# New geodetic constraints on the role of faults and blocks vs. distribute strain in the Nubia-Arabia-Eurasia zone of active plate interactions

Semih Ergintav, Michael Floyd, Demitris Paradissis, Hayrullah Karabulut, Philippe Vernant, Frédéric Masson, Ivan Georgiev, Ali Özgün Konca, Uğur Doğan, Robert King, et al.

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## To cite this version:

Semih Ergintav, Michael Floyd, Demitris Paradissis, Hayrullah Karabulut, Philippe Vernant, et al.. New geodetic constraints on the role of faults and blocks vs. distribute strain in the Nubia-Arabia- Eurasia zone of active plate interactions. Turkish Journal of Earth Sciences, 2023, 32 (3), pp.248-261. 10.55730/1300-0985.1842 . hal-04723628

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1-1-2023

# New geodetic constraints on the role of faults and blocks vs. distribute strain in the Nubia-Arabia-Eurasia zone of active plate interactions

SEMİH ERGİNTAV

MICHAEL FLOYD

DEMITRIS PARADISSIS

HAYRULLAH KARABULUT

PHILIPPE VERNANT

See next page for additional authors

Follow this and additional works at: https://journals.tubitak.gov.tr/earth

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Part of the Earth Sciences Commons

## Recommended Citation

ERGİNTAV, SEMİH; FLOYD, MICHAEL; PARADISSIS, DEMITRIS; KARABULUT, HAYRULLAH; VERNANT, PHILIPPE; MASSON, FREDERIC; GEORGIEV, IVAN; KONCA, ALİ ÖZGÜN; DOĞAN, UĞUR; KING, ROBERT; and REILINGER, ROBERT (2023) "New geodetic constraints on the role of faults and blocks vs. distribute strain in the Nubia-Arabia-Eurasia zone of active plate interactions," *Turkish Journal of Earth Sciences*:

Vol. 32: No. 3, Article 2. https://doi.org/10.55730/1300-0985.1842

Available at: https://journals.tubitak.gov.tr/earth/vol32/iss3/2
# New geodetic constraints on the role of faults and blocks vs. distribute strain in the Nubia-Arabia-Eurasia zone of active plate interactions

## Authors

SEMİH ERGİNTAV, MICHAEL FLOYD, DEMITRIS PARADISSIS, HAYRULLAH KARABULUT, PHILIPPE VERNANT, FREDERIC MASSON, IVAN GEORGIEV, ALİ ÖZGÜN KONCA, UĞUR DOĞAN, ROBERT KING, and ROBERT REILINGER
# New geodetic constraints on the role of faults and blocks versus distributed strain in the Nubia-Arabia-Eurasia zone of active plate interactions

Semih ERGINTAV<fnref n="1" />*, Michael FLOYD<fnref n="2" />, Demitris PARADISSIS<fnref n="3" />, Hayrullah KARABULUT<fnref n="4" />, Philippe VERNANT<fnref n="5" />, Frederic MASSON<fnref n="6" />, Ivan GEORGIEV<fnref n="7" />, Ali Özgün KONCA<fnref n="4" />, Uğur DOĞAN<fnref n="8" />, Robert KING<fnref n="2" />, Robert REILINGER<fnref n="2" />

**Received:** 12.05.2022 • **Accepted/Published Online:** 14.09.2022 • **Final Version:** 28.04.2023

**Abstract:** We present a broad view of present-day motions and deformations derived from uniform processing of GNSS observations within the Nubia-Arabia-Eurasia zone of plate interaction. The new observations we present provide a ~29% increase in the number of velocity determinations, a reduction in average station spacing from ~76 km to ~39 km, and an improvement in velocity uncertainties (for <1 mm/year), from 180 to 578 sites compared to our prior published solution (Reilinger et al., 2006). We use these new constraints to better evaluate the role of faults and blocks in controlling the character of continental deformation within the zone of plate interactions. Simple elastic block models show that internal deformation of the region occurs in large part on mapped, seismically active fault systems, indicating elastic behavior of the seismogenic crust (above ~15 km). For example, eastern central Anatolia, an area of > ~126,000 km<fnref n="2" />, bounded by the North and East Anatolian Faults exhibits internal velocity differences of <0.5 mm/year, indicating strain rates of < ~1.5 nanostrain/year. Geodetically constrained fault slip rates obtained from this simplified approach are comparable to geologic rates, indicating that major faults have controlled the recent geologic evolution of the region (i.e. 5–10 Myr). The pattern of present-day deformation, including increasingly fast motions towards the Hellenic trench, and the roughly simultaneous opening of all the major Mediterranean basins in the early Miocene with the slowing of the Nubia-Eurasia convergence, support conceptual models that foundering and rollback of the subducted Nubian slab beneath the Aegean is the primary mechanism responsible for present-day motion and internal deformation of the Anatolian-Aegean region.

