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Surveys in GeophysicsAn International Review JournalCovering the Entire Field ofGeosciences and RelatedAreas ISSN 0169-3298Volume 32Number 3 Surv Geophys (2011)32:271-290DOI 10.1007/s10712-010-9109-8
Shallow and Deep Structure of aSupradetachment Basin Based onGeological, Conventional Deep SeismicReflection Sections and Gravity Data inthe Buyuk Menderes Graben, WesternAnatolia
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Shallow and Deep Structure of a Supradetachment BasinBased on Geological, Conventional Deep SeismicReflection Sections and Gravity Data in the BuyukMenderes Graben, Western Anatolia
G. Cifci • O. Pamukcu • C. Coruh • S. Copur • H. Sozbilir
Received: 6 September 2010 / Accepted: 15 November 2010 / Published online: 3 December 2010� Springer Science+Business Media B.V. 2010
Abstract The Buyuk Menderes Graben is a depression in the Menderes core complex of
western Turkey. The region is one of the most rapidly deforming regions of continental
crust in the world and has exceptionally high seismic activity. In this study, shallow and
deep seismic studies were conducted at the Buyuk Menderes graben. These studies
included surface geological mapping and two seismic reflection sections. Detailed mod-
elling was performed with the seismic study. In addition to these, a moving windows
power spectrum was applied to the Bouguer gravity profile data of the study area. Since no
deep well is available in this area, the geological interpretation of the seismic stratigraphy
is based on the correlation with the surface geology, this was combined with the major
reflections and the seismic facies observed along the profiles, and, thus, four main seismic
units can be distinguished in the basin fill. Structural features of the basin is driven by a
complex extensional faults system, consisting of a low-angle, S-dipping Buyuk Menderes
detachment and by its synthetic and antithetic splays, bordering the opposite flanks of the
basin. As a result of conventional deep seismic reflection sections and gravity data, three
layers were defined in the study area. The first layer occurs at a thickness of 6 km, and the
second layer is between 13 and 18 km. The third layer is at *33 km and may also
G. Cifci (&)Institute of Marine Sciences and Technology, Dokuz Eylul University, 35340 Inciraltı, Izmir, Turkeye-mail: [email protected]
O. PamukcuEngineering Faculty, Department of Geophysics, Dokuz Eylul University,Buca Tınaztepe Campus, Buca, Izmir, Turkey
C. CoruhDepartment of Geological Sciences, Virginia Polytechnic Institute and State University,1046 Derring Hall, Blacksburg, VA 24061, USA
S. CopurTurkish Petroleum Co., Exploration Group, 06520 Ankara, Turkey
H. SozbilirEngineering Faculty, Department of Geology, Dokuz Eylul University, Buca Tınaztepe Campus,Buca, Izmir, Turkey
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emphasize Moho depth. The Buyuk Menderes graben has three clear reflectors which are
base sediments, brittle-ductile transition, Moho and faults that show a half-graben floored
by a detachment. The Moho depth is comparable with previous estimates. According to the
results obtained, Bouguer gravity and seismic results are very much consistent with each
other. It was observed that at the depths determined from seismic and gravity data, the
distribution percentage of earthquake focal depths also rises.
Keywords Deep seismic reflection � The Buyuk Menderes graben �Western Anatolia � Gravity
1 Introduction
Western Anatolia forms one of the most seismically active and rapidly extending regions in
the world and is currently experiencing an approximately N–S continental extension since at
least Miocene time. The N–S extension in the region has resulted in many Neogene to
Quaternary continental basins trending mainly in E–W and NE–SW directions (Sengor et al.
1985; Yılmaz et al. 2000). The E–W-trending basins (e.g. Gediz and Buyuk Menderes
grabens) are characterised by two unconformity-bounded infills: the older one is related to
Miocene detachment faulting and core-complex formation and the younger is related to high-
angle normal faulting controlled the Plio-Quaternary lateral alluvial fans and axial river
deposits that fill the graben floor (Yılmaz et al. 2000; Sozbilir 2001; Bozkurt and Sozbilir
2004, 2006; Sen and Seyitoglu 2009; Cifci and Bozkurt 2009). The activity of the bounding
high-angle normal faults is shown by numerous earthquakes (Arpat and Bingol 1969; Sengor
et al. 1985; Eyidogan and Jackson 1985). North of the Gediz and Buyuk Menderes graben are
NE-trending basins that are generally oriented at high-angles to the E–W-running basins.
The best-known of these are the Gordes, Demirci, Selendi, and Usak-Gure basins (Seyitoglu
1997; Bozkurt 2003; Purvis and Robertson 2005; Ersoy et al. 2008, 2010; Fig. 1).
