Modelling tectonic deformation along the North-Anatolian Fault in the Sea of Marmara
Introduction
The North Anatolian Fault (NAF) is a right-lateral, >1200 km-long continental transform fault that separates the Eurasian and Anatolian plates (Fig. 1) (Şengör et al., 2005). In its eastern part, the NAF is constituted by a single fault strand that experiences almost pure strike-slip deformation. To the west, it separates into two major branches, the Northern branch (N-NAF) and the Southern branch (S-NAF) (Fig. 1) that accommodate predominantly transtensive deformation. According to GPS modelling, the northernmost of these branches (NAF-N in Fig. 1) takes up most Eurasian-Anatolian relative motion, about 24 ± 1 mm/yr (McClusky et al., 2003). Geodynamic models explain the transtension pattern observed in the Marmara basin as a consequence of Anatolia escaping towards the west, with its rate of counterclockwise rotation progressively increasing westward in response to the Hellenic subduction (McClusky et al., 2000). The NAF-N pull-apart system of the Sea of Marmara creates deep tectonic depressions that reach over 1200 m below sea level, separated by structural highs (Fig. 1). Despite the formation of the Sea of Marmara being elegantly explained as a consequence of major oversteps along the westward propagating NAF (Barka and Kadinsky-Cade, 1988; Armijo et al., 1999; Şengör et al., 2005), there remain unsolved issues regarding its recent tectonic evolution and present activity, issues that are particularly critical for reliable earthquake scenarios in a region of high seismic hazard. According to historical catalogues (Ambraseys, 2002) the Sea of Marmara and other adjacent regions along the NAF are sites of major earthquakes (Mw ≥ 7) with a rather regular periodicity of about 250–300 years along specific fault segments. The delimitation of these segments and analysis of their mutual interaction through time is particularly complex in the Sea of Marmara due the presence of releasing and restraining bends. This structural complexity has led to contrasting interpretations. Existing models assume: (1) the presence of a single through-going fault (Le Pichon et al., 2001); (2) a sequence of pull-apart basins with northwest-trending normal faults and ENE-trending strike-slip faults (Armijo et al., 2002); or (3) a major negative flower structure (Laigle et al., 2008). Models of seismic hazard depend strongly on the assumed tectonic model because the length of potentially rupturing seismogenic segments differs significantly between the different model reconstructions.
Here we focus on structural analysis of the Cinarcik Basin, the easternmost deep basin (about 1200 m) in the Sea of Marmara. The basin is bounded to the north by the so called Cinarcik segment of the NAF-N that connects the Izmit segment to the east with the Central High to the west (Fig. 2). This segment is inferred to have been tectonically loaded by the 1999 Mw = 7.4 Izmit earthquake which ruptured the fault through the entire Gulf of Izmit (Gasperini et al., 2011a). For this reason, describing the position and geometry of active faults and reconstructing their recent deformation history can provide key information for reliable seismic risk scenarios in the Istanbul metropolitan area.
In particular, our work will address the following topics:
- 1)
Can restraining and releasing bends along a transcurrent-type fault act as barriers to stress transfer and earthquake slip? Or do they help earthquake propagation, as suggested by Cunningham and Mann (2007)?
- 2)
Does co-seismic strain release on the Izmit segment directly affect strain accumulation on the Cinarcik segment?
- 3)
Since the Izmit segment is (and has been) oriented at an angle relative to the Central High segment towards the west (about 10°), could this difference be responsible for the compressive deformation observed at the NW edge of the Cinarcik basin?
To analyse these problems, we used a 3D scaled sandbox model which reproduced the NAF-N segmentation as imaged by geophysical data, i.e., by morphobathymetry and seismic reflection profiles across the Cinarcik Basin. Our model experiment was successful in reproducing observed deformation patterns along the basin, and therefore the eastern part of the NAFN, and gives us insights on possible stress transfer mechanisms between fault segments near the Istanbul metropolitan area.
Section snippets
Tectonic setting
The Cinarcik basin is the easternmost sub-basin of the Sea of Marmara, located ~20 km to the southeast of Istanbul (Fig. 1). It is about 50 km long, 18 km wide, and reaches a maximum depth of 1270 m. The basin is positioned on the extensional side of the prominent Tuzla fault bend (Fig. 2). A second major bend, the Istanbul bend, connects the Cinarcik basin with the Central High segment to the west (Fig. 2).
Seismic reflection profiles indicate that the southern and the northern margins of the
Analogue models: setup and material
The analogue model experiments described in this paper were designed to simulate the eastern portion of the NAF-N in the Sea of Marmara, and to obtain further insights on the relationships between the present segmentation and the evolution of the major fault (Fig. 2). In particular we focussed on how the different orientations of the fault segments can influence the kinematics of the pull-apart basin and the propagation of strain in a dextral transtensional regime. Model results were compared
Cross-sections
The setup geometry imposed to the model fault generated through transtensional and transpressional deformation with distributed and focused patterns that follow the along-strike segmentation of the fault. This is visible in the 3D perspective view of the final model step, after 7 cm of cumulative displacement (Fig. 6). Overall, the basin topography shows a graben corresponding to the Cinarcik basin depocenter, and the development of a topographic high at the western end of the basin, in
Experimental limitations
Before exploring how the analogue models correlate with marine geological/geophysical data, we should underline their possible limitations. First of all, a typical limiting factor in sandbox modelling is the lack of fluids permeating the “experimental” crust, both in host rock pores and localised within shear zones. It is known that pore fluid pressure is a major factor that shapes deformation patterns and fault activity in nature (e.g., Chester et al., 1993). In our case, being the Cinarcik
Conclusions
The Cinarcik basin, a tectonic depression along the North Anatolian Fault in the Sea of Marmara, is bounded in its northern edge by a seismogenic fault that connects the Istanbul metropolitan area to the Izmit fault segment which ruptured in 1999 during a Mw 7.4 earthquake. We successfully reproduced tectonic deformations in this area with a simple analogue model characterised by a velocity discontinuity imposed at its base by a moving basal plate that has an edge profile which reproduces the
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thank Associate Editor Ramon Carbonell, and anonymous reviewers for their constructive feedback that helped improve the quality of the manuscript. This work was fostered by visits funded by the FLOWS-COST Action ES1301 “FLOWS” (EU-COST ES 1301). Analogue models were produced in the Analogue Modelling Laboratory “E. Costa” of the Department of Chemistry, Life Sciences and Environmental Sustainability, University of Parma, Italy. We thank J. P. Morgan for his help with the science and writing.
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