International Geology ReviewVol. 54, No. 1, January 2012, 33–50

Plio-Quaternary development of the Baklan–Dinar graben: implicationsfor cross-graben formation in SW Turkey

Alper Gürbüz*, Sonay Boyraz and M. Tariq Ismael

Mühendislik Fakültesi, Jeoloji Mühendisligi Bölümü, Ankara Üniversitesi, 06100 Tandogan, Ankara, Turkey

(Accepted 24 May 2010)

Western Turkey is characterized mainly by E–W, NW–SE, and NE–SW-trending grabens. The reverse V-shaped geometryof the Baklan–Dinar graben is formed by coupling of the NE–SW-trending Baklan and the NW–SE-trending Dinar grabens,and is critical to an understanding of the structural evolution of the Aegean region. Its geometry indicates biaxial extensionin the region, as proposed in the current literature (e.g. Temiz et al. 1997; Cihan et al. 2003). The Acıgöl and Burdur grabensparallel the axis of the Baklan graben in the south, and these three basins terminate against the Dinar fault. The BaklanBasin is bounded along both margins by the Baklan and Çal left-lateral oblique faults. Basin-fill deposits of the Baklan Basinthicken towards the Dinar fault, and the basin floor dips northeastward. In addition, geomorphological characteristics of theBaklan–Dinar graben indicate that the Dinar fault is relatively more active than the faults of the Baklan Basin. All availablemorphologic, structural, borehole, and geophysical data from the Baklan–Dinar graben define a cross-graben structure thatdeveloped during Plio-Quaternary uniaxial NE-directed extension in SW Turkey. In this framework, the Dinar fault plays therole of a breakaway fault of this formation. The increase in the normal components of the northwestern left-lateral obliquebasin-bounding faults of the Baklan Basin is explained by the gradual steepening of the pitch of the slip vectors towards thenortheast, because of capturing of the extension by the Dinar breakaway fault.

Keywords: extensional tectonics; cross-graben; Plio-Quaternary; Baklan–Dinar graben; Western Anatolia; Aegean region

Introduction

Western Turkey is one of the most seismically activeregions in the world, and the region is rapidly beingdeformed by extensional tectonics (e.g. Jackson andMcKenzie 1984; Eyidogan 1988; Taymaz et al. 1991;Reilinger et al. 1997; Ambraseys and Jackson 1998;Bozkurt 2001) (Figure 1A). As a result, western Turkeyis also one of the best-studied continental extensionalregions. It is characterized mainly by E–W-trendinggrabens (e.g. Büyük Menderes and Küçük Menderes) andNW–SE (e.g. Denizli and Soma) to NE–SW-directed (e.g.Gördes and Usak-Güre) relatively short cross-grabens thatdeveloped as a result of regional extension (Sengör 1987;Sengör et al. 1985; Bozkurt 2001).

The Baklan–Dinar graben has a different structuralgeometry in this framework (Figure 1B). This angulargraben formed by the coupling geometries of NE–SW-trending Baklan and NW–SE-trending Dinar cross-grabensaround the town of Isıklı (Figure 2). The landscape reflectsa biaxial extensional regime in the uniaxial extensionalframework and is very important for a fuller understandingof the structural evolution of western Turkey.

This study is based on relationships between tecton-ics and geomorphology of the Baklan–Dinar graben, and

∗Corresponding author. Email: agurbuz@eng.ankara.edu.tr

demonstrates the presence of Plio-Quaternary basin devel-opment and active faulting in SW Turkey. Its angulargeometry reflects the complexity of the general E–W-trending graben structure of western Turkey. The geologyof the area is also complex, with high seismicity rates.Thus, the understanding of its evolution will be informa-tive as to the extensional regime in the adjacent Aegeanregion.

Geological setting

Turkey is one of the most actively deforming regions in theworld. Its neotectonic framework is a result of the continen-tal collision of the Arabian and Eurasian plates in easternTurkey, the related westward escape of the Anatolian plate,and the subduction of the African plate towards the east,the North and the East Anatolian fault zones in the northand east, and the Aegean–Cyprian subduction zone in thesouth and west of Turkey (Figure 1A).