The E–W trending basins are represented by gravity anomalies of up to 40–50 mGal,
which corresponds to a Neogene sediment accumulation of up to 2–3 km in the graben
floors (Gurer et al. 2002; Sarı and Salk 2006). The generalised crustal models in the
Aegean area are derived from older seismic refraction lines (e.g. Makris 1978). To date, no
detailed study of the extensional basins in the area has been conducted to determine the
underlying crustal structures using seismic reflection data. Despite the fact that consider-
able research efforts are being directed towards understanding the geo-dynamics of the
Aegean region, our understanding of deep crustal structures in western Turkey is incom-
plete compared to many other regions of continental Europe and the Eastern Mediterranean
Sea. The Aegean region is well known for its geothermal resources and numerous hot
springs. The Turkish Petroleum Corporation reported the occurrence of hydrocarbons
within the sedimentary fill of the Gediz graben near Alasehir (Cifci and Bozkurt 2009).
Recently, outcrop observations supported by subsurface data, drilled wells and a two-
dimensional (2D) seismic survey were applied to the Gediz graben by Cifci and Bozkurt
(2009) and Demircioglu et al. (2010). Similar multi-disciplinary studies were not per-
formed on the Buyuk Menderes Graben prior to our study.
This collaborative study by Turkish Petroleum Corporation (TPAO), Dokuz Eylul
University, the Virginia Polytechnic Institute and the State University Department of
Geological Sciences images the deep crustal structures and ascertains the local crustal
thickness. The variations in amplitude and lateral extension were determined for the Buyuk
Menderes Graben. In this paper, the results and interpretations from these seismic
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reflection lines are presented as the first deep crustal study of the area based on seismic
reflection data (Fig. 2).
Two shallow cross-sections were interpreted within the scope of the seismic study. In
the next stage, the depth distribution was determined by applying a moving windows
power spectrum to the Bouguer gravity sections in the N–S and W–E directions (Spector
Fig. 1 Geological map of western Anatolian Basins (compiled from Bozkurt and Park 1994; Sozbilir 2001;Ozer and Sozbilir 2003; Bozkurt and Sozbilir 2004; Isık et al. 2003; Bozkurt 2001; Sozbilir 2005). Note, theGediz and Buyuk Menderes Graben are bounded by detachment faults along the southern and northernmargins, respectively
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and Grant 1970). In the final stage, the shallow-deep seismic interpretations and gravity
outcomes were evaluated together with the focal depths of the earthquakes in the region.
As a result, the reflective surfaces within the crust, the lateral variations and the faulting
were determined.
2 Geology of the Buyuk Menderes Graben
The Buyuk Menderes Graben contains the valley of the Buyuk Menderes River, which is
150 km long and 10–20 km wide. The graben is bordered by well-developed normal fault
systems along its length. The rock units exposing in the vicinity of Buyuk Menderes
Graben can be classified into two groups as the basement and basin fill units.
2.1 Pre-Neogene Basement Units
Metamorphic rocks belonging to Menderes Massif constitute the pre-Neogene basement
which is an extensional metamorphic core complex in the western Anatolian extensional
province (Bozkurt 2001). The northern border of the Menderes Massif is the Simav
detachment, which separates the Menderes Massif from higher structural units, including
the Afyon and Tavsanli zones and the Izmir-Ankara ophiolites and ophiolitic melange (Isik
and Tekeli 2001). The massif is bordered by the metamorphosed Lycian nappes in the
South (Sengor and Yilmaz 1981; Rimmele et al. 2003; Pourteau et al. 2010; Fig. 1).
The stratigraphy of the Menderes Massif was based principally on the metamorphic
sequences of the Cine submassif where a Pan-African core sequence is overlain by a
Palaeozoic–Mesozoic metasedimentary cover sequence (Schuiling 1962). However, recent
studies show that this simple subdivision is incorrect (e.g. Bozkurt et al. 1993; Ring et al.
1999; Lips et al. 2001; Gessner et al. 2001; Ozer and Sozbilir 2003). According to these
studies, the massif is made up of four tectonoctratigraphic units which are, from bottom to
top, rudist-bearing metamorphic rocks of the Bayındır unit, high-grade metamorphic unit
intruded by Pan-African and Triassic granites, the metasedimentary Bozdag unit, gneiss-
Fig. 2 Seismic profiles in study area. Deep seismic profile is from 101 to 1,462, 1–5 is the W–E profile,11–3 is N–S profile
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dominated Cine unit with Pan-African granitic protoliths, and the Selimiye metasedi-
mentary unit of Permo–Triassic age (Ozer and Sozbilir 2003; Erdogan and Gungor 2004;
Okay 2001; Gessner et al. 2001). The contacts between the tectonostratigraphic units have
been the subject of much debate; they have been interpreted as a thrust, extensional
detachment or reactivated thrust. Details of the stratigraphic and structural features of the
pre-basin-fill units is beyond the scope of this paper, but is reviewed in recent literature
(Bozkurt and Oberhansli 2001; van Hinsbergen 2010; van Hinsbergen et al. 2010a, b).
