GEOLOGICA CARPATHICA, OCTOBER 2006, 57, 5, 337—346
www.geologicacarpathica.sk
Three-directional extensional deformation and formation of
the Liassic rift basins in the Eastern Pontides (NE Turkey)
YENER EYÜBOƒLU
1
,
OSMAN BEKTA
1
, AYSEL EREN
3
, NAFIZ MADEN
3
, RAH AN ÖZER
2
and WOLFGANG R. JACOBY
4
1
Department of Geology, Gümü hane Engineering Faculty, Karadeniz Technical University, Gümü hane, Turkey; eyuboglu@ktu.edu.tr
2
Department of Geology, Karadeniz Technical University, 61080-Trabzon, Turkey; obektas@ktu.edu.tr
3
Department of Geophysics, Karadeniz Technical University, 61080-Trabzon, Turkey; seren@ktu.edu.tr
4
Institut für Geowissenschaften, Johannes Gutenberg-Universität Mainz, Saarstr. 21, D-55099 Mainz, Germany
(Manuscript received October 14, 2004; accepted in revised form October 6, 2005)
Abstract: The Eastern Pontide magmatic arc (NE Turkey) was rifted by the polyphase extensional tectonic regimes in the
Early Jurassic. While alternated volcanics and siliciclastic sedimentary rocks accumulated during the episodic tectonic
subsidences, thermal subsidence is manifested by sedimentation of the red pelagic limestones of the Ammonitico Rosso
during the Pliensbachian. The trends of the Liassic basins extending in NW-SE, E-W, NE-SW directions coincide with the
gravity and magnetic lineament anomalies and corresponding fault zones that are responsible for the paleotectonic and
neotectonic evolutions of the Eastern Pontide magmatic arc. These mutual relationships suggest that the faults making up
the architecture of the Liassic basins might have operated during the Paleozoic, Mesozoic and Cenozoic times in different
manners. Neptunian dikes, filled by the early rift siliciclastic and following fossiliferous red pelagic limestone implying
the repeated extensional tectonic regimes, are also parallel to the main extensions of the Liassic basins. The poles of the
contemporaneous neptunian dikes suggesting two or three extensional conjugate fracture systems are in accordance with
the dip directions of the rift sediments accumulated in the same conjugate normal fault systems. Assuming that the Liassic
basins with Ammonitico Rosso are coeval, multi-armed rift basins might have opened by the mode of the three-
directional extension rather than reactivation of faults in the different times.
Key words: Eastern Pontides, Liassic rift basins, extension, gravimetry, magnetics, sedimentary dikes.
Introduction
The Eastern Pontide orogenic belt extends along the
southeastern coast of the Black Sea and comprises three
subtectonic units from north to south: the northern (mag-
matic arc), southern and axial zones (back-arc) (Fig. 1).
Jurassic multiple extension within the Eastern Pontide
orogenic belt (NE Turkey) caused the formation of a
failed triple rift system trending in NW, E-W and NE di-
rections. First Liassic rifting of the magmatic arc is
characterized by the asymmetrical half-grabens and ac-
cumulation bimodal volcanics and associated coarse
clastics. However, during and subsequent to the first rift-
ing, these rift-related basins experienced short-lasting
thermal subsidence and so pelagic limestone of Ammo-
nitico Rosso deposited as post rift sediments. The second
rifting of the Jurassic began with second alternations of
the bimodal volcanics and epiclastics. A following, sec-
ond long-lasting thermal subsidence caused building up
of the Upper Jurassic-Lower Cretaceous carbonate plat-
form on the rift-related sediments. Though there are
many sedimentological studies on the Liassic rifting of
the Eastern Pontide magmatic arc (Yôlmaz et al. 1996 and
2006), tectonic aspects of the rift-related basins are lack-
ing. Therefore we intend to present some geological evi-
dence and the outline of the geophysical properties of
the Liassic basins to interpret the kinematic and dynamic
analysis of Liassic rifting.
In this article, we also present some evidence for three-
dimensional deformation of the Liassic rifting of the
Eastern Pontides by using the Liassic neptunian dikes
(Bekta & Çapkônoûlu 1997; Bekta et al. 2001), dip
analysis of the Liassic bedding and possibly fault-related
folding.
