Extensional Neotectonic Regime through the NE Edge of the Outer Isparta Angle, SW Turkey: New Field and Seismic Data


Abstract: The Akşehir-Afyