Xavier Le Pichon, Corné Kreemer, and Nicolas Chamot-Rooke
From east to west, the western extension of the North Anatolian Fault (NAF) follows the northern margin of the Sea of Marmara, cuts across the Gallipoli Peninsula through the Ganos Fault, joins the northern margin of the Saros Trough and finally the southern margin of the Sporades Trough. The Sporades Trough is the symmetric of the trough that forms the northern Sea of Marmara. Both are 30-35 km wide, U-shaped, isostatically compensated, 150 km long basins. The Saros Trough on the other hand is a narrow 10-15 km wide, V-shaped, uncompensated valley. This composite 500 km long section of fault is now the site of pure dextral strike-slip at a rate of 24 mm/yr throughout. We have argued earlier (Le Pichon et al., EPSL, 2001) that the present trace of this purely dextral strike-slip portion of NAF was established within the last few hundred thousand years within a previously existing North Marmara Sea basin in which the motion was transtensional. The Saros Trough appears to be geologically quite recent and was probably formed during this Late Pleistocene kinematic transition. On the other hand the western North Aegean Trough system was formed much earlier about 10 Ma ago by NE-SW extension. The NAF abruptly ends to the west as it abuts northern Greece where the motion is absorbed by a system of extension with clockwise rotation. The Gulf of Corinth is related to this system and its Late Pleistocene age is similar to the age of the kinematic reorganization in the Sea of Marmara and to the age of the Saros Trough. Thus the whole NAF west of the Gallipoli peninsula is of Late Pleistocene age that is the age of a major kinematic reorganization. Its young age implies a total offset that may not exceed 10 km and this small offset may in turn explain why there is no anisotropy detectable in the mantle parallel to the fault, at the difference of the mantle below the San Andreas Fault for example. A remarkable character of this western NAF is that it appears to be trapped by preexisting troughs in which it tends to follow one of the margins. The margins of these troughs are the sites of strong heterogeneity of material. There is a vertical offset of the basement of several kilometers and the crust in the trough is highly sheared and faulted. This heterogeneity is confirmed in the Sea of Marmara as it results there in asymmetric interseismic strain. We propose that these preexisting troughs, because of their built in asymmetry, act as traps for the NAF through a process similar to the one advocated by Ben Zion (this session).