GeolCarp_Vol62_No4_345

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GEOLOGICA CARPATHICA

, AUGUST 2011, 62, 4, 345—359 doi: 10.2478/v10096-011-0026-7

Introduction

Most of the collisional and non-collisional plate boundaries on
Earth are in juxtaposition with curved topographic features in
plan-view. The Japan, Mariana, Carpathian, Aegean, Cypriote
arcs, the Himalayan collision zone or the non-collisional Bo-
livian orocline, are well-known examples of such features.

Weil & Sussman (2004) classified these curved structures

into three main categories considering their origin and evo-
lution. The first group constitutes “oroclines”. These are the
orogens,  which  originate  from  linear  structures  and  have
been  rotated  around  a  vertical-axis  through  time  during  a
protracted  deformation.  The  second  group  comprises  “pri-
mary  arcs”,  which  attain  their  curvature  during  the  initial
stages of deformation. The third group involves “progressive
arcs”.  These  structures  attain  their  curvature  progressively
throughout a mountain belt’s deformation history. All types
of  orogenic  curvatures  can  be  determined  by  paleomag-
netism because of the ability to obtain the path and amount
of rotation of each block around a vertical-axis (Schwartz &
Van  der  Voo  1984;  Eldredge  et  al.  1985;  Weil  &  Sussman
2004). Paleomagnetic results may also be correlated with ki-
nematic data in order to assess the spatiotemporal trends of
deformation of a mountain belt.

The role of oroclinal bending in the structural evolution of

the Central Anatolian Plateau: evidence of a regional

changeover from shortening to extension

ERMAN ÖZSAYIN and KADI

R DI

Hacettepe University, Department of Geological Engineering, Tectonic Research Laboratory, 06800 Ankara, Turkey;

eozsayin@hacettepe.edu.tr

(Manuscript received February 10, 2010; accepted in revised form March 17, 2011)

Abstract: The NW—SE striking extensional I

nönü-Eski ehir Fault System is one of the most important active shear zones

in Central Anatolia. This shear zone is comprised of semi-independent fault segments that constitute an integral array of
crustal-scale faults that transverse the interior of the Anatolian plateau region. The WNW striking Eski ehir Fault Zone
constitutes the western to central part of the system. Toward the southeast, this system splays into three fault zones. The
NW striking Il

ca Fault Zone defines the northern branch of this splay. The middle and southern branches are the Yeniceoba

and Cihanbeyli Fault Zones, which also constitute the western boundary of the tectonically active extensional Tuzgölü
Basin. The Sultanhan

Fault Zone is the southeastern part of the system and also controls the southewestern margin of the

Tuzgölü Basin. Structural observations and kinematic analysis of mesoscale faults in the Yeniceoba and Cihanbeyli Fault
Zones clearly indicate a two-stage deformation history and kinematic changeover from contraction to extension. N-S
compression was responsible for the development of the dextral Yeniceoba Fault Zone. Activity along this structure was
superseded by normal faulting driven by NNE-SSW oriented tension that was accompanied by the reactivation of the
Yeniceoba Fault Zone and the formation of the Cihanbeyli Fault Zone. The branching of the I

nönü-Eski ehir Fault System

into three fault zones (aligned with the apex of the Isparta Angle) and the formation of graben and halfgraben in the
southeastern part of this system suggest ongoing asymmetric extension in the Anatolian Plateau. This extension is compat-
ible with a clockwise rotation of the area, which may be associated with the eastern sector of the Isparta Angle, an oroclinal
structure in the western central part of the plateau. As the initiation of extension in the central to southeastern part of the
I

nönü-Eski ehir Fault System has similarities with structures associated with the Isparta Angle, there may be a possible

relationship between the active deformation and bending of the orocline and adjacent areas.

Key words: Central Anatolian Plateau, I

nönü-Eski ehir Fault System, Isparta Angle, Tuzgölü Basin, extensional

deformation, neotectonics, orogenic plateau evolution, oroclinal bending.

