Volcanic and deformation history of the Bodrum resurgent caldera system (southwestern Turkey)

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Journal of Volcanology and Geothermal Research 136 (2004) 71–96

Volcanic and deformation history of the Bodrum resurgent

caldera system (southwestern Turkey)

I. Ulusoya,*, E. Cubukcua, E. Aydara,*, P. Labazuyb, A. Gourgaudb, P.M. Vincentb

aHacettepe University Department of Geological Engineering, 06532, Beytepe Ankara, TurkeybUniversity Blaise Pascal, UMR-CNRS 6524, 5 rue Kessler, 63038 Clermont-Ferrand, France

Received 9 May 2003; accepted 29 March 2004

Abstract

The volcanic rocks of the Bodrum Peninsula, in SW Turkey and NE of the Hellenic Arc, outcrop over an area of 138 km2. A

monzonitic intrusion is exposed in the western part of the peninsula. Upper Miocene volcanism is represented by high-K (HK)-

andesitic, andesitic lava flows and pillows, sparse HK-andesitic and dacitic lava domes and associated block-and-ash flows. A

HK-andesitic ignimbrite sequence with two stratigraphic units is associated with the collapse of a complex caldera system.

Breccias, formed as a result of slumping of the caldera walls are observed inside the caldera. Post-caldera activity is represented

by HK-andesitic, HK-basaltic andesitic lava flows, domes and associated block-and-ash flows. Numerous dykes, HK-andesitic

and shoshonitic in composition cut all volcanic units.

The structure of the Bodrum caldera was investigated using SPOT image, digital elevation model (DEM), aerial photographs

as well as field data. The Bodrum caldera is a NE–SW-elongated, semi-elliptical, deeply eroded caldera with dimensions of

18.7� 7.7 km. It is partly submerged in the SW part. The complex caldera system can be described in terms of two structural

domains. The collapse of the Dagbelen domain is interpreted as a piston type subsidence, while the Karakaya domain represents

a piecemeal collapse. Both domains exhibit two separate resurgence events. The elongation of the caldera may be related to pre-

existing regional tectonic structures. The caldera is also affected and cut by late stage faults related to regional extensional

events.

Moreover, pre-caldera volcanism is dispersed and cannot be related to a pre-existing stratovolcano. Bodrum volcanism is

therefore interpreted as a complex ignimbritic shield volcano.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Bodrum; Turkey; caldera; resurgence; tectonics; remote sensing

1. Introduction

The Bodrum area is a peninsula located in SW

Turkey and NE of the Hellenic arc, covering 250 km2

(Fig. 1). It is historically well-known; Herodotus, the

0377-0273/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jvolgeores.2004.03.016

* Corresponding authors.

E-mail addresses: [email protected] (I. Ulusoy),

[email protected] (E. Aydar).

‘‘father of history’’, was born in Bodrum (Halicarnas-

sos) in about 484 BC. Bodrum is also the site of one of

the seven wonders of the world: the Mausoleum built

by Artemisia II in honour of her husband King

Mausolos. It is also a popular holiday resort region

where most of the geological features are hidden by

extensive construction.

The Aegean region exhibits strong seismic activity

and complex, rapidly changing tectonics (Dewey and

Fig. 1. Geological sketch map of the Bodrum peninsula (geochemical rock descriptions are taken from Cubukcu, 2002).

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I. Ulusoy et al. / Journal of Volcanology and Geothermal Research 136 (2004) 71–96 73

S�engor, 1979). Two main tectonic regimes occur in the

western Anatolian region. The first is a paleotectonic

regime characterized by N–S compression (Kurt et al.,

1999). The second is a neotectonic regime, defined by

N–S extension and collapse of E–W-directed grabens.

Dewey and S�engor (1979) offered a widely accept-

ed model for the cause of extension. Extension of the

Aegean region is related to the southwestern escape of

the Anatolian plate (McKenzie, 1972) in response to

convergent movement of the Arabia–Africa and Eur-

asian plates along the Bitlis collision zone (Dewey and

S�engor, 1979). This westward escape of the Anatolian

plate relative to Africa is compensated by the Hellenic

subduction (Dewey and S�engor, 1979). The south and

southwestern escape of Anatolia (30–35 mm/year,

Kahle et al., 1998), points to the western Aegean being

affected by a counterclockwise rotation (Walcott and

White, 1998; Rotstein, 1984; Reilinger et al., 1997).

While there is an agreement regarding an N–S

extensional regime, there are differing ideas on the

cause and onset time of the extension. Dewey and

S�engor (1979) proposed Upper Miocene and Kurt et

al. (1999) proposed Upper Miocene–Pliocene. How-

ever, Seyitoglu and Scott (1991) placed the extension

in the Early Miocene and Walcott and White (1998)

at Oligocene–Early Miocene.

Volcanic rocks outcrop in the west of the penin-

sula, while the eastern part is covered by limestone.

Previous works mainly focused on the petrology and

geochemistry of magmatic rocks (e.g., Ercan et al.,

1984; Robert et al., 1992; Kurt and Arslan, 2001;

Cubukcu, 2002). In addition, Robert et al. (1992)

doubted the formation of ‘‘resurgent dome-like’’

structures. Genc� et al. (2001) considered the Bodrum

area as a stratovolcano with secondary volcanic

centres. Altunkaynak and Yilmaz (2000) described

a stratovolcano, Kizilcamandira, 5� 5 km in size,

located in the SW of the peninsula. Moreover,

Altunkaynak and Yilmaz (2001) defined the Turgu-

treis stratovolcano as having five evolutionary

stages: pre-cone phase, cone-building phase, climac-

tic phase related to a very small caldera (300� 700

m), post-caldera phase and late phase.

Based on new data (SPOT image, DEM, aerial

photographs as well as field data), we focused our

work on a more complex caldera system than previ-

ously described, including its boundaries, resurgence

events and relation to tectonics.

2. Geological outlines

The volcanic rocks of the peninsula are distributed

over an area of 138 km2. Robert and Montigny (2001)

defined the peninsula as part of a chemical province

including the Kos and Patmos islands, called the

Dodecanese Province. Robert et al. (1992) defined

the mafics of Bodrum as two groups: ultrapotassic

and shoshonitic rocks. Cubukcu (2002) summarised

the characteristics of the peninsula’s chemical evolu-

tion as follows: magmatism commences with a mon-

zonitic I-type pluton followed by high-K (HK)

calcalkaline intermediate lava and associated block-

and-ash flows. The volcanism attains a shoshonitic

character with evacuation of the magma chamber by

caldera forming ignimbritic eruptions. Volcanism

ceases with medium-K calcalkaline intermediate prod-

ucts. All the geochemical volcanic descriptions are

taken from Cubukcu (2002).

Radiometric dating of the volcanism falls within

the range of 11.2–9.3 Ma (Pis�kin, 1980; Robert etal., 1992; Fahmi et al., 1997; Robert and Montigny,

2001), while some dykes and domes are younger

(7.8 and 7.5 Ma, respectively; Robert and Mon-

tigny, 2001). The contemporaneous emplacement of

volcanic rocks and monzonitic intrusions (11.2F 1.6

Ma; Pis�kin, 1980), located in the western tip of the

peninsula (Fig. 1), was demonstrated by radiometric

dating.

Although western Turkey has experienced crustal

extension since the lower Miocene, in the Bodrum

area, the extension-related E–W-trending structure

(Gokova graben, illustrated in Fig. 1) originated in

the late Miocene–Pliocene period (Kurt et al., 1999).

In the seismic reflection studies applied to the

Gokova graben, Kurt et al. (1999) found WNW–

ESE-oriented faults, younger than the E–W-oriented

faults which are graben-related. NW–SE faults in the

vicinity of the Bodrum Peninsula have also been

observed by previous researchers (i.e., Ercan et al.,

1984).

2.1. Pre-caldera activity

The volcanism of the Bodrum Peninsula started

with HK-andesitic, andesitic lava flows (sometimes

pillow facies), sparse HK-andesitic and dacitic lava

domes and associated block-and-ash flows, considered

I. Ulusoy et al. / Journal of Volcanology and Geothermal Research 136 (2004) 71–9674

as belonging to the pre-ignimbrite period. The radio-

metric age of this initial phase of volcanism is

11.2F 1.6 Ma (Pis�kin, 1980).Pre-caldera volcanism is not widely developed in

the Bodrum Peninsula. It is a dispersed volcanism,

poorly extended. Only four monogenetic dome-like

edifices, and sparce lava flows have been observed.

Such volcanism cannot be related to a previous

stratovolcano.

2.2. Caldera forming eruptions

Two stratigraphic sequences are associated with

ignimbritic emplacement: Kale ignimbrites and

Akvaryum ignimbrites (Ulusoy, 2002). They are

HK-andesitic in composition and fall approximately

within the same time span: 10.9F 0.3 Ma (Fahmi et

al., 1997).

2.2.1. Kale ignimbrites

The castle (meaning in Turkish ‘‘Kale’’) of Bod-

rum was built by the knights of St. John. The walls

of the castle were built with ignimbrite. The first

ignimbritic sequence outcrops within a limited area

between Koyunbaba bay and Sivrikaya (Fig. 1)

where it fills N-, NE- and W-oriented valleys. Kale

ignimbrite covers an area of 3 km2, with an estimat-

ed volume of 0.21 km3. The apparent thickness

reaches 70 m.

In the western part of the peninsula, in Koyun-

baba bay, a poorly sorted ignimbritic unit exhibits

medium to well-welded rheology (with 15% clasts)

with fiamme texture. Computed ‘‘flattening ratios—

FR’’ (height/length) for the pumices show that the

FR of coarse pumices (0.1) are higher than the fine

ones (0.2; 0.3). The welded texture of the ignim-

brite, lahar-like block-and-ash flow deposits overly-

ing the ignimbrite, the shallow sea sediments

overlying this block-and-ash flow deposits and the

pillow lavas near Geris village at 130 m altitude are

evidence of the submarine emplacement of the Kale

ignimbrite, at least, along the western shores.

Kale ignimbrite is also observed at Sivrikaya,

with a coarse-lithic-rich outcrop. Two distinct tex-

tural layers are observed: a matrix-supported layer

with rare, but coarse (10–15 cm) clasts lies at the

base (Fig. 2a); a second clast-supported layer with

coarse to very coarse (up to 4 m, Fig. 2b) clasts

overlies the first layer. The transition is sharp

between these two layers. The matrix-supported

layer is green and composed of a coarse-ash sized

matrix.

In the southern part of Sivrikaya, a reworked

ignimbrite tongue extends into the caldera (Fig. 2c).

2.2.2. Akvaryum ignimbrites

They are well exposed in ‘‘Akvaryum bay’’ (Fig.

1). This second sequence outcrops widely in the

south, southeast, central and northern parts of the

peninsula. The area covered is about 25 km2. The

calculated volume, using the measured stratigraphic

sections and direct current resistivity measurements

(Table 1) is 8.34 km3. Numerous air fall and

pyroclastic flow deposits interstratified with block

and ash flow deposits occur at the top of the

sequence. They are generally non welded and

sometimes hydrothermally altered, near Mandira

village and Bagla cape. The total thickness reaches

about 330 m.

Akvaryum ignimbrites comprise four eruption

phases (A, B, C and D phases). The stratigraphic

columnar sections of the Akvaryum, Turkbuku,

Gundogan, Mandira Hill, Bitez and Bodrum sites

are given in Fig. 3. Their sedimentologic and erup-

tive properties are summarized in Fig. 4 and Table 2.

Photographs of some key units described in Fig. 3,

Fig. 4 and Table 2 are shown in Fig. 5.

2.3. Resurgence and post-caldera activity

The central part of the peninsula is topographi-

cally higher than the surrounding area. Two main

domes with N–S elongation appear at the centre of

the peninsula: the Yakakoy and Karakaya domes.

Intensively hydrothermalized rocks cover this part.

The rocks are silicified and/or clayey; some zones

contain pyrites and manganese mineralizations. Im-

portant welded breccia deposits are found in the

central part. Tilted shallow-sea sediments are ex-

posed near Gurece and Yakakoy. The Karakaya

dome is cut by many porphyritic HK-andesitic

dykes, with coarse sanidine crystals and mafic

enclaves (Fig. 6).

