The intra-pontide suture zone, northern turkey: an updated view

The tectonic setting of Turkey can be described as a puzzle where several continental terranes are boubded by ophiolite-bearing suture zones (Sengör and Yılmaz, 1981). Among these suture zones, the Intra-Pontide Suture Zone (IPSZ) is probably the less known one. The IPSZ is an east-west trending, suture zone that stretches for about 600 km dividing the Istanbul-Zonguldak terrane, in the north, from the Sakarya terrane, in the south. This suture zone is well exposed along the Daday-Arac-Kursunlu and Tosya – Emirköy geotraverses in northern-central Turkey where its tectonic features can be fully reconstructed. The study of the two geotraverses indicates that the imbricate stack of the IPSZ consists of several distinct tectonic units, all characterized by the occurrence of mafic rocks. These units include the Aylı Dağ ophiolite Unit, the Arkot Dağ Mèlange and three metamorphic units (the Daday, Domuz Dağ and Devrekani Units). This imbricate stack is probably the result of several episodes of out-of-sequence thrusts that affected the whole IPSZ. The Aylı Dağ Unit (Göncüoglu et al., 2012) includes an about 5 km-thick, not metamorphic ophiolite sequence topped by the middle Bathonian to early Callovian radiolarian cherts. In addition, a metamorphic sole is present at the base of the serpentinized peridotites. The geochemical evaluation of pillow-basalts and dykes highlights subduction-related characteristics, similar to IAT- and BABB-type lavas generated in a back-arc oceanic basin. The Arkot Dağ Mèlange (Göncüoglu et al., 2014) consists of an assemblage of slide-blocks, with different size and lithology, enclosed in a Late Santonian sedimentary matrix. The slide-blocks also include ophiolitic lithologies, mainly represented by basalts, but gabbros and peridotites are also found. The slide-blocks of basalts display affinites to IAT- and BABB-type magmas, signifying the involvement of subduction component, whereas no MORB-like basalt have been found. The age of the radiolarian chert blocks ranges between Middle-Late Triassic (Tekin et al., 2012) to Late Cretaceous. The Daday unit (cf. Martin Unit by Okay et al., 2013) is characterized by metasedimentary and metabasic slices deformed under low-grade blueschist metamorphic facies conditions (in the Early Cretaceous (ca. 110 Ma; Rb–Sr method on mica; Okay et al., 2012). The metasedimentary rocks include mica-bearing schists, fine-grained marbles and quartzites, which inclue detritial zircons mainly derived from an Early Carboniferous felsic magmatic source. The metabasic lithologies, on the other hand, comprise actinolite-bearing schists and Na-amphibole-bearing varieties possibly derived from basaltic and gabbroic protoliths. The metabasic rocks have a wide range of chemical compositions, displaying N-MORB-, E-MORB-, OIB- BABB- and IAT-type signatures. The Devrekani Unit is represented by an assemblage of amphibolites, marbles and micaschists showing a metamorphic climax developed under upper amphibolite facies in the Late Jurassic time (ca. 163 Ma; mica Ar-Ar geochronology; Marroni et al., 2014). Amphibolites recorded pressures of ~0.80–0.99 GPa and temperatures of ~600°C. Comparable P-T conditions were obtained for the micaschist (P ~0.86 GPa and T ~ 640°C)(Marroni et al., 2014). The amphibolites display E-MORB-, OIB- BABB- and IAT-type signatures. The Domuz Dağ Unit is in turn mainly represented by eclogites and amphibolites with minor micaschist, gneisses and marbles deformed under eclogite facies metamorphism (490°C and 1,7 Gpa; Okay et al., 2006) of Early Cretaceous age (ca. 105 Ma; mica Ar-Ar geochronology; Okay et al., 2006). The three metamorphic units, referred by Okay et al (2006; 2013) as the Central Pontide Supercomplex, represent subduction-related mèlanges developed at different depths in a subduction zone during the Middle Jurassic-Early Cretaceous time span. This subduction zone probably developed since Early Jurassic in the Intra-Pontide oceanic basin (IPOB) as result of the convergence between Istanbul-Zonguldak and Sakarya continental margins. The occurrence of calc-alkaline volcanic rocks of Late Cretaceous in the sedimentary cover of Istanbul-Zonguldak terrane strongly suggests that the subduction was north-dipping. In this framework, the ophiolites from the Aylı Dağ Unit can be interpreted as the remnants of a Late Jurassic back-arc basin opened above this north-dipping subduction. As result of the continuous convergence, the back-arc basin started to close in the Late Cretaceous time leading to the obduction of the Aylı Dağ ophiolites onto the Sakarya continental margin. During the obduction, the Arkot Dağ Mèlange originated. In the Late Paleocene, the final closure of the IPOB led to development of the imbricate stack of tectonic units that characterizes the IPSZ, whose structures are sealed by the Early Eocene shallow-water deposits.