GeolCarp_Vol63_No1_13

www.geologicacarpathica.sk

GEOLOGICA CARPATHICA

, FEBRUARY 2012, 63, 1, 13—32 doi: 10.2478/v10096-012-0001-y

Structural pattern and emplacement mechanisms of the

Krížna cover nappe (Central Western Carpathians)

ROBERTA PROKEŠOVÁ

, DUŠAN PLAŠIENKA

and RASTISLAV MILOVSKÝ

Institute of Landscape Research, Faculty of Natural Sciences, Matej Bel University, Cesta na amfiteáter 1, 974 01 Banská Bystrica,

Slovak Republic; roberta.prokesova@umb.sk

Department of Geology and Paleontology, Comenius University, Mlynská dolina G, 842 15 Bratislava, Slovak Republic;

plasienka@fns.uniba.sk

Geological Institute, Slovak Academy of Science, Ďumbierska l, 974 11 Banská Bystrica, Slovak Republic; milovsky@savbb.sk

(Manuscript received January 10, 2011; accepted in revised form June 9, 2011)

Abstract: The Central Western Carpathians are characterized by both the thick- and thin-skinned thrust tectonics that
originated during the Cretaceous. The Krížna Unit (Fatric Superunit) with a thickness of only a few km is the most
widespread cover nappe system that completely overthrusts the Tatric basement/cover superunit over an area of about
12 thousands square km. In searching for a reliable model of its origin and emplacement, we have collected structural
data throughout the nappe body from its hinterland backstop (Veporic Superunit) to its frontal parts. Fluid inclusion (FI)
data from carbonate cataclastic rocks occurring at the nappe sole provided useful information about the p-T conditions
during the nappe transport. The crucial phenomena considered for formulation of our evolutionary model are: (1) the
nappe was derived from a broad rifted basinal area bounded by elevated domains; (2) the nappe body is composed of
alternating, rheologically very variable sedimentary rock complexes, hence creating a mechanically stratified multi-
layer; (3) presence of soft strata serving as décollement horizons; (4) stress and strain gradients increasing towards the
backstop; (5) progressive internal deformation at very low-grade conditions partitioned into several deformation stages
reflecting varying external constraints for the nappe movement; (6) a very weak nappe sole formed by cataclasites
indicating fluid-assisted nappe transport during all stages; (7) injection of hot overpressured fluids from external sources
(deformed basement units) facilitating frontal ramp overthrusting under supralithostatic conditions. It was found that no
simple mechanical model can be applied, but that all known principal emplacement mechanisms and driving forces
temporarily participated in progressive structural evolution of the nappe. The rear compression operated during the
early stages, when the sedimentary succession was detached, shortened and transported over the frontal ramp. Subse-
quently, gravity spreading and gliding governed the final nappe emplacement over the unconstrained basinal foreland.

Key words: Slovakia, Central Western Carpathians, Krížna Nappe, structural evolution, fluid inclusions, driving forces,
emplacement mechanisms.

Introduction

Thrust sheets or nappes (in Alpine terminology) are the ele-
mentary  geological  structures  in  compressional  orogens
everywhere  in  the  world.  The  basic  definition  (see  McClay
&  Price  1981  or  Merle  1998)  considers  the  nappe  to  be  a
large-scale,  allochthonous  tectonic  sheet-like  body,  which
was  displaced  along  a  basal,  originally  nearly  horizontal
fault  (either  contractional,  or  extensional,  depending  on  the
emplacement  mechanism).  The  most  commonly  used  divi-
sion of thrust sheets is based on the presence or absence of
crystalline basement rocks in a sheet. If the basement rocks
are  widely  involved,  the  thrust  is  a  so-called  thick-skinned
(or, alternatively, a basement nappe). The thin-skinned thrust
sheets (superficial or cover nappes) consist of areally exten-
sive,  but  relatively  thin  (a  few  kilometers)  blocks  of  preva-
lently  non-metamorphosed  sedimentary  rocks,  which  have
been displaced on a thrust plane to the distance reaching sev-
eral  tens  of  kilometers.  Most  cover  nappes  completely  lost
connection with their homelands and their root areas are only
tentatively identifiable.

Since their discovery in the nineteenth century, the mecha-

nisms of nappe emplacement have been the main interest of

geologists all around the world. As a result, a number of me-
chanical models were developed. Considering the driving
forces, they can be categorized as gravitational and compres-
sional ones (see Merle 1998 for review).

The first category invokes gravity as the force causing later-

al  rock  movement.  The  motion  results  in  loss  of  potential
gravitational  energy  of  the  system  in  all  gravity  models.
Gravity gliding (or sliding) requires sliding of the thrust sheet
over a generally down-dipping basal slope. Frictional gliding
(Hubert & Rubey 1959) and viscous gliding (Kehle 1970) are
two end-members subcategories of the gravity gliding models.
Gravity-driven  internal  distortion  of  rock  mass  accompanied
by vertical flattening and lateral extension (Ramberg 1981) is
the main principle of gravity spreading models.

Compressional stresses applied at the rear of the thrust sheet

are the main driving forces of compressional models. Although
the first to be proposed, compressional models were rejected
due to so-called “mechanical paradox” (Smoluchowski 1909)
until the plate-tectonics theory was formulated. Later on, the
popular critical wedge model was introduced by Chapple
(1978) and worked out in detail by Davis et al. (1983), Dahlen
et al. (1984) and their followers. Instead of a single thrust
sheet, this model rather considers orogen-scale thrust belts.

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GEOLOGICA CARPATHICA, 2012, 63, 1, 13—32

Overlooking the nature of driving forces, a weak base in-

duced by mechanically weak rocks (cf. Kehle 1970), overpres-
sured fluids (cf. Hubert & Rubey 1959) or other less known
strain-softening mechanisms seem to play a key role in the
motion of thrust sheets.

Unlike the various thrust and nappe units in the world, lit-

tle attention has been paid to the Central Western Carpathian
superficial  nappe  emplacement  mechanisms  that  could  be
deduced  from  the  strain  analysis.  Virtually,  this  is  due  to  a
non-simple  relationship  between  strain  and  stress  in  such
brittle-ductile deformed units for which cleavages and folds
are the main structural features (Debacker et al. 2008). Like-
wise, the role of the overpressured base during emplacement
of these cover nappes, although proposed from the hydrotec-
tonic  phenomena  (Jaroszewski  1982;  Plašienka  &  Soták
1996; Milovský et al. 1999), has not been confirmed by ex-
perimental  data.  The  only  exceptions  are  the  work  of
Milovský et al. (2003) in which the role of supra-lithostatic
fluid  pressure  at  the  sole  of  the  Muráň  Nappe  (part  of  the
Silica  Nappe  System)  was  discussed  in  detail;  and  research
of  Jurewicz  &  Slaby  (2004)  in  the  paraautochthonous
Giewont thrust in the Tatricum unit.

This contribution presents the possible emplacement

mechanisms of the Krížna Nappe System as they can be inter-
preted from deformation features and p-T data obtained from
the Krížna Nappe sole. Furthermore, we discuss the extent to
which this model fits (or disproves) formerly presented paleo-
tectonic scenarios for the evolution of the Krížna Nappe
(Plašienka & Prokešová 1996; Plašienka 1999).

Geological setting

The Western Carpathians, the northernmost part of the Eu-

ropean  Alpides,  are  a  north-vergent  stack  of  several  crustal-
scale  (thick-skinned)  and  cover  (thin-skinned)  nappe
superunits  (e.g.  Froitzheim  et  al.  2008).  Thin-skinned  thrust
sheets – Krížna, Choč and Silica Nappe units (ordered uphill)
in the central part of the Western Carpathians (Central West-
ern Carpathians – CWC) were displaced from their original
substratum  during  mid-Cretaceous  times  (Andrusov  1968;
Mahe  1986; Plašienka 1999).

The Krížna Nappe System (or the Fatricum, cf. Andrusov

1973) is the most representative thin-skinned thrust unit in the
CWC. Most of geological and paleotectonic evidence confirm
that this thin (1—3 km), but areally extensive ( ~ 12,000 km

cf.  Jacko  &  Sasvári  1990;  Fig. 1a,b)  thrust  sheet  body  has
been transported over the underlying Tatric Superunit to a dis-
tance of several tens of kilometers (40—60 km) as a relatively
coherent body (see Biely & Fusán 1967; Jaroš 1971; Plašienka
1983,  1995b,c,  1996,  1997,  1999  and  references  in  these
works). In contrast to the higher Central and Inner Western
Carpathians  nappe  systems  (Hronicum,  Turnaicum,  Silici-
cum),  that  are  typical  “rootles”  thrust  units,  the  Krížna
Nappe  is  exposed  from  its  root  area  up  to  the  frontal  zone
(Mahe  1983). This feature makes the Krížna Nappe the most
suitable for structural studies, although its post-emplacement
structural  pattern  has  been  affected  by  numerous  younger
tectonic events.