In the NE central part of the peninsula, another

dome-shaped body rises: the Dagbelen dome. The

top of the dome is 451 m high and its diameter

Fig. 2. The first of the two main ignimbritic sequences of the peninsula: Kale ignimbrites at Sivrikaya location, on the caldera rim. (a) Proximal

ignimbrite emplacement at Sivrikaya location, border of clast supported and matrix supported parts are indicated with dashed line. (b) Coarse

lithics up to 4 m in the clast supported part at Sivrikaya location. (c) Tongue-like resedimentation into the caldera (small arrows point out the slip

cracks) with landslide from caldera rim.


reaches 3 km. Its estimated volume is 4.25 km3.

This dome is largely affected by hydrothermal

alterations. Silicified limestone and a small HK-

andesitic lava flow were seen to overlie the dome.

The outer part of this structural doming, which is

not affected by hydrothermalism, is covered by

ignimbrites and block-and-ash flow deposits in the

northern part, as well as by recrystallised limestone

in the eastern part (Fig. 1).

Slumped and piled ignimbritic blocks belonging to

the inner wall of the caldera are observed along this

geological limit. A cone-sheet with a 32j dip out-

crops at the northern part of the Dagbelen dome (Fig.

1). It differs from the other dykes of the peninsula by

its structural emplacement, rhyolitic composition and

glassy texture.

The southwestern part of the Dagbelen dome is

covered by polylithologic breccias (Fig. 1). In addi-

tion to these breccias, many strike slip and dip slip

faults are observed between Dagbelen and Yakakoy

(Fig. 1).

The ignimbritic sequences are capped by HK-

basaltic andesitic, HK-andesitic, basaltic andesitic

lava flows, domes and block-and-ash flow deposits.

Numerous HK-andesitic and shoshinitic dykes are

observed inside and outside the caldera. The lavas

Table 1

Thickness and resistivities of the underground layers at different measurement locations

Unit Resistivity Thickness Unit Resistivity Thickness

Yakakoy 1 Yakakoy 2

Breccia (?) 121.4 0.69 Alluvion 39.42 1.15

Block-and-ash flow 24.76 1 Block-and-ash flow 11.38 3.06

Ignimbrite 71.05 1.41 Ignimbrite 75.66 2.07

Ignimbrite 27.43 79 Ignimbrite 16.9 88.15

Lava flow 185.2 Lava flow 176.2

Gumusluk 1 Gumusluk 2

Alluvion 13.28 1.61 Kos Ignimbrite 28.48 2.55

Breccia 155.2 6.2 Lava flow 92.11 7.13

Salty unit (?) 3.67 5.04 Salty unit (?) 6.93 3.9

Lava/pluton (?) 181.5 Metamorphic 87.4

Islamhaneleri 1 Islamhaneleri 2

Alluvion 130.2 0.84 Alluvion 76.45 0.73

Debris deposits 44.96 7.63 Debris deposits 42.73 5.19

Lava flow 121.6 Lava flow 134.9 28.64

Hydrothermally 20.62

Turgutreis 1 altered limestone

Alluvion 25.07 0.96

Kos Ignimbrites 12.76 3 Turgutreis 2

? 22.78 45.92 Alluvion 23.68 1.02

? 40.71 Kos Ignimbrites 10.37 1.81

Hydrothermally 69.22 8.46

Geris altered lava flow

Alluvion 41.98 1.02 ? 26.53

Ignimbrite 27.94 20.96

Lava flow 150.8 42.43 Yalikavak

Sediment (?) 43.21 Alluvion 45.2 1.88

Ignimbrite 24.77 42.61

Akvaryum Lava flow 84.26

Alteration/surface 20.49 0.96

Pumice layer 2.5 0.52 Kargi bay

Ignimbrite 89.87 0.52 Alteration/surface 37.14 0.62

Water rich layer 1.73 6.09 Ignimbrite 13.34 3.04

Ignimbrite 61.59 60 Ignimbrite 29.7 25.48

Fall-back (?) 5.79 Ignimbrite 12.33

Yahsi

Surface 59.27 0.37

Recrystallised limestone 21.8 0.74




Recrystallised limestone 18.41

Resistivity is given in Vm and thickness is in m; measurement locations are given in Fig. 3.


and dykes of this stage have been dated between 9.3

and 7.5 Ma (Pis�kin, 1980; Robert and Montigny,

2001).

The most recent volcanic products are pumiceous

flow deposits related to the ‘‘Kos Plateau Tuffs’’

(Smith et al., 1996; Allen et al., 1999; Allen, 2001),

Fig. 3. Correlation of the stratigraphic sections, locations of resistivity profiles and stratigraphic sections. Locations of resistivity profiles and stratigraphic sections are indicated in the

small map. Letters of the unit names indicated with small arrows are coded as follows: ‘‘Na’’, block-and-ash flow unit; ‘‘I’’, ignimbrite unit; ‘‘Fb’’, fall-back unit; ‘‘A,B,C,D’’,

eruption phases).

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Fig. 4. Eruption phases of Akvaryum ignimbrites in correlation with the physical stratigraphic properties: pumice and clast sizes in flow and fall units, resedimentation.

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Table 2

Sedimentologic and eruptive properties of the Akvaryum ignimbrites according to the eruption phases

Phases A B C D

Dating Cumulate sample

from Bagla cape

11F 0.7 Ma

(40Ar/39Ar; Robert

et al., 1992)

10.9F 0.3 Ma

(fission track ;

Fahmi et al., 1997)

Apparent

thickness

17 m 21 m 60 m 266 m

Rheology IA4 Well-welded,

colour changes

from red at

bottom to

white at top.

Generally medium

welded IB1 is white,

IB2 is faded pink and

IB5 is white coloured.

Generally medium welded.

Flow units are white in colour.

Generally medium

to well-welded

IA1, IA2,

AI3

Non- to

moderately

welded.

Pale-white,

pink coloured.

Sedimentology Four flow

units (IA1,

IA2, IA3,

IA4)

Similar grain

size distribution

except IA1

(1m for lithics).

IA1, IA2

and IA3 are

medium-sorted,

pumice poor

ignimbrites.

IA4 is a

well-sorted,

pumice-rich

ignimbrite.

Five flow

units

(IB1– IB5)

IB1 is poorly

sorted, reverse

graded. Pink

coloured fine

ash fall units

(FbB2) separate

IB1 and IB2.

IB2 is a coarse

(up to 1,2 m)

lithic bearing

ignimbrite. IB3

exhibits lapilli

size pumice and

lithic bearing

base level,

followed by a

matrix supported

layer. IB4 is

reverse graded.

Initial fall-back

units are followed

by four flow

units (IC1, IC2,

IC3 and IC4).

