GeolCarp_Vol63_No1_3

www.geologicacarpathica.sk

GEOLOGICA CARPATHICA

, FEBRUARY 2012, 63, 1, 3—11 doi: 10.2478/v10096-012-0002-x

The impact of Outer Western Carpathian nappe tectonics on

the recent stress-strain state in the Upper Silesian Coal Basin

(Moravosilesian Zone, Bohemian Massif)

JIŘÍ PTÁČEK

, RADOMÍR GRYGAR

, PETR KONÍČEK

and PETR WACLAWIK

Institute of Geonics AS CR, v.v.i., Ostrava, Studentská 1768, 708 00 Ostrava-Poruba, Czech Republic; ptacek@ugn.cas.cz

VŠB, Technical University of Ostrava, Institute of Geological Engineering, Ostrava, Czech Republic; radomir.grygar@vsb.cz

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

Abstract: The Upper Silesian Coal Basin (USCB) represents a typical foreland basin developed during the Variscan
orogenic phase of the Late Carboniferous. Later, during the Alpine orogeny the Outer Western Carpathian nappes were thrust
over the post-Variscan foreland, to which the USCB belongs. Due to this complex tectonic history, redistribution of stress
fields occurred in the post-Variscan basement. Furthermore, post-Variscan denudation processes probably also contributed to
recent stress regimes. Nevertheless, the impact of the West Carpathian orogeny can be regarded as the most significant
influence. The in-situ measurement of recent stress fields in deposits of the Karviná Formation of the USCB and structural
analysis of the Czech part of the USCB, has focused on verification of the structure and stress interference of the Carpathian
nappes and post-Variscan foreland basement. In the southernmost part of the Karviná Subbasin, the easternmost domain of the
USCB, situated in the apical zone of the Variscan accretionary wedge, hydrofracturing and overcoring stress measurements
have been recorded in coal seams from selected coal mines. The data have been supplemented by interpretation of focal
mechanism solutions of mine induced seismic events. Measurements of recent in-situ stress regimes in the Karviná Formation
of the USCB indicate a dominant generally NW—SE orientation of the maximum horizontal compression stress. The results
demonstrate that the stress-strain regime in the Karviná Formation in the Variscan Upper Carboniferous basement is signifi-
cantly influenced by the stress field along the Outer Western Carpathian nappes front. Besides improving our understanding
of recent regional stress fields within an area of mutual structural-tectonic interference by both the Variscan and Alpine
orogenies, the measured data may contribute to more optimal and safer mining activities in the coal basin.

Key words: Variscan orogeny, Outer Western Carpathians, Upper Silesian Coal Basin, paleostress, recent stress fields.

Introduction

The Upper Silesian Coal Basin (USCB) represents the apical
domain  of  the  Variscan  accretionary  wedge  (foreland  coal-
bearing  molasse),  which  is  now  a  part  of  the  epi-Variscan
basement (e.g. Grygar & Vavro 1995; Kandarachevová et al.
2009; Grygar & Waclawik 2011; etc.). Its Variscan tectonic
pattern was structurally also affected by tectonic loading of
the  West  Carpathian  nappes  and  by  sedimentary  loading  of
the  West  Carpathian  Foredeep.  Amongst  others,  these  im-
pacts are apparent in the southern part of USCB (referred to
as the Karviná Subbasin).

The definition of neotectonics was discussed by more au-

thors  (e.g.  Hancock  &  Williams  1986;  Karabanov  et  al.
1994; Hók et al. 2000; etc.), but the Hók et al. (2000) con-
cept of the definition of neotectonics and recent stress-strain
fields has been adopted. They consider neotectonic stress in
the Western Carpathians to have been brought about by tec-
tonic  movements  from  Pliocene  to  recent  times.  Neverthe-
less, we believe that the definition of neotectonic stress will
vary  for  different  localities  depending  on  the  local  tectonic
evolution.  Therefore,  for  neotectonic  development  of  the
southern part of the USCB, it is more appropriate to consider
the period from the Oligocene to Recent. Consequently, we
regard contemporary stress states measured in the coal mines
of the USCB as representing the recent stress regime.