The Krížna Nappe System is composed mainly of Mesozo-

ic sediments dominated by carbonate lithology sheared off
their mostly disappeared original basement along a décolle-
ment horizon of Upper Scythian shales to form a far-reach-
ing allochthonous body. Now it overlies various Tatric cover
units and is overlain by the higher Hronic (Choč) cover
nappe system (Fig. 1c).

Although its substratum mostly disappeared, the close spa-

tial  relations  between  the  (southern)  rear  parts  of  the  Krížna
cover  nappe  and  northern  parts  of  the  thick-skinned  Veporic
Superunit are clear. Here the sedimentary Krížna successions
grade from their allochthonous position (Fig. 1c) to the sedi-
mentary  envelope  of  the  North  Veporic  basement,  which  al-
ready forms the backstop of the Fatric thrust system (i.e. the
Ve ký Bok and Lučatín Units). Therefore, a widely accepted
idea  is,  that  the  Krížna  Nappe  System  as  a  part  of  the  Fatric
Superunit  (cf.  Plašienka  (2003)  originated  from  the  area  of
stretched  and  thinned  continental  crust  (Zliechov  Basin)
flanked by elevated domains of the South Tatric Ridge to the
north  and  the  North  Veporic  continental  margin  to  the  south
(e.g. Plašienka 1999, 2003 and references therein).

Mechanical stratigraphy of the Krížna Nappe

Along with a stress field state, strain rates and p-T condi-

tions, the lithology of deformed rock units is one of the main
factors controlling the mode of deformation. It plays a signifi-
cant role especially in deformation of upper crustal sedimenta-
ry  rocks  where  the  contrast  in  rheology  between  deformed
strata  is  occasionally  high  and  layers  should  accommodate
strain in a different way. The numerous primary (e.g. bedding
or compositional layering) and secondary (cleavage, joint sys-
tems)  planar  anisotropies  represent  the  weakness  zones  that
can be predisposed for slips in any phase of deformation.

From the lithostratigraphic point of view, the Krížna Nappe

is  generally  composed  of  Middle  Triassic  to  lower  Upper
Cretaceous  strata.  While  the  Triassic  rocks  are  uniform
throughout the nappe body, the Jurassic and Cretaceous sedi-
ments  are  subdivided  into  the  restricted  ridge  or  slope-type
Vysoká  Succession  and  prevailing  basinal  Zliechov  Succes-
sion  (e.g.  Andrusov  1968;  Mahe   1983;  Froitzheim  et  al.
2008).  Most  of  the  sedimentary  infill  of  the  Zliechov  Basin
has been detached from its original substratum along a Lower
Triassic horizon of the Werfenian shales and evaporites. The
underlying Permoscythian clastic sequence remained attached
to the basin substratum that disappeared by underthrusting be-
neath  the  northern  Veporic  thrust  wedge  (Plašienka  1999,
2003).  A  small  part  of  this  substratum  forms  rare  tectonic
slices at the base of the Krížna Nappe in its rear parts (e.g. the
Staré  Hory  Unit  –  Jaroš  1971,  Plašienka  1999;  and  the
Rázdiel Unit – Hók et al. 1994, 1997).

From the physical point of view, the detached Zliechov

Succession  (Fig. 2)  is  a  typical  sedimentary  multilayer  com-
plex  (cf.  Ramsay  &  Huber  1987)  with  upward  increasing
anisotropy induced by variable thickness and rheology of de-
formed  strata.  In  this  sense,  six  main,  rheologically  different
complexes were defined in the detached Zliechov Succession
in order to fit the purpose of this work:

PROKEŠOVÁ, PLAŠIENKA and MILOVSKÝ

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GEOLOGICA CARPATHICA, 2012, 63, 1, 13—32

T3 – up to 300 m thick horizon of variegated shales and

evaporites with dolomite and sandstone beds (Carpathian
Keuper Formation) in the roof of T2. Due to its weakness and
plasticity it plays a second-order décollement function;

J—K1

– the Jurassic (including also Rhaetian) to Lower

Cretaceous  (Valanginian)  succession  is  a  relatively  thick
(100—500 m) complex of alternating well-bedded strata (vari-
ous  types  of  limestones,  cherts  and  marlstones,  e.g.  Fatra,
Kopieniec,  Allgäu,  Osnica  Formations  –  see  e.g.  Michalík
2007) of different thickness and competency with a high ten-
dency to buckling strain and interlayer slip. Its original thick-
ness  is  frequently  reduced  due  to  macroboudinage  of
sequences situated above the Carpathian Keuper;

Fig. 2. Mechanical stratigraphy of the Fatric Zliechov sedimentary succession.

– incompetent and weak complex

( ~ 300 m) of the Lower Cretaceous (Valan-
ginian to Aptian) marls and marly limestones
(Mráznica and Párnica Formations);

K2 – the uppermost Middle Cretaceous

(Albian—Cenomanian)  syn-tectonic  (syn-de-
formational)  flysch  sequence  (Poruba  For-
mation)  deposited  immediately  before  and
during thrusting.

More detailed stratigraphy of the Zliechov

and Vysoká type successions can be found in
many works (e.g. Mahe 1983; Lefeld et al.
1985; Michalík 2007).

Obviously three weak horizons can be de-

fined in the Krížna Nappe successions.
While the role of the T1

as basal décolle-

ment has been known for a long time, the
role of the T3 and K1

has not been fully

appreciated until recent time (e.g. Plašienka
1999). Especially the role of the K1

newly recognized. It deserves more atten-
tion and will be discussed in detail, here.

Methods

To investigate the structural pattern of

displaced Fatric successions with a view to
the  Krížna  Nappe  emplacement  mecha-
nisms, field profiles parallel to the XZ prin-
cipal deformation plane, namely parallel to
assumed  nappe  transport  direction  (S—SSE
to  N—NNW), were selected for the structural
analysis.  Field  profiles  were  studied  prefera-
bly  in  the  area  defined  as  “A”  in  Fig. 1a,  al-
though  some  additional  profiles  were  also
selected  in  areas  “B”  and  “C”.  Additionally,
the results of structural analysis carried out in
area  “D”  and  presented  in  the  works  of
Plašienka (1983, 1995a) have been considered
in the “Discussion”. Conventional methods of
field structural analysis (e.g. Ramsay & Huber
1983,  1987;  Price  &  Cosgrove  1990)  were
used.  Mesoscopic  structures,  including  folds,
cleavages,  shear-band-like  structures,  stylo-
lites, and stretching lineations were assembled

into structural paragenesis succession. Their orientation param-
eters were evaluated in the Lambert’s equal area projection by
using software GEOrient v9.4 (Holcombe 2010).

To gain insight into the p-T regime on the Krížna sole

thrust, basal cataclasites (rauhwackes) from near-root areas of
the  Krížna  Nappe  were  sampled  for  fluid  inclusion  analyses.
Samples were cut for macroscopic study of textures and clas-
tic  lithologies,  thin  sections  were  prepared  for  petrographic
study and the rest of the material was digested in diluted acetic
acid.  Euhedral  authigenic  minerals  were  hand-picked  from
light  and  heavy  fractions  of  insoluble  residues,  separated  in
methylbromide.  Newly-formed  crystals  of  quartz  up  to
1.5 mm in size were mounted in resin and doubly polished for

KRÍŽNA NAPPE – STRUCTURAL PATTERN AND EMPLACEMENT MECHANISMS (WESTERN CARPATHIANS)

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GEOLOGICA CARPATHICA, 2012, 63, 1, 13—32

study  of  fluid  inclusions.  Microthermometric  observations
were  performed  on  a  Linkam  THMSG-600  heating-freezing
stage mounted on an Olympus BX-51 microscope. Calibration
was  performed  using  temperature  of  melting  in  natural  pure
CO

inclusions in quartz (—56.6 °C) and synthetic K

(398 °C).  Low  temperature  data  ( < 100 °C)  were  measured
with the precision of  ± 0.2 °C at a rate of 1 °C/min. An uncer-
tainty  of  ± 2 °C  is  estimated  for  the  temperatures  above
100 °C measured at a rate of 10 °C/min.

Results

Structural analysis

In consequence of the above described lithological as-

pects,  downward  increasing  competence  and  upward  in-
creasing  ability  to  fold  may  be  expected  in  the  Fatric
successions.  Since  different  structural  styles  and  different
deformation mechanisms reflect the same tectonic history in
the  above  defined  rock  complexes,  they  are  characterized
separately in this section.

