Determination of the Sense of Shear Using the Orientation of Shear Band Foliation in Mylonites: Field Evidence from the Keban Complex, Eastern Turkey

Abstract

The structural features of the northern part of the Bitlis suture zone of the Alpine-Himalayan orogenic belt (Turkey) show intense internal deformations. The variable lithology and microstructural features of the high strain zones commonly found in the Keban complex are characterized by mylonitic texture. The Keban complex were formed in ancient continental margin sedimentary sequences of the northern branch of the Neo-Tethys. Single sets of shear band foliations occur within mylonite zones composed of strongly foliated phyllitic psammites. Two well defined microscopic criteria, the tails of the augen structures and smaller scale shear zones which were formed between the relatively undeformed pod shaped aggregates, are used to deduce the sense of shear in a lithologically inhomogeneous brittle-ductile shear zone of the Keban complex. The sense of shear determined in this way from the anastomosing part of the shear zone is then applied to the orientation of shear band foliation in the same shear zone. This microscopic evaluation of the structures suggests that the acute angle between the shear band foliation and the mylonitic foliation points in the shear direction as proposed by earlier research which were based on experimental studies. Therefore, the determination of sense the of shearing in the field using the orientation of shear band foliations in mylonites is suggested.