The lithic sizes

in IC3 and IC4

change in

different localities

and reach 10 cm

in Akvaryum,

36 cm in

Bodrum or

44 cm in

Turkbuku.

Variation in

pumice grain

size is roughly

constant and

reaches

maximum

25 cm in all

localities.

Phase D started

with pink coloured

fall-back products,

exposed mainly

in three localities:

Turkbuku,

Akvaryum,

Bodrum (FbD1).

They are 85, 120

and 425 cm thick,

respectively.

The ignimbritic

sequence of phase D

is composed of

interstratifications

of ignimbrite and

block-and-ash

flow deposits,

observed in

Numerous

thin fall-back

layers at

the base

Max 172 cm

total thickness,F

max,Pumice:

5 cm,F

max,Lithics:

3.3 cm

Fall-back

units at

the base

Up to 5.1 m

total thickness.F

max,Pumice:

1–5 cm,F

max,Lithics:

1–5 cm

northern area.

(continued on next page)


Phases A B C D

Special

property

Well observed in Akvaryum

and Turkbuku outcrops.

Wide extension, most voluminous eruption products Ignimbrite and

block-and-ash flow

interstratifications.

Clearly recognisable

at the north of the

peninsula.

Reworked units clearly

visible at the bottom

of the Bodrum serie

separate phases A and B.

The reworked ash layers

between phase B and

phase C represent a break-

off period in eruptive activity

Observed

area

Southern and northern

parts of the peninsula

Southern, Northern,

Eastern parts.

Southern, Northern,

Eastern parts.

Especially

concentrated on

the northern half

of the peninsula.

Table 2 (continued)


exposed on the western and south western coastal

areas and emplaced 161 ka ago.

3. Image analysis

Information on the physical nature of terrains

and associated structures can be obtained from

digital elevation model (DEM) which enables a

morpho-structural analysis of the topography (Fro-

ger et al., 1998). Satellite images are commonly

used for detecting and analysing tectonic and vol-

canic structures (Bellier and Sebrier, 1994; De Silva

and Francis, 1991; Froger et al., 1998; Adıyaman et

al., 2001; Saintot et al., 1999). Satellite Pour

Observer la Terre (SPOT) satellite image, DEM

images (for both subaerial and submarine environ-

ment) and aerial photographs were used to investi-

gate the volcano-structural properties of the Bodrum

Peninsula where most of the geological features are

hidden by extensive construction. DEM images are

obtained by the digitisation of 2D topographic and

bathymetric maps.

The satellite image (SPOT-4 acquired in July 8,

1999 with a HRVIR instrument) has four bands

(spectral bandwidths are as follows: Band 1: 0.50–

0.59 Am, Band 2: 0.61–0.68 Am, Band 3: 0.79–

0.89 Am, Band 4: 1.58–1.75 Am) and a resolution

of 20 m (coordinates of the image centre: Latitude

37j11V29VV, Longitude 27j34V49VV). Twenty-four

possible combinations of the original four bands of

image were tested for display enhancement qualities.

Nine of the combinations provided valuable results

for structural analysis. A combination of the 4th,

2nd and 1st bands was used as red, green and blue

bands. The image was not originally georeferenced,

though we focused on a subset of 30.5� 32 km2

from the entire image and georeferenced the image

using ground control points. One of the basic

methods used for the image processing, was radio-

metric enhancement technique: ‘‘Look Up Table’’

(LUT) (Schott, 1997; Richards, 1993). Filtering

techniques for edge detection were applied to define

the lineaments.

Morphological analysis is the best method for de-

fining the structure of calderas, because calderas pres-

ent particular morphologies such as topographic rims.

Lipman (1984, 1997) successfully defined the widely

accepted major structural and morphological elements

of a simplified caldera model: a topographic rim, inner

topographic wall, bounding faults, a structural caldera

floor surrounded by these faults, intracaldera fill and

the underlying magma chamber or solidified pluton.

For young calderas with steep walls (i.e., Nemrut

caldera, Turkey, Aydar et al., 2003), it is easy to

detect the topographic rim. But for ancient calderas,

erosion highly affects the topographic rim, so pre-

served rims and flanks are not obvious. Generally,

topographic caldera rims exhibit elliptic and circular

structures.

The DEMs of the peninsula were obtained by

digitising the elevation contour lines of nine 1:25000

scaled topographic maps. The DEM was generated

using the Krigging (with linear interpolation) method.

Fig. 5. Photographs of some key units in the Akvaryum ignimbrites. (a, b) Flow and fall-back units of phase B located at Akvaryum bay.

(c, d) Pink-colored, last fall-back unit of phase C. (e) First ignimbrite unit of phase D, at Gundogan. (f) Same ignimbrite unit of phase D

at Mandira hill.


Fig. 6. (a) Coarse sanidine crystals. (b) Mafic enclaves from a HK-andesitic dyke at Karakaya dome.


The slope image of DEM was generated for a geo-

morphologic approach. A final strong image showing

the slope and elevation relationship (Fig. 7a) was

obtained using the slope image combined with the

coloured image using the colour density slicing meth-

od. Circular features were defined with the final image.

We also used digital elevation model of bathym-

etry (DEMB), after digitising and interpolating the

depth points of a 1:100000 scaled bathymetry map

of the surrounding sea.

Finally, the stereoscopic analyses of 136 pairs of

1:25000 scaled aerial photos of the region were

carried out.

4. Bodrum caldera

The formation of ash-flow calderas by roof collapse

over an underlying shallow magma reservoir is now

widely admitted as accompanying explosive eruptions

that involve magmatic volumes of several cubic kilo-

meters (Lipman, 1997). The eruptions of Kale and

Akvaryum delivered about 8.5 km3 of ignimbrites s.l.

and triggered the collapse of the caldera. According to

our field data and image analysis, the Bodrum Penin-

Fig. 7. DEM, DEMB and SPOT satellite images. (a) The slope image deri

color density slicing method. (White arrows indicate the three main circu

(Black arrows and line indicate the fourth circular morphologic feature in th

the circular features on land.) (c) SPOT image of the peninsula with tw

geological features.

sula exhibits an ignimbritic caldera with complex

collapse and resurgence mechanisms.

Satellite images yield productive databases to

reveal hidden calderas, which are difficult to define

because of their large size and erosional factors.

