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

The Tosya basin is located in the bending segment of the North Anatolian Fa ult (NAF) in Turkey. We have obtained original observations on the neotecto nics from SAR ERS images, Digital Elevation Model (DEM) and field structura l analysis. Regional Neogene deformation is characterised by the occurrence of several basins that are superimposed in time and space. They result fro m differently oriented movements since 12 Ma, including southwestward motio n along a fault subparallel to the NAE We propose a model of polyphased tec tonics related to the displacement of several individualised blocks. In the first stage (Tortonian), the North Tosya block has moved toward the N250 d egrees azimuth, parallel to the dextral N70 degrees-striking segment of the NAE As a consequence, a triple-junction-related compatibility basin was op ened at the intersection with a N60 degrees to N30 degrees-striking fault. This pattern is similar to the Karliova corner where the NAF and the East A natolian Fault meet. In the second stage (Early Pliocene-Middle Pleistocene ), a segment of the former N70 degrees-NAF was abandoned and the NAF propag ated eastward to form a N90 degrees-striking segment (N90 degrees-NAF), cut ting 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 compress ion in the Tosya basin. In the third stage (Holocene), the South Tosya bloc k moved toward N240 degrees, obliquely to any of the NAF segments. This has resulted in the formation of two Holocene pull-apart type basins along the previous N60 degrees to N30 degrees-striking fault while extensional fault s were formed in the South Tosya block. Estimated dextral displacement alon g the NAF is 5.9 to 8.5 km at this stage. This model of blocks moving in di fferent directions, including Holocene local movements toward N240 degrees, means that the NAF can be considered not to be a simple transform fault. O ur model implies that the N90 degrees-NAF was non-existent before the Early Pliocene. (C) 1998 Elsevier Science B.V. All rights reserved.