Permoscythian complex P—T1

(A,B)

The Upper Scythian shales and evaporites (“Werfen” For-

mation) have long been regarded as the main décollement

along which the Fatric sedimentary succession was detached.
Unfortunately,  rare  preservation  and  scarce  outcropping  of
Werfenian shales does not allow us to study the strain in rocks
comprising this primary basal décollement. Nevertheless, the
structural overprint of underlying Lower Scythian clastics was
studied  at  several  localities  near  Banská  Bystrica  where  tec-
tonic  slices  of  the  Fatric  basement  (Staré  Hory  Unit)  are
present. In some horizons the quartzites with shale intercala-
tions are refolded to recumbent folds (Fig. 3a,b) with roughly
E—W  trending  sub-horizontal  folds  axes.  Their  character  and
localization suggest a passive amplification of initial perturba-
tions during shearing (Cobbold & Quinquis 1980; Hanmer &
Passchier 1991) and can be a result of differential movements
during  decoupling  of  Krížna  sedimentary  successions.  The
plastic  behaviour  of  quartzite  beds  was  conditioned  by  their
(original?) gypsum cement.

Interestingly, the thickness of the Permoscythian sediments

is highly variable in this area (it ranges from 100 m to 800 m)
but their absence is also not uncommon (Jaroš 1965). Al-
though it cannot be definitely stated whether this absence is
due to primary (sedimentary) or secondary (tectonic) reasons,
the second one seems to be more probable.

Carbonate complex T2

Thick (up to 700 m) T2 carbonate complex constitutes a

lower rigid frame of the Krížna sedimentary succession. The

Fig. 3. Typical structural pattern of the Krížna near sole-thrust rocks: a, b – folds in Lower Scythian clastics near Baláže village; c – rauh-
wackes outcropping in the Donovaly road-cut; d – carbonate tectonic breccias (rauhwackes) – the same locality as “c”.

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high  proportion  of  dolomites,  poor  bedding  and  insignificant
content of incompetent beds caused the whole complex to act
as  a  huge  single  layer  (the  C-type  multilayer  cf.  Ramsay  &
Huber 1987, p. 418) controlling the micro- and mesostructural
character of lower part of the nappe – namely low buckling
instability and high ability to brittle deformation. Large-scale
duplexes and megafolds are typical map-scale features mainly
in  the  frontal  and  rear  parts  of  the  Krížna  Nappe  where  they
were  described  as  northward  plunging  “digitations”  (e.g.
Polák  et  al.  1979;  Mahe   1985)  and  partial  thrust  units  and
megafolds  (e.g.  Plašienka  1983,  2003).  All  Mesozoic  Fatric
and North Veporic successions (together with massive T2 car-
bonates  and  basement  rocks)  suffered  more  ductile  strain  in
the  rear  areas  (i.e.  in  the  Ve ký  Bok  and  Lučatín  Units  –
Plašienka  1995a;  Soták  &  Plašienka  1996)  where  low-grade
metamorphism was established (Plašienka et al. 1989).

Unlike these rear zones, the mesostructural character of

the  T2  complex  in  allochthonous  Fatric  units  (i.e.  Krížna
Nappe  System)  reflects  solely  the  brittle  strain  (Fig. 3c).
Several  generations  of  contractional  (stylolites,  weak  frac-
ture  cleavage),  shear  and  extensional  brittle  structures  with
rather  complicated  cross-cutting  relationships  are  wide-
spread  in  this  complex  and  indicate  a  changeable  stress  re-
gime which is best explainable by fluctuating fluid pressure.
The  intensity  of  brittle  overprint  increases  toward  the  base
(that is the sole of the nappe) where fine-grained cataclasites,
tectonic  breccias  and/or  rauhwackes  as  a  final  result  are
common  (Fig. 3d).  Some  authors  (e.g.  Plašienka  &  Soták
1996)  envisaged  that  high  pore-fluid  pressure  was  the  main
mechanism of their development. Moreover, in several frontal
areas “megastylolitic” contact of T2 carbonates with underly-
ing  massive  limestones  of  the  autochthonous  Tatric  succes-
sions (described also by Jaroszewski 1982 and Jurewicz 2005,
2007)  apparently  resulted  from  complete  dissolution  of  the
carbonate breccia bodies. All these facts suggest a significant
role of overpressured fluids at the sole of transported Krížna
thrust sheet. Therefore, special attention has been devoted to
such phenomena in a chapter below.

Carpathian Keuper Formation (T3)

Regardless of its high original anisotropy (shales with

evaporites  alternate  with  dolomite  and/or  sandstone  beds),
structures originating from buckling instability are rare in this
complex. Instead, monoclinal position is typical except in the
areas  where  these  rocks  are  incorporated  into  T2-megastruc-
tures  (i.e.  duplexes  and/or  megafolds  in  the  frontal  and  rear
zones  of  the  nappe  system).  Thus  the  structural  character  of
the T3 seems to be partly conditioned by the underlying rigid
T2  complex  by  impeding  small-scale  folding  (Treagus  &
Fletcher 2009). Instead folds, low-angle (with respect to origi-
nal layering) planar fabric (cleavage, partly pervasive) in the
shales  and  small  thrusts  in  the  competent  beds  are  common.
En-echelon types of fractures, slightly elongated pebbles and
fine  stretching  lineation  suggest  that  differential  movements
and shearing occurred along this horizon. The role of the T3
complex  as  a  secondary  detachment  horizon  is  revealed  by
several partial nappe units occurring in the frontal parts of the
Krížna Nappe System (e.g. Ďurčiná, Drietoma, Manín or Belá

Units containing the Vysoká-type successions), which do not
contain the T2 complex and were fully detached along the T3
layer (Plašienka 1999).

Jurassic to Lower Cretaceous complex (J—K1)

Evolution of a rich spectrum of mesostructures (i.e. out-

crop-scale structures) in this complex was conditioned by its
rheological properties. Obvious pre-existing planar anisotro-
py was further intensified by layering rotation and develop-
ment of secondary foliations as deformation proceeded. This
led  to  an  increase  of  J—K1  heterogeneity,  which  was  origi-
nally induced by variable thickness and viscosity contrast of
deformed  beds.  Numerous  planar  discontinuities  prone  to
slip resulted in complicated structural overprint. Thus appar-
ently  unrelated  structures  could  have  originated  during  the
same deformational event as a result of rotation of pre-exist-
ing planar anisotropies from contractional to extensional sec-
tor of incremental strain ellipsoids.

Although the structural successions described below are re-

ally the result of progressive deformation history, we are using
the traditional deformation stages approach for their descrip-
tion as the most practical for further interpretations.

The oldest widespread mesostructural paragenesis D1

comprising the F

folds, S

cleavage and related structures, is

markedly developed in the J3—K1 sub-complex consisting of
well-layered and foldable micritic limestones. Rotated (now
overturned or recumbent), flattened and segmented F

meso-

folds (Fig. 4a,b and h) are the most frequent remnants of the
D1 stage. They mostly represent later strongly sheared, flat-
tened and rotated small-scale folds related to large-scale
folds and thrusts. They are associated with pressure solution
cleavage S

moderately south-dipping in the normal and sub-

horizontal in the inverted fold limbs, where it is also more
pervasive – intensified due to a later deformational event.
Sub-horizontal to gently plunging L

lineations (fold axes or

hinge  lines  and  cleavage/bedding  intersection)  with  mean
E—W  to  ENE—WSW  trends  (Figs. 5a,d  and  6a,b)  suggest  an
origin  in  compressional  stress  field  with  N—S  to  NNW—SSE
(in present coordinates) oriented maximal compression axis

Reorientation of primary layering (S

) and development of

secondary foliation (S

) during the D1 stage led to an increase

of anisotropy and heterogeneity in the J—K1 complex and en-
hanced its ability to slip. Therefore, deformation frequently
occurred by sliping along suitably oriented pre-existing
(mainly S

) planes. Thus besides rotation, flattening and seg-

mentation of F

folds, new associations of shear-like struc-

tures and/or thrust-related F

folds are the most common

results of the D2 stage. Sub-horizontal to gently south-dipping
(compressional) shears with “top to the N—NW” kinematics
(Fig. 5b) are either reactivated S

)-planes (Fig. 4c) or new-

ly formed C

-shears in domains with steeply dipping S

fabric (Fig. 4d,e). NW-directed tectonic transport may also be
indicated by mineral or object stretching lineation and/or
S

-plane striations (Fig. 4f,g) observed mainly in J1-J2 suc-

cessions (see also Kováč & Bendík 2002).

The bedding-cleavage relationships in this case correspond

well to the “flexural flow” model of a thrust sheet overcoming
a frontal ramp (Sanderson 1982). As will be shown later, this

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frontal ramp might be paleogeographically identified with the
South Tatric Ridge, which the Krížna Nappe had to overcome
during the early stages of thrusting, namely the D

deforma-

tion stage (Fig. 12b). It might also be inferred, that remagneti-
zation of the Krížna Nappe body in a southward inclined
position (hinterland dipping duplex – Grabowski 2000;
Grabowski et al. 2009) occurred during this “ramp” stage.