Anguita et al. (1991) used Landsat images for the

semicircular features in the central part of the Trans-

Mexican Volcanic belt and nine collapse calderas

were defined in the same region (Anguita et al.,

2001).

Digital elevation models, SPOT4 image and the

aerial photos of the region were used to shed light on

the structural properties of the Bodrum caldera. Faults

and lineaments were determined by aerial photos and

SPOT (Fig. 7c) image. Field data are sketched on a

geological map (Fig. 1).

4.1. Caldera morphology and structure

4.1.1. Northeast segment: Dagbelen body

Three large, main circular features were defined

with DEM (Fig. 7a: white arrows) in the investigat-

ed area. The first, around the Dagbelen dome (Fig.

7a), characterises a semi-circular depression. The

NW flanks of this depression represent typical

ved from the DEM combined with the image of DEM colored with

lar feature on the land.) (b) Digital elevation model of bathymetry.

e sea delimited with the � 30 m isobath. Black dashed lines indicate

o main structural faults (dashed black lines) and some important



caldera wall morphology. The depression surround-

ing the Dagbelen dome exhibits concentric fractures,

detectable both on the images (Fig. 8a) and in the

field. These fractures correspond to the ring-faults,

bounding the caldera floor (Fig. 8a). It is known that

the caldera boundaries may be exposed in highly

eroded calderas (Lipman, 1997). Ring-faults, thought

to unambiguously define plate (piston) subsidence,

have been described in different localities (Lambert,

1974; Lipman, 1976; Fridrich et al., 1991) and in

many experimental studies (Walter and Troll, 2001;

Marti et al., 1994; Roche et al., 2000). The depres-

sion around the structural dome is interpreted as a

geological limit. Polylithologic breccias observed

SW of the dome are evidence of the accumulation

of rock fragments along the structural boundary of

the caldera (Fig. 1). This boundary separates the NE

and SW part of the caldera system. The abundance

of faults is related to extensive structural activity.

4.1.2. Northwest segment: Karakaya and Geris areas

The second and third circular features appear as

ridges along a chain of peaks. The second one

describes an arc from Karakaya to SW of Dagbelen

(Fig. 7a). The arcuate shaped ridge from Karakaya

to Dagbelen also displays both the rim and a

boundary fault with a vertical dip (Fig. 8b). Along

this structural boundary, hydrothermally altered

rocks, monolithologic and polylithologic breccias

outcrop inside the caldera. The elements of the

polylithologic breccia were deposited within the

caldera by landslide along the inner wall, after

caldera collapse or synchronously with caldera sub-

sidence. The breccias were formed by the later

injection of lava, leaking into this lithic pile during

post-caldera volcanism (Fig. 9). Monolithologic

breccias result from hydrothermal alteration devel-

oped in the joint systems, formed during autobrec-

ciation of the host rock.

Near Geris, the structural boundary discussed

above is cut by a dextral fault and displaced by

850 m (Fig. 8b). The structural boundary expected

to appear at the southern part of this rim may have

been buried by the slide accumulations at the foot of

Fig. 8. Structural features derived from the SPOT image representing the

peninsula, (b) around Karakaya and Geris villages at the central part and (

lines: faults, dashed lines: lineaments, double lines: dykes, lines with tria

caldera wall, as observed south of Sivrikaya area

(Fig. 2c). The coarse lithics at the Sivrikaya location

(Fig. 2b) probably reflect a vent-proximal emplace-

ment near the caldera wall.

4.1.3. South segment: area between Turgutreis and

Gurece

The third circular feature follows a curve from

south of Turgutreis to Gurece (Fig. 7a). The steeper

upper parts, the flattening down-slope, and the con-

cave profile of the inner slopes of both the second

and third arc are typical features of an inner caldera

wall. The southern part of the caldera boundary lies

between Turgutreis and Gurece village (Fig. 8c). It is

geologically obvious that the inner part of the

structure is constituted by slope-debris deposits in

high land, and by alluvium in low land (Fig. 1).

Such debris accumulation originating from the ero-

sion of the adjacent volcanic highlands has been

previously observed (Lipman, 1997). This debris

accumulation probably covers the structural bound-

ary of the caldera. A lineation within the caldera and

sub-parallel to the caldera rim is observed both on

SPOT image and aerial photos (Fig. 1). The outer

part of the structure is mainly covered by Akvaryum

ignimbrites and post-caldera deposits, in the form of

lava flows and block-and-ash flows. The topographic

wall has moved due to the strike slip faults, oriented

mainly NW–SE (Fig. 8c).

4.1.4. Submerged southwestern segment

The fourth main circular feature is defined by

DEMB, below sea level (Fig. 7b). A morphologic

limit starting at Gumusluk dives into the sea and

follows the Catal, Sariot and Tulluce islands

through Turgutreis, where it meets the land again

(note the � 30 m isobaths surrounding the islands

in Fig. 7b). The average depth of the region

between the rim bordered by the islands and the

land is � 15 m. Depth at the outer part of the rim

suddenly drops to � 70 m and this depth represents

the average depth for a large region. This fourth

circular feature defines the submerged part of the

caldera.

area (a) around Dagbelen dome at the northeastern segment of the

c) Turgutreis–Gurece region at southern part of the peninsula (solid

ngles: caldera border).


Fig. 9. Breccia formations in the caldera, on the Karakaya dome (western central part). (a) Lenticular injection in the breccia (dashed lines


4.2. Widening of the caldera and resurgence

A NE–SW elongated caldera collapsed in the Bod-

rum Peninsula. A three-dimensional image of the

peninsula is proposed, combining DEM and satellite

image of the peninsula, showing the main structural

and morphological features of the caldera (Fig. 10).

Two main structural bodies are present within the

caldera: the Karakaya domain in the SW and the

Dagbelen domain in the NE (Fig. 10). They are

separated from each other by polylithologic breccias

and cross-cutting faults.

Some units of Akvaryum ignimbrites were found

on many outcrops of the peninsula, while some were

outline the injection) and (b, c) breccia– injection relationship.

found only in a limited location, interpreted as the

activation of many vents. Caldera forming eruptions

are generally associated with many syn-activated

vents (Allen, 2001). The ignimbrites outcrop on a

large arc-like area, indicating that they were erupted

from ring fractures. The deposits of the initial phases

of caldera forming are observed in southern, western

and northern parts of the peninsula, while the prod-

ucts of the last phase (phase D) outcrop only at the

northeastern half of the peninsula. The caldera col-

lapse in the western half, the Karakaya domain, is

related to the emplacement of Kale ignimbrites and

phases A, B, C of Akvaryum ignimbrites (Fig. 11b).

