Abstract:

The Bolu Basin in northwestern Turkey, situated in the western part of the North Anatolian Fault Zone (NAFZ), displays the neotectonic features of a pull apart basin. The long axis of the basin extends east-west, parallel to the fault zone and Bolu city, situated in the central part of this basin, was extensively damaged during the 17 August 1999 M= 7.4 and 12 November 1999 M= 7.2 earthquakes. The master strand of the North Anatolian Fault Zone cuts through the basin close to its southern edge and movement on this strand has caused tilting of the basin floor towards the south because of a small dip slip component. Almacık Mountain, to the west of the Bolu Basin, is interpreted as a plate flake and appears to play a role in the bifurcation of the North Anatolian Fault Zone around the Düzce Basin, to the west of Bolu. Thus the surface fracture associated with the right-lateral strike-slip Gölyaka-Kaynaşlı segment of the NAFZ (which caused the 12 November 1999 M= 7.2 earthquake in this region), can be traced along the northern flank of Almacık Mountain and may extend into the middle of the Bolu Basin from the west. The northern boundary of this basin is controlled mainly by an E-W-striking oblique-slip normal fault with a right-lateral strike-slip component but a major NE-SW-trending younger fracture (Kocasu Fault) has also influenced the kinematic behaviour of this sector of the NAFZ and the adjacent basin. It is concluded that the Bolu Basin opened as a pull-apart basin since the Early Pleistocene between the northern boundary faults and southern master strand, and within the complex stress-field reflected in this still-active fault regime. It has continued to develop in this style, despite the regional transpressional stress field prevailing in the western sector of the northwards-convex North Anatolian Fault Zone. New radiometric dates obtained from travertine deposits developed along the NAFZ master strand on the southern border of the Bolu Basin show that the basin is older than 3 x 105 years. Dip-slip normal faults observed in the poorly consolidated Quaternary fluvial sediments forming the basin floor display both ENE-WSW and N-S trends, in accordance with the transtensional kinematics of a pull-apart. The 4.5 metre co-seismic right-lateral displacement in the middle part of the Gölyaka-Kaynaşlı segment of the NAFZ that occurred during the 12 November 1999 earthquake appears to have loaded stress on to the eastern part of this segment, possibly causing it to propagate eastwards, into the middle of the Bolu pull-apart basin and creating an east-west-trending high strain zone north of Bolu city, suggesting the route of the possible continuation of the segment.

Keywords: