FOLDING AND FRACTURING: EXAMPLES FROM RHENOHERCYNIAN ZONE AND HELLENIDES 153
GEOLOGICA CARPATHICA, 54, 3, BRATISLAVA, JUNE 2003
RELATIONSHIPS BETWEEN FOLDING AND FRACTURING
IN OROGENIC BELTS: EXAMPLES FROM THE RHENOHERCYNIAN
ZONE (GERMANY) AND THE EXTERNAL HELLENIDES (GREECE)
SOTIRIOS KOKKALAS, PARASKEVAS XYPOLIAS, IOANNIS K. KOUKOUVELAS*,
and THEODOR DOUTSOS
Department of Geology, University of Patras, 26500 Patras, Greece; *email@example.com
(Manuscript received April 30, 2002; accepted in revised form December 12, 2002)
Abstract: The kinematic similarities of small-scale structures in the Rhenohercynian Zone and the external zones of the
Hellenides are illustrated. Both orogenic domains comprise asymmetric folds verging toward the foreland and are af-
fected by extensional joints (ac and bc) associated with hybrid joints as well as with shear fractures (hk0
Joints were formed during layer bending which took place throughout the fold evolution and were symmetrically ar-
ranged with respect to principal stresses
. Although the magnitude of the stress axes changed during folding,
their orientation with respect to layering remained constant: one of them is orientated perpendicular to the layering,
whereas the other two are parallel and perpendicular to the dip direction of the layering respectively. As expressed by ac
and hybrid joints the extension normal to the dip direction of layering progressively increased during fold tightening.
Layer parallel shear responsible for the asymmetry of the folds caused joint rotation toward the fold hinge within the long
limb. Furthermore, these movements controlled at both limbs the formation of bc and associated hybrid joints during the
Key words: Rhenohercynian, Hellenides, folding, extensional joints, hybrid joints, shear fractures.
Complexities in the analysis of small-scale structures in fold-
and-thrust belts are produced due to the coexistence of exten-
sional, compressional and wrench structures, each one close
to the other. This coexistence appears to give conflicting in-
formation regarding the stress system during folding (see also
Price & Cosgrove 1990).
Small-scale structures have long been used to delineate the
temporal progression of deformation. We use the term small-
scale structures to cover small faults, small folds, stylolites,
cleavage, shear-zones, veins and joints. From these small-
scale structures, the fractures that include joints and small
faults are the most important for understanding fluid circula-
tion and mineral precipitation within rocks and have been an-
alyzed in this contribution. Adopting the nomenclature of
Dennis (1972), Hancock (1985) and Price & Cosgrove
(1990), fractures that range in size from centimeters to tens of
meters can be divided into three groups (Fig. 1): (a) Dilation
fractures or Mode I cracks characterized by open space with
or without mineral precipitation. (b) Shear fractures bearing
horizontal slickenlines and (c) oblique extension fractures or
hybrids characterized by oblique slip as well as fractures of
intermediate type. The attitude of fractures related to folds are
described with respect to three orthogonal reference axes in
three different ways: (a) The layer-control system (Price 1967;
Stearns 1967), in which the three reference axes are oriented
parallel to layer strike, layer dip and normal to layering. (b)
The layer- and the fold-control system as introduced by Price
(1967) and Hancock (1985), in which the reference axes are
parallel to the fold axis, parallel to the layering and normal to
the fold axis, and normal to layering (Fig. 1A). (c) The fold-
control system (Hobbs et al. 1976), in which the axes are par-
allel to the fold axis, parallel to the axial surface and normal to
the fold axis, and normal to the axial surface. Note that most
of these classifications were used to describe fractures related
to symmetrical folds.
Natural examples and mechanical analyses in fold-and-
thrust belts have documented that fold asymmetry due to sim-
ple shear is common during fold development (Sanderson
1979). In addition flexural-slip mechanism is important
through fold evolution and accompanied by slip along com-
petent units (Chester et al. 1991; Gray & Mitra 1993; Tanner
1989; Ohlmacher & Aydin 1995, 1997; Cooke & Pollard
1997; Couples et al. 1998). Competent units are defined by
surfaces that host slip (see also Couples et al. 1998) and can
include one thick and massive bed or a series of beds showing
similar mechanical behavior.
