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Institut für Geologie und Paläontologie, Universität Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria

(Manuscript received April 24, 1997; accepted in revised form October 15, 1997)


Semiductile to ductile deformation phenomena within unmetamorphic, fine-grained, pelagic limestones of

the Northern Calcareous Alps (NCA) in the Eastern Alps were studied. The investigated pelagic limestones include
the Triassic Hallstatt Limestone, the Liassic Adnet Limestone, the Middle Jurassic Strubberg Marl and the Upper
Jurassic Oberalm Limestone. These pelagic limestones and marls display structures related to semiductile to ductile
deformation, including shear planes, solution seams, stylolites and dynamic recrystallization similar to structures in
protomylonites due to strata-parallel simple shear. The structures preferentially developed along clay-rich layers and
along boundaries between more competent marly limestones showing a protomylonitic appearence due to disjunctive
anastomosing foliation. Along the boundaries aragonite and fine-grained calcite were dissolved and insoluble clay
minerals were concentrated. Increasing strain resulted in the development of a penetrative foliation, generating S-C
fabrics even within competent limestone layers. Furthermore, mesoscale out-of-sequence shear planes caused decom-
position of competent limestone layers into clasts and nodules. These clasts acted as rigid objects within a more
viscous, argillaceous matrix. When ideally oriented, asymmetric pressure shadows were generated around these nod-
ules and 


-clasts were developed. The transition from massive limestone beds to nodular layers depends on silt and

clay contents. While limestones with low clay content were structurally resistant to deformation, clay-rich limestones
were easily deformed. The Adnet and Hallstatt limestones formed décollement horizons accomodating high strain
during Cretaceous nappe stacking and thrusting within the NCA, while the Strubberg and Oberalm limestones were
involved during Tertiary transpressive overprint in large strike-slip faults and thrusting within an associated triangle

Key words:

 Eastern Alps, pelagic limestone, plasticity, deformation mechanisms, deformation partitioning.


Fine-grained pelagic limestones are widespread, among oth-
ers, within passive continental margin sequences and occur in
external fold-thrust belts of many orogens. These commonly
comprise variegated micritic limestones of the „ammonitico
rosso“-type, widespread in Paleozoic and Mesozoic sequences
of the circum-Mediterranean Alpine mountain belts. The typi-
cal fabric is domainal including internally undeformed, often
fossil-rich lenses and nodules that are surrrounded by stylolitic
seams. Compared with recent settings the deposition of these
limestones is often assumed to have occurred on deep pelagic
swells that were protected from siliciclastic input from the hin-
terland. Domainal fabrics are usually interpreted to result from
sedimentary processes, especially from an interplay between
carbonate deposition and dissolution (e.g. Jurgan 1969; Jen-
kyns 1974; Mullins et al. 1980). A possible tectonic origin for
the stylolitic foliation in pelagic limestones was previously de-
scribed by Tucker (1973) and Alvarez et al. (1978).

Burkhard (1990) demonstrated that even at temperatures

between 160–350


C micritic limestones can plastically de-

form. Furthermore, various workers (e.g. Schmid 1982;
Schmid et al. 1987) showed that calcite is very susceptible to
annealing under elevated temperatures (starting with green-
schist facies conditions). Experiment-based deformation
mechanism maps for calcite predict either pressure solution

(Rutter 1976) or creep mechanisms (Schmid 1982) for low
temperatures (160–350


C), small grain sizes (up to 10 



and slow strain rates (10


 to 10





We describe and analyse semiductile to brittle deformation

within pelagic limestones of the Northern Calcareous Alps
(NCA; Eastern Alps; Fig. 1) which have not been affected by
any temperature higher than diagenetic conditions (200±50



There, semiductile to ductile fabrics of pelagic limestones,
similar to those known from ductile mylonites within meta-
morphic sequences, reached their final appearence by super-
imposition of sedimentary and structural processes. We show,
furthermore, that these ductilely deformed limestones acted as
décollement levels during Alpine nappe stacking. Descriptions
of tectonic structures within studied pelagic limestones follow
basic work including, e.g. Bell & Etheridge (1973), Dietrich &
Song (1984), Hancock (1985), Groshong (1988) and Carrio-
Schaffhauser et al. (1990).

