GeolCarp_Vol64_No5_383

www.geologicacarpathica.sk

GEOLOGICA CARPATHICA

, OCTOBER 2013, 64, 5, 383—398 doi: 10.2478/geoca-2013-0026

Introduction

The  highly  arcuate  external  Miocene  thrust  belt  of  the  Car-
pathians  and  its  foredeep  (Fig. 1)  formed  in  Miocene  times
(Matenco et al. 2003, 2007, 2010). Continental blocks of dif-
ferent provenance referred to as Mega-Units (ALCAPA, Tisza,
Dacia; see Csontos & Vörös 2004; Schmid et al. 2008 and ref-
erences  therein)  are  presently  located  between  the  Bohemian
and Moesian promontories (Fig. 1), in an area that was, until
Mid-Miocene  times,  occupied  by  the  so-called  Carpathian
embayment,  probably  partly  underlain  by  old  oceanic  litho-
sphere (Balla 1987; Csontos & Vörös 2004; Ustaszewski et al.
2008).  Before  their  final  emplacement  into  the  Carpathian
embayment  these  three  Mega-Units  underwent  Cretaceous
orogeny.  Our  area  of  investigation  is  located  in  the  internal
Eastern  Carpathians  that  are  a  part  of  the  Dacia  Mega-Unit.
Cretaceous  orogeny  in  the  Eastern  Carpathians  propagated
eastwards until Paleogene times (Matenco et al. 2003). In the
Miocene  the  Eastern  Carpathians,  including  partly  oceanic
accretionary  prisms  accreted  in  Cretaceous  times  (e.g.  the
Ceahlau  and  Black  Flysch  Nappes  of  Fig. 2;  Săndulescu
1975),  collided  with  the  European  foreland,  thereby  closing

Thermal history of the Maramure area (Northern

Romania) constrained by zircon fission track analysis:

Cretaceous metamorphism and Late Cretaceous to

Paleocene exhumation

HEIKE R. GRÖGER

, MATTHIAS TISCHLER

, BERNHARD FÜGENSCHUH

and

STEFAN M. SCHMID

Statoil ASA, Forusbeen 50, 4035 Stavanger, Norway; heigr@statoil.com

Institut für Geologie und Paläontologie, Universität Innsbruck, Innrain 52, Bruno Sander Haus, 6020 Innsbruck, Austria;

bernhard.fuegenschuh@uibk.ac.at

Institute of Geophysics, ETH, CH-8092 Zürich, Switzerland; stefan.schmid@erdw.ethz.ch

(Manuscript received February 22, 2013; accepted in revised form June 5, 2013)

Abstract: This study presents zircon fission track data from the Bucovinian nappe stack (northern part of the Inner
Eastern Carpathians, Rodna Mountains) and a neighbouring part of the Biharia nappe system (Preluca massif) in order
to unravel the thermal history of the area and its structural evolution by integrating the fission track data with published
data on the tectonic and sedimentary evolution of the area. The increase of metamorphic temperatures towards the SW
detected by the zircon fission track data suggests SW-wards increasing tectonic overburden (up to at least 15 km) and
hence top NE thrusting. Sub-greenschist facies conditions during the Alpine metamorphic overprint only caused partial
annealing of fission tracks in zircon in the external main chain of the Central Eastern Carpathians. Full annealing of
zircon points to at least 300 °C in the more internal elements (Rodna Mountains and Preluca massif). The zircon fission
track central and single grain ages largely reflect Late Cretaceous cooling and exhumation. A combination of fission
track data and stratigraphic constraints points to predominantly tectonic differential exhumation by some 7—11 km,
connected to massive Late Cretaceous extension not yet detected in the area. Later events such as the latest Cretaceous
(“Laramian”) juxtaposition of the nappe pile with the internal Moldavides, causing exhumation by erosion, re-burial by
sedimentation and tectonic loading during the Cenozoic had no impact on the zircon fission track data; unfortunately it
prevented a study of the low temperature part of the Late Cretaceous exhumation history.

Key words: Cretaceous, Eastern Carpathians, Romania, Rodna Mountains, Alpine metamorphism, thermochronology,
zircon fission track analysis.

the Carpathian embayment and forming the external Miocene-
age flysch belt (Matenco et al. 2010).

A number of publications investigating the invasion of these

continental blocks into the Carpathian embayment (e.g. Balla
1987;  Royden  &  Báldi  1988;  Ratschbacher  et  al.  1991a,b;
Csontos et al. 1992; Csontos 1995; Fodor et al. 1999; Sperner
et  al.  2005;  Schmid  et  al.  2008;  Ustaszewski  et  al.  2008)
considerably improved our understanding of the Tertiary tec-
tonic  evolution.  In  contrast,  the  Cretaceous  and  Early  Paleo-
gene  history  of  the  Tisza  and  Dacia  Mega-Units  themselves,
forming the backbone of the Carpathian arc (Burchfiel 1980;
Săndulescu  1988,  1994;  Csontos  1995;  Csontos  &  Vorös
2004; Schmid et al. 2008), is still ill constrained.

In the area of investigation the Dacia Mega-Unit consists

of the so-called Bucovinian nappe stack (Fig. 2). This nappe
stack is built up, from bottom to top, by the Infrabucovinian
Nappe,  followed  by  the  Subbucovinian  and  Bucovinian
Nappes  (Săndulescu  1994).  These  Cretaceous-age  nappes
that  constitute  the  internal  parts  of  the  Eastern  Carpathians
are  the  lateral  equivalents  of  the  Getic  and  Supragetic
Nappes  of  the  Southern  Carpathians  that  can  be  followed
into  the  Sredna  Gora  and  Serbo-Macedonian  Units  of  the

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GEOLOGICA CARPATHICA, 2013, 64, 5, 383—398

duction and closure of the intervening South Apuseni—Tran-
sylvanian  oceanic  units  (Transylvanides – Săndulescu
1988;  Vardar-Mure   zone – Csontos  &  Vorös  2004).  The
Transylvanides,  consisting  of  Middle  Triassic  to  Middle/
Upper Jurassic ophiolites (Săndulescu 1994), were obducted
and  subsequently  thrusted  onto  the  Bucovinian  nappe  pile
(Săndulescu 1988, 1994).