**Key words:** Anatolia, Aegean, GNSS deformation, subduction, geodynamics, Mediterranean

## 1. Introduction

(Note: In this paper we use “Anatolia” to refer to all of Turkey between the North and East Anatolian faults and the Aegean coast; “Aegean region” refers to the Aegean Sea and adjacent Peloponnese; “East Anatolia” (E Anatolia)” refers to the eastern section of the deforming Anatolia plate, not to the geographical definition of eastern Anatolia.)

The geology of the Anatolian-Aegean region, and indeed the Eastern Mediterranean, has been the focus of numerous early studies (see Dixon and Robertson, 1984, for overview), leading to the region becoming a “laboratory” for developing the principles of Plate Tectonics (e.g., McKenzie, 1972; Le Pichon and Angelier,

1979; Şengör and Yılmaz, 1981; Royden, 1993; Jackson, 1994). Subduction of the Tethys Ocean has dominated tectonic processes along the southern margin of Eurasia for the past >200 Myr (e.g., Agard et al., 2011). Since the early Miocene (~20 Myr), the active tectonics of the region has resulted from the interaction between the late stages of subduction of the Nubian oceanic lithosphere along the Hellenic-Cyprus subduction system, and the early stages of continental collision between Arabia and Eurasia, that at present involves an ~600 km-wide deformation zone (Jolivet and Faccenna, 2000; McQuarrie et al., 2003; McQuarrie and van Hinsbergen, 2013). Geological, seismological, and plate tectonic observations

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<footnote n="1">¹Department of Geodesy, Kandilli Observatory and Earthquake Research Institute, Boğaziçi University, İstanbul, Turkey</footnote>

<footnote n="2">²Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA</footnote>

<footnote n="3">³Department of Topography, National Technical University of Athens, Athens, Greece</footnote>

<footnote n="4">⁴Department of Geophysics, Kandilli Observatory and Earthquake Research Institute, Boğaziçi University, İstanbul, Turkey</footnote>

<footnote n="5">⁵Geosciences Laboratory, University of Montpellier, Montpellier, France</footnote>

<footnote n="6">⁶Institute of Physics of the Globe, University of Strasbourg, Strasbourg, France</footnote>

<footnote n="7">⁷National Institute of Geophysics, Geodesy and Geography, Bulgarian Academy of Sciences, Sofia, Bulgaria</footnote>

<footnote n="8">⁸Department of Geomatics Engineering, Yıldız Technical University, İstanbul, Turkey</footnote>

<footnote>* Correspondence: semih.ergintav@boun.edu.tr</footnote>
have provided important constraints on the evolution of the Anatolian-Aegean region including the decoupling of Anatolia and the Aegean from Eurasia and Arabia with the development of the North and the East Anatolian Faults (~11 and 5 Myr, respectively; e.g., Şengör et al., 2004; Yılmaz et al., 2006; Şengör and Yazıcı, 2020), backarc extension of the Aegean and Corinth Gulf (Jackson, 1994; Armijo et al., 1996), and the ~north-south extension in western Anatolia (e.g., Şengör et al., 1984; Bozkurt and Satır, 2000). Furthermore, plate tectonic reconstructions and geologic investigations of paleo-fault slip rates suggest relatively steady rates of motions for Nubia (since ~11 Myr) and Arabia (since > 30 Myr) with respect to Eurasia, providing a roughly uniform temporal context in which to investigate relationships between plate interactions and lithospheric tectonics (McQuarrie et al., 2003; Hatzfeld and Molnar, 2010).

Beginning in the late 1980s, geodetic observations, most notably from Global Navigation Satellite Systems (GNSS), have allowed quantification of the spatial and temporal behavior of active deformation within the zone of interaction of the Arabia, Nubia and Eurasia plates, providing precise constraints on plate motions and broad-scale continental deformation (e.g., Oral et al., 1993; Smith et al., 1994; Le Pichon et al., 1995; Barka and Reilinger 1997; McClusky et al., 2000; Briole et al., 2000; Reilinger et al., 2006; Floyd et al., 2010; Nocquet, 2012). In this study, we present an updated GNSS velocity field for the Anatolia-Aegean region

and surrounding areas (Figure 1). We take the approach of developing local reference frames for the Aegean and east Anatolia to better resolve subtle variation of motions within rapidly moving (i.e., with respect to Eurasia) regions. We use this broad view of plate motions and deformations to investigate the role of Nubian lithosphere subduction along the Hellenic-Cyprus subduction zone, and continental collision between Arabia and eastern Anatolia, in the active deformation of the Anatolia-Aegean region. Furthermore, to investigate the distribution of the deformations within the Anatolian plate, we developed a simplified block model. Two key results of this study are, within the resolution of present geodetic observations (~<0.5 mm/year): firstly, the seismogenic crust (above ~15 km depth) deforms elastically, with a very broad area bounded by the North Anatolian Fault (NAF) and East Anatolian Fault (EAF) rotating with negligible internal strain; secondly, increasing rates of motion directed towards the Hellenic trench supports models in which foundering of the subducting Nubian slab along the Hellenic subduction zone is the principal driver of western Anatolia-Aegean motion and internal deformation (Le Pichon and Kreemer, 2010; Özeren and Holt, 2010; Reilinger and McClusky, 2011; Royden and Facenna, 2018). In support of this interpretation, our new observations demonstrate a close relationship between surface deformation and slab breaks/contortions along the Hellenic and Cyprus subduction zone system (Karabulut and Özbakır, 2018; Karabulut et al., 2019a).

