RESEARCH Lithospheric structure and the isostatic state of Eastern Anatolia

12 Pages·2017·2.77 MB·English

RESEARCH
Lithospheric structure and the isostatic state of Eastern
Anatolia: Insight from gravity data modelling
Rezene Mahatsente*, Gökay Önal, and Ibrahim Çemen
DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF ALABAMA, TUSCALOOSA, ALABAMA 35487, USA
ABSTRACT
Eastern Anatolia, Turkey, is a part of the Alpine-Himalayan collisional belt where continental crust is relatively thin for a collisional belt.
The region contains part of the Zagros suture zone, which formed during collision of the Arabian and Anatolian plates in the Miocene. It
is underlain by a low-velocity zone associated with asthenospheric flow in the uppermost mantle.
We constructed gravity models of the crust and upper-mantle structures to assess the driving mechanism of asthenospheric flow
and the isostatic state of Eastern Anatolia. Our density models are based on terrestrial and satellite-derived gravity data, and they are
constrained by receiver function and seismic tomography. The gravity models show significant lithospheric thickness variations across
the Anatolian and Arabian plates. The lithospheric mantle in Eastern Anatolia is thinner (~62–74 km) than the Arabian plate (~84–95 km),
indicating that part of the Anatolian mantle lithosphere might have been removed by delamination. The lithospheric removal process
might have occurred following the detachment of the Arabian slab in the Miocene. Widespread Holocene volcanism and high heat flow
in Eastern Anatolia can be considered as evidence of lithospheric delamination and slab break-off. The upward asthenospheric flow and
subsequent asthenospheric underplating beneath Eastern Anatolia might have been induced by both delamination and slab break-off.
These two processes may account for the rapid uplift of the Anatolian Plateau. There is a residual topography of ~1.7 km that cannot be
explained by crustal roots. Based on our gravity models, we suggest that part of the eastern Anatolian Plateau is dynamically supported
by asthenospheric flow in the upper mantle.
LITHOSPHERE; v. 10; no. 2; p. 279–290; GSA Data Repository Item 2018111 | Published online 22 February 2018 https://doi.org/10.1130/L685.1
INTRODUCTION 2003). This is much less than the 100–125 km thickness of the cold and
stable mantle lithosphere in the Arabian Shield and Iranian Plateau (e.g.,
The Eastern Anatolia region, Turkey, with an average elevation of 2 km Angus et al., 2006). Consequently, delamination and slab break-off models
above sea level, is a classic example of a young continental collision zone have been proposed to explain the thin lithosphere (e.g., Al-Lazki et al.,
(Fig. 1). The geodynamic evolution of the region involved major ocean 2003; Gök et al., 2003; Keskin, 2003; Şengör et al., 2003; Faccenna et
closures (e.g., Şengör and Yılmaz, 1981; Okay and Tüysüz, 1999; Okay al., 2006; Lei and Zhao, 2007; Göğüş and Pysklywec, 2008; Toksöz et al.,
et al., 2010) due to subduction of oceanic lithosphere and continental col- 2010; Biryol et al., 2011; Koulakov, 2011; Fichtner et al., 2013; Bartol and
lision of the Arabian and Eurasian plates (e.g., Keskin, 2003; Faccenna et Govers, 2014). The rapid topographic uplift in Eastern Anatolia between
al., 2006; Göğüş and Pysklywec, 2008; Koulakov, 2011). The complex the late Miocene and early Pliocene might be attributed to the dynamic
combination of these processes resulted in the present-day crustal structure and isostatic effects of delamination, slab break-off, and a compressional
of Eastern Anatolia and surrounding regions. These structures include the regime between the Arabian and Eurasian plates (Keskin, 2003; Şengör et
Zagros fold-and-thrust belt, the north, northeastern, and east Anatolian fault al., 2003; Faccenna et al., 2006; Göğüş and Pysklywec, 2008).
zones, the Anatolian Plateau, and east Anatolia volcanic centers (Fig. 1). The upper-mantle structure of Eastern Anatolia has been imaged in
During the 1970s and 1980s, crustal thickening due to continental col- various body and surface wave tomography experiments (Al-Lazki et al.,
lision was proposed by several models to explain the geodynamic evolu- 2003; Gök et al., 2003; Lei and Zhao, 2007; Toksöz et al., 2010; Biryol et
tion of the eastern Anatolian Plateau and high topography (e.g., Şengör al., 2011; Salaün et al., 2012; Koulakov, 2011; Fichtner et al., 2013; Delph
and Kidd, 1979; Şengör and Yılmaz 1981; Dewey et al., 1986; McKenzie et al., 2015). The lithosphere in Eastern Anatolia is underlain by a low-
and Bickle, 1988). To test the proposed geodynamic models, the Eastern velocity zone (Pn velocity = 7.6–7.9 km/s), and this has been interpreted
Turkey Seismic Experiment Project was conducted in the 1990s (Sandvol as anomalously hot asthenosphere in the uppermost mantle (Toksöz et
et al., 2003). The results of the project suggested that mantle lithosphere is al., 2010; Biryol et al., 2011; Salaün et al., 2012; Koulakov, 2011; Fich-
either absent or extremely thin beneath the eastern Anatolian Plateau (Al- tner et al., 2013; Delph et al., 2015). The hot asthenospheric flow in the
Lazki et al., 2003; Gök et al., 2003; Sandvol et al., 2003). The thickness upper mantle might have affected the crustal and lithospheric structure
of the lithospheric mantle in the Eastern Anatolian region is ~60 km (e.g., in Eastern Anatolia (e.g., Keskin, 2003). The presence of low-velocity
Pearce et al., 1990; Al-Lazki et al., 2003; Gök et al., 2003; Sandvol et al., structures within the lower crust might be attributed to the low-velocity
zone in the uppermost mantle (e. g., Pamukçu and Akçığ, 2011; Warren
*Corresponding author: [email protected] et al., 2013; Delph et al., 2015).