Studies relating folding to stages of fracturing indicate that
jointing appears to develop under the influence of fluid pres-
sure and temperature (i.e., Skarmenta & Price 1984; Srivasta-
va & Engelder 1990; Gray & Mitra 1993).
In this paper, we present data pertaining to the orientation
and kinematics of fractures developed during folding by tan-
gential longitudinal strain. In this process extension or stretch-
ing acts all along the outer arc of the folds tangentially, while
compression acts along the inner arc. It is shown that the frac-
ture network in short and long limbs of the asymmetric folds
can be divided into bending related fracture assemblages
which were strongly affected by an imposed shear. Shear
154 KOKKALAS, XYPOLIAS, KOUKOUVELAS and DOUTSOS
strain in fold-and-thrust belts can be generated during the
overthrust of the internal parts of the orogen onto the under-
plated external parts (i.e., Gray & Mitra 1993) along crustal
scale shear zones. Two regions, the Rhenohercynian Zone of
the Variscan orogenic belt (Germany) (Figs. 2, 3 and 4) and
the external Hellenides of the Alpine orogenic belt (Greece)
(Figs. 5, 6 and 7) are studied.
The analysed stations were selected where the enveloping
surfaces of folds can be recognized and this enables reliable
identification of the short and long limbs of folds. Then orien-
tation data were collected in the stations using the selection
method (Davis 1984), wherein sets of fractures are visually
determined and three to seven orientations are measured on
each fold limb; 25 measurements were the minimum for a sta-
tion, unless the exposure was limited. The selection method is
commonly used in study of fractures (i.e., Hancock 1985; En-
gelder & Geiser 1980; Dunne 1986). The stations were classi-
fied into domains by limb dip (030
overturned) (Dunne 1986). The second step of data collection
includes mesoscopic descriptions of the small-scale structures,
the dihedral angles between sets of conjugate fractures, the
overprinting relationships between fractures, the types and at-
titude of vein fills, the attitudes of stylolites, evidence for dis-
placement along joint surfaces, the amount of movement along
slip surface and the direction of slickensides. In the final step
of data collection, thickness of beds, slickensides of bedding
planes, small-scale folds, duplexes developed on the bedding
planes, and attitude of normal and thrust faults which cut one
or more beds or mechanical units were measured. The collect-
ed data were plotted on four equal area nets following the limb
dip classification into four domains, three of them for the long
limb and one for the short limb. For the consideration of frac-
ture distribution in the equal area nets we follow the methodol-
ogy by Dunne & Hancock (1994). Analytically, we recognize
the occurrence of single sets of fractures and sets of fractures
displaying dispersion about the mean orientation on these ste-
reonets. As a single set we recognize a maximum with disper-
sion less than 10
, while maxima reflecting the presence of
two sets display dispersion of >10
. Finally, when the 2
gles between the sets range between 10 to 50
corresponds to hybrid joints, while 2
angles ranging between
correspond to shear fractures.
The case of the Rhenohercynian Zone
The geology of the Rhenohercynian Zone
The Rhenohercynian Zone represents a Devonian to Lower
Carboniferous passive continental margin accumulating 3
12 km thick marine clastics and carbonates (Meyer & Stets
1980). The southern parts of this margin comprise telescoped
slope and rise sequences bordering a small oceanic basin be-
tween the Rhenohercynian and Saxothuringian Zones (Fig. 2)
(Dittmar & Oncken 1992). Contractional movements started
during the earliest Late Devonian, resulted in a SE-directed
subduction causing the consumption of the intervening ocean-
ic basin as indicated by the onset of flysch sedimentation and
the evolution of a magmatic arc in the south on the Mid-Ger-
man crystalline rise (Plesch & Oncken 1999). The orogenic
front propagated from south to the north from ca. 325 Ma to
300 Ma, causing the formation of NW-verging map scale syn-
clines and anticlines (Wunderlich 1964; Ahrendt et al. 1983).
