Tectonic evolution of the Ganos segment of the North Anatolian Fault (NW Turkey)

Abstract

We analyzed the paleostress field, ongoing deformation, meso- to micro-scale faulting, cataclasis, fault rock alteration and veining within turbidite and limestone sequences at the Ganos Fault which represent a major branch of the North Anatolian Fault Zone in NW Turkey. Fault damage was found to occur across a several kilometers wide zone. Effects of faulting are shown by localized subsidiary brittle faults and fault rock alteration in the turbidites as well as fault breccia formation in the limestone sequence. Microseismicity along the Ganos Fault cluster at two locations, the more pronounced being located offshore at a fault bend associated with a change from a transpressional to a transtensional regime. Kinematic analysis reveals a dextral strike-slip regime with components of normal and thrust faulting. Along strike paleostress orientation at the Ganos Fault is rather uniform. Deformation mechanisms and fluid inclusion data from quartz and calcite veins suggest that fault-related quartz veins were formed at temperatures between 170 and 250 °C and pressures between 40 and 120 MPa. Fault-related calcite vein growth occurred during a temperature decrease from 170 °C to 70 °C with pressures likely below 50 MPa. Fluid inclusion and stable isotope data show that the fluids are predominantly of meteoric origin and migrated upwards into the fault. Pure CH₄ inclusions in quartz also suggest a biogenic or thermogenic methane origin.

Introduction

The North Anatolian Fault Zone (NAFZ) is the most active plate-bounding strike-slip fault in Europe and has developed in the framework of the northward moving Arabian plate and the Hellenic subduction zone where African lithosphere is subducting below the Aegean (e.g. Sengör, 1979). Extending along 1600 km between Eastern Anatolia and the North Aegean, it predominantly forms a right-lateral strike-slip plate boundary that slips at an average rate of 10–24 mm/yr (e.g. Barka, 1992, Reilinger et al., 2006). During the last century, the NAFZ has ruptured over 900 km of its length in a series of large events starting in 1939 near Erzincan in Eastern Anatolia and propagating toward the West (Ambraseys, 1970, Barka, 1992). The last events in the broader Marmara region were the 1912 Mw = 7.3 event on the Ganos Fault (e.g. Ambraseys, 2001, Altunel et al., 2004) at the western end and the Izmit Mw = 7.4 and Düzce Mw = 7.1 events of 1999 (e.g. Tibi et al., 2001, Barka et al., 2002) at the eastern end. The Sea of Marmara represents a more than 100 km long seismic gap that did not rupture since 1766 and that is believed being capable of generating two M ≥ 7.4 earthquakes within the next decades (e.g. Hubert-Ferrari et al., 2000).

Seismogenic faulting occurs within the upper 5–20 km of the crust and is therefore not accessible to direct observations. As a result, geological field investigations of faults exhumed from seismogenic depth, geochemical analyses of fault rocks and geophysical studies have been used to constrain the evolution of fault zones and to test fault zone models (e.g. Chester and Logan, 1986, Schulz and Evans, 2000, Janssen et al., 1998, Janssen et al., 2004).

In this paper we examine the fault-related deformation of the Ganos Fault (GF), as one major branch of the NAFZ in NW Turkey (Fig. 1a). Despite the important role of the GF for seismic activity, tectonics and morphology of the Ganos-Saros region (e.g. Hancock and Erkal, 1990, Tüysüz et al., 1998, Okay et al., 1999, Yaltirak, 2002, Yaltirak and Alpar, 2002, Okay et al., 2004, Altunel et al., 2004, Seeber et al., 2004, Zattin et al., 2005) the knowledge of fault structures, fault evolution (including fault kinematics and deformation conditions), seismicity, and fault–fluid relations is rather incomplete. In this study we address these issues examining the structure and seismicity of the GF using structural and geochemical investigations and low-detection threshold seismic monitoring.

Spatial distribution of the microseismicity provides information about active faulting and focal mechanisms allow to determine the local faulting regimes. The evaluation of meso- and microscopic structures of the exhumed fault portions provides direct information about the fault structure and composition of fault rocks and the evolution of the fault zone. Fluid inclusion analyses from quartz and calcite veins are used to examine faulting deformation conditions. Stable isotope data help to relate fluid involvement to faulting processes. Four locations close to the fault trace were examined in detail (Fig. 1b).