The pre-Alpine metamorphic evolution of the Bucovinian

nappes in the study area and that of the similar Biharia nappe
system  of  the  Apuseni  Mountains  is  quite  well  established
(Kräutner 1988, 1991; Pană & Erdmer 1994; Voda & Balintoni
1994; Strutinski et al. 2006; Balintoni et al. 2010; Balintoni &
Balica  2013).  The  grade  and  age  of  Alpine  metamorphic
overprint, however, is still ill constrained (Săndulescu et al.
1981;  Pană  &  Erdmer  1994;  Dallmeyer  et  al.  1996,  1998;
Strutinski et al. 2006; Culshaw et al. 2012). The low degree
of metamorphism of Permian to Lower Cretaceous sedimen-
tary units separating the individual nappes (Fig. 2) led to the
view that late Early Cretaceous (Aptian/Albian) nappe stack-
ing occurred under sub-greenschist metamorphic conditions
(Săndulescu et al. 1981). Later publications revealed Alpine-
age greenschist facies metamorphic overprint (Pană & Erdmer
1994;  Dallmeyer  et  al.  1996,  1998;  Balintoni  et  al.  1997;
Strutinski  et  al.  2006;  Culshaw  et  al.  2012).  An  Alpine-age
metamorphic  overprint  is  particularly  well  documented  for
the  so-called  Rodna  horst  (Fig. 2),  exposing  the  Infrabu-
covinian  nappes  (Dallmeyer  et  al.  1998;  Strutinski  et  al.
2006; Culshaw et al. 2012). The same holds for the Preluca
massif in the SW corner of our area of investigation classi-
cally  assigned  to  the  Biharia  nappe  system  (Fig. 2;  Rusu  et
al.  1983;  Strutinski  et  al.  2006)  and  the  bulk  of  the  Biharia
nappe system in the Apuseni Mountains located further to the
SW  (Fig. 1;  Dallmeyer  et  al.  1996;  Strutinski  et  al.  2006;
Kounov & Schmid 2013).

This zircon fission track study complements structural

(Tischler et al. 2007), sedimentological (Tischler et al. 2008),
paleomagnetic  (Márton  et  al.  2007)  and  apatite  fission  track
(Gröger  et  al.  2008)  investigations  in  the  Maramure   area  of
northern Romania. In Gröger et al. (2008) the zircon fission
track data has been used combined with apatite fission track
data to constrain the Tertiary exhumation and final emplace-
ment  of  the  Tisza-Dacia  block  in  the  Eastern  Carpathians.
This study offers a new interpretation of the data in combina-
tion  with  published  metamorphic  information.  Firstly,  the
zircon fission track data allow us to discuss the degree and age
of Alpine metamorphism. Secondly, the zircon data also pro-
vide  information  on  the  subsequent  exhumation  of  the  Bu-
covinian  nappe  stack  of  the  Eastern  Carpathians  (Fig. 2)  in
Cretaceous times since zircon fission tracks were not annealed
during subsequent burial in Cenozoic times; this was the case
for the apatite fission tracks (Gröger et al. 2008). A third target
concerns the comparison of the Bucovinian nappe stack with
basement units of the neighbouring Biharia nappe system ex-
posed  in  the  North  Apuseni  Mountains,  classically  attributed
to the Tisza Mega-Unit (e.g. Haas & Péró 2004).

Since annealing of fission tracks in zircon occurs in a

temperature range of 200—350 °C (Hurford 1986; Yamada et
al. 1995; Tagami et al. 1996) our results will be discussed in
the context of published geochronological data from the Bu-

covinian nappe stack that record the higher temperature his-
tory (K-Ar and

Ar/

Ar thermochronology – Dallmeyer et

al. 1998; Strutinski et al. 2006; Culshaw et al. 2012). Since
apatite  fission  tracks  have  been  fully  annealed  during  post-
Cretaceous  burial  (Gröger  et  al.  2008),  we  also  will  use
stratigraphic  data  (Szasz  1973;  Kräutner  et  al.  1978,  1983;
Săndulescu  et  al.  1991)  to  further  constrain  the  Late  Creta-
ceous to Early Paleogene exhumation history of the area.

Geological setting

The Infrabucovinian Nappe is the tectonically deepest unit

of the Bucovinian nappe stack and laterally corresponds to the
Getic  Nappe  of  the  Southern  Carpathians  (Schmid  et  al.
2008). It is exposed in a series of windows (Fig. 2). The win-
dow  in  the  Rodna  horst  (Kräutner  1988),  surrounded  by  the
Subbucovinian  Nappe,  is  one  of  the  largest.  The  tectonically
highest  Bucovinian  Nappe  is  found  in  the  most  external,
meaning  the  northeastern  part  of  the  Eastern  Carpathians
(Fig. 2).  The  bulk  of  the  Bucovinian  nappe  stack  consists  of
pre-Mesozoic  basement,  the  Mesozoic  cover  being  thin  and
only sporadically preserved, particularly in the case of the In-
frabucovinian and Subbucovinian Nappes. The pre-Mesozoic
basement of the Preluca massif (SW corner of Fig. 2) is attrib-
uted to the Baia de Arie  Nappe of the Biharia nappe system
(Rusu et al. 1983; Strutinski et al. 2006). Its Mesozoic cover is
not preserved.

A Precambrian amphibolite facies basement, derived from

Proterozoic  sediments  (Rebra,  Negrisoara  and  Bretila  series;
Kräutner  1938,  1988)  predominates  within  the  Bucovinian
nappe  pile.  In  addition,  the  Subbucovinian  and  Bucovinian
Nappes feature a series composed of Cambrian sediments and
eruptive  rocks  (Tulghes  series)  whose  greenschist  facies
metamorphic overprint has been dated by K-Ar methods as Or-
dovician  (450—470 Ma;  Kräutner  1988,  1991,  and  references
therein).  According  to  Kräutner  (1991),  the  Variscan  and  Al-
pine orogenies only locally caused greenschist facies overprint.

The Infra- and Subbucovinian Nappes also feature a post-

Caledonian  sedimentary  cover  (Repedea,  Rusaia  and  Cim-
poiasa  series – Kräutner  1991;  Rodna  series – Voda  &
Balintoni  1994).  Palynological  data  indicate  a  Silurian  to
Mississippian  depositional  age  of  these  series  (Săndulescu
et  al.  1981;  Kräutner  1988,  1991,  and  references  therein).  A
Variscan  prograde  greenschist  facies  metamorphic  overprint
has been dated by K-Ar age data as Pennsylvanian (310 Ma;
Kräutner  1991).  However,  in  the  case  of  the  Rodna  horst,
Balintoni  et  al.  (1997)  proposed  the  greenschist  facies  over-
print to be Alpine in age. Moreover, Balintoni et al. (1997)
interpreted  the  so-called  “Rodna  series”  to  represent  post-
Variscan Jurassic cover, due to similarities in structural posi-
tion  and  lithology  with  metamorphosed  Jurassic  cover  units
exposed in the Vaser window.

A post-Variscan Permian to Lower Cretaceous cover, with

highly  variable  facies  types,  characterizes  the  different
nappes, except for the basement of the Rodna horst that lacks
non-metamorphic Permo-Mesozoic cover (Săndulescu et al.
1981;  Săndulescu  1994).  Common  to  all  tectonic  units  are
Middle  Triassic  dolomites,  an  Upper  Triassic  hiatus  and

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GEOLOGICA CARPATHICA, 2013, 64, 5, 383—398

tions (i.e. Săndulescu et al. 1981). However, syntectonic
greenschist facies metamorphism in the vicinity of nappe con-
tacts was locally described, especially in the case of the Rodna
horst (Kräutner et al. 1978, 1982, 1983, 1989; Dallmeyer et al.
1998; Strutinski et al. 2006; Culshaw et al. 2012) and in the
case of the Vaser window (Balintoni et al. 1997).