Major thrusts, producing antiformal stack-type structures at
depth are rooted in a crustal scale decollement at a depth of
1316 km (Oncken 1998). Internal deformation within the
Rhenohercynian Zone is controlled by the flexural-slip mecha-
nism (Fig. 4) associated with frictional sliding and pressure so-
Fig. 1. Relationships of (A) dilational and (B) shear fractures with a
fold. Fabric axes following the layer- and the fold-control system.
(C) Ideal relationships of ac, bc, hk0
fractures to a stereo-
net. Legend for explanation of symbols in the ideal stereonet and to
show nomenclature after Stearns (1967) and Hancock (1985), for
more explanations see text.
FOLDING AND FRACTURING: EXAMPLES FROM RHENOHERCYNIAN ZONE AND HELLENIDES 155
Fig. 2. Simplified geological map of the Rhenohercynian Zone showing major thrusts and transport direction (after Meyer & Stets 1980) and
interpretative block diagram of the main Variscan structures in Central Europe (modified, after Martin & Franke 1985).
lution (Cloos & Martin 1932; Breddin 1956; Plessmann
1966). Pressure-temperature metamorphic conditions are
higher to the south (estimated P»6 kbar and T»350
(Meisl 1990) and decrease toward the north (Fig. 2). Meta-
morphism is estimated at the anchizone conditions below the
Siegen Thrust (Meyer et al. 1986).
Long limb deformation
In order to refine symmetrical relationships between frac-
tures and bedding orientation we summarize fracture data col-
lected in places where the bedding attitude is nearly horizon-
tal, moderately and steeply dipping.
Common fractures affecting the nearly horizontal long
limbs are represented by a set of NE-trending and NW- or SE-
steeply dipping joints that are classified as bc joints (Fig. 3,
nets A1 and A2). Irregular fracture planes, and typical absence
of slickensides as well as little or no offset along the bedding
plane characterize these joints. However, in case the attitude
of the fractures is more than 10
of the mean orientation, the
bc joints display oblique slickensides. Thus we interpret joints
with attitude dispersion of less than 10
as tension or Mode I
cracks, while fractures with a large dispersion in the NW- and
SE-quadrant (Fig. 3, nets A1 and A2) are regarded as hybrid
bc characterized by oblique extension. The opening of joints
ranges between 0.51.5 cm and in some cases they have
quartz filling. A NW-trending joint set dipping steeply either
to the NE or to the SW is typical for almost all horizontal long
limbs, those sets following the layers are classified as ac
joints. The kinematics as well as the attitude of this joint set is
similar to the bc joints showing a scatter over the sectors of the
equal area net (Fig. 3, nets A1 and A2) and thus this set in-
cludes tension, as well as hybrid fractures which are character-
ized by oblique extension.
The fracture network affecting the moderately inclined long
limbs is similar to the networks appearing on the nearly hori-
zontal limbs. Sets of fractures recognized on these limbs are
classified as ac, bc and hybrids of these two sets. However, on
these limbs two additional conjugate fracture systems were
recognized, which form an acute angle about the a and b refer-
ence axes (Figs. 1B, 3; nets B1 and B2), respectively. Mesos-
copic structural analysis of these fractures indicates that most
of them are characterized by horizontal slickensides and thus
they are classified as shear fractures. Additional extension in
these limbs is represented by planar NW-facing normal faults
(Fig. 3, net B2: N
) displaying small dip-slip displace-
ments. The sense of slip is defined by the displacement of
marker beds in the schist. Moreover, a series of en-echelon
planar or sigmoidal veins showing oblique slip kinematics are
developed, in respect with these normal faults, which are clas-
sified as oblique extensional fractures. Offset on these oblique
slip faults range from a few centimeters to 5 m and their
lengths may exceed 50 m with the possibility of much longer
dimensions along strike in the Altenburg-Altenahr area
(Fig. 2, Measurement stations 1 to 4).
Steeply dipping limbs are affected by all previous described
fractures with the hk0
shear fractures as the most prominent
(Fig. 3, net C1 and C2). Another joint maximum at the SW
quadrant of the equal area net C1 in the Fig. 3 cannot be clas-
sified according to the layer- and the fold-controlled system
and may be attributed to post orogenic movements of the area.