In this article, the influence of Alpine tectonics on the

structural  patterns  and  in  consequence  on  the  recent  stress-
strain regime in the Variscan basement, including the USCB,
is discussed. Discussion is based on in situ horizontal stress
measurements  and  their  interpretation.  The  stress  measure-
ments  were  obtained  using  hydrofractures  and  what  is  re-
ferred  to  as  the  Compact  Conical  Ended  Borehole
Overcoring (CCEBO) method, and by interpretation of focal
mechanisms of important seismic events induced by the coal
mining  activity.  The  main  purpose  of  the  research  is  to  re-
veal,  as  accurately  as  possible,  recent  stress-strain  distribu-
tions.  This  would  be  very  useful  in  guiding  coal  mining
activities in this part of the USCB.

Variscan tectonics of the USCB

The USCB forms an integral part of the Moravosilesian re-

gion of the Bohemian Massif (Fig. 1). The USCB corresponds
to the West European zones of the sub-Variscan coal-bearing
molasse (e.g. Grygar & Vavro 1995; Dopita et al. 1997; etc.).
The  recent  structural  framework  of  the  coal-bearing  deposits
represents only the erosional relicts of an originally more ex-
tensive system of partially, more or less connected sedimenta-
ry  basins  located  in  the  zone  of  the  Brunovistulian  foreland
(see  Fig. 1).  These  Carboniferous  coal-bearing  formations

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GEOLOGICA CARPATHICA, 2012, 63, 1, 3—11

they present the typical features of a synorogenic accretionary
wedge. The stacked pile of Silesicum crystalline nappes (Cháb
et al. 2010) in the western domain of the Moravosilesian area
(Fig. 2) represents what is referred to as a backstop structure
(e.g. Davis et al. 1983; Lallemand et al. 1992; Jamison 1993;
etc.). Due to the structural-tectonic activity, more complex
tectonic styles participated in the structural pattern of both the
flysch foredeep and the foreland basin of the USCB.

The complicated fold-thrust pattern in the western part of

the USCB, in comparison with the relatively simple tectonic
style of the eastern Karviná Subbasin, has been commented
on by Dopita et al. (1997). The distribution of tectonic style
is  also  determined  by  the  character  of  the  Brunovistulian
basement and by its regional position in the apical domain of
the  Variscan  accretionary  wedge.  The  USCB  is  noticeably
asymmetrical  in  both  WNW—ESE  and  NNW—SSE  direc-
tions. These two polarities, longitudinal and transverse ones,
are responsible for its present structural-tectonic framework
(e.g. Grygar et al. 1989; Nawrocki 1993).

The Czech part of the USCB could, from the point of view

of  the  longitudinal  polarity,  be  subdivided  into  two  basic
structural  domains.  The  boundary  line  corresponds  to  the
Orlová  fault-propagation  fold  structure  (Grygar  &  Waclawik
2011).  Whilst  the  westerly  domains  (Ostrava  and  Petřvald
Subbasins)  display  more  complex  and  complicated  tectonic
styles,  the  area  to  the  east  of  the  Orlová  structure  (Karviná
Subbasin)  is  distinguished  by  the  predominance  of  transten-
sional normal fault tectonics. The transverse structural polarity
of the NNW—SSE direction is evident from the more compli-
cated tectonic styles in the more northerly parts of the above
group  of  subbasins.  The  intensity  of  deformation  decreases
southwards,  due  to  the  variable  kinematics  and  deformation
intensity of the Variscan accretionary wedge. Both structural
directions  are  equally  important  in  the  general  scheme  of
Variscan regional stress fields (Grygar et al. 1989).

The Orlová structure was previously generally regarded as

the  easternmost  fold-thrust  structure  of  the  Moravosilesian
Variscan  foredeep.  On  the  other  hand,  the  Karviná  Subbasin,
which lies to the east of the Orlová structure, is in direct contact
with  the  Outer  Carpathian  nappes.  A  more  detailed  picture  of
the structural tectonic pattern of the Karviná Subbasin is given
in  Fig. 5.  A  transtensional  paleodynamic  regime  dominates

there. Many of the normal faults are combined with strike-slip
movements (transtensional faults, etc.; Grygar et al. 1989).

The Karviná Formation coal seams dip at very low angles,

which usually do not exceed 10° to 15°. In the western part,
open  and  gentle  fold  structures  are  defined  –  the  Suchá
Anticline and Suchá Syncline, both of which are parallel to
the Orlová fault-propagation fold structure. Both Suchá fold
structures  are  genetically  linked  with  normal  fault  kinemat-
ics. In the easternmost part of the Karviná Subbasin, the fold
structures  are  not  present.  The  folding  is  also  linked  to  re-
gional longitudinal faults, which brought about mostly anti-
thetic rotation (tilting) of the fault blocks.