2.2 Basin Fill Units
The basin fill consists of four sedimentary packages that formed on the metamorphic rocks
of the Menderes Massif (e.g. Sozbilir and Emre1991; Emre and Sozbilir 1997; Bozkurt
2000; Sen and Seyitoglu 2009). These are, from bottom to top, the early-middle Miocene
Haskoy Formation, late Miocene Gokkırantepe formation, late Pliocene–Pleistocene
Asartepe formation and Quaternary alluvium (Sozbilir and Emre 1991; Sen and Seyitoglu
2009). Detailed descriptions of the sequence stratigraphic units are given in Sozbilir and
Emre (1991), Emre and Sozbilir (1997), Seyitoglu and Scott (1992), Cohen et al. (1995),
Bozkurt (2000) and Sen and Seyitoglu (2009). However a brief description of the basin
infill is given below.
2.2.1 Haskoy Formation (SS-I)
The Haskoy formation is a shale-dominated sequence with lateral alluvial fan delta and
lacustrine deposits (Sozbilir and Emre 1991; Cohen et al. 1995). This unit crops out around
Haskoy and between Sultanhisar and Guvendik village. It rests unconformably on the
metamorphic rocks of the Menderes Massif. Coarse-grained polygenetic boulder con-
glomerates form the basal section of the formation. This facies is bounded by a
NS-trending oblique normal fault around Dereagzı village. Toward the East, this facies is
graded laterally and vertically into fine-grained siliciclastic sediments containing bitumi-
nous shale and thin coal lenses and ends with a sandy limestone facies at the top. This
sequence was interpreted by Yılmaz et al. (2000) and Gurer et al. (2009) as the deposits of
a North–South trending depression formed by an East–West extensional regime during the
Early(?)-Middle Miocene prior to the development of the East–West extensional basins.
However, recent studies contradict this view and suggest that the shale-dominated suc-
cession formed under the control of E–W striking detachment fault showing top-to-the
North East sense of shear (Cemen et al. 2006; Sen and Seyitoglu 2009). Recent mag-
netostratigrafic data yielded an Early-Middle Miocene age for the coal-bearing Haskoy
formation in the area (Sen and Seyitoglu 2009).
2.2.2 Gokkırantepe Formation (SS-II)
After the oldest sedimentary sequence was deposited, the basin was the site of lateral
alluvial fan and axial fluvial facies of reddish-coloured clastic sediments of the
Gokkırantepe formation (Sozbilir and Emre 1991; Bozkurt 2000; Sen and Seyitoglu 2009).
The Gokkırantepe formation consists of reddish clastic sediments that is characterised by a
conglomerate and sandstone intercalations. The conglomerate is coarse-grained, poorly
sorted, and polygenetic. The sources of the conglomerate are the metamorphic rocks and
the SS-I sediments. Above the reddish conglomerate and sandstone intercalations, the SS-II
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(Figs. 3, 4, 6) is composed of green-yellowish sandstone and mudstone alternations
together with minor interbedded lignites. These two sequences have formed on the upper
plate of the Buyuk Menderes Detachment, which accommodated large-magnitude regional
extension during Miocene time (Emre and Sozbilir 1997; Sen and Seyitoglu 2009).
2.2.3 Asartepe Formation (SS-III)
The unconformably overlying sedimentary sequence, the Asartepe formation, consists of
lateral alluvial clastic sediments which have formed in front of the high-angle normal
faults. These sediments comprise massive to moderately-bedded, poorly-compacted
boulder conglomerates with alternations of sandstones and mudstones. Unay et al. (1995)
assigned a Late Pliocene–Pleistocene age for the SS-III sediments (Figs. 3, 4, 6) on the
basis of mammal fauna.
Fig. 3 A longitiudunal seismic profile in association with the interpreted cross-section showing thesequence stratigraphic units (see Fig. 2 for location of the section)
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2.2.4 Alluvium (SS-IV)
The Alluvium is composed of lateral alluvial and axial graben floor sediments deposited in
the modern Buyuk Menderes graben (SS-IV, Figs. 3, 4, 6). The lateral alluvial fans are of
diverse size and are controlled by the E–W-trending high-angle normal faults. These fans
are sourced from the N–S-directed streams and grade into fine-grained basin–floor sedi-
ments along the axial Buyuk Menderes River.