The post-tectonic (with respect to the “Austrian” phase)

Upper  Cretaceous  cover  (Figs. 2,  3)  includes  Cenomanian-
Turonian  conglomerates  and  sandstones,  discordantly  over-
lain  by  Turonian—Coniacian  (silty)  marls  (Kräutner  et  al.
1978,  1983;  Săndulescu  et  al.  1991).  Within  the  Borsa  gra-
ben  Santonian  to  Maastrichtian  conglomerates  are  docu-
mented above a second unconformity (Szasz 1973; Kräutner
et  al.  1983).  Similar  Cenomanian  to  Maastrichtian  deposits
are  widespread  within  and  at  the  rims  of  the  Transylvanian
Basin  and  are  generally  interpreted  to  have  been  deposited
during crustal extension (e.g. Krézsek & Bally 2006).

Thrusting resumed in the Late Maastrichtian when the ex-

humed “Austrian” nappe-pile and its post-tectonic Late Cre-
taceous  cover  thrusted  the  underlying  Black  Flysch  and
Ceahlau Nappes (“Laramian” phase; Săndulescu 1982, 1994;
Matenco  et  al.  2003).  This  thrusting  is  contemporaneous
with  the  formation  of  the  Danubian  nappes  in  the  Southern
Carpathians (Schmid et al. 1998; Matenco & Schmid 1999).
The Maastrichtian collisional event was also followed by up-
lift and erosion in the Central East Carpathian chain, leading
to  a  paleo-relief,  for  example,  associated  with  a  basement
high in the area of the Rodna horst (Săndulescu et al. 1991;
Tischler  et  al.  2007;  Gröger  et  al.  2008).  During  the  Paleo-
cene  erosion  dominates  within  most  of  the  study  area
(Fig. 3).  Only  west  of  the  Borsa  graben  and  in  the  Preluca
massif are Paleocene continental deposits, namely the shales
and sandstones  of  the  Jibou  Formation, seen  to  unconform-
ably  overlie  the  basement  (Rusu  et  al.  1983;  Săndulescu  et
al. 1991; see Figs.  2 and 3).

Post-“Laramian” Paleogene sedimentation started with

typically terrestrial conglomerates of Ypresian (in case of the
Borsa graben – Kräutner et al. 1983) to Lutetian age (Prislop
conglomerate). These are followed by lithologically variable
Lutetian  to  Priabonian  marine  sediments  (Fig. 3)  deposited
in  sag  basins  (Krézsek  &  Bally  2006).  Platform  carbonates
are  preserved  in  the  Rodna  horst  and  in  the  southern  and
eastern parts of the study area (Iza limestone – Dicea et al.
1980; De Brouker et al. 1998; Sahy et al. 2008). Deepening
towards  the  northwest  is  indicated  by  a  change  from  plat-
form  carbonates  towards  marls  and  distal  turbidites  (Vaser
and Viseu Formations – Săndulescu et al. 1991). The maxi-
mum  thickness  of  the  Eocene  sediments  in  the  study  area
(around  1000 m)  is  found  immediately  west  of  the  Borsa
graben  (Fig. 3).  The  existence  of  an  Eocene  paleorelief  is
also supported by the observation that Oligocene sediments
locally directly overlie the basement units of the Rodna horst
(Kräutner et al. 1982).

Deposition of thick siliciclastic flysch (“Transcarpathian

flysch”) started in the Early Oligocene, reflecting flexural
bending at the onset of convergence between ALCAPA and
the Tisza—Dacia Mega-Units (Tischler et al. 2008) and lead-
ing to considerable burial of all underlying units (Dicea et al.
1980). Final exhumation of the pre-Mesozoic basement units

is the result of Miocene brittle tectonics and erosion during
the final stages of juxtaposition of the Tisza—Dacia Mega-
Units against the European margin (Tischler et al. 2007,
2008; Márton et al. 2007; Gröger et al. 2008).

Method – zircon fission track analysis

Fission track (FT) analysis (overview in Wagner & van

den Haute 1992) is a radiometric dating procedure. The sam-
ples  in  this  study  are  analysed  using  the  external  detector
method,  calculating  single  grain  ages  (Gleadow  1981).  The
age is calculated from the ratio between spontaneous fission
tracks  (Ns),  counted  on  a  defined  square  on  the  grain,  and
tracks  induced  by  thermal  neutrons  (Ni),  counted  on  the
equal square on a uranium-free external detector (Fig. 4).

Latent fission tracks are only stable below a critical tem-

perature  range,  namely  the  partial  annealing  zone,  wherein
tracks start to anneal and finally fade. The zircon partial an-
nealing  zone  (ZPAZ)  has  been  addressed  in  experimental
(e.g. Yamada et al. 1995) and empirical studies (e.g. Hurford
1986; Tagami et al. 1996; Tagami & Shimada 1996). While
the lower temperature limit at  ~ 200 °C (Tagami et al. 1996)
is generally agreed, the upper temperature limit is still a mat-
ter  of  debate,  ranging  between  300  and  400 °C  (Yamada  et
al. 1995). Indeed this large temperature range might at least
partly  be  related  to  different  geodynamic  settings  (fast  ex-
huming/high-grade  rocks  vs.  deposition/burial/slow  exhu-
mation) and the effect of a-recoil damage (Rahn et al. 2004;
Timar-Geng  et  al.  2006).  For  our  interpretations  we  take  a
temperature  range  of  240 ± 50 °C  (Hurford  1986),  which  is,
within error bars, in accordance with other authors (Zaun &
Wagner 1985; Tagami et al. 1996).

Samples have been processed using conventional crush-

ing, sieving, magnetic and heavy liquid separation (bromo-
form, methylene iodide). Zircon grains were mounted in
PFA® Teflon, polished and etched for 12—24 hours in an eu-
tectic melt of NaOH/KOH (relation 16/23 g) at 225 °C to re-
veal the

238

U fossil fission tracks. Irradiation was carried out

at the High Flux Australian Reactor (HIFAR) at Lucas
Heights, New South Wales with neutron fluxes monitored in
CN1. Muscovite was used as an external detector and etched

Fig. 4. Zircon fission track ages are calculated based on the ratio of
spontaneous tracks (Ns) counted on the grain (a) and induced tracks
(Ni) counted on a uranium free mica detector (b).

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GEOLOGICA CARPATHICA, 2013, 64, 5, 383—398

with the aid of the windows software TrackKey (Dunkl 2002).
All ages mentioned are central ages (Galbraith & Laslett
1993) and errors are quoted at the 1 level.