The Isparta Angle (IA) in the western sector of the Central

Anatolian highland is an example of protracted orocline evo-
lution. This inverse, broadly V-shaped morphotectonic feature
was  first  defined  by  Blumenthal  (1951)  as  the  “Courbure
d’Isparta” (Isparta bend). It is located to the north of Antalya
Bay in southern Turkey, immediately to the north of the Ae-
gean  and  Cyprian  arcs  in  the  eastern  Mediterranean  (Fig. 1).
Previous  studies  have  clearly  identified  three  major  nappe
sheets  as  an  integral  part  of  the  triangle,  which  formerly
formed linear structures in the area of present day of the IA.
The Antalya nappes originate from the southern part of Neote-
thys and were emplaced onto the Tauride Carbonate Platform
during late Early Paleocene (Uysal et al. 1980). The Bey ehir-
Hoyran nappes to the east, derived from the northern branch
of Neotethys, were thrusted onto the central Tauride platform
during  two  consecutive  stages  (Campanian—late  Lutetian)
(Piper et al. 2002). Similarly, to the west, the Lycian nappes
have the same origin and record a two-step emplacement onto
the western Tauride platform (Late Oligocene-late Langhian)
(Piper  et  al.  2002).  Paleomagnetic  studies  suggest  a  30°
counter clockwise rotation of the western part of the IA (Kis-
sel & Poisson 1987; Morris & Robertson 1993), while a 40°
clockwise  rotation  have  been  determined  for  the  eastern  part
(Kissel et al. 1993).

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GEOLOGICA CARPATHICA, 2011, 62, 4, 345—359

The possible relationship between the IA and active defor-

mation in the neighbouring regions has not been assessed
previously. As a neighbouring array of active structures, the
I

nönü-Eski ehir Fault System (I

EFS) is one of the most im-

portant shear zones in Central Anatolia. It lies between Mt
Uludag˘ (city of Bursa) to the west and the town of Sultan-
han

to the southeast (Fig. 1). Furthermore, the southeastern

segment of this extensional system controls the western mar-
gin of the extensional Tuzgölü Basin, one of the most impor-
tant intracontinental basins in Central Anatolia. The
southeastern part of this fault system, which we have analy-
sed in detail, has similar strike and deformation history com-
pared to those observed in the IA during its neotectonic
period. The principal aim of this paper is therefore, to unravel
the structural evolution of the southeastern part of the I

EFS

and to characterize the relationship between the IA and I

EFS

to better understand the evolution of the central part of the
Central Anatolian Plateau.

Tectonic framework

Anatolia and adjacent areas result from the continental

collision between the northward moving African-Arabian
and the quasi-stationary Eurasian plates ( engör & Y

lmaz

1981;  engör et al. 1985). This has been associated with the
formation  of  four  main  neotectonic  phenomena.  The  first
one is the Aegean-Cyprian Arc, where the African plate sub-
ducts beneath the Anatolian Plate to the north (Papazachos &
Comninakis 1971; McKenzie 1978;  engör & Y

lmaz 1981;

Meulenkamp et al. 1988; Mart & Woodside 1994). The sec-

ond one is the sinistral Dead Sea transform-fault system ac-
commodating motion between Africa and Arabia ( engör &
Y

lmaz 1981; Gülen et al. 1987; DeMets et al. 1990; Barka

&  Reilinger  1997;  Reilinger  et  al.  1997).  Consequently,
Anatolia is forced to move westward. This tectonic escape is
accommodated  by  the  North  Anatolian  and  East  Anatolian
fault systems, which are dextral and sinistral intracontinental
strike-slip  faults,  respectively  ( engör  1979;

engör &

lmaz 1981; Barka 1992) (Fig.1).

