The structure of the orhaneli ophiolitic suite

Abstract

Orhaneli ophiolite is a Late Cretaceous ophiolitic suite obducted over the Late Cretaceous high-pressure rocks of the Tavşanlı Zone. It extends over approximately 43 km in length and 14 km in width. It is part of the Neotethyan ophiolite belt along the southern side of the Izmir-Ankara-Erzincan Suture. This study aims to evaluate the internal structure of the Orhaneli ophiolite. In other words, by determining the geometric relationships of structural elements, it aims to make inferences about the positions of these structures relative to the paleo-horizontal plane and paleo-ridge axis. Additionally, the study seeks to gain insights into the flow and deformation conditions by examining the deformation mechanisms of these units. In this study, it has been determined that the Orhaneli ophiolite consists of three tectonic domains separated by north-south trending thrust faults. Domain 1 is located at the easternmost part and is composed of upper mantle rocks. The structures in this domain predominantly exhibit sub-vertical dips and N-S trends. Domain 2 is located at the central part of the study area and comprises the upper mantle, Moho Transition Zone (MTZ), and lower crustal rocks. The mantle structures within this domain exhibit an N-S trend with sub-vertical dips. This unit consists of harzburgite, dunite, and pyroxenite. The transition from the mantle to crust (MTZ) is represented by a 1 km thick, high-strain zone. The structures within this zone are generally dipping to the east at angles of 50-60°. The Moho Transition Zone is predominantly composed of mylonitic gabbro and high-strain peridotites. The lower crust is represented by mafic-ultramafic cumulates. The structures within the lowermost part of the cumulates are dipping to the east parallel to the Moho. Stratigraphically upward, the structures progressively steepen and show a transition to the westward dip. Domain 3, located in the westernmost part of the study area, represents deeper sections of the mantle. It is dominantly composed of harzburgite and dunite. The absence of pyroxenites distinguishes the mantle rocks of this domain from the others. Domain 2 is used for structural analysis due to that it consists of a considerable portion of the ophiolitic sequence. The structures within the MTZ and the lowermost parts of the layered gabbro are considered paleo-horizontal references. According to this reference, the structures within the mantle were dipping with approximately 40° before the emplacement. The boundary between the mantle and crust is defined by a 1 km thick shear zone. The structures within this zone are in a sub-horizontal position. The structures within the crust are horizontal in the lower parts and gradually steepen toward the upper parts. In the Orhaneli ophiolite, the evidence of dynamic recrystallization is well-preserved, indicating the limited influence of static recrystallization in the region. The mantle tectonites in the Orhaneli ophiolite are represented by high-temperature (1200-1250°C) structures that are partially overprinted by low-temperature (800-1000°C) structures. The high-temperature deformation is evident by grain boundary migration (GBM) and subgrain rotation (SGR) recrystallizations of olivine and pyroxene. Later developed low-temperature deformation predominantly occurs through kinking, deformation twinning, and bulging (BLG) recrystallization. High-temperature and low-temperature deformations represent asthenospheric and lithospheric flow, respectively. The fast-spreading ridges are characterised by gentle dipping mantle structures. The low-angle relationships between the mantle structures and the Moho, the high thickness of the lower crustal rocks (>5.5 km), the continuity of crustal structures (magmatic layerings), as well as the abundance of asthenospheric flow structures (high-temperature deformation), indicate that the Orhaneli ophiolite has formed in a fast-spreading ridge.