<figure><img src="image_5.png" /><figcaption>**Figure 1.** GNSS velocities and 95% confidence ellipses shown with respect to Eurasia (see text and Table S1 for details). Thin black lines show active faults (Ganas et al., 2013; Emre et al., 2013). Abbreviations: EUR = Eurasia, NAT = North Aegean Trough, KTF = Kephalonia Transform fault, HSZ = Hellenic subduction zone, CSZ = Cyprus subduction zone, AN = Anatolia, AR = Arabia, NAF = North Anatolian Fault, EAF = East Anatolian Fault.</figcaption></figure>
## 2. GNSS data and analysis

GNSS velocities and associated uncertainties used in this study cover a period of 27 years from 1984 to 2018. All GNSS observations were analyzed uniformly in a single solution using the GAMIT/GLOBK software (Herring et al. 2018) as part of the updated version of a velocity solution covering the Eastern Mediterranean (Reilinger et al., 2006), following the approach described in Floyd et al., (2022). Hence, our velocity field is estimated from a uniform processing and is isolated from any effects of combined published velocity fields, obtained by the optimization of their reference frames in a common datum (e.g., Nocquet, 2012).

Daily solutions for each station were combined into position time series. All discontinuities in the time series (including coseismic displacements) were defined and estimated simultaneously with velocities, where velocities are equated before and after the discontinuity. At this stage, sites which have less than 2.4 years duration are excluded from our solution to minimize the potential bias introduced by seasonal variations (Blewitt and Lavallée, 2002; Blewitt and Lavallée, 2003). If statistically significant changes in velocity are found, multiple velocities are retained over different periods. In the case of significant postseismic motion, a few years of data after the event are excluded if the observations are sparse or otherwise fit with an additional logarithmic decay function in addition to the linear velocity (Ergintav et al., 2009; Ergintav et al., 2014). The only exception is the rupture zone of 1999 İzmit event (Mw 7.6). Within this zone, researchers reported ongoing shallow deformations (Çakır et al., 2012; Özarpacı et al., 2021) and we could not remove this local deformation anomaly from our velocity field but we isolated it during our interpretations.

To define the reference frame, a total of 14 IGS core stations included in the processing are used to estimate a consistent transformation (translation and rotation) to align our network velocity solution to the ITRF2014 (Altamimi et al., 2016). Euler pole parameters from the Altamimi et al. (2017) plate motion model were used to rotate the velocity field into a Eurasia-fixed reference

frame. The weighted root-mean-square (WRMS) misfit of our velocity field to the reference frame is $(e, n, u) = (0.17, 0.14, 0.38)$ mm/year (Floyd et al., 2022). Finally, we removed any sites whose formal (1-sigma) velocity uncertainties in either horizontal component is greater than 1.5 mm/year.

Within the frame of this study, we interpret only observations within the Anatolia-Aegean domain (Figure 1, Table S1). Table 1 summarizes the improvements in the new velocity field in our study area (longitudes 19.0°E-44.0°E, latitudes 33.7°N-43.0°N).

## 3. Analysis approach for the velocity field

In this study, instead of interpreting our observations with respect to Eurasia (Figure 1), we defined local reference frames to focus on subtle variations within the Anatolia-Aegean region. Figure 2 shows the same velocity field (Table S1) plotted with respect to E Anatolia, accomplished by rotating the full velocity field to minimize the relative motions between the GNSS sites circled in blue (their names are marked in Table S1). These sites were selected because of the small intersite relative velocities, and the low level of shallow seismicity in E Anatolia (Figure 3). To mark the low-level seismicity zones, we used the reviewed ISC catalog (Bondár and Storchak, 2011) (depth < 40 km, Mw > 4.5; between 1964 and 2022) that has accurate spatial resolution for this kind of classification (Figure 3).

Following Floyd et al. (2022), we use the same approach to better illustrate the deformation of the Aegean, using the same sites of Floyd et al. (2022) to minimize velocities and define a local Aegean reference frame (Figure 4, Table S1); as for E Anatolia, these sites are located in the area of low shallow seismicity in southwest Anatolia (Figure 3). Site names are marked in Table S1. Table 2 shows estimated Euler poles within these reference frames, relative to ITRF2014. The predicted internal motions (<1 mm/year) are around the upper bound for stable plate interiors (e.g., Argus and Gordon, 1996; Gordon, 1998), confirming the high degree of coherence of the selected zones.