It is evident from the map in Fig. 5, that the framework of

the Karviná Subbasin is characterized by longitudinal normal
faults  striking  in  a  submeridional  direction  (NNE—SSW  to
NNW—SSE)  and  transverse  normal  faults  striking  generally
W—E  (to  WNW—ESE).  These  are  mainly  steep  normal  faults
(dips 50°—70°). Most of them display evidence of strike-slip ki-
nematics (Danys & Sivek 1976). This is an example of transcur-
rent dextral strike-slip faulting often with an “en echelon” fault
pattern (Grygar et al. 1989; Grygar & Welser 1994). The main
genetic role belongs to the submeridional Karviná Graben and
its  transverse  Dětmarovice  Graben  (see  Fig. 3).  The  Dětmaro-

Fig. 2. Schematic structural cross-section across the accretionary wedge of the Moravosilesian Zone. Contour diagrams correspond to poles
of bedding and/or main cleavage system.

Fig. 3. Variscan stress-strain model of the Czech part of the USCB
based on paleostress analysis.

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vice Graben is part of a major structure of higher regional or-
der – the Dětmarovice Shear Zone (Grygar et al. 1989). The
main regional tectonic zones (Dětmarovice Shear Zone and
Karviná Graben) split up the Karviná Subbasin into four blocks
with different vertical structural positions and different inter-
nal structural frameworks (Grygar et al. 1989 – see Fig. 5).

Apart from what is referred to as the Central Thrust (see

Fig. 5),  no  other  thrust  structures  have  been  identified  in  the
Dětmarovice Shear Zone. However, during extension of min-
ing to lower stratigraphic levels in the easternmost part of the
Karviná  Subbasin,  new  thrust  systems  structures  and  corre-
sponding  ductile  deformation  structures  were  discovered
(Grygar et al. 1998; Ptáček 1999; Waclawik 2009; Grygar &
Waclawik  2011).  Amplitudes  of  the  above  very  flat,  mostly
intraformational thrusts give rise to vertical magnitudes of up
to  20—30  meters.  This  system  of  recently  discovered  thrust
structures  (referred  to  as  the  Eastern  Thrusts  –  Waclawik
2009;  Grygar  &  Waclawik  2011)  generally  strike  NE—SW
(Fig. 5). This thrust system represents the most easterly thrust-
ing of the Variscan accretionary wedge of the Moravosilesian
Zone as a whole.

Alpine tectonics of the Outer Western Carpathians

In the Late Cretaceous, a major foreland basin system de-

veloped  in  the  Outer  Carpathian  zone,  dominated  by  silici-
clastic shelf, and deep-water flysch sedimentation formed in
the  Outer  Carpathian  zone.  The  Inner  and  Outer  Western
Carpathians are separated by the Pieniny Klippen Belt suture
zone  (e.g.  Golonka  &  Pícha  2006).  The  Outer  Western
Carpathians are composed of Jurassic to Miocene mostly flysch
deposits,  which  built  up  into  several  nappes.  As  is  evident
from  Fig. 6,  during  the  Paleogene  and  Neogene  they  were
thrust as an accretionary wedge over the European foreland of
consolidated Variscan and pre-Variscan Brunovistulian base-
ment,  overlain  by  Miocene  deposits  within  the  Carpathian
Foredeep  (e.g.  Plašienka  et  al.  1991;  Plašienka  1997,  1999;
Golonka & Picha 2006; etc.).

