27. The North Anatolian Fault and the North Anatolian Shear Zone

The North Anatolian Shear Zone (NASZ) and its most prominent member, the North Anatolian Fault (NAF), initiated some 11 million years ago, together form the northern boundary of the westerly extruding Anatolian Scholle. The NAF has had a remarkable seismic activity between 1939 and 1999 in which the westward migrating earthquake sequence created surface ruptures amounting to about two-thirds of its total length of 1600 km, leaving unbroken only the Marmara Segment, to the west, and the Yedisu Segment, to the east. Both the NASZ and the NAF are located within a broad zone of soft subduction-accretion material forming the suture fill of both the Palaeo- and Neo-Tethyan oceans. In general, the NASZ becomes wider from east to west in harmony with the widening of the zone of accretionary complexes and it reaches its maximum width in the Marmara Lobe. The NAF generally follows a very prominent valley from the Karlıova Triple Junction in the east, to the town of Bolu in the west. Farther to the west, the NAF bifurcates into two strands probably resulting from the existence of structures already established in the west as a result of the Aegean extension. There are many major river courses that cross the NAF where they bend in a clockwise fashion because of the dextral displacement of the fault. In the east, the tributaries of the Fırat (Euphrates) are deflected along a very narrow corridor, but further to the west, other major rivers display a broader zone of dextral deflection. The decreasing cumulative offset from east to the west suggests a diachronous character for the NAF that formed by the progressive strain localisation in this westerly widening right-lateral shear zone. The localisation of the NAF happened during the late Miocene in the extreme east of the shear zone and then gradually tore farther and farther westward at an average rate of some 13 cm/year until it finally reached its present position in the west some 200.000 years ago, although this extreme youth in the west is not yet universally agreed upon. This nucleation did not deactivate the earlier broad shear zone, but left some elements still active, creating earthquakes and shaping the topography, but at incomparably smaller rates.