<table><caption>**Table 1.** Improvements in the new velocity field compared with Reilinger et al. (2006).</caption><thead><tr><td>Velocity field</td><td># sites</td><td>Velocity uncertainties in both horizontal components</td><td>Minimum station distance</td></tr></thead><tbody><tr><td>Reilinger et al., 2006</td><td>223</td><td>220 sites (98.7%) ≤ 2.0 mm/year<br>180 sites (80.7%) ≤ 1.0 mm/year<br>16 sites (7.2%) ≤ 0.5 mm/year</td><td>75.8 km</td></tr><tr><td>This study</td><td>783</td><td>726 sites (92.7%) ≤ 2.0 mm/year<br>578 sites (73.8%) ≤ 1.0 mm/year<br>298 sites (38.1%) ≤ 0.5 mm/year</td><td>39.3 km</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Figure 2.** GNSS velocities and 95% confidence ellipses shown with respect to the eastern part of Anatolia. Sites within the blue circle were used to define the reference frame (their names marked in Table S1 as R2). Abbreviations: NAT = North Aegean Trough, Pel = Peloponnese, DSF = Dead Sea fault, KTF = Kephalonia Transform fault, NAF = North Anatolian Fault, EAF = East Anatolian Fault, TG = Tuz Gölü fault.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Figure 3.** Shallow (depth < 40 km, Mw > 4.5) seismicity (from: Reviewed ISC Bulletin, 1964–2022) (Bondár and Storchak, 2011). Circles show areas used to define the reference frames in Figures 2 and 4. Thin black lines show active faults (Ganas et al., 2013; Emre et al., 2013). As expected, the activity coincides well with the fault zones.</figcaption></figure>

To further quantify internal deformation within the Anatolia-Aegean region, we developed a simplified block model (McCaffrey, 2002), using the velocity field, with respect to E Anatolia (Figure 2) and major, seismically

active faults (Figure 5; see Table S2 and Figure S1 for fault slip rates). Fault locking depths are constrained to 16 km, based on the depth of earthquakes along fault zones (Figure 3) (Wright et al., 2013). Many researchers
<figure><img src="image_8.png" /><figcaption>**Figure 4.** GNSS velocities and 95% confidence ellipses in and around the Aegean Sea region shown with respect to the southwestern Aegean (as Figure 2, see text for discussion). The blue circle shows the area of low seismicity identified in Figure 3 and the sites within were used to define the reference frame (their names marked in Table S2 as R3).</figcaption></figure>

published different block models for the region (e.g., Reilinger et al., 2006; Aktuğ et al., 2009; Seyitoğlu et al., 2022), and others have investigated geodetic fault slip rates along the North and East Anatolian Faults directly from cross-fault observations (e.g., McClusky et al., 2000; Cavalié, and Jónsson, 2014; Ergintav et al., 2014; Vernant, 2015; and the references therein). Comparisons of geodetic fault slip rates are generally comparable to well-determined, longer-term geologic slip rate estimates (e.g., Hubert-Ferrari et al., 2002; Reilinger et al., 2006; Özbakır et al., 2017; Zabcı, 2019; Özbey 2022). Although we report model slip rates (Figure S1), the highly simplified block model is more appropriate to illustrate subtle variations within slowly deforming areas, providing upper bounds on the low internal deformation of large regions of the upper crust bordered by major fault zones.

Finally, to investigate possible links between processes in the upper mantle and the surface, we correlate the residuals of our highly simplified block model with tomographic images, including P wave velocity perturbations with respect to the AK135 velocity model (Kennett, 1995) at 100 km depth (Figure 6). The details of tomography are given in Karabulut et al. (2019a and 2019b).

**4. Kinematics of the Anatolia-Aegean region**

The velocity field shown in Figure 1 with respect to Eurasia (Table S1) better constrains well-known GNSS results for the active tectonics of the Anatolia-Aegean region referenced in the “Introduction”, including (1) decoupling of Anatolia-Aegean from Eurasia along the NAF and its extension across the North Aegean Trough system and the
<table><caption>**Table 2.** Cartesian Euler Vectors and associated 1-sigma uncertainties (±) for the E Anatolia (EANATOLIA), West Aegean (WAEGEAN) reference frames, relative to ITRF2014. Abbreviations: Wx, Wy, Wz = Euler rotation rates around the X,Y,Z cartesian axes in degrees per million years (deg/Myr). RhoXY, RhoXZ, RhoYZ are the correlations between the X, Y, and Z rotation estimates.</caption><thead><tr><th rowspan="2">Plate</th><th rowspan="2">ω<sub>x</sub><br>(deg/Myr)</th><th rowspan="2">±</th><th rowspan="2">ω<sub>y</sub><br>(deg/Myr)</th><th rowspan="2">±</th><th rowspan="2">ω<sub>z</sub><br>(deg/Myr)</th><th rowspan="2">±</th><th colspan="3">Covariance matrix</th></tr><tr><th>ρx</th><th>ρy</th><th>ρz</th></tr></thead><tbody><tr><td>EANATOLIA</td><td>1.008722</td><td>0.075427</td><td>0.543127</td><td>0.057285</td><td>1.020384</td><td>0.076967</td><td>0.999</td><td>1.000</td><td>0.999</td></tr><tr><td>WAEGEAN</td><td>0.050898</td><td>0.101442</td><td>0.147426</td><td>0.044009</td><td>0.158303</td><td>0.083776</td><td>0.999</td><td>1.000</td><td>0.999</td></tr></tbody></table>