The Outer Western Carpathians belt represents the immedi-

ate contact of the Alpine orogeny with the most easterly part
of the Bohemian Massif. The thin-skinned type of Alpine ac-
cretionary wedge of the Outer Carpathian flysch belt consists
of numerous tectonostratigraphic units (e.g. Oszczypko 1998;
Šefara  et  al.  1998).  The  most  external  units,  the  Menilite-
Krosno Group are characterized by a Late Cretaceous to Late
Eocene succession of variegated shales, Early Oligocene me-
nilitic  silicites,  and  the  Late  Oligocene  to  Early  Miocene
Krosno-type flysch (Mahe  1991). The structural tectonic pat-
tern of the Outer Western Carpathians belt represents the typi-
cal structure of an accretionary wedge (e.g. Oszczypko 1998).
The Subsilesian and Silesian Units may be better classified as
a  continuation  of  the  Alpine  molasse  into  the  territory  of
northeastern Moravia. Thick Late Cretaceous to Eocene deep-
water  flysch  deposits  characterize  the  internal  Magura  Unit,
which,  during  the  sedimentation,  was  separated  from  the  ex-
ternal units by the Silesian Ridge. The Magura Unit, also re-
ferred  to  as  the  Magura Group  of Nappes,  is  correlated  with
the Rhenodanubian Flysch of the Alps (Eliáš et al. 1990).

Generally top-to-NW movement of the nappes of the Outer

Carpathians over the Variscan basement (Bohemian Massif
including the Brunovistulian Pan-African basement) com-
menced probably in the Oligocene and continued in the early
Tortonian (Menčík et al. 1983; Bielik et al. 2002; Golonka &
Picha 2006). As a consequence, superposition and reactivation
of Variscan fault patterns of the Variscan foreland basement,
including the USCB terrane, also took place.

Stress-strain model of the USCB

A paleostress analysis, based on a complex structural-tec-

tonic analysis and slickenside measurements on thrust faults,
resulted in our interpretation of a WNW—ESE to NW—SE orien-
tation of maximum compression (Grygar et al. 1989; Grygar
& Vavro 1995; Havíř 2001). This conclusion is also based on
structural  and  paleostress  investigations  of  thrust-fold  struc-
tures  in  the  eastern  domain  of  the  Variscan  flysch  foredeep
(Culm  facies  of  Hradec-Kyjovice  Formation  in  the  strati-
graphic footwall of the coal-bearing Ostrava Formation) west-
wards of the western limit of the USCB (see Fig. 1). Typical
out-of-sequence  thrust  structures  were  observed  not  only  in
the above Moravosilesian flysch domain, but also in the Upper
Silesian  Coal  Basin,  controlled  by  widespread  sedimentary
bedding  parallel  (intrafolial)  shearing  (for  “progressive  easy-
slip thrusting” see Gayer et al. 1991). Čížek & Tomek (1991),
on the basis of drilling and seismic profiling of the eastern part
of  the  flysch  foredeep,  recognized  detachment  and  thrusting
between  Culm  facies  and  Devonian  limestone  facies  in  the
cover of the Brunovistulian foreland. Similar kilometer scale
thrusts were identified and documented in detail in the Czech
part of the Karviná Subbasin of the USCB (Grygar et al. 1989;
Grygar & Waclawik 2011).

The stress-strain model of the Variscan orogeny is present-

ed in Fig. 3. It was derived by a complex structural-tectonic
analysis of the USCB, and corresponds to the late Variscan
deformation phase of the USCB. The directions of the princi-
pal  horizontal  axes  of  the  generalized  regional  strain  ellip-
soid are marked. The positive signs represent compressional
quadrants (approximately NW—SE direction), which have re-
sulted in structurally higher, uplifted segments of the Ostrava
and Karviná Subbasins. The larger complex arrows describe
the  relative  perpendicular  movements  of  constituent  seg-
ments and simultaneously strike-slip movement sense on the
conjugate  shear  zones.  The  diagram  confirms  the  different
stages  of  deformation  within  the  Dětmarovice  Shear  Zone
(strike  direction  approximately  WNW—ESE)  and  deforma-
tion in the southern part of the Ostrava Subbasin. Almost the
same  dirrections  of  maximum  compression  were  confirmed
by interpretation of recent earthquakes located on the neotec-
tonically rejuvenated fault systems in the eastern part of the
Sudetes (Špaček et al. 2006).

Stress-strain model of the Outer Carpathians

In the region of the Karviná Subbasin, neither direct nor

indirect recent stress measurements have been recorded from

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the foredeep of the Outer Western Carpathians at the imme-
diate  contact  with  the  Bohemian  Massif.  The  directions  of
paleostress  fields  may  be  interpreted  from  analyses  of  the
main compressive forces in the partial Godula Nappe in the
Beskydy  Mts  (Menčík  et  al.  1983).  The  directions  of  the
main  compressive  forces  for  Tortonian  and  late  Tortonian
folding  events  are  documented  in  Fig. 4.  Whilst  Tortonian
directions of maximal horizontal compression were interpret-
ed  as  NW—SE,  based  on  paleostress  analyses  in  the  Godula
Nappe  (Menčík  et  al.  1983),  the  direction  of  maximal  hori-
zontal compression in the late Tortonian was rotated to a N—S
direction. Menčík et al. (1983) asserted that the NW—SE di-
rection was not influenced by elevations in the post-Variscan
autochthonous  basement,  but  were  controlled  by  synsedi-
mentary  movements.  The  direction  of  the  late  Tortonian
stage of the Alpine orogeny, exhibits a dominant northward
direction of thrusting (see Fig. 4).

