GeolCarp_Vol52_No5_263

GEOLOGICA CARPATHICA, 52, 5, BRATISLAVA, OCTOBER 2001

263 — 275

VARIATION OF DEFORMATION MECHANISMS WITHIN THE

PROGRESSIVE—RETROGRESSIVE MYLONITIZATION CYCLE OF

LIMESTONES: BRUNOVISTULIAN SEDIMENTARY COVER (THE

VARISCAN OROGENY OF THE SOUTHEASTERN BOHEMIAN MASSIF)

PETR ŠPAČEK

, JIŘÍ KALVODA

, EVA FRANCŮ

and ROSTISLAV MELICHAR

Department of Geology and Paleontology, Masaryk University, Kotlářská 2, 61137 Brno, Czech Republic; *vajgl@sci.muni.cz

Czech Geological Survey, Leitnerova 22, 60200 Brno, Czech Republic

(Manuscript received December 7, 2000; accepted in revised form June 13, 2001)

Abstract: This study deals with the calcite mylonites of the Brunovistulian sedimentary cover developed in the frontal
thrust area of the Moravian nappe units. The inhomogeneous structure of sedimentary protoliths allowed the analysis of
the contrasting behaviour of calcite in matrix and porphyroclasts and the interpretation of microfabric evolution during
deformation  under  low  temperature  conditions.  Several  stages  of  microfabric  evolution  characterizing  progressive  as
well as retrogressive deformation are distinguished. Generally, the progressive phase of mylonitization is characterized
by grain growth in the matrix and the grain size reduction of the porphyroclasts leading to a stress-induced equilibration
of  grain  size.  During  the  initial  deformational  stages  the  calcitic  porphyroclasts  deformed  brittlely  and  the  strain  was
strongly  localized  into  the  ductile  matrix.  With  continuing  evolution  the  onset  of  the  dynamic  recrystallization  of
porphyroclasts  occurred,  which  obviously  preceded  a  significant  grain  growth  in  the  matrix.  With  rising  temperature
during  deformation,  grain  growth  predominated  after  grain  size  homogenization  was  finished.  The  lack  of  effective
dynamic  recovery  along  the  stages  of  the  progressive  low  temperature  phase  of  deformation  is  discussed.  Core-and-
mantle  structures  which  are  characteristic  of  the  initial  stages  of  progressive  deformation  carry  microfabric  features
which document the dominance of grain boundary bulging and/or nucleation recrystallization. Formation of subgrains
within  the  porphyroclasts  is  only  a  rarely  observed  feature  which  probably  could  not  lead  to  significant  grain  size
reduction.  The  higher  effectiveness  of  nucleation  and  recrystallization  via  migration  of  grain  boundaries  compared  to
subgrain rotation mechanism could be a consequence of high fluid content. Large-scale thrusting within the Brunovistulian
basement is shown by the juxtaposition of calcitic and quartzitic mylonites with deformational microstructures reflecting
pronounced  contrasts  of  deformational  styles.  Fully  plastic  vs.  fully  brittle  behaviour  of  quartz  represents  the  most
pronounced indicator of different deformational conditions between the lower unit of the Svratka Dome and the other
domains of the Brunovistulian basement. In the lower tectonic unit of the Svratka Dome the microtructures of calcite
mylonites indicate stresses which were about four times lower than in the other two parts of the Brunovistulicum. Despite
the deformational contrasts, the values of illite crystallinity measured do not show any spatial gradient which could be
linked  with  the  distribution  of  the  contrasting  deformational  microstructures.  The  paleothermometric  data  which  are
available to date suggest maximum paleotemperatures of 250—300 °C for all three studied domains of the Brunovistulian
basement and it is suggested that the difference of Variscan peak temperatures between the three compared domains of
the basement was not higher than several dozens of °C. The observed deformational contrasts can thus be explained by
an abrupt change of deformation mechanisms in both calcite and quartz at temperatures around 300 °C.

Key words: Eastern Variscan front, Brunovistulicum, inhomogeneous limestones, mylonitization, dynamic recrystal-
lization, microstructures.

Introduction

Microfabric  studies  of  strained  homogeneous  calcite  aggre-
gates have been described relatively frequently, both in nature
(e.g. Dietrich & Song 1984; Heitzmann 1987; Burkhard 1990;
Covey-Crump & Rutter 1989; Busch & Van der Pluijm 1995)
and experiments (e.g. Schmid et al. 1977, 1980, 1987; Rutter
1974;  Rutter  et  al.  1994;  Walker  et  al.  1990).  However,  re-
search into the mylonitization of inhomogeneous carbonates,
which are the most abundant in nature, is rather sparse. There-
fore we attempted to give a detailed study of the development
of  such  inhomogeneous  limestones  in  low-temperature  (LT)
deformational stages.

In the southeastern part of the Bohemian Massif, the tecton-

ic contact of the allochthonous domain of Variscan orogen (the
Moldanubian and Moravian nappe units) and the per-autochth-

onous  pre-Variscan  basement  (the  Brunovistulicum)  is  ex-
posed (Matte et al. 1990). The Devonian and Lower Carbonif-
erous carbonate-clastic sedimentary cover of the Brunovistuli-
cum  is  strongly  sheared  in  the  thrust  area  under  anchimeta-
morphic and very low-grade metamorphic conditions (Schul-
mann  et  al.  1991).  Calcite  mylonites  from  this  highly  de-
formed sedimentary sequence were studied along orogen-per-
pendicular  profiles,  which  cross-cut  the  foot-wall  of  the
Moravian nappe units and proximal parts of their foreland.

As the microstructure of sedimentary protoliths was gener-

ally inhomogeneous, the mylonites served as a suitable object
for the comparison of the deformational behaviour of the ma-
trix vs. the porphyroclasts within the given spectra of LT de-
formational conditions. The analysis of deformational micro-
fabric allowed the interpretation of its evolution during the
progressive transformation of limestones into mylonites and

VARIATION OF DEFORMATION MECHANISMS WITHIN THE

PROGRESSIVE—RETROGRESSIVE MYLONITIZATION CYCLE OF

LIMESTONES: BRUNOVISTULIAN SEDIMENTARY COVER (THE

VARISCAN OROGENY OF THE SOUTHEASTERN BOHEMIAN MASSIF)

264 ŠPAČEK et al.

during retrogressive degradation under decreasing tempera-
tures. On the basis of the observed deformational microstruc-
tures it was possible to make a comparison between three do-
mains of the Brunovistulicum in terms of dominant deforma-
tional regimes in calcite aggregates.

Geological settings

The Variscan assembly of the allochthonous units and the

Brunovistulian basement within the eastern Bohemian Massif
are related to the Devonian-Late Carboniferous dextral oblique
collision  of  the  Armorican  terranes  (Moldanubian  and
Saxothuringian  Terrane)  with  the  Brunovistulian  Terrane  –
outer  part  of  Laurussia  (Matte  et  al.  1990;  Kalvoda  1995,
2001). Three main units have been distinguished in the colli-
sional  zone:  Moldanubicum  (Suess  1912)  with  high  grade
Variscan  metamorphism  (Suess  1912,  1926;  Cháb  &  Suk
1977),  Moravian  Zone  (Suess  1912)  with  medium  grade
Variscan metamorphism (Suess 1912, 1926; Štípská & Schul-
mann 1995) and Brunovistulicum (Dudek 1980), in which the
Variscan  metamorphism  is  low-grade  and  occurs  only  in  a
close contact with the overlying tectonic units (Schulmann et
al. 1991; Franců et al. 1999). According to some authors the

nappe units of the Moravian Zone were derived from the high-
est parts of the imbricated Brunovistulian basement (e.g. Frasl
1983; Fritz & Neubauer 1993; Štípská & Schulmann 1995), as
indicated mainly by similar radiometric ages of the Brunovis-
tulian granitoids and sheared orthogneiss bodies of the Mora-
vian nappes (van Bremen et al. 1982; Morauf & Jäger 1982).