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MAHATSENTE ET AL.
A
B
Figure 1. (A) Simplified tectonic map of Turkey and surrounding regions (Şengör et al., 1985; Barka, 1992). (B) Regional map
of eastern Turkey with topographic relief and Holocene volcanoes. The digital elevation model is from the Shuttle Radar
Topography Mission (SRTM; Jarvis et al., 2008). The circles represent earthquakes (M ≥4.2) that occurred between 1985 and
2016. The earthquake data were obtained from Boğaziçi University Kandilli Observatory and Earthquake Research Institute
(http://koeri.boun.edu.tr/sismo/2/earthquake-catalog/). The red dashed lines show the locations of the 2.5-dimensional
(2.5-D) gravity models. Abbreviations: NAFZ—North Anatolian fault zone, EAFZ—East Anatolian fault zone, DSFZ—Dead
Sea fault zone, NEAFZ—Northeast Anatolian fault zone.
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| RESEARCH
Lithospheric structure and isostatic state of Eastern Anatolia
The presence of widespread Miocene to Pleistocene volcanism across gravity models, representative of the eastern, central, and western parts of
the region and the existence of hot asthenospheric material in the upper Eastern Anatolia, and we discuss the dynamic implications of the models.
mantle suggest that delamination and slab break-off might have occurred
in the late Miocene (Al-Lazki et al., 2003; Gök et al., 2003; Keskin, 2003; GEOLOGIC OVERVIEW
Şengör et al., 2003; Faccenna et al., 2006; Lei and Zhao, 2007; Göğüş and
Pysklywec, 2008; Toksöz et al., 2010; Biryol et al., 2011; Koulakov, 2011; Although Eastern Anatolia has a long geological history since the
Fichtner et al., 2013; Bartol and Govers, 2014). Distinguishing between fragmentation of Rodinia in the late Proterozoic, only the neotectonic
the two processes is not easy, since both processes equally explain the evolution of the region will be discussed here. The reader is referred to
genesis of widespread volcanism and the dynamic topography in the Şengör and Yılmaz (1981), Yılmaz (1993), Keskin (2007), and Şengör
region. Although several studies have been carried out in Eastern Anatolia et al. (2008) for a comprehensive discussion of the geology of Eastern
to understand the crust and upper-mantle structure, the driving mechanism Anatolia and surrounding regions.
of asthenospheric flow in the uppermost mantle beneath Eastern Anatolia Neotectonic deformation in the eastern Anatolian Plateau was initiated
and isostatic state are not well understood (Şengör et al., 2003; Keskin, during the Arabian-Eurasian collision in the early Miocene. The collision
2007; Pamukçu and Akçığ, 2011). formed the thrust faults of the Zagros fold-and-thrust belt and initiated
In this paper, we assessed the lithospheric structure and isostatic state of contraction and shortening across Eastern Anatolia due to the northward
Eastern Anatolia based on gravity data modeling. The main purpose of this motion of the Arabian plate (e.g., Şengör and Yılmaz, 1981; Perinçek and
study was twofold: (1) to determine the detailed lithospheric structure of Çemen, 1990; Yılmaz, 1993; Faccenna et al., 2006). The collision and
Eastern Anatolia down to a depth of 250 km, and (2) to interpret the grav- associated plate indentation increased the accumulation of stress across
ity model to determine the driving mechanism of asthenospheric flow and Eastern Anatolia and led to the formation of the North and East Anatolian
residual topography in the region. Our density model is based on gravity fault zones in the late Miocene (Şengör et al., 1985; Perinçek and Çemen,
data from the European Improved Gravity Model of the Earth (EIGEN- 1990; Çemen et al., 1992; Yılmaz, 1993; Faccenna et al., 2006). Both the
6C4; Förste, et al., 2015). To reduce ambiguity inherent in potential field North and East Anatolian faults are responsible for the westward lateral
interpretations, the gravity model was constrained using results from seis- motion of the Anatolian plate (Figs. 1 and 2). Contraction and strike-
mic-reflection, seismic-refraction, and earthquake tomography studies in slip tectonics across the region are still active (e.g., Şengör et al., 2003;
Eastern Anatolia. In this paper, we present four 2.5-dimensional (2.5-D) Yılmaz, 2017). However, few extensional basins, probably associated with
oE oE oE
oN
oN
Figure 2. Simplified geologic map
of Eastern Anatolia, showing the
main tectonic features and volcanic
centers (after Keskin, 2007). Abbre-
viations: EATF—East Anatolian
transform fault, E-K-P—Erzurum-
Kars Plateau, NATF—North
Anat ol ian transform fault, NEAFZ—
Northeast Anatolian fault zone.