The following deformation stages within the central North-

ern Calcareous Alps produced semiductile deformation phe-
nomena in the pelagic limestones: (1) mid-Cretaceous top-to-
the NNE thrusting of the Juvavic nappes; (2) Late Cretaceous
syn-Gosau transtension in the entire nappe pile of the NCA;
(3) Oligocene NE–SW contraction, and (4) Miocene N–S con-
traction (Schweigl 1997; Perreson & Decker 1997). Further
deformation stages produced only brittle deformation struc-
tures, apart from folds.

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362                                                                               SCHWEIGL and NEUBAUER

Geological setting

The Northern Calcareous Alps represent an allochthonous

cover nappe complex within the Alpine orogen (e.g. Toll-
mann 1976; Linzer et al. 1995). They comprise a several ki-
lometres thick, mainly carbonatic passive continental margin
sequence of Permian to early Late Cretaceous age. In the
central part the NCA are mainly built up by three different
tectonic units, from base to top (Fig. 1): (i) the Tirolic
Nappe Complex (e.g. Osterhorn Mountains), (ii) the Lower
Juvavic Nappe Complex (e.g. the Hallein Unit) and (iii) the
Upper Juvavic Nappe Complex (e.g. Berchtesgaden
Nappe). Illite-crystallinity data from the Carnian and Per-
mo-Scythian sequences showed diagenetic conditions for
the northern and central part of the central NCA (Kralik et
al. 1987). The NCA suffered low grade metamorphic over-
print (270–320


C) only along its southern edge (Schramm

1982; Kralik et al. 1987; Gawlick et al. 1994). Apatite fis-
sion track ages between 149 and 143 Ma (Hejl & Grund-
mann 1989)  demonstrated that the central Northern Calcar-
eous Alps had already cooled below ca. 100


C by the end

of Jurassic, synchronous with sediment deposition, and
have not been heated above this temperature since that time.

Spötl et al. (1996) obtained similar ages for the thermal over-
print based on 




Ar dating of authigenic feldspar and

fluid inclusion studies of the same region. Our own apatite
fission track data revealed that the temperature was high
enough only in the southern sectors of the central NCA to
reset apatite tracks (Schweigl 1997). The temperature re-
mained, therefore, below c. 150



Four different formations with pelagic limestones are inter-

calated within thick carbonate sequences of the NCA nappe
complex (Figs. 1, 2). These are from base to top: (1) the red,
nodular, Liassic Adnet Limestone; (2) grey to black Middle
Jurassic Strubberg Marl and Limestone; (3) the grey, chert-
rich, Upper Jurassic Oberalm Limestone; and (4) red to grey,
condensed, Triassic Hallstatt Limestones of the Lower Juvav-
ic Nappe Complex. The first three types of pelagic limestones
belong to the Tirolic Nappe of the central NCA. The Hallstatt
and Adnet Limestones are quite similar: both have been de-
posited on deep marine swells, both are condensed fossil-rich
limestones (e.g. Tollmann 1976). The Adnet limestones have
more clay content and are structurally less competent than the
Hallstatt limestones. The Strubberg marls and limestones
have been deposited in an oxygen-poor basin and contain si-
liciclastic material derived from turbidites. They sometimes

Fig. 1.

 Structural map of the central Northern Calcareous Alps with distribution of the studied pelagic limestones. A–A´ locates section

shown on Fig. 2. L.F.= Lammer Fault, K.L.T.F.= Königsse–Lammertal–Traunsee Fault.

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suffered the low grade metamorphism of the southern part of
the central NCA. The widespread Oberalm limestones were
also deposited in a basin and contain a lot of cherts (Gawlick
1996). They include some metres thick competent limestone
layers and marly intercalations.