Furthermore, the eruption of the last phase (D) led to

Fig. 10. 3D image of the peninsula generated by combining the DEM and SPOT image of the peninsula (small block diagram at the upper left

shows the block resurgence near Yakakoy dome; a white dashed line borders the resurgent area and the undeformed area in the Karakaya

domain).

Fig. 11. Morphologic evolution of the volcanic edifice.



the collapse of the Dagbelen area (Fig. 11c), thus

widening the caldera to the NE. Such nested calderas

have been previously observed (i.e., Campi Flegrei,

Orsi et al., 1996). Ongoing erosional processes as

well as tectonic events, such as uplift, collapse,

played a main role in the present morphology of

the peninsula (Fig. 11f).

4.2.1. Karakaya domain

The Karakaya domain exhibits the monzonite

body and two segments of a structural dome: the

Karakaya and Yakakoy dome segments (Figs. 10 and

11d). The caldera floor presents an uneven, rough

topographic surface. A monzonitic pluton, in the

middle of the Karakaya domain (Figs. 1, 8 and 10),

is probably outcropped due to erosion and tectonic

control. Magma chambers, preserved as solidified

plutons or batholiths, are exposed in many deeply

eroded ash-flow calderas, as evidenced by petrologic

and age correlations with erupted volcanics (Lipman,

1997). Such plutons are commonly emplaced at a

depth of a few kilometers.

Karakaya resurgence represents one of the two

main resurgence events of the peninsula, occupying

the western and the central parts of the caldera. A

NE–SW valley (Fig. 10) divides this resurgence into

two segments (Karakaya and Yakakoy). Doming of

Yakakoy resulted in the uplifting of shallow sea

sediments and recrystallized limestone in the caldera,

which slopes at up to 45j. The Karakaya dome is

formed of monolithologic and polylithologic brec-

cias. En-echelon emplacement of violet, lenticular

magmatic injections occur in the breccias from

bottom to top of the dome (Fig. 9a). Lenticules are

1.5 m thick and 10–80 m long. We consider these

lenticules as syn-generated with the upward driving

forces related to resurgence.

The western flanks of the Yakakoy dome consist

of valleys, sequentially arranged and oriented E–W

representing en-echelon uplifted caldera blocks (Fig.

10). Such structures have been observed in fault

controlled resurgent calderas such as the Ischia,

Pantelleria islands (Orsi et al., 1991; Acocella and

Funiciello, 1999) and Campi Flegrei in Italy (Orsi et

al., 1999). Orsi et al. (1991) defined a model of this

type of resurgence called simple-sheering block

resurgence. The model implies that at the beginning

of the deformation, high-angle inland-dipping mar-

ginal detachments—which may result from reacti-

vated caldera fractures—define the edges of the

resurgent block. To avoid physical instability, the

uplifting block has to tilt, causing an internal defor-

mation of the block through a simple-shear mecha-

nism (Orsi et al., 1991). The remaining part of the

floor of the Karakaya domain is not deformed (Fig.

10) as Orsi et al. (1991) shown in the Ischia and

Pantelleria islands.

The generation of resurgence in the form of

separate blocks highlights the piecemeal subsidence

of this part of the caldera (Karakaya domain).

Piecemeal subsidence of caldera blocks, which have

more than one collapse centre, is also observed on

the caldera floor morphology (Acocella and Funi-

ciello, 1999). The Karakaya domain is a thin and

long elliptical structure related to more than one

collapse centre. The abrupt changes in the grain size

of the ignimbrites and reworked layers between

eruption phases represent fluctuating eruption inten-

sity and break-off periods in eruptive activity. The

presence of numerous plinian fall-back units eviden-

ces the periodicity of the eruption column and cloud

generation during several volcanic phases. Several

sequential activities from multiple vents may also

lead to a piecemeal collapse.

4.2.2. Dagbelen domain

The Dagbelen domain is differs from Karakaya.

It rises as a dome-shaped body in the middle of

the surrounding depression (Fig. 10). This depres-

sion forms a structural zone with ring-faults and a

cone-sheet.

The doming of the area, following plate type

subsidence, represents the second resurgent activity

of the caldera.

After the collapse of the Karakaya domain, con-

tinuing eruptive activity led to the collapse of the

Dagbelen domain. Phase D products, observed only

at the northern part of the peninsula, are considered as

responsible for the collapse of the Dagbelen domain.

The relatively weak activity related to block-and-ash

flow and ignimbrite deposits of phase D can be

explained by sequential dome eruptions and ash col-

umns (Eichelberger and Westrich, 1981; Fink, 1983)

or by gas loss through the permeable conduit wall

(Eichelberger et al., 1986; Fink et al. 1992). More

stable conditions of eruptive intensity and the smaller


circular structure of the collapsed area are taken as

evidence of plate type subsidence for the Dagbelen

domain.

Ring-faults can accommodate uplift as well as

subsidence (Lipman, 1997). Domical magmatic resur-

gence at the Lake City caldera in Colorado occurred

along the same faults that earlier accommodated cal-

dera collapse (Hon, 1987) with magma then being

partially injected as ring-dykes (Lipman, 1997). The

resurgence of the Dagbelen dome was accompanied by

ring-faults acting in the opposite direction to their

former direction. A cone-sheet in the northern part of

the dome is considered as contemporaneous with

resurgence. Strong resurgence processes may induce

newly formed and/or reactivated fractures, connected

to a regional and/or a local stress field (Acocella and

Funiciello, 1999). A ring-fault in the northern part of

the dome cuts the thickest dyke (N33E/67SE) of the

peninsula. Walter and Troll (2001), in their experimen-

tal study, proposed that the event of circular faults

cutting the radial fractures requires at least one caldera

subsidence followed by doming or resurgence. For-

mation of cone-sheets, ring-dykes and radial dykes is

generated as a function of the stress field near the

Fig. 12. Rose diagrams representing fault and dyke directions at

magma chamber, which is subjected to multiple intru-

sion and collapse stages (Lafrance and John, 2001).

5. Volcano-tectonics

5.1. Faults and lineaments

The faults and the lineaments were defined using

satellite images, aerial photos and field studies. Faults

and lineaments of the Bodrum Peninsula were inves-

tigated in three groups according to their directions:

caldera boundary faults, intra-caldera faults and outer

caldera faults.