oN
oE oE oE
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MAHATSENTE ET AL.
the rapid uplift of the topography and subsequent gravitational forces, of the Anatolian plate between the North and East Anatolian fault zones,
are controlled by normal faults (Göğüş and Pysklywec, 2008). After the and northeast motion of the Anatolian plate northeast of the Bitlis-Zagros
rapid uplift of the region between the late Miocene and early Pliocene, the suture zone (Reilinger et al., 2006). Most of the shortening in the eastern
eastern Anatolian Plateau experienced widespread Pliocene–Quaternary Anatolian Plateau is being accommodated by lateral motion of the Anato-
volcanism (Fig. 2), represented by calc-alkaline to alkaline volcanic rock lian plate along the North and East Anatolian fault zones (Reilinger et al.,
sequences (Pearce et al., 1990; Keskin, 2003; Şengör et al., 2003). Geo- 2006, 2010). The southern part of the eastern Anatolian Plateau moves
chemical studies indicate that the volcanic centers in the region consist faster than the northern part, and the rate of motion gradually decreases
of enriched asthenospheric material (Keskin et al., 1998; Keskin, 2003). from south to north (Reilinger et al., 2006, 2010; Şengör et al., 2008;
The eastern Anatolian Plateau consists of different tectonic units Yılmaz, 2017).
accreted during the Late Cretaceous to Early Tertiary (Fig. 2). The East-
ern Rhodope–Pontide metamorphic massif is located in the northern part DATA AND METHODS
of Eastern Anatolia (Şengör and Yılmaz, 1981). The massif is overlain by
a thick volcano-sedimentary rock sequence (Yılmaz, 1993). The Eastern Gravity Database
Anatolian accretionary complex is located in the middle of Eastern Anato-
lia and trends northwest-southeast. The complex consists of remnants of a The gravity data used for this study are based on the European Improved
subduction-accretion complex, including a Late Cretaceous–age ophiolitic Gravity Model of the Earth (EIGEN-6C4; Förste et al., 2015). The EIGEN-
mélange and Paleogene-age flysch sequences (e.g., Şengör and Yılmaz, 6C4 geopotential model is a spherical harmonic representation of the
1981). The flysch sequences in the north are older than the ones in the gravitational field of Earth up to degree and order of 2190 (spatial resolu-
south, indicating that the area gradually became shallower from north to tion ~ 8 km). The model is constrained by terrestrial and satellite gravity
south (e.g., Şengör and Yılmaz, 1981; Yılmaz, 1993). The Bitlis-Poturge data from the LAGEOS (Laser Geodynamics Satellites), GRACE (Gravity
Massif is exposed in the southernmost portion of Eastern Anatolia (Fig. 2) Recovery and Climate Experiment), and GOCE (Gravity Field and Steady-
and is composed of medium- and high-grade metamorphosed sediments State Ocean Circulation Explorer) satellite missions (Förste et al., 2015).
and igneous rocks formed between Late Cretaceous and middle Eocene The spherical representation is based on the World Geodetic System 1984
time (Okay and Tüysüz, 1999; Yılmaz, 1993). The Miocene Bitlis-Zagros (WGS1984) reference system.
suture zone marks the closure of the southern branch of the Neotethys The spatial resolution of the Earth Gravitational Models (e.g.,
Ocean (Fig. 2). Shallow-marine deposits and collision-related subaerial EGM2008, EIGEN-6C4) depends on the availability of high-quality
volcanic units are Neogene to Quaternary in age, and the volcanic units land gravity data (Köther et al., 2012; Gutknecht et al., 2014; Godin and
become younger from north to south/southeast (Keskin, 2003). Harris, 2014). We would like to emphasize that the land gravity data
In Eastern Anatolia, global positioning system (GPS) measurements in the EIGEN-6C4 model are based on the Earth Gravitational Model
(Fig. 3) clearly show north-south shortening, coherent westward escape (EGM2008; Pavlis et al., 2012), which includes all available land gravity
oN
Figure 3. Digital elevation map of
Eastern Anatolia and surround-
ings, showing global positioning
oN system (GPS) velocities relative to
the Eurasian plate. The elevation
data are from the SRTM30 PLUS
(http://topex.ucsd.edu/WWW
_html /srtm30 _plus.html; Reilinger
et al., 2010).
oN
oE oE oE oE
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Lithospheric structure and isostatic state of Eastern Anatolia
data. Existing land gravity data in Eastern Anatolia are available at 2–5 km
station intervals (Ates et al., 1999; Ekinci and Yiğitbaş, 2015). The land
gravity data sets from Eastern Anatolia are provided to the National Geo-
spatial-Intelligence Agency (NGA) by external organizations in Turkey
without any restriction and are included in the EGM2008 (Data Reposi-
tory Fig. DR11; Pavlis et al., 2012) and EIGEN-6C4 models (Förste et
al., 2014). In some areas, the available land gravity data sets are included
with restrictions (e.g., Himalaya). Their use is limited to a resolution cor-
responding to 15 arc-minute area-mean value (Fig. DR1 [see footnote 1];
Pavlis et al., 2012).