The pelagic limestones often occur along major décolle-

ments along which thrust displacement during Cretaceous
and Tertiary nappe stacking occurred, while thick, massive
shallow water limestone and dolomite form the competent,
stiff interior of those nappes (Fig. 2). The studied samples
are from the décollement levels of the Tirolic Osterhorn
Mountains and the Lower Juvavic Hallein Unit and from
various units along the Lammer and Königssee-Lammertal-
Traunsee faults (Fig. 1).

Field observations

The about 40 m thick, Jurassic Adnet Formation is a nodu-

lar, phacoidal, relatively thin-bedded, clay-rich, micritic
limestone with rich fossil faunas including ammonoidea,
well exposed in quarries close to Adnet (Figs. 1, 2, 3). The
Adnet Formation itself is divided into several members: The
Adnet Limestone sensu strictu represents a micritic red to
green, pelagic limestone and the red Adnet Marl comprises a
much higher (40 %) clay content than the former (e.g. Toll-
mann 1976). The Adnet Scheck represents a submarine brec-
cia of Adnet Limestone with clasts within a matrix of white
sparite (e.g. Jurgan 1968; Böhm et al. 1995).

Our studies show that stylolitic Adnet Limestone (sensu

stricto) forms several strongly deformed structural levels. In

all these levels anastomosing stylolitic seams occur subparal-
lel, oblique and perpendicular to the bedding forming a dis-
junctive anastomosing stylolitic cleavage (according to the
nomenclature proposed by Powell 1979). Shear planes, solu-
tion seams and stylolites preferentially developed along clay-
rich layers and along boundaries of competent limestone
beds within the Adnet Limestone (Fig. 4a). Some stylolitic
seams are oblique to both bedding and the main stylolitic
foliation. In general these portions of the Adnet Limestone
have a protomylonitic character, following the definitions
given by Wise et al. (1984) and Heitzmann (1985). Along
these stylolitic seams aragonite and very fine-grained cal-
cite were dissolved and insoluble clay minerals were con-
centrated. Increasing deformation resulted in the develop-
ment of a penetrative foliation generating S-C-fabrics
(Berthé et al. 1979) even within competent limestone layers
while slip accumulated (Fig. 4a). During advanced stages
of deformation mesoscale out-of-sequence shear planes
caused decomposition of competent limestone layers into
boudin-like clasts and nodules, respectively, that sometimes
contain remnants of fossils (e.g. ammonoidea). These clasts
acted as rigid particles within a less viscous, argillaceous
matrix. When ideally oriented, asymmetric pressure shad-
ows were generated around these nodules until 



were developed (Fig. 4a). Biogenic and other clasts are flat-
tened and partly dissolved along clast edges. The transition
between more or less nodular beds depends on the silt and
clay contents. While limestones with very little clay con-
tents are structurally competent, clay-rich limestones may
be structurally responsive. Offset of sedimentary dikes
(Fig. 4b) with crinoidal infill along cm-scaled, internally

Fig. 2.

 Cross section with Adnet, Strubberg and Oberalm limestones which were used as décollement horizons. Hallstatt limestones are

not exposed in this section. For location of the section, see Fig. 1.

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364                                                                               SCHWEIGL and NEUBAUER

finely foliated shear zones developed subparallel to bedding
planes clearly proves the formation of ductile shear zones in
otherwise nearly undeformed limestone. The lineation on the
stylolitic cleavage is only weakly developed. It trends NNE
to NE (Fig. 3). Strata-parallel slickensides of the Adnet For-
mation prove top-to-the NNE displacement (Fig. 3).