The main geological features of the Eastern Pontide
orogenic belt
The Eastern Pontides correspond to a part of the active
continental margin extending 600 km along the eastern
part of the Black Sea coast (Fig. 1). It remains debatable
whether the Eastern Pontides were the northern active con-
tinental margin of Gondwana during the Cretaceous (Dew-
ey et al. 1973; Bekta et al. 1984; Bekta 1986, 1987;
Chorowicz et al. 1998), or the southern active continental
margin of Eurasia during that time ( engör et al. 1981;
Adamia et al. 1981). According to Bekta et al. (1999), the
Eastern Pontide orogenic belt comprises three subtectonic
units, from north to south: the northern (magmatic arc)
zone, the southern zone and the axial zone (back-arc)
(Fig. 1). Bouger gravity, magnetic and residual gravity
338
EYÜBOƒLU, BEKTA , EREN, MADEN, ÖZER and JACOBY
anomalies (Fig. 8A,B,C) and geological map criteria imply
that polygonal networks of the extensional faults with
NW, E-W, NE directions are responsible for the formation
of the Mesozoic basins in the Eastern Pontides. The
blocks of the Hercynian basement in the southern and axi-
al zones such as Agvanis and Pulur metamorphic massifs
and Gümü hane-Köse granites are rhomboid or lozenge-
shaped in plan view and are framed by zig-zag shaped pa-
leofault systems (Fig. 1). The alignment of the Upper
Cretaceous calderas in the northern zone and of the Kop
peridotites in the axial zone, are also controlled by these
basement-involved fault systems. The fault framework of
NW, E-W, NE directions makes up the block-faulted tec-
tonic style of the Eastern Pontides. Block coupling along
these three weakness zones produced a diagnostic en ech-
elon arrangement of the orthogonal drag and drape folds
(Schlische 1995), possibly reflecting the trace of the base-
ment fault at the surface. The block fault framework
formed the Mesozoic basins in three main distinctive
weakness or tectonic zones on the Eastern Pontide mag-
matic arc evolved on the southward subduction zone of
the Paleotethys (Bekta et al. 1999). Neptunian dikes, ex-
posed near Gümü hane in the southern zone, were formed
by the polyphase rifting of the magmatic arc during the
Jurassic and Cenomanian (Fig. 2).
The first rifting is related to the break-up of the granitic
basement of the Hercynian whereas the later two were re-
lated to the break-up or drowning of the carbonate plat-
form of the Upper Jurassic—Lower Cretaceous (Fig. 2).
Sedimentological pattern of the Liassic rift basins
During the Liassic, the Eastern Pontides were rifted in
NW, E-W, NE directions (Figs. 1 and 3). These multi-direc-
tional rift basins are considered to be contemporaneous due
to the Ammonitico Rosso facies in each basin. The syn-rift
strata of the basins form asymmetric prism indicating depo-
sition in half-grabens (Fig. 4). The prisms contain volcano-
sedimentary clastics and pelagic sediments ranging from 0
to 1000 m thickness (Yôlmaz 2002). The vertical columns
of the rift sediments form two megacycles that thin and be-
come more fine-grained upward (Fig. 2). Each megacycle
ended with the accumulation of the red pelagic limestones
(Ammonitico Rosso); this is interpreted as reflecting the fill-
ing of a marine basin created by faulting. Asymmetry of the
sediment prism and facies changing within them indicate
the position and dip direction of the normal faults varying
in north, northwest, and northeast multi-directions indicat-
ing three-directional extensional deformation and forma-
tion of polygonal networks of extensional faults.
Field observation carried out in the Gümü hane (Fig. 3)
and Kelkit areas imply that subsidence of the Liassic ba-
sins occurred in three stages: (1) an initial phase of the
Fig. 1. Main tectonic features and tectonic zones of the Eastern Pontides. 1 – Paleozoic metamorphic basement, 2 – Paleozoic granites,
3 – serpentinite, 4 – undifferentiated Mesozoic and Cenozoic rocks, 5 – platform carbonates, 6 – mainly Mesozoic sedimentary rocks,
7 – Cretaceous and Eocene arc volcanics, 8 – Upper Cretaceous and Eocene arc granites, 9 – caldera or dome, 10 – orthogonal drape
and drag folds 11 – fault, 12 – thrust fault, 13 – normal fault, 14 – study area. NAF – North Anatolian Fault, NEAF – Northeast
Anatolian Fault.