The compilation map of recent stress fields, published by

Hók et al. (2000), presents stress measurements from a vari-
ety of other authors in addition to the original measurements
of horizontal stress. Furthermore, the results of Polish inter-
pretation of stress fields in the Outer Western Carpathians
are also quoted (Hók et al. 2000). The paleostress develop-
ment of the Alps and Outer Western Carpathians is discussed
by more authors (e.g. Zuchiewicz 1994; Peresson & Decker
1997; Márton & Fodor 2003). On the basis of the available
data, the direction of maximum horizontal compressional de-
formation in the Polish part of the Outer Western Carpathian
belt is roughly N—S, whereas in the western part of the zone
of collision with the Bohemian Massif, this direction has ro-
tated to an approximately NW—SE direction (Fig. 4).

Horizontal stress directions in the northern part of the Outer

Western Carpathians are published in the World Stress Map
(Heidbach et al. 2009). The results represent the data gained
from  borehole  breakout  analyses  of  variable  quality.  All
quoted results of horizontal stress measurements or interpre-
tations  are  shown  in  the  compilation  map  in  Fig. 4.  Apart
from  the  directions  interpreted  by  Menčík  et  al.  (1983)  for
the late Tortonian stage of the Alpine orogeny, they generally
correspond to each other.

Methods and results of recent stress measurements

For the purposes of our project, three different methods

were used. The hydrofracturing method is one the most com-
monly used methods for delineating recent stresses in the
Ostrava-Karviná Coal Field. Measurement of the stress tensor
by utilization of special conical gauge probes was a second
method employed, and the third method involved the interpre-
tation of focal mechanisms of seismic events monitored by the
Regional Seismic Network of the Czech part of the USCB.

Hydrofracturing is a borehole test method used for stress

state assessment in rock masses in the vicinity of boreholes
(e.g.  Amadei  &  Stephansson  1997;  Nakamura  et  al.  1999;
Haimson & Cornet 2003; etc). The non-deformed section of
a borehole (commonly 30 m deep) is chosen and two rubber
packers are pressurized so that they adhere to the walls. Wa-
ter  is  pumped  into  the  sealed  up  section,  and  pressure  is
gradually  raised  until  a  fracture  is  initiated  in  the  wall  of
borehole.  The  orientation  of  the  fracture  is  obtained  by  use
of  oriented  packers,  which  are  imprinted  on  the  borehole

Fig. 4. Compilation map of horizontal stresses measured and/or interpreted in the
Outer Western Carpathian area.

wall by the newly initiated fractures. Values
of  breakdown  pressure,  reopening  pressure
and  shut-in  pressure  are  recorded,  so  that
measurements  of  the  horizontal  components
of  the  principal  stresses  can  be  calculated.
The  technique  is  based  on  similar  methods
depending  on  the  geomechanical  conditions
in the vicinity of the borehole.

Recently, measurements of stress tensor

changes  have  been  obtained  using  special
conical gauge probes. These probes were de-
veloped  by  the  Institute  of  Geonics  AS  CR
(Staš  et  al.  2005)  on  the  basis  of  theoretical
and  practical  experience  gained  from  using
the Compact Conical End Borehole Overcor-
ing  (CCBO)  method  (Obara  &  Sugawara
2003). This method of long term monitoring
of stress tensor changes by CCBM (Compact
Conical-ended  Borehole  Monitoring)  is  de-
rived from a compact conical ended borehole
overcoring method. By omission of the over-
coring phase (which results in destruction of
the measured point for the next measurement
during  standard  CCBO)  a  long  term  probe
life is achieved. This allows us to measure a
stress tensor change in relation to a reference
stress state. Our apparatus is constructed for
76 mm diameter boreholes, where the bottom

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GEOLOGICA CARPATHICA, 2012, 63, 1, 3—11

of the borehole is shaped with an apical angle of 60° by the
special conical drill bits. The probe is water-proof and uses
6 pairs of mutually perpendicular gauge sensors placed with
a standard configuration on the conical surface. The appara-
tus is developed with two variants: for overcoring (CCBO),
and for long term monitoring (CCBM). Measurement of de-
formation on every gauge and A/D data processing are con-
trolled by a microcomputer inside the probe. Digital data can
be stored either in the internal memory of the probe when in
autonomous mode, or can be sent to an external control unit.