In the Tauglboden area near the Schmittenstein Mountain

we observed steep ductile shear zones in the Adnet Lime-
stones  that produced nodular limestones. The contacts be-
tween the shear zones and less deformed limestone are very
sharp (Fig. 4b). The semiductile shear zones also developed
as pinch-and-swell structures where pull-apart structures
were filled with calcite (Fig. 4c). Another common feature in
these shear zones is the grain size reduction and S-C fabrics.
Outside of the semiductile shear zones en-echelon tension
gash veins and fibrous slickensides with the same orientation
and shear sense occur. This indicates that cataclastic and

ductile mechanisms were at work during the same stress and
kinematic conditions.

In summary, the nodular, marly members of the Adnet For-

mation appear semiductilely deformed in all major expo-
sures. A principal feature is the presence of a stylolitic folia-
tion largely subparallel to the sedimentary bedding, and
sometimes overprinted by steep shear zones crosscutting
bedding and foliation.

The Oberalm Limestone is a fine-grained, grey, pelagic,

Upper Jurassic limestone with chert lenses. It reaches a
thickness up to 800 meters and the individual beds are deci-
metres to metres thick. In the Trattberg area the Oberalm
Limestone is partly transformed into a coarse grained marble
with S-C planes and ecc-planes (ecc: extensional crenulation
cleavage: Platt & Vissers 1980; Fig. 4d, e). An anastomosing
stylolitic cleavage is spaced on the scale of centimetres. The
thick beds are foliated, cm-thick layers of marble and less de-
formed limestone. The disjunctive cleavage is overprinted by
shear bands. These structures document formation of dip-slip
faults on the frontal edge of a triangle structure (Fig. 2).

Similarly, the rocks of the pelagic Strubberg Formation of-

ten show S-C fabrics and shear bands within semiductile
shear zones (Fig. 4f). The grey to black Middle Jurassic
Strubberg Formation consists of micritic, radiolaria-bearing
limestones, siliceous limestones, marls and marly lime-
stones. These display similar deformational structures as the
Adnet Limestone. The Strubberg Formation can reach a
thickness of  200 metres and is restricted to the Lammer Val-
ley (Fig. 1). A ductile fabric with S-C fabric and shear planes
is common although microfossils are well-preserved within
clasts sourrounded by anastomosing foliation.

The Hallstatt Limestone is a colourful, greyish to reddish,

Ladinian to Norian limestone with a rich pelagic fauna. The
thickness of the thick bedded limestones is about 200 metres
(Tollmann 1976). Stylolitic seams, nodular structures and
shear bands are present throughout the Hallstatt Limestone in
the study area. Ecc- and S-C planes can be observed along
fault zones (Fig. 4g) in the Hallein Unit.


During deformation, twinning and grain size reduction due

to recrystallization of originally coarser grained rocks have
been the most important mechanisms in all studied samples
of all four types of fine-grained pelagic limestones. Thin sec-
tions of the Adnet limestones, taken from shear zones, show
high degrees of pressure solution, elongation and rotation of
grains and biogenic components, e.g. lamellibranchiata
shells or crinoidea columnalia (Fig. 5a). Internally less de-
formed nodules developed by pressure solution and shear
band formation (Fig. 5b).

Semiductile shear zones within otherwise undeformed

Adnet Limestone show the development of a stylolitic folia-
tion, strong flattening of calcite grains, fossils, and other
clasts (Fig. 5b). Clasts and calcite grains show serrated
boundaries caused by pressure solution (Fig. 5b). Grain size
reduction occurs by two mechanisms: (i) breakage of coarse

Fig. 3.

 Structural map of the quarries at Adnet. The stretching lin-

eations of the semiductile deformation structures are parallel to
the NNE–SSW oriented, strata-parallel fibrous slickensides.

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Fig. 4.