Western Turkey has been experiencing extensional tec-tonics. The proposals on the origin of this extension can begrouped into four models: (1) the tectonic escape modelconsiders that westward escape of the Anatolian platealong the North and East Anatolian fault zones followedthe collision between the Arabian and Eurasian plates

ISSN 0020-6814 print/ISSN 1938-2839 online© 2012 Taylor & Francishttp://dx.doi.org/10.1080/00206814.2010.496543http://www.tandfonline.com

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Figure 2. Physiographic image of SW Turkey from SRTM data.

since ∼12 Ma (McKenzie 1972; Dewey and Sengör 1979;Sengör 1979, 1987; Sengör et al. 1985); (2) the back-arc spreading model based on the extension caused bythe southeastward migration of the Aegean trench system(McKenzie 1978; Le Pichon and Angelier 1979; Jacksonand McKenzie 1984; Kissel and Laj 1988; Meulenkampet al. 1988; Avigad et al. 1997; Jolivet et al. 1998;Thomson et al. 1998); (3) the orogenic-collapse modelsuggests that the extension in the region is related to the lateorogenic collapse following the closing of Neotethys bythe collision between the Sakarya zone and the Anatolide-Tauride platform along the Izmir-Ankara suture zone(Dewey 1988; Seyitoglu and Scott 1991, 1992, 1996;Seyitoglu et al. 1992; Seyitoglu and Scott 1994; McCluskyet al. 2000); and (4) the episodic, two-stage graben modelis a combination of the above mechanisms. According tothis model, the extension occurs in two distinct structuralstyles that are separated by an interval of N–S crustal short-ening (Koçyigit et al. 1999; Bozkurt 2000, 2001, 2003,2004; Yılmaz et al. 2000; Cihan et al. 2003; Koçyigit andÖzacar 2003; Purvis and Robertson 2004, 2005; Bozkurtand Rojay 2005; Koçyigit 2005; Koçyigit and Deveci,2007; Gürer et al. 2009b).

There is controversy concerning the age of develop-ment of the grabens. The three major views are (1) thebasins started to form during the late Miocene (McKenzie1972; Sengör and Yilmaz 1981; Sengör et al. 1985;

Sengör 1987); (2) the grabens began to form during thelate Oligocene–early Miocene, and have been continu-ously evolving ever since (Seyitoglu et al. 1992, 1996;Sen and Seyitoglu 2009; Demircioglu et al. 2010); (3) thebasins date from the Plio-Quaternary (Koçyigit et al. 1999;Bozkurt 2000; Sarıca 2000; Yılmaz et al. 2000; GürerYilmaz 2002; Gürer et al. 2001, 2006, 2009).

The Baklan–Dinar graben is situated between twotectonostratigraphic units in the southeastern margin of theWest Anatolian Extensional Province (Sengör et al. 1985;Bozkurt 2001): the Menderes massif in the north and theMugla nappes (also known as Lycian nappes) in the south(Figure 1C). The Menderes massif is an autochthonousPan-African basement, overlain by pre-Devonian-Eoceneschists and marbles and Mesozoic-early Cenozoic plat-form carbonates (Özer et al. 2001). The Mugla nappesform a tectonic package, consisting of slices of the east-ern Taurides metamorphic basement, which is overlain byMesozoic platform carbonates, and the dismembered ophi-olite and volcano-sedimentary units that form the upper-most nappe (Yılmaz et al. 2000). The Mugla nappes, wherethe study area and other similar grabens are located, isbounded by the Fethiye-Burdur fault zone (FBFZ) in thesouth. The FBFZ is an oblique fault with a left-lateralstrike-slip component (Taymaz and Price 1992; Temizet al. 1997; Koçyigit et al. 2000; Yagmurlu 2000; Güreret al. 2004, 2009). This fault zone is the northeastern

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continuation of the Pliny fault zone of the Aegean arcsystem (Dumont et al. 1979; Barka et al. 1995; Eyidoganand Barka 1996; Aksu et al. 2009; Hall et al. 2009)(Figure 1A).

The study area is located in the east of the Ispartaangle and characterized by three NE–SW-trending grabens:Baklan, Acıgöl, and Burdur (Figure 2). These basins aredefined by Price and Scott (1994) as half-grabens, becauseof major NW-dipping, listric basement faults that boundthem to the southeast sides. This graben system is boundedby the NW–SE-trending Dinar fault to the northeast. Thisregion experienced a NE–SW-directed extensional regimein late Miocene time (Temiz et al. 1997). The biaxial NE–SW and NW–SE-trending extension has been active in thestudy area since the late Pliocene (Price and Scott 1994;Temiz et al. 1997; Cihan et al. 2003; Verhaert et al. 2006).This could be related to the westward migration of theAnatolian plate and the rollback of the African slab belowthe Aegean arc (Cihan et al. 2003; Gürer et al. 2009)(Figure 1A). Also, some researchers suggest that the recentearthquake activity in the area indicates a biaxially orientedextension (Taymaz and Price 1992; Eyidogan and Barka1996).