Some second-order structures, which divide the Central

Anatolian region into smaller blocks, also exist. These are the
NE  striking  sinistral  Central  Anatolian  Fault  System  and  the
NW striking dextral Tuzgölü Fault Zone (Fig. 1). The Central
Anatolian  Fault  System  splays  off  from  the  North  Anatolian
Fault  System  near  the  city  of  Erzincan  (Yeti   1978,  1984;
Yeti   &  Demirkol  1984;  Dirik  &  Göncüog˘lu  1996;  Koçyig˘it
& Beyhan 1998) (Fig. 1). The Tuzgölü Fault Zone bounds the
eastern  margin  of  the  extensional  Tuzgölü  Basin  (Ar

kan

1975; engör et al. 1985; Dirik & Göncüog˘lu 1996; Çemen et
al. 1999; Dirik & Erol 2003). This zone is cut by the Central
Anatolian Fault System to the south of the city of Nig˘de. In
addition, the Ak ehir Fault Zone and the I

EFS are prominent

structures in Central Anatolia (Fig. 1). The NW-striking
Ak ehir Fault Zone lies between the cities of Afyon and
Karaman. It is located northeast of the IA and characterized
by NE-dipping normal faults (Koçyig˘it & Özacar 2003). The
I

EFS, approximately 450 km long, is a NW- to WNW-striking

mega-shear system, which constitutes the transition between a
contractional tectonic regime to the east of Tuzgölü and an ex-
tensional tectonic regime to the west-southwest. The Eski ehir
Fault Zone, between the city of Bursa and the town of Sivri-

Fig. 1. Simplified map showing major neotectonic structures in Turkey and surrounding area (simplified from Çiftçi 2007).

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GEOLOGICA CARPATHICA, 2011, 62, 4, 345—359

hisar, forms the WNW-striking western part of this system
(Figs. 1, 2). This zone consists of several dextral faults with a
normal component of motion (Altunel & Barka 1998;
Ocakog˘lu & Aç

kal

n 2009). Southeast of Sivrihisar, the sys-

tem changes its strike to NW and splays out into the Il

ca,

Yeniceoba, and Cihanbeyli Fault Zones (Koçyig˘it 1991a; Çe-
men et al. 1999; Dirik & Erol 2003; Dirik et al. 2005; Koçyig˘it
2005) (Fig. 2). The NW striking Il

ca Fault Zone constitutes

the northern branch of this system (Koçyig˘it 1991a). It is char-
acterized by SW dipping dextral faults with a normal compo-
nent. The Yeniceoba Fault Zone (YFZ) is the central branch
with NE dipping normal fault planes. Superimposed slicken-
lines, representing a two-stage deformation, are also encoun-
tered on the YFZ (Özsay

n & Dirik 2005, 2007). The first set

of  fault  striations  indicates  dextral  strike-slip  faulting,  while
the second generation of slickenlines indicates normal faulting
with  a  minor  dextral  strike-slip  component.  The  Cihanbeyli
Fault  Zone  (CFZ)  comprises  the  southeastern  branch  of  this
system having several SW and NE dipping faults with normal
fault kinematics (Fig. 2). Both the YFZ and CFZ, which con-
trol  the  western  margin  of  the  Tuzgölü  Basin,  are  cut  by  the
NNE  striking  Alt

nekin Fault Zone (Dirik & Erol 2003)

(Fig. 2). The southeastern part of the I

EFS is the Sultanhan

Fault Zone located in the eastern part of the Alt

nekin Fault

Zone (Özsay

n & Dirik 2005, 2007). Although no slip data is

available from the Sultanhan

Fault Zone, normal faulting can

be inferred from seismic reflection data from the southern part
of the Tuzgölü region (Ar

kan 1975; Ug˘urta 1975) (Fig. 2).

Stratigraphy

To better understand the temporal and spatial evolution of

normal faulting in the western part of the Tuzgölü region, it
is necessary to review the available stratigraphic data. The
units located in the study area can be divided into two main
groups. Here units older than Oligocene-Miocene are regard-
ed as “basement” units, while younger strata correspond to
cover units (Fig. 3).

“Basement” units

The oldest exposed basement units are located in the

northern, northwestern and southern sectors of the study area.

Fig. 2. Tectonic map of the study area and surrounding regions (modified after Dirik & Göncüog˘lu 1996; Göncüog˘lu et al. 1996; Dirik
2001; Dirik & Erol 2003; Koçyig˘it & Özacar 2003; Özsay

n & Dirik 2007).