339
EXTENSIONAL DEFORMATION AND FORMATION OF THE LIASSIC RIFT BASINS (NE TURKEY)
stretching or first tectonic subsidence gave rise to tilting
of the blocks and formation of the asymmetric basins. As
the basal coal-bearing sandstones accumulated in the
southern depositional environment of a short-lived swamp
area (Ravnas & Steel 1998) near Kelkit, a volcano-sedi-
mentary package accumulated in the northern narrow deep
troughs on the northward tilted block in the northern dep-
ositional area (Gümü hane). (2) A subsequent phase of the
gradual thermal subsidence, during which the deposition-
al basins expanded to bury the earlier border faults and so
progressively younger condensed sediments of Ammoniti-
co Rosso onlapped onto the basement and formed neptu-
nian dikes on the uplifted hanging wall of the normal
faults (Fig. 4). (3) Recurrent tectonic subsidence deepened
previous basins in which a second level of the coal-bear-
ing sandstones accumulated at the end of the Liassic rift-
ing in the Kelkit area. Failed Liassic rifting ends during
the Late Jurassic, and rift-related sediments are overlain
by the Upper Jurassic-Lower Cretaceous neritic carbon-
ates during long-lived thermal subsidence (Fig. 2).
Descriptive analysis of the Liassic neptunian dikes
Neptunian dikes are sedimentary dikes in which the in-
filling sediments are derived from above, in contrast to
some sedimentary dikes with filling injected from below
(Winterer & Sarti 1994). The passive continental margins
of the Mesozoic Tethys in the circum-Mediterranean re-
gion display a great variety of neptunian dikes that are
used for understanding the tectonic evolution of these
margins (Bekta et al. 2001). Liassic neptunian dikes, out-
cropping separately in the Hur and Kôrôklô Valleys, 10 km
south of Gümü hane, were developed in the Hercynian
granites and metamorphics and also in the early rift-relat-
ed sandstones implying that Hercynian basement was de-
formed by the Liassic multiphase extensional regimes
(Fig. 3). The dimensions of the mesoscopic neptunian
dikes varies from 20 cm to 18 m in length and from 0.5 cm
to 40 cm in width in the NW, E-W, NE directions parallel
to regional fault systems of the Eastern Pontides. The fact
that the granitic wall rocks of the neptunian dikes in the
Hur Valley (Gökdere village) are not generally sheared
though cross-cutting or conjugate shapes of the neptunian
dikes may imply that some of them correspond to shear
fractures included in the Paleozoic granites (Fig. 3). Some
neptunian dikes are displaced by the younger cracks or
faults. All the dikes outcropping in Gökdere village along
the granitic contact are filled and covered by the pelagic
limestones of Ammonitico Rosso suggesting that neptu-
nian dikes developed during the thermal subsidence of the
granitic horsts. Another neptunian dike exposure and
some associated calcite veins are seen in an area of 1 km
2
on each side of the Kôrôklô Valley near Kov village
(Fig. 3). They are included in the basement Paleozoic
metamorphic rocks and early Liassic sandstones. Succes-
sive neptunian dikes are filled with early rift-related sand-
stone and later pelagic limestones of Ammonitico Rosso
Fig. 2. Stratigraphic column showing rifting periods of the Mesozoic and inversion of the Cenozoic in the southern zone of the Eastern
Pontides.
3
4
0
EYÜBO
ƒ
LU,
BEKTA,
EREN,
MADEN,
ÖZER
and
JACOBY
Fig. 3. A – Geological map of the Gümü hane and Köse granites including Mesozoic extensional and compressional fault systems and their longitudinal and transversal drag-drape folds.
1 – Kôrôklô metamorphics (Paleozoic), 2 – Gumushane and Kose granites (Paleozoic), 3 – Liassic volcanic and sedimentary rocks, 4 – Mesozoic and Cenozoic rocks, 5 – strike and
dip direction of the beds, 6 – orthogonal drag and drape folds, 7 – thrust fault. B—C – Neptunian dike maps of the Gokdere and Kov villages, respectively.