The structure of the southern part of the collisional zone is

well exposed in two incomplete tectonic windows: the Thaya
Dome in the south and the Svratka Dome in the north (Fig. 1).
A similar lithotectonic zonation has developed in both of
them, as described, for example, by Schulmann et al. (1991,
1994) and Fritz & Neubauer (1993):

1 (bottom). Granitoids, metagranitoids and migmatites of the

Brunovistulian Cadomian basement with their metasedimentary
host rocks and Paleozoic sedimentary cover (Dudek 1980; Fin-
ger et al. 1989, 1995; Bosák 1980; Batík & Skoček 1981),

2. Moravian nappe units composed mostly of metasedi-

ments, pre-Variscan Bíteš orthogneiss and its metasedimentary
host rocks (e.g. Schulmann et al. 1991),

3. Mica-schist zone with metasediments, amphibolites and

orthogneiss bodies (Suess 1908),

4 (top). Moldanubian nappes with high-grade granulites,

paragneisses, migmatites and relics of eclogites (Matějovská
1975; Jenček & Dudek 1971; Vrána et al. 1995).

Fig. 1. Schematic map of the area studied and a conceptual profile through the main units of the collision zone.

DEFORMATION MECHANISMS WITHIN THE MYLONITIZATION OF LIMESTONES 265

Characteristic features of this sequence are inverted Bar-

rovian metamorphic zoning, metamorphic foliation parallel
with lithological boundaries and N to NE trending lineations
subparallel to the axes of tight folds (Schulmann et al. 1991).
In the upper units, E-W stretching lineations are common
(Fritz & Neubauer 1993).

In the foreland of the Moravian and Moldanubian nappe

units, the Brunovistulian basement is exposed in the Brno
batholith and has been verified through many boreholes be-
neath the Paleozoic cover on the eastern slopes of the Bohemi-
an Massif. In this eastern part of the collisional zone different
lithotectonic zonation is developed:

1 (bottom). Cadomian granitoids of the Brunovistulicum with

its metamorphosed host-rocks and Devonian-Lower Carbonif-
erous pre-flysh sedimentary cover (e.g. Dudek 1980; Leich-
mann 1996; Hanžl & Melichar 1997; Finger et al. 2000;
Dvořák 1995),

2. several km thick flysh sequence of Viséan age (Dvořák

1973; Rajlich 1990; Čížek & Tomek 1991).

The Viséan flysh sequence is strongly folded (e.g. Rajlich

1990)  and  Čížek  &  Tomek  (1991)  proved  the  existence  of
large-scale east-vergent thrusts imbricating both the flysh se-
quence  and  the  Brunovistulian  basement  with  its  tectonized
pre-flysh sedimentary cover. In this part of the orogen, the duc-
tile deformation of the rocks is non-penetrative and thick zones
of mylonitization occur only in close proximity to the Moravi-
an nappes. Stretching lineation and associated kinematic indi-
cators  show  top-to-the-NNE  shearing  (e.g.  Bábek  &  Janoška
1997).  In  general,  the  degree  of  Variscan  deformation  and
metamorphism  is  very  low  and  further  decreases  to  the  east
where Paleozoic sequences rest autochthonously on the Bruno-
vistulian basement.

Within the Brunovistulian basement, two parts with different

development during the Variscan orogenesis can thus be distin-
guished. The first is in the westernmost part with the Moravian
nappes in the hanging wall and the second in the eastern part is
covered by Viséan flysch nappes. The boundary of these two
sections is covered by Westphalian-Autunian sediments of the
Boskovice Graben. The exact meaning of the tectonic contact
of units beneath the sedimentary infill of the graben is still not
well understood.

For the tectonic evolution of the collisional zone the forward

thrust propagation model was suggested by Schulmann et al.
(1991), Fritz & Neubauer (1993) and Fritz et al. (1996). The
thrusting began under HT conditions and continued during
gradual cooling to LT conditions. The deformation regimes
were changing continuously from top-to-the-N shear through
top-to-the-E shear to E-W coaxial extension. The simultaneous
activity of non-coaxial shearing in the lower units and the co-
axial extension in the upper units has been assumed. This mod-
el explains the complex structural evolution and systematic de-
crease of isotopic ages of metamorphism from the uppermost
tectonic to the lowest tectonic levels.

Analytical methods

The transformations of the primary structures of inhomoge-

neous carbonate sediments into a deformation fabric of carbon-
ate mylonites were analysed. Dozens of samples were collect-

ed  in  both  parts  of  the  basement  –  from  the  foot-wall  of
Moravian nappe unit in the western part and from the foreland
of  the  Moravian  nappes  in  the  eastern  part  of  the  basement
(Figs.  1  and  6).  The  key  steps  in  the  analytical  procedure,
which should have provided data for the interpretation of de-
formation mechanisms and conditions, were:

1) the description and quantification of the optical deforma-

tion microstructures and lattice preferred orientations (LPOs);

2) the correlation of the microfabric with the temperatures

of deformation.

Microstructures

Thin and ultra-thin (< 10 µm) sections were prepared from

oriented  samples  cut  parallel  to  XZ  and  ZY  planes  of  finite
strain  and  were  examined  under  optical  and  SEM  micro-
scopes.  Grain-shape  analyses  of  the  coarse-grained  domains
were carried out using a polarizing microscope—digital cam-
era—computer arrangement. Images of the thin sections were
captured in two or three different polarizer/analyzer positions
in order to identify the maximum number of grain-boundaries
(Burkhard 1990). The two or three images obtained were pro-
jected on a horizontally oriented screen in a slideshow mode
and the grain boundary networks were then produced by man-
ually outlining the grains onto transparent foil. For the SEM
morphological  analyses  of  the  fine-grained  aggregates,  pol-
ished XZ and XY slabs of the selected samples were etched in
a  1%  hydrochloric  acid  solution  for  10  seconds  and  coated
with  gold-film.  Secondary  electron  photomicrographs  were
obtained from 30—50° tilted samples after tilt-correction.

Quantitative processing of the grain boundary networks was

carried out using ImageTool 2.0 software. In this paper, only
the grain size parameter is used for the characterization of mi-
crostructures. The grain size (D) is defined as the diameter of
a circle with the same area as the grain being measured, that is

D = sqrt(4

grain area/

Such a definition of grain size gives the most realistic val-

ues which are independent of grain shape. For stress calcula-
tions we used the Rutter paleopizometer (Rutter 1995) calcu-
lated for the grain boundary migration (GBM) recrystal-
lization mechanism:

log

= 2.22 + 0.37 log d — 0.30 (log d)

where

is the differential stress and d is the grain size, and

the median values of grain size were used as suggested by
Ranalli (1984).

The content of dolomite and other secondary phases was

examined with an electron microprobe in selected samples in
order to assess their potential influence on the deformational
processes.

The stretch (S = original length/finite length of the deformed

object) was measured at pressure fringes, deformed peloids
and ooids, boudinaged clasts and other strain-markers in order
to demonstrate the strain magnitude of the distinguished mi-
crostructural types.

Lattice Preferred Orientations (LPO)

If the evolution of the mylonites was to be reconstructed, it

was necessary to determine the mechanisms operative during
the deformation. Therefore the LPO were measured. Their in-

266 ŠPAČEK et al.

tensity is related to the magnitude of the intracrystalline defor-
mational mechanisms and their ratio to the other mechanisms
of deformation (e.g. Casey & McGrew 1999). X-ray diffrac-
tion texture analysis was used for the comparison of the fabric
geometries and intensities of the distinguished microstructural
types. The measurements were carried out in the laboratory of
of Military Technical Institute of Protection in Brno, using a
Siemens D-500 texture goniometer. Textures were measured
using reflection geometry on thin slabs of the rock which had
been cut parallel to the macroscopic foliation (XY plane). Fil-
tered CuK

1+2

rays were used and maximum tilt was 80°. The

data were further tilt-corrected, using a tilt scan on a powder
sample and processed with popLA software (Kallend et al.
1991).