North-vergent thrust-related F

folds (associated with S

cleavage  steeply  inclined  to  the  south)  are  frequent  in  more
competent (e.g. majolica and biancône type) limestones in the
rear  zone  of  the  Krížna  Nappe  (Banská  Bystrica  surround-
ings).  Axial  directions  of  F

folds (accompanied by S

cleavage-bedding intersection lineation) are sub-horizontal
with E-W trend, and so nearly homoaxial with the F

folds

(Fig. 4h). All the types of D2 structures are more obvious in
the near-rear and dorsal parts of the Krížna Nappe.

Extensional shear-like structures with generally “top to the

N” kinematics referred to as D3 structural association have the
position of reactivated S

) planes slightly to moderately in-

clined (10—50°) towards the NW—NE and/or newly formed
S-C-like shears (C

) slightly inclined (10—35°) towards the

NW—NE in areas with strong south-dipping cleavage S

(Figs. 5, 6 and 7). Their origin in the stress field with vertical
to steeply foreland-inclined

is most probable, although

their slightly variable kinematics (from “top to the NW” to
“top to the NE”) can be a result of partly unrestricted (both lat-
erally and frontward) forward movement. North-vergent D

shears are the most conspicuous in the dorsal and near-frontal
areas of the Krížna Nappe (Fig. 6c,d). Towards the rear zone

Fig. 6. Stereograms showing orientation of main D1 and D3 tectonic mesostruc-
tures in dorsal and near-frontal areas of the Krížna Nappe (lower hemisphere of
Lambert’s equal area projection): a, b – F1 fold axes – Ružomberok surround-
ing (a), Strážovské vrchy Mts and Krivánska Malá Fatra – East (b); c, d – D3
shear bends with “top to the North” kinematics – Nízke Tatry Mts (Northern
slopes) and Ružomberok surrounding (c), Strážovské vrchy Mts (d).

two  conjugate  systems  appear  (Fig. 5c,f,g)  that,
along  with  crenulations  (Fig. 7f)  and  boudinage
of more competent beds, suggest sub-vertical flat-
tening component of strain in this area during D3.
Segmentation and boudinage of competent mem-
bers is more obvious at map-scale, where they are
revealed  by  the  lack  of  some  stratigraphic  mem-
bers in the deep-water Jurassic successions. They
have been identified from many areas in the CWC
(Bujnovský  1979;  Mahe   1985;  Nemčok  et  al.
1993;  Polák  et  al.  1997b).  The  fact  that  boudi-
naged formations are situated mostly between two
incompetent complexes – the Carpathian Keuper
Formation below and the Lower Cretaceous marl-
stones above – supports this suggestion.

Numerous extensional faults with variable ori-

entation and other related brittle structures should
be  associated  with  the  post-emplacement  D4
stage.  Most  of  them  can  be  related  to  the  Paleo-
gene/Neogene  gravitational  collapse  and  lateral
extension of the CWC nappe stacks leading to fast
cooling and exhumation of CWC crystalline com-
plexes confirmed by fission track data (e.g. Kováč
et al. 1994; Danišík et al. 2008, 2010) and/or by
movements  in  the  Central  Slovak  Fault  System
(Kováč & Hók 1993).

Mid-Cretaceous flysch complex – K2

The Albian—Cenomanian Poruba Formation is accumulated

mostly in the frontal parts of the Krížna Nappe. Foreland-ward
increase of thickness and coarsening upward indicate the syn-
tectonic  nature  of  this  flysch  complex.  The  geometry  of
synsedimentary  structures  such  as  olistoliths,  slump  breccias
and  slump  folds  together  with  paleo-current  markers  in  the
Vlkolinec Breccia Formation (Jablonský & Marschalko 1992)
situated  on  the  base  of  the  Poruba  Formation  indicate  NNW
paleoslope inclination  as a result of compression and thrusting
in the rear part of the Krížna Nappe accompanied by foreland
flexing (Plašienka 1999). At the same time, the Poruba Forma-
tion  shows  a  structural  independence  from  the  underlying
Krížna complexes, which indicates its partial detachment and
a  relative  forward  movement  during  the  final  stages  of  the
nappe emplacement.

p-T regime on the nappe sole

Basal tectonic breccias – occurrence and petrography

Since the carbonate hydraulic breccias (so-called rauhwack-

es) are a common rock type in the soles of all the CWC cover
nappes  including  the  Krížna  Nappe,  some  authors  proposed
that  overpressured  fluids  must  have  played  an  important  role
during  their  emplacement  (Plašienka  &  Soták  1996).
Rauhwackes  are  valuable  nappe-base  material  because  they
are able to provide information on the p-T regime at the time
of  the  nappe  displacement.  Supra-lithostatic  pressures  deter-
mined  from  the  basal  rauhwackes  of  the  Muráň  Nappe

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(Milovský et al. 2003) challenged us to obtain analogous data
from the sole of the Krížna Nappe to detect the p-T regime ac-
companying its emplacement.

Basal cataclasites, similar to those described from the

Muráň Nappe (Milovský et al. 2003), were also observed on
the foot of the Krížna Nappe, mainly in its rear zones, where
it reposes on anchizonally metamorphosed Tatric cover units
(Fig. 5a).  The  cataclasites  are  very  variable  in  appearance,
from monomict varieties with boxwork textures and leached
dedolomite  fragments  to  highly  polymict  breccias,  contain-
ing  fragments  of  carbonates  and  shales,  scarcely  of  crystal-
line  rocks  (gneiss  in  breccias  from  Kalište  –  Hrubý  vrch
Hill).  Textures  of  hydrofracturing  and  dedolomitization  are
ubiquitous. The newly-formed mineral assemblage compris-

Fig. 7. Typical D3 structures in Jurassic to Lower Cretaceous Krížna Nappe rocks: a—e – shear-like D3 structures: Nízke Tatry Mts – I a-
novo Valley (a, b) and upčianska dolina Valley (c, d), Belianske Tatry – Kopské sedlo-Hlúpy (e); f – subvertical flattening crenulation
cleavage (Banská Bystrica-Kostiviarska).

es  quartz,  pyrite,  dravite  tourmaline,  microcline,  albite  and
anhydrite (which is only present as inclusions in other min-
erals).  The  matrix  consists  of  newly-crystallized  calcite  ce-
ment,  contaminated  by  microclasts  and  clay  minerals.  We
propose that basal tectonic breccias have formed during par-
tial nappe movements by grinding wallrocks of the thrusting
plane  and  reworking  of  frontal  debris  of  the  advancing
nappe. The characteristic petrographic features of basal rau-
hwackes are illustrated in Fig. 8a—c.

Fluid inclusions

The quartz, tourmaline and feldspars from basal tectonic

breccias contain numerous fluid inclusions (FI), which were

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investigated  (in  quartz  only)  to  constrain  the  p-T  conditions
and chemical properties of included fluids. Clearly neoformed
minerals are devoid of matrix calcite inclusions and they ap-
pear  as  crystallized  into  open  space,  yet  they  are  often  frac-
tured and besides primary FI (trapped during crystal growth)
they contain secondary inclusions, trapped along healed frac-
ture planes. These features (analogous to those studied previ-
ously by Milovský et al. 2003) lead us to conclude that fluids
were  trapped  in  crystallizing  minerals  synkinematically,  and
thus refer to conditions of thrusting events.

Primary fluid inclusions (FI) are either scattered or form

small isolated groups. Secondary FI are arranged in planar ar-
rays along healed cracks. Inclusions are of irregular shape,
typically up to 10 micrometers in size (Fig. 8d). At room tem-
perature, they contain aqueous liquid (L), vapour bubble (V)
and crystal of halite (H). All populations showed homoge-
neous volumetric phase ratios of individual FI (Fig. 9), and no
other coevally trapped phases were observed, thus ruling-out
entrapment of heterogeneous fluid.

Phase transformations were observed upon heating of com-

pletely frozen fluid inclusions, from approximately —100 °C

Fig. 8. Characteristic petrographic features of basal rauhwackes: a – typical texture with cavities after leached dolomite fragments in cal-
cite matrix (Patočiny); b – newly crystallized calcite cement of incompletely dedolomitized clasts often encloses relic rhombs of dolomite
(Kalište-Hrubý vrch); c – large crystal of newly-formed quartz, growth zones marked by solid inclusions of anhydrite, tourmaline and
mica hint at a polyphase growth (Kalište-Hruhý vrch); d – array of fluid inclusions in crystal of newly-formed quartz (Patočiny).

up to temperature of total homogenization. Homogenization
successions were invariably as follows:

– first melting or “eutectic” temperature at —65 to

—40 °C;

Gradual hydration of halite above the first melting tem-

perature;

I – melting of ice at —25.3 to —12.2 °C;

Hh – metastable dissociation of hydrohalite up to

+ 15 °C;

– homogenization of vapour bubble in liquid phase

from 65 to 192 °C;

H – dissolution of halite crystal at 193 to 344 °C.