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They  comprise  an  Upper  Cretaceous  ophiolitic  mélange,
mainly  composed  of  Triassic  crystalline  limestone  blocks,
serpentinites, radiolarian cherts, and gabbros. This unit is un-
conformably overlain by Upper Maastrichtian—Paleocene red
beds. These clastics grade upward into yellow, shallow ma-
rine  carbonates  of  Late  Paleocene—Early  Eocene  age.  This
sequence begins with sandy-clayey limestone and continues
with alternating marl and sandstone layers. A thick-bedded,
fossiliferous  limestone  follows  at  the  top  of  this  sequence
(Çemen et al. 1999) (Figs. 3, 4).

Cover units

Separated by an angular unconformity, the Oligocene-

Miocene  Gökda˘g  Formation  constitutes  the  oldest  cover
rocks in the study area and exclusively consists of terrestrial
sediments  (Göncüo˘glu  et  al.  1996)  (Fig. 3).  It  overlies  the
basement  units  with  an  angular  unconformity.  This  unit  is
characterized by alternating brick- to red-coloured conglom-
erates and sandstones at the base and continues upward with
yellow-  to  green-coloured  alternations  of  gypsum-bearing
claystone,  mud-  and  sandstone.  The  Gökda˘g  Formation  is

Fig. 3. Generalized tectono-stratigraphic columnar section of the study area.

tween  older  and  younger  units  exposed  in  the  study  area;
(2)  strike,  dip,  and  slip-lineation  measurements  from  fault
planes to decipher different deformational phases that affect-
ed  the  study  area.  Angelier’s  Direct  Inversion  Method  ver-
sion  5.42  was  used  to  analyse  fault-slip  data  (Angelier
1991). For the definition of the paleostress field, the nature
of the vertical/sub-vertical stress axis and the value of ratio
were  taken  into  account  (Angelier  1994).  Stress  fields  may
vary  from  radial  extension  (

vertical, 0 < < 0.25), exten-

sion (

vertical) with pure extension (0.25< < 0.75) and

transtension (0.75< < 1), to strike-slip stress fields (

verti-

cal), with pure strike-slip (0.25< < 0.75), transtension
(0.75< < 1) and transpression (0< < 0.25), or to compres-
sion (

vertical), with pure compression (0.25< < 0.75) and

transpression (0< < 0.25) (Delvaux et al. 1997). Radial
compression (

vertical, 0.75< < 1) has been rejected from

the calculation, being considered inconclusive. In order to cal-
culate principal stress directions and to determine the different
deformational regimes, a total of 171 slip-data were measured
from fault planes at 14 stations. 132 slip-data were previously
published to characterize the recent activity of both the Cihan-
beyli and Yeniceoba Fault Zones (Özsay

n & Dirik 2007).

overlain by the Pliocene Cihanbeyli For-
mation. This unit is characterized by alter-
nating

carbonate-cemented

polymict

conglomerates and sandstone layers at the
base  and  white-coloured,  thick-bedded,
porous,  lacustrine  limestone-claystone  al-
ternations  at  the  top.  Özsay

n & Dirik

(2007) defined the Ku ça Member, having
a limited exposure to the south of the town
of  Yeniceoba  within  this  formation
(Figs. 3, 4). This member comprises thick-
bedded,  alternating  conglomerate  and
sandstone  at  the  bottom,  and  sandy  clay-
stone-mudstone alternations at the top. Al-
ternating mudstones and tuffites constitute
the  uppermost  level  of  this  member.  The
Cihanbeyli  Formation  grades  into  the
Pleistocene Tuzgölü Formation (Ulu et al.
1994), including poorly consolidated con-
glomerates and sandstones with carbonate,
gypsum,  and  sulphate  deposits  in  the  up-
per levels. These units are unconformably
overlain  by  Quaternary  alluvial-fan  grav-
els,  fluvial  terrace  deposits,  colluvium,
and  recent  alluvium  associated  with  the
I

nsuyu River (Figs. 3, 4).

Structural analysis

Method of study

Two types of structural data were col-

lected in the field: (1) strike and dip mea-
surements  of  the  bedding  planes  to
clarify  both  deformation  adjacent  to
faults  and  the  angular  relationships  be-

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Characteristics of unconformities

Three unconformities were identified in the study area.

