TÜCAUM 30. Yıl Uluslararası Coğrafya Sempozyumu International Geography Symposium on the 30th Anniversary of TUCAUM 3-6 Ekim 2018 /3-6 October 2018, Ankara

Doğu Anadolu Yüksek Platosu’nun Manto Destekli Asimetrik Dom Yapısının Jeomorfolojik Analizi

The geomorphic patterns of a mantle supported asymmetrical dome structure of the East Anatolian High Plateau

Erkan Yılmaz*1, C. Erdem Bekaroğlu1, Şule Gürboğa2 1Ankara Üniversitesi, Coğrafya Bölümü 2Maden Tetkik ve Arama Genel Müdürlüğü

Öz: Doğu Anadolu yüksek platosu deniz seviyesinden ortalama 2 kilometrelik yüksekliği, yaygın volkanik arazileri ve sıkışma tektoniğinin ürünü olan K-G yönünde kısalmış ve kalınlaşmış kabuk yapısıyla çarpıcı bir morfolojiye sahiptir. Bununla birlikte güncel jeofizik çalışmalar, yüksek platonun beklenenden yaklaşık 10 km daha ince bir kabuk kalınlığına (~45 km) sahip olduğunu ortaya koymaktadır. Yeni jeolojik yorumlarla birleştirildiğinde, bu durum bölgede manto litosferinin olmadığını, kabuğun doğrudan sıcak manto tarafından desteklendiğini göstermektedir. Bu çalışmada, manto destekli Doğu Anadolu domunun kendisini morfolojik olarak yeryüzüne nasıl yansıttığını anlamak amacıyla bölgedeki havzalar iki morfolojik indisle analiz edilmiştir. Bunlardan ilki olan drenaj havzası asimetri faktörü (AF), bölgeyi kesen 39° K ve 40° K paralellerinin güneyindeki havzaların (Bingöl, Arıcak and Lice-Kulp vd.) sistematik olarak yüksek AF değerlerine sahip olduğunu ve güneye doğru çarpıldığını; aynı hattın kuzeyindeki havzaların ise (Kelkit, , , Kağızman-Tuzluca, Ağrı) düşük AF değerlerine sahip olduğunu ve kuzeye doğru çarpıldığını göstermektedir. Vadi tabanı genişliği-vadi yüksekliği oranı (Vf) indisi ise, platonun kuzey bölümü (batıda Kelkit’ten doğuda Kağızman- Tuzluca havzaları) boyunca düşük değerler sergilemektedir. Bu hat, aynı zamanda AF değerlerinin birbirinden ayrıldığı zona oturmaktadır. Havza bazlı analizlerden elde edilen sonuçlar, bölgesel ölçekte değerlendirildiğinde, Doğu Anadolu yüksek platosunun ekseni kuzeye kaymış asimetrik bir dom yapısına sahip olduğunu, dom ekseninin aynı zamanda maksimum dikey tektonik yükselim hattına oturduğunu ortaya koymakta ve plato altındaki mantonun domu destekleyen en aktif kısmının bu zonun altında yer alabileceğine işaret etmektedir. Anahtar Kelimeler: Doğu Anadolu Yüksek Platosu, asimetrik dom yapısı, morfometrik indisler

Abstract: The East Anatolian High Plateau displays spectacular morphology and structure with its average ~2 km height (apsl), widespread volcanism and N-S shortened thick crust resulted from the contractional tectonics. However, recent geophysical studies suggest that this high plateau has a crustal thickness of ~45 km, ~10 km thinner than that of expected. Combined with its geology, this situation shows that the region is lack of mantle lithosphere but supported by hot mantle. In this study, we used two simple geomorphic indices to understand possible surface manifestations of this mantle supported dome structure in the region. The first indice, drainage basin asymmetry factor (AF), indicates that basins south of the 39° N and 40° N exhibit systematically higher values suggesting that basins in this part of the plateau distorted southward. In contrast, basins north of the 39° N and 40° N show generally lower values. Although and Pasinler basins are in a symmetrical state (AF ~50), remaining basins like Kelkit, Tekman, Horasan, Kağızman-Tuzluca and Ağrı are clearly asymmetrical and distorted northward. The second indice, ratio of valley floor width to valley height (Vf), exhibits minimum values in the northern part of the plateau starting from Kelkit in the west to Kağızman-Tuzluca basin in the east. This concave shaped arc exactly coincides with the dome axis where AF values diverge. Our basin-based

