Establishing the structure of the Cretaceous Neotethyan Orhaneli ophiolite, NW Turkey

Orhaneli ophiolite is a Late Cretaceous ophiolitic suite, obducted over the Late Cretaceous high-pressure rocks of the Tavşanlı Zone that represents the subducted part of the southern passive continental margin. It is part of the Neotethyan ophiolites related to the Izmir-Ankara-Erzincan Suture. The present work aims to decipher the inner structure of the Orhaneli ophiolite. This implies constructing the geometrical relationships between structural elements and to evaluate their original positions relative to the paleo-horizontal and paleo-ridge axis.

The Orhaneli ophiolite comprises three tectonic domains separated from each other by N-S trending east-vergent thrusts. The middle domain comprises mantle harzburgite, dunite, pyroxenite, and crustal layered gabbro and cumulate peridotite. The Moho transition zone is represented by a 1 km thick, highly sheared zone that consists of serpentinite and mylonitic gabbro. Mylonitic gabbro has a layered-laminated structure and is very well lineated. Mantle structures (compositional layerings and foliations) are dominantly sub-vertically dipping with the N-S trend. While the layered gabbros are dipping to the east with 65° near the Moho, the dip direction progressively changes to the west stratigraphically upward. The eastern domain is the tectonic repetition of the mantle section of the middle domain. Foliations and compositional layerings strike N-S and sub-vertical dips. The western domain corresponds to relatively lower parts of the mantle which consists of harzburgite and dunite. The absence of pyroxenites distinguishes the mantle rocks of this domain from the others.

It is observed that (1) there is a low-angle relationship between the mantle structures and the lower parts of the layered gabbro, (2) layered gabbros are progressively steepening stratigraphically upward, (3) the boundary between the lithospheric mantle and the crust is strongly sheared.