3-Discussion
In the following paragraphs, the two contrasting views proposed on the nature of the eastern Anatolian crust are discussed based on geological, geophysical, and geochemical data;
1-an ophiolitic mélange-accretionary complex forms the basement, which was developed during the demise of the Neo-Tethyan Ocean. Later, it was trapped between the approaching continents (Fig 6A) (Şengör and Kidd 1979; Şengör et al. 2008).
2- The entire eastern Anatolia is underlain by an old and thick continental crust (Fig 6 B) (Topuz et al., 2017).
Both views listed above were based on inconclusive evidence. The following critical data set are needed to provide verifications for the two hypotheses:
a-detailed geological data to examine the applicability of the view on the regional scale, including comparative data to correlate the eastern Anatolian geology with the neighboring tectonic belts (i.e., with the Southeastern Anatolian Orogenic Belt to the south and the Pontide Belt to the north to provide evidence how each hypothesis fits with the geology and tectonic evolution of the neighboring regions in a time-space reference.
b-geophysical data, particularly seismic data, to enlighten the nature of the basement.
The hypothesis proposed by Topuz et al. (2017), is based on the observations basically from two metamorphic rock inliers in eastern Anatolia (Fig 2) and the following interpretations; on the SE Anatolia and Pontide
1- ‘’Metagranite obtained from the metamorphic rock outcrops have a Late Ordovician–Early Silurian protolith and were therefore connected with the Menderes Massif of western Anatolia, which displays a similar protolith”.
The old metamorphic minerals obtained from eastern Anatolia’s metamorphic inliers are expected to be similar to those obtained from the Menderes Massif because the metamorphic massifs of Western, Central, and SE Anatolian orogenic belts share the Pan African-Gondwanan origin (Şengör and Yılmaz 1981).
2- ‘’Multiple continental fragments separated by oceanic accretionary complexes are absent in eastern Anatolia”.
This interpretation dismisses the essential characteristics of the Anatolian orogen, including the eastern Turkey where the orogen was developed by accretions of small continental fragments following the closure of the separating oceans (Şengör and Yılmaz 1981; Yılmaz and Şengör,1985; Dercourt et al. 1986; Dilek, 2006; Robertson et al. 2012; Barrier et al. 2018; McNab et al. 2018). Regional heating generated during the plate reorganization is stated to have caused major obduction of these ophiolites (Hassig et al. 2013, 2016 a, b; Roland 2020).
3- ‘’the metamorphic mineral assemblages that crop out in the metamorphic rocks of the region, display medium to high T metamorphisms, but not high-pressure metamorphism”.
This interpretation disregards the critical geological data obtained from the neighboring orogenic belts (see the two accompanying papers in this volume on the Pontide and SE Anatolian orogenic belts by Yılmaz et al.). Particularly the recent studies on the SE Anatolian metamorphic massifs indicate that the ophiolitic rocks and the Bitlis Massif underwent an initial HP metamorphism, followed by an HT metamorphic phase during the Late Cretaceous-Early Eocene period (Roland et al. 2012; Oberhansli et al. 2012; 2014; Pourteau et al. 2013; Awalt and Whitney 2018; Yılmaz 2019). The Bitlis Massif, a fragment of continental crust involved in northward subduction together with the oceanic lithosphere (the Berit metaophiolite; Yılmaz 2019) under the Eastern Anatolia where, they were buried down to 35-65 km at depths (Oberhansli et al., 2014). Their exhumation occurred mainly during the Early Eocene period (Roland et al., 2012; Oberhansli et al., 2014; Yılmaz, 2019). A transgressive sequence was deposited above the elevated metamorphic massifs during the Middle Eocene (Yiğitbaş 1989; Yiğitbaş and Yılmaz 1996; Yılmaz 1993; Yılmaz 2019). The metamorphic massifs were then thrust over the ophiolitic mélange during the Late Eocene before they were tectonically emplaced as a giant nappe package onto the Arabian Plate in the Late Miocene (Yılmaz 2019). Consequently, the southeast Anatolian metamorphic massifs represent allochthonous belts (Fig 7B) resting tectonically on the Late Cretaceous-Middle Eocene metamorphic ophiolites (the Kızılkaya Metamorphics and the Berit Metaophiolite), and non-metamorphic ophiolites (The Göksun Ophiolite) and an accretionary complex (Fig 7B) extending from the eastern Anatolia (Fig 7A). Tens of kilometers of the nappe transport of the metamorphic massifs may be estimated from the Muş region in the North to the Sason region in the South (Figs 7A and B) (Yılmaz 2019 and the references therein).