Neotectonic Structures Indicating Extensional and Contractional Strain within Pliocene Deposits Near the NW Margin of the Niksar Pull-Apart Basin, Turkey

Abstract

The Niksar basin lies between the 1939 and 1942 earthquake fault segments of the North Anatolian Fault Zone (NAFZ) and has been described as a pull-apart basin. Its structural, drainage and depositional systems generally support such a model. The extensional structures indicate normal faults, oblique-slip faults with a normal component, and geomorphological expressions such as sag ponds and are attributed to NE-SW extension related to strike-slip movements along the WNW-ESE principle displacement zone (PDZ) during the progressive evolution of the basin. The extension direction makes an angle of 50-60^0 with the orientation of the PDZ of the strike-slip fault system indicating a significant degree of transtension compared with the basement. The general trend of the transtensional structures makes an angle, ranging from 17^0 to 32^0, with the associated strike-slip faults. The value is generally compatible with the theoretical models. The indicator of contractional strain is the presence of NE-SW trending fold axes which plunge at approximately 4^0 to 8^0 towards either S or N indicating NW-SE dextral transpression. The most important result obtained in this study is that the NAFZ, at least in this particular segment, does not present pure strike-slip movement. The displacements generally have a dip-slip component, either reverse or normal, and dextral oblique-slip movement is dominant along the course of the NAFZ in this region.