Results

Location of samples and list of zircon fission track data

All zircons analysed were taken from samples of the pre-

Permo-Mesozoic  basement.  Most  samples  were  taken  in
close contact to the autochthonous sedimentary cover of the
basement  in  order  to  provide  independent  stratigraphic  and
thermal  control.  Fig. 5  shows  their  location  on  a  geological
map  and  depicts  the  central  ages  and  radial  plots,  Table 1
lists the results of the 32 samples analysed.

Samples M01-09 and M12-M14 were taken along a hori-

zontal strike-parallel profile going from the Maramure
Mountains in the NW to the Central East Carpathian chain in
the SE (see Figs. 2 and 5). The area of the Rodna horst (Fig. 2)
was sampled in greater detail (Fig. 5). The samples collected

there  include  three  vertical  profiles:  two  across  the  Subbu-
covinian Nappe (R2 with 4 samples and R4 with 3 samples)
and  one  across  the  Infrabucovinian  Nappe  (R3  with  6  sam-
ples).  These  are  complemented  by  four  samples  from  the
northern part of the Rodna horst (R1-1 from the Subbucovin-
ian  Nappe;  R5-1,  R5-2,  R5-4  from  the  Infrabucovinian
Nappe  and  covering  a  WSW-ENE  profile).  Three  samples
derive from the Preluca massif in the SW (P1—P3; Fig. 5).

Zircon fission track data

All central ages (except for that of M08), found along the

horizontal profile from the Maramure  Mountains to the Cen-
tral East Carpathian chain (Fig. 5; Table 1, row 1—12) scatter
between  Late  Jurassic  and  Cenomanian  (162.3—96.1 Ma).
Some  of  these  samples  (M01—M04,  M09),  all  located  more
to  the  SW,  show  Cenomanian  central  ages  (99.7—96.1  Ma)
and pass the Chi-Square test (

> 5 %; Table 1, column 12).

Hence this group of samples indicates Cenomanian cooling
after full annealing of the zircon fission tracks. Central ages

Code

Locality X Locality Y Locality Z

[m]

No.

Grains

[ 10

–2

]

[ 10

–2

]

[ 10

–2

]

[%]

Central Age

±1σ [Ma]

M01

24.496670 47.729790

540

17.637 1291 4.932

361 0.385 3065 34 96.6±7.6

M02

24.560710 47.753720

580

10.939

367 3.010

101 0.377 3065 13 95.7±13.5

M03

24.586640 47.772540

630

8.688 1468 3.545

599 0.580 3605 81 99.7±6.8

M04

24.628100 47.791450

680

7.007

436 3.054

190 0.597 3605 84 96.1±9.5

M05

24.667090

47.804300

745

9.844

2366

3.720

894

0.586

3605

<5 107.5±7.4

M06

24.698590

47.790850

790

12.907

2543

3.335

657

0.591

3605

<5 162.3±13.0

M07

24.736840

47.773640

835

8.663

2193

2.501

633

0.614

3605

10 146.1±10.5

M08

24.770543 47.690263

820

12.309 2703 5.232 1149 0.369 3065 <5

61.3±4.7

M09

24.833619 47.647505 1660

12.615 2501 3.218

638 0.350 3065 22

96.2±6.5

M12

25.112014

47.603497

985

13.443

164

2.541

0.377

3065

21 139.7±29.5

M13

25.128220

47.571246

930

25.389

1812

5.871

419

0.354

3065

23 107.6±8.5

M14

25.279510

47.478662

850

13.767

2830

3.060

629

0.338

3065

<5 107.0±9.1

23.574760 47.430957

315

7.960 1010 2.522

320 0.373 3065 55

82.8±6.6

23.628772 47.509842

215

15.574 1919 4.740

584 0.342 3065 38

78.8±5.6

23.686807 47.488712

610

12.838 3336 4.087 1062 0.381 3065 <5

84.5±5.5

R1-1

24.559360 47.597860 1550

9.173

996 3.113

338 0.346 2967 25

71.8±6.1

R2-2

24.597260

47.414920

1105

8.041

981

3.238

395

0.574

3605

78 100.1±7.6

R2-3

24.595860 47.415710

885

9.881 1326 3.622

486 0.357 2967 <5

68.2±6.4

R2-4

24.590430 47.419300

705

11.520 2312 3.443

691 0.352 2967 12

82.8±5.9

R2-5

24.583480 47.421240

600

12.109 1625 3.368

452 0.335 2967 49

84.7±6.1

R3-1

24.620652 47.533782 2020

18.830 2550 5.568

754 0.335 3065 47

79.5±5.1

R3-2

24.582850 47.423290 1465

7.415 1642 3.902

864 0.625 3605 62

83.0±5.5

R3-3

24.608990 47.528930 1310

7.626

842 3.877

428 0.620 3605 37

85.6±6.8

R3-4

24.604450 47.526330 1155

9.477

607 6.011

385 0.614 3605 50

68.1±5.5

R3-5

24.597630 47.523200 1005

11.482 1751 5.705

870 0.608 3605 14

84.9±5.9

R3-6

24.587980 47.518690

945

9.224

934 4.977

504 0.603 3605 11

78.5±6.4

R4-1

24.920150 47.500800 1638

7.516 1710 2.193

499 0.366 3065 28

87.8±6.3

R4-3

24.941010 47.494710

980

11.694

107 3.716

34 0.362 3065 86

80.0±16.2

R4-4

24.960230 47.490430

700

13.472 1923 4.589

655 0.369 3065 <5

77.0±5.8

R5-1

24.546451 47.552021 1150

20.411

635 5.754

179 0.346 3065 47

86.2±8.4

R5-2

24.449131 47.460078 1245

15.094 1556 5.345

551 0.324 2967 90

64.4±4.5

R5-4

24.872870 47.596160 1270

16.118 2517 5.033

786 0.358 3065 10

81.2±5.5

Table 1: Zircon fission track data. All samples have been analysed using the external detector method (Gleadow 1981) with a zeta value
(Hurford & Green 1983) of 141.40 ± 6.33 (Fish Canyon Tuff standard, CN1). Code – sample code; Locality X – Latitude in decimal
degrees; Locality Y – Longitude in decimal degrees; Loc. Z [m] – altitude above sea level; No. Grains – number of grains counted;

s [ 10

—2

] – spontaneous track density; Ns – number of spontaneous tracks counted; i [ 10

—2

] – induced track density;

Ni – number of induced tracks counted; d [ 10

—2

] – standard track density; Nd – number of standard tracks counted;

[%] – Chi-Square probability (Galbraith 1981). Central Age ± 1 [Ma] – zircon fission track central age (Galbraith & Laslett 1993).

