Osman Bektas

Migration of the Upper Cretaceous subduction-related volcanism towards the back-arc basin of the eastern Pontide magmatic arc (NE Turkey)

The Eastern Pontide active continental margin extends along the eastern part of the Black Sea with an E-W trend and comprises the parallel oriented northern, southern and axial zones. Each zone is fault bound and distinguished by distinctive magmatism, lithofacies and stratigraphy. In the northern zone, from the coast of the Black Sea to Torul, the cessation of Liassic bimodal volcanism and accumulation of anoxic carbonates of the Valanginian-Barremian was followed by bimodal volcanism which evolved from a tholeiitic (TH)-calc-alkaline (CA) to high K-CA composition. These volcanics erupted sporadically in asymmetric deep basins until the end of the Senonian. In the southern zone, around Torul and Gumushane, Liassic bimodal volcanism and the accumulation of the Malm-Cenomanian platform carbonate was succeeded by Campanian volcanism, i.e. much later than volcanism in the northern zone. A second epoch of CA-high K-CA andesitic volcanism occurred in the southern zone and is intercalated with a Campanian red pelagic limestone which is also widespread in the northern zone, and known as the marker lithologic unit in the eastern Pontides. South of the southern zone, in the Bayburt-Maden area, the pelagic limestone of the Early Cretaceous and overlying ophiolitic olistostromal melange (which contains Cenomanian MOR, WP and IA basaltic pillow lavas) are overlain by high-K calc-alkaline Campanian andesitic volcanic rocks, implying that the ophiolitic melange was formed in a back-arc environment. The major and trace element geochemistry and REE patterns of the Upper Cretaceous volcanic rocks suggest a variation from hydrous IA to anhydrous back-arc melting conditions on the subduction zone and the migration of arc magmatism laterally from the northern to the southern zones during the Senonian. This change to dehydration melting and the migration of the subduction-related Late Cretaceous volcanism towards the south in the eastern Pontide Magmatic arc, requires a south-dipping subduction polarity during the Senonian. Copyright # 1999 John Wiley & Sons, Ltd.

Mid-Cretaceous Olistostromal Ophiolitic Mélange Developed in the Back-arc Basin of the Eastern Pontide Magmatic Arc, Northeast Turkey

Geological and geochemical characteristics of the Mid-Cretaceous olistostromal ophiolitic mélange exposed in the southern part of the eastern Pontide magmatic arc indicates that it originally formed during the drifting stage of a back-arc basin (Neotethys). This view conflicts with the popular idea that this ophiolitic mélange represents a typical tectonic mélange or an accretionary prism developed in the fore-arc of the eastern Pontide magmatic arc by northward subduction of Neotethys or Paleotethys during the Cretaceous. This Mid-Cretaceous ophiolitic mélange is composed of the three distinct lithological facies that may reflect a strike-slip cycle—from transtensional to transpressive tectonic regimes—in the deep spreading troughs of pull-apart basins. The lower part of the mélange is represented by redeposited carbonate rocks that occur as an upward thinning and fining sequence. The middle level of the mélange comprises basaltic pillow lavas, hyaloclastics, and intercalated pelagic sediments such as radiolarite, mudstone, and red pelagic limestone. The upper part of the mélange is represented by an olistolith-olistostromal facies that includes sandstone, siltstone, marl, and intervening olistoliths and olistostromes—mainly limestone, peridotite-gabbro, and metamorphic blocks derived from the continental shelf and a nearby peridotitic-metamorphic ridge. Trace-element and REE contents of the basaltic rocks belonging to the Mid-Cretaceous ophiolitic mélange show distinct geotectonic settings (IA, WP, MOR) with various enrichment and depletion trends with respect to MORB and chondrite. Moreover, this ophiolitic mélange is overlain by analcime-bearing Upper Cretaceous-Paleocene(?) shoshonitic basalts in the Bayburt-Maden area. These geological and geochemical data imply that mélange formed in the back-arc basin of the eastern Pontide magmatic arc.

Successive Extensional Tectonic Regimes during the Mesozoic as Evidenced by Neptunian Dikes in the Pontide Magmatic Arc, Northeast Turkey

The eastern Pontide magmatic arc extends ∼600 km in an E-W direction along the Black Sea coast and was disrupted by a series of fault systems trending NE-SW, NW-SE, E-W, and N-S. These fault systems are responsible for the formation of diachronous extensional basins, rift or pull-apart, in the northern, southern, and axial zones of the eastern Pontides during the Mesozoic. Successive extensional or transtensional tectonic regimes caused the abortive Liassic rift basins and the Albian and Campanian pull-apart basins with deep-spreading troughs in the southern and axial zones. Liassic, Albian, and Campanian neptunian dikes, which indicate extensional tectonic regimes, crop out within the Paleozoic granites near Kale, Gumushane, and the Malm–Lower Cretaceous platform carbonates in Amasya and Gumushane. These neptunian dikes correspond to extensional cracks that are filled and overlain by the fossiliferous red pelagic limestones. Multidirectional Liassic neptunian dikes are consistent with the general trend of the paleofaults (NE-SW, NW-SE, and E-W), and active dextral North Anatolian fault (NAF) and sinistral Northeast Anatolian fault (NEAF) systems. The Albian neptunian dikes in Amasya formed in the synthetic oblique left-lateral normal faults of the main fault zone that runs parallel to the active North Anatolian fault zone (NAFZ). Kinematic interpretation of the Liassic and Albian neptunian dikes suggests N-S extensional stress or northward movement of the Pontides along the conjugate fracture zones parallel to the NAFZ and NEAFZ. This northward movement of the Pontides in Liassic and Albian times requires left-lateral and right-lateral slips along the conjugate NAFZ and Northeast Anatolian fault zones (NEAFZ), respectively, in contrast to the recent active tectonics that have been accommodated by N-S compressional stress. On the other hand, mutual relationships between the neptunian dikes and the associated main fault zone of Campanian age extending in an E-W direction in the Kale area, Gumushane suggest the existence of a main left-lateral transtensional wrench zone. This system might be accommodated by the counterclockwise convergence of the Turkish plate with the Afro-Arabian plate relative to the Eurasian plate, and the southward oblique subduction of Paleotethys beneath the eastern Pontide magmatic arc during the Mesozoic.