To develop the 2.5-D gravity models of the deep crust and upper-
mantle structure of Eastern Anatolia, we downloaded the free air anomaly
of the region between 38°E and 37°N and 44°E and 41°N from the data
portal of the International Centre for Global Earth Models (ICGEM: http://
icgem.gfz-potsdam.de/ICGEM/). Then, we computed the complete Bou-
guer anomaly of the study area using Gravity Terrain Correction code
Bouguer anomaly (mGal)
(GTeC; Cella, 2015). The complete Bouguer anomaly (Fig. 4) is based on
spherical cap and terrain corrections up to a radius of 168 km. The cor-
-27.3 -55.0 -75.3 -91.8 -104.4 -118.9 -131.5 -147.0 -159.6 -174.1 -219.6
rections are based on elevation data from the Shuttle Radar Topography
Figure 4. Complete Bouguer anomaly map of Eastern Anatolia from the
Mission (SRTM; Jarvis et al., 2008) and a standard reduction density of
European Improved Gravity Model of the Earth (EIGEN-6C4; Förste et al.,
2.670 g cm–3. 2014). Red triangles show Holocene volcanoes. The numbers in the map
show the lithosphere-asthenosphere boundaries (stars) and Moho depths
Initial Model and Data Constraints (crosses) in the region from receiver function and seismic tomography
(Angus et al., 2006; Kind et al., 2015; Delph et al., 2015). Abbreviations:
NAFZ—North Anatolian fault zone, EAFZ—East Anatolian fault zone,
In this paper, we discuss four 2.5-D gravity models, representative of
NEAFZ—Northeast Anatolian fault zone.
the western, central, and eastern parts of the Eastern Anatolia region. The
models depict the deep crust and upper-mantle structure of the Anatolian
and Arabian plates at 38°E, 41°E, 42°E, and 44°E longitude and span Different lithospheric thickness values obtained from receiver func-
from 37°N to 41°N (Fig. 1). tion analysis show that the Arabian lithosphere is thicker, ranging from
To reduce the ambiguity inherent in potential field interpretations, the 75 to 160 km, than the Eastern Anatolian lithosphere, which ranges from
densities and geometric structures of the sediments, crust, lithospheric 40 to 90 km (e.g., Angus et al., 2006; Özacar et al., 2008; Zor, 2008;
mantle, and asthenosphere were constrained by velocity models from Pasyanos et al., 2014; Kind et al., 2015). Low Pn velocities (7.6–7.9
receiver function analysis and seismic tomography (e.g., Örgülü et al., km/s) dominate the uppermost mantle of Eastern Anatolia, and these
2003; Piromallo and Morelli, 2003; Zor et al., 2003; Reiter and Rodi, 2006; are interpreted as indicative of asthenospheric underplating beneath
Lei and Zhao, 2007; Özacar et al., 2008; Gans et al., 2009; Toksöz et al., Eastern Anatolia (Al-Lazki et al., 2003; Gök et al., 2003; Lei and Zhao,
2010; Biryol et al., 2011; Koulakov, 2011; Salah et al., 2011; Fichtner et 2007; Toksöz et al., 2010; Biryol et al., 2011; Koulakov, 2011; Fichtner
al., 2013; Pasyanos et al., 2014; Delph et al., 2015). et al., 2013).
The crustal thickness in Eastern Anatolia, as determined from receiver Table 1 shows the velocities of sediments, crust, lithospheric mantle,
function and seismic tomography, ranges from 30 to 55 km (e.g., Zor et and asthenosphere in Eastern Anatolia, along with the corresponding
al., 2003; Angus et al., 2006; Özacar et al., 2008; Gök et al., 2011; Gökalp, densities. The density values of major tectonic units were derived from
2012; Tezel et al., 2013; Vanacore et al., 2013; Pasyanos et al., 2014; empirical relationships between P-wave velocities and densities of rocks
Schildgen et al., 2014; Delph et al., 2015), and the sediment thickness at relevant pressure and temperature conditions (Sobolev and Babeyko,
ranges from 0 to 5 km, based on the global sediment map determined by 1994; Nafe and Drake, 1957). The velocity and geometric structures were
seismic data (Laske and Masters, 1997). used to constrain the 2.5-D gravity models of Eastern Anatolia.
TABLE 1. P-WAVE VELOCITIES AND DENSITIES OF MAJOR TECTONIC UNITS IN EASTERN ANATOLIA
Tectonic structure P-wave velocity Density References
(km s–1 ) (g cm–3)
Sediment 4.55–4.98 1.70–2.90 Maden et al. (2009); Yılmaz et al. (2010); Salah et al. (2011); Tezel et al. (2013); Warren et al. (2013)
Upper crust 4.68–6.3 2.50–2.90 Reiter and Rodi (2006); Özacar et al. (2008); Maden et al. (2009); Yılmaz et al. (2010); Gök et al. (2011); Salah et al.