Pre-Neogene rock succession around the Baklan–Dinargraben consists of non-metamorphosed rocks that overlie a

metamorphic basement (Figure 3). The Neogene succes-sion begins with conglomerates that are covered uncon-formably by an interbedded sandstone, claystone, marl,and limestone unit deposited in a lacustrine environ-ment. This unit contains lignites with an economic reserve(about 5 million tonnes (Ünal et al. 1990)), and repre-sents some invertebrate and vertebrate fossils, the agesof which range from early Miocene to early Pliocene(Platen 1967; Konyalı 1970; Dubertret 1973). To thenorth of the graben, Pliocene deposits containing pele-cypod and gastropod fauna are represented by lacustrinelimestones (Konak et al. 1986). Along the northern edgeof the graben, carbonate-cemented conglomerate uncon-formably overlies the basement rocks and represents theinitiated activity of neotectonics in the latest stages ofthe Pliocene in the region. Towards the basin, there isa fluvio-lacustrine sequence that contains red-, brown-,green- and yellow-coloured claystone, marl, siltstone, andintercalated conglomerate lenses. The Plio-Quaternary for-mations deposited under the control of active tectonismare represented by alluvial fan and fluvio-lacustrine sedi-ments. They consist of a sequence (clayey, silty complexcomprising sand, sandy clay with pebble, gravel lenses)mainly resulting from the evolution of the deposition fromlacustrine towards fluvial environment.

Figure 3. Geological map of the study area. The pre-Quaternary units are from the MTA (2002) and the Quaternary units are fromthis study.

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Seismicity

The Baklan–Dinar graben and its surrounding area isknown to have a long historical earthquake period extend-ing through Byzantium, Seljuk, and Ottoman times(Ambraseys and Finkel 1987). In the study area, mostof the historical and instrumental seismicity is related tothe activity of the NE–SW-trending NW-dipping Baklanfault and the NW–SE-trending SW-dipping Dinar fault(Figure 4).

Historical earthquakes of 400 BC, 88 BC, and AD53 damaged the ancient city of Apamea Kibotos (themodern town of Dinar) (Strabon; Ergin et al. 1967; Soysalet al. 1981; Guidoboni et al. 1994). However, there areno records of large earthquakes between AD 53 and 1875(Altunel et al. 1999). The 1875 (Io = IX–X) earthquakeoccurred on the NE–SW-trending Baklan fault (Ambraseys1975, 1988). In this earthquake, 1300 people lost theirlives. Later studies related to this earthquake determinedthat a rupture zone of 20 km had occurred in a regionbetween Dinar and Çivril (Pınar and Lahn 1952).

In the instrumental period, 330 people died in the 1925(Io = VII–IX; M = 6) earthquake that occurred on theBaklan fault (Ambraseys 1975, 1988). The next earthquakein the graben was the 1933 Çivril earthquake (Io = VIII;M = 5.7). In this earthquake, 20 people were killed andover 200 houses were heavily damaged.

On 1 October 1995, the study area was struck by aM s = 6.1 (Pınar 1998) earthquake (Mw = 6.2, Koral 2000;ML = 5.9, Yalçinkaya and Alptekin 2005) located at Dinar(Figure 4). In this devastated town, which originally hada population of 35,000, 96 people were killed 260 wereinjured, and over 25,000 people left homeless (Erdik et al.1995; Saroglu et al. 1995; Demirtas et al. 1996; Pınar 1998;Altunel et al. 1999). A 10 km-long surface rupture wasobserved following the earthquake (Saroglu et al. 1995;Demirtas et al. 1996; Eyidogan and Barka 1996; Koralet al. 1997). The total length of the NW-striking Dinar faultis about 75 km (Pınar 1998; Koral 2000). The palaeoseis-mological work of Altunel et al. (1999) into the surfacerupture of the 1995 earthquake shows that two large eventshave occurred on the Dinar fault during the past 3500 years,even larger than the last event.

Morphotectonics

The purpose of this article is to use geomorphic and struc-tural data to describe the evolution of drainage network,basin-margin and basin-fill morphologies, and discuss theirmeanings in the neotectonic framework of the region.Geomorphic markers have been used to evaluate the upliftof basin margins and the responding basin-fill. Field stud-ies, geological maps, 1:25,000-scaled topographic maps,

Figure 4. Structural setting and instrumental seismicity of the Baklan–Dinar graben. Focal mechanism solutions: 1 October 1995 Dinarearthquake is from Tan et al. (2008) and 13 July 2009 Çal earthquake is from Tan O. (2009, personal communication).