* İletişim yazarı: Erkan Yılmaz, e-posta: [email protected]

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TÜCAUM 30. Yıl Uluslararası Coğrafya Sempozyumu International Geography Symposium on the 30th Anniversary of TUCAUM 3-6 Ekim 2018 /3-6 October 2018, Ankara analyses clearly suggest that the high plateau has an asymmetrical dome shape with an axis slipped northward. The dome axis also marks the highest rate of vertical tectonic uplift zone, which possibly defines the area where the most active part of the mantle located beneath the plateau. Keywords: Eastern Anatolia High Plateau, asymmetric dom structure, morphometric indices

1. Problem Definition The East Anatolian High Plateau (EAHP) is a part of the Alpine-Himalayan mountain belt (Şengör and Kidd, 1979) and its paleotectonics consists of three different units: The Eastern Rhodope-Pontid Arc in the north, The East Anatolian Accretionary Complex in the middle, and the Bitlis-Zagros Suture in the south (Fig. 1; Şengör and Yılmaz, 1981). The continent-continent collision-related paleotectonic contractional tectonic regime started during the Middle Miocene in the Eastern Anatolia, along the Bitlis-Zagros Suture Zone (Yılmaz, 1993). The convergence between Arabian and Eurasian plates has resulted in closure of the southern branch of the Neotethyan Ocean diachronously and changed the overall paleogeography dramatically in the area. Due to the post-collisionary intracontinental convergence across the suture zone, during which the East Anatolian Accretionary Complex experienced crustal shortening and thickening, the EAHP was tightened as a ~2 km high plateau and characterized by folds, thrust to reverse faults, ramp basins, and fissures (Koçyiğit et al., 2001). Following the termination of the intracontinental convergence, the palaeotectonic stress regime was replaced by the contractional-extensional tectonics during the Early Pliocene that also marks the initiation time of the neotectonic period in the region (Bozkurt, 2001). Neotectonics of the Anatolia is characterized by three major elements: formation of two major intra-continental transform faults (North Anatolian Fault Zone and East Anatolian Fault zone) and westward motion of Anatolian wedge over African plate (Le Pichon and Angelier, 1979). The neotectonic period in the Eastern Anatolia, on the other hand, is mainly characterized by dextral and sinistral strike-slip faults, strike-slip basins and N-S trending fissures (Koçyiğit et al., 2001). However, recent geophysical studies (Al-Lazki et al., 2003; Gök et al., 2003; Sandvol et al., 2003; Türkelli et al., 2003; Zor et al., 2003; Maggi et al., 2005; Barazangi et al., 2006) have shown that the crustal thickness of EAHP, geologically located on the East Anatolian Accretionary Complex (Şaroğlu and Yılmaz, 1987), on average is 45 km which is ~10 km thinner than earlier considerations (Şengör, 1980). The crustal thickness varies from ~38 km in the south to ~50 km in the northern highlands of the region. Such geologically supported (Şengör et al., 2003) surprising evidence suggest that the Eastern Anatolia has a dome structure and is supported directly by the upper mantle, not by a thick crust, due to the slab break off beneath the EAHP (Keskin et al., 1998). Although the overall picture of this geodynamics is relatively well documented, its connection with the surface processes has not been properly established so far: How these geological and geophysical developments have been manifested on the surface? Hence, our purposes appear as follows: (1) Depicting the basic surface geometry of the so-called dome structure of the EAHP by using two simple geomorphic indices, (2) Connecting geological and geophysical processes with the geomorphological nature of the region. This, we believe, is a unique approach that may promote geosciences of this spectacular plateau.

