Quaternary tectonics of the North Anatolian fault zone and its environs

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The westernmost portion of the North Anatolian fault zone (NAFZ) has been believed to be of transitional nature from the dominantly-transcurrent tectonic regime in the east to the extensional tectonic regime in western Turkey and the Aegean Sea. We analyzed late Quaternary surface deformation in and around this : portion of NAFZ, using SPOT and COSMOS imagery ; limited field observations were employed as ground truth. The following results were obtained :1. Fault offsets on Pleistocene geomorphic surfaces are visible on satellite imagery, whereas those developed on Holocene surfaces are not. Active fault traces detected on the satellite imagery coincide almost exactly with the surface breaks associated with the 1944, 1957 and 1967 earthquakes.2. NAFZ west of the Mudurnu Valley branches into two active fault zones : the lzmit-Sapanca Fault Zone on the north, and the lznik-Mekece fault on the south. The Iznik-Mekece fault is dominantly strike-slip and consists of a simple, continuous strand ; a considerable dipslip component exists on its western portion. In contrast, late Quaternary deformation in the lzmit-Sapanca Fault Zone is quite paradoxical. Our interpretation of satellite imagery and limited field observations indicate that Quaternary surface deformation in this fault zone is compressional, although fault plane solutions of microearthquakes (Evans et al., 1985) suggest that transtensional movements are presently dominant. We speculate that the geometry of faults at seismogenic depths (>6 km) is different from those at the surface.