Temperature ranges for particular samples are given in

Table 1. First melting temperatures are equal to, or slightly
lower than true eutectic temperatures and depend on associa-
tion of coexisting solid salt species with liquid electrolyte (ab-
breviations of daughter-phase mineral names are used here as
follows: I – ice, H – halite, Hh – hydrohalite NaCl · 2H

Ca4 – CaCl

· 4H

O, Ca6 – antarcticite CaCl

· 6H

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Mg8 – MgCl

· 8H

O, Mg12 – MgCl

· 12H

O). In our sam-

ples  they  cluster  between  A)  —65  and  —55 °C,  B)  —55  and
—53 °C and C) —52 and —40 °C. Groups A) and B) may refer
to  eutectic  melting  in  metastable  associations  Ca4 + H + I  at
—70 °C and Mg8 + H + I at  —55 °C, respectively (Davis et al.
1990). Span of the group C) covers eutectic temperatures in
stable  associations  Mg12 + Ca6 + I  at  —52.2 °C,  Ca6 + Hh + I
at —51.6 °C and Ca6 + I at —49.8 °C (Linke 1965a,b; Spencer
et al. 1990). Summarizing this, trapped fluids contain over-
saturated brines from the system Na-Ca-Mg-Cl. Salinity ex-
pressed  in  wt. %  of  NaCl  equivalents  was  calculated  using
the equations of Sterner et al. (1988) at the homogenization
temperature of halite and is denoted on the right y-axis of the
diagram in Fig. 10. Overall range is 29.4 to 41.9 wt. %.

In one inclusion (LLČV-2-1-1) a sylvite daughter crystal

was present at room temperature, but it dissolved in aqueous
liquid at 106 °C after bubble homogenization and prior to
halite dissolution. This succession allowed us to determine
the NaCl/KCl weight ratio to 0.99 and overall NaCl + KCl sa-
linity to 42.3 wt. % (Sterner et al. 1988).

Final homogenization temperatures are dispersed in an un-

usually wide range, stretching over 150 °C, between 193 and
344 °C (Fig. 10). Spans for particular localities Bal-1, Bar-1,
Dono-62, KalHV-1, LLČV-1, Pat-1 are 99, 75, 150, 84, 98,

Fig. 9. Fluid inclusions in newly formed quartz crystal at room temperature (Patočiny): a – near-
ly identical phase ratios of three-phase fluid inclusions suggest trapping of homogeneous fluid;
b – three-phase FI contain an NaCl-saturated aqueous liquid (L), vapour bubble (V) and cubic
crystal of halite (H).

Explanations: Bal – Baláže, Bar – Barboriná, Dono – Donovaly, KalHV – Kalište-Hrubý vrch,
LLČV – Liptovská Lúžna-Čierny vrch, Pat – Patočiny, T

– first melting or “eutectic” temperature,

I – melting of ice, T

–homogenization of vapour bubble in liquid phase, T

H – dissolution of halite

crystal at 193 to 344 °C.

Table 1: Summary of temperatures of important phase transformations, calculated salinities
(Sterner et al. 1988) and pressures (Brown & Lamb 1989).

Sample

(°C)

I (°C)

(°C)

H (°C)

wt. % NaCl

P (kbar)

Bal

–47 to –40

–13.6 to –12.2

107–192

198–297

31.8–37.9

0.25–2.42

Bar

117–175

269–344

35.9–41.9

2.02–4.03

Dono

–62 to –43

–25.3 to –21.9

83–123

211–294

29.4–37.7

1.78–3.55

KalHV

–62 to –41

119–161

240–324

34.1–40.1

1.61–2.97

LLČV

–65 to –44

–23.0

65–152

193–318

31.5–39.6

1.83–2.97

Pat

–44.5

90–137

241–307

34.1–38.7

1.83–3.79

66 °C,  respectively.  The  TmH
spans  for  individual  FI  populations
in  quartz  grains  are  mostly  within
30 °C,  exceptionally  more,  up  to
147 °C (Dono 62—3).

Pressure determination is based

on the equation of the state for sys-
tem H

O—NaCl of Zhang & Frantz

(1987), modified by Brown &
Lamb (1989), using the Th and T

to  calculate  molar  volume  and  iso-
chore slope in two-phase L+H field.
The values of pressure at TmH must
be taken with a certain caution due
to  approximation  of  complex  natu-
ral  brine  to  the  theoretical  NaCl—
H

O binary. In the overall TmH

span,  the  pressure  also  has  an  ab-
normally  wide  span  from  0.25  to
4.03 kbar,  with  variations  for  par-
ticular localities up to 2.17 kbar and
for  particular  FI  populations  up  to
1.62 kbar (Fig. 11a).

The whole p-T dataset as well as

the  plots  for  particular  localities
scatter  in  elongated  fields  approxi-
mately  parallel  to  isochores  in  the
liquid  field  of  the  NaCl—H

O sys-

tem (Fig. 11a). In p-T plots for par-
ticular  FI  populations  in  individual
quartz crystals, two distinct patterns
appear  repeatedly:  (i)  a  transverse,
nearly  isochore-parallel  trend,  and
(ii)  a vertical,  nearly  isothermic
trend  (Fig. 11b).  However,  the  di-

rections of these trends remain unclear, as we do not know
the trapping succession of individual FI.

Discussion

The evolution of the Krížna Nappe has been discussed in sev-

eral papers (e.g. Biely & Fusán 1967; Jaroszewski 1982; Mahe
1983;  Jacko  &  Sasvári  1990;  Plašienka  &  Prokešová  1996;
Plašienka 1999). Evolutionary models presented there are based
mainly  on,  indisputably  important,  paleogeographic  and  paleo-
tectonic evidence. However, results obtained by structural analy-
sis together with p-T data providing basic information about the
nappe-base regime can be crucial for detection of emplacement
mechanisms  of  shallow-crustal  thrust  units.  Several  stimulating
discussions have been written on this topic (e.g. Coward & Kim
1981;  Sanderson  1982;  Merle  1986,  1989,1998;  Geiser  1988;
Gray  &  Willman  1991;  Grant  1992;  Schulz-Ela  2001)  from
which the following most important issues for such debate arose:

Character of the nappe sole and p-T regimes operating

on the basal thrust plane;

Degree of tectonic inversion;
Strain partitioning, deformation gradient and progressive

deformation history;

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Fig. 11. Pressure calculated at temperatures of total homogenization according to equations of Brown & Lamb (1989) for NaCl—H

O sys-

tem. Dashed curves represent isochores in one-phase liquid field. Left – data for all studied samples, right – example of two prevailing p-T
trends in FI populations: open symbols – “transverse” trend, solid symbols – “vertical” trend.

Fig. 10. Histograms of T h (dashed bars) and TmH (solid bars) in fluid inclusions.
Salinity expressed in NaCl equivalents is on the right y-axis. Sample names are
indicated below histogram axes, numbers of measurements above them – in
dashed letters for T h and in solid letters for T mH.

Displacement gradient and trajectory;
The scale of observation.

Character of the nappe sole and p-T regime in the basal
thrust zone

Existence of a weak décollement is the most important

feature of all the nappe emplacement models. A weak base

can be formed by easily-deformable rocks such as evaporites
or shales (e.g. Kehle 1970; Davis & Engelder 1985; Jaumé &
Lilie  1988),  mylonites  with  pseudo-viscous  behaviour
(Schmid  et  al.  1981;  House  &  Gray  1982;  Wojtal  &  Mitra
1986), “superplastic” calcite tectonites (Schmid et al. 1977;
Schmid  1982),  high  fluid  pressure  (Hubert  &  Rubey  1959)
or other softening mechanisms.

The concept of the Krížna Nappe detachment along the

weak horizon of the Upper Scythian (Werfenian)
shales  and  evaporites  is  almost  classical  in  the
Slovak geological literature. This role of the Wer-
fenian shales can be inferred from their rheology
and  stratigraphic  position  between  two  rigid
members.  Since  these  rocks  are  only  rarely  pre-
served, the base of the Krížna Nappe is more fre-
quently  formed  by  massive  Triassic  carbonates
(T2)  with  a  thin  horizon  of  carbonate  tectonic
breccias  (rauhwackes)  at  their  sole.  Processes  of
pressure  solution,  recrystallization  and  alteration
have  been  recognized  in  the  Krížna  Nappe-base
(Bac-Moszaszvili  et  al.  1981;  Jaroszewski  1982;
Jurewicz  2005,  2007)  also  in  its  frontal  position
where classic rauhwackes are not present.

The widely occurring phenomena of hydraulic

brecciation and reaction-softening due to dedolo-
mitization point to high activity of pervasively
circulating fluids along the basal thrusting plane.
Fluid inclusions (FI) in newly-formed minerals
were trapped in the course of these fluid-rock in-
teractions and thus record their dynamics.

In the studied FI populations, no obvious evi-

dence of heterogeneous trapping of brine and sol-
id halite was observed. This suggests trapping at
the halite liquidus by temperature decrease. Thus
they do not necessarily represent exact trapping
temperatures, and may also be regarded as their
lowest limit. The extremely wide p-T ranges may
then be explained in two ways: (i) only the high-

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est data reflect true trapping p-T conditions, namely trapping
at,  or  close  to  halite  liquidus,  while  all  other  inclusions
trapped  unsaturated  brines.  Large  variations  in  salinity  are
expected  during  trapping  of  individual  FI  populations;  (ii)
the  p-T  data  represent  the  true  trapping  conditions,  which
implies trapping at NaCl saturation level. It may be achieved
either by steady saturation of circulating fluids in the course
of  temperature  increase,  or  oversaturation/precipitation  in
the course of cooling.

With the first alternative, the trapping conditions come

constrained at some 2.4 to 4.0 kbars and 290 to 340 °C. The
second  alternative  proposes  large  p-T  variations  during  en-
trapment, mostly along “transverse”, or “vertical” trends de-
scribed in the results section. The “vertical”, nearly isothermal
trends may represent either the pressure rise, or drop.  They
are  typically  recorded  in  populations  of  secondary  FI  on
healed  crack  planes,  that  is  trapped  at  episodic  cracking  of
quartz grains. On the other side, “transverse”, near-isochoric
trends  dominate  among  primary  inclusions,  and  may  repre-
sent  either  gradual  cooling  or  gradual  heating  during  grain
growth. Considering the usual orogenic geothermal gradients
of 25—30 °C in the upper crust and published illite-crystallini-
ty  data  (Plašienka  et  al.  1989)  the  lower  temperature  limit  is
roughly  consistent  with  diagenetic  overprint  of  the  Krížna
Nappe, but the upper temperature limit is anomalous and should
be interpreted in terms of focused hydrothermal fluid flux.

Two sources of heat come into account, which may also

combine: (i) intrinsic – frictional heating by pervasive cata-
clasis in course of kinematic events; (ii) external – injections
of hot overpressured metamorphic fluids from the underlying
Tatric  basement  units.  The  latter  involves  an  external  fluid
budget,  which  presumably  mixed  with  autochthonous  fluids,
circulating in basal formations. Both may well explain the ob-
served trends, if we putatively link (or loop) them in the sense:
“transverse”  upwards  –  “vertical”  downwards.  We  suggest
two different mechanisms responsible for such hypothetic flu-
id  p-T  pulsing:  a  “frictional  heating”,  whereby  thrusting  is
a driving  force  for  p-T  pulse;  and  a  “hot  injection”  of  over-
pressurized fluid, which in turn triggers the thrusting. Causali-
ties of both mechanisms are summarized in Table 2.

Since we do not have time-series of FI p-T data, the pre-

ferred scenario must be deduced from indirect hints. The “fric-
tional heating” mechanism is simpler, as it only assumes one
fluid budget. On the other side it can hardly explain coeval
precipitation of quartz, anhydrite and calcite, which have op-

Mechanism

Frictional heating

Hot injection

Drive Thrusting

Fluid injection

1. standstill
190–210 °C 1.6–1.7 kbar

Pre-kinetic: basinal brines in basal formation (Scythian shales and sandstones with evaporites), circulating or stagnant
at “ambient” diagenetic conditions

2. prograde
transverse path upwards

Kinetic: detachment and thrusting frictional heating

adiabatic thermal pressuring

Pre-kinetic: injections of hot overpressured basement
fluids: fluid mixing isochoric heating + pressuring

3. peak
300–340 °C 3–4 kbar

Kinetic: main thrusting, maximum frictional heat
production

Pre-kinetic: culminating fluid influx from the basement

4. collapse

transverse or vertical path
downwards

Post-kinetic: thrusting arrest breakdown of p-T peak
by gradual isochoric cooling of fluid in sealed porosity

Kinetic: fluid pressure surpassed the shear strength of basal
rocks detachment collapse of p-T peak by adiabatic
decompression

Table 2: The two hypothetical mechanisms of p-T pulsing on Krížna Nappe base – synopsis of events, their causes and p-T regime of fluids.
The terms adiabatic and isochoric are approximative.

posite  temperature  dependence  of  solubility  (quartz  prograde
while  anhydrite  and  calcite  retrograde).  The  “hot  injection”
mechanism however expects mixing of deep hot fluids, rich in
silica, with cooler formation waters, saturated by sulphate and
carbonate. The silica may thus precipitate due to temperature
drop  and  sulphates  due  to  temperature  increase  at  the  same
time. Pressure and temperature spans would then mirror vari-
ous mixing ratios of basement fluids with formation waters.

Another hint may be the spatial distribution of basal tecton-

ic  breccias.  We  found  them  exclusively  in  the  rear-part  of
Krížna Nappe, where it is thrust over anchizonally metamor-
phosed South Tatric complexes (Donovaly cover unit – e.g.
Rakús et al. 2003) capable of generating large amounts of hot
overpressured fluids. Moreover, the South Tatric realm creat-
ed  an  elevated  area  in  the  time  of  the  Krížna  Nappe  transla-
tion. Thus a compressional regime of nappe emplacement can
be  proposed  to  overcome  this  frontal  ramp  (i.e.  South  Tatric
Ridge).  Since  a  compressional  stress  field  helps  to  contain
overpressured fluids (Sibson 2004) due to higher pressure gra-
dient in convergent tectonic settings (Petrini & Podladchikov
2000), supralithostatic pressures (fluid vs. lithostatic pressure
ratio  v > 1)  could  have  been  achieved  episodically  at  this
stage. The scarcity of typical rauhwackes in more frontal parts
may  be  explained  by  insufficient  temperature  gradient  be-
tween  substrate  and  nappe  rocks.  Cavernous  calcitic  rocks
with  evidence  of  leaching  and  pressure  solution  present  in
these  frontal  zones  (Jaroszewski  1982;  Plašienka  &  Soták
1996;  Jurewicz  &  Słaby  2004)  confirm  that  fluids  surely
played a key role also during the northward translation of the
nappe from the South Tatric Ridge. However, p-T conditions
at  the  nappe  sole  during  this  stage  remain  unknown.  If  the
gravitational  regime  of  the  nappe  emplacement  is  applied,  a
maximal  fluid  overpressure  had  to  be  limited  by  overburden
pressure (e.g. Sibson & Scott 1998). On the other hand, hydro-
static  to  lithostatic  fluid  pressure  fluctuation  (0.4 > v < 1.0)
could have effectively facilitated nappe movements via accel-
eration of load-weakening of the basal zone leading to its epi-
sodic failure (Sibson 1993) in the gravitational regime.

Degree of tectonic inversion

Although tectonic inversion is probably very common in

compressional orogens at all scales, its intensity is not easily
recognizable due to the high intensity of compression (e.g
McClay & Buchanan 1992).

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In the case of the Krížna Nappe, the north-south oriented (in

the present geographical coordinates) Jurassic extension of the
Zliechov  Basin  has  been  predicted  (e.g.  Plašienka  2003).
Michalík  (2007)  proposed  a  transtensional,  pull-apart  model
for the Zliechov Basin formation. Accordingly, the extension-
al  faults  in  the  northern  part  of  the  Zliechov  Basin  were  in-
clined  to  the  south,  and  so  compatibly  with  Cretaceous
compression. Huge north-ward plunging recumbent folds (so-
called  “digitations”)  in  the  frontal  zone  of  the  nappe  (e.g.
Polák  1979;  Mahe   1985,  1986)  were  probably  initiated  by
contractional reactivation of originally extensional faults asso-
ciated with roll-over anticlines. On the other hand, inclination
of extensional faults at the southern part of the Zliechov Basin
should not be estimated with doubt because both cases, sym-
metrical  or  asymmetrical  extension,  were  possible.  Although
extensional faults were inclined against the direction of Cre-
taceous  compression  (i.e.  to  the  north)  in  the  case  of  sym-
metrical  extension  and  their  simple  reactivation  was  thus
improbable, rotation and turnover of these faults, softened by
underthrusting to greater depth beneath an advancing orogenic
wedge, have been proposed by Plašienka (1999, 2003).

The present tectonic situation in the “root” area of the

Krížna  cover  nappe  reveals  a  total  inversion  of  the  Zliechov
Basin and detachment of its sedimentary infill followed by di-
minishing of its former basement substratum by underthrust-
ing  below  the  overriding  Veporic  thrust  wedge  (Plašienka
2003).  In  places,  the  toe  of  the  Veporic  thrust  sheet  directly
overthrusts the southern Tatric margin, which meant that the
former  basinal  area  was  entirely  sutured.  This  thrust  fault  is
known  as  the  Čertovica  “line”  in  the  Carpathian  literature
(Biely & Fusán 1967; Andrusov 1968).