 Deformational structures within Adnet, Oberalm, Hallstatt

and Strubberg Formations. a) Shear zones with anastomosing foli-
ation, shear bands, solution seams, stylolites and nodules of inter-
nally undeformed limestone. Adnet Limestone, Adnet. b) Offset of
a sedimentary (neptunian) dyke by a subhorizontal ductile shear
zone. Adnet Limestone, Adnet. c) Pull apart structure (dilational
jog) along a ductile shear zone within the Adnet Limestone of the
Tauglboden area, one kilometre SW of the Schmittenstein. d) Ecc-
and S-C planes within a semiductile shear zone, Oberalm Lime-
stone, Trattberg. e) Detail of Fig. 4d with S-C planes; s plane = sty-
lolitic cleavage. f) Mylonitic Strubberg Limestone with foliation
and shear bands, Lammer valley. g) Shear band and S-C planes
within a semiductile shear zone. The foliation represents the s-plane.
Hallstatt Limestone along the Königsse–Lammertal–Traunsee Fault
southeast of the Hoher Göll.

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Fig. 5.

 Microfabrics of the Adnet and Oberalm Limestones. a) Formation of nodules by pressure solution and shearing within semiductile

shear zones. The nodules are internally undeformed. Note aligment and sigmoidal shape of filaments (shells) within shear zones. Adnet
Limestone. Scale bar: 40 

mm. b) Shear bands and pressure solution seams play an important role within shear zones. Parallel orientation

of shells and flattening of biogenic clasts within shear zones. Adnet Limestone. Scale bar: 40 

mm. c) Dynamic recrystallization of calcite

within small shear zones. Adnet Limestone. Scale bar: 5 

mm. Note that new grains are twinned. d) Broken quartz grain. Nucleation re-

crystallization of calcite within the veins of the quartz grain. Cataclastic Oberalm Limestone; scale bar: 40 

mm. e) Ductile shear zone with

twinned calcite grains and recrystallized grains is cut by a calcite vein, which is offset by a stylolitic cleavage. Oberalm Limestone. Scale
bar: 10 

mm. f) Ductile shear zone with dynamic recrystallization by subgrain rotation. Old calcite grains are strongly flattened and paral-

lel-oriented. At the boundary between the shear zone and the undeformed pelagic limestone cataclasis (arrows) occurs. The ductile shear
zone is cut by a calcite vein. Oberalm Limestone. Scale bar: 40 

mm. g) Calcite grains showing grain boundary migration, subgrain forma-

tion, twinning and undulose extinction. Oberalm Limestone. Scale bar: 10 

mm. h) Dynamic recrystallization of calcite. Old grains are

twinned and new grains show a polygonal grain shape. Oberalm Limestone. Scale bar: 5 


clasts and calcite grains; and (ii) dynamic recrystallization of
coarser calcite grains within narrow zones (Fig. 5c). By this
deformation the compact Adnet limestones disintegrate into
nodules, resulting in the nodular texture (Fig. 5a, b). The oc-
currence of nodule-rich and competent layers within the Ad-
net limestones appears to be solely a result of strain parti-
tioning during strata-parallel simple shear deformation (Fig.

Shells of lamellibranchiata are oriented parallel to the foli-

ation within shear zones (Fig. 5a, b) and became thinned,
disintegrated and elongated at the same time due to ductile
deformation. They are also partly dissolved by pressure solu-
tion. Calcite grains are often twinned, with up to three twin
sets per grain, strongly elongated, and bounded by pressure
solution seams. Subgrains are common along edges of calcite
grains. Shear bands can often be observed within shear
zones, sometimes grading into S-C fabrics.

Shear zones in the Oberalm Limestone show cataclastic to

ductile deformation. Quartz grains are broken; resulting gaps
are filled with calcite by nucleation recrystallization (Fig. 5d).
Relic calcite grains are strongly elongated (Fig. 5e), indicating
intracrystalline gliding as one principal deformation mecha-
nism. Within small ductile shear zones calcite grains show de-
creasing length-width ratios towards the centre of the shear
zone (Fig. 5f). Twin lamellae are often bent (Fig. 5e). Shear
zone boundaries to undeformed country rocks are very sharp
(Fig. 5e, f). The contact zone between ductile shear zones and
country rocks shows cataclastic deformation and pressure so-
lution (Fig. 5f). S-C fabrics developed in the interiors of the
shear zones. Dynamically recrystallized calcite grains bear
serrated and lobate grain boundaries (Fig. 5g) and parallel ori-
ented grain shapes (Fig. 5f). Grain boundary migration, sub-
grain formation and undulose extinction together with twin-
ning indicate further important deformation mechanisms
(Fig. 5g). Dynamic recrystallization of calcite (Fig. 5h) and
cataclasis are responsible for grain size reduction.