341
EXTENSIONAL DEFORMATION AND FORMATION OF THE LIASSIC RIFT BASINS (NE TURKEY)
of the Pliensbachian testifying to alternating extensional
tectonic regimes during the Liassic.
Kinematic and dynamic analysis of the Liassic
neptunian dikes
Because joints are kinematically enigmatic structures
their interpretation has generated controversy (Hancock
1985). For example, Scheidegger (1983) regards many
joints, especially those belonging to orthogonal vertical
sets, as shears. The same sets have been interpreted as
comprising extensional fractures. If we classify the shape
of the neptunian dikes in Hur and Kôrôklô Valleys (Figs. 5
and 6), they are grouped as I-shaped (possibly unidirec-
tional extension jointing) and T-shaped (possibly two epi-
sodes of the orthogonal systematic extensional joints),
Fig. 4. Liassic tectonic and following thermal subsidences resulted
in the formation of neptunian dikes in the granitic and metamor-
phic Hercynian basement.
Fig. 5. Main geometric shapes of the neptunian dikes exposed in the
Gokdere and Kov villages. A – X-shaped (Fig. 7E), B – Y-shap-
ed, C – L-shaped (Fig. 7A), D – T-shaped, E – I-shaped
(Fig. 7B,C,D,F and G), F – V-shaped (Fig. 7A).
and L-, V-, Y-, X-shaped conjugate or hybrid conjugate
joints (Hancock 1985). So it might be concluded that nep-
tunian dikes in both areas formed mainly in extensional
fractures and less frequently in conjugate fracture systems.
On the other hand the mesoscopic fracture systems of the
neptunian dikes correspond to the macro-fracture systems
of the Eastern Pontides. We interpreted regional exten-
sional directions operating during the Early Jurassic by
using the poles of the neptunian dikes in the Hur and
Kôrôklô Valleys as in the NE, N, and NW directions (Fig. 7).
If we assume that two or three conjugate fracture systems
of the Liassic neptunian dikes are coeval, these character-
istic fracture systems arranged in orthorhombic symmetry
(Oertel 1965; Reches 1983a,b) may be caused by three-di-
mensional strain rather than by multiple phases of faulting
or pre-existing basement faults. Synchronous opening of
the multi-direction Liassic rifts, in which characteristic
limestone of the Ammonitico Rosso accumulated, testify
to the triaxial extensional deformation of the Hercynian
basement. On the other hand, multi-directional block-edge
folds (Fig. 3), dogleg structures, trap-door block and angu-
lar unconformity caused by block tilting are evidence for
the extensional fault style of the Liassic.
The dip direction method as a tool for estimating
regional kinematics in extensional terranes
Scott et al. (1995) presented a stimulating and perhaps
widely applicable method to determine the regional maxi-
mum extensional direction in extensional terranes on the
basis of dip direction of the sediments (Ring & Betzler
1995; Moustafa 1996). Though the Eastern Pontide oro-
genic belt experienced a multi-stage extensional history
during the Mesozoic and a reverse reactivation of the nor-
mal faults during the Cenozoic (Fig. 2), experimental and
theoretical studies (Mandal & Chattopadhyay 1995) have
shown that the dip direction of the normal faults and of
bedding could not be changed significantly during inver-
sion tectonics. So we established the poles to bedding of
the Liassic rift-related sediments, and obtained the mean
dip directions or multi-extensional directions as NW-SE,
N-S, NE-SW (Fig. 7). These are consistent with those ob-
tained from the neptunian dikes of the Liassic.
Liassic rifting and inversion
Mesozoic basins in the southern zone of the Eastern
Pontides evolved from the Liassic rifting through passive
continental margin to the deep troughs with sea-floor
spreading of the middle and Upper Cretaceous. Diachro-
nous compressive tectonic regimes are responsible for the
closing of these deep basins and inversion of the exten-
sional faults before the Eocene. During inversion tecton-
ics, regional contraction can reactivate pre-existing
extensional faults as reverse faults (Cooper & Williams
1989; Williams et al. 1989; Letouzey et al. 1990). Howev-
er, reverse reactivation may not take place in every do-
342
EYÜBOƒLU, BEKTA , EREN, MADEN, ÖZER and JACOBY
Fig. 6. Field photographs of the neptunian dikes. Gr – Paleozoic granites, Mt – Metamorphic rocks, Ls – Liassic sandstone, AL – Am-
monitico Rosso limestone.