The first one is between the basement units and the Gökda˘g
Formation. This relationship cannot be seen in the study area
sensu  stricto,  but  outcrops  in  the  vicinity  of  the  villages  of
Kandil  and  Sincik  (NW  of  study  area  boundary)  clearly  re-
veal  this  relationship.  The  second  unconformity  occurs  be-
tween  the  Gökda˘g  and  the  Cihanbeyli  Formations.  This
unconformity is key to understanding the timing and change
in  regional  kinematic  regimes,  as  the  virtually  horizontal
Cihanbeyli Formation covers the Gökda˘g Formation and old-
er units. The third unconformity involves the disconformity
between the Tuzgölü and Cihanbeyli Formations. Here, ero-
sional surfaces of the Cihanbeyli Formation in the Yeniceoba
Plain are covered by the Tuzgölü Formation.

Attitude of faults and folds

Fault kinematics was determined either by slickenlines or

chatter marks (e.g. Phillipson 2003; Dirik 2005), where
available. In addition, we used drag folds, the nature of hori-
zontal and vertical offsets, juxtaposition of different-aged
units, and cross-cutting relationships. The timing of faulting
and deformation are estimated by the age of the stratigraphic
units bounded by unconformities and cross-cutting relation-
ships.

Thrust faulting characterizes the basement units. These

structures are exposed around the villages of Hac

ömero˘glu

and Ku ça (Figs. 5, 6). These structures are the vestiges of an
older  contractional  tectonic  regime.  The  Gökda˘g  Formation
is commonly overthrust by ophiolitic mélange; in places, Pa-
leocene terrestrial clastics and Eocene limestones constitute
the hanging wall. The series of anticlines and synclines, hav-
ing  NW  strike  and  observed  in  the  Gökda˘g  Formation,  are
strongly  controlled  by  these  thrusts.  These  tectonic  bound-
aries  are  sealed  by  Pliocene  and  Quaternary  units  or  have
been subsequently cut by younger normal faults.

Younger normal faults cutting all units in the region are

ubiquitous. The average strike of these faults is 310°N, with
steeply  dipping  NE  and  SW  fault  planes.  Slickenlines  on
these fault planes are well preserved and alluvial-fan deposi-
tion at different scales is still in progress associated with the
vertical  offsets.  Normal  faulting  has  created  the  Cihanbeyli
Graben,  which  is  approximately  8 km  long  and  1 km  wide,
and  located  to  the  northwest  of  the  town  of  Cihanbeyli
(Fig. 4). Maximum vertical offset in this graben is measured
to  be  50 m,  based  on  the  Cihanbeyli  Formation  limestones.
Importantly, there are normal faults with much smaller off-
sets, which cut recent slope debris and terrace deposits of the
I

nsuyu River near the village of I

nsuyu, attesting to the on-

going nature of this phase of deformation (Fig. 7).

There are also E-W-striking normal faults, which have a

minor sinistral strike-slip component. These faults constitute

Fig. 4. Geological map of the study area (from Özsay

n 2007).

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GEOLOGICA CARPATHICA, 2011, 62, 4, 345—359

wedge-like structures together with the NW-striking normal
faults. One of these wedges is located at the village of I

nsuyu

where the centre part of the wedge is downthrown by NW
and E—W striking faults. The E-W striking faults may repre-
sent accommodation structures (e.g. Faulds & Varga 1998),
which kinematically connect oppositely dipping faults.

Fault-slip analysis

The first data set characterizes a N—S to NNW—SSE compres-

sional stress regime (Fig. 8). At station # 12,

is vertical and

the value represents pure compression for this data set (Ta-
ble 1). In the field, macroscale deformation features pertaining
to this deformation phase are primarily characterized by thrust
faults with north to northeast vergence and backthrusts with

southwest  vergence.  At  the  outcrop
scale these thrusts juxtapose basement
units  and  the  Gökdag˘  Formation.
Slickenline  overprinting  relations  on
fault  planes  are  observed  at  station
# 11 (Fig. 9). In the first phase,

vertical, and a pure strike-slip (dex-
tral) stress regime represents the de-
formation according to

value

(Table 1). This episode of faulting is
responsible for the tectonic boundary
between the Paleocene red beds and
the Gökdag˘ Formation.