Shortening is controlled by SE-dipping reverse faults (Fig. 3,
net C2: T
and Fig. 4A and B) in similar orientation with pre-
viously analysed normal faults.
156 KOKKALAS, XYPOLIAS, KOUKOUVELAS and DOUTSOS
The distribution of shear fractures belonging to hk0
forms an acute angle with the bedding reaching up to 30
(Fig. 3, nets B1, B2 and C1, C2). This can be explained by
forward rotation of formed fractures during layer parallel slip
Short limb deformation
The short limb was affected by ac, hk0
(Fig. 3, nets D1, D2, E1 and E2). Bedding perpendicular veins
allied with hk0 shear fractures are common and are often dis-
placed by bedding parallel shear (Fig. 4C). In the Taunus at
the southern parts of the Rhenohercynian Zone, where the
short limbs belong to upright isoclinal fold, hk0
(Fig. 3, E2, Doutsos & Prufert 1986). Limb shortening is ex-
pressed by wedge folds (sensu Cloos 1961), layer parallel
slip, top-to-NW small scale thrust faults, continuous and dis-
junctive cleavage and kink bands.
Thrusts cutting up from the base to the top of competent
sandstone layers illustrate the early stage of ramp-related fold-
ing or wedge folding and imply thickening of layers where
layers were sub-horizontal. Subsequently bedding was rapidly
steepened to a nearly vertical position and then the short limb
extended by gently SE-dipping and NE-trending small-scale
thrust faults that cut the beds (Fig. 3, net D2: N
These minor thrusts, which were formed after the steepening
of the short limb, essentially accommodate distributed shear
away from the major thrust planes.
The case of the external Hellenides
The geology of the external Hellenides
The Apulia platform in the external Hellenides represents a
Mesozoic passive continental margin comprising a 24 km
thick sequence of calcareous rocks, evaporites and cherts
(Bernoulli & Laubscher 1972) (Fig. 5). During the Late Juras-
sic, this margin was subdivided by rifting into two shallow-
water platform areas namely the Tripolitsa and the pre-Apulia
Zones, which were separated by the deep-water Ionian Basin
(Auboin 1959; Karakitsios 1995). To the east the Apulian
margin was thinned and passed gradually into a thin pelagic
sequence of cherty limestones and radiolarites known as the
Fig. 3. Equal area lower hemisphere projections illustrating clustering of mesofracture in the Rhenohercynian Zone. Nets (A1) to (C1)
correspond to long limbs, and nets (D1) and (E1) correspond to short limbs. Data in equal area net (E1) comes from the Taunus area
(modified, after Doutsos & Prufert 1986). Equal area nets (A2) and (E2) show labelling of clusters in respect to the mean bedding plane
(B) and following Hancocks (1985) classification. For details see text.
FOLDING AND FRACTURING: EXAMPLES FROM RHENOHERCYNIAN ZONE AND HELLENIDES 157
Fig. 4. The asymmetrical Schuld fold at Altenahr-Kreuzberg location, first described by
H. Cloos, in Lower Devonian schists. (A) General view, photo was taken looking SE. (B)
Detail of the long limb (the right part of the photo A) deformed by normal faults, compass
for scale. (C) Detail of the short limb to the left of the monument, the photo shows layer
slip expressed by the offset of a quartz vein, pencil for scale.
Fig. 5. Simplified geological map and cross-section (modified, after Xypolias & Dout-
sos 2000) of the external Hellenides showing isopic zones, major thrusts, and analysed
cross-sections. Inset shows the location of the study area in the Greek Peninsula.