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TÜCAUM 30. Yıl Uluslararası Coğrafya Sempozyumu International Geography Symposium on the 30th Anniversary of TUCAUM 3-6 Ekim 2018 /3-6 October 2018, Ankara

Figure 1: Simplified tectonic map of the Eastern Anatolia showing major paleotectonic and neotectonic units. NAFZ: North Anatolian Falt Zone, EAFZ: East Anatolian Fault Zone. Other abbreviations point the basin locations. White linear lines are the selected cross sections that can be seen in Fig 4. Active fault lines (black) adapted from Emre et al., 2013. Within the framework of paleotectonic and neotectonic geological history of the Eastern Anatolia and in the light of the recent geophysical findings, we put forward following hypotheses in terms of required geomorphology of the region: H1: Mantle supported dome structure of the plateau must be reflected in basin geometries. Basins should be distorted systematically in one direction on one flank and opposite on the other flank of the dome. H2: The zone separating flanks of the dome, characterized by different distortion patterns of basins, must point the location of the dome axis. H3: The dome axis should experience the highest vertical tectonic uplift rate, which manifests itself in basin floor elevations. Average altitudes of basin floors on or near the dome axis, accordingly, should be higher than rest of the basin floors in the region. H4: Basin floors situated at the highest elevations on the dome axis must indicate the area where the most active part of the mantle located beneath the plateau. 288

TÜCAUM 30. Yıl Uluslararası Coğrafya Sempozyumu International Geography Symposium on the 30th Anniversary of TUCAUM 3-6 Ekim 2018 /3-6 October 2018, Ankara

2. Method We used two simple geomorphic indices to highlight the surface expressions of the mantle supported dome structure in the region. The first one known as drainage basin asymmetry factor (AF) and expressed by the following equation (Keller and Pinter, 1996): AF= 100 (Ar / At) Here, Ar is the area of the basin to the right part of the river cutting across the basin and At is the total area of the drainage basin. In ideal conditions, it is expected that AF value of a basin should be equal to 50 (symmetrical state). Any unstable setting would give rise a deflection from normal (AF= 50) value (asymmetrical state). In this study, a value lower (higher) than 50 indicates that the basin distorted northward (southward). Note that only basins whose long axes extend in E-W direction have been taken into account in measurements. The second one known as the ratio of valley floor width to valley height (Vf) and defined by the following equation (Bull and McFadden, 1977):

Vf = 2Vfw / [(Eld – Esc) + (Erd – Esc)]

Where Vfw is the width of the valley floor, Eld and Erd are the elevations (apsl) of the left and right divides of the valley, and Esc is the elevation (apsl) of the valley floor. Lower Vf values in any basin suggest that the river system is actively downcutting the valley in response to vertical uplift whereas higher values imply that the river system is eroding the valley laterally due to the relative vertical tectonic stability. In order to avoid any sort of local disturbances (e.g. lithological differences, differences in stream sections etc.), Vf values have been measured in three different (upstream, middle, downstream) segments of each basin and only average value of the three presented here. Whereas AF indice requires basin-wide measurements, such a condition is not an imperative for Vf calculations. Nevertheless, Vf indice was also calculated for each basin since there is no compelling indice or method to distinguish lithological contribution from its tectonic counterpart in extrabasinal areas. As sumed up in the introduction part, basins in the EAHP are characterized by strike-slip deformations in neotectonic period so that basin-wide analyses of Vf indice more or less equalizes interbasinal comparasions. We would like to underline that extrabasinal areas reflect not real but enterpolated values in all figures. 3. Results 3.1. Asymmetrical Dome Shape We measured drainage basin asymmetry factor (AF), a sensitive geomorphic index for basin asymmetry, to find out whether there is a systematic distortion pattern in basin orientations. As seen in (Fig. 2), there appears a clear one. The most striking feature is that basins south of the 39° N and 40° N exhibit systematically higher values which indicate that basins in this part of the plateau distorted southward. Palu, Elazığ and Pütürge basins in the SW part of the plateau are being exceptions (reflecting a local disturbance possibly because of the East Anatolian Fault Zone), maximum values (AF >80) are reached between 40° E and 41° E, concentrating on Bingöl, Arıcak and Lice-Kulp basins. In contrast, basins north of the 39° N and 40° N show generally lower values. Although Erzurum and Pasinler basins are in a symmetrical state (AF ~50), remaining basins like Kelkit, Tekman, Horasan, Kağızman-Tuzluca and Ağrı are clearly asymmetrical and distorted northward.