Corinth Gulf; (2) broad scale, counterclockwise rotation of Anatolia; (3) southwest motion of the Peloponnese and southern Aegean; and (4) rapid motion of the Hellenic trench over the subducting Nubian plate. Moreover, the new velocity field shows that the southward motion in northern Greece and the Balkans extends at least 300 km to the north of the Aegean Sea and involves clockwise rotation towards the western Hellenic trench.

Figure 2 shows the same velocity field with respect to E Anatolia, estimated by the methodology described in Section 3. This perspective illustrates well the internal deformation of Anatolia; in particular, right-lateral, elastic strain accumulation on the NAF, left-lateral motion across the EAF, and low internal deformation of Anatolia east of ~33°E, an area of over 126,000 km² with statistically significant velocities < 0.5 mm/year, indicating strain rates of < ~1.5 nanostrain/year. West of ~32°E, in SW Anatolia, velocities increase (due to the extension of Anatolia to the southwest) and rotate counterclockwise towards the Hellenic trench, inducing ~north-south extension across western Anatolia reaching ~20 mm/year (e.g., Aktuğ et al., 2009; Floyd et al., 2010). Additionally, new, subtle variations in the velocity field are apparent. For example, there is no resolvable convergence of Arabia with Anatolia across the EAF. Deformation is also transferred, at least in part, from the Dead Sea Fault Zone to the EAF in the northern part of Arabia and shows the spatial initiation stage of the diffuse tectonics in East Anatolia (Gomez et al., 2020). In the Marmara, there is no elastic strain accumulation resolvable around the southern branches of NAF and this part shows coherent internal deformation with the other parts of Anatolia at large scale. In the west, the Büyük Menderes graben system has a key role for the partitioning of north-south velocities.

To similarly illustrate deformation of the Aegean, we follow the same strategy used for E Anatolia to minimize velocities for defining a local reference frame (Table S1) using the same sites used by Floyd et al. (2022) located in the area of low shallow seismicity in the southwest Aegean (Figure 3); their names are marked in Table S1. This central Aegean reference frame (Figure 4) minimizes the large southwest motion of the southern Aegean and

Peloponnese as a whole (~35 mm/year with respect to Eurasia, Figure 1), isolating the internal deformation of the Aegean. The small residual velocities throughout a broad area of the southern Aegean, northwest Crete, and the eastern Peloponnese (~50,000 km², Figure 4), and the close correlation between the region of low internal deformation (<1.5 mm/yr) and low levels of shallow seismicity (Figure 3), attest to the representative velocities of the sites used to define the local Aegean reference frame. This perspective illustrates well the concentration of deformation on the North Aegean Trough and Corinth Gulf (e.g., McClusky et al., 2000; Briole et al., 2000), low internal strain rates of the central and southern Aegean, and rapid trench-ward motion of the eastern side of the Hellenic subduction zone, and to a lesser extent the western side (western Peloponnese) (see Floyd et al., 2022, for a detailed description and interpretation).

# 5. Discussion

## 5.1. Kinematics

The velocity fields in Figures 1, 2, 4, and 5b provide an opportunity to investigate the contributions of distributed strain within crustal blocks, and deformation localized on block-bounding faults, to the overall deformation of the Anatolia-Aegean region. The continuously increasing GNSS velocities rotating toward the Hellenic subduction zone (Figure 1) have been interpreted to suggest that deformation is not confined to faults but can occur, at least in some areas, via aseismic, broadly distributed strain (e.g., Hatzfeld et al., 1997; Aktuğ et al., 2009; England et al., 2016; Barbot and Weiss, 2021). Figures 2 and 4 indicate broad areas of very low internal strain between the NAF and EAF in east Anatolia (E of 32°E), and within the Aegean Sea/Peloponnese, respectively. This low level of internal strain is remarkable given that E Anatolia is moving at ~20–25 mm/year and the Aegean at ~35 mm/year with respect to Eurasia, all occurring within the complex zone of the collision of Arabia and Nubia with Eurasia. As indicated by the simple block model in Figure 5, a broad region of western Turkey and the Aegean coast serves as an accommodation zone where deformation includes north-south extensional strain in southwestern-most
<figure><img src="image_9.png" /><figcaption>**Figure 5.** Simplified block model using only major, seismically active faults. Thin black line shows active faults (Ganas et al., 2013; Emre et al., 2013) and white lines indicate the border of the block model. (a) GNSS velocities plotted, with respect to E Anatolia reference frame for comparison to the model residuals. (b) GNSS residuals from the block model. Rectangles mark the GNSS sites affected by ongoing postseismic deformation of the Mw7.4/7.2, İzmit/Düzce earthquakes (IZ) (see Figure S2), and deformations of the very active zones around the Halkidiki Peninsula (HP). Abbreviations: BM = Büyük Menderes graben system, NAF = North Anatolian Fault, EAF = East Anatolian Fault, DSF = Dead Sea Fault, BP = Biga Peninsula, IA = Isparta Angle, KTJ = Karliova triple junction, PEL = Peloponnese.</figcaption></figure>