Narrow (centimetre to metre thick), semiductile shear zones

with shear bands, foliations and stylolites occur in the Hallstatt
Limestone of the Hallein Unit and along the Königssee–Lam-
mertal–Traunsee fault (Fig. 1), too. In areas between shear
bands strongly elongated calcite grains were dynamically re-
crystallized. Insoluble clay minerals are concentrated along
pressure solution seams.

Displacements of calcite veins along shear zones, elonga-

tion of calcite grains, shear bands and stylolitic foliation can

be found in marls and limestones of the Strubberg Forma-
tion. Grain boundaries of calcite are serrated due to pressure
solution. Quartz grains show undulose extinction, partly ser-
rated to lobate grain boundaries and the formation of sub-
grains. But the temperature reached was obviously too low
to permit dynamic recrystallization of quartz. On the other
hand calcite displays widespread dynamic recrystallization.
Mylonitic Strubberg marls and limestones with extreme
grain size reduction, elongation of calcite grains, shear band
formation, folding, and the associated development of a axi-
al plane foliation occur within a 10–100 m wide shear zone
in the Lammer Valley (Fig. 6a). Depending on the primary
layering of differing grain sizes and mineral composition,
fine-grained and clay-rich layers preferentially formed duc-
tile mylonites, while coarse-grained, calcite-dominated
lithologies display cataclastic fabrics (Fig. 6b). S-C fabrics,
shear bands and stylolites developed in cataclastically and
ductilely deformed layers (Fig. 6b, c). Partly because the
amount of finite strain was so high, only quartz grains sur-
vived as clasts in the mylonitic calcitic matrix (Fig. 6c).
Clastic quartz grains are undulose, sometimes corroded or
broken. Relictic calcite grains are strongly elongated and
their twin lamellae are bent. Cataclastically deformed calcite
grains, cut by shear bands, are surrounded by a mylonitic cal-
citic matrix (Fig. 6b). Calcite grains can display undulose ex-

Shear strain determination

Two methods of shear strain determination on the Adnet

limestones in the quarries of Adnet (Fig. 2) were used: a) an
absolute shear strain determination using the offset of a sedi-
mentary dyke along a ductile shear zone (Fig. 4b) according
to a method described by Ramsay (1980) and Ramsay & Hu-
ber (1983); and b) the analysis of two-dimensional strain
from the preferred orientation of lines, derived from bivalvia
shells, described by Panozzo (1984).

Absolute shear strain 


 within the ductile shear zone that off-

sets the neptunian dyke in the Adnet Formation was calculated
by the ratio between the offset distance and the thickness of
the shear zone. The strain measurement was used for high
strain Adnet Limestone. Absolute shear strain was calculated:


 = tan


 = offset/thickness  (of the shear zone).

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368                                                                               SCHWEIGL and NEUBAUER

The average absolute shear strain 


 resulted in a value of ca.

42. Minimum and maximum value, depending on the change
of the thickness of the shear zone along strike range between
22 and 268.

The Panozzo method (Panozzo 1984) is based on distribu-

tion of originally randomly distributed lines in a rock vol-
ume. Therefore, we made thin sections parallel to foliation
of ten Adnet Limestone samples containing shear zones
from one quarry of Adnet (Salzburg). The thicknesses of the
shear zones range between some millimetres and one centi-
metre. Then we digitized the bivalvia shells within and out-
side the semiductile shear zones with the help of the Panoz-
zo program created by R. Ott (see Ratschbacher et al. 1994).
The shear strain was calculated by:



 = a / b (long and short axis of strain ellipse).