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from  locations  more  to  the  NE,  i.e.  towards  the  present-
day erosional front of the Bucovinian nappe stack, are older
and  range  between  Late  Jurassic  and  Early  Cretaceous
(162.3—107.0 Ma; M05—M07, M12—M14). The large spread
in central ages from this external sub-group reflects the ob-
served  scattering  of  single  grain  ages  between  Late  Creta-
ceous  and  Paleozoic  (oldest  single  grain:  310 Ma,  M06);
most of them fail the Chi-square test (

< 5 %; Table 1, col-

umn 12).  Consequently  the  zircons  from  this  sub-group  are
interpreted  as  having  been  partially  annealed  prior  to
Cenomanian cooling. The Paleocene central age obtained for
M08 (61.3 Ma) forms an exception. Its extraordinarily young
age is most likely the result of hydrothermal overprint induc-
ing  partial  annealing,  caused  by  a  Miocene  volcanic  body
nearby  (Pécskay  et  al.  1995).  Consequently,  sample  M08
will be excluded from further discussions.

All zircon FT central ages taken from the Preluca massif

(Fig. 5; Table 1, row 13—15) and the Rodna horst (Fig. 5; Ta-
ble 1, row 16—32) show a spread between 100.1—64.4 Ma, a
time interval that covers the entire Late Cretaceous period.
Most of the zircon FT central ages from the Rodna horst are of
Coniacian to Campanian (89.3—70.6 Ma) age and most of them
pass the Chi-Square test (

> 5 %; Table 1, column 12), which

indicates full annealing prior to Late Cretaceous cooling.

Figure 6 presents the distribution of zircon single grain

ages for specified groups of samples and compares this dis-
tribution  with  the  spread  in  nominal  central  ages  for  the
same groups of specimens in order to better assess the degree
of  annealing  that  occurred  during  Early  Cretaceous  meta-
morphism. In the Maramure  Mountains and the Central East
Carpathian  main  chain  (Fig. 6a,b)  differences  depending  on
the structural position within the Early Cretaceous nappe pile
are apparent. In the case of the Bucovinian and Subbucovinian
Nappes  (Fig. 6a)  no  clear  peak  is  discernable  in  the  single
grain age distribution, suggesting only partial annealing during
the  Early  Cretaceous  orogeny.  Samples  from  the  tectono-
stratigraphically  lowest  Infrabucovinian  Nappe,  however,
show a relatively well-defined peak at around 100 Ma, coin-
ciding  with  the  small  spread  in  central  ages  (Fig. 6b),  sug-
gesting full Cretaceous-age annealing.

Within the Rodna horst both structural units appear to be

fully annealed (Fig. 6c,d). The distributions of the single
grain ages as well as the spread of the central ages are very
similar regardless of tectonic position. Moreover, no age vs.
altitude relation was detected in the case of the vertical pro-
files in the Rodna horst (Fig. 5). The three samples from the
Preluca massif show single grain age distribution similar to
that of the Rodna horst (Fig. 6e), suggesting full annealing
again, though testified by far less single grains.

In order to evaluate the geological significance of the sur-

prisingly large spread of central ages found in the Rodna
horst, vertical profile R2 is examined in more detail. Fig. 7a—b
shows two neighbouring samples from this profile that reflect
a large difference in central age (R2-2: 100.1 ± 7.6 Ma; R2-3:
68.2 ± 6.4 Ma) but at the same time shows a similar spread of
single grain ages. Note that this overall spread of single grain
ages is roughly the same as that observed when looking at all
the samples taken from the Rodna horst (Fig. 7c). This indi-
cates that the differences in central ages observed in the Rodna

horst have no geological significance and are merely of statis-
tical  relevance.  A  large  spread  in  single  grain  ages  such  as
shown in Fig. 7a,b can be aggravated by inhomogeneous an-
nealing behaviour, resulting in shifts of single grain age dis-
tributions  within  one  sample.  Apart  from  cooling,  alpha
radiation  damage  is  another  important  factor  influencing  the
annealing behaviour of zircon grains (Gleadow 1981; Kasuya
& Naeser 1988; Rahn et al. 2004). The alpha radiation damage
accumulates  in  relation  to  the  entire  grain  age  and  uranium
content  of  the  grain  (Gleadow  1981;  Timar-Geng  et  al.
2006).  Strong  reduction  of  alpha  radiation  damage  requires

Fig. 6. Age distribution of the single grain zircon FT ages and spread
in  the  calculated  central  ages  from  groups  of  specimens.  a  –  ages
from  the  Bucovinian  and  Subbucovinian  Nappes  of  the  Maramure
Mountains  and  Central  East  Carpathian  chain;  b  –  ages  from  the
Infrabucovinian  nappes  of  the  Maramure   Mountains;  c  –  ages
from the Subbucovinian nappes in the Rodna horst; d – ages from
the Infrabucovinian nappes in the Rodna horst; e – ages from the
Preluca massif.

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higher  temperatures  than  those  needed  for  FT  annealing
(Rahn et al. 2004 and therein). Due to these considerations,
the zircon central ages in the Rodna horst will not be geologi-
cally  interpreted  individually.  On  the  other  hand,  the  clear
peak of the single grain age distribution in Campanian, as seen
in Fig. 7c, assembling all the samples from the Rodna horst, is
interpreted  as  documenting  the  approximate  time  when  the
samples from the Rodna horst cooled through the ZPAZ.

In summary, the new zircon FT data indicate a significant

metamorphic imprint during Early Cretaceous nappe stacking,
followed  by  Late  Cretaceous  cooling  and  exhumation.  Full
annealing  during  this  event  is  indicated  for  (1)  the  Infrabu-
covinian Nappe of the Maramure  Mountains and the Central
East  Carpathian  chain,  (2)  the  Infrabucovinian  and  Subbu-
covinian  Nappes exposed in the area of the Rodna horst and
(3) the Biharia nappe system (Baia de Arie  Nappe) exposed
in the Preluca massif. Hence metamorphic temperatures must
have  exceeded  300 °C  (upper  limit  of  ZPAZ;  Hurford  1986;
Yamada et al. 1995) in these areas. Cooling and exhumation is
of  Late  Cretaceous  age  within  the  entire  studied  area.  In  the
case of the Rodna horst cooling across the ZPAZ occurred in
Campanian times (Fig. 7c). However, the rather tight cluster-
ing of central ages in the case of samples M01—M04 and M09
from the more internal (SE) parts of the Maramure  Moun-
tains and the Central East Carpathian chain, and the Cenom-
anian  age  of  the  post-tectonic  cover,  are  evidence  that
cooling  started  earlier,  namely  during  the  Cenomanian  in
these  areas.  Apatite  fission  tracks  have  been  fully  annealed

during renewed burial in the Miocene (Gröger et al. 2008)
that occurred in the context of thrusting of the easternmost
tip of ALCAPA (Pienides) over Tisza—Dacia (Tischler et al.
2007) and related flysch sedimentation (Tischler et al. 2008).
Hence, they cannot provide additional information regarding
the Cretaceous exhumation history.