(2011); Tezel et al. (2013); Warren et al. (2013); Delph et al. (2015)
Lower crust 5.32–7.23 2.50–3.18 Reiter and Rodi (2006); Özacar et al. (2008); Maden et al. (2009); Yılmaz et al. (2010); Gök et al. (2011); Salah et al.
(2011); Tezel et al. (2013); Warren et al. (2013); Delph et al. (2015)
Mantle lithosphere 7.8–8.28 3.22–3.30 Al-Lazki et al. (2003); Piromallo and Morelli (2003); Reiter and Rodi (2006); Lei and Zhao (2007); Özacar et al. (2008);
Maden et al. (2009); Yılmaz et al. (2010); Simmons et al. (2011); Tezel et al. (2013); Koulakov (2011)
Asthenosphere 7.6–8.3 3.15–3.35 Piromallo and Morelli (2003); Reiter and Rodi (2006); Lei and Zhao (2007); Pasyanos and Nyblade (2007); Maden
et al. (2009); Simmons et al. (2011); Biryol et al. (2011); Koulakov (2011); Pasyanos et al. (2014)
Note: The densities were derived from P-wave velocities using empirical relations (Sobolev and Babeyko, 1994; Nafe and Drake, 1957).
1 GSA Data Repository Item 2018111, Figure DR1: Land and ocean gravity anomaly data used to develop the EGM2008 model: (a) data availability, and (b) data source
identification (Pavlis et al., 2012), is available at http://www.geosociety.org/datarepository/2018, or on request from [email protected].
LITHOSPHERE | Volume 10 | Number 2 | www.gsapubs.org 283
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MAHATSENTE ET AL.
TABLE 2. DENSITY VALUES USED IN THE FINAL GRAVITY MODEL
Tectonic units Density Density Density Density Average tolerable
(g cm–3 ) (g cm–3 ) (g cm–3 ) (g cm–3 ) variations
Profile @ 38°E Profile @ 41°E Profile @ 42°E Profile @ 44°E (± g cm–3 )
Sediment 2.55 2.55 2.55 2.55 0.108
Upper crust 2.71–2.78 2.69–2.73 2.67–2.73 2.68–2.72 0.021
Lower crust 2.8–2.83 2.72–2.88 2.71–2.85 2.75–2.82 0.031
Mantle lithosphere 3.27 3.27 3.27 3.27 0.017
Asthenosphere 3.24–3.32 3.23–3.32 3.23–3.32 3.23–3.32 0.023
In order to determine the deep crust and upper-mantle density structure (<200 km) were attenuated at a height of 50 km above the surface. The
of Eastern Anatolia, we applied two modeling approaches: First, 2.5-D gravity anomalies at heights >50 km are regional in nature. In the second
forward modeling of the Bouguer anomaly was performed using GM-SYS step, the Bouguer gravity anomalies were filtered using a low-pass filter
Gravity and Magnetic Modeling Software (Geosoft Oasis Montaj, 2017), for various cutoff values. The cutoff wavelength (300 km) was selected
which makes use of the method of Talwani et al. (1959) and Talwani and based on similarity of amplitudes of gravity anomalies obtained using
Heirtzler (1964) to calculate gravity anomalies. We would like to empha- the upward continuation and wavelength filtering methods.
size here that the strike lengths of the geological units in the study area The regional gravity anomaly map of Eastern Anatolia (Fig. 5) does
are not long enough to assume 2-D modeling. The strike length of a linear not suggest direct spatial correlation between the locations of volcanic
structure should be at least 4–5 times the width of the geologic features for centers and the broad long-wavelength regional gravity anomaly, although
2-D modeling. The 2-D and 2.5-D gravity modeling techniques are gener- the centers are associated with rapid asthenospheric upwelling beneath
ally applicable to profiles nearly perpendicular to linear structures. The Eastern Anatolia. This implies that the regional gravity anomalies of East-
main difference between 2-D and 2.5-D modeling is that a 2-D model has ern Anatolia are mainly due to density variations in the deep crust and
an infinite strike length, whereas a 2.5-D model has a finite strike length. upper-mantle structures.
In the second step, to improve the fit of the calculated gravity to the Three distinct positive and negative anomalies can be identified on the
observed data, the density values of all tectonic units along each of the regional gravity map (Fig. 5): The negative anomaly in the northeastern
cross sections were inverted using a ridge-regression algorithm, taking all part of the map area (north of the Bitlis-Zagros suture zone) correlates
available geological information into consideration. The errors in the mis- with the eastern Anatolian Plateau, and the two positive anomalies in
fits were minimized in a least-square sense. The density values of the final the north and southwestern sections of the map coincide with Black Sea
gravity models and their average tolerable variations are given in Table 2. and Arabian plates, respectively. The transition from high- to low-grav-
The 2.5-D gravity models were set perpendicular to geologic strike, ity anomalies (gravity gradients) in Eastern Anatolia is marked by four
and the strike length was constrained by the physical limits of geologic major tectonic structures (Figs. 4 and 5). This indicates that the deforma-
units. The modeling space, and hence the mass therein, is a small fraction tion associated with major tectonic structures in Eastern Anatolia most
of the entire Earth. This has an effect on the forward gravity modeling, probably affects the deepest part of the lower crust. The residual gravity
because the levels of the observed and modeled gravity values are not iden- anomalies of Eastern Anatolia, as obtained from high-pass filtering, are
tical and can result in offsets between the measured and modeled gravity shown in Figure 6. The residual gravity anomalies range from 93 mGal to
values. To account for the effect of the surrounding mass on the forward –64 mGal (Fig. 6) and appear to correlate inversely with the topography,
gravity modeling, the dimension of the modeling space was extended to
10,000 km beyond the dimension of the 2.5-D gravity models.