Fig. 4 A transverse seismic profile in association with the interpreted cross-section showing the sequencestratigraphic units (see Fig. 2 for location of the section)
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3 Structural Geology of the Buyuk Menderes Graben
Two different stages of crustal extensional events, an extensional detachment faulting
followed by a rift stage, characterise the province (Seyitoglu and Scott 1992; Kocyigit et al.
1999; Bozkurt 2001; Lips et al. 2001; Bozkurt and Sozbilir 2004). Three types of major
faults occurred during these two stages along the northern margin of the Buyuk Menderes
graben: (1) low angle-normal faults; (2) N–S-trending oblique normal faults; and (3)
approximately E–W-trending high-angle normal faults.
3.1 Buyuk Menderes Detachment Fault
The northern margin of the Buyuk Menderes graben has a South-dipping low-angle normal
fault (the Buyuk Menderes detachment fault) that separates a sequence of high-grade
metamorphic gneisses and a Neogene sedimentary rock succession in its hangingwall from
the marble-intercalated mylonitized schists in its footwall (Emre and Sozbilir 1997; Lips
et al. 2001; Gessner et al. 2001; Cemen et al. 2006). The basin fill deposits rest uncon-
formably on the hanging wall gneissic sequence but are in fault contact with the underlying
rudist-bearing footwall sequence. This result suggests that the gneiss klippen and the
overlying synextensional sedimentary sequence were transported as extensional alloch-
thons (Emre and Sozbilir 1997) to the South along the Buyuk Menderes detachment fault.
The Buyuk Menderes detachment was first mapped by Emre and Sozbilir (1997) as a re-
activated thrust fault (Lips et al. 2001). The same fault was interpreted by Okay (2001) as a
thrust along which the older high-grade gneisses were brought over the younger marble-
intercalated mylonitized schists during the Eocene contractional tectonics in the region.
However, Gessner et al. (2001) also re-interpreted this surface as a low-angle normal fault.
The Buyuk Menderes detachment fault is characterised by semiductile to brittle fault
behaviour (Lips et al. 2001). Some ductile fabrics indicate northward tectonic transport of
the hanging wall gneissic sequence on top of the rudist-bearing sequence. These fabrics
pre-date subsequent top-to-South detachment faulting (Lips et al. 2001; Cemen et al.
2006). These findings suggest that the Buyuk Menderes detachment is a reactivated
structure showing earlier semi-ductile top to the north sense of tectonic transport over-
printed by top to the South tectonic transport.
3.2 N–S-Trending Strike-Slip Fault
Two lines of N–S-trending strike-slip faults were observed North of Nazilli and North of
Kuyucak (Fig. 1). These faults act as cross or accommodation faults in the northern margin
of the Buyuk Menderes graben.
3.3 E–W-Trending High-Angle Normal Faults
These faults are South-dipping high-angle normal faults that have formed step-wise
topography along the Buyuk Menderes graben. The sedimentary sequences have been
brittly deformed by these high-angle normal faults that strike West-Northwest and
Northeast. Three structural blocks have been defined between these faults. These blocks
have been tilted against the fault planes, resulting in step-wise topography. The blocks are
younger towards the basin. The youngest one forms the boundary between all of the older
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units and the alluvium. This fault is characterised by triangular facets, fault scarps and
alluvial fans of diverse size on the hanging wall.
4 Seismic Data Acquisition and Processing
Figure 2 shows the locations of the seismic profiles in the study. The deep data acquisition
lines are marked accordingly. Because of economic and practical considerations, a DSP-1
profile in the E–W direction was chosen as the key line for the deep data acquisition, and
the recording time was 16 s.
As part of the collaborative data acquisition, the field parameters were chosen to
accommodate TPAO’s petroleum exploration program, and the recording length was
increased to 16 s to receive signals from depths of up to 45 km (Fig. 5 DSP-1). The data
acquisition for regular lines was carried out by a TPAO crew using a 240-channel
recording system with 25 and 50-m geophone groups and shot point spacing. A 250-m shot
point spacing was utilised for the crustal seismic profiling with a 16-s recording length and
10 Hz vertical geophones. An explosive source of 4 kg at about 21 m depth was also used.
The geophones and explosive source resulted in nominal 12-fold deep seismic reflection
data with a length of 16 s. These data were acquired at offsets from the data in the Nazilli
area. The data were recorded with long offsets of up to 36 km in the East–West direction.
These measurements resulted in nominal sevenfold data with a recording length of 16 s.
