The deformation mechanisms of Upper Cretaceous Neotethyan Orhaneli ophiolite, NW Turkey

Orhaneli ophiolite is an Upper Cretaceous ophiolitic suite obducted over the Late Cretaceous high-pressure rocks. It covers 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. The lithological and structural mapping of the Orhaneli ophiolite revealed that the mantle rocks, the Moho Transition Zone (MTZ), and the ophiolitic lower crust are exposed along the region but the subvolcanic and volcanic sequences are missing. The study of the deformation mechanisms of available three units is our research.

The mantle rocks in the Orhaneli ophiolite comprise harzburgite (~50%), dunite (~40%), websterite, and clinopyroxenite (~10%). Harzburgite and dunite are coarse-grained and show well-developed L-S tectonic fabric. Websterite and clinopyroxenite are coarse/very coarse-grained with granular texture. The mantle tectonites (harzburgite and dunite) in the region are characterized by widespread high-temperature (1200-1250 °C) deformation partially overprinted by low-temperature (800-1000 °C) deformation. The grain boundary migration (GBM) and subgrain rotation (SGR) recrystallizations are the dominant mechanisms of the high-temperature deformation in this unit. The subsequent low-temperature deformation predominantly proceeded through subgrain rotation (SGR), and bulging (BLG) recrystallizations accompanied by kinking and twinning. Contrarily to the mantle tectonites, the pyroxenite (websterite and clinopyroxenite) predominantly shows low-temperature deformation structures. They are mainly deformed through the kinking of the pyroxene grains, however, high-temperature deformation structures also exist. A possible explanation is that the pyroxenite predominantly deformed through viscous flow under spreading center conditions.

The MTZ in the Orhaneli ophiolite is a ~1 km thick, strongly sheared zone between the mantle and lower crustal rocks. It mainly consists of serpentinite, layered gabbro, and mylonitic peridotite. The serpentinite, the most prevalent lithology in this zone, commonly shows anastomosing foliation. The layered gabbro mainly consists of orthopyroxene and plagioclase. It is characterized by thin and continuous layers of plagioclase and orthopyroxene. In some cases, these layers are transposed into isoclinal folds with detached limbs by continuous, layer-parallel, simple shearing. The stretching lineation is well-developed and defined by plagioclase and orthopyroxene. The mylonitic peridotite mainly consists of olivine and orthopyroxene. It is characterized by ribbons of orthopyroxene and elongated aggregates of olivine within a fine-grained olivine and pyroxene-rich matrix. The orthopyroxene ribbons indicate high-strain conditions within the MTZ. The aggregates of olivine suggest that the SGR recrystallization is an important mechanism of deformation within this zone.

The lower crust in the Orhaneli ophiolite comprises cumulates of gabbro, peridotite, pyroxenite, and anorthosite, in order of prevalence. Peridotite is more abundant in the stratigraphically lower sections of the crust and it diminishes stratigraphically upward. The serpentinization of peridotite is commonly over 90%. In general, the crustal section does not show evident plastic deformation. The gabbroic rocks commonly show magmatic foliation defined by the preferred orientation of undeformed plagioclase and pyroxene grains. This suggests that the crustal section is mainly deformed through viscous flow in the magmatic state.