The second data set represents a

NNE—SSW  to  NE—SW  oriented  ex-
tensional stress regime (Fig. 8). This
data  set  can  be  subdivided  into  two
groups.  The  first  group,  character-
ized  by  NNE—SSW  extension  has
low   ratios, which indicate pure ex-
tension (stations 2, 7, 9, 10) and radi-
al extension (stations 4, 5, 6, Table 1
and  location  in  Fig. 8).  The  second
subgroup, showing a NE—SW orient-
ed tensional stress regime, has higher

ratios, which is compatible with

pure  extension  (stations 3,  8,  11—2,
13,  Table 1  and  location  in  Fig. 8).
At  station  # 1  the    ratio  indicates  a
radial-extensional stress regime with
NNW—SSE  orientation.  The  defor-
mation  linked  to  this  data  set  is  pri-
marily  characterized  by  normal
faults, which cut the Cihanbeyli For-
mation  and  Quaternary  alluvium.  At
the outcrop scale these normal faults
are  observed  dominantly  in  the
Cihanbeyli  Formation  and  its  con-
tacts with the Gökdag˘ Formation.

Fault kinematic data from faulted

Quaternary alluvium and terrace de-
posits are similar to those measured
on structures developed in the
Pliocene

Cihanbeyli

Formation.

Fig. 5. Detailed geological map and the cross-section of the Hac

ömerog˘lu and surrounding re-

gions (from Özsay

n 2007).

This clearly shows that the extension processes, which initi-
ated in Early Pliocene, are still active. The regional distribu-
tion of earthquake epicentres also supports the notion of
ongoing tectonic activity in these zones (Fig. 10).

Result of structural analysis

Taken together, field observations of macro- and mesoscale

faults and paleostress analyses clearly document two different,
and  subsequent  tectonic  phases  in  western  Central  Anatolia.
The older phase pertains to N—S to NNW—SSE oriented com-
pression,  which  is  observed  in  the  pre-Pliocene  units  on  the
YFZ. During this phase the thrust faults and folds affecting the
Upper Cretaceous ophiolitic mélange, Paleocene red beds, Pa-
leocene—Lower  Eocene  limestones  and  the  Gökda˘g˘  Forma-

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Table 1: Field information and kinematic analysis results of slip-data measurements.