The data sets were first processed at Turkish Petroleum Corporation’s processing centre
using a standard initial processing sequence. Later, special seismic data processing was
performed on the deep seismic data at Virginia Polytechnic Institute and State University in
the Department of Geological Sciences using Focus/Disco (Paradigm Geophysical) seismic
data processing packages, and modules were modified and developed. The data processing
steps for the deep seismic data included de-multiplexing, geometry definition, editing,
eliminating bad traces and correcting polarity reversals (at least four times at different
steps), datum plane statics correction, coherency filtering to eliminate noise at different time
intervals and dips for each shot, normal move-out corrections using the determined
velocities, sorting into common-shot-point and common-mid-point gathers, stacking (sev-
eral stack techniques were tested), applying residual statics (multi-pass with multi-steps),
filtering and scaling for signal enhancement, deconvolution, and spectral balancing.
Because of the deep targets (up to 45 km) and relatively short line lengths (36 km with fold
taper-in and taper-out) of the deep profile, no migration scheme was applied.
4.1 Stratigraphic and Structural Interpretations of Seismic Profiles
The geometry of the Buyuk Menderes Graben has been reconstructed by interpreting two
seismic profiles; one is transversal to the basin (WSW–ENE), whilst the other one is
longitudinal (NNW–SSE). Since no deep well is available in this area, the geological
interpretation of the seismic stratigraphy is based on the correlation with the surface
geology; this was combined with the major reflections and the seismic facies observed
along the profiles, and, thus, four main seismic units can be distinguished in the basin fill.
The basin fill unconformably overlies the pre-Neogene basement, consisting of meta-
morphic units of The Menderes Massif exposed at the surface along the basin’s flanks. The
contact between the basement and the overlying basin fill sediments is generally marked by
a strong reflection, that can be easily explained considering the sharp increase of acoustic
impedance at the sediments/basement interface.
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Fig. 5 a The whole original seismic reflection profile, b some parts of the original seismic reflection profile.Ondulation is shown with solid line
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The first marker (Figs. 3, 4) shows the bottom of the recent sediments. From the top of
the section, a second marker (Fig. 4), which is a strong reflector, represents the Miocene
limestone (dark blue) between Horizons III and II in the West and between Horizons III
and I in East. The lower Seismic stratigraphic unit I (SS-I), is characterized by lens-shaped
reflections which thicken toward the basin centre. Along the transverse section, the
thickness of the unit apparently decreases from South to North, while along the longitudial
section, the SS-I is characterised by much more regular and laterally continuous reflectors.
The unit can be related to a lacustrine succession of the Haskoy formation, mainly con-
sisting of fine-grained sediments, with lenses of coarser material close to the basin’s flanks.
The overlying Seismic stratigraphic unit II (SS-II) comprises relatively irregular, lens-
shaped reflections, characterised by high amplitude and low continuity, possibly corre-
sponding to coarse, alluvial sediments and can be considered as the equivalent of the
Gokkırantepe formation. The lens-shaped geometries of the SS-I and SS-II indicate their
syn-tectonic nature.
Seismic stratigraphic unit III (SS-III) overlies the SS-II and is characterized by much
more regular and laterally continuous reflectors overlapping onto the underlying units
(Fig. 6). The reflectors have a tendency to thicken near the depocentre of the graben. They
also appear more continuous towards the southern and northern flank of the basin.
Above this sequence, a thin nearly transparent uppermost unit (SS-IV) occurs; the ill-
defined SS-IV corresponds to the Quaternary alluvial and fluvial sediments of the modern
Buyuk menderes basin, whilst SS-III is correlated with the Plio-Pleistocene Asartepe
formation, exposed along the Buyuk Menderes Graben.
The profiles also show significant images of the structural setting: we focused on the
normal faults, bordering the basin and affecting the basin fill units. In general, the profiles
provide a reliable image at depth of the major faults recognised at the surface, showing that
the northern and the southern flanks of the basin are bordered by two sets of normal faults,
dipping towards S and N respectively, which can be regarded to as synthetic and antithetic
splays of the major S-dipping Buyuk Menderes detachment fault. It is clear that both SS-I
and SS-II were developed in the hanging wall of the Buyuk Menderes detachment acting as
a growth fault.
The seismic data show that the Buyuk Menderes detachment fault is interpreted to have
a listric geometry. and roots into sub-horizontal and undulating reflectors at about 2.5 km
(2–2.3 s). A border fault at the north side exhibits a slope of 208-30� and some of the faults
in the basin have slopes greater than 50�. The topographic changes are related to the fault
planes and their geometries. The side of the detachment fault is shown (Fig. 3) in a
perpendicular profile in the W–E direction (Profile 1–5 in Figs. 2, 4). A continuous seis-
micity (Profile 11–3 in Fig. 2, N–S) was recorded along those faults. Several geothermal
water sources were observed along the active fault surface in the deformed areas like
Germencik, Salavatlı, and the Buharkent areas in the Buyuk Menderes graben.
