An Oligocene ductile strike-slip shear zone; the Uludag Massif, northwest Turkey; implications for the westward translation of Anatolia

The Uludag Massif in northwest Turkey represents an exhumed segment of an Oligocene ductile strike-slip shear zone that is over 225 km long and has approximately 100 km of right-lateral strike-slip displacement. It forms a fault-bounded mountain of amphibolite-facies gneiss and intrusive Oligocene granites. A shear-zone origin for the Uludag Massif is indicated by: (1) its location at the tip of the active Eskisehir oblique-slip fault, (2) pervasive subhorizontal mineral lineation in the gneisses with a right-lateral sense of slip, (3) foliation with a consistent strike, (4) the presence of a subvertical synkinematic intrusion, and (5) the alignment of the Eskisehir fault, synkinematic metagranite, and the strike of the foliation and mineral lineation. The shear zone nucleated in amphibolite-facies gneisses at peak pressure-temperature (P-T) conditions of 7.0 kbar and 670 degrees C, and it preserves Eocene (49 Ma) and Oligocene (36-30 Ma) Rb/Sr muscovite and biotite cooling ages. The shear zone was active during the latest Eocene and Oligocene (38-27 Ma), as shown by the crystallization and cooling ages from synkinematic granite. A 27 Ma postkinematic granite marks the termination of shear-zone activity. The 20-21 Ma apatite fission-track (AFT) ages indicate rapid exhumation during the early Miocene. A 14 Ma AFT age from an Uludag gneiss clast deposited in a neighboring Neogene basin shows that the shear zone was on the surface by the late Miocene. Results of this study indicate that during the Oligocene, crustal-scale right-lateral strike-slip faults were transporting crustal fragments from Anatolia into the north-south-extending Aegean; this implies that the westward translation of Turkey, related to the Hellenic slab suction, started earlier than the Miocene Arabia-Eurasia collision.