Station

Easting

Northing

# of slip data

Stress axes

Unit and / or boundary

= 140°

/ 69°

487500 4280040

= 256°

/ 10°

= 349°

/ 18°

0.206

Cihanbeyli Formation limestones

= 242°

/ 68°

479757 4284687

= 099°

/ 18°

= 005°

/ 12°

0.273

Quaternary terrace deposits and alluvium

= 001°

/ 69°

479757 4284687

= 153°

/ 18°

= 246°

/ 9°

0.390

Cihanbeyli Formation limestones

= 017°

/ 75°

478629 4285420

= 110°

/ 1°

= 200°

/ 15°

0.233

Cihanbeyli Formation limestones

= 031°

/ 71°

474388 4287855

= 293°

/ 3°

= 202°

/ 19°

0.167

Cihanbeyli Formation limestones

= 021°

/ 71°

479757 4284687

= 280°

/ 4°

= 189°

/ 19°

0.229

Cihanbeyli Formation limestones

= 013°

/ 75°

442677 4314339

= 105°

/ 1°

= 195°

/ 15°

0.315 Gökdağ Formation clastics

= 311°

/ 66°

439019 4321525

= 158°

/ 22°

= 064°

/ 10°

0.366

Paleocene basement — Cihanbeyli Formation
limestones boundary

= 156°

/ 84°

452110 4314142

= 272°

/ 3°

= 002°

/ 5°

0.261

Cihanbeyli Formation limestones

= 030°

/ 85°

454535 4317437

= 294°

/ 1°

=204°

/ 5°

0.337

Serpentinite basement — Cihanbeyli Formation

limestones boundary

= 180°

/ 1°

470340 4306800

= 071°

/ 88°

11–1

= 270°

/ 2°

0.522

Gökdağ Formation clastics — Cihanbeyli Formation
limestones boundary

= 206°

/ 63°

470340 4306800

= 346°

/ 22°

11–2

= 082°

/ 16°

0.422

Gökdağ Formation clastics — Cihanbeyli Formation
limestones boundary

= 335°

/ 6°

472729 4304637

= 244°

/ 8°

= 099°

/ 80°

0.740

Eocene limestones (basement) — Serpentinite
(basement) thrust

= 098°

/ 77°

472366 4297396

= 323°

/ 9°

= 232°

/ 9°

0.498

Cihanbeyli Formation Kuşça Member clastics

= 151°

/ 77°

487500 4280040

= 272°

/ 7°

= 003°

/ 11°

0.425 Quaternary

alluvium

tion, which is coeval with the second stage of the YFZ. Our re-
sults are at odds with previously suggested evolutionary mod-
els. In these models, the YFZ and CFZ constitute the western
boundary faults of the Tuzgölü Basin formed during the Late
Cretaceous (Erol 1969; Ar

kan 1975; Görür & Derman 1978;

Ünalan & Yüksel 1978; Çemen & Dirik 1992; Göncüo˘glu et
al. 1996; Çemen et al. 1997; Çemen et al. 1999; Dirik & Erol

2003). As the YFZ is the only fault zone representing a two-
stage deformation, the CFZ cannot be the principal boundary
fault for the western part of the basin. Furthermore, the previ-
ous  models  proposed  normal  faulting  or  strike-slip  faulting
with  a  normal  component  of  movement,  suggesting  a  large
component of transtension for the YFZ. However, our results
indicate  dextral  strike-slip  faulting  during  the  first  stage  of

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GEOLOGICA CARPATHICA, 2011, 62, 4, 345—359

faults during N-S oriented shortening (Fig. 11c,d). The I

EFS

is one of the most important fault structures that were formed
as a result of pure shear (derived from N—S oriented shorten-
ing of Central Anatolia), resulting in a dextral strike-slip
fault extending to Thrace in NW of Anatolia. According to
various authors (e.g. Yalt

rak et al. 1998; Sak

nç et al. 1999;

Yalt

rak 2002), movement in the Thracian part of the I

EFS

continued until this structure was cut by the NAFS in Late Mi-
ocene—Early Pliocene time. This pure shear regime thus gener-
ated the YFZ as part of the I

EFS for the first time (1

phase,

dextral strike-slip) (Fig. 11e).

In the Late Miocene—Pliocene, the Tuzgölü Basin experi-

enced NNE—SSW oriented extension. Due to this extension,
accommodation space was generated resulting in deposition
of the lacustrine Cihanbeyli Formation (Fig. 11f). This tec-
tonic regime has continued to the present day. During this
phase, the boundary faults of the basin were reactivated and
the dextral YFZ changed its kinematic character, becoming a
normal fault in the latest Pliocene. In addition, the CFZ, which
branches from the YFZ and constitutes several dip-slip normal
faults, developed as the most important neotectonic structure
during this period (Fig. 11g). The Pleistocene Tuzgölü Forma-
tion was also cut by normal faults of the YFZ associated with
the formation of the Cihanbeyli Graben (Fig. 11h,i).

Discussion and conclusion

The I

EFS which cuts the Anatolian Plate transversely has

played an important role in the Central Anatolian Plateau and
the evolution of the Tuzgölü Basin. Importantly, this region
records the kinematic changeover from contraction to regional
extension. The western margin of the Tuzgölü Basin has been
controlled by the Yeniceoba, Cihanbeyli and Sultanhan

Fault

Zones of the I

nönü-Eski ehir Fault System (I

EFS) since the

Late  Miocene.  Field  studies  and  kinematic  analyses  on  the
Yeniceoba Fault Zone (YFZ) reveal a two-stage deformation
history.  This  two-stage  history  comprises  earlier  N-S  com-
pressional stress regime, which established the YFZ as a dex-
tral strike-slip fault, and a subsequent NNE—SSW extensional
regime that has reactivated the former structures of this zone.
The Cihanbeyli Fault Zone (CFZ) is composed of NE and SW
dipping  dip-slip  normal  faults,  which  represent  the  same  ex-
tension direction observed for those faults of the second stage
of  the  YFZ.  This  situation  is  an  important  indication  for  the
CFZ  being  younger  than  the  YFZ.  Furthermore,  the  normal
faults cutting recent alluvium at the easternmost parts of both
zones and the distribution of earthquake epicentres clearly at-
test to the ongoing extension.

