‘b’ and ‘c’ in Fig. 3B, and possi-
bly ‘d’ discussed further below): (i) According to geo-
logical record, the area was subjected to normal marine
sedimentation during Jurassic to Early Cretaceous
times, where the total thickness of sediments hardly ex-
ceeded ~ 2 km and therefore sedimentary burial caus-
Table 1:
Fission
track
data
a
.
23
THERMAL EVOLUTION OF THE MALÁ FATRA MOUNTAINS (CENTRAL WESTERN CARPATHIANS)
ing resetting of the ZFT thermochronometer in the Early
Cretaceous is rather unlikely. (ii) To date there has never
been a distinct Early Cretaceous tectonic event reported from
the Western Carpathians or from the analogous units in the
Eastern Alps. (iii) The only well known tectonic event in the
Western Carpathians during Cretaceous time is thrusting
(nappe-stacking) in the Cenomanian—Turonian period, when
the Tatric crystalline basement, including the MF, was tec-
tonically buried by Mesozoic nappes (Plašienka et al. 1997).
However, the thrusting is of Cenomanian—Turonian age
(Plašienka et al. 1997), which is clearly younger than the
measured ZFT ages.
Danišík et al. (2008a) argued that Cenomanian-Turonian
thrusting is recorded by ZFT ages of ~ 100 ± 10 Ma in the Žiar
Mts, located ~ 30 km south of the MF (for location see Fig. 4).
Fig. 3. A – Chronostratigraphic chart of the study area and surrounding regions with relevant geodynamic events (Plašienka et al. 1997; Lexa
et al. 2000; Frisch & Gawlick 2003). B – Schematic thermal trajectories reconciling the data: speculative – dashed lines, convincing – solid
lines; APAZ – apatite partial annealing zone; ZPAZ – zircon partial annealing zone (according to Brandon et al. 1998), see text for explana-
tion. C – ZFT single grain age distribution showing difference between samples MF-1, 3, 6 and MF-2.
The authors correlate the event with the Eo-Alpine orogeny in
the Eastern Alps (e.g. Frisch & Gawlick 2003), where the
Austroalpine basement, which is an analogue of the Tatric
basement in the CWC, also experienced peak conditions of
metamorphism at ~100±10 Ma (e.g. Thöni & Jagoutz 1992;
Thöni & Miller 1996). Therefore, one possible resolution of
the ZFT data in the MF is by partial resetting of the ZFT sys-
tem by Cenomanian-Turonian thrusting (cooling curve ‘b’ in
Fig. 3B). An additional argument supporting the partial re-
setting at ~ 100 Ma is the single grain age spectrum
(Fig. 3C), where none of the zircons yielded ZFT age young-
er than ~ 100 Ma.
There are also other alternative explanations of the data. It is
not clear whether the first suggested interpretation (i.e. a cool-
ing event in the Early Cretaceous; cooling curve ‘a’ in
24
DANIŠÍK, KOHÚT, BROSKA and FRISCH
Fig. 3B) should be completely dismissed. A growing number of
Jurassic to Early Cretaceous ZFT ages found in other crystalline
bodies (Kováč et al. 1994; Danišík et al. 2007b; Plašienka et al.
2007; Danišík unpublished data) as well as sparse occurrences
of Jurassic/Cretaceous magmatic rocks reported from other
parts of the Western Carpathians (Hovorka & Spišiak 1988;
Spišiak & Hovorka 1997; Spišiak & Balogh 2002) might indi-
cate that there was indeed a distinct thermal event at that time.