Interpretation and discussion

Combination of zircon FT data with other constraints on
Cretaceous metamorphism and cooling of the Bucovinian
nappe stack

The combination of the zircon FT central age data with a

compilation of K-Ar (Strutinski et al. 2006) and

Ar/

Ar data

(Dallmeyer  et  al.  1998;  Culshaw  et  al.  2012)  data  from  the
same area enables better estimation of the maximum tempera-
tures reached during the Cretaceous. Fig. 8 provides tempera-
ture estimates based partly on the upper limit of the ZPAZ (at
least 300 °C; Hurford 1986; Yamada et al. 1995) and partly on
widely accepted temperatures for argon retention in muscovite
(400 ± 25 °C; von Blanckenburg et al. 1989; Hames & Bowring
1994).  The  same  Fig. 8  also  summarizes  available  age  con-
straints. Note that, in the case of the Rodna horst, zircon cen-
tral  ages,  which  failed  the  Chi-Square  test  (

< 5 %), and so

do not represent cooling ages (R2-3 and R4-4), are omitted.
Note that the colour coding of the ages in Fig. 8 is not identi-
cal for all data since some of the sources for radiometric ages
only provide age groups and not individual ages.

The data indicate that metamorphic temperatures increase

from  NE  to  SW.  Along  the  north-easternmost  rim  of  the
study  area  sub-greenschist  facies  metamorphic  conditions
(200—300 °C)  are  indicated  by  (1)  the  partial  annealing  of
zircon fission tracks, (2) sub-greenschist facies metamorphic
grade  of  Permian  to  Lower  Cretaceous  sedimentary  cover
and (3) undisturbed pre-Alpine K-Ar and

Ar/

Ar mica ages

(Dallmeyer et al. 1998; Strutinski et al. 2006). Temperatures
between 300—400 °C are inferred further to the SW based on
(1) fully annealed zircon fission tracks and (2) pre-Alpine
K-Ar and

Ar/

Ar ages (Dallmeyer et al. 1998; Strutinski et

al. 2006). In the area of the Rodna horst still further SW both
pre-Alpine and Cretaceous-age K-Ar and

Ar/

Ar ages are

found (Dallmeyer et al. 1998; Strutinski et al. 2006; Culshaw
et al. 2012). The Cretaceous muscovite

Ar/

Ar ages group

around 95 Ma (Cenomanian) and only occur close to Alpine
nappe contacts. This implies temperatures of around 400 °C
during Cretaceous nappe stacking, which is corroborated by
the greenschist facies microstructures found in Alpine-age
tectonites (Culshaw et al. 2012).

Nearby age data were projected onto a schematic cross-sec-

tion across the Rodna horst (Fig. 8). The contact between In-
frabucovinian  and  Subbucovinian  Nappes  is  openly  folded
with NW-SE striking fold axes. Fold axial planes dip 80° to-
wards  the  NE.  Note  that  the  Borsa  graben  is  delimited  by
Miocene age faults (Tischler et al. 2007). Assuming a geother-
mal gradient of 25 °C/km and a surface temperature of 25 °C
the  observed  temperatures  along  the  Central  East  Carpathian
chain  (200—300 °C)  and  in  the  south-westernmost  corner  of

Fig. 7. a,b – Comparison between the zircon FT single grain and
central ages of neighbouring samples R2-2 (a) and R2-3 (b) taken
from the Subbucovinian nappes in the Rodna horst. Note that both
samples show a similar spread in single grain ages, although the
central ages are rather different. In these cases the two central ages
merely indicate the position of the most pronounced peaks within
that spread. c – Histogram of single grain ages assembled for all
samples from the Rodna horst. The peak in the Campanian approxi-
mates the time when these samples cooled through the ZPAZ.

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fission tracks in zircon, followed by Late Cretaceous cooling
and  exhumation.  Fig. 9,  also  clearly  shows,  together  with
Fig. 8, that degree of annealing and thus metamorphic grade
increase from the area of the Maramure  Mountains and the
Central East Carpathian chain in the NE towards the Rodna
horst  and  the  Preluca  massif  located  more  to  the  SW.  This,
together  with  the  observation  that  the  degree  of  annealing
also  depends  on  the  position  within  the  Bucovinian  nappe
stack  (Fig. 6a—b),  provides  clear  evidence  that  (1)  this  an-
nealing is the consequence of nappe stacking during the Early
Cretaceous orogeny, (2) that nappe stacking was most proba-
bly top NE and (3) that the Preluca massif attributed to the
Biharia nappe system and whose annealing behaviour is sim-
ilar to that of the Rodna horst area is also very probably part
of  this  same  nappe  stack.  The  southwestward  increasing
tectonic  overburden  provided  by  the  overriding  South
Apuseni—Transylvanian  oceanic  units  provided  the  neces-
sary  overburden  for  Early  Cretaceous  metamorphism  that
reached greenschist facies conditions in the internal parts of
our  working  area.  SW-NE  oriented  Early  Cretaceous  short-
ening is also indicated by the NW-SE trending fold axes and
predominant  metamorphic  lineations  (Balintoni  et  al.  1997;
Culshaw  et  al.  2012)  associated  with  pre-Cenomanian  fold-
ing of the Austrian nappe pile (profile of Fig. 8). Secondary
ENE-WSW-oriented lineations of Culshaw et al. (2012) pos-
sibly reflect top ESE transport during Late Cretaceous exten-
sion (see below).

The available thermochronological data in the working

area (Dallmeyer et al. 1998; Strutinski et al. 2006; Culshaw
et al. 2012) do not allow us to precisely constrain the age of
this Early Cretaceous metamorphic overprint. On the scale of
the entire Biharia nappe system—Transylvanian Basin—Eastern
Carpathians  orogenic  system  (see  Schmid  et  al.  2008  and
Matenco et al. 2010 for a larger scale overview), and based
on  biostratigraphic  evidence  (Săndulescu  1975,  1984)  the
Early  Cretaceous  orogeny  started  during  the  Hauterivian  to
Barremian,  which  means  about  130 Ma  (Fig. 9a),  with  the
onset  of  syntectonic  sedimentation  recorded  in  the  South
Apuseni—Transylvanian Nappes (Fene  Formation) and at the
base  of  the  Transylvanian  klippen,  on  top  of  the Bucovinian
nappe  stack,  in  the  Eastern  Carpathians  (Săndulescu  1984;
Kounov & Schmid 2013).

During the latest stages of the Early Cretaceous orogeny the

Bucovinian  nappe  pile  became  gently  folded  and  juxtaposed
against the more external Black Flysch and Ceahlau Nappes.
The end of the Early Cretaceous orogeny is dated by Cenom-
anian and Turonian conglomerates and sandstones that over-
step the folded nappe contacts (Ianovici & Dessila-Codarcea
1968).  Cenomanian  or  even  older  (Aptian)  post-tectonic
cover is also known to overstep the contact of the Bucovin-
ian—Getic nappe system with the underlying Ceahlau Unit in
the  southern  part  of  the  Eastern  Carpathians  (Bucegi  Con-
glomerate – Stanley & Hall 1978). In our working area tim-
ing  of  the  end  of  Early  Cretaceous  nappe  stacking  in  the
Cenomanian  roughly  coincides  with  Cenomanian  zircon  FT
central ages (samples M01—M04 and M09 from the more in-
ternal parts of the Maramure  Mountains) and with Cenoma-
nian

Ar/

Ar muscovite ages (Dallmeyer et al. 1998;

Culshaw et al. 2012) documenting cooling below 400 °C.

