Abstract:

The variation in the motion sense of Anatolian platelet in Aegean Sea resulted in a strike-slip tectonic regime and related neotectonic domain, the central to northern Aegean neotectonic province, which also includes both western Marmara Sea and Biga Peninsula. Our study focuses mostly on the Gülpınar-Tuzla earthquake area located at the southwestern tip of Biga Peninsula, which is controlled by the southern major strand of the North Anatolian Fault System (NAFS). The strand consists of two sections, the onshore Biga and offshore Babakale-Skyros sections. Both sections are seismically very active. The Gülpınar-Tuzla earthquake area is composed of the Paleozoic metamorphic rocks, the Oligo-Miocene granitoid pluton, Lower-Middle Miocene calc-alkalic volcanic rocks and the Upper Miocene-Pliocene sedimentary sequence. All of these rocks, which formed and deformed (folded to tilted) in palaeotectonic period, are overlain with an angular unconformity by the Quaternary neotectonic basin fill, that is nearly flat-lying except for the faulted basin margins. Both the onshore and offshore sections of the southern strand are linked to each other in terms of the structures characterizing the Babakale pull-apart basin and the Gülpınar-Tuzla earthquake area. The latter is shaped by the NE-trending Gülpınar (GFZ) and Yenice-Gönen Fault Zone (YGFZ), the ENE-trending Edremit fault zone (EFZ), the WNW-trending Tuzla Fault Zone (TFZ) and three strike-slip basins (Ayvacık, Behramkale and Tuzla basins) developed along them. Some segments of both the TFZ and GFZ were reactivated by the occurrences of seven moderate-to small-sized independent earthquakes and related aftershocks over 2760. Five of the independent earthquakes have an origin of oblique-slip normal faulting, while the rest two seismic events are strike-slip faulting in origin. Focal mechanism solution diagrams of these two groups of earthquakes reveal that the Gülpınar-Tuzla area is under the control of a strike-slip neotectonic regime, which commenced in Early Quaternary time owing to the major inversion in extensional palaeotectonic regime. This is also supported by the palaeostress analysis of slip-plane data measured on fault slickensides. The uniform slip rates on both the YGFZ and EFZ are 10.8 mm/yr and 7.3 mm/yr, respectively.

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