Progressive deformation history, strain partitioning, defor-

mation gradient

Concerning progressive deformation history, two aspects

seem to be important: i) structural associations developed as a
consequence of rear compression are typical for the first two
deformational stages D1 and D2, although the structural pat-
tern of D2 can be characterized by larger tendency to slip (i.e.
compressional  shear).  Together  with  increasing  intensity  of
contractional  strain  towards  the  rear  part  of  the  nappe  these
facts  suggest  rear  compression  as  the  driving  force  at  early
stages of the Krížna Nappe’s evolution; ii) the structural asso-
ciation linked to the extensional shear mode of strain, typical
of the D3 deformation stage, is connected with a changed tec-
tonic regime moving the Krížna Nappe from rear compression
to  gravity-controlled  modes  of  emplacement.  Distribution  of
extensional  shears  over  the  whole  thrust  unit  (although  con-
centrated  to  mechanically  suitable  horizons)  most  closely
matches  with  the  gliding-spreading  emplacement  mechanism
(cf.  Merle  1998)  during  this  stage.  Both  additional  attributes
of  D3,  namely  gradually  varying  character  of  extensional
shears from conjugate system in the near-rear zones to clearly
“top  to  the  N”  kinematic  types  towards  the  frontal  zones,  as
well  as  increasing  intensity  of  D3  from  the  base  towards  the
top is in agreement with this model.

Deformed Krížna successions are a typical example of parti-

tioned deformation. Strain partitioning along the vertical pro-

file  primarily  depended  on  the  rheological  properties  of  the
deformed  multilayer  (i.e.  existence  of  weak  and  strong  com-
plexes),  thus  above  all  it  is  manifested  by  concentration  of
strain in mechanically suitable horizons. Position in the thrust
unit (e.g. base vs. upper level) is crucial for the vertical defor-
mation  gradient,  which  means  the  concentration  of  simple-
shear component in the basal zone and pure shear component
in the top (mainly in the rear zone).

The deformation gradient along the longitudinal profile

(parallel  to  the  XZ  principal  plane  of  deformation)  reflects
several important aspects. (i) Primarily facies and lithologi-
cal variability (deep-water vs. shallow-water succession and
well-bedded incompetent vs. mostly massive competent stra-
ta, respectively), as well as the existence of extensional fea-
tures (mainly faults) suitable for compressional reactivation
should  be  noted.  Northward  plunging  recumbent  folds  and
duplexes in the frontal part of the nappe reflect contractional
reactivation  of  extensionally  weakened  margins  of  the
Zliechov Basin at early stages of the nappe’s evolution (con-
trolled  by  compression)  followed  by  their  rotation  and  for-
ward  rigid  translation  controlled  by  gravity.  Partial  units
detached  along  higher  weak  horizons  (i.e.  Carpathian  Keu-
per  Formation)  primarily  reflect  the  facial  changes  in  the
Fatric sedimentary area emphasized by the increasing impor-
tance  of  higher  décollements  induced  by  the  gradually
changed  regime  of  deformation  in  a  substantial  part  of  the
Krížna  Nappe.  (ii)  Position  within  the  thrust  unit  coupled
with variability of both the basal slope angle and mechanical
properties of the nappe base – are the most important from
this point of view and can be accommodated by a changing
mode of internal strain in a transported wedge-shaped thrust
unit.  (iii)  Changing  emplacement  mechanisms  also  play  an
important role. Stronger and multi-stage contractional defor-
mation  recorded  in  the  rear  part  of  the  nappe  and  its  North
Veporic  backstop  (Ve ký  Bok  and  related  units,  Plašienka
1983, 1995a, 1999) reflects the “rear compression” acting on
this zone for the whole time the of the nappe’s evolution. Ex-
tensional  shear  strain  with  strong  pure-shear  component  in
the near-rear areas and simple shear with distinct “top to the
~ N” kinematics towards the frontal zones developed as a re-
sult of the gravity controlled final emplacement.

Displacement gradient and displacement trajectory

The displacement gradient is considered one of the key fea-

tures differencing the two main genetic categories of the
nappe units, namely those generated by rear compression from
those driven by gravity (Merle 1998). Although this criterion
can be simply applied for thrust units considered in isolation, a
problem should arise if the same unit is assessed in a broader
context (i.e. in orogen scale).

Simply, studying amounts of displacement in the Mesozoic

Fatric successions seems to make clear that the maximum dis-
placement (up to 60 km) should be set for the Krížna Nappe
front while travelling to the rear zone these successions create
an autochthonous sedimentary cover of the North-Veporic
basement (i.e. zero displacement should be set with respect to
this basement). Such a displacement gradient would be char-
acteristic for gravity controlled emplacement mechanisms, es-

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GEOLOGICA CARPATHICA, 2012, 63, 1, 13—32

pecially for gravity spreading. However, evaluating displace-
ments on a larger scale, the Krížna Nappe must be considered
as  the  frontal  part  of  the  joint  Fatric-Veporic  thrust  system
overriding  the  Tatric  Superunit  during  the  mid-Cretaceous
shortening. The basal displacement plane did not fade out in
the rear part of the Krížna Nappe. Instead it slopes downward
to  the  base  of  crustal-scale  Veporic  thrust  wedge  (i.e.  thin-
skinned thrust unit passes into the thick-skinned one). Conse-
quently,  amount  of  displacement  at  the  Tatric-Veporic
Čertovica suture should be roughly approximated by the width
of  the  disappeared  Fatric  substratum  originally  separating
these two realms, which was at least 100 km (a minimal width
of the Zliechov Basin, cf. Plašienka 1999). From this point of
view,  the  displacement  gradient  problem  appears  to  be  more
complex and needs to be evaluated not only spatially, but also
temporally.

Obviously, the Krížna Nappe has been translated in front

of the Veporic Superunit during the early stages when the
rear compression was the main driving force. After the Tat-
ric-Veporic collision, the forward movement of the Veporic
thick-skinned sheet stopped and the Krížna Nappe moved
forward independently due to gravity on a foreland-inclined
paleoslope.

Retracing the exact displacement path of an allochthonous

unit  is  an  even  more  complex  problem.  Main  complications
summarized by Merle (1998) arise from the character of dis-
placement of shallow-crustal thrust units which typically com-
bines  two  components:  internal  strain  and  rigid  translation.
Frequently  the  displacement  related  to  internal  strain  can  be
negligible  in  comparison  with  the  displacement  produced  by
rigid translation and their directions can be very different. In
addition,  there  can  be  no  particular  relationship  between  the
displacement achieved by internal strain and the displacement
accomplished by rigid translation. Therefore, the displacement
produced  through  rigid  translation,  very  important  in  cover
nappes, is hardly determined (in direction) or quantified by the
methods  of  structural  geology  and  can  be  studied  only  by
means of paleogeographic reconstructions and paleomagnetic
measurements.

Pre- or syn-thrusting paleomagnetic data from the Krížna

Nappe  were  reported  by  Kruczyk  et  al.  (1992),  Grabowski
(1995,  2000)  and  Grabowski  et  al.  (2009,  2010).  These  data
show a systematic variation from the west (Malé Karpaty Mts)
to  the  east  (Tatra  Mts)  –  the  western  localities  show  Creta-
ceous  paleodeclinations  rotated  counterclockwise  (CCW)  up
to  70°,  further  east  in  the  Strážovské  vrchy  and  Malá  Fatra
Mts this CCW rotation decreases to 60—25°, in the Nízke Tat-
ry  Mts  it  is  only  20°,  then  no  rotation  was  detected  in  the
Chočské vrchy and Western Tatra Mts, while the easternmost
sites in the Eastern Tatra Mts and the Ružbachy “island” have
already  shown  a  clockwise  (CW)  rotation  20—50°.  This  fan-
wise arrangement of paleomagnetic declinations remains pre-
served also after subtracting the Late Tertiary CCW rotation of
the  whole  Western  Carpathian  orogenic  system  by  some  80°
(Grabowski & Nemčok 1999; Márton et al. 1999; Grabowski
2010). Although particular directions might have been affect-
ed also by local tectonic phenomena, such as the vertical axis
block rotation within wrench fault zones, or slight relative ro-
tations  of  individual  “core  mountains”  (Hrouda  et  al.  2002),

the  fanwise  pattern  of  paleomagnetic  directions  might  be  in-
terpreted in terms of oroclinal bending (Kruczyk et al. 1992;
Grabowski 2010). Since the paleodeclinations are also subpar-
allel to the presumed transport directions of the Krížna Nappe
deduced from the kinematic criteria (Prokešová 1994; Kováč
&  Bendík  2002;  Plašienka  2003),  it  is  highly  probable  that
they  are  also  roughly  parallel  to  the  translation  pathways  of
the  Krížna  Nappe  System.  Furthermore,  the  paleomagnetic
and transport directions are normal to the trace of a suture after
closure of the Zliechov Basin – the Čertovica line, which is
also slightly northward-convexly bended. The Krížna Nappe
directions are also comparable to those from the underlying
Tatric  Mesozoic  complexes,  at  least  in  the  Tatra  Mts
(Grabowski  1997).  All  these  data  collectively  indicate  that
the  Krížna  Nappe  was  emplaced  in  its  recent  position  as  a
coherently  moving  body  without  any  considerable  internal
distortions  at  the  orogenic  scale.  After  emplacement,  the
Krížna  Nappe  created  an  originally  straight  belt  that  was
bent  shortly  afterwards,  namely  during  the  Late  Cretaceous
but  before  the  onset  of  sedimentation  in  the  Central  Car-
pathian Paleogene Basin.