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and digital elevation models are combined to illuminate theeffects of structural features on morphology.

Drainage network

The drainage network of the Baklan–Dinar graben is shownin Figure 5. It is formed by secondary hydrographic net-works connected to the Büyük Menderes River, one of themost important rivers in Turkey. These streams, locatedat the northern, southern, and southwestern ends of thegraben, respectively, flow perpendicular to the NW–SEand NE–SW-trending basin margins. The drainage basinhas a watershed that follows the main elevations of theregion. To the NNE of the graben, the watershed is moredeveloped than the other margins and this is also sug-gestive of high tectonic activity. The drainage pattern inthis area is dentritic, and the lithology is represented bymainly carbonate rocks. The streams that drain to the northof the Akdag Mountains feed Lake Isıklı and then con-nect to the Büyük Menderes River. To the southeast ofLake Isıklı, in the Dinar Basin, a stream shows linear-ity, flowing parallel to the basin elongation towards LakeGök. Lake Isıklı and Lake Gök are connected by an artif-ical waterway like its natural form in the past. Theselakes are separated from each other by a large alluvial fandeposited by the Akçay stream (Figure 3). To the west ofLake Isıklı, the Küfi stream, which is the largest streamof the drainage basin that feeds Lake Isıklı and the BüyükMenderes River, used to be directly connected to the BüyükMenderes River before the year 1952. At this date, the Küfi

stream connected to Lake Isıklı with a channel (Figure 5).To the west, in the NE–SW-trending Baklan Basin, thestreams which drain into the basin are short and ephemeral.Many of them cannot reach the Büyük Menderes River.In the southeast of the Baklan Basin, a circular drainagebasin developed on the Besparmak mountains (Figure 5).This is the single important stream network in this basin.The circular character of this drainage basin represents alesser tectonic uplift rate in this area during the Quaternaryperiod. In the southwest of the Baklan Basin, the watershedis more irregular and the streams do not reach the BüyükMenderes River.

Basin-margin morphology

Fault-controlled slopes

The presence of stepped normal faults is a common char-acteristic in regions of extensional tectonics and plays animportant role in slope development. The slopes bound-ing a basin exhibit different morphologies depending ontheir lithology and structural setting. The basin floor of theBaklan–Dinar graben lies at about 820 m a.s.l., where theouter and inner margins of this reverse V-shaped grabenreach elevations of about 2450 m (Akdag) and 1600 m a.s.l.(Besparmak) at the Dinar and Baklan basins, respectively(Figure 6). Ten slope profiles cross-cutting the mountainfronts of the studied graben are used to determine thetopographic knickpoints to evaluate possible effects ofbasin-margin faults on basin development. The first sixbelong to the outer margin of the graben and the followingfour profiles to the inner margin.

Figure 5. Drainage map of the Büyük Menderes River in the study area.

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Figure 6. Slope profiles along the margins of the Baklan–Dinar graben. Slope breaks pointed with lines interpreted as faults.

The outer margin of the graben, especially the easternflank, has the steepest slopes. According to the profiles onthis margin, SP4 and SP5 show the stepped fault scarpsand palaeosurfaces, whereas the others present only onemajor scarp (Figure 6). The palaeosurfaces on the Akdagare located at elevations of about 1600 and 1100 m anddefined as belonging to the late Miocene and Pliocene ages,respectively (Ceylan 1998). The eastern flank of this mar-gin appears strongly controlled by the activity of the Dinarfault. However, the presence of some structural evidencefrom the basin-margin faults, in addition to geomorphicdata, can be taken as pointing to neotectonic period activityat this fault.

In a field survey along the Dinar fault, clear normalfault planes (Figure 7) and kinematic indicators (Figure 8)were detected. The western flank of the outer margin,controlled by the Çal fault, has a completely different geo-morphic expression with respect to the eastern one. In thefield, we observed a few fault plane data points that shownormal faulting on this flank towards the northeast. Theybelong to the neotectonic period, but no tensor could becalculated because of the data population (n < 4); theywere probably not preserved because of their rheology.However, the mountain–piedmont junction and the slope

profiles show clear straight ENE–WSW trends, typical offault-controlled slopes (Figure 6).

The eastern and western flanks of the inner margin arecompletely different from each other in their morphologicexpressions (Figure 6). The western flank has the steep-est slopes with respect to the eastern flank. This flank iscontrolled by a prominent fault scarp known as the Baklanfault. As shown in the SP7 profile, which cross-cuts thisfault, two major fault scarps are represented in the mor-phology (Figure 6). Compared to the scarps of the Dinarfault, the Baklan fault presents low vertical displacement.