Rose diagrams of the faults were drawn using the

frequency method (Fig. 12). Most of the boundary

faults are in NE–SW direction. Faults out of the

caldera are mainly in NW–SE, and are probably

related to the WNW–ESE-directed tectonic faults in

the Gokova graben.

The Rose diagram of intra-caldera faults and

lineaments defines two main groups of orientations

(Fig. 12). The first (NW–SE) group represents outer

caldera faults. We consider that they were formed by

the caldera boundary, out of the caldera and in the caldera.


post-caldera tectonic events. The second group is

directed E–W, ENE–WSW and WNW–ESE, and

we consider it to be related to the collapse and

resurgence events. The E–W-oriented dykes of the

Karakaya dome and E–W-oriented valleys at the

western side of the Yakakoy dome (Fig. 10) are

compatible with the E–W direction of the intra-

caldera faults and lineaments.

5.2. Fault striae investigations

Fault striae measurements were carried out on a

dyke, two lava flows and two consolidated fall-back

units. Such measurements permit an interpretation of

structural events according to Sparner et al. (1993).

Fault striae analyses were computed with Fault-

KinWin software, programmed by Allmendinger,

R.W. This method is detailed in Marrett and Allmen-

dinger (1990) and in Cladouhos and Allmendinger

(1993).

The results of fault striae analyses (Fig. 13) show

that the extensional tectonic regime affected the pen-

insula in a NNW–SSE direction. Kokkalas and Dout-

sos (2001), emphasized a NNW–SSE-directed, Upper

Miocene aged regional extensional regime on Kos

Island located f 35 km southeast of Bodrum. This

regime also explains the strike slip movements along

Fig. 13. Fault straie solutions. (a) Beach-ball illustration of the fault plan

Plunge: 61j). (b) Stereonet illustrations of faults and strikes (arrows show

the caldera boundary. Calculated r1, r2 and r3 values

(Fig. 13) are compatible with the values calculated on

Kos Island (Table 3) by Kokkalas and Doutsos (2001).

5.3. Dykes

The observed dykes were investigated in two

main groups: intracaldera dykes and dykes out of

caldera. These groups differ by texture, direction and

thickness.

Intracaldera dykes are light coloured rocks and

exhibit a porphyritic texture with coarse crystals. They

are mostly HK-andesitic rocks. Dykes intruded out of

the caldera are also porphyritic but no coarse crystals

are observed. They are mafic rocks characterised by

shoshonite, banakite, absarokite and HK-andesitic

composition.

Rose diagrams prepared using the frequency meth-

od were used to interpret the directions of the dykes.

Dykes out of the caldera are grouped in NNW–SSE

and NE–SW directions, indicating that the dykes

were intruded radially. In elliptical volcanic edifices,

radial dykes are expected to be numerous along the

long and small axes of the edifice (Nakamura, 1977).

Occurrence of similarly oriented faults in and out of

the caldera, with NNW–SSE-directed dykes (Fig.

12), emphasizes the effect of regional tectonics.

e solutions (nodal planes: Azi: 206j, Plunge: 51j and Azi: 322j,the movement of hanging wall) and main stress axes.

Table 3

Azimuth and dip values of principal stress axes calculated at

Bodrum peninsula and Kos Island by fault striae measurements

r1 (j) r2 (j) r3 (j)

Bodrum

Peninsula

82/6 347/38 180/52 This work

Kos Island 85/13 323/66 180/20 Kokkalas and

Doutsos (2001)

Values were given as azimuth/dip angle.


Intracaldera dykes are oriented along WNW–ESE

and ENE–WSW directions (Fig. 12). Such directions

are similar to those of intracaldera faults, so these

directions are related to collapse and resurgence

events. Probably, the faults generated during piece-

meal subsidence were reactivated during resurgence

and acted as preferential pathways for dykes.

5.3.1. Dyke thickness

The thickness of the dykes was measured in the

field and plotted on a graph according to their dis-

Fig. 14. Illustration of thickness of the dykes of the peninsula, as a functio

and outer part of the caldera.

tances from the caldera boundary (Fig. 14). Dyke

thicknesses decrease away from the caldera boundary

both towards the interior and exterior of the caldera.

The decrease in thickness of dykes located outside the

caldera, away from the caldera boundary, denotes a

decrease in stress per unit volume, caused by tumes-

cence away from the edifice centre. Decrease in the

thickness of intra-caldera dykes away from the caldera

boundary may point to an increase in stress per unit

volume through the centre of a piecemeal caldera.

5.3.2. Using dykes to calculate crustal extension

Dykes are cracks opened and filled by magma.

Thus, the extension generated by dyke intrusion may

be calculated using the geometric parameters of

dykes (Marinoni and ve Gudmundsson, 2000). Most

methods concerning the extension generated by

dykes do not take account of dip angles. A new,

simple method proposed by Marinoni (2001) includ-

ing dips was applied to seven dykes whose geometric

properties were measured accurately. The computed

n of distance from the caldera boundary through the caldera interior

Fig. 15. Graph of the horizontal component of extension due to dyke injection.


maximum horizontal extension for the Bodrum Pen-

insula is 7.21 m, at a 140j azimuth defining N40W

direction (Fig. 15).

6. Discussion and conclusion

6.1. Volcanic events

The Bodrum Peninsula was previously described in

the Dodecanese Province as part of a shoshonitic serie

(Robert and Montigny, 2001; Cubukcu, 2002). More-

over, the volcanic vents were previously defined as

stratovolcanoes. But we consider it to be an ignimbritic

complex shield volcano (Vincent, 1960; Gourgaud and

Vincent, 2004). Of course, few dispersed lava domes,

related block-and-ash flows and lava flows are ob-

served and are considered as pre-caldera events. Two

major ignimbritic sequences can be distinguished: Kale

and Akvaryum. Akvaryum ignimbrite deposits com-

prised four phases (A, B, C, D). A, B and C phases are

widely represented throughout the peninsula and con-

sidered as responsible for the main pair of Bodrum

calderas, the Karakaya domain. Phase D deposits occur

in the northern and eastern areas. The Bodrum caldera

widened to the NE, forming the Dagbelen domain,

related to the eruption of phase D, and characterized by

block-and-ash flow, ignimbrite and fall deposits. Al-

ternation of eruptive styles, i.e., dome-forming and

plinian eruptions, is quite common in silicic volcanoes

(Martel et al., 2000). Moreover, two main resurgence

events are observed in the complex caldera. After the

collapse events, volcanism continued mainly with lava

flows, lava domes and block-and-ash flows. The inter-

nal part of the caldera was widely subjected to hydro-

thermal alteration.

