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). LITHOSPHERE© 2018 The Authors. | Volume Gold 10Open | Number Access: 2 This | www.gsapubs.org paper is published under the terms of the CC-BY-NC license. 279 Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/2/279/4102788/279.pdf by guest on 28 September 2021 MAHATSENTE ET AL. A B Figure 1. (A) Simplified tectonic map of Turkey and surrounding regions Ş( engör 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. 280 www.gsapubs.org | Volume 10 | Number 2 | LITHOSPHERE Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/10/2/279/4102788/279.pdf by guest on 28 September 2021 Lithospheric structure and isostatic state of Eastern Anatolia | RESEARCH 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).