Pindos zone which bordered the Pindos
Ocean (Smith et al. 1979; Degnan & Rob-
ertson 1998) (Fig. 5). In the Late Eocene
plate convergence commenced and the Pin-
dos Ocean subducted eastwards below the
Pelagonian microcontinent (Fig. 5) (Tem-
ple 1968; Pe-Piper & Koukouvelas 1992;
Doutsos et al. 1993, 2000; Xypolias &
Koukouvelas 2001). The Pindos Zone was
separated from its basement and was high-
ly deformed into a series of imbricate
thrust sheets that were finally emplaced
over the Tripolitsa Zone to the west (Xypo-
lias & Doutsos 2000). As a result the
Apulian margin was thickened, uplifted
and subdivided into synorogenic flysch ba-
sins, which were formed during the south-
westward propagation of thrusting and
folding until the Middle Miocene time
(Richter 1978). Internal deformation in the
Apulian platform occurred under non-
metamorphic conditions at burial depths
not greater than 5 km (Kisch 1981). This
deformation was predominantly achieved
by pressure solution and fracturing (Xypo-
lias & Doutsos 2000) (Fig. 7).
Long limb deformation
In the initial stages of folding, fold long-
limbs have dips between 530
and are af-
fected by bc- and ac-joints as well as hk0
joints (Fig. 6, net A1 and A2). The cluster
in the north sector of the net corresponds to
a complex double cluster including ac joint
and hybrid ac joints and hk0
most of the clusters appeared in the net A1
display dispersion more than 10
mean orientation and thus we infer that hy-
brid fractures are widespread during this
In many places stylolites perpendicular
to the bedding plane indicate contraction of
bedding parallel to the dip direction. In
other places, and especially where the fold
limbs dip at 2030
, stylolites are widened
to become bed-normal calcite-filled veins
(Fig. 7a). In addition stratal extension and
limb thinning are accommodated by nor-
mal faults. Normal faults form a conjugate
NNW-trending set with dominant ENE-
dipping faults (Fig. 6, net A2: N
Those faults have small displacements
(1 cm to 1 m), and most of these are con-
fined in the competent units. Normal faults,
duplexes and small folds in weak beds sug-
gest accommodation to the westward
nappe transportation and therefore are clas-
158 KOKKALAS, XYPOLIAS, KOUKOUVELAS and DOUTSOS
sified as transport related small-scale
With progressive folding and limb rota-
tion, limbs dip range between 3055
an additional conjugate system of hk0
shear fractures can be observed (Fig. 6,
nets B1 and B2). Earlier stylolites and cal-
cite veins within the limestone beds are
curved as they approach their neighbour-
ing strong interlayers or are offset by bed-
parallel movements. Bed parallel offset
range from few mm to more than 30 cm,
and their curvature as they approach the
bedding plane indicates a systematic top-
to-the-west sense of shear. Limb shorten-
ing is commonly expressed by east dip-
ping reverse faults (Fig. 6, nets B1 and B2
and Fig. 7c). SW-dipping back-thrusts
displace limbs, which dip more than 50
to the hinterland (Fig. 6, net B2: T
During the final stages of limb rotation,
where limbs dip range between 6085
ac joints become more pronounced, while
shear fractures appear to be less
prominent. Tightened folds are character-
Fig. 6. Equal area lower hemisphere projections of structural data from representative stations in external Hellenides. Nets (A1) to (C1) il-
lustrate clustering of fractures in long limbs with respect to sedimentary layering and fold hinge lines, nets (D1) and (E1) illustrate clustering
of fractures in short limbs from representative stations and the Klokova Anticline respectively. The equal area nets (A2) to (E2) show label-
ling of fracture clusters to long and short fold limbs, nomenclature as in Fig. 3.
Fig. 7. Key structural observations in the Pindos Zone. (a) Stylolites perpendicular to the
bedding widened to calcite veins, lens cap for scale. (b) A lift-off fold affected by east dip-
ping out-of sequence thrusts, field of view is 20 m. (c) A series of east dipping small scale
thrust faults affecting a steeply dipping long limb, hammer for scale. (d) Sub-vertical sty-
lolitic cleavage affecting the inverted fold limb, coin for scale. Photos (a) to (c) were taken
looking north while photo (d) was taken looking south.
FOLDING AND FRACTURING: EXAMPLES FROM RHENOHERCYNIAN ZONE AND HELLENIDES 159
ized by a close chevron core, which broadens up-section in an
open conjugate box fold (Fig. 7b) forming box lift-off folds
(sensu Mitra & Namson 1989). Local stylolitic cleavage of
this structural stage forms a small angle or is parallel to the
bedding planes (Fig. 7d).