This hypothesis might be supported by an analogous situa-
tion in the Western and Central Alps, where numerous ZFT
ages in the range 220—100 Ma commonly occur, but their
meaning is not entirely clear due to lack of supportive argu-
ments from the geological record. An elegant solution was
suggested by Vance (1999), who ascribed these ages to high
heat flow related to mantle upwelling associated with rifting
and opening of different branches of the Tethys Ocean. Per-
haps the CWC were affected by a similar thermal pulse from
the mantle, which affected the ZFT thermochronometer but
left no other evidence in the geological record. It is worth
mentioning that there is some record of Mesozoic magmatic
activity in the CWC that might be supportive of this interpre-
tation. For instance, Hovorka & Spišiak (1988) and Spišiak &
Hovorka (1997) argued that during late Early Cretaceous
times, a climax of the extensional period in the CWC was
marked by small extrusions of hyalobasanitic lavas of upper-
mantle origin. Further, there are several occurrences of small
sill intrusions of alkali lamprophyres reported from several
basement granitic rocks, that were dated at 115—93 Ma (Spišiak
& Balogh 2002).
The final possible interpretation is based on sample MF-2
(Eocene age), which is so far the youngest ZFT age ever re-
ported from a Tatric crystalline complex in the CWC. While
there is inherent danger in drawing broad conclusions from
single ages, there are also other examples of Eocene ZFT ages
found in other crystalline bodies in the CWC (e.g. in Tribeč
Mts, Považský Inovec Mts and Malé Karpaty Mts; Kováč et
al. 1994; Danišík et al. 2007b; Danišík unpublished data), and
we, therefore, argue that this age cannot be ignored.
Unlike the previous three samples, the single grain age
spectrum of the sample MF-2 forms a distinct peak, is narrow,
and ranges from 32 to 55 Ma (Fig. 3C). Such a spectrum is
typical for relatively quickly cooled samples. Thus we inter-
pret this sample as a record of a distinct thermal event in the
Middle Eocene (cooling curve ‘d’ in Fig. 3B). There are two
possible interpretations: the age may record (i) the cooling of
the basement following the thermal peak reached during the
Cenomanian—Turonian nappe-stacking; or (ii) an independent
thermal event in the Middle Eocene.
Although we are not sure which option is correct, we tend to
prefer the latter as it shows some similarities to data collected
in other parts of the CWC. Namely, the ZFT age is almost
identical with the age of 46±3 Ma measured by whole-rock
K-Ar analysis on a metabasalt from an olistolith in the Belice
Unit in the northern part of the Považský Inovec Mts (Putiš et
al. 2008) and the ZFT age of 53±12 Ma from the Tribeč Mts
granite reported by Kováč et al. (1994). The meaning and ro-
bustness of both ages are questionable – whole-rock K-Ar
dating on basalts is not a particularly powerful tool, moreover
the authors report no analytical results, just refer to ‘own un-
published data’ (Putiš et al. 2008). The ZFT age of Kováč et
al. (1994) does not meet standard international criteria in
terms of analytical and statistical requirements. Nevertheless,
Fig. 4. Spatial distribution of AFT ages reported from crystalline complexes in the CWC, where a clear younging trend towards the former
plate boundary (Pieniny Klippen Belt) is visible. Unexposed segments of the Pieniny Klippen Belt are indicated by dashed line. AFT data
compiled from the following studies: Burchart (1972), Krá (1977), Kováč et al. (1994), Struzik et al. (2002), Baumgart-Kotarba & Krá
(2002), Danišík et al. (2004, 2007b, 2008a,b, 2009).
25
THERMAL EVOLUTION OF THE MALÁ FATRA MOUNTAINS (CENTRAL WESTERN CARPATHIANS)
Putiš et al. (2008) pointed out that this age fits exactly the age
of the Early Tertiary orogenic event that was related to colli-
sion between the European and the Adriatic plate and was well
documented in the Northern Calcareous Alps (Frisch &
Gawlick 2003). Putiš et al. (2008) suggest that this part of the
Považský Inovec Mts was underthrust and metamorphosed in
the Eocene and Early Tertiary orogeny this would also apply
to the CWC. We speculate that the Eocene age found in the
MF records the same Early Tertiary orogenic event. The same
event might also be responsible for partial resetting of the rest
of the samples (cooling curve ‘c’ in Fig. 3B). This interpreta-
tion is, however, not in agreement with the tectonic model
proposed for the Eocene. Applying the model of Kázmér et al.