Anatolia, and counterclockwise rotation of the Isparta Angle region that is superimposed on the broader rotation of Anatolia as a whole. The available geodetic observations are not sufficient to determine whether deformation in the accommodation zone is confined to faults or represents uniform aseismic strain, although the association with seismicity is apparent (Figure 3). We note that the larger

trenchward motions in the Peloponnese, compared to those along the eastern Hellenic subduction zone are an artifact due to the greater number of well-determined GNSS velocities in the Peloponnese that bias the overall motion of the block towards the west.

As indicated in Figure 5, the main, well-known faults in our simplified block model account for the large
majority of the observed deformation throughout the Anatolia-Aegean region, reducing the residual RMS of the velocities internal to Anatolia and the Aegean (Figures 2 and 4) by ~50%. It is notable that the large majority of strain accumulation across the NAF from the Karlıova triple junction to ~32°E (~700 km) can be accounted for with a single fault with uniform locking depth, and a single velocity for the Anatolian block (Vernant, 2015) (Figure S2). Together these observations attest well to the plate-like behavior of E Anatolia. However, further west where the Mw 7.6/7.4 İzmit/Düzce earthquakes occurred, and west of the main coseismic break in the Marmara region, the observed cross fault velocity profile requires a significantly shallower locking depth (<5 km; profile NAF10 on Figure S2). We interpret this as strain release that may be due to postseismic afterslip below the coseismic fault (Figures S2 and S3) (Ergintav et al., 2009; Çakır et al., 2012; Aslan et al., 2019; Özarpacı et al., 2021); possibly an indirect observation of a fault healing process. In the Sea of Marmara, the variations in fault coupling of fault segmentations occur as identified by Ergintav et al. (2014) (profile NAF10, 11 and 12 on Figure S2). Although the data are sparser, the same is roughly the case for the EAF (verified by InSAR by Cavalié and Jónsson 2014; Walters et al., 2014).

Residual velocities in western Anatolia are likely related to the Eskisehir fault zone (~3–4 mm/year) and Büyük Menderes (~5-6 mm/year) active graben system (Figure 5, e.g., Altunel and Barka, 1998; Bozkurt and Satır, 2000; Ocakoğlu, 2007), and those in southwestern Anatolia, east of the Hellenic subduction zone, to rotation of the Isparta Angle towards the Cyprus subduction zone (Tiryakioğlu et al., 2013). None of these well-known active faults were included in our simplified model. Scattered model residuals north and south of the western NAF are associated with deep, ongoing postseismic effects of the 1999 İzmit/Düzce earthquake sequence (Figure S3) as we discussed, above. Large residuals (>8 mm/year) in the Halkidiki (Thessaloniki) Peninsula, north of the Aegean Sea that show north-south extension is likely related to a system of seismically active, east-west-striking, normal faults traversing the peninsula (Martinod et al., 1997; Goldsworthy et al., 2002).

Furthermore, the North and East Anatolian Faults, (fault parallel velocities are 18-20 mm/year and 8-10 mm/year, respectively) and the Dead Sea Fault (fault parallel velocity is around 5 mm/year within our study area) in this simplified model have slip rates (Figure S1) that generally agree within uncertainties with longer-term geologic estimates (see Vernant, 2015) and Zabcı, 2019) for the North Anatolian Fault, and Gomez et al., (2020) and Reilinger et al., (2006), for the Dead Sea and East Anatolian Faults, respectively). We take the large reduction in internal strains of the Anatolia-Aegean region, with a very simplified block model, and the fact that many mapped,

seismically active faults were not included in the model (Figures 3, 5a, and 5b), as evidence that the seismogenic crust (above ~15 km) behaves elastically, even within this zone of complex, interacting geodynamic processes.

