The North Anatolian Fault is a 1200 km long strike-slip fault system connecting the East Anatolian convergent area with the Hellenic subduction zone and, as such, represents an intracontinental transform fault. It began forming some 13–11 Ma ago within a keirogen, called the North Anatolian Shear Zone, which becomes wider from east to west. Its width is maximum at the latitude of the Sea of Marmara, where it is 100 km. The Marmara Basin is unique in containing part of an active strike-slip fault system in a submarine environment in which there has been active sedimentation in a Paratethyan context where stratigraphic resolution is higher than elsewhere in the Mediterranean. It is also surrounded by a long-civilised rim where historical records reach well into the second half of the first millennium BCE (before common era). In this study, we have used 210 multichannel seismic reflexion profiles, adding up to 6210 km profile length and high-resolution bathymetry and chirp profiles reported in the literature to map all the faults that are younger than the Oligocene. Within these faults, we have distinguished those that cut the surface and those that do not. Among the ones that do not cut the surface, we have further created a timetable of fault generation based on seismic sequence recognition. The results are surprising in that faults of all orientations contain subsets that are active and others that are inactive. This suggests that as the shear zone evolves, faults of all orientations become activated and deactivated in a manner that now seems almost haphazard, but a tendency is noticed to confine the overall movement to a zone that becomes narrower with time since the inception of the shear zone, i.e., the whole keirogen, at its full width. In basins, basin margins move outward with time, whereas highs maintain their faults free of sediment cover, making their dating difficult, but small perched basins on top of them in places make relative dating possible. In addition, these basins permit comparison of geological history of the highs with those of the neighbouring basins. The two westerly deeps within the Sea of Marmara seem inherited structures from the earlier Rhodope–Pontide fragment/Sakarya continent collision, but were much accentuated by the rise of the intervening highs during the shear evolution. When it is assumed that below 10 km depth the faults that now constitute the Marmara fault family might have widths approaching 4 km, the resulting picture resembles a large version of an amphibolite-grade shear zone fabric, an inference in agreement with the scale-independent structure of shear zones. We think that the North Anatolian Fault at depth has such a fabric not only on a meso, but also on a macro scale. Detection of such broad, vertical shear zones in Precambrian terrains may be one way to get a handle on relative plate motion directions during those remote times.
When did the North Anatolian fault reach southern Marmara, Turkey?