However, Coniacian to Campanian zircon FT central ages

from  the  Rodna  horst  and  the  Preluca  massif  indicate  that
these more internal areas cooled slowly and were still within the
ZPAZ (i.e. 200—300 °C) in Cenomanian times. The close spa-
tial neighbourhood of Coniacian to Campanian zircon FT cen-
tral ages and Cenomanian sediments in the eastern part of the
Rodna  horst  (Fig. 10)  thus  indicates  7—11 km  differential  ex-
humation between the internal area of the Rodna horst and the
more  external  Maramure   Mountains  and  Central  East  Car-
pathian chain since the Cenomanian, assuming again a geother-
mal gradient of 25 °C/km and a surface temperature of 25 °C.

Late Cretaceous extension and exhumation

The Coniacian to Campanian zircon FT central ages

(Figs. 5 and 6c—d) and the single grain zircon FT ages peak-
ing in the Campanian (Fig. 7c) from the internal areas of the
Rodna horst and the Preluca massif (Fig. 9b—c) indicate sub-
stantial  Late  Cretaceous  cooling  and  exhumation,  responsi-
ble  for  much  of  the  7—11 km  differential  exhumation
between  the  internal  area  of  the  Rodna  horst  and  the  more
external Maramure  Mountains and Central East Carpathian
chain mentioned earlier. Most of this differential exhumation

Fig. 10. Sketch map showing a belt with retrograde greenschist fa-
cies  metamorphic  overprint  (Kräutner  et  al.  1978,  1983)  running
sub-parallel  to  parallel  to  the  base  of  the  Upper  Cretaceous  sedi-
ments on the eastern margin of the Rodna horst. This belt may serve
as a normal fault that would explain the finding of zircon FT central
ages  (Coniacian  to  Campanian)  that  are  younger  than  the  age  of
deposition of the neighbouring Upper Cenomanian sediments.

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probably  occurred  mainly  by  extension,  although  distinct
Upper Cretaceous structures have not yet been mapped in the
area.  On  the  other  hand  a  broad  zone  of  greenschist  facies
retromorphism in the Subbucovinian basement, running par-
allel  to  the  contact  with  the  Upper  Cretaceous  sedimentary
cover (Kräutner et al. 1978, 1983) has been mapped and pos-
sibly marks a low angle top E shear zone of Late Cretaceous
age,  allowing  for  the  exhumation  of  the  basement  units  of
the  Rodna  horst  (Fig. 10).  A  possible  continuation  of  this
feature  into  the  area  of  the  Borsa  graben  (Figs. 8  and  10),
which  is  bounded  by  Miocene-age  faults  (Tischler  et  al.
2007)  and  unfortunately  overprints  Cretaceous  structures,
could  explain  the  presence  of  syntectonic  Santonian  to
Maastrichtian conglomerates in the area of this graben (Szasz
1973)  as  well  as  the  presence  of  Infrabucovinian  and  Bu-
covinian  Nappes  at  similar  altitudes  south  and  north  of  the
Borsa graben, respectively (Fig. 8).

Late Cretaceous extension leading to the exhumation of

metamorphic  domes  and  contemporaneous  sedimentation  is
widespread within the ALCAPA (e.g. Neubauer et al. 1995)
and Tisza—Dacia Mega-Units (e.g. Willingshofer et al. 1999;
Schuller et al. 2009). Although the exact timing and geody-
namic context of Late Cretaceous basins may be different in
different parts of the ALCAPA and Tisza—Dacia Mega-Units
such  Late  Cretaceous  basins  are  often  collectively  coined
with  the  term  “Gosau”  or  “Gosau-type”  basins  (see  discus-
sion  by  Willingshofer  et  al.  1999).  The  term  “Gosau”  was
originally  defined  in  the  Eastern  Alps  and  used  for  piggy-
back basins interpreted to have formed in the upper plate of
an active margin related to the subduction of the South Pen-
ninic  Ocean  (“external”  Gosau  of  the  Northern  Calcareous
Alps – e.g. Wagreich & Faupl 1994), and/or, for basins that
are believed to have formed due to massive extension and as-
sociated  exhumation  of  metamorphic  rocks  within  over-
thickened  crust  (“internal”  Gosau  of  the  internal  upper
Austroalpine basement nappes – e.g. Neubauer et al. 1995).

Late Cretaceous sedimentary basins within the Tisza and

Dacia Mega-Units have some similarities with the “internal”
Gosau of the Eastern Alps in that they post-date Cretaceous-
age  nappe  stacking  and  are  associated  with  the  exhumation
of  previously  stacked  nappe  piles.  However,  we  do  not  re-
gard  the  term  “Gosau”  for  such  Late  Cretaceous  sediments
within the Tisza and Dacia Mega-Units as particularly useful
since there appear to be two periods of Late Cretaceous ex-
tension in these Mega-Units: (1) a first one affects units at-
tributed to the Dacia Mega-Unit and starts in Late Albian to
Cenomanian  times  (basal  siliciclastics  of  our  working  area;
Fig.  9a),  roughly  contemporaneous  with  the  sedimentation
of the Bucegi Conglomerate of the Eastern Carpathians and
the Valea lui Paul Formation of the Biharia nappe system in
the Apuseni Mountains (Bleahu & Dimian 1967; Kounov &
Schmid 2013). (2) A second period of extension and sedimen-
tation  lasted  from  Turonian  to  Maastrichtian  times  (“Gosau”
deposits of the northern Apuseni Mountains attributed to the
Tisza  Mega-Unit  –  e.g.  Săndulescu  1994;  Schuller  et  al.
2009; Kounov & Schmid 2013). In the case of the Apuseni
Mountains  zircon  FT  central  ages  from  the  South  Apuseni
Mountains indicate that cooling of parts of the Biharia nappe
system  (Vidolm  Nappe),  a  part  of  the  Dacia  Mega-Unit,

started  already  in  Albian—Cenomanian  (112—96 Ma)  times
(Kounov  &  Schmid  2013).  Zircon  FT  central  ages  from  the
Baia de Arie   Nappe of the Biharia nappe system of the Apus-
eni  Mountains  yielded  central  ages  between  (101—69 Ma;
Kounov & Schmid 2013). This age range is very similar to
that  found  in  the  Preluca  massif  of  our  working  area,  sup-
porting  correlation  of  the  basement  of  the  Preluca  massif
with that of the Baia de Arie  Nappe in the Apuseni Moun-
tains  (Rusu  et  al.  1983;  Strutinski  et  al.  2006),  both  being
part of the Dacia Mega-Unit. The most prominent time of ex-
tension  and  contemporaneous  sedimentation  of  Late  Creta-
ceous deposits in the Tisza Mega-Unit of the North Apuseni
Mountains occurred, however, in Turonian to Maastrichtian
times (94—65 Ma; Schuller 2004); zircon FT central ages in
this  part  of  the  Apuseni  range  between  89—71 Ma  (Kounov
& Schmid 2013).