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TÜCAUM 30. Yıl Uluslararası Coğrafya Sempozyumu International Geography Symposium on the 30th Anniversary of TUCAUM 3-6 Ekim 2018 /3-6 October 2018, Ankara

Figure 2: Asymmetrical dome shape of the East Anatolian High Plateau. Drainage basin asymmetry factor (AF) calculated for each E-W trending basin. Lower (higher) values indicate that basins distorted northward (southward). This N-S oriented distortion pattern of basins on the plateau suggests that the EAHP has a dome shape, which also verifies H1. On the other hand, there is a zone separating northward and southward oriented basins, passing through between Kelkit and Erzincan, Ilısu and Erzurum, Tekman and Hınıs, Ağrı and Karasu basins. As a logical consequence, this zone constitutes the dome axis. Although AF index was measured only in basins, extrabasinal areas do not interfere its extension in the region. The so-called dome axis has an arc shape and its location not on the center but slipped northward, which not only implies that the dome has a symmetrical surface geometry but also verifies H2. 3.2. Vertical Tectonic Uplift Vf indice, an indirect measurement for detecting relative rate of vertical tectonic movements in river courses, is calculated for each basin to assess the variations of the uplift rates in the EAHP. Results are striking (Fig. 3). There are two different zones where the lowest Vf values (< 1) reached. The first one is in the southern section of the region and extends from Pütürge basin in the west to Mutki basin in the east. Since this zone is located on the SE Taurids, a mountain belt that extends along the Bitlis-Zagros Suture Zone, the southern limit of the continent-continent collision between Arabian and Eurasian plates, such low values can be considered not only as expected but also as a test. However, more importantly, the second zone where Vf indice also exhibits minimum values is located in the northern part of the plateau starting from Kelkit in the west to Kağızman-Tuzluca basin in the east. This concave shaped arc exactly coincides with the dome axis where AF values diverge (Fig. 2 and 3). Areas outside of these two minimum Vf zones show moderate to low values, showing the same pattern of tectonic activity, relatively.

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TÜCAUM 30. Yıl Uluslararası Coğrafya Sempozyumu International Geography Symposium on the 30th Anniversary of TUCAUM 3-6 Ekim 2018 /3-6 October 2018, Ankara

