Analysis of the North Anatolian Shear Zone in Central Pontides (northern Turkey): Insight for geometries and kinematics of deformation structures in a transpressional zone

Highlights

• We performed a structural study along the North Anatolian Shear Zone in Turkey.

• Along major strike-slip faults the deformation distributes in wide shear zones.

• Fault-Fold relationship was compared with theoretical models.

• The documented strain partitioning is more complex than predictions of models.

• Our structural model is compatible with a crustal-scale flower-structure.

Abstract

The western part of the North Anatolian Shear Zone at the southern boundary of the Central Pontides in Turkey, was investigated in the Kurşunlu-Araç area by means of a geological-structural field study. In this area the North Anatolian Shear Zone results in a transpressional deformation zone that extends between two master faults striking parallel to the main shear direction. The main systems of structures identified in the deformation zone appear to be oriented parallel to the directions predicted by Riedel theoretical model. Nevertheless, the strain partitioning is more complicated than predicted by theory. The structural analysis suggests a polyphase deformation characterized by a steady component of transcurrence associated with alternance of compression and extension. Along each of theoretical directions the combination of double verging structures can be observed, with folds and thrust surfaces root into high-angle shear zones, according to flower-type geometries. The discrepancies of directions, kinematics and geometries from theoretical models are due to transpressive and/or transtensive nature of the deformation. According to the observed outcropping structures, we propose a conceptual model for the North Anatolian Shear Zone, interpreting it as a crustal-scale positive flower structure.

Introduction

Regional-scale strike-slip faults are widespread tectonic features of the earth's crust that accommodate large-scale motions of lithospheric plates (e.g. Wilson, 1965). These structures are characterised by steeply dipping, straight development and prominent geomorphic features (Cunningham and Mann, 2007 with references).

The strike-slip deformation is often not concentrated along a single fault but is distributed along interlinked systems of faults and shear zones bounding blocks according to a complex strain-partitioning model at different scales (Oldow et al., 1990, Tikoff and Teyssier, 1994, Jones and Tanner, 1995, Dewey et al., 1998).

The development of strike-slip zones in a mechanically heterogeneous crust results in the reactivation of pre-existing tectonic features as older suture zones (White et al., 1986, Holdsworth et al., 1997).

The North Anatolian Fault (NAF) (Ketin, 1948, Ketin, 1957, Allen, 1969, Ambraseys, 1970, Şengör, 1979, Barka, 1992) is a major continental dextral strike-slip fault, which extends for approximately 1400 km from the Karliova basin in the east to the Aegean Sea in the west (Taymaz et al., 1991, Kocyigit et al., 2001, Yılmaz et al., 2002, Şengör et al., 2003). The NAF is the most important structural element of the North Anatolian Shear Zone (NASZ), a complex deformation zone of regional scale. In agreement with Sengor et al. (2005), we regard the NASZ as the entire shear zone consisting of all of the structures belonging to the NAF system, developed in correspondence with the Tethyan suture zones. As noted by these authors, the evolution of the NAF cannot be discussed independently of the entire shear zone represented by the NASZ.

Because the NASZ is one of the world's major active fault systems, the NASZ time-space evolution is matter of debate. Therefore, a considerable number of studies on paleoseismology, seismicity, paleomagnetism, GPS measurements, geophysical modelling, geodynamics and global tectonics regarding the NASZ and the Anatolian plate have been produced and published. On the contrary, the geological studies of the geometry and kinematics of the tectonic structures associated with the NASZ are scarce (Andrieux et al., 1995, Bozkurt and Koçyiğit, 1996, Dhont et al., 1998, Sunal and Tüysüz, 2002, Ozden et al., 2008) and mostly concentrated on the western part of the NASZ (Neugebauer, 1995, Yaltırak et al., 1998, Okay et al., 1999, Yaltırak and Alpar, 2002, Yiğitibaş et al., 2004, Ozden et al., 2008).

This contribution illustrates the results of a geological-structural field study carried out along a transect across the central part of the NASZ at the southern boundary of the Central Pontides in the Kurşunlu-Araç area. The characterisation of the geometries and kinematics of the studied tectonic structures, which suggest the presence of main transpressional systems, indicates that strain partitioning played an important role in the deformation evolution of the NASZ. The described structures and their possible relationships with inherited tectonic structures are interpreted within a regional crustal setting.