The geodynamic scenario for Late Cretaceous extension in

the Tisza and Dacia Mega-Units is a matter of debate. Some
authors proposed in situ orogenic wedge collapse following
thickening of the continental crust (e.g. Willingshofer et al.
1999).  Others  related  the  deposition  of  Upper  Cretaceous
sediments  to  a  fore-arc  basin  scenario  (Schuller  2004;
Schuller  et  al.  2009).  Extension  related  to  the  formation  of
the  Late  Cretaceous  Apuseni—Banat—Timok—Sredna  Gora
magmatic belt (von Quadt et al. 2005) behind the N-directed
subduction of the Neotethys in the Aegean area is yet another
possibility.  Our  data  document  long-lived  and  surprisingly
large  amounts  of  Late  Cretaceous  extensional  unroofing  of
the  Bucovinian  nappe  stack  in  Cenomanian  to  Campanian
times.  This  makes  a  fore-arc  scenario  rather  unlikely  and
favours  the  orogenic  wedge  collapse  model  proposed  by
Willingshofer et al. (1999).

Latest Cretaceous thrusting (“Laramian”phase) and Paleocene
exhumation

During the “Laramian” phase in the Maastrichtian, the

Ceahlau and Black Flysch Units were thrusted onto the most
internal  nappes  of  the  Moldavides,  carrying  the  mid-Creta-
ceous Bucovinian nappe stack along in “piggy-back” fashion.
This thrusting caused exhumation by erosion in the more ex-
ternal  part  of  the  study  area,  expressed  by  the  erosion  of
much  of  the  Upper  Cretaceous  cover  and  a  period  of  non-
deposition during the Paleocene in much of the working area
(Figs. 3  and  9).  In  the  Preluca  massif,  however,  continental
shales and sandstones (Jibou Formation) were deposited di-
rectly onto the pre-Mesozoic basement during the Paleocene,
which  requires  erosion  of  all  former  Cretaceous-age  cover
sometime during the latest Maastrichtian and/or Early Paleo-
cene.  We  found  no  Paleocene-age  central  ages  (with  the
exception  of  M09  for  which  hydrothermal  overprint  is  in-
voked). The apatite fission track and (U-Th)/He thermochro-
nology available from the Apuseni Mountains (Merten et al.
2011; Kounov & Schmid 2013) and the Eastern Carpathians
(Merten et al. 2010), however, provide evidence for consid-
erable  amounts  of  latest  Cretaceous  to  Paleogene  exhuma-
tion  that  must  also  have  affected  our  working  area  in  a
temperature range below some 200 °C that we could not ex-
plore due to later re-heating.

396

GRÖGER, TISCHLER, FÜGENSCHUH and SCHMID

GEOLOGICA CARPATHICA

GEOLOGICA CARPATHICA, 2013, 64, 5, 383—398

Eocene burial and Miocene exhumation

Renewed burial started in the Eocene with the sedimenta-

tion  of  conglomerates,  followed  by  marls  and  platform  car-
bonates  (Fig. 9a).  It  continued  in  Oligocene  times  with
flysch sedimentation, starting with fine-grained siliciclastics
coarsening into sand dominated flysch units. Our data show
that related burial metamorphism did not even reach temper-
atures to allow for partial annealing of zircon fission tracks,
as  indicated  by  the  complete  lack  of  zircon  FT  single  grain
ages younger than Eocene (Fig. 9). As a consequence of ero-
sion  after  Miocene  shortening  and  strike-slip  faulting  the
basement  units  of  the  Preluca  massif  and  the  Bucovinian
nappe  stack  only  underwent  minor  differential  exhumation
(Gröger et al. 2008). This indicates that Miocene-age differ-
ential exhumation was modest in the area and did not exceed
some 2 km. The total amount of Miocene-age exhumation in
the Eastern Carpathians is in the order of 5—7 km (Gröger et
al. 2008; Merten et al. 2010), which is substantially less than
that reported for Late Cretaceous times by the present study,
which must substantially exceeds 10 km in view of our esti-
mate for 7—11 km differential exhumation alone.

Conclusions

Our zircon FT study provides two major constraints re-

garding the Cretaceous history of the Bucovinian nappe
stack of the Eastern Carpathians:

(1) Early Cretaceous (“Austrian”) nappe stacking is inferred

to have been top to the NE. While only sub-greenschist facies
were reached in the main Maramure  Mountains and Central
East Carpathian chain (Fig. 9d), temperatures of about 400 °C
are documented for the areas of the more internal Rodna horst
(Fig. 9c) and the Preluca massif (Fig. 9b) attributed to the Bi-
haria nappe system of the Apuseni Mountains. This tempera-
ture gradient is interpreted as the result of increasing tectonic
overburden (up to about 15 km) towards more internal units,
partly  provided  by  the  South-Apuseni—Transylvanian  nappe
stack  within  the  Dacia  Mega-Unit,  comprising  both  the  Bi-
haria nappe system and the Bucovinian nappe stack.

(2) The zircon FT central and single grain ages largely re-

flect  Late  Cretaceous  cooling  and  exhumation.  Differential
exhumation  by  some  7—11 km,  indicated  by  a  combination
of  FT  data  and  stratigraphic  constraints,  point  to  massive
amounts of Late Cretaceous extension accompanied by pre-
dominantly tectonic exhumation that must have substantially
exceeded 10 km. Due to the lack of structural data this mas-
sive extension is documented for the first time.

Later events such as the latest Cretaceous (“Laramian”)

juxtaposition of the nappe pile with the internal Moldavides,
causing  exhumation  by  erosion,  re-burial  by  sedimentation
and tectonic loading during the Cenozoic had no impact on
the zircon FT data but prevented a study of the low tempera-
ture  part  of  the  Late  Cretaceous  exhumation  history.  The
presence  of  Upper  Cretaceous  sediments  deposited  on  ex-
humed pre-Permian basement indicates exhumation of parts
of the basement of the Bucovinian nappe stack to the earth’s
surface in Late Cretaceous times.

Acknowledgments: We are most grateful for the excellent in-
troduction into the study area and its geology provided by M.
Săndulescu and L. Matenco and their ongoing support. Fruit-
ful discussions with I. Balintoni, D. Radu and especially C.
Strutinski are also highly appreciated. F. Neubauer is grate-
fully acknowledged for discussion of still unpublished Ar/Ar
data from the study area. And finally the careful reviews by
Ioan Balintoni and Ernst Willingshofer further improved the
manuscript during the publishing process. Financial support
by the Swiss National Science foundation (NF-Project
Nr. 21-64979.01, granted to B.F) is gratefully acknowledged.

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