Figure 3: Vertical tectonic uplift in the East Anatolian High Plateau. The ratio of valley floor width to valley height (Vf) indice calculated for each E-W trending basin. Lower Vf values in any basin suggest that the river system is actively downcutting the valley in response to vertical uplift whereas higher values imply that the river system is eroding the valley laterally due to the relative vertical tectonic stability. When combined with the AF index, Vf results clearly depict that the axis of the asymmetrical dome structure of the EAHP is also characterized by the highest vertical tectonic uplift rates (Fig. 4), which also verify H3. Indeed, as shown in (Fig. 4), average elevations (apsl) of basins on or near the dome axis are systematically higher than other basins located on the plateau (except Kağızman-Tuzluca basin that will be discussed in the next part). Those basins are not only located at the highest elevations compare to others, but also their locations are bounded by minimum Vf and AF values. 4. Discussion and Conclusion Before the continental collision between Arabian and Eurasian plates along the Bitlis-Zagros Suture Zone, Anatolian peninsula had a peneplain-type (Davis, 1899) lowland topography under the tropical climatic conditions (Erol, 1981). From the Middle Miocene onwards, collision related contractional tectonics, followed by intracontinental convergence, elevated and shaped the Eastern Anatolia which is now characterized by folds, intermountain basins, and widespread volcanism that started around 11 Myr ago in NE part and is getting younger in the southern part of the region (Keskin et al., 1998). 11 Myr ago was also a time during which the subducting lid of the rigid Arabian plate under the EAHP was broken off due to the partial melting in the asthenosphere (Şengör et al., 2003). Although the initial paleogeography of the Eastern Anatolia was shaped under the contractional tectonics during the paleotectonic period, its regional uplift and present heights both in basins and extrabasinal areas should have directly produced by a mantle supported crustal structure regionally after the Late Pliocene when the strike-slip faulting became the prevailing type of neotectonic deformation. This also explains how undeformed Pliocene and Pleistocene formations are now found at high elevetions in the EAHP, previously interpreted by epirogenesis (Erinç, 1953).

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The dome structure of the Eastern Anatolia was already revealed by several geophysical studies and here we would like to focus on one of the most striking ones (Zor et al., 2003). In that study, authors investigated the crustal structure of the Anatolian plateau by using receiver functions obtained from the teleseismic recordings of a 29 broadband PASSCAL temporary network and their Moho depth data are presented in (Fig. 4) together with the surface data. Although minimum AF and Vf values do not coincide with the minimum moho depths across the Eastern Anatolia, it can be seen that minimum AF and Vf zones are actually underlain by probable mantle convection cells spesifically evident in (Fig. 4a and 4b). Thus, we think that it must be this force to shape the overall asymmetrical dome structure of the EAHP. An interesting pattern appears in (Fig. 4c) where both Moho depths and basin elevations contradict the last two hypotheses put forward (H3 and H4). The first contradiction deals with the fact that Moho depths show thicker part of the crust especially under Kağızman basin where AF and Vf values decrease (Fig. 4c). The second one demonstrates that Kağızman and Ağrı basins are not located on the highest part of the plateau when taken into account only basin floor elevations. Those contradictions could be explained as such: (a) the Eastern Anatolia between 43° E and 44° E is characterized by large amount of volcanic centers which concentrated around north of the Lake Van. It seems that the crust in this part of the region has been elevated by mantle convection regionally, which explains why Kağızman and Ağrı basins do not constitute the highest parts in the cross section. (b) Ages of the volcanics clearly show that volcanic units sequentially get younger from north to south (Keskin, 2003), which accordingly implies that the erosion rates of the rocks decrease towards the south, causing present high elevations of the basins along this section of the plateau. Besides, the asymmetrical dome axis of the EAHP is not confined by any particular strike-slip fault system, rather this zone with its concave-arc shaped extension cuts across both the dextral Northern Anatolian Fault Zone and the sinistral Kağızman Fault Zone (Şengör and Yılmaz, 1981; Şaroğlu et al., 1992). This particular situation indicates that the depicted geomorphic pattern is not produced by any particular fault system-related mechanism. These converging evidence show that the EAHP is characterized by an asymmetrical dome shape with its northward slipped axis that experiences the highest rates of vertical tectonic uplift driven by magma convection cells. This conclusion which rather than being a random pattern is actually required by geodynamics of the region.

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Figure 4: Selected cross sections (a, b, c) in the East Anatolian High Plateau with Moho depths adapted from Zor et al., 2003. Note that minimum AF and Vf values (yellow bars) coincide minimum Moho depths locally, showing potential active mantle cells that sustain the dome structure of the plateau.

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