The maximum and minimum horizontal components of re-

cent principal stresses (S

and S

), have been measured in

28  localities  in  the  region  of  interest  since  1994  using  the
hydrofracturing  method  (e.g.  Amadei  &  Stephansson  1997;
Haimson  &  Cornet  2003;  etc).  However  only  25  were  suit-
able  for  the  purposes  of  interpretation  of  horizontal  stress
components. The other boreholes were deformed and closed
for the probe. The measurements were recorded at a depth of

600—800 m  beneath  the  surface  in  the  Karviná  Subbasin.
Eight new hydrofrac measurements were obtained at the same
depth  during  2008  and  2009  (see  Fig. 5).  The  first  group  of
measurements  in  2008  was  obtained  in  localities  next  to  the
main Variscan tectonic structures. Although most of the mea-
sured  strain  directions  could  be  influenced  by  local  stress
fields in the vicinity of mine workings, the interpreted stress
directions are close to published Variscan kinematic directions
(Grygar  &  Vavro  1995).  For  example,  the  maximum  hori-
zontal stress direction (S

), interpreted from the 2008 mea-

surements between the Stonava and Albrechtice faults,
corresponds to the maximum horizontal stress direction (S

)

of the Variscan kinematic model. In the follow-up stage, stress
measurements have been obtained at different distances from
the faults and finally in the central parts of tectonic blocks in a
relatively unfractured rock. Compilation of all hydrofrac mea-
surements and interpreted compression directions are shown
in Fig. 5. On the diagram, indicated by different arrows, are

horizontal stress measure-
ments using the new CCBO
or CCBM method (Staš et
al. 2006), described above.

Interpretation of focal

mechanisms

seismic

events, monitored in USCB,
is  based  on  the  principal
seismic  moment  tensor  in-
version method (e.g. Aki &
Richards  1980;  Lund  2000;
Stec  2009;  etc.).  The  seis-
mic events with high energy
emissions were analysed.

More than 250 seismic

events of magnitude from
1.2 to 2.3 (of energy 10

J to

J) were monitored in the

easternmost  part  of  the
Karviná  Subbasin  in  2008
and as many as 190 in 2009.
Analyses of the focal mech-
anism  for  each  seismic
event  significantly  expand-
ed  the  information  on  local
stress  fields  in  the  vicinity
of  excavated  coal  seams.
The  first  results  indicated
that  interpreted  stress-strain
directions  in  some  cases
replicated the assumed hori-
zontal  components  S

and

Nevertheless,

focal

mechanism interpretations
demonstrate

considerable

variability  of  the  compres-
sive component of the hori-
zontal  stresses  (Fig. 5).  We
consider  this  to  reflect  the
importance  of  stresses  in-
duced by mining.

Fig. 5. Compressional components of horizontal stresses
derived from hydrofrac, conical gauge probe measure-
ments and “beach ball” diagrams of analysed seismic
events with the main Variscan faults of the Karviná Sub-
basin in the background.

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The influence of the Alpine orogeny on the

southeastern part of the Upper Silesian Coal Basin

As stated in the preamble, it is crucial that we understand

the neotectonic development of stress fields and principally
the recent stress regime, which is most probably influenced
by the Alpine orogeny. Earlier workers (e.g. Roth et al. 1962)
assumed that the Bohemian Massif, representing the Alpine
foreland, played an important role in relation to the Carpathian
nappes and their internal structural development. On the other
hand, Hók et al. (2000) consider the influence of the
Carpathian nappes on the structural tectonic framework of the
Variscan basement unlikely. According to them, in the Tertia-
ry, the Variscan basement was already consolidated.

