PETROLOGICAL EVOLUTION OF OPHIOLITE ASSEMBLAGES EXPOSED NEAR THE NORTH ANATOLIAN FAULT IN THE TOKAT MASSIF, NORTHEASTERN TURKEY

The North Anatolian Fault (NAF) is a significant and active dextral strike-slip fault that extends ~1600 km E-W throughout northern Turkey from the Karliova triple junction in the east to the North Aegean Trough in the west. The NAF may have formed in the late Miocene or Pliocene. Located in the Sakarya Zone of the eastern Pontides, the Tokat Massif is cut by the NAF and is described as Permian-Triassic metamorphic, volcanic, and sedimentary group of rocks that is located either between the towns of Amasya and Susehri or ~100 km further east between Corum and Susehri. In this area, the NAF is located near the Izmir-Ankara-Erzincan suture, but the role of this paleosuture in facilitating more recent movement has been unexplored. An additional unknown is the petrological evolution of ophiolite assemblages exposed along the suture in the Tokat Massif. Deciphering the assembly of the Tokat Massif is important for understanding the evolution of the NAF as the structure cuts through the region and facilitates the development of basins and locally developed thrust faults. Structures that facilitated ophiolite exposure may have reactivated during the development of the NAF.

The stratigraphy of the Tokat Massif is debated. According to one interpretation, the Tokat Massif is split into three tectonic units that make up a subduction-accretion complex: the Tokat, Yesilirmak, and Akdagmadeni groups (Yilmaz and Yilmaz 2004). The Yesilirmak group is made up of an ophiolitic mélange and fore-arc sequences. The ophiolitic mélange is part of the North Anatolian Ophiolite Belt that begins in Izmir and continues eastward 1000 km to the Lesser Caucasus. The Akdagmadeni group consists of gneiss, amphibolite, schist, quartzite and marble. These rocks have initially undergone amphibolite facies metamorphism, then cataclastic metamorphism during the Late Cretaceous. The Tokat Group, which comprises most of the Tokat Massif, is comprised of a pre-Jurassic metamorphic group called the Turhal metamorphics and a Silurian to Triassic heterogeneous unit called the Devecidag Mélange. Others characterize the central Pontides as various imbricated piles of Pre-Tethyside and Tethyside tectonic units that have been fragmented by the NAF. During the formation of the Tokat Massif, each orogenic phase was overprinted onto a previous one, creating a complicated assemblage. Rojay (1995) classifies the tectonostratigraphic units of the Amasya region into 7 units based on age, lithostratigraphic evolution, internal organization and location: the Tokat Group, the Devecidag Complex, the Amasya Group, the North Anatolian Ophiolitic Mélange, and 3 metasedimentary units.

The debated stratigraphy of the Tokat Massif is likely due to the region’s complex tectonic history. Providing geochronologic and geochemical information from metamorphic and igneous assemblages within the massif is essential to understanding its role and relationship within the Pontides. To address this, ophiolite samples were collected from the Tokat Massif and analyzed for their major and trace elements. Although the samples display alteration features, their incompatible element concentrations can be used to speculate about their volcanic history due to their immobility during metamorphism. Major element concentrations of our samples are similar to tholeiitic island-arc basalts exhibited by the subalkaline nature of Na₂O+K₂O vs. SiO₂ as well as FeO/MgO vs. SiO₂ ratios. Y vs. Cr and V vs. TiO₂ data for all our samples also suggest island arc tholeiitic sources. Chondrite-normalized REE diagrams for ophiolites display negative slopes with slight LREE enrichment and flat patterns for HREE. Overall REE abundances are between 15-20X chondritic. This is typical for transitional MORB types and suggests chemical contributions from an enriched mantle source.

Age data for rocks displaced by the NAF in the Tokat Massif is lacking, yet this information is key for deciphering the region’s past tectonic history and role in facilitating more recent deformation. We recently found micron-sized zircon grains in clusters in our ophiolite samples from the Tokat Massif. These grains have the potential to be dated using an ion microprobe, which will be used to constrain the timing of crystallization and the relationship of the rocks to ophiolites elsewhere in the Pontides. The age data and kinematic studies of structures in the region will be used to compare the relationship of units in the Tokat Massif across the Pontides.

Rojay (1995) Geologica Romana 31, 329-350. Yilmaz and Yilmaz (2004) Turkish Journal of Earth Sciences 13, 231-246.