The clusters of shear fracture poles are not perpendicular to
the bedding showing similar behaviour to the already de-
scribed examples of the Rhenohercynian Zone (Fig. 3 nets B2
and C2), which suggest joint rotations due to layer parallel
Short limb deformation
Deformation associated with the progressive rotation of
short limbs is characterized by the following joint-pattern: (a)
ac-joints and hk0
shear fractures, typical for the long limbs
and (b) a new conjugate system of 0kl
shear fractures (Fig. 6,
nets D1, D2, E1, E2). The latter system of fractures exhibits
multiple sets of slickenlines with the oldest slickenlines show-
ing oblique-slip overprinted by dip-slip extensional move-
ments. Well-developed normal faults in the Skolis and Kloko-
va Anticlines show similar attitude with 0kl
and exhibit offsets in the range from centimetres to hundreds
of meters (Fig. 6, net E2: N
During the initial stages of fold limb rotation, layer-parallel
extension is expressed by a set of conjugate normal faults
(Fig. 6, net D1 and D2: N
), which have planar geome-
tries and progressively become inactive. During the limb rota-
tion west-dipping normal faults (Fig. 6, net E2: N
into a thrust position (Fig. 6, net E2: T
). In addition m-scale
displacements, brecciation and cm-scale arrays of sigmoid en
echelon fractures characterize these faults. The kinematics on
these en echelon faults suggests that these synthetic faults are
hybrid shear fractures. Steepened to the near vertical position
the fold limbs are affected by a well-preserved stylolitic cleav-
age. This newly formed cleavage dips about 60
northeast and transects the bed parallel cleavage, forming a
lozenge shaped stylolitic pattern (Fig. 7d).
Our geometrickinematic analysis reveals that folds in the
two fold-and-thrust belts comprise a similar fracture pattern of
ac, bc, hybrid, hk0
Stress distribution during fracturing
At the initial stages of folding (Fig. 3, net A2 and Fig. 6, net
A2) an orthogonal set of joints (ac and bc) and hybrid ac-, bc-
joints formed in the long limb, involving approximately syn-
chronous horizontal extension in two directions: parallel and
perpendicular to the fold axis (Fig. 8a). In order to interpret
this orthogonal set of joints in terms of stress orientation we
adopted Hancocks (1985) proposal where: the direction of
maximum principal stress
remains parallel to the intersec-
tion direction between the sets, the intermediate
principal stresses lie on the joint planes and per-
pendicular to the
principal stress axis (Fig. 8a). This mech-
anism is possible when rapid 90
switching of roughly equal
For the stress interpretation of shear fractures we used the
Navier-Coulomb criterion of failure where the orientation of
bisects the acute angle formed by the shear fractures. Thus,
for the hk0
axis is parallel to the dip direction of bed-
is perpendicular to the bedding and the
allel to the fold axis (Fig. 8b). The common occurrence hk0
joints within the same lithological horizon, as well
as the formation of these joints in every stage of folding (Figs.
3, 6 and 8) lead us to assume that
Fig. 8. Summary model showing progressive deformation and associ-
ated fractures, the folds are markedly asymmetrical. Fracture label-
ling in respect to sedimentary layering and fold hinge lines. From top
to bottom the first three models correspond to long limbs while the
fourth represents the deformation in the short limb.
160 KOKKALAS, XYPOLIAS, KOUKOUVELAS and DOUTSOS
within them are symmetrically arranged about the dip of the
layers (Figs. 3 and 6, nets D2 and E2).
Furthermore, our analysis indicates local stress distribution
within folds that in general differs strongly from the regional
stress field controlled by the collision of plates (Zoback & Zo-
back 1991). Each fracture set studied in this work marks the
orientation of a local stress field during the progressive devel-
opment of fold bend, with an interchange of the principal
1. Data from detailed structural analysis of road cuts sug-
gest that the fracture network in the two fold-and-thrust belts
show strong similarities. Common fractures are classified as
ac- bc-joints and their hybrids and hk0
. Once structures
were formed they appear to be continuously active and new
fractures of the same attitude were formed during the later
stages of deformation.