343
EXTENSIONAL DEFORMATION AND FORMATION OF THE LIASSIC RIFT BASINS (NE TURKEY)
main of the thrust belt. Letouzey et al. (1990) have shown
that reverse slip occurs along the deeper segments of the
normal faults but not along shallower fault segments,
which typically dip more steeply. Some field investiga-
tions (Hatcher 1981) have shown that many normal faults
have not been reactivated, and that contraction is accom-
modated by new low-angle thrust. Normal faults reactivat-
ed only in areas subjected to large extension prior to
contraction. Mandal & Chattopadhyay (1995) have shown
experimentally that there are two modes of reverse reacti-
vation which depend on dip and spacing of faults. In
Mode 1 fault blocks undergo rigid rotation during the late
contraction and pre-existing normal faults are reactivated
as reverse faults. In Mode 2 faults are reactivated in re-
verse movements without rigid block rotation. As a result,
the dips of faults and layers do not change during contrac-
tion. Unlike Mode 1 reactivation, the layers remain tilted
Fig. 7. Poles to Liassic neptunian dikes and their mean extensional
directions (A and B: Gökdere village; C and D: Kov village). Poles
to bedding and mean dip directions for the Liassic rift sediments
(E and F). Dip analysis of the Liassic bedding and orientation of the
Liassic neptunian dikes may imply three axial extensional deforma-
tion and formation of the three pairs conjugate normal faults (G).
even after fault offsets vanish. If we assume that Mode 1
and 2 are applicable methods for the reactivation of the
Liassic extensional faults in the Eastern Pontides, we can
deduce maximum extensional directions and dip direc-
tions of faults in extensional terranes on the basis of dips
of the rift-related sediments.
Modern seismicity of the Eastern Pontides is mainly
controlled by the faults corresponding to the lineament
of the gravity and magnetic anomalies and extensions of
the Mesozoic—Cenozoic basins implying that active
faults are superimposed on the paleofaults (Figs. 1 and 8).
The multiple directions of the Mesozoic—Cenozoic folds
have mutual relationships with faults suggesting that the
folds were formed by the faults in the extensional and/or
compressional deformations.
Fault-related folding in the Eastern Pontides
Fold orientations in NW, E-W, NE-SW, that are parallel
to the fault systems are the main characteristic features of
the Eastern Pontide structural style. This mutual relation-
ship between faults and folds strongly suggests that folds
were formed by the extensional and compressional fault
systems during the opening and closing of the Mesozoic
and Cenozoic basins, respectively. The overall size,
shape, and trend of the fold in the Mesozoic and Cenozo-
ic sediments reflects the size, shape, and trend of the
basement blocks, such as the Hercynian Gümü hane-
Köse granites around Gümü hane city (Fig. 3). On the
other hand, the reactivation of synrift normal faults of
the paleo-Atlas rifts inverted previous half-grabens into
anticlinal structures, with the axis of the half-graben cen-
tred below the axis of the inverted anticline. The result-
ing inverted fold geometries are controlled by the
geometries of the extensional planar or listric faults
(Beauchamp et al. 1996). Therefore, if we take into ac-
count the opening and closing of the Liassic grabens ex-
tending in the NW-SE, E-W, NE-SW directions, it can be
concluded that longitudinal Liassic synclines and anti-
clines in the same directions as faults, may correspond to
the Liassic extensional folding and formation of the half-
grabens (drag folds or rollover folds, Schlische 1995) and
to later compressional folding and closing of the half-
grabens (Mitra 1993).
Interpretation of the gravity and magnetic
anomaly maps of the Eastern Pontide orogenic belt
The characteristic magnetic and gravity features of the
Eastern Pontides orogenic belt are summarized as fol-
lows:
1 – The large variations in amplitude and steep gradi-
ents in both gravity (Bouger and residual gravity map)
and magnetic data define distinct northern, southern and
axial zones, as seen in the geological map of the Eastern
Pontides, and are compatible with the structures of the
block fault tectonics (Fig. 1).