Preliminary results have shown that the LPO patterns of all

samples are very similar and that it was not necessary to calcu-
late orientation distribution functions. The LPO intensities of
the samples measured were compared in the incomplete
(

= 0—80°) pole figures of (018) planes (e-poles).

Additionally, crystallographic orientations of the coarse

grains were measured in several thin sections using optical po-
larizing microscope with a U-stage. This “semi-domainal”
LPO analysis allowed the LPO of the porphyroclasts and that
of the whole samples to be measured separately.

Paleothermometry

The illite crystallinity of clay fractions from shales associat-

ed with the mylonitized limestones was measured in order to
estimate  the  maximum  reached  paleotemperatures.  Clay-size
material was separated from 8 rock samples after removing the
cements  such  as  carbonates,  organic  matter  and  iron  oxides
(Jackson 1975). Clay fraction < 2 µm was collected by centrif-
ugation for determination of illite crystallinity. Oriented slides
were analysed both air-dry and after vapour glycolation using
X-ray  diffractometer  Philips  PW  1830  (generator)  and  PW
3020  (goniometer)  with  0.02°  step  from  2  to  50  °2Q.  Illite
crystallinity index (IC) was measured as peak width in

∆

half maximum (PWHM) of the (001) basal reflection of illite
(Kübler 1967) using background stripping and peak-fitting.
The results were calibrated to international standards (Warr &
Rice 1994).

Microstructures and their interpretations

The results of microfabric analyses allow several basic

groups of (proto-)mylonites with similar features to be distin-
guished. In the following discussion, these groups of micro-
structures will be identified with the letters A—E. The inter-
preted mechanisms of deformation are shown in a schematic
diagram in Fig. 3.

Microstructures A.

Weakly deformed protoliths

Rocks of this type retain their original inhomogeneous

structure and the sedimentary attributes of their protoliths. The
most typical composition includes a micritic matrix (d ~ 4 µm)
and a wide-ranging assemblage of various parts of fossil or-
ganisms, which together with boudinaged veins and other con-

stituents composed of sparite, will be referred to as “porphyro-
clasts”.

Strain markers, for example, the calcite-filled pressure fringes

around quartz clasts and deformed peloids, indicate that matrix
suffered substantial strains with minimum stretch values up to
S = 4.5. In spite of this high degree of strain, the micrite in the
matrix does not exhibit substantial microstructural changes.

The coarse-grained porphyroclasts are not substantially in-

ternally strained and figure as rigid bodies passively flowing
in  a  ductile  environment.  Locally,  book-shelf  fracturing,  mi-
croboudinage,  twinning  and  only  slight  undulose  extinction
within the coarse grains can be observed in the samples with
higher  finite  strains  or  with  the  load-bearing  framework  of
coarse grains. The distribution of the c-axes measured in por-
phyroclasts is random to strong with a single maximum near
the  pole  to  the  foliation,  depending  on  the  strain  magnitude
(not shown in the figure, in strongly deformed types the c-axes
distribution  is  very  similar  to  that  in  Fig.  2d).  Bulk  (whole
sample) X-ray LPO is much weaker, increasing with the high-
er volume of the clasts (Fig. 2a).

Fabric development of this microstructure took place in a

semi-ductile regime. Strain partitioning due to grain size inho-
mogeneity, with preferential localization into fine-grained ma-
trix, is characteristic. For the matrix, the interpreted dominant
mechanism  of  the  deformation  is  grain  boundary  sliding
(GBS)  with  accommodation  by  diffusion  transfer  at  grain
boundaries. The main arguments for GBS are negligible mi-
crostructural  changes  in  spite  of  high  strains  and  the  weak
LPO. The high water content in the limestones during defor-
mation is indicated by frequent pressure fringes and stylolites
in the micrite. It is very likely that the fluids have played an
important role in the diffusive matter-interchange between the
grains and could thus have caused a substantial weakening of
the  matrix.  During  the  deformation,  the  strength  of  coarse
grained porphyroclasts was much higher than that of the fine-
grained matrix (compare Fig. 7a). Local high stress conditions
resulted in brittle fracturing, twinning and incipient intracrys-
talline slip within the porphyroclasts.

Microstructures B.

Protomylonites

A mantled porphyroclasts/matrix structure is characteristic

for this group (Fig. 7b). Core-and-mantle structure grains are
free of clay minerals and in most cases can be easily distin-
guished from the matrix which appears darker under transmit-
ted light. The bulging of the grain boundaries and the forma-
tion of oval-shape recrystallized grains (Fig. 7c) takes place
preferentially at twin lamellae and boundaries of clasts.

This type of microstructure is interpreted as a result of grain

boundary bulging (GBB) or nucleation recrystallization
(Drury & Urai 1990; Mercier et al. 1977). High aspect ratios
of the porphyroclasts were generated mainly by the superposi-
tion of GBM on the twin lamellae (Fig. 7b). Within the matrix,
non-distinctive grain growth up to d < 10 µm also indicates the
incipient activity of grain boundary migration. The formation
of subgrains and rotation recrystallization are less pronounced
and do not lead to significant reduction in grain size. Despite
the occurrence of mantled porphyroclasts, which one is tempt-
ed to interpret as the product of subgrain rotation, the lack of
recovery in the grains suggests the dominance of the GBB

DEFORMATION MECHANISMS WITHIN THE MYLONITIZATION OF LIMESTONES 267

and/or nucleation mechanisms of recrystallization. The recrys-
tallized grains produced by grain size reduction of the porphy-
roclasts are usually larger in size than those resulting from the
grain growth of the matrix micrite (Table 1). This discrepancy
can be explained by the low rate of GBM in the matrix grains
as a result of their low level of internal strain and/or the inhibi-
tory  effect  of  the  secondary  phase  (Olgaard  &  Evans  1988).
The LPO of these types of protomylonites have features simi-
lar to that of the microstructures A: a strong single maximum
of  the  c-axes  close  to  the  pole  of  foliation  in  the  porphyro-
clasts,  which  is  weakened  in  the  LPO  of  the  whole  sample
(Fig. 2b,c,d). The activity of GBS is likely to have persisted in
both the matrix and the domains of recrystallized grains as in-
dicated by the lack of microstructural change in spite of strong

deformation of the fine-grained domains. Boudinaged bio-
clasts indicate minimum stretch values of S = 6.1.

Microstructures C.

Mylonites

These types of rocks are composed of a relatively coarse-

grained matrix (d = 20—50 µm) with relict porphyroclasts. Pres-
sure fringes indicate minimum stretch values of S = 6.0. Grain
aspect ratios vary between 1.5 and 4. All the grains of the ma-
trix  and  the  porphyroclasts  are  optically  strain  free,  grain
boundaries  are  almost  straight,  slightly  curved  or  bulbous.
Twin lamellae are rare, straight and were probably produced
during cooling due to high thermoelastic anisotropy of calcite
(Rosenholtz & Smith 1949). The LPO is similar both for ma-
trix and porphyroclasts, showing single maximum of e-poles
(and  c-axes  respectively)  close  to  the  pole  of  foliation  (Fig.
2e).  Because  the  grains  lack  undulose  extinction  and  a  sub-
grain  microstructure  and  the  grain  boundaries  are  frequently
bulbous, we attribute the finite microstructure to GBM-domi-
nant  recrystallization.  Nevertheless,  twinning  and/or  disloca-
tion glide are also likely to have played a substantial role dur-
ing  earlier  phases  of  the  deformation  as  indicated  by  high
aspect ratios and straight boundaries of the grains and the en-
hanced intensity of LPO.