**5.2. Contribution to long-debated geodynamic problems**
The well-defined counterclockwise rotation and increase in rates of surface motion in western Anatolia and the Aegean toward the Hellenic Subduction Zone have been attributed to (1) extrusion of Anatolia caught between the Arabian indenter and Eurasia (e.g., Şengör and Yılmaz, 1981), a hypothesis similar to that proposed for the India-Eurasia collision (e.g., Tapponier and Molnar, 1977); (2) rollback of the subducting Nubian ocean lithosphere along the Hellenic subduction zone (e.g., Le Pichon and Angelier, 1979; Royden, 1993; Burchfiel et al., 2000; Allmendinger et al., 2007; Le Pichon and Kreemer, 2010; Royden and Faccenna, 2018); and (3) flow within the lithosphere due to gravitational potential energy (GPE) differences between the high eastern Turkey Plateau and the Aegean/Hellenic trench (e.g., England and McKenzie, 1982; Jackson and McKenzie, 1984; Houseman and Molnar, 1997; England et al., 2016).

As shown by our systemic approach to analyzing the updated GNSS observations, the notion that Anatolia is at present being squeezed out of the Arabia-Eurasia collision zone is difficult to reconcile with the absence of shortening (Figure 2) across the Arabia-Eurasia boundary (Bitlis-Zagros suture zone and East Anatolian Fault), the lack of thrust faulting, the dominance of strike-slip and extensional earthquake mechanisms throughout Anatolia (e.g., Jackson, 1994; Jackson and McKenzie, 1984), and the increase in GNSS velocities from eastern to western Turkey (Figure 2, Figure S1). However, it is clear from plate reconstructions and the geologic evolution of the collision zone that the initial collision at ~25–30 Myr involved major indentation of Arabia into Eurasia and associated deformation and uplift of the Turkish-Iranian plateau (e.g., Şengör et al., 2003; McQuarrie et al., 2003; McQuarrie and van Hinsbergen, 2013). The transition from compression and uplift to the present-day tectonics of eastern Anatolia that are dominated by westward motion and velocity gradient towards the Hellenic subduction zone apparently became dominant with the development of the North Anatolian fault at ~11 Myr (Şengör et al., 2004). This interpretation is supported by a recent analysis of the Tuz-Gölü fault zone in eastern central Anatolia that reports a change from thrust faulting to strike slip faulting, albeit with a slow rate (<3 mm/year), shortly after the formation of the NAF (Özbey, 2022). The low level of possible slip on the fault is further illustrated by the small residual relative velocities in eastern central Anatolia (Figure 2).

There is no doubt that the sublithospheric mantle flows, as this is required by Plate Tectonics, and the ocean lithosphere
penetrates into the mantle. The Nubia-Eurasia plate collision
itself provides further evidence that sublithospheric processes
associated with the subducting Nubian slab influence
surface deformation. The GNSS velocities (Figure 2) show
that counterclockwise rotation of the Antalya-Isparta Angle
region, is superimposed on the broader counterclockwise
rotation of southwestern Anatolia, possibly responding
to the subducting Nubia slab beneath Cyprus (Güvercin
et al., 2021) that may be retarding faster westward motion
of Cyprus (Figure 5). The fragmented structure east of the
Hellenic subduction zone (Spakman and Wortel, 2004; Biryol
et al., 2011; Karabulut et al., 2019a) also contributes to this
region being more responsive to slab geometry and dynamics
(Sternai et al., 2014). Figure 6 shows a seismic tomographic
image of the region at 100 km depth, below the overriding
Anatolian-Aegean lithosphere (Karabulut et al., 2019a,
2019b). There is a correlation between the velocity variations
at depth in the tomographic images and block model residuals
at the surface (Figure 6). The GNSS residuals computed
from the simplified block model show large residuals in the
region between the Hellenic and Cyprus subduction zones
(Figure 6). While the residuals are relatively small along the
plate boundaries (NAF and EAF) indicating narrow zones

of deformation, they are much larger and diffused over the
regions of the observed slab segmentations.

Anomalous trenchward motions at both ends of the Hellenic subduction zone are associated with breaks and/or sharp warping of the subducting plate (Spakman and Wortel, 2004; Barka and Reilinger, 1997; Biryol et al., 2011; Pearce et al., 2012; Karabulut et al., 2019b; Floyd et al., 2022). We suggest that the eastern slab tear detaches the Hellenic slab from the more shallow-dipping Cyprus slab beneath the eastern side of the subduction zone and back arc, allowing the eastern side of Hellenic slab to founder and inducing southwest extension of the overriding plate. We further point out that the eastward extent of rapid Anatolian extension (~32°E, Figures 2 and 5) corresponds roughly to the western edge of the Cyprus slab at ~33°E (Figure 6; see also, Biryol et al., 2011, their figure 7 and Karabulut et al., 2019a), perhaps allowing mantle to flow around the subducting plate, thereby promoting western Anatolia motion towards the trench (Le Pichon and Kreemer, 2010). Indeed, we suggest here that many features of Anatolian deformation may result from or be modified by the dynamics of the subducted plate and associated mantle flow.

