Long term deformation and earthquake history of the Ovacik fault (Eastern Turkey): Implications and contributions to the intra-plate deformation of Anatolia

The post-collisional convergence between the Eurasian and the Arabian plates and the subduction along the Hellenic Trench (the Eurasian and the African plates) lead to the deformation of Anatolia. The Anatolian Block in which the westward escape tectonics with respect to Eurasia is accommodated along the North and East Anatolian shear zones (NASZ and EASZ). Anatolia incorporate almost all kind of structural elements from strike-slip dominated (especially in the Central Anatolian 'ova' Province) to extensional (the Western Anatolia Extensional Province) and compressional (the Eastern Anatolia Province of Shortening) tectonic features within this complex tectonic regime. In addition to plate boundary elements, the NASZ and the EASZ, Anatolia is internally deformed by many NE-striking sinistral and NW-striking dextral faults. Moreover, all tectonic elements in the Anatolian Block, which all carry certain amount of deformation, give an opportunity to study the intra-plate deformation of this particular place. Microseismisity also indicates that the interior of the Anatolian Block has a considerable degree of deformation. The instrumental seismicity represents spatially a detailed, but only a screenshot of the earth's dynamics when we consider the long history of faults. Therefore, any data about palaeoearthquakes provide a temporal control on the behaviour of active tectonic structures, which is important on understanding the long-term deformation of intra-plate regions. The Ovacık Fault (OF), which is the northeastern member of the Malatya-Ovacık Fault Zone (MOFZ), is close to the eastern boundary of the Anatolian Block. Although the tectonic geomorphology is very impressive along the OF, this fault is just poorly known in terms of active tectonics except geodetic and geologic slip rate estimates. This study covers geology, tectonic geomorphology along the strike of the fault and a palaeoseismological study to understand the distribution and character of the deformation along and around this particular structure. The geology of the region starts with the Keban metamorphics (PzMzk) at the bottom of the stratigraphy, which is unconformably overlaid by olistostromal facies of the Kemaliye Formation (Kk). The Munzur Limestone (Mzm) is thrusted over to both of these units while it is overlain by the south vergence thrust of the Cretaceous ophiolites (Cop). The Tertiary cover deposits (Tc), which include mainly limestones and fine grain clastic rocks, unconformably overlies all older units. All of these units are cut by the intrusion of the Çöpler granitoids (g). The Yamadağ Volcanic Complex (Ty), which widely spreads at the southwestern part of the study region, is composed of basalts, andesites and dolerites (Fig. 3.2). The Quaternary sediments such as debris flow deposits (Qd), alluvial fan deposits (Qaf), morains (Qm) and alluvium (Qal) have best exposures in the Ovacık Basin. The region is affected by polyphase deformation processes, which lead to the formation of folding and faulting. The open folds generally have hinges that plunge to the E-NE. Moreover, according to poles of M-planes, I identified at least two different deformation phases, which can be summarized to be oblique and strike slip dominated. I also studied the tectonic geomorphology by using geomorphic indices (such as hypsometric curve and integral, longitudinal channel profiles, channel steepness and concavity and mountain front sinuosity) to characterize the deformation along the OF and the surrounding region where the regional morphology was shaped with a joint-effect of palaeo- and neo-tectonics and other morphological factors such as palaeoglacial processes. The highest hypsometric integral (HI), steepness and concavity values, which indicates the 'young age' topography related with the uplift, generally occur along the Munzur Mountains in the east and around the Kemaliye Region in the west. The high HI and steepness at the Kemaliye Region, where there is negligible evidence for palaeoglacial and karstic activity, is interpreted to be the result of tectonic uplift. This sudden tectonic uplift of the Kemaliye Region is based on the change of the fault's strike, which generates a transpressional setting. This compressional deformation is not only expressed within the geomorphic signals in the steepness and concavity analyses, but it is also well defined with kinematic features along fault surfaces and the exposure of a single palaeoseismological trench at Yukarı Yuvacık Site. This trench site is morphologically characterized by a linear valley with at least two terrace treads, which are delimited from the bedrock at each side of the valley. This first palaeoseismological study along the OF reveals three palaeoevents in the last 7000 years within the fluvial sediments. Radiocarbon dating of stratigraphic layers yields an average recurrence of eathquakes 2400±765 years, which suggests a higher seismic potential for the OF than the previous suggestions. Furthermore, the main motivation is to understand the role of the OF in the deformation of Anatolia together with other intra-plate structures. Although there are still many gaps, one of the most possible mechanism can be the existence of a higher convergence strain zone as a result of convergence between Eurasia and Arabia, which does not only rotate the strike of the OF, making it to gain a compressional component of motion, but also affect other parallel/sub-parallel fault zones in the same sense (Şengör et al., 1985; Zabcı et al. submitted). In summary, the results of my study simply suggest that the OF is an active seismogenic sinistral strike-slip fault with an average recurrence of earthquakes 2400±765 years within the Anatolian Block. Along the strike of the OF, it shows all type of tectonic structures from strike slip to extensional and compressional due to the geometric changes in the fault's strike. Although the deformation character varies, the cumulative deformation, which slip rate is calculated as approximately 2.5 mm/yr from the age and offset of the Karasu River, is constant along all length of the fault. The OF and other intra-plate faults of the Anatolian Block carry the whole deformation, which penetrates from boundary condition, heteregeously and creates big scale cataclastic flow as deformation mechanism of Anatolia.