Locally, sharply terminated prolate lens-shaped domains are

developed in these types of mylonites, representing strained
stems of Amphipora sp. These domains are composed of rela-

Fig. 2. Sample pole figures of e-planes and c-axes of the main microstructural types. All data are projected to the plane of foliation in
equal-area projection, lower hemisphere. a—c, e, f – X-ray diffraction data in incomplete pole figures; intensities are expressed as multi-
ples of a random distribution (m.r.d.). d – c-axes distribution of porphyroclasts in microstructure B. Contours at 0, 2, 4, 6, 8, 10, 12, 14,
16 and 18 %. Notice the strong LPO of porphyroclasts vs. weak LPO of whole sample in the microstructural types B and the increasing
intensity of the preferred orientation from Type B to Type D with relatively constant distribution geometry. The patchy pattern of the dis-
tribution is due to the high content of large grains in the samples.

Fig. 3. The interpreted dominant deformational mechanisms oper-
ative  in  the  distinguished  stages  of  mylonitized  limestones.  GBS
–  grain  boundary  sliding,  GBB  –  grain  boundary  bulging,  DG
– dislocation glide,  GBM – grain boundary migration, SGR –
subgrain  rotation  recrystallization,  nucl.  –  nucleation,  BPT  –
brittle-plastic  transition.

268 ŠPAČEK et al.

tively coarse grained calcite aggregates of equant, strain free
grains with slightly curved boundaries.

Microstructures D.

Coarse grained marbles

In the most mature mylonites mesoscopic indicators (sheath

folds, boudinaged clusters of dolomite) suggest stretch values
of S > 10. In these types characteristic homogeneous domains
are developed with a uniform grain size which varies with the
volume  of  the  dispersed  phyllosilicates  and  dolomite  (Table
1). Porphyroclasts composed of calcite are absent. In the do-
mains with a small amount of secondary phases, the grain size
usually reaches 100—120 µm. All grains are strain free, having
slightly  curved  to  lobate  boundaries.  Grain  aspect  ratios  are
usually  > 2.5  and  occasionally  domains  with  equant  coarse
grains with grain boundaries meeting in 100—140° triple junc-
tions can be observed (Fig. 7d). The c-axes distribution pattern
shows a strong single maximum close to the pole of foliation
(Fig. 2f). Rare twin lamellae are straight, and were probably
produced during cooling. The microstructural features of these
types suggest the dominance of GBM recrystallization mecha-
nism.  However,  strong  LPO  indicates  substantial  activity  of
intracrystalline  deformation  during  the  evolution  of  these
types, whose microstructures could have been overprinted in
the latest phases of the deformation.

Microstructures E.

Retrogressively deformed marbles

In some areas, grain size reduction of the coarse grains oc-

curs within D types and narrow shear zones are developed af-
fecting coarse-grained aggregates of microstructure D (Table
1). The old grains are polygonized into subgrains and newly
formed grains usually have a crystallographic orientation very
close to that of their host grains (Fig. 7e). As the grain bound-
aries of the recrystallized grains are interpenetrating and bul-
bous, we suggest that the grain size reduction is an effect of
the combination of both subgrain rotation and GBM mecha-

nisms. These structures are attributed to the onset of retrogres-
sive deformation during incipient cooling. Further low temper-
ature deformation of some domains generated strong twinning
and undulose extinction of the coarse grains.

Paleothermometry

Illite crystallinity was measured in clay fractions of eight

samples from both the Svratka and Thaya Domes and western
margin of the Brno batholith. Two samples were excluded
from further processing because of their high content of ex-
pandable smectite and chlorite.

The values of illite crystallinity index (IC) range from 0.20

to 0.35°

∆

(Table 2) and indicate higher part of very low-

grade metamorphic (VLGM) conditions with probable maxi-
mum paleotemperatures of 250—320 °C (calibration after Frey
& Robinson 2000). The thermal alteration is more advanced
than  in  most  Paleozoic  rocks  of  the  Drahany  Upland  which
show  mainly  lower  VLGM  conditions  (Franců  et  al.  1999).
The results of IC thermometry are consistent with the data of
Bosák (1984) who analysed the degree of kerogen graphitiza-
tion in dark carbonate rocks of the lower units of the Svratka
Dome  and  of  the  western  margin  of  the  Brno  batholith.  He
concluded  that  in  most  samples  from  the  Svratka  Dome  the
maximum  temperature  did  not  exceed  300  °C  and  that  the
grade of thermal alteration of organic matter was considerably
lower in the carbonates of the Brno batholith’s western mar-
gin. Similar conclusions were made recently by Ulrich (2000)
who analysed the carbon and oxygen isotopic composition of
six  carbonate  samples  from  two  localities  in  the  lowermost
unit of the Svratka Dome. Using the graphite-calcite thermom-
eter and calibration of Covey-Crump & Rutter (1989) he stated
that  the  maximum  temperatures  in  the  graphite-rich  marbles
did not exceed 300 °C for a longer period of time. In the lime-
stones of the Brno batholith’s western margin paleotempera-
tures of 250—300 °C are indicated by the degree of conodont

Table  1:  Grain  size  values  and  calculated  paleostresses  of  selected  typical  samples  of  the  calcite  mylonites  studied.  In  some  domains,
stresses  were  not  calculated  for  the  reasons  expressed  by  abbreviations:  sp  –  high  content  of  secondary  phase  (possible  inhibition  of
grain  growth),  nu  –  not  well  understood  mechanisms  of  origin  (inhomogeneous  tectonofacies  with  ambivalent  characteristics),  ss  –
non-recrystallized primary sedimentary structures, crm – combination of recrystallization mechanisms.

tectonic domain microstructure sample no., locality

microstructural domain (and responsive deformation phase)

d (µm, median)

diff.

(MPa)

eastern

s88b, Čebín

matrix

4.0

eastern

s88b, Čebín

recrystallized grain mantles (peak metamorphosis)

6.2

211

eastern

s90-1, Čebín

matrix

5.0

eastern

s90-1, Čebín

recrystallized grain mantles (peak metamorphosis)

8.2

203

eastern

s51a, Lažany

matrix

5.41

eastern

s51a, Lažany

recrystallized grain mantles (peak metamorphosis)

6.7

209

eastern

s188, Šebetov

recrystallized grain mantles (peak metamorphosis)

8.0

204

western

s45-1, Kadov

matrix

4.3

western

s45-1, Kadov

recrystallized grain mantles (peak metamorphosis)

7.2

207

western

s170, Květnice

fine grained domains

12.3

western

s170, Květnice

coarse grained domains

26.9

western

s 239, Vohančice

fine grained domains

19.6

western

s 239, Vohančice

coarse grained domains

27.3

western

s166-1, Dranč

coarse grained domains (peak metamorphosis)

100.5

western

s171, Lažánky

fine grained domains

30.6

western

s171, Lažánky

coarse grained domains (peak metamorphosis)

115.6

western

s158, Lažánky

coarse grained domains (peak metamorphosis)

89.4

western

s151, Dřínová

coarse grained domains (peak metamorphosis)

102.4

western

s157, Vohančice

coarse grained domains (peak metamorphosis)

111.5

western

s157, Vohančice

fine grained domains (incipient retrogression)

38.9

crm

western

s157, Vohančice

fine grained domains (advanced retrogression)

24.4

crm

DEFORMATION MECHANISMS WITHIN THE MYLONITIZATION OF LIMESTONES 269

Table 2: Illite crystallinity values of clay fractions from shales as-
sociated with the mylonitized limestones. BB – Brno batholith’s
western margin, TD – lower tectonic unit of the Thaya Dome, SD
– lower tectonic unit of the Svratka Dome. (*) – associated tec-
tonofacies are marked with capital letters in parentheses.

alteration. The black colour of the conodonts without the tones
of brown corresponds to conodont colour alteration index
(CAI) 5—5.5 (see Frey & Robinson 2000 for calibration).