(2003), in the Middle Eocene the crystalline basement of the
MF should be covered by sediments of the CCPB, whose
thickness would have had to be more than ~ 8 km (assuming a
closure temperature of ~ 240 °C, a cooling rate of 10 °C/Myr,
and a paleo-geothermal gradient of 30 °C/km). That is not re-
alistic in our opinion but it is possible that the burial was not
solely sedimentary but also had a tectonic component.
In summary, as discussed above, interpretation of ZFT data
is extremely difficult. The only conclusion from ZFT that can
be made with confidence is that after the exposure in the Late
Permian (see section 2), the basement must have been reheat-
ed to temperatures sufficient to reset ZFT system. We dis-
cussed three scenarios that can explain the observed age
pattern and to certain degree incorporate presently accepted
tectonic models for the CWC: Jurassic/Cretaceous thermal
event related to elevated heat flow (cooling curves ‘a’ in
Fig. 3B), Cenomanian-Turonian thrusting (cooling curve ‘b’
in Fig. 3B) and Eocene orogeny (cooling curve ‘c’ in Fig. 3B).
It is, however, clear that with more ZFT data, new models will
be proposed and ZFT system can reveal many surprising facts
about the evolution of the CWC.
Implication for metamorphic evolution
Despite the uncertainty in the interpretation, the ZFT data
provide important information on the metamorphic history of
the MF: ZFT data clearly show that the basement reached tem-
perature conditions sufficient to fully reset the ZFT system
( > 210 °C, lower limit of zircon PAZ) during the Mesozoic
and/or Cenozoic and would have undergone very low-grade
metamorphism. This agrees with conclusions of Faryad &
Dianiška (2003) who, citing textural relations and mineral
compositions in the MF granitoids, argued for an Alpine very
low-grade metamorphic overprint (P-T conditions: ~ 0.3 GPa
at ~ 300 °C) of the basement. The overprint did not exceed
~ 350 °C as shown by Variscan mica Ar-Ar ages (Hók et al.
2000) obtained from undeformed granites (assuming
~ 350 °C closure temperature of Ar-Ar system in muscovite;
McDougall & Harrison 1988).
AFT data
Unlike ZFT data, the interpretation of AFT data is straight-
forward. Our data reproduce within 1 sigma error with AFT
data reported by Krá (1977) and Kováč et al. (1994), but
have much higher precision (1 sigma errors < 8 %). The data,
combined with thermal modelling, allowed us to constrain
the cooling episode to Middle to Late Miocene times be-
tween ~ 13 and 9 Ma, when the basement cooled from tem-
peratures above ~ 130 °C to ~ 70 °C, assuming a slightly
higher temperature range of PAZ typical for Cl-rich apatites
(e.g. Carlson et al. 1999). Our AFT data cannot explain what
happened within the basement prior to the cooling onset. We
interpret this cooling event in terms of exhumation of the
basement because the timing is corroborated by the sedimen-
tary record: the first clastic material derived from the MF are
pebbles from Mesozoic nappes deposited during Late Bade-
nian to middle Pannonian times (14.8—9.1 Ma) and the crys-
talline basement was first exposed to erosion in the Late
Pannonian (9.1—8.1 Ma; Hók et al. 1998). The portion of
clastic material derived from the basement increased in the
Pliocene and totally dominates the Quaternary sediments
(Hók et al. 1998, 2000), indicating uplift of the range and
creation of the present-day topography.
Modelled cooling trajectories can be translated into exhuma-
tion rates, if a reasonable paleo-geothermal gradient is assumed.
Adopting a value of 30 °C/km results in average exhumation
rates of > 1000 m/Myr for a time period of 13—9 Ma, and
~ 200 m/Myr for time period between ~ 9 Ma and present.
Considering the limited resolution of the modelled cooling path,
the maximum and minimum exhumation rates for the fast cool-
ing stage could range from ~ 4000 to ~ 400 m/Myr, and for the
slow cooling stage, from ~ 200 to > 50 m/Myr. These num-
bers are likely biased by the chosen paleo-geothermal gradient
value, however, there is at least a two-fold difference between
pre- and post-9 Ma cooling rates. If the geothermal gradient
had not changed in the last 13 Myr, the data imply that since
post-mid-Miocene (i.e. since ~ 13 Ma) about ~ 3.5 km of over-
burden has been removed, with the majority (
≥2.6 km) being
removed between 13 and 9 Ma.