Alluvial fan morphology

Tectonic controls may influence sediment production in thesource area, and, together with gross topography, appearprimarily to control fan location, setting, and geometryrather than fan-sediment sequences (Harvey et al. 2005).Thus, we have studied this type of sequences to determinethe impact level of active tectonism by fixing their spatialvariations along the basin margins.

Basin-margin deposits of the Baklan–Dinar graben arecomposed of low-gradient alluvial fans that have beenfed by transversal streams flowing through the surround-ing mountains. Principal fans can be distinguished along

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Figure 7. Examples of normal fault planes along the Dinar fault. FP, fault plane; Tr, travertine; SD, slope debris.

Figure 8. Schmidt lower hemisphere, equal-area projection of principal stress axes constructed from fault-slip data on theDinar fault.

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the eastern flank of the outer and western flank of theinner margins of the graben. These deposits, which hidethe lower lacustrine sequence, appear to control the NNEgradient of the basin floor.

The eastern flank of the outer margin has the steep-est slope and presents an important debris deposition atits foot. It represents the source of high-angle young fansdeveloped over talus and older fans of the basin-margindeposits. Along this flank, Plio-Pleistocene alluvial fandeposits that belong to the activity of the Dinar fault consistof grey, clast-supported, carbonate-cemented conglomer-ates in the lower levels (Figure 9A), and are dominated bylate Quaternary alluvial fan deposits and uncemented slopedebris in the upper levels (Figure 9B). Along the Dinarfault, a large alluvial fan is located between Lake Isıklı andGök. As mentioned before, this fan morphology separatesthese two lakes. There is a waterway connection betweenthese lakes that traces the geometry of the distal fan(Figure 3).

Furthermore, as shown in Figure 10A, clear evidence ofhorizontal displacement on the morphology of Oligocenedeposits represents a left-lateral movement on both mar-gins of the Baklan Basin, approximately 30 km in length. In

the southern part of the western flank of the inner margin,well-developed alluvial fan sequences of the Pliocene areobservable by their large extending morphologies. On thismargin, towards the basin, right-lateral strike-slip faults aredetermined by the morphology of the northern edges ofthese clastics (Figure 10B). They would be related to thetorsion in this side because of counterclockwise rotationthat developed as a result of Plio-Quaternary movementson the left-lateral basin-bounding faults. Except this, on thewestern flank of the outer margins and the eastern flank ofthe inner margins of the Baklan Basin, Quaternary allu-vial fans are observable by their idealized morphologies(Figure 10C and 10D).

Basin-fill morphology

Basin-floor topography

To understand the impact of active tectonism on the basinfloor, eight topographic profiles were taken between thegraben flanks (Figure 11). The profiles PP1 and PP5 areparallel to the basin’s long axes, and the rest of the pro-files are perpendicular to these axes. As shown in PP1, theBaklan Basin is separated by a threshold. The southwardly

Figure 9. (A) Carbonate-cemented conglomerates of the Plio-Pleistocene; (B) general view of the southwestward-facing scarp of theDinar fault and the Quaternary alluvial fan and talus deposits.

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Figure 10. (A) Left-lateral displacements on the morphology of the study area (χ = 10 km). (B) Right-lateral movements on the ridgemorphology of the Pliocene clastic deposits in the southwest of the Baklan Basin. (C,D) Quaternary deposits that were controlled bythe active faults on the northwest and southeast margins of the Baklan Basin. Qal, quaternary alluvial fan; Qfl, quaternary fluvial; Qfl-lc,quaternary fluvio-lacustrine deposits.

draining Büyük Menderes River drains northwestward tothe middle of the Baklan Basin because of this elevatedarea. There is a palaeolake plain towards the north of thisthreshold. This Pleistocene lake was named Çivril-Isıklıpalaeolake by Kazanci et al. (2009). They suggested thatthis palaeolake drained because of its capture by the BüyükMenderes River to the north. The PP2 that cut across theDinar Basin contains two recent lake basins (Lake Isıklıand Gök), which are separated from each other by a largeQuaternary alluvial fan. The general gradient of the basinfloor of the Dinar Basin slopes towards the north. In theBaklan Basin, PP2 and PP3 show a northwestward gra-dient of the basin-floor, whereas the PP4 presents a lowdegree of gradient towards the southeast. To the DinarBasin, PP6 represents a relatively flat floor and two fault-controlled margins. PP7 and PP8 show a northeastwarddipping basin-floor because of the activity of the Dinarfault.