2. Layer parallel shear and the separation of the succession
into different mechanical units is an important mechanism for
understanding dispersion of fracture poles over stereonet sec-
tors. In our work the effect of the layer-parallel shear resulted
in progressive increase of the angle between the hk0
shear fractures and the bc great circle.
3. Although stress orientation remains constant with respect
to the layer, rapid 90
switching of the stress axes are inferred
from the coexistence of ac-, bc-joints and hk0
4. Extension normal to the dip direction of layering increas-
es during fold tightening.
Acknowledgments: We acknowledge the support of Enter-
prise Oil Limited, Hellenic Petroleum, M.O.L. and ARCO,
Grants No. 18631, 18632. We are also grateful to Prof. W.
Meyer who provided helpful comments during 1985 field
trips in Rhenohercynian Externides and read parts of this
manuscript. Detailed reviews by Duan Plaienka, and two
anonymous referees considerably helped to improve the
manuscript and are gratefully acknowledged.
Ahrendt H., Clauer N., Hunziker J.C. & Weber K. 1983: Migration
of folding and metamorphism in the Rheinische Schieferge-
birge deduced from K-Ar and Rb-Sr Age determinations. In:
Martin H. & Eder F.W. (Eds.): Intracontinental Fold Belts.
Springer, Berlin, 323338.
Auboin J. 1959: Contribution a letude geologique de la Grece
septentionale: les confins de lEpire et dela Thessalie. Ann.
Geol. Pays Hellen. 10, 1483.
Bernoulli D. & Laubscher H. 1972: The palinspastic problem of
the Hellenides. Eclogae Geol. Helv. 65, 107118.
Breddin H. 1956: Die tektonische Deformation der Fossilien im
Rheinischen Schiefergebirgen. Z. Dtsch. Geol. Gesell. 106,
magnitudes through the fold evolution. The fluid pressure
within the rocks, which changed during folding, could be an
important factor controlling the switching of the
axes (see discussion by Price & Cosgrove 1990). The ac and
associated hybrid joints become abundant as folds amplified
(Fig. 3, 6, 8c and d) suggesting an increase of extension paral-
lel to the fold axis. This is supported by volumetric measure-
ments in slates in the southern part of the Rhenohercynian
Zone where the stretching rates parallel to the fold axis range
up to 15 % (Dittmar et al. 1994).
In the strongly deformed southern parts of the Rheno-
hercynian Zone in the Taunus where the metamorphism in-
creased reaching the greenschist facies (Meisl 1990) the domi-
nance of hk0
joints indicates strongly extension parallel to
the fold axis. Nevertheless it is important to note that the
above described stress distribution is restricted within the lay-
ers and can differ from the stress distribution affecting the
whole fold-and-thrust belt.
The role of layer parallel shear
Both fold-and-thrust belts of the Rhenohercynian Zone and
the external Hellenides have a similar tectonic position as they
were underplated beneath earlier oceanic domains, which
were destroyed in the central parts of both orogens. The main
compressive direction is oriented perpendicular to the struc-
tural grain of the belts and is inclined to the stratigraphic dis-
continuities resulting from the formation of asymmetric folds.
The long and short limbs of folds show some differences re-
garding the formation of fractures (Fig. 8).
The bc and associated hybrid joints are abundant within the
long limbs and scarce within the short limbs of the folds. This
can be explained by the different position of the two limbs in
the strain ellipsoid produced by the layer parallel shearing. As
the asymmetrical fold initiated, the long limb lay in the exten-
sional field of the strain ellipsoid and bc joints changed,
whereas the short limb lay in the contractional field of this el-
lipsoid and bc joint formation was hampered.
The prevalence of ac and hybrid joints in the short limbs in-
dicates that extension parallel to the fold axis is greater in the
short limbs. The presence of 0kl
joints on the short limbs of
the folds in the external Hellenides (Fig. 6, net D2 and E2)
support this conclusion.
Only in the first stages of folding (Fig. 3, net A1 and Fig. 6,
net A1) joints are symmetrically arranged about the dip of lay-
er and hence they can be named according to Hancocks ter-
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Although small joint rotations at the short limb are also ob-
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