RESULTS AND DISCUSSION
Gravity Anomalies and Analysis
The Bouguer gravity map of Eastern Anatolia contains long- and short-
wavelength gravity anomalies (Fig. 4). The gravity map reveals broad
regional negative and positive Bouguer anomalies over the Anatolian
and Arabian plates, respectively. A negative Bouguer anomaly as low as
−219 mGal coincides with the highest elevation in Eastern Anatolia, and
the anomaly increases with decreasing elevation to the NW and SW of
the eastern Anatolian Plateau.
To better understand the gravity anomalies of Eastern Anatolia, we
applied upward continuation and wavelength filtering. The upward con-
tinuation process enhances long-wavelength anomalies and attenuates
short wavelengths. The low-pass or long-wavelength filter suppresses
Regional gravity anomaly (mGal)
short-wavelength anomalies and enhances regional long-wavelength grav-
ity anomalies. Practically, both low-pass filtering and upward continuation
-23.1 -56.5 -75.9 -90.5 -106.2 -121.4 -133.6 -145.1 -157.8 -168.2 -177.9
accentuate long-wavelength anomalies of deep origin. In the first step, the
Figure 5. Regional gravity anomaly map of Eastern Anatolia. Four major
Bouguer gravity anomalies were continued upward to various heights.
tectonic features of the region are Bitlis-Zagros suture zone, East Anato-
The purpose of this process is to help us select the cutoff wavelength lian fault zone (EAFZ), North Anatolian fault zone (NAFZ), and Northeast
for the low-pass filtering. We found that most of the short wavelengths Anatolian fault zone (NEAFZ). Red triangles show Holocene volcanoes.
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Lithospheric structure and isostatic state of Eastern Anatolia
Residual gravity anomaly (mGal)
Residual topography (meter)
92.5 29.9 19.5 10.9 4.8 -1.4 -6.3 -11.2 -17.3 -25.9 -64.0
-3,166 -1,004 -526 -258 -48 86 296 449 602 870 1,712
Figure 6. Residual gravity anomaly map of Eastern Anatolia. Red trian-
gles and dashed black line show Holocene volcanoes and the location of Figure 7. Residual topography of Eastern Anatolia based on Airy isos-
Lake Van, respectively. Abbreviations: NAFZ—North Anatolian fault zone, tasy model. Red triangles show Holocene volcanoes. Abbreviations:
EAFZ—East Anatolian fault zone, NEAFZ—Northeast Anatolian fault zone. NAFZ—North Anatolian fault zone, EAFZ—East Anatolian fault zone,
NEAFZ—Northeast Anatolian fault zone.
indicating a crustal root beneath the eastern Anatolian Plateau that causes
a low in the gravity anomaly. residual values (~1.7 km), indicating that the eastern Anatolian Plateau
may be undercompensated. This further implies that the crustal thickness
Isostatic State of Eastern Anatolia beneath the eastern Anatolian Plateau may not be sufficient to isostatically
support the observed topography. This is in agreement with some of the
The region beneath Eastern Anatolia may not have a sufficient crustal recent studies in Eastern Anatolia (e.g., Faccenna et al., 2014; Komut,
root to explain the observed topography (cf. e.g., Zor et al., 2003; Vana- 2015). Thus, the Anatolian plateau may be partly compensated by another
core et al., 2013). Thus, part of the eastern Anatolian Plateau may not be mechanism, most probably by asthenospheric flow in the uppermost man-
isostatically compensated. To assess the isostatic state and compensation tle. Other possible compensation mechanisms include density heterogene-
mechanism of Eastern Anatolia, we determined residual topography of the ity in the crust and upper mantle, as well as thickness of the lithosphere.