Basin-sole geometry

To draw the basin-sole geometry, data are available inthe form of borehole and geophysical information. Thereare several boreholes in the Baklan–Dinar graben, drilledby the General Directorate of State Hydraulic Works ofTurkey (DSI) over the past 50 years.

Boreholes are especially present in the Çivril-Isıklıarea, where the Baklan and Dinar basins intersect, becauseof attempts by the state to determine the groundwaterreservoir of the basins. Some drilled borehole data areavailable for the west of the Baklan basin, but it is relatedto the determination of the lignite reserve of the Neogeneformations and is not useful to the main pursuit of ourstudy. Six of DSI’s corelogs around the Gümüssu andIsıklı (Figure 12) are used to define the sole geometry ofPlio-Quaternary formations. In the cores, Pliocene bluishand greenish grey claystone and marl were drilled at 9th,74th, and 90th metres (Line-1), and 36th, 66th, and 84th

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Figure 11. Basin-floor topographic profiles across and parallel to the Baklan (PP1–3) and Dinar grabens (PP5–8). Slope breaks pointedwith lines interpreted as faults on profiles.

metres (Line-2) along two lines. The basin-sole geometryshows that the Dinar and Baklan basins have asymmetricsoles, and defines the activity of the Dinar fault, becauseof the increased thickness of fluvial and debris depositstowards it.

The asymmetric basin-sole geometry of the graben isproved by the data obtained from the geophysical resistiv-ity survey by Özpınar (1978) applied around the town ofDinar (Figure 13). According to resistivity data, the thick-ness of the Plio-Quaternary deposits increase towards theDinar fault and agree with the northeastward gradient ofthe basin-sole geometry.

In the Baklan Basin, however, lacustrine deposits ofthe Pliocene crop out towards the southwest and along thewestern part of the Büyük Menderes River. Towards thenorthern edge of the graben, these deposits are drilled at anapproximately 100 m depth around the town of Çivril. Allthese data indicate that the Baklan Basin has a northwardasymmetric sole geometry like the Dinar Basin, but on alongitudinal axis.

Discussion

Earlier workers regarded the NE–SW-trending Baklan,Acıgöl, and Burdur basins in SW Turkey as simple

half-grabens (Price and Scott 1991, 1994; Taymaz andPrice 1992). They defined these grabens by the NW-dipping, active listric faults that bound them to theirsoutheast sides. In the framework of those studies, the NW–SE-trending graben (Dinar graben) is defined as a passivestructure. Some other workers suggested that these NE-trending grabens originated as orogen-top rifts (intramon-tane grabens), the result of the late Neogene NW–SE exten-sion in the region after the placement of the Mugla nappesin the late Serravalian (Alçiçek 2007; ten Veen et al. 2009).

However, many researchers define the studied areaand surrounding region as an active biaxial extension,especially after the 1995 Dinar earthquake (Mw = 6.2)on the Dinar fault (Koçyigit 1984; Westaway 1990; Temizet al. 1997; Cihan et al.2003; Verhaert et al. 2006).This system is recognized by structural data from thefield and seismicity of the region. But, in the BaklanBasin, our recognition of relative tectonic uplift ratesfrom the drainage network, gradient of the basin-floortopography and basin-sole geometry, spatial features ofalluvial fans, left-lateral faults with displacements onthe morphology towards the southwest of the basin, andaccommodated left-lateral strike-slip focal mechanismsolution of the last earthquake in the Baklan Basin (13 July

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Figure 12. Locations and stratigraphy of boreholes that were drilled by General Directorate of State Hydraulic Works of Turkey (DSI)in the northern part of the Baklan–Dinar graben. North-northeastward asymmetrical basement geometry of the Plio-Quaternary depositscould be surveyed by these core logs.

2009 Çal earthquake, M = 4.2; Tan, O., 2009, personalcommunication) (Figure 4) compel a reinterpretation ofthe Plio-Quaternary evolution of the region.

The deformation in western Turkey is accompanied bynormal faulting over many years, as represented by field-based earlier studies. After the most recent earthquakes inthe Izmir Bay region (1992, Mw = 6.0; 2003, Mw = 5.7;2005, Mw = 5.8), which revealed a strike-slip motion,some scientists discuss active strike-slip faulting as a pos-sibility in western Turkey (e.g. Ocakoglu et al. 2004, 2005;Benetatos et al. 2006; Aktar et al. 2007; Uzel and Sözbilir2008). Uzel and Sözbilir (2008) defined a right-lateralshear zone (Izmir-Balıkesir Transfer Zone) in the west-ern section of western Turkey towards the northern Aegeanin the extensional framework of the region. However, left-lateral shear zones are also known, through both offshoreand onshore studies, to affect the southeastern Aegean(Peters and Huson 1985; Duermeijer et al. 1998; Mascle

et al. 1999; ten Veen and Postma 1999; Huguen et al. 2001;ten Veen and Kleinspehn 2002, 2003; van Hinsbergen et al.2007; Aksu et al. 2009; Hall et al. 2009).