The seismic reflection (Profile DSP-1 in Fig. 2) data reveal many continuous East and
West dipping reflectors in the sedimentary section (Fig. 5) down to 2 s. Discontinuous and
patchy reflectors are present in the lower crust (6–12 s or 18–36 km) after the relatively
poor reflective upper crust below the sediments (2–6 s or 6–18 km). The patch reflectors at
about 11–12 s represent the bottom of the lower crust at about 33 km (Fig. 5). Some of the
deeper patch reflections are relatively strong, and the reflections at about 11 s define the
base of the lower crustal reflectors. This base is defined by the zone of the Moho
discontinuity.
A highly anomalous mantle or a kind of crust-mantle mix must be assumed for the
structure of the transition zone. A differentiation process of intruding lavas or melting of
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the lower crust could be in progress (Meisner 1986). One of the explanations is that there is
no mantle lithosphere, and that the crust is underlain directly by asthenosphere (van
Hinsbergen et al. 2010a, b).
Further stations of the reflections become more and more discontinuous and scarce.
Only weak indications of the Moho or other crustal boundaries were observed (Fig. 5). The
most prominent reflection packages in the entire crust are 0–3, 5–7 and 8–12 s in three
levels. Three major zones are much deeper than the brittle detachment system. Syncline
structures are present. The intersection points of the foliation, which has detachments, with
Fig. 6 Interpreted seismic stratigraphic units based on the surface geology and seismic profiles(stratigraphy ans sedimentary facies of the formations are taken from Sozbilir and Emre 1991; Seyitogluet al. 2009)
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the mid-crustal reflectivity (extensional deformation) exhibit some mid-crustal reflection
packages at about 5–7 s along with the structures in the upper crust.
The zone of detachment is about 10 km deep, where the listric faults turn horizontally.
Here, a major zone of the detachment must have developed during the rifting period. This
zone marks the transition from brittle to ductile behaviour in a hot and wet crust during the
rifting stage. The good mark of the fault zones at large depths and the reason for the
reflections, which are relatively strong and continuous, remain unexplained. The exposed
fault zones show strong mylanitisation. On the basis of earthquake studies, a large magma
body should be present at mid-crustal depths. Therefore, this thin and flat-lying magma
body is responsible for the strong reflectors in both kinds of seismic sections. To explain
the strong reflected events, this body must be assumed to have a layered structure. The
crust-mantle transition was observed in the form of discontinuous reflections. The reflec-
tions of the zones of detachment (and metasediments) appear in the upper crust, and at
lower crust levels, intrusions with a lamellation character are present. Spreading
asthenospheric flow patterns or intrusions of magmas into the crust and subsequent
modification of the crust are general features of the area. Indications of the Moho dis-
continuity are present at about 11 s (about 33 km). This discontinuity dips to the West and
has strong reflectivity on the right part of the section.
5 Gravity Applications in the Study Area
In the second stage of the application, evaluations were made based on the gravity data of
the area (Fig. 7). Large gravity anomalies with highly negative amplitudes are present in
the region.
The moving windows power spectrum method was applied to 4 cross-section values at a
length of 70 km and in the N–S direction. The power spectrum values were deduced from
the gravity map of the study area (Spector and Grant 1970) (Figs. 8, 9). As a result of the
application, depth variations were observed between the western and eastern parts of the
area.
The structural depths were determined again by applying the moving windows power
spectrum method to the data of the W–E trending Bouguer cross-sections that reflect the
540000 560000 580000 600000 620000 640000 660000 680000 700000
meter
4160000
4180000
4200000
4220000
met
er
-55
-20
15
50
Ayd
in Naz
illi
mgal
Fig. 7 Gravity anomaly map and studied profiles (from Turkish Oil Company TPAO, interval 1 km)
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deep-reflection seismic section (Fig. 10). The depth increases to the East of Buyuk
Menderes around Nazilli. A structural difference probably exists between the West and
East side of the graben, and the depth increases from the West to the East. The depths of
the region are shown inside the rectangle in Fig. 10. These depths may show the effects of
faulting in the N–S trending seismic cross-sections.
6 Discussion and Conclusion
The study area is one of the most tectonically active regions in the world and has been
extending in the N–S direction since at least the Early Miocene (Bozkurt 2001). The N-
tending strike-slip faults of the West Anatolian extensional province appear to be subor-
dinate, shallow features that accommodate differential extension between the hanging-wall
block compartments that move on enormous low-angle normal faults (Sengor et al. 1985).
The E–W trending normal faults bounding the hanging-wall blocks root into (Seyitoglu
et al. 2002) or cut the current low-angle normal faults (Kocyigit et al. 1999; Yılmaz et al.