region (Fig. 7) from differences between observed and calculated isostatic
topography based on the Airy isostasy model. The observed topographic Lithospheric Structure and Driving Mechanism of
data were obtained from the SRTM (Jarvis et al., 2008). The isostatic topog- Asthenospheric Flow in Eastern Anatolia
raphy was determined based on crustal thickness derived from receiver
function analysis (Tezel et al., 2013) and the Crust1.0 model (Laske et al., Our gravity models (Fig. 8) along longitude 38°E, 41°E, 42°E, and 44°E
2013). We assumed a global mean crustal thickness of 31.2 km for the are representative of the western, central, and eastern sectors of Eastern
Airy isostatic model (Watt, 2015). The assumed crustal and mantle densi- Anatolia and show the crust and upper-mantle structure of the Arabian
ties for the Airy model were 2.750 g cm–3 and 3.250 g cm–3, respectively. and Anatolian plates. As shown in Figure 8, the long-wavelength gravity
The reliability of the calculated residual topography depends on the anomaly of Eastern Anatolia is well explained in terms of a thin lithosphere
accuracy of crustal thickness and density data. To quantify the effects of and anomalous asthenosphere in the uppermost mantle. There are signifi-
density variation in the calculated residual topography and determine the cant variations in the lithospheric structure of Eastern Anatolia. The litho-
corresponding uncertainty, we used a range of density values for the crust spheric mantle beneath Eastern Anatolia is thinner (~62–74 km) than the
(2.7–2.8 g cm–3) and upper mantle (3.2–3.35 g cm–3). The effect of density Arabian plate (~84–95 km), indicating that the mantle lithosphere beneath
variation of the residual topography is in the order of 50 m. We did not Eastern Anatolia may have been delaminated. This is in agreement with
include the contribution of mantle lithosphere to isostatic compensation, results of S-wave receiver function studies in Eastern Anatolia (Fig. 9; cf.
because the available information on the depth of the lithosphere-asthe- e.g., Angus et al., 2006; Kind et al., 2015). The lithospheric instability and
nosphere boundary beneath Eastern Anatolia is limited (cf. e.g., Angus the subsequent delamination of the Anatolian mantle might have occurred
et al., 2006; Kind et al., 2015). However, this limitation did not affect the following slab break-off in the region. Several receiver function studies
results of our analysis, because the contribution of the continental crust have determined the presence of the detached Arabian slab at the base of
to isostatic compensation is more than the lithospheric mantle (Faccenna the transition zone (Lei and Zhao, 2007; Özacar et al., 2008; Zor, 2008;
et al., 2014). This should be due to the contrasting composition between Biryol et al., 2011). Thus, the upward asthenospheric flow and subsequent
continental crust and lithospheric mantle. asthenospheric underplating in the uppermost mantle beneath Eastern
The residual topography map (Fig. 7) shows negative and positive Anatolia might be attributed to both detachment of the Arabian slab in the
topographic anomalies (residuals) over Eastern Anatolia where negative Miocene and delamination of the Anatolian mantle in the late Miocene.
and positive residuals indicate over- and undercompensation conditions. The lithospheric delamination of the Anatolian continental mantle and
The high topography in Eastern Anatolia is characterized by positive slab break-off might have induced asthenospheric flow beneath Eastern
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MAHATSENTE ET AL.
38oN 39oN 40oN
al)
G
m
y (
al
m
o
n
a
y
vit
a
Gr
+41 +41
*67
Figure 8. 2.5-dimensional (2.5-D)
gravity models of Eastern Ana-
tolia (EA). (A) Profile along 38°E.
(B) Profile along 41°E. (C) Profile
along 42°E. (D) Profile along 44°E.
The locations of the profiles are
shown in Figure 1. The white circles
are earthquake hypocenters. The
numbers in the models show the
lithosphere-asthenosphere bound-
aries (stars) and Moho depths
(crosses) in the region from receiver
function and seismic tomography
(Angus et al., 2006; Kind et al.,
2015; Delph et al., 2015). The white
dashed lines show the inferred loca-
tions of the Bitlis-Zagros suture,
East Anatolian fault zone, North
38oN 39oN 40oN Anatolian fault zone, and Northeast
Anatolian fault zone. Abbreviations:
AP—Anatolian plate, EA—East-
ern Anatolia, BZS—Bitlis-Zagros
Gal) suture, EAFZ—East Anatolian fault
m
y ( zone, NAFZ—North Anatolian fault
al zone; NEAFZ—Northeast Anatolian
m
no fault zone. (Continued on following
y a page).
vit
a
Gr
37oN 38oN 39oN 40oN 41oN
+40 +45
*70
*75
*93
286 www.gsapubs.org | Volume 10 | Number 2 | LITHOSPHERE
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| RESEARCH
Lithospheric structure and isostatic state of Eastern Anatolia
38oN 39oN 40oN
al)
G
m
y (
al
m
o
n
a
y
vit
a
Gr
+40 +40
*68 *65
*94
Figure 8 (continued).
38oN 39oN 40oN
al)
G
m
y (
al
m
o
n
a
y
vit
a
Gr
+46 +44
*68 *73
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MAHATSENTE ET AL.
oE oE oE oE oE oE oE oE oE oE
Figure 9. S-wave receiver function of Eastern Anatolia showing common
conversion point (CCP) stacking image at 40°N (Angus et al., 2006). Abbre-
viations: UCD—upper-crust discontinuity; LAB—lithosphere-asthenosphere
boundary; IB—Iranian block; NAF—North Anatolian fault; EAAC—East
Anatolian accretionary complex. The letters U and D highlight regions of
thin and thick lithosphere, respectively.