<figure><img src="image_10.png" /></figure>

**Figure 6.** P wave velocity perturbations with respect to the AK135 model at 100 km depth (reference velocity is 8.1 km/s) showing the break in the subducting Nubia slab near the junction of the Hellenic and Cyprus subduction zones, the warp (or break) in the slab beneath the western side of the Hellenic subduction zone, and the eastern edge of the subducted slab beneath central Anatolia (Karabulut et al., 2018, 2019a) that corresponds to the location where Anatolia begins to extend and rotate towards the Hellenic subduction zone. Black arrows show the residual GNSS velocities of the simplified block model. Other map features are as described for Figures 1, 2, and 5
The absence of earthquakes below the upper crust (~15 km in western Anatolia and ~20 km in eastern Anatolia (Figure 3) and low GNSS residuals from our simplified block model (Figure 5b) are consistent with models where the lower crust is weaker than the upper crust, possibly as a result of thin lithospheric mantle beneath the Aegean-Anatolian region (e.g., Houseman and Molnar, 1997; Mutlu and Karabulut, 2011; Karabulut et al., 2018; Karabulut et al., 2019a). While the weaker lower crust is expected to deform in response to geodynamic processes, the contribution of GPE to tectonic interactions associated with the late stages of Nubia-Eurasia plate convergence is still debatable. We also note that the Arabian plate has penetrated at least 400 km into Eurasia's southern boundary since ~25 Myr with minimum slowing of Arabia (e.g., McQuarrie et al., 2003; Hatzfeld and Molnar, 2009), consistent with weak continental lithosphere, possibly weakened by a few 100 Myr of Tethys northward subduction (e.g., Şengör and Yılmaz, 1981; Barazangi et al., 2006).

The coincidental timing of the opening of the Mediterranean basins with the slowing of the rate of Nubia-Eurasia convergence in the early Miocene (McQuarrie et al., 2003), including the Alboran basin (Cloetingh et al., 1992), the Tyrannian basin (Dewey et al., 1989; Krijgsman and Garces, 2004), and the Aegean basin (Le Pichon and Angelier, 1979; Jackson, 1994; Jolivet and Faccenna, 2000), suggests that processes directly related to the slowing of Nubia, rather than those associated with the Arabia collision in E Anatolia, or processes internal to the Anatolia-Aegean lithosphere, are responsible for the initiation of basin extension. Turcotte and Shubert (2002, pg. 242-244) derive a simple analytical expression relating the dip angle of the subducting slab to the convergence rate. The model implies that, all else being equal, slab dip increases as the convergence rate decreases, supporting the hypothesis that slowing of Nubia-Eurasia plate convergence causes sinking of the subducting slab that, in turn, causes the plate interface to migrate out over the slab, inducing rapid extension in the overriding plate (e.g., Le Pichon X and Kreemer C, 2010; Reilinger and McClusky, 2011; Royden and Faccenna, 2018).

## 6. Conclusions

The principal results of our analysis of the most recent GNSS velocity field in and around the Anatolian-Aegean region are (1) the upper, seismogenic crust of this complexly deforming region, which involves continental collision

and ocean subduction, behaves elastically and, to the best of our ability to measure crustal strain, deformation of the surface is controlled mostly by faulting; (2) there is no resolvable convergence of Arabia with Anatolia across the East Anatolian Fault that is the principal active boundary between these plates; (3) extensional strain rate and rotation of GNSS velocities in Anatolia and the Aegean region towards the Hellenic subduction zone, and the coincidence in the timing of extension of the major Mediterranean basins with slowing of Nubia-Eurasia convergence, support the hypothesis that the subducting slab is a dominant process responsible for motion and internal deformation of the Anatolia-Aegean region; and (4) comparison of surface deformation with a subsurface seismic tomographic image indicates that segmentation of the Hellenic and Cyprus slabs has a first-order influence on the active tectonics of the overriding Aegean-Anatolian domain.

## Acknowledgments, samples, and data

We gratefully acknowledge late Prof. Dr Aykut Barka (Istanbul Technical University) for initiating GPS-based geodynamic studies in Turkey, Dr Kim Kastens (Lamont-Doherty Geological Observatory) for leading early GPS investigations in the Aegean, and the many individuals and institutions that contributed to GNSS survey observations and maintained continuous GNSS stations in the Anatolian-Aegean region. This research was supported in part by Massachusetts Institute of Technology (MIT) International Science and Technology Initiatives (MISTI) Grant no: 202048, and The Scientific and Technological Research Council of Turkey (TÜBİTAK) Project no: 114Y250.

We would like to thank the reviewers for the constructive suggestions as well as for their time to review the manuscript.

We have used GNSS data that are in the public domain (UNAVCO, IGS, EUREF) and in the supplementary materials of the previously published studies and open data repositories. We also provide the calculated velocity fields, with respect to selected velocity reference frames in the Auxiliary Section of this manuscript.

## Supplementary Data

Supplementary data can be accessed at the following link: https://aperta.ulakbim.gov.tr/record/252404#.ZFziZS9Bxqs

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