Average shear strain measured by the orientation of shells

from twelve analysed shear zones of the thin sections resulted
in a value of  4. The shear strain value ranged between 2 and
10.  Average shear strain of the irregularly oriented shells out-
side the shear zone within nearly undeformed Adnet lime-
stones resulted in a shear strain value of  1.4.

The shear strain values of the Panozzo method are much

lower than the values of ductile shear zones which offset the
neptunian dykes because within the semiductile shear zones
strain is partially stored in twinning, in shear bands and by
pressure solution that is not measured by the Panozzo method.
The shear strain determined by the Panozzo method yields an
average strain value for the semiductilely deformed limestone.
The strain analysis support the idea of strain partitioning and
presence of high strain zones within the Adnet Limestone.


From the above presented structural data it appears that all

four studied pelagic limestone formations partly include a
domainal protomylonitic fabric. In ductile shear zones the
Hallstatt, Adnet, Strubberg and Oberalm limestones show a
clearly developed planar fabric and parallel arrangement of
minerals and biogenic components, e.g. shells.

In shear zones the pelagic limestones show clear indica-

tions of both ductile and cataclastic deformation. Cataclasis
is characterized by fracturing of the minerals like calcite and
quartz and by frictional slip within the Hallstatt, Strubberg
and Oberalm limestones. The grain size reduction is
achieved by cracking and fracturing of minerals and compo-
nents, e.g. fossils. Fracturing in calcite and quartz grains
ceased with grain size reduction. Minerals and biogenic
components mostly lack preferred orientation.

The minerals and fossils are strongly elongated and cor-

roded by pressure solution. Pressure solution played an im-
portant role during ductile deformation processes in pelagic
limestones, especially at the formation of nodular lime-
stones. There is ample evidence in naturally deformed rocks
that pressure solution is a common deformation mechanism
at relatively low temperatures (e.g. Alvarez et al. 1978;
Schmid 1982). The amount of pressure solution was not de-

Fig. 6.

 Microfabrics in marls and limestones of the Strubberg For-

mation. a) Strubberg Marl with slaty cleavage, with a kink fold
and second axial plane foliation that is a disjunctive cleavage. My-
lonitic Strubberg Marl; scale bar: 40 

mm. b) Brittle to ductile de-

formation in a shear zone. Calcite grains are broken, undulose, cut
by shear bands and show cataclastic fissures. Ductilely deformed
layers consist of fine-grained calcite and clay minerals. Protolith
was a turbiditic sequence within the Strubberg Marl. Scale bar: 10
mm. c) Shear bands and foliation (s) appearing as a zonal crenula-
tion cleavage. Quartz clasts are flattened, corroded and sometimes
rotated. Initial stage for 

s- or d-clast formation with recrystalliza-

tion tails is achieved. Strubberg Marl; scale bar: 10 


terminable because solution and redeposition are mostly very
distant in space.

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                             SEMIDUCTILE DEFORMATION IN PELAGIC LIMESTONES AT DIAGENETIC CONDITIONS                         369

Oblique stylolites, asymmetric pressure shadows of 



clasts, shear bands and S-C fabrics were used as kinematic
indicators. Those prove a dominant strata-parallel simple
shear deformation with top-to-the NNE displacement of
hanging wall units during semiductile deformation (Schwei-
gl & Neubauer 1997).

The calcite recrystallized dynamically; the quartz shows

undulose extinction and subgrains developed. The grain
size reduction was achieved by dynamic recrystallization.
Two types of dynamic recrystallisation can occur within
these ductile shear zones: the more common one is dynamic
recrystallization by subgrain rotation, called rotation recrys-
tallization (Guillope & Poirier 1979), mainly concentrated
in grain boundary regions (Fig. 5h). The second one is nu-
cleation recrystallization (Guillope & Poirier 1979), caused
by a nucleation and growth mechanism and concentrated in
relatively undeformed domains liked calcite veins (Fig. 5g).
This mechanism produced smaller grains than rotation re-

A study by Olsson (1974) on various calcite rocks of dif-

ferent grain sizes showed that in the temperature range of


C yield stresses increase with decreasing grain size.

