Suşehri basin: an active fault-wedge basin on the North Anatolian Fault Zone, Turkey

Abstract

The Suşehri section of the North Anatolian Fault Zone (NAFZ) is dominated by an active fault-wedge basin. Its recent configuration is a Quaternary strike-slip depression superimposed on a larger, fault-controlled fluvial to lacustrine setting inherited from the Pliocene. In and around the Suşehri fault-wedge basin, the NAFZ is characterized by the North Anatolian master fault (NAMF) or principal displacement zone (PDZ) and four second-order fault sets, namely the Kelkit, Akçaaǧil, Beydeǧirmeni and Ekenek fault sets. The NAMF cuts through the basin. The fault sets border the margins of the basin and impart a double divergence character to the NAMF which is proposed as an evolutionary model for the Suşehri fault-wedge basin. The Suşehri basin fill deposits of Pliocene age rest on the quiet and shallow-marine limestones of Burdigalian age with a sharp angular unconformity. Besides, the upper half of the Pliocene basin fill deposits is dominated by the well-developed syndepositional mesofractures, which reveal the existence of a right-lateral shearing at the time of deposition. These observations also imply the emergence of a new tectonic regime after the Burdigalian by which time the Suşehri basin had experienced a subsidence of about 660 m to 1150 m during its evolution. On the other hand, at the south-easternmost end of the Suşehri basin, the Lutetian volcano-sedimentary sequence of the Yeşilyayla Group is cut and displaced right-laterally by about 35 km, which is the detectable minimum offset on the NAFZ. Finally, during the 1939/12/27 Erzincan earthquake, the Erzincan-Erbaa segment of the NAFZ was reactivated and several modern faults were produced. Along these modern faults some natural and man-made features were displaced right-laterally. These offsets vary in decreasing order from 7.5 m near the epicenter at the southeastern end to 3.7 m away from it at the northwestern end of the Erzincan-Erbaa segment of the NAFZ.