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The Hazar Basin is a 25 km-long, 7 km-wide and 216 m-deep depression located on the central section of the East Anatolian Fault zone (eastern Turkey) and predominantly overlain by Lake Hazar. This basin has been described previously as a pull-apart basin because of its rhombic shape and an apparent fault step-over between the main fault traces situated at the southwestern and northeastern ends of the lake. However, detailed structural investigation beneath Lake Hazar has not been undertaken previously to verify this interpretation. Geophysical and sedimentological data from Lake Hazar were collected during field campaigns in 2006 and 2007. The analysis of this data reveals that the main strand of the East Anatolian Fault (the Master Fault) is continuous across the Hazar Basin, connecting the two segments previously assumed to be the sidewall faults of a pull-apart structure. In the northeastern part of the lake, an asymmetrical subsiding sub-basin, bounded by two major faults, is cross-cut by the Master Fault, which forms a releasing bend within the lake. Comparison of the structure revealed by this study with analogue models produced for transtensional step-overs suggests that the Hazar Basin structure represents a highly evolved pull-apart basin, to the extent that the previous asperity has been bypassed by a linking fault. The absence of a step-over structure at the Hazar Basin means that no significant segmentation boundary is recognised on the East Anatolian Fault between Palu and Sincik. Therefore, this fault segment is capable of causing larger earthquakes than recognised previously. [ABSTRACT FROM AUTHOR]