344
EYÜBOƒLU, BEKTA , EREN, MADEN, ÖZER and JACOBY
Fig. 8. Gravity (A), magnetic (B) and residual gravity (C) maps of the Eastern Pontides (from MTA,Turkey) may imply three different
major subzones outlined by the three fracture systems in NW, E—W, NE directions.
345
EXTENSIONAL DEFORMATION AND FORMATION OF THE LIASSIC RIFT BASINS (NE TURKEY)
2 – In contrast to the southern zone, which has nega-
tive residual gravity anomalies, the northern and axial
zones have positive residual gravity anomaly. These im-
ply that the Eastern Pontide orogenic belt had horst and
graben structures or block-fault tectonics in Mesozoic and
Cenozoic times (Fig. 8C).
3 – Lineaments which are expressed as linear magnetic
and gravity anomalies in NW, E-W, NE directions are in-
terpreted as originating from fault zones. The trends of the
fault zone are parallel to the ultramafic, metamorphic and
granitic massifs implying that the emplacements of the
massifs are controlled by these faults.
4 – Basins of the Mesozoic and Cenozoic correspond
to the magnetically calm areas which constitute the belts,
a maximum of 30—40 km wide and 100—150 km long, in
NW, E-W and NE directions. They coincide approximate-
ly with the edge of the blocks as defined by the gravity
anomalies.
5 – A series of strong positive circular magnetic anom-
alies are seen especially in the northern zone. They are
clearly granites and/or calderas that have been emplaced
along the zones of faults.
Discussion and conclusion
Models of rift basins commonly depict a relatively sim-
ple geometry produced during a single, sometimes pro-
tracted episode of extension (Gibbs 1987).
In recent years, many studies show four or six sets of
faults with orthorhombic symmetry (Freund & Merzer
1976; Reches 1983a) or a zig-zag and polygonal pattern
of normal faults in extensional regions (Lonergan et al.
1998; Lonergan & Cartwright 1999; Nieto-Samaniego
1999; Tikof & Fossen 1999). Such patterns are also ob-
served from small-scale faults to the regional zig-zag pat-
tern of the rift valleys (Collby & Susanne 1998). These
patterns of faults were classically explained as the results
of multiple phases of faulting or as being due to pre-exist-
ing basement faults. However, in some cases penecontem-
poraneous development of three or four sets of faults is
either evident or very probable. Such a fault pattern was
produced by (Oertel 1965) in a clay cake subjected to
stretching in a three-dimensional strain field. Similarly,
Reches (1983b) produced such patterns in cubes of sand-
stone, granite and limestone that were subjected to com-
pression in a three-dimensional field. According to
Reches’s slip model, fault patterns such as defined above
can form in a single phase of faulting, as the effect of a
three-dimensional strain field. As three-dimensional states
of strain are common in nature, it seems that the present
analysis of the zig-zag shaped Liassic faulting of the East-
ern Pontides (Fig. 1) is an appropriate approach for the in-
terpretation of faults in the field.
The Eastern Pontide magmatic arc was rifted in NW, E-W
and NE multi-directions during the Early Jurassic. As the
characteristic red pelagic limestones of the Ammonitico
Rosso deposited at the same stratigraphic level in the zig-
zag shaped rift basins, it is considered that the openings of
the rift system with multi-directions were synchronous. Dis-
tributions of the poles to the Liassic neptunian dikes filled
and covered with Ammonitico Rosso imply that extension-
al deformation of the Liassic rift basins occurred in a three-
dimensional strain field rather than multiple phases of the
faulting. Extensional block tilting and dip direction of the
Liassic sediments in NW-SE, N-S, NE-SW directions testify
that Liassic rifting occurred by three pairs of conjugate nor-
mal faults parallel to the multi-direction rift, or three-direc-
tional deformation is responsible for the Liassic rifting.
Acknowledgments: We thank Mehmet Arslan for correct-
ing the English of the manuscript. Field studies of this
work were supported by funds from Karadeniz Technical
University research Grands 21.112.005.4.
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