Not only are stress intensity and temperature responsible for
the kind of deformation seen in pelagic limestones, but also
the clay content, fluid content and grain size of the pelagic
limestones. In experiments on calcite rocks Schmid (1982)
and Schmid et al. (1987) demonstrated that in addition to the
strain rate, temperature and differential stress, the grain size
is a parameter of significant importance for the flow law con-
ditions. The work hardening effect of a small grain size is
due the fact that grain boundaries are obstacles to free propa-
gation of slip and twinning (Schmid 1982). At high stresses a
larger grain size leads to lower strength. The dominance of
strata-parallel semiductile shear zones within fine-grained
limestones, intercalated within coarse-grained shallow water
limestone and dolomite, leads to the conclusion that the fine
grain size represents the governing factor of shear zone for-

Also the primary thickness of individual layers has an im-

portant influence whether ductile or brittle deformation dom-
inates. Due to strain partitioning different deformation mech-
anisms occurred under the same stress conditions in the same

A change from pressure solution mechanism into diffu-

sional creep or towards intracrystalline plasticity for fine-
grained calcite rocks is only achieved by differential stresses
in excess of 100 MPa (Schmid 1982). This implies that the
pelagic limestones could have suffered such stresses.

All considered, pelagic limestones form major décolle-

ment levels and flats along which thrust displacement during
Cretaceous to Tertiary nappe stacking and transpression oc-
curred (Schweigl 1997), while massive shallow water lime-
stones and dolomites form the competent stiff interior of
nappes (Fig. 3). There are also other pelagic limestones of
the central NCA forming subordinate décollement levels,
e.g. the Early Cretaceous Schrambach marls.

Micritic pelagic limestones can suffer semiductile defor-

mation with protomylonitic fabrics under diagenetic to low

grade metamorphic conditions. Important deformation mech-
anisms like pressure solution, dynamic recrystallization of
calcite, twinning and grain boundary sliding of calcite within
pelagic limestones can also operate at these low temperature
conditions: old calcite grains of about 50 


m grain size re-

crystallize in grains of some 


m. Creep by grain boundary

sliding leads to a superplastic behaviour (see also Schmid et
al. 1987), that implies extreme ductility in the extension of
materials. Quartz in such an environment only shows undu-
lose extinction, some minor elongation of grains and devel-
opment of subgrains. The primary bedding, the grain size and
the clay contents of the pelagic limestones are responsible
for strain concentration and resulting ductile or cataclastic
deformation. In the same shear zone, cataclastic and ductile
deformation can occur simultaneously depending on grain
size and original mineral composition of the individual layer.
At extreme mylonitization only quartz survives and is some-
times rotated, but real porphyroclasts, as defined by Passchi-
er & Simpson (1986), with recrystallization of porphyroclas-
tic material within pressure shadows, were not found. The
contacts between shear zones and surrounding, undeformed
limestones are always very sharp.

In summary the following deformation mechanisms acted

under low temperature conditions within the shear zones of
the pelagic limestones: i) cataclastic flow, ii) intracrystalline
deformation like dislocation glide and twinning, iii) glide
and climb (power law creep) and iv) pressure solution. A fi-
nal conclusion is that ramp-flat geometries of imbricated
passive continental margin sequences that mostly contain
carbonates are largely controlled by the presence of pelagic


Johann Genser improved the English

of the paper. Gerhard Amann and Hans Steyrer helped with
computer facilites and drawings. We acknowledge the com-
ments of Stefan Schmid (Basel) on an earlier version of the
manunscript, and helpful suggestions by the journal review-
ers Alex Maltman (Aberystwyth) and Milan Mišík (Bratisla-
va). The studies have been supported by the Grant P9918-
GEO of the Austrian Research Foundation.


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