Indirectly, and with limited reliability, the available paleo-

thermometric  data  which  are  summarized  in  Table  3  can  be
supported by the features of deformation microstructures ob-
served  in  quartz.  In  metamorphosed  basement  crystalline
rocks and Devonian conglomerates and sandstones of the low-
er tectonic unit of the Svratka Dome, quartz aggregates carry
clusters  of  recrystallized  quartz  grains  with  a  similar  c-axis
orientation  and  intensely  sutured  grain  boundaries.  Well-de-
veloped  low-angle  boundaries  and  subgrains  inside  relic  old
grains are commonly found (Fig. 7g). This fabric is indicative
of  deformation  by  dislocation  creep  with  recovery,  subgrain
rotation recrystallization and grain boundary migration being
operative. The features of the microstructure thus correspond
to  the  fully  plastic  deformation  regime  3  of  Hirth  &  Tullis
(1992). After Stöckhert et al. (1999), the steady-state medium
stress  dislocation  creep  of  quartz  in  the  fully  plastic  regime
can only be effective at temperatures above the closure tem-
perature for K-Ar and Rb-Sr systems of biotite, that is above
ca.  310±30  °C.  In  contrast,  in  the  eastern  tectonic  domain,
quartz is brittlely deformed, lacking any traces of intracrystal-
line slip (Fig. 7h). We consider similar pressure of fluids dur-
ing the deformation of the quartz aggregates in both units. It
seems to be a reasonable assumption that in the eastern part of
the Brunovistulian basement (external part of the orogen) the
brittle quartz was not deformed at higher strain rates than the
plastic quartz in the lower tectonic units of the Svratka Dome
(more internal part of the orogen). Assuming this, the observed
deformational  microstructures  of  quartz  indicate  that  in  the

eastern part of the Brunovistulian basement (Brno batholith’s
western margin), the deformation was taking place under tem-
peratures which were probably lower than 310±30 °C.

The lack of significant IC differences between the compared

domains  of  the  Brunovistulian  basement  (Table  3)  suggests
that  the  metamorphic  transformation  of  smectite  into  illite
reached  similar  stages  in  the  studied  rocks  of  the  Brno
batholith’s western margin, the Thaya Dome and the Svratka
Dome and that the paleotemperature differences are below the
detection limit of the IC paleothermometer.

Considering potential errors of the paleothermometers and

the variation of the IC values measured, it can be stated that
the  maximum  paleotemperatures  under  which  the  studied
rocks  were  deformed  probably  lie  between  250  and  300  °C
(Table 3). As was discussed above, the deformational features
of both quartz and calcite indicate that in the Brno batholith
and  the  lower  unit  of  the  Thaya  Dome  the  maximum  pale-
otemperatures were somewhat lower than in the lower unit of
the Svratka Dome. However, the paleothermometric data con-
strain the maximum difference of Variscan peak temperatures
between the three compared domains of the basement to sever-
al dozens of °C only (see Table 3).

The development of mylonitic stages

The sequence of microstructures A—E which has been dis-

tinguished above can be seen as a succession of frozen-in stag-
es  within  the  process  of  the  deformational  and  metamorphic
transformation  of  sediments.  Several  facts  justify  such  an
opinion:  the  analogous  lithostratigraphic  position  of  calcite
mylonites,  their  overlapping  biostratigraphic  ranges  and  the
mutual transitions of microstructural stages. A schematic dia-
gram  of  the  model  is  presented  in  Fig.  4.  It  is  necessary  to
stress the fact that unquestionable primary sedimentary mark-
ers – fossil organisms – are common within the microstruc-
tures  A,  B  and  C.  Microprobe  analyses  revealed  a  calcitic
composition of the fossils. This is the main argument for inter-
preting  the  A—D  sequence  as  a  product  of  progressive  my-
lonitization under increasing metamorphic conditions. During
the retrogressive deformation phases, the fine-grained matrix
of  Type  C  was  probably  reworked  again,  but  a  significant
change in the microstructure did not occur. The Type C micro-
structures  never  acquired  the  features  of  D  types  during  its

Table 3: Table of paleothermometric data of the studied area which are collected from the available literature.

tectonic domain* lithology

locality

IC (

∆°2Θ)

BB (A)

clayey limestone

Újezd u B. (s168)

0.30

BB (A)

shale

Újezd u B. (s168b)

0.28

BB (B)

clayey limestone

Šebetov (s111)

0.25

BB (A)

shale

Chudčice (s161)

0.20

TD (B)

shale

Skalice (s221)

0.35

SD (D)

shale

Lažánky (s237)

0.24

Brno batholith’s western margin

Lower tect. unit of the Thaya Dome

Lower tect. unit of the Svratka Dome

graphite/calcite thermometer (Ulrich 2001 with calibration after Covey-Crump & Rutter 1989)

–

max. 300 °C

CAI (Špaček 2001, calibration after Frey & Robinson 1999)

>250 °C

–

graphitization of kerogen (Bosák 1984)

<300 °C

max. 300 °C

illite crystallinity (this work, calibration after Frey & Robinson 1999)

250–320 °C

dynamic recrystallization of calcite (this work, after Burkhard 1990)

min. 250 °C

>250 °C

plastic deformation of quartz (this work, after Stöckhert et al. 1999)

–

>310±30 °C

270 ŠPAČEK et al.

evolution. It was never fully homogenized and primary sedi-
mentary structures were not completely destroyed. Thus, only
the phase during which the microstructures E were produced is
interpreted as retrogressive. Generally, the progressive phase
of mylonitization is characterized by the grain growth of the
matrix and a grain size reduction of the clasts leading to a
stress-determined equilibrium of the grain size. With rising
temperature during deformation, the homogenization of grain
size was finished and grain growth predominated. In retrogres-
sive phase, grain size reduction occurred due to a decreasing
temperature and increasing stress.

Mechanisms of recrystallization

One of the most surprising features one can observe in the

mylonitized sequence of carbonates studied is the dominance
of GBM and the lack of effective dynamic recovery within the
microstructures of the progressive low temperature phase of
deformation.

Recovery represents the process of ordering the lattice de-

fects, originated during intracrystalline slip, into subgrain
boundaries which leads to a decrease in the internal strain en-
ergy of the crystal. When dislocations are continuously added
to subgrain boundaries, the misorientation of the subgrains in-
creases and new grains are formed. This process of new grains

formation  is  referred  to  as  subgrain  rotation  recrystallization
(SGR)  and  produces  diagnostic  core-and-mantle  structures
with (sub-)grains increasingly misoriented towards the exter-
nal parts of mantle (Guillopé & Poirier 1979; Lloyd & Free-
man 1994). If the temperature is high enough to enable order-
ing of the lattice defects, continuous recovery-accommodated
dislocation creep can operate. However, when the temperature
is too low, recovery cannot keep pace with the tangling of the
dislocations during intracrystalline slip, newly formed disloca-
tions  cannot  move  and  strain  hardening  of  the  lattice  occurs
(e.g. White 1977).

In our study, small (d ~ 8 µm) recrystallized grains of the

mantled porphyroclasts in high stress mylonites B do not show
any optical filiation to the host grains and their shape indicates
the activity of GBB and/or nucleation (Fig. 7b,c). Although ul-
tra-thin sections were used for the observation of microstruc-
tures, the formation of small-sized subgrains within the coarse
grains was found only sporadically.

We therefore explain the recrystallization of the clasts in mi-

crostructures B as the product of a GBB-dominant process.
Our observations lead us to the conclusion that during the pro-
gressive, low temperature deformation of the limestones stud-
ied, SGR was not capable of reducing the coarse grains into a
steady-state size. Recovery and SGR produced only relatively
large (sub-)grains which must have been further reduced by
more effective GBB.