-3.9
-3.2
-2.5
-1.8
-1.1
km
560000 570000 580000 590000 600000 610000 620000
meter
4180000
4190000
met
er
NazilliAydin
Fig. 8 Shallow depths obtained from the results of application of power spectrum to N–S trending cross-sections in Fig. 6
-8
-6.8
-5.6
-4.4
-3.2
km
560000 570000 580000 590000 600000 610000 620000
meter
4180000
4190000
met
er
NazilliAydin
Fig. 9 Deep depths obtained from the results of application of power spectrum to N–S trending cross-sections in Fig. 6
284 Surv Geophys (2011) 32:271–290
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2000), which are commonly exposed in culminations known the Menderes Metamorphic
core complexes (Bozkurt and Park 1994; Bozkurt and Oberhansli 2001; van Hinsbergen
2010, van Hinsbergen et al. 2010a, b).
The graben systems may have become active at 11–7 Ma and are now 1.5 km deep
relative to the surrounding topography. They are associated with gravity anomalies that
range from -50 to ?50 mGal, indicating Neogene sediment accumulations of up to
2–3 km in the graben floors (Sarı and Salk 2006). So far, no individual extensional basin
has yet been investigated to determine the related underlying crustal section.
Distribution deep reflectors with detachment faults that root into deep reflectors suggest
a master decollement about 13–18 km deep (Fig. 5) The exposed fault zones show strong
mylonitization, a dramatic disturbance and alteration of the original rock structure. These
mylonite zones occur in the middle or lower crust. Environmental conditions such as high
pore pressure or the sandwich structure of several branches of a mylonite zone and a
cataclastic layer may lead to the observed continuously strong reflectivity of many fault
zones.
The deep fault in the study area accounts for the dense geothermal potential in the
region. According to the deep seismic section (DSP-1), undulations exist within the Moho
(Fig. 5). A large magma body should be present at mid-crustal depths, and the crust gets
thinner. The undulation within the upper crust also affected the gravity data (Fig. 10).
In the results obtained from the DSP-1 profile (Fig. 5), the upper boundary of the lower
crust begins at 5–7 s. This profile was monitored about 12 km on average, and the values
of the power spectrum are indicated within the rectangular region in Fig. 10. The moni-
toring was present at this depth in the W–E trending DSP-1 seismic profile but not in the
outcomes of power spectrum obtained from the N–S gravity anomaly values. These results
prove that this structure has a N–S direction. The deepest detachment fault present in the
N–S seismic profile (Fig. 4) governs the region.
As seen in the E–W seismic section and geological data (Fig. 3), two types of faults
occur in the N–S direction. Some of these faults intersect the graben filling, and the others
0
2
4
6
8
10
12
14
560000 570000 580000 590000 600000 610000 620000
meter
kmNazilliAydın
EW
Fig. 10 Results of application of moving windows power spectrum to W–E gravity cross-section in Fig. 6
Surv Geophys (2011) 32:271–290 285
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are located above the main detachment fault. Some of the indicated N–S trending faults
intersect the entire graben filling. This result indicates that the N–S trending faults evolved
after the grabens. This model is differs from the cross-graben model of Sengor et al.
(1984).
According to the N–S compiled seismic and geological data, the deepest detachment
fault governs the region, and the other faults operate on it. This fault splits the sedimentary
and the metamorphic rocks and forms the boundary. The structural elements defined by
Sozbilir and Emre (1991) continue with the packages designated I, II, III, and IV in the N–
S trending seismic section.
A decrease in the amplitudes of the gravity anomalies was observed from the beginning
of the Buyuk Menderes graben to the West of Aydın until Nazilli (Fig. 7). This decrease in
the gravity anomaly was observed where the sediments are thicker or the density drops
because of the high temperature. In this study, there are highly negative amplitude-gravity
anomalies within the detachment fault zone. The reason for this is either the estimation of
high heat flow in the region (Dolmaz et al. 2005) or the increase in the sediment thickness
of the fault which deepens towards the South, to Nazilli (Fig. 4).
Because the N–S gravity data in the selected window is from the Buyuk Menderes
graben, the shallowest structural depths range between 2 and 3 km. A structure was found
below. This structure changes between 5 and 7 km, and the deepest structure is between 10
and 17 km. These results are consistent with the structures observed in seismic profiles.
Besides, the gravity profile and the depths of possible structures in W–E cross-section
(Fig. 10) are coherent with the depths of possible structure transition determined in the
deep seismic cross-section (Fig. 5) having the same co-ordinates.
The seismicity in Western Anatolia displays swarm-type activity with remarkable
clustering of low-magnitude earthquakes in time and space. This seismicity is closely
associated with major tectonic structures in the area. The focal depths of the earthquakes in
Western Anatolia range from very shallow to 50 km (Yılmazturk et al. 1999; Akyol et al.