Teper & Sagan (1995) considered the influence of the load-

ing of the Variscan basement by the Outer Carpathian nappes,
and the influence of the stress fields along their front. This in-
fluence is most intense in the southern part of the USCB and
decreases to the north. The impact of the sedimentary loading
was  simultaneously  enhanced  by  the  Miocene  sedimentary
filling of the sub-Variscan autochthon. As already stated, Mio-
cene sediments overstepped the eroded Carboniferous surface
during the Karpatian. The thickness of the sedimentary filling
oscillated and reached up to 1000 meters. According to Teper
&  Sagan  (1995),  the  maximum  horizontal  stress  component
rotated from the original E—W direction to a N—S direction, a
principal direction of the Carpathian overthrusting, and subse-
quently to the contemporary approximately NE—SW direction.
It is necessary to point out, that the entire neotectonic stress-
strain interpretation of Teper & Sagan (1995) was based on fo-
cal  mechanism  solutions  of  the  seismic  events  induced  by
mining. The interpretations are, no doubt, very interesting, but

we consider, based on our experiences with the interpretation
of seismicity in the Karviná Subbasin, that their relevance to
the entire USCB is limited.

Our view, based on the structural and morphotectonic inves-

tigation of the mutual interaction of the Alpine and Variscan
orogenies  in  the  USCB  area,  has  been  presented  previously
(Grygar & Jelínek 2003). Three structural levels are developed
in the USCB area. The analyses demonstrate a good correla-
tion between the structural framework in Variscan structures,
observed  relatively  precisely  by  measurements  in  the  coal
mines  and  the  Brunovistulian  basement,  and  simultaneously
help us to explain the mutual relationships between the struc-
tural pattern of the USCB and the present-day epi-Alpine re-
lief  of  the  Outer  Carpathian  belt.  The  Alpine  reactivation  of
the  Variscan  fault  structures  was  also  very  significant  (see
Fig. 6). There is a causal genetic coincidence with the tectonic
role of both the post-Variscan and Brunovistulian foreland. It
influenced the dynamics, kinematics and internal structures of
the  Carpathian  nappes.  Contemporaneously,  the  Carpathian
Foreland was modified due to tectonic loading by the Outer
Carpathian  nappes  and  the  sedimentary  loading  of  the
Carpathian  Foredeep  sedimentary  fill  (Fig. 6).  This  loading
influenced  the  development  of  a  lithospheric  flexure  of  the
Alpine  Foreland  and  consequently  rejuvenation  of  the
Variscan, initially by reactivation of subequatorial, fault sys-
tems (Fig. 6).

Conclusions

Measurements of recent in-situ stress regimes in the Karviná

rocks formation of the USCB using the hydrofrac method

Fig. 6. Schematic interpretation of (A) lithospheric flexure and consequent tilting of the epi-Variscan foreland due to tectonic loading by
Alpine nappes and (B) 3D-model of buried paleorelief of the Czech part of the USCB.

PTÁČEK, GRYGAR, KONÍČEK and WACLAWIK

GEOLOGICA CARPATHICA

GEOLOGICA CARPATHICA, 2012, 63, 1, 3—11

presented here, indicate a dominant generally NW—SE orien-
tation  of  the  maximum  horizontal  compressional  stress
(Fig. 5). The results demonstrate that the stress-strain regime
in the Karviná Formation in the Variscan Upper Carbonifer-
ous  basement  is  significantly  influenced  by  the  stress  field
distributed along the Outer Western Carpathian nappes front.
The relations and mutual interference of the tectonic patterns
of both the post-Variscan basement and the Outer Carpathian
nappes,  however,  is  open  to  discussion  due  to  the  fact  that
both  the  Variscan  (WNW—ESE  to  NW—SE)  and  Alpine
(NW—SE  to  NNW—SSE)  maximum  compressional  stresses
are similarly oriented (see Fig. 1). In our opinion, there is lit-
tle  doubt  that  the  most  significant  control  on  the  stress  re-
gime  within  the  collision  domain  of  the  epi-Variscan
foreland  and  Outer  Western  Carpathians  since  Oligocene
times, has come from the West Carpathian orogenic system.
Anyway  the  recent  stress  state  in  the  Karviná  Subbasin,
based on the in situ measurements, is influenced by the stress
fields of the Outer Western Carpathian nappes front.

Acknowledgment: The research of stress-strain in USCB is
financially supported by the Grant Agency of the Czech Re-
public (Project No. 105/08/1625).

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