Figure 10. Shear wave velocity model of Eastern Anatolia at 40°E
(Delph et al., 2015). Abbreviations: BZS—Bitlis-Zagros suture; EAFZ—
Anatolia. The NE-SW–oriented fast polarization direction of the upper-man- East Anatolian fault zone; IAESZ—Izmir-Ankara-Erzincan suture zone;
tle seismic anisotropy in Eastern Anatolia may be considered as evidence NAF—North Anatolian fault; NEAFZ—Northeast Anatolian fault zone;
Elv—elevation; L. crust—lower crust.
of the direction of recent asthenospheric flow (Sandvol et al., 2003; Biryol
et al., 2010; Yolsal-Çevikbilen, 2014; Vinnik et al., 2016). The polarization
direction is nearly parallel to plate motion and may have provided driving topographic uplift and widespread Holocene volcanism in the region. The
forces in a NE-SW direction (e.g., Vinnik et al., 2016). Thus, the astheno- models, based on EIGEN-6C4 data, show that the lithospheric mantle in East-
spheric flow and subduction in the Caucasus region may have driven north- ern Anatolia is thinner (~62–74 km) than the Arabian plate (84–95 km) north
eastward plate motion. The asthenospheric flow beneath Eastern Anatolia of the Bitlis-Zagros suture zone. These data support the hypothesis that the
accounts for rapid topographic uplift in the late Miocene and early Pliocene, Anatolian lithosphere is delaminated and might have induced asthenospheric
high heat flow, and widespread Holocene volcanism. The highest heat-flow flow beneath Eastern Anatolia. The lithospheric instability and the subsequent
value (105 mW m–2) was observed in the Eastern Pontides orogenic belt, delamination probably occurred following slab break-off in the region in the
associated with Neogene and Quaternary volcanism (Maden and Öztürk, late Miocene. The asthenosphere probably ascended to the base of the thin
2015). The exact contributions of delamination and slab break-off to the lithosphere following the slab break-off and delamination. The ascension
overall upward asthenospheric flow, and hence to the dynamic topogra- of asthenosphere accounts for the rapid topographic uplift and extensive
phy and volcanism in eastern Anatolia, is not easily determined, because melting that resulted in widespread Holocene volcanism across the region.
the composition of magma derived from asthenospheric flow induced by The densities of the lower crust (2.71–2.88 g cm–3) beneath the central
delamination and slab break-off may be the same. Moreover, delamination and eastern parts of Eastern Anatolia are less than the average density of
and slab break-off could occur during and after subduction. the crust in the region (2.80–2.95 g cm–3). This indicates that the crustal
The crustal structure of Eastern Anatolia is segmented, much like the rocks are probably thermally affected by asthenospheric underplating.
underlying lithospheric mantle. The crustal thickness in Eastern Anatolia Based on our gravity models, we suggest that the hot asthenospheric
increases with increasing elevation and ranges from 33 km in the west to material in the uppermost mantle might have induced thermal weaken-
46 km in the east (Fig. 8), similar to previous crustal thickness estimates ing in the overlying crust.
for the eastern Anatolian Plateau (Fig. 9 [Angus et al., 2006; Motavalli- Our residual topographic model shows that part of the eastern Anato-
Anbaran et al., 2016] and Fig. 10 [cf. e.g., Delph et al., 2015]). The crust lian Plateau is not isostatically supported. There is residual topography
becomes thicker north of the Bitlis-Zagros suture zone. This might be up to ~1.7 km that cannot be explained by a crustal root. The residual
attributed to the ongoing collision between the Arabian and Eurasian topography in Eastern Anatolia is probably supported by rising hot asthe-
plates. The crust might have been affected by the hot asthenospheric nosphere in the uppermost mantle. However, this interpretation does not
material in the uppermost mantle. The densities of the upper and lower exclude the possibility that the Anatolian Plateau may partly be compen-
crust in the central and eastern sectors of Eastern Anatolia are less than sated by density heterogeneity in the crust and upper mantle, as well as
the average density of the crust in the region, indicating that the crust thickness of the lithosphere.
may be thermally weakened (Fig. 8). The receiver function analysis by
Delph et al. (2015) also suggested that the shear wave velocity of the ACKNOWLEDGMENTS
crust beneath the Anatolian plate is slower than the global average crustal This work was supported by a College Academy of Research, Scholarship, and Creative
Activity (CARSCA, a unit of the College of Arts and Sciences, University of Alabama) grant to
shear wave velocity of 3.4 km s–1. We interpret the low densities in the
Mahatsente. We thank the Turkish Petroleum Corporation (TPAO) for supporting Gökay Önal
upper and lower crust beneath the central and eastern sectors of Eastern during his M.S. studies at the Department of Geological Sciences, University of Alabama. We
Anatolia to indicate a thermally altered crust. also thank the two anonymous reviewers for thoughtful and constructive comments, which
were of great help in the preparation of the final version of this paper. The data used in this
study are available via file transfer protocol (FTP) by contacting the authors.
CONCLUSIONS
REFERENCES CITED
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vide a possible explanation of the density and geometric structures and mographic Pn velocity and anisotropy structure beneath the Anatolian Plateau (eastern
Turkey) and the surrounding regions: Geophysical Research Letters, v. 30, no. 24, p. 8043,
the driving mechanism of asthenospheric flow that resulted in the rapid https:// doi .org /10 .1029 /2003GL017391.
288 www.gsapubs.org | Volume 10 | Number 2 | LITHOSPHERE
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