Abstract:

Although offshore multi-channel seismic reflection and multi-beam bathymetric studies were intensively carried out in the Sea of Marmara especially after the August 17, 1999 İzmit earthquake (Mw 7.4), some basic problems remain unsolved, for example whether the seafloor was cut by a double fault system or just by a single fault. The researchers advocating the double fault system assumed that most of the deformation occurred along the margins of the basins and concluded that the classical model of pull-apart basin formation along releasing bends or stepovers within an east−west-trending dextral strike-slip system prevails. An alternative and completely different proposition proposed by some other researchers suggested that the seafloor was cut by a single, continuous strike-slip fault that does not coincide with the basin margins and hence does not fit the pull-apart model. These two apparently opposing ideas in fact, are not in conflict with each other; but instead define successive events. The fault geometry in the Sea of Marmara simply reflects a transition from complex pull-apart basins into single strike-slip faults. All the pull-apart basins in the world, regardless of offset geometry, evolve progressively from narrow grabens bounded by oblique-slip link faults to wider rhombic basins flanked by terraced basin sidewall fault systems. Field observations and analogue models show that in the later stages of this widening, the cross-basin faults cut the floor of the pull-apart basins and link the offset principal displacement zones. The jogs and therefore the pull-apart basins become extinct and strike-slip faults become straight as time passes. This development history of the faults and accompanying basins is valid also for the North Anatolian Fault and the basins in the northern Sea of Marmara as well as those in the Gulf of İzmit and for the other sections of the fault on land displaying two sets of faults, the older being inactive.

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