2006; Taymaz and Price 1992; Taymaz 1993, 1996; Taymaz et al. 1990, 1991, 2007).
Large earthquakes in Western Anatolia usually centre close to the base of the brittle upper
crust. The normal faults are approximately planar from the depth of nucleation (typically
about 10 km) to the surface, and the dips are between 30� and 60� (Jackson 1987; Taymaz
et al. 1990). In our study area, the distributions of the focal depths of the earthquakes were
investigated at various depth scales for the quakes with magnitude larger than 2 as
determined by the U.S. Geological Survey (USGS, http://www.usgs.gov). The distribution
of the earthquake focal depths (Fig. 11) shows that 36 % of all of the earthquakes in the
region occur at depths between 7 and 13 km. The earthquakes between 1 and 7 km
dominate. The distribution decreases in the range of 13–25 km and rises again between 25
and 31 km.
The distribution of the focal depths of the earthquakes between Aydın and Nazilli is
consistent with both the depths where possible magma intrusions exist (Fig. 5) and the
power spectrum results obtained from the gravity cross-section (Fig. 10). The percentage
of the earthquake focal distribution that corresponds to the structure at deep seismic
sections decreases from an average of 6–12 s (Fig. 5). After this zone, the percentage of
earthquake distribution increases. Shallow-focus earthquakes were observed in areas where
low-viscosity layers extend. The reflections that represent faults were followed down to
about 12–16 km, which is the depth of the postulated transition zone from the rigid upper
crust to the ductile lower crust.
The Menderes massifs, which generally trend E–W, are large areas identified by neg-
ative Bouguer gravity anomalies. In the Aegean region, Makris (1978) integrated the
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results of seismic and gravity data. He suggested the presence of a transitional type of crust
rather than oceanic or continental crust. According to him, the Moho occurs at about 30 km
depth over the central Aegean and to the South of the arc and rises to 22 km over the
Cretan Sea. The Western Anatolia region does not fit the local Airy isostatic model. This
finding is consistent with the idea that 6 km of the Western Anatolian lithosphere may be
more resistant to the stresses induced by scaled geological flexure over long time scales
(Pamukcu and Yurdakul 2008). The results obtained in this study are consistent with the
results of the seismological studies performed by Zhu et al. (2006) and Akyol et al. (2006),
with the gravity study conducted by Ankaya and Akcıg (1998) and with the isostasy study
carried out by Pamukcu and Yurdakul (2008). In almost all of these studies, the mean
crustal thickness was determined to be 33 km. The seismic layer at 6 km corresponds to
the durable portion within the crust in the previous work of Pamukcu and Yurdakul (2008).
In regions that possess this degree of shallow elastic thickness, problems like e.g. unrigid
medium, magmatic intrusion exist in the lower crust. The structure observed within the
lower crust in the DSP-1 profile (Fig. 5) probably decreases the rigidity of the crust.
According to gravity and geological studies (Figs. 8, 9), the structural elements in the
West and East parts of the area vary. Therefore, the seismic interpretation of Aydın and its
surroundings in the West will be the topic of future study.
As a result of the study, Buyuk Menderes graben comprises three clear reflectors which
could be named as base sediments, brittle-ductile transition, Moho and faults that show a
half-graben floored by a detachment. The Moho depth is at a comparable degree with the
previous works of researchers (Zhu et al. 2006). According to the results obtained from
Bouguer gravity and seismic data, the possible structure depths especially at 6 km and
13 km are very much consistent with each other. The distribution percentage of focal
depths seems to rise at the certain depths determined from seismic and gravity data.
Acknowledgments We greatly appreciate the support at various stages in the project of the Turkish OilCompany (TPAO) for data and the Department of Geological Sciences at the Virginia Polytechnic Instituteand State University for processing the data. Gunay Cifci was supported by The Scientific and TechnicalResearch Council of Turkey (TUBITAK) and the Regional Geophysics Lab at Virginia Polytechnic Instituteand State University during data processing. The language editing was done by Elsevier Language EditingService. We would like to express our special thanks to D.J.J. van Hinsbergen whose comments andcriticisms have greatly improved the earlier version of the paper.
27.5 27.7 27.9 28.1 28.3 28.5 28.737.5
37.6
37.7
37.8
37.9
38
Ayd
in
Naz
illi
1 to 7 7 to 13 13 to 19 19 to 25 25 to 31 31 to 37 37 to 41
% 24% 36% 16% 4
% 8% 0
% 12
Logitude
Lat
itude
Fig. 11 Focal distribution statistics with 6 km interval regarding Buyuk Menderes Graben
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