Polyphased block tectonics along the North Anatolian Fault in the Tosya basin area (Turkey)

Abstract The Tosya basin is located in the bending segment of the North Anatolian Fault (NAF) in Turkey. We have obtained original observations on the neotectonics from SAR ERS images, Digital Elevation Model (DEM) and field structural analysis. Regional Neogene deformation is characterised by the occurrence of several basins that are superimposed in time and space. They result from differently oriented movements since 12 Ma, including southwestward motion along a fault subparallel to the NAF. We propose a model of polyphased tectonics related to the displacement of several individualised blocks. In the first stage (Tortonian), the North Tosya block has moved toward the N250° azimuth, parallel to the dextral N70°-striking segment of the NAF. As a consequence, a triple-junction-related compatibility basin was opened at the intersection with a N60° to N30°-striking fault. This pattern is similar to the Karliova corner where the NAF and the East Anatolian Fault meet. In the second stage (Early Pliocene–Middle Pleistocene), a segment of the former N70°-NAF was abandoned and the NAF propagated eastward to form a N90°-striking segment (N90°-NAF), cutting the former Tosya block and basin into two parts. The North Tosya block has moved again and this new geometry has permitted a South Tosya block to move parallel to the NAF but with a higher rate which has induced compression in the Tosya basin. In the third stage (Holocene), the South Tosya block moved toward N240°, obliquely to any of the NAF segments. This has resulted in the formation of two Holocene pull-apart type basins along the previous N60° to N30°-striking fault while extensional faults were formed in the South Tosya block. Estimated dextral displacement along the NAF is 5.9 to 8.5 km at this stage. This model of blocks moving in different directions, including Holocene local movements toward N240°, means that the NAF can be considered not to be a simple transform fault. Our model implies that the N90°-NAF was non-existent before the Early Pliocene.