Fig. 4. Schematic diagram of microfabric development in mylonitized sequence of limestones. Notice the different peak grades reached
in the lower tectonic unit of the Svratka Dome and the other two domains of the Brunovistulian basement. Objects in diagrams: black –
quartz, gray – micrite and microsparite, white – coarse-grained calcite. BB – Brno batholith’s western margin, TD – lower tectonic
unit of the Thaya Dome, SD – lower tectonic unit of the Svratka Dome.

DEFORMATION MECHANISMS WITHIN THE MYLONITIZATION OF LIMESTONES 271

Fig.  5.  The  suggested  model  of  recrystallization  mechanisms  in  a
complete  deformation  path  of  mylonitized  limestones.  Individual
microstructures  are  marked  with  capital  letters  A—E.  See  text  for
explanations.

A suggested model of recrystallization development is

shown in the Fig. 5. According to Lloyd & Freeman (1994),
the  velocity  of  grain  boundary  migration  processes  is  deter-
mined by the relative crystallographic characteristics of adja-
cent grains, the driving forces, temperature and the structure of
the  boundary.  Driving  forces  include  mainly  lattice  defects,
elastic energy and grain-boundary energy, and always lead to a
decrease in internal strain energy. In our case, as the SGR did
not lead to sufficient grain size reduction, each increment of
continuing deformation raised the internal strain of the large
grains and could accelerate the grain boundary bulging. Hip-
pertt & Egydio-Silva (1996) presented arguments for the activ-
ity of solution-reprecipitation process during the deformation
of quartz which can be concurrent with solid state recrystalli-
zation. Thus the high content of water in the system, which is
indicated  by  frequent  markers  of  solution  transfer,  probably
also significantly increased the GBB or nucleation rate (com-
pare also with Tullis & Yund 1982). The facilitating of grain
boundary migration resulting from a high water content would
explain the contradiction of our model to observations of some
other  authors  who  suggest  that  SGR  is  more  effective  than
GBB under lower temperatures (Schmid et al. 1987). The re-
crystallization mechanisms of calcite in LT conditions could
be analogous to those of quartz, in which grain boundary mi-
gration-dominant structures have even been described for dry
samples (Hirth & Tullis 1992).

Deformational contrasts and the imbrication of the

Brunovistulicum

It has been demonstrated by many authors that inverse pro-

portionality between stress and recrystallized grain size exists

(e.g. Twiss 1977; Kohlstedt & Weathers 1980). Thus, under a
constant strain rate, the increase in recrystallized grain size is
due to a decrease in material strength. Analogously, grain size
distribution within a mylonitized sequence can be viewed as
the  result  of  metamorphic  grade  (i.e.  temperature)  variation.
We therefore attempted to assess the differences between the
deformation grade of the two domains of the deformed Bruno-
vistulian basement. The most effective and reliable method of
relative  paleostress  estimation  seems  to  be  the  comparative
measurement of grain size in the peak grade microstructures of
individual tectonic domains. Uniform recrystallized grain size
within broad domains justifies the presumption of steady state
creep  (e.g.  Twiss  1977;  Michibayaschi  1993).  Facies  with
small-scale grain size variations were not taken into account in
the paleostress calculations. It was assumed that the coarsest
recrystallized  grain  size  within  the  defined  groups  of  micro-
structures  represents  the  peak  metamorphic  conditions.  In
many coarse grained domains of facies D, the lack of anneal-

Fig. 6. Simplified sketch of the studied area showing the distribu-
tion  of  distinguished  microstructural  types.  Circles  with  numbers
indicate  sampled  localities  which  are  referred  to  in  the  tables  and
figures. 1 – Šebetov (s111), 2 – Újezd u Boskovic (s168, s168b),
3  –  Lažany  (s51a),  4  –  Čebín  (s88b,  s90-1),  5  –  Chudčice
(s2056,  s161),  6  –  Kadov  (s45-1),  7  –  Skalice  (s221),  8  –
Lažánky  (s171,  s158,  s237),  9  –  Vohančice  (s157,  s239),  10  –
Květnice (s170), 11 – Dřínová (s151) and Dranč (s166-1).

DEFORMATION MECHANISMS WITHIN THE MYLONITIZATION OF LIMESTONES 273

ing is evidenced with the shape of the grain boundaries, internal
strain and polygonization of the grains. It can be stated that after
reaching peak temperatures and the localization of the retrogres-
sive deformation into narrow zones, no substantial grain growth
took place in the domains which are now coarse grained. The
stress calculations thus should not be affected by static recrystal-
lization. In Table 1, the calculated paleostresses are given for
several typical samples, using the Rutter paleopiezometer (Rut-
ter 1995) for the GBM recrystallization mechanism.

If we compare the stress values of the facies B and D, which

represent  the  peak  metamorphic  conditions  in  the  eastern  and
western parts of the Brunovistulian basement respectively, we
can see a significant difference. Stresses four times lower in the
lower tectonic unit of the Svratka Dome than in the other two
parts  of  the  Brunovistulicum  are  in  accordance  with  the  ob-
served  deformation  regimes  of  quartz.  As  the  three  compared
parts  of  the  Brunovistulian  basement  are  eroded  to  a  similar
lithostratigraphic  level,  the  tectonic  juxtaposition  of  the  con-
trasting facies must have occurred during late tectonic phases.
The main contrasts however, are not seen between the two parts
of the Brunovistulicum with different tectonostratigraphic zona-
tion, which were defined above, but they lie between the lower
tectonic unit of the Svratka Dome and the other two domains of
the  Brunovistulicum  –  the  Brno  batholith’s  western  margin
and the lower tectonic unit of the Thaya Dome (Fig. 6).

Summary and concluding remarks

The analysis of the mylonites assembly developed within

the major thrust system of the Central European Variscan
orogeny revealed several characteristic features:

1) A lack of effective dynamic recovery within the progres-

sive  low  temperature  phase  of  deformation.  Microstructural
features  of  core-and-mantle  structures  developed  in  calcite
protomylonites provided evidence of GBB-dominance of the
recrystallization  process.  In  this  phase,  the  SGR  mechanism
produced only relatively large grains, which must have been
further reduced by GBB and/or nucleation. Recovery was ef-
fective  only  during  grain  size  reduction  in  the  retrogressive
phase. This can be explained with a higher rate of GBB under
lower temperatures, which could have been increased as a re-
sult of a high fluid content.

2) The onset of grain size reduction of porphyroclasts prior

to distinct grain growth in matrix.

During the incipient mylonitization of inhomogeneous mi-

critic  limestones,  the  porphyroclasts  and  matrix  display  con-
trasting  rheological  behaviour  and  strong  strain  localization
into a superplastic matrix occurs. At a certain point of mylo-
nite development, which is probably temperature-determined,
GBB is facilitated and the steady-state dynamic recrystalliza-
tion of clasts sets in. The recrystallized grain size is very close
to the grain size in the matrix. Assuming that an inverse pro-
portionality  between  grain  size  and  stress  is  valid,  the  exist-
ence of approximate stress homogeneity must be considered in
naturally  deformed  calcite  aggregates.  A  distinct  to  almost
complete dynamic recrystallization of the porphyroclasts and
minimum microstructural changes of the matrix indicates sig-
nificant strain rate differences between the porphyroclasts and
the matrix.

3) Variscan large-scale thrusting within the Brunovistulian

basement is indicated by the juxtaposition of facies with con-
trasting microstructures which reflect incompatible peak-grade
conditions. The main deformational contrasts can be observed
between the lower tectonic unit of the Svratka Dome and the
other two domains of the Brunovistulicum – the Brno
batholith’s western margin and the lower tectonic unit of the
Thaya Dome.