The scale of observation

As it was pointed by Schultz-Ela (2001), this problem is

highly underestimated in structural geology and appears spo-
radically in the literature. Nevertheless, the scale of observa-
tion  appears  to  be  a  very  important  factor,  which  may
markedly influence our subjective assessment of the problem
(for  example:  cataclastic  flow,  which  is  the  brittle  deforma-
tional  mechanism  on  the  microscale,  is  regarded  as  a  ductile
process on the macroscale).

In our discussion, the scale of observation is important for

the evaluation of the final mechanism of the Krížna Nappe’s
emplacement. We have relevant evidence that it was gravity-
controlled.  In  the  shallow  crustal  environments,  which  are
characteristic of the Krížna Nappe’s evolution, gravity gliding
is  conventionally  proposed  as  a  typical  nappe  emplacement
mechanism. For this mechanism concentration of strain solely
at the base of the nappe is typical. In the Krížna Nappe, how-
ever,  strain  is  more  distributed  across  the  moving  thrust  unit
mainly  in  the  final  emplacement-related  stages  (D3).  There-
fore,  gravity  spreading  should  be  proposed  as  a  contributory
emplacement mechanism.

Usually, the main argument that restricts this mechanism

to “very weak or very hot” rocks is a doubt that rocks can be-
have  viscously  in  shallow  crustal  conditions  (e.g.  Mandl
1988). However, there are at least two higher weak horizons
in the Krížna Succession for which spreading processes are
possible  –  the  Carpathian  Keuper  and  Lower  Cretaceous
marls. In addition, the problem of gravity spreading appears
differently when changing the scale of appreciation. Gravity
spreading of the whole orogenic wedge is a widely accepted
event that may be accommodated by a number of processes.
Gravity gliding of small undistorted blocks of rocks as well
as  faulting,  deformation  along  deformation  bands  or  brittle
shear zones and / or pressure solution are some of them. Es-
pecially the last named process, which is one of the most im-
portant  deformation  mechanisms  in  carbonate  rocks,  is

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GEOLOGICA CARPATHICA, 2012, 63, 1, 13—32

considered analogous to the deformation of a viscous material
(Rutter 1976, 1983).

Bearing this in mind and taking into account that a large

part of the Krížna Nappe has been translated as a foreland
slopped wedge (accumulation of the flysch of the Poruba For-
mation in its frontal areas) over a foreland inclined plane after
overcoming the South Tatric Ridge (frontal zones of the
Krížna Nappe rest on deep-water Tatric Poruba flysch, while
its rear parts lie on the South Tatric Ridge) it is more than pos-
sible that the final emplacement of the Krížna Nappe has been
accommodated by interaction of both gravity mechanisms (i.e.
gliding—spreading).

Conclusions

The Krížna cover nappe of the Central Western Carpathians

was  characterized  as  an  areally  extensive,  but  comparatively
thin  allochthonous  body  continuously  overriding  the  Tatric
substratum.  Originally,  the  Krížna  Nappe  sedimentary  com-
plexes  were  deposited  in  a  broad  basin  that  originated  by
Lower Jurassic rifting and subsequent subsidence of a portion
of  the  widespread  Triassic  carbonate-clastic  shelf  area.  The
central part of this rift furrow is known as the Zliechov Basin
with the Jurassic—Lower Cretaceous pelagic Zliechov Succes-
sion.  The  northern  margin  of  the  Zliechov  Basin  juxtaposed
the southern edge of the present Tatric Superunit, which was

represented by the South Tatric Ridge domain passing into an-
other  rifted  basinal  domain  further  northwards.  The  Vysoká-
type,  comparatively  shallow-water  successions,  which  also
became constituents of the Krížna Nappe later, were deposited
on  this  northern  slope.  The  southern  margin  was  also  repre-
sented  by  more  shallow-water  successions  (Ve ký  Bok,
Lučatín).  These  mostly  remained  confined  to  their  basement
substratum  and  formed  the  northern  toe  of  the  thick-skinned
Veporic  thrust  sheet  afterwards.  The  Zliechov  Succession  is
terminated by the synorogenic flysch complex – the Albian—
Cenomanian  Poruba  Formation,  which  heralded  the  onset  of
the basement shortening due to underthrusting of its basement
substratum  below  the  prograding  Veporic  thrust  wedge.  The
basin  closure  was  completed  by  the  Turonian.  This  was  also
the time of the final emplacement of the Krížna Nappe in su-
perposition  over  the  youngest  sediments  of  the  Tatric  Supe-
runit – basinal shales and turbidites of Early Turonian age.

On the basis of analysis and interpretation of both the

small-scale  and  large  structures,  the  structural  evolution  is
partitioned  into  several  stages  of  progressive  deformation
that  record  the  changing  boundary  conditions  in  different
parts of the nappe body at different time levels. At the same
time, the nappe body is regarded as a lithologically variable
and rheologically stratified multilayer unit with presence of
three potential décollement horizons exhibiting a downward-
ly increasing significance for the nappe transport. The basal
décollement  followed  the  horizon  of  Upper  Scythian  shales

Fig. 12. Tectonic evolution of the Krížna Nappe (not to the scale, for the detailed explanations see the text).

and  evaporites  and  was
transformed  into  a  nappe
sole  during  the  later  stages
of  emplacement.  The  mid-
dle décollement horizon oc-
curred in the Upper Triassic
shales  and  evaporites  (Car-
pathian  Keuper  Formation)
and  was  important  for  de-
tachment  of  some  frontal,
Vysoká-type  partial  units.
The highest décollement al-
lowed  a  partially  free
movement  of  the  youngest
member  of  the  Zliechov
Succession  –  the  Poruba
Formation.

Considering the available

data,  we  have  defined  the
most  important  phenomena
that  would  constrain  further
thoughts  about  the  origin
and  emplacement  of  the
nappe. These are as follows:
(1)  complete  inversion  and
suturing of a broad rifted ba-
sinal  area  bounded  by  ele-
vated  domains;  (2)  strain
partitioning  within  a  me-
chanically  stratified  multi-
layer  unit;  (3)  décollement
horizons;  (4)  weak  nappe

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GEOLOGICA CARPATHICA, 2012, 63, 1, 13—32

sole; (5) stress and strain gradients, both vertical and hori-
zontal; (6) structural associations of several deformation
stages; (7) external constraints for the nappe movement.

In general, no simple mechanical model for the nappe em-

placement  can  be  applied.  It  appears  that  all  potential  em-
placement  mechanisms  and  driving  forces  temporally  and
spatially  participated  in  the  progressive  structural  evolution
of the nappe. The rear compression provided by the backstop
Veporic  thrust  sheet  operated  during  the  early  stages,  when
the  sedimentary  succession  of  the  Zliechov  Basin  was  de-
tached,  shortened  and  transported  over  the  frontal  Tatric
ramp. Subsequently, gravity spreading and gliding governed
the  final  nappe  emplacement  over  the  unconstrained  Tatric
basinal foreland. In other words, the nappe was first pushed
to overcome the frontal ramp (Fig. 12a,b), which was in fact
a narrow ridge elevation. In this high structural position, the
nappe  body  experienced  gravity  spreading  and  partial  de-
tachment  of  small  diverticulates  that  glided  furthest  to  the
north  (Vysoká,  Manín,  Klape  and  analogous  units  –  cf.
Plašienka  1995;  Fig. 12c).  After  the  Zliechov  Basin  was
completely closed, the main nappe body was pulled downs-
lope by gravity and glided to its final position (Fig. 12d).

This doubled up-down ramp model seems to be unique

worldwide and makes the Krížna Nappe a model structure of
thin-skinned thrust tectonics.

Acknowledgments: This research was supported by the Sci-
entific Grant Agency of the Ministry of Education of the Slo-
vak  Republic  and  the  Slovak  Academy  of  Sciences  (VEGA
Projects 1/0388/10, 1/0157/10 and 1/0744/11) and Slovak Re-
search  and  Development  Agency  (APVV  Projects  0081-10,
51-008305). We are also thankful to Edyta Jurewicz and Jacek
Grabowski  who  improved  the  quality  of  the  paper  with  their
comments and corrections from their review of the paper.

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