6.2. Caldera and resurgence

Most of the large volume ignimbrite emplace-

ments can be directly or indirectly related to caldera

formation (Smith, 1979). Ignimbrites were erupted

from the ring fractures generated during caldera

collapse. The emplacement of two ignimbritic

sequences was responsible for the collapse of the

NE–SW, partially submerged elongated Bodrum cal-

dera complex, 18.7� 7.7 km wide. Many submerged

calderas are known around the world (such as

Rabaul, Papua New Ginea; Aira, Japan; Santorini,

Kos, Greece). The Bodrum caldera exhibits a com-

plex resurgence with two domains (Karakaya and

Dagbelen), separated by two crosscutting sinistral

faults and polylithologic breccias.

The Bodrum caldera system exhibits multiple-

block collapses and relevant resurgent doming, such

as the Dagbelen resurgent dome and the Karakaya

resurgence. The Dagbelen domain with its typical

radial-faults and peripheral concentric faults is inter-

preted as a plate type subsidence with resurgence

occurring as a central, near-perfect dome. Karakaya

resurgence occurs as an uplift of small plates, rather

like consecutive terraces, bounded by crosscutting


small faults. Such resurgence is related to a previous

piecemeal type subsidence.

The interior of the caldera is widely affected by

hydrothermal alteration, related to the resurgence

event. The breccias are considered to be synchronous

with resurgence, as observed at Karakaya and SW of

Dagbelen. Block-and-ash flow deposits that have

been subjected to occasional hydrothermal alteration,

also occur within the caldera.

The topographic caldera area covers about 98 km2

and the collapsed area is estimated at 58 km2. The

average overall slope of the inner topographic walls

is 16j. Such lower slopes, dipping gently (10–15j),are the only parts preserved in many eroded calderas,

where the inner wall is expressed as an irregular

unconformity between pre-caldera and caldera-filling

rocks (Lipman, 1976, 1997).

The Bodrum caldera is a small–medium sized

caldera (Table 4) when compared with other calderas.

Low collapse and collar angle values demonstrate the

high erosional effects. An important volume of erup-

Table 4

Geometric properties of Bodrum caldera and some worldwide calderas in

Caldera Diameter

(km)

Topographic

area

(km2)

Structural

area

(km2)

Collaps

(m)

Bodrum caldera 7.7� 18.7 98 58 220

Acıgol caldera, Turkey 12.2� 12.3* 150 200

Gollu Dag caldera,

Turkey

f 10� 11* 113 300

Nemrut caldera, Turkey 8.5� 7 47 32.2 700

Nigorikava caldera,

Japan

2.5 4.9 0.2 1500

Crater Lake caldera, USA 9 64 20 1000

Aira caldera, Japan 20 314 113 2000

Creede caldera, USA 24 452 154 2000

La Garita, USA 50 1963 1256 2000

*Refer to the values calculated.

tive products is thought to be emplaced below the

sea. The difference between the topographic volume

and the caldera related ignimbrite volume (Table 4)

infers an underwater emplacement.

6.3. Caldera elongation and tectonics

Fault striae measurements indicate a NNW–SSE

extensional local regime. The NNW–SSE exten-

sional regime was effective during and after Upper

Miocene volcanism. Such results are compatible

with Kokkalas and Doutsos’s (2001) studies. The

NE–SW-elongated and semi-elliptic Bodrum calde-

ra is oblique in relation to the regional extensional

regime. Acocella et al. (2003), explained similar

observations in Ethiopia by pre-existing structural

features. A volcanic edifice grows in relation to an

extensional regime and the vents on its flanks are

expected to be aligned perpendicular to the exten-

sion axes (Nakamura, 1977; Adiyaman, 2000).

Structural features responsible for the elongation

different sizes

e Topographic

volume

(km3)

Structural

volume

(km3)

Caldera-related

ignimbrite

volume

(km3)

Collar

angle

(j)

From

21.6 12.8 >8.5 16 This work

30 28.2 Mouralis

et al. (2002)

33.9 21.7 Mouralis

et al. (2002)

32.9 22.5 37 *

3.9 1.2 7 56 Lipman (1997),

Aramaki (1984)

79 59 27 Lipman (1997),

Aramaki (1984)

636 452 300 27 Lipman (1997),

Aramaki (1984)

965 692 22 Lipman (1997),

Aramaki (1984)

6961 6280 22 Lipman (1997),

Aramaki (1984)


of the Bodrum caldera appear to have been formed

under the effect of a previous tectonic regime.

Some NE–SW-directed structural and topographic

features are observed in the Bodrum Peninsula and

its surroundings, which bear witness to such a pre-

existing local regime. Mes�hur and Yoldemir (1983)

also observed NE–SW-elongated faults, 80–90 km

in length, near the Gokova gulf. Kurttas� (1997)

emphasized the directional variations of tectonic

structures and lineaments near the Gokova gulf,

between N30E and N50E.

When calderic and structural features are evalu-

ated together, the effects of post-caldera tectonism

are obvious. Some faults observed both within and

outside of the caldera were active after caldera

collapse. Some intracaldera faults, linked to the

subsidence and resurgence events were later used

by dyke emplacements.

A N40W horizontal component of the extension

due to dyke intrusions was found. The thickness of the

dykes observed in the peninsula decreases away from

the caldera boundary towards the interior and exterior

of the caldera. This situation is interpreted as varia-

tions of stress within the edifice.

Acknowledgements

This work benefited from a research grant from the

French Ministry for Foreign Affairs and the French

Embassy in Ankara. The satellite image was supplied

by the French CNRS (UMR6524). The field expenses

for Turkish participants were financed by Hacettepe

University (Ankara, Turkey). The authors especially

thank to Prof. Dr. S.D. Weaver (Editor) and the

reviewers Prof. Dr. G. Orsi and Prof. Dr. F. Anguita for

their helpful comments on the manuscript and V.

O’Dwyer for improving the English expression.

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Volcanic and deformation history of the Bodrum resurgent caldera system (southwestern Turkey)