Recent temporal change in the stress state and modern stress field along the North Anatolian Fault zone (Turkey)
The North Anatolian Fault (NAF) is a dextral strike-slip fault which runs about 1400 km from east to west, from the Karliova triple junction to the Aegean domain. It was active during the Plio-Quaternary as a consequence of the collision between the Eurasian and Arabian plates. The NAF intracontinental deformation zone contributes with the sinistral East Anatolian Fault to the westward extrusion of Anatolia as a consequence of the northward drift of Arabia. The Central NAF zone forms a northward convex bend where the eastern NAF N110°E-trending segment extending from Karliova is connected to the western NAF N75°E-trending segment running to Aegea. Analysing the slip occurring in major earthquakes around the NAF, we show that the present-day stress pattern along the NAF agrees with stress magnitude and strike variations from east to west. The stress regime along the NAF is NNW-trending ?Hmax (?1) transpression in eastern NAF; this progressively changes westwards to NW-trending ?1 transtension in central Anatolia. This lateral variation means a relative decrease of the ?Hmax magnitude, which evolves progressively to an extensional regime (?1 vertical) around the westernmost NAF, characterized by a NNE-trending ?hmin (?3). The drastic change in the tendency of the regional stress state in the North Anatolian block, from compressional to extensional, is effective within the central NAF bend. The stress regime determined by inversion of slip vectors measured on fault planes of various scales confirms the present-day transtensional regime in the central part of the NAF bend. However, the fault kinematic analysis of Quaternary stress states within the Central NAF indicates that this stress state was not continuous throughout the Quaternary. Indeed, chronologies of fault slip vectors provide evidence for two distinct Quaternary regional strike-slip stress states within this zone. Both states have consistent NE- and NW-trending ?3 and ?1 axes, respectively, but have significantly different R values. The change in the strike-slip stress regime probably occurred in the middle Pleistocene. The older mean stress state is characterized by a N142 ± 8°E-trending ?1, a N52 ± 13°E-trending ?3 and a mean arithmetic Rm value of 0.75, which indicates that the regional stress regime is transpressional. The younger strike-slip regime is characterized by a N142° ± 14°E ?1 axis, a N52° ± 10°E ?3 axis and a mean Rm of 0.24, which indicates the transtensional character for this regime. The low R values of the stress deviators related to the recent stress state reflect normal-component slips. These temporal and spatial stress changes along the NAF result from the coeval influence of forces due to the Aegean subduction in the west and to the northward drift of Arabia in the east. Over these boundary forces, shear is superimposed along the NAF, which accommodates the Anatolia extrusion. However, the timing of the temporal stress change permits the suggestion that the Quaternary stress regime variation in North Anatolia is mainly due to the influence of the Aegean domain.