Morphological, structural, and seismological dataattained in the Baklan–Dinar graben can be interpreted bya system known as the cross-graben model in the literature(Sengör 1987). The model developed by (Sengör 1987) wasoriginally applied to the E–W-trending Gediz graben andthe NE-trending Gördes, Demirci, Selendi, and Usak-Gürebasins to its north (Bozkurt 2003). According to thismodel, in the NE-trending basins, the faults that boundthe grabens have oblique-slip natures and have acquireda strike-slip configuration as illustrated in the modelin Figure (14A), rather than typical extensional basins(Sengör 1987; Bozkurt 2003). These basins lie on the hang-ing wall of a breakaway fault and the basin-bounding faults(accomodation faults) of the cross-grabens do not cut thebreakaway fault (Sengör 1987; Bozkurt 2003).

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Figure 13. Simplified geological map and E–W cross sections of the Dinar Basin according to resistivity data (after Özpınar 1978).Circles indicate the recording stations.

The cross-graben nature of the NE-trending BaklanBasin can be interpreted in this scenario because ofthe left-lateral oblique basin-bounding faults in the Plio-Quaternary period. However, the Acıgöl and the Burdurbasins are oriented parallel to the Baklan Basin and rep-resent a system scene. The Burdur Basin is bounded to thesouth by the NE-trending left-lateral oblique FBFZ, whichis interpreted as the landward continuation of the Pliny-Strabo trench (Barka et al. 1995, 1997; Barka and Reilinger1997; Gürer et al. 2004; Aksu et al. 2009; Hall et al. 2009).This graben system terminates against the NW-trendingDinar graben. The Dinar graben is bounded to the NE mar-gin by the Dinar fault, which might play a major structuralrole in the neotectonic framework of SW Turkey and isconsidered as a breakaway fault (Barka et al. 1995; Temizet al. 1997; Sintubin et al. 2003). The fault extends fromnortheastern ends of the Burdur graben in the south to theBaklan graben in the north with a length of approximately75 km. The fault has a SW-dipping normal fault plane,from whose footwall Mesozoic limestones, Palaeocenemelange, and Oligocene clastics were exhumed. Price andScott (1994) mentioned a left-lateral component for theBaklan fault but they tried to explain the development ofthe Baklan graben within a classical NW–SE extensionframework. They claimed that there is no evidence fora position for the Dinar fault in the cross-graben model

of Sengör (1987). First, they suggested that there is nodeepening towards the Dinar fault, and second they sug-gested that there is no seismic record on the Dinar fault.As we represented from the borehole data reviewed forthe study area, there is a northeastward deepening towardsthe Dinar fault. However, as mentioned before, the Dinarfault proved its activity in the 1995 earthquake. After thisearthquake, Temiz et al. (1997) stated this activity relatedto biaxial extension in the region. They suggested thatthe cross-graben model of Sengör (1987) is suitable forthe region. Theoretically, their adapted model reflects thestrike-slip movements on the margins of this NE-trendingsystem as left- and right-lateral displacements, in keepingwith Sengör’s (1987) model. However, they skipped thestrike-slip faults necessary to describe the movements onthe boundaries of the inner horsts and grabens (Baklan,Acıgöl, and Burdur) in the system and they tried to definethe development of this system only with normal faultingthat would belong to the NE–SW and NW–SE extension.

According to morphotectonic observations in theBaklan–Dinar graben, the northwest and southeast mar-gins of the Baklan Basin are controlled by left-lateraloblique faults like the Burdur and Acıgöl basins, and thesebasins are bounded on their northeast ends by the Dinarbreakaway fault (Figure 14B). All these aspects appear toindicate a cross-graben formation. However, unlike from

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Figure 14. (A) Cross-graben model of Sengör (1987) (after Bozkurt 2003). (B) Adaptation of cross-graben model to Baklan–Dinargraben.