It seems very likely that the recrystallized grain size within

the mylonitized sequence of limestones studied is due to dif-
ferential stress variations. Assuming this, it is quite surprising
that temperature differences probably not exceeding 50 °C re-
sulted in such dramatic changes of differential stresses and re-
sponsive  microstructures  (compare  microstructures  A  and  B
with D). Actually, we are not the first to have observed indica-
tions of such contrasting behaviour of calcite aggregates under
natural LT deformation conditions. Burkhard (1990) examined
the change of the microfabric of micritic limestones strained
under a natural temperature gradient. He found out that up to
250  °C,  grain  size  distributions  were  indistinguishable  from
the sedimentary protolith. Above 280 °C, an increase in grain
size in micritic limestones occurred along with an increase in
the preferred orientation of the lattice and grain-shape. Behr-
mann (1983) described distinct variations in the microfabric of
calcite  mylonites  strained  at  about  300  °C.  Microstructural
features of the tectonofacial succession, which were described
above, provide evidence of the dominance of GBB process or
nucleation  during  the  progressive  part  of  LT  mylonitization.
Burkhard  (1990)  also  suggested  a  grain  boundary  migration
mechanism  for  grain  growth  in  epizonally  strained  micrites.
Grain boundary migration has a first order dependence on tem-
perature (e.g. Guillopé & Poirier 1979) and it can be expected
that  the  variation  of  finite  microstructures  within  the  lime-
stones  strained  under  LT  conditions  is  due  to  a  significant
change of GBB effectiveness at temperatures around 300 °C.

Acknowledgments: We thank the creators of ImageTool 2.0
for  its  free  provision  at  http://www.uthscsa.edu/dig/
itdesc.html.  The  constructive  reviews  of  K.  Schulmann,  D.
Plašienka  and  an  anonymous  reviewer  are  highly  acknowl-
edged. The research was supported by Grant Agency of Czech
Republic through Grant No. 205/98/0751 and by research plan
J07/98:143100004.

References

Bábek O. & Janoška M. 1997: Tectonic evolution of the Konice-Mladeč

belt: Structural analysis and facies disjunction. Fac. Rer. Nat., Acta
Univ. Palackianae Olomuc., Geol. 35, 31—35.

Batík P. & Skoček V. 1981: Lithological development of the Paleozoics

in the Thaya Batholith eastern margin. Věst. ÚÚG 56, 6, 337—347
(in Czech).

Behrmann J.H. 1983: Microstructure and fabric transitions in calcite

tectonites from the Sierra Alhamilla (Spain). Geol. Rdsch. 72, 2,
605—618.

Bosák P. 1980: Sedimentology of the Devonian in the Tišnov Brunnides

and Brno Unit s.s., Tišnov area. Unpublished PhD. Thesis. Charles
University, Prague, 1—200 (in Czech).

Bosák P. 1984: Organic matter in the Devonian carbonates of the Tišnov

area. Čas. Mineral. Geol. 29, 1, 41—53 (in Czech).

Burkhard M. 1990: Ductile deformation mechanisms in micritic lime-

274 ŠPAČEK et al.

stones naturally deformed at low temperatures (150—350 °C). In:
Knipe R.J. & Rutter E.H. (Eds.): Deformation mechanisms, rheolo-
gy and tectonics. Geol. Soc. Spec. Publ. 54, 241—257.

Busch J.P. & Van der Pluijm B.A. 1995: Calcite textures, microstructures

and rheological properties of marble mylonites in the Bancroft shear
zone, Ontario, Canada. J. Struct. Geol. 17, 677—688.

Casey M. & McGrew A.J. 1999: One-dimensional kinematic model of

preferred orientation development. Tectonophysics 303, 131—140.

Cháb J. & Suk M. 1977: Regional metamorphism in Bohemia and Mora-

via. Knih. ÚÚG 50, 156 (in Czech).

Čížek P. & Tomek C. 1991: Large-scale thin-skinned tectonics in the east-

ern boundary of the Bohemian Massif. Tectonics 10, 273—286.

Covey-Crump S.J. & Rutter E.H. 1989: Thermally induced grain

growth of calcite marbles on Naxos island, Greece. Contr. Miner-
al. Petrology 101, 69—86.

Dietrich D. & Song H. 1984: Calcite fabrics in natural shear environ-

ment, the Helvetic nappes of western Switzerland. J. Struct. Geol.
6, 19—32.

Drury M.R. & Urai J.L. 1990: Deformation-related recrystallization pro-

cesses. Tectonophysics 172, 235—253.

Dvořák J. 1973: Synsedimentary tectonics of the Paleozoic of the Draha-

ny Upland (Sudeticum, Moravia, Czechoslovakia). Tectonophysics
17, 359—391.

Dvořák J. 1995: Stratigraphy of the Moravo-Silesian zone. In: Dallmeyer

R.D., Franke W. & Weber K. (Eds.): Pre-Permian geology of Central
and Eastern Europe. Springer, Berlin, 477—489.

Dudek A. 1980: The crystalline basement block of the outer Carpathians

in Moravia – Brunovistulicum. Rozpr. Čs. Akad. Věd, Ř. Mat. Přír.
90, 8, 85.

Finger F., Höck V. & Steyrer H.P. 1989: The granitoids of the Moravian

Zone of north-east Austria – Products of a Cadomian active conti-
nental margin? Precambrian Res. 45, 235—245.

Finger F., Frasl G., Dudek A., Jelinek E. & Thöni M. 1995: Cadomian

Plutonism in the Moravo-Silesian Basement. In: Dallmeyer R.D.,
Franke W. & Weber K. (Eds.): Pre-Permian geology of Central and
Eastern Europe. Springer, Berlin, 495—507.

Finger F., Hanžl P., Pin C., Von Quadt A. & Steyrer H.P. 2000: The

Brunovistulian: Avalonian Precambrian sequence at the eastern end
of the Central European Variscides? In: Franke W., Haak V., Oncken
O. & Tanner D. (Eds.): Orogenic Processes: Quantification and
Modelling in the Variscan Belt. Geol. Soc. London Spec. Publ. 179,
103—112.

Franců E., Franců J. & Kalvoda J. 1999: Illite crystallinity and vitrinite

reflectance in Paleozoic siliciclastics in the SE Bohemian Massif.
Geol. Carpathica 50, 5, 365—372.

Frasl G. 1983: Zur Geologie des Kristallins und Tertiars der weiteren

Umgebung von Eggenburg. Österreichische Geologische Bundesan-
stalt, Excursion Guide.

Frey M. & Robinson D. (Eds.) 1999: Low-grade metamorphism. Black-

well Science, London, 1—313.

Fritz H., Dallmeyer R.D. & Neubauer F. 1996: Thick-skinned versus thin-

skinned thrusting: Rheology controlled thrust propagation in the
Variscan collisional belt (The southeastern Bohemian Massif, Czech
Republic—Austria). Tectonics 15, 6, 1389—1413.

Fritz H. & Neubauer F. 1993: Kinematics of crustal stacking and dis-

persion in the southeastern Bohemian Massif. Geol. Rdsch. 82,
556—565.

Guillopé M. & Poirier J.P. 1979: Dynamic recrystallization during creep

of single crystalline halite: an experimental study. J. Geophys. Res.
84, 5557—5567.

Hanžl P. & Melichar R. 1997: Brno Massif: a section through the ac-

tive continental margin or a composed terrane? Krystalinikum
23, 33—58.

Heitzmann P. 1987: Calcite mylonites in the Central Alpine root zone.

Tectonophysics 135, 207—215.

Hippertt J. & Egydio-Silva M. 1996: New polygonal grains formed by

dissolution-redeposition in quartz mylonite. J. Struct. Geol. 18,
1345—1352.

Hirth G. & Tullis J. 1992: Dislocation creep regimes in quartz aggregates.

J. Struct. Geol. 14, 145—159.

Jackson M.L. 1975: Soil chemical analysis. Advanced course. Published

by the author. Dept. Soil Sci., Univ. Wisconsin, Madison, 1—895.