Morphological features of the I

EFS and locations and/or

discontinuities of some structures observed on both the YFZ
and CFZ suggest spatial disparities in the magnitude of ex-
tension for the central to southeastern part of the I

EFS.

These characteristics include:

– The branching of the I

EFS into three fault zones at the

town of Sivrihisar (Fig. 4).

– The location of the Cihanbeyli Graben in the southeast-

ern part of the CFZ, this structure discontinues at the village
of I

nsuyu (Fig. 6).

– Separation of the boundary faults of the Ku ça halfgra-

ben from the YFZ. This structure is also located at the south-
eastern part of the YFZ (Figs. 6, 10).

– Location of the faults cutting recent sediments in the

easternmost parts of both fault zones (Fig. 9).

– Existence of secondary faults, linking YFZ and CFZ

(Fig. 6).

Based on these observations, it is suggested that the south-

eastern part of the I

EFS is extending more than the central

part. This circumstance requires a clockwise rotation of the
area between the Ak ehir Fault Zone and the central to
southeastern part of the I

EFS. Previous studies clearly indicate

a  40°  clockwise  rotation  in  the  eastern  sector  of  the  IA  (e.g.
Kissel et al. 1993), which took place during the Late Miocene
to Pliocene (Frizon de Lamotte et al. 1995; Piper et al. 2002;
Poisson  et  al.  2003a).  As  the  timing  of  these  movements  is
similar to the timing of extension inferred for the study area,
the  trigger  mechanism  of  the  extension  for  the  southeastern
part  of  the  I

EFS may be associated with oroclinal bending.

This rotation in the eastern limb of the orocline has been fol-
lowed  by  NE—SW  extension  during  the  Late  Pliocene  and
Quaternary  (Koçyig˘it  &  Özacar  2003;  Poisson  et  al.  2003b),
which is manifested the active tectonic regime in the Tuzgölü
Basin.  This  phase  of  extension  can  also  be  verified  for  the
western part of the neighbouring Konya Basin, which is locat-
ed to the east of the IA (Eren 2003a,b) (Fig. 12).

If the apex of the IA is inferred to be the vertical axis of

the  rotation,  the  amount  of  extension  in  the  neighbouring
area  of  the  orocline  caused,  is  expected  to  be  low  near  the
axis  and  would  be  accommodated  by  few  normal  faults;
however,  further  away  the  amount  of  extension  would  be
greater. Although we are not able at present to assess the to-
tal amount of extension in both areas, the fanning out of ex-
tensional  structures,  their  ubiquity  and  spatial  extent
suggests to us that such a mechanism is viable. A first-order
reflection of this disparate NE—SW extension is the evolution
of normal faulting of the YFZ (2

phase). Due to ongoing ro-

tation, the extension in the southeastern part of the I

EFS in-

creased and caused the formation of the CFZ, which separates
from the YFZ near the town of Sivrihisar. Seismic reflection
profiles clearly indicate the normal faulting characteristics of
the Sultanhan

Fault Zone. In this scenario, the Alt

nekin

Fault Zone acts as a transfer fault that balances the amount of
extension between the northwestern (the CFZ and YFZ) and
southeastern parts (the Sultanhan

Fault Zone) of the system.

Acknowledgments: This study was financially supported by
Hacettepe University Scientific Research Unit Project BAB-
02 02 602 012. The author is grateful to Manfred Strecker,
Erdin Bozkurt, Ug˘ur Kag˘an Tekin and Serkan Üner for their
fruitful discussions, help and suggestions to improve this
manuscript.

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