the Sengör (1987) model, the NE-trending cross-grabensare not controlled by different lateral offsetted strike-slipfaults. Sengör (1987) expressed in his theoretical modelthat the cross-grabens are controlled by both left- andright-lateral faults (accomodation faults) on their margins(Figure 14A). The case of the Baklan graben would berelated to the dominated regime of left-lateral shearingin the region to the north of the FBFZ, which resultedfrom the oblique convergence between the southeasternAegean region and the northward moving African plate.According to the initiation of the activity of the Dinarfault, which demonstrates NE–SW extension in the lateMiocene and left-lateral faulting along both margins of theBaklan Basin since the Pliocene period, the earlier activityof these basins is related to same stress state that resultedfrom the pulling of the Anatolian plate towards the south-west, because of the northward subduction of the Africanplate along the Aegean arc. In contrast to the previousbiaxial extension proposals in the literature, we suggestthat a NE-trending uniaxial extension would be enough tocause the Plio-Quaternary evolution of cross-grabens in theregion (Figure 13). From this perspective, the increase inthe rate of the normal components of the left-lateral basin-bounding faults of the Baklan Basin towards the northeastof the Dinar breakaway fault would be explained by themodel shown in Figure 15. This model is adapted fromMouslopoulou et al. (2007). They developed this model toexplain the gradual steepening of the pitch of the slip vec-tors in the fault geometry of the North Island Fault Systemin New Zealand that intersects and terminates against theTaupo Rift.

Kazancı et al. (2009) suggested that Pleistocenedeposits in the Büyük Menderes graben are very thin ver-sus the current large catchment area (ca. 24,300 km2) of theBüyük Menderes River that flows throughout the graben.They proposed that the Büyük Menderes River capturedthe Çivril-Isıklı palaeolake in the latest Pleistocene ageunder the control of tectonics and surface processes anddoubled its own drainage area. The quantity of its Holocenedeposits is relatively large because of this capturing pro-cess. In the Baklan Basin, a counterclockwise rotationoccurred because of defined left-lateral oblique faults inthe northwest and southeast margins; the basin-floor of

Figure 15. Schematic block diagram showing the left-lateralfaulting in the Baklan Basin. Slip vectors steepen northwardwhere the basin-bounding fault of the Baklan Basin intersectswith the Dinar breakaway fault because of captured extension.

this basin also uplifted in the SW and tilted northward toopposite flank. As a result, today’s Büyük Menderes Riverin the Baklan–Dinar graben drains northward after a coun-terclockwise rotation and uplift in the southwest of theBaklan Basin during the Quaternary period.

Conclusions

The Baklan–Dinar graben consists of the coupledgeometry of the NE–SW-trending Baklan and the NW–SE-trending Dinar basins. According to the morphologicand seismic data that have been acquired for the BaklanBasin, this graben is bounded on the NW and SE marginsby left-lateral oblique faults. The same type of informationand additional structural, borehole, and geophysical datathat have been obtained from the Dinar graben indicatenormal faulting and basin deepening towards the Dinarfault. The latter structure extends from the northeast endof the Baklan Basin to the northeast end of the BurdurBasin and bounds three NE-trending Plio-Quaternarybasins with a contrary orientation. In SW Turkey, thelate Miocene origination of the NW-trending Dinar faultthat bounds the Dinar graben, and the following devel-opment of the NE-trending left-lateral oblique faults ofthe Baklan Basin in the Pliocene, are apparently related

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to NE–SW-directed extension in the region as a resultof an oblique convergence of the southeastern Aegeanregion and the African plate. During the Plio-Quaternaryperiod, the last stage of extension along the same axiscontrolled the pre-existing basin-bounding faults in thecross-graben system. However, from this perspective,the origination and active controlling of the Acıgöl andBurdur grabens located in the south of the studied regionwould be evaluated in the same framework, because oftheir left-lateral basin-bounding faults known from theliterature. The counterclockwise rotation that occurred inthe southwestern end of the Baklan graben because ofleft-lateral strike-slip movements in both the northwest andsoutheast margins, as well as related uplift in the southeastand the northward tilt and development of the Çal Basinin the north, resulted in the capture of the Çivril-Isıklıpalaeolake that was located in the Baklan–Dinar grabenin the Pleistocene, as a result of the northward draining ofthe today’s Büyük Menderes River.

AcknowledgementsThe authors are grateful to Nizamettin Kazancı (AnkaraÜniversitesi) for his guidance in the field, to Gürol Seyitoglu(Ankara Üniversitesi) and Ömer Feyzi Gürer (KocaeliÜniversitesi) for their constructive reviews that improved themanuscript significantly, and to Camille Wengreen, who revisedthe English of the manuscript. Yasar Suludere (JEMIRKO) andÖzlem Makaroglu (Istanbul Üniversitesi) are specially thankedfor assisting in the fieldwork. And we are also thankful to DSIÇivril Bölge Müdürlügü for welcoming us during our fieldstudies.

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