Jenček V. & Dudek A. 1971: Relationship of the Moravicum and Mold-

anubicum at the western border of the Dyje Dome. Věst. ÚÚG 46, 6,
331—338.

Kallend J.S., Kocks U.F., Rollet A.D. & Wenk H.R. 1991: Operational

Texture Analysis. Mater. Sci. Engin., A132 1—11.

Kalvoda J. 1995: Devonian sedimentary basins of East Avalonian margin

in Moravia. Geologické výzkumy na Moravě a ve Slezsku v roce
1994, 48—50 (in Czech).

Kalvoda J. 2001: Upper Devonian—Lower Carboniferous foraminiferal

paleobiogeography and Perigondwana terranes at the Baltica-Gond-
wana interface. Geol. Carpathica 4, 52, 205—215.

Kohlstedt D.L. & Weathers M. 1980: Deformation-induced microstruc-

tures, paleopiezometers, and differential stresses in deeply eroded
fault zones. J. Geophys. Res. 85, B11, 6269—6285.

Kübler B. 1967: La crystallinite de l´illite et les zones tout a fait su-

perieures du metamorphisme. In: Etages Tectoniques, Colloque de
Neuchatel, 1966, A la Baconniere, Neuchâtel, 105—122.

Leichmann J. 1996: Geologie und Petrologie des Brünner Massifs. Un-

published PhD Thesis, University of Salzburg.

Lloyd G.E. & Freeman B. 1994: Dynamic recrystallization of quartz un-

der greenschist conditions. J. Struct. Geol. 16, 6, 867—881.

Matějovská O. 1975: The Moldanubicum gneiss series of southwestern

Moravia and its relation to granulites. Věstník ÚÚG 50, 345—351.

Matte P., Maluski H., Rajlich P. & Franke W. 1990: Terrane boundaries in

the Bohemian Massif: Result of large-scale Variscan shearing. Tec-
tonophysics 177, 151—170.

Mercier J.C.C., Anderson D.A. & Carter N.L. 1977: Stress in the lito-

sphere: inferences form steady steate flow of rock. Pure Appl. Geo-
phys. 115, 199—226.

Michibayaschi K. 1993: Syntectonic development of a strain-independent

steady-state grain size during mylonitization. Tectonophysics 222,
151—164.

Morauf W. & Jäger F. 1982: Rb-Sr whole rock ages for Bitesch Gneiss,

Moravicum, Austria. Terra Cognita 2, 60—61.

Olgaard D.L. & Evans B. 1988: Grain growth in synthetic marbles with

added mica and water. Contr. Mineral. Petrology 100, 246—260.

Rajlich P. 1990: Strain and tectonic styles related to Variscan transpres-

sion and transtension in the Moravo-Silesian Culmian basin, Bohe-
mian Massif, Czech Republic. Tectonophysics 174, 351—367.

Ranalli G. 1984: Grain size distribution and flow stress in tectonites. J.

Struct. Geol. 6, 443—447.

Rosenholtz J.L. & Smith D.T. 1949: Linear thermal expansion of calcite,

var. Iceland spar and Yule marble. Amer. Mineralogist 34, 846—854.

Rutter E.H. 1974: The influence of temperature, strain rate and interstitial

water in the experimental deformation of calcite rocks. Tectonophys-
ics 22, 311—334.

Rutter E.H. 1995: Experimental study of the influence of stress, tempera-

ture, and strain on the dynamic recrystallization of Carrara marble.
J. Geophys. Res. 100, 24651—24663.

Rutter E.H., Casey M. & Burlini L. 1994: Preferred crystallographic ori-

entation development during the plastic and superplastic flow of cal-
cite rocks. J. Struct. Geol. 16, 1431—1446.

Schmid S.M., Boland J.N. & Paterson M.S. 1977: Superplastic flow in

finegrained limestone. Tectonophysics 43, 257—291.

Schmid S.M., Paterson M.S. & Boland J.N. 1980: High temperature flow

and dynamic recrystallization in Carrara marble. Tectonophysics 65,
245—280.

Schmid S.M., Panozzo R. & Bauer S. 1987: Simple shear experiments on

calcite rocks: rheology and mirofabric. J. Struct. Geol. 9, 747—778.

Schulmann K., Ledru P., Autran A., Melka R., Lardeaux J.M., Urban M.

& Lobkovicz M. 1991: Evolution of nappes in the eastern margin of
the Bohemian Massif: a kinematic interpretation. Geol. Rdsch. 80/1,
73—92.

Schulmann K., Melka R., Lobkowicz M., Ledru P., Lardeaux J.M. & Au-

tran A. 1994: Contrasting styles of deformation during progressive
nappe stacking at the southeastern margin of the Bohemian Massif
(Thaya Dome). J. Struct. Geol. 16, 355—370.

Špaček P. 2001: Microtectonics and stratigraphy of the Paleozoic lime-

stones in the Brunovistulian SW margin. Unpublished PhD. Thesis,
Masaryk University, Brno (in Czech).

DEFORMATION MECHANISMS WITHIN THE MYLONITIZATION OF LIMESTONES 275

Štípská P. & Schulmann K. 1995: Inverted metamorphic zonation in a

basement-derived nappe sequence, eastern margin of the Bohemian
Massif. Geol. J. 30, 385—413.

Stöckhert B., Brix M.R., Kleinschrodt R., Hurford A.J. & Wirth R. 1999:

Thermochronometry and microstructures of quartz – a comparison
with experimental flow laws and predictions on the temperature of
brittle-plastic transition. J. Struct. Geol. 21, 351—369.

Suess F.E. 1908: Die Beziehungen zwischen dem moldanubischen und

moravischen Grundgebirge in dem Gebiet von Frain und Geras.
Verh. Geol. Reichsanst. 393—412.

Suess F.E. 1912: Die moravischen Fenster und ihre Beziehung zum

Grundgebirge des Hohen Gesenks. Denkschr. K. Akad. Wiss., Wien,
88, 541—631.

Suess F.E. 1926: Intrusionstektonik und Wandertektonik im varizcischen

Grundgebirge. Wien, 268.

Tullis J. & Yund A. 1982: Grain growth kinetics of quartz and calcite ag-

gregates. J. Geology 90, 301—318.

Twiss R.J. 1977: Theory and applicability of a recrystallized grain-size

palaeopiezometer. Pure Appl. Geophys. 115, 227—244.

Ulrich S. 2000: Deformation microstructures and comparative rheology

of marble and quartzite in natural strain gradient. Unpublished PhD.
thesis. Charles University, Prague.

Van Bremen O., Aflalion A., Bowes D.R., Dudek A., Mísař Z., Povondra

P.  &  Vrána  S.  1982:  Geochronological  studies  of  the  Bohemian
Massif,  Czechoslovakia,  and  their  significance  in  the  evolution  of
Central  Europe.  Transactions  of  the  Royal  Society  of  Edinburgh:
Earth Sci. 73, 89—108.

Vrána S., Blumel P. & Petrakakis K. 1995: Moldanubian Zone, metamor-

phic evolution. In: Dallmeyer R.D., Franke W. & Weber K. (Eds.):
Pre-Permian geology of Central and Eastern Europe. Springer, Ber-
lin, 453—466.

Walker A.N., Rutter E.H. & Brodie K.H. 1990: Experimental study of

grain-size sensitive flow of synthetic, hot-pressed calcite rocks. In:
Knipe R.J. & Rutter E.H. (Eds.): Deformation mechanisms, rheolo-
gy and tectonics. Geol. Soc. Spec. Publ. No. 54, 259—284.

Warr L.N. & Rice H.N. 1994: Interlaboratory standardization and calibra-

tion of clay mineral crystallinity and crystallite size data. J. Meta-
morphic Geol. 12, 141—152.

White S.H. 1977: Geological significance of recovery and recrystalliza-

tion processes in quartz. Tectonophysics 39, 143—170.