The implication for exhumation of crystalline bodies in the
Western Carpathians
Our results significantly refine an exhumation pattern of in-
dividual basement complexes in the CWC, which better fits
the lateral tectonic extrusion models placing faulting and ex-
humation of crystalline bodies in the middle and post-Middle
Miocene (circa post-13 Ma; Ratschbacher et al. 1991a,b;
Sperner et al. 2002). Furthermore, with our data, a much clear-
er younging trend towards the orogenic front is evident
(Fig. 4). The figure shows that internal massifs retain mostly
Paleogene or Cretaceous AFT ages, even though some of
them experienced a distinct reheating in the Neogene related
to mantle upwelling, volcanic activity, and increased heat
flow as documented by thermal modelling results based on
track length data and also by apatite (U-Th)/He data (Danišík
et al. 2004, 2008a,b). The internal massifs were thus in a
‘colder’ environment (i.e. < ~ 120 °C) during the Tertiary. In
contrast, all the external massifs located closest to the orogen-
ic front (i.e. MF and High Tatra Mts) show almost exclusively
Middle Miocene or younger AFT ages, which indicates their
residence in a relatively ‘hotter’ environment (i.e. > ~ 120 °C)
in the Tertiary. Since there is no evidence of volcanic activity
in the external Tatric belt, we interpret the ‘hotter’ environ-
26
DANIŠÍK, KOHÚT, BROSKA and FRISCH
ment in terms of deeper burial of the massifs prior to their fi-
nal exhumation.
We would like to emphasize that any interpretation of the
observed AFT age pattern in terms of surface uplift, rock up-
lift or uplift without specification of reference point or eleva-
tion level (e.g. Kováč et al. 1994; Hók et al. 1998, 2000;
Sperner et al. 2002) is incorrect. Instead it should be always
kept in mind that the reference frame for the FT system is the
thermal structure of the crust and not the Earth’s surface.
Lastly, tectonic models for the Tertiary evolution of the
Western Carpathians incorporating AFT data have to consid-
er the fact that the majority of AFT ages are not cooling ages
but apparent ages.
Conclusions
New AFT and ZFT data enabled us to constrain the thermo-
tectonic evolution of the MF Mts. The most important results
are summarized as follows:
– The Variscan crystalline basement of the MF Mts was
heated to temperatures above ~ 210 °C and was affected by a
very low-grade Alpine overprint as recorded by ZFT data. The
time and origin of the heating remains unclear. We propose
three explanation that need to be corroborated by future re-
search: Jurassic/Cretaceous thermal event related to increased
heat flow associated with mantle upwelling, Cenomanian—
Turonian (Eo-Alpine) thrusting and Eocene orogeny related to
collision between the European and the Adriatic plate;
– The AFT ages constrain a cooling event between ~ 13
and 9 Ma, which we interpret in terms of exhumation of the
basement in the course of lateral tectonic extrusion;
– The investigated MF together with the High Tatra Mts are
the ranges with the youngest exhumation history among all the
crystalline complexes of the CWC as they record the youngest
cooling AFT ages. This is in good agreement with their position
as the external massifs located closest to the orogenic front and
fits with the well-known spatial and temporal migration of pro-
cesses from internal to external portions of the CWC.
Acknowledgments: This study was financed by the German
Science Foundation
the German Science Foundation, John
de Laeter Centre of Mass Spectrometry, Slovak Research
and Development Agency (APVV-549-07) and VEGA
project GA7076
. We thank D. Mühlbayer-Renner and D.
Kost (Tübingen) for their careful sample preparation. An
earlier version of the manuscript benefited from construc-
tive reviews by M. Kováč (Bratislava University), J. Majka
(Uppsala University), I. Petrík (SAS Bratislava) and M.
Svojtka (CAS Prague). English language was improved by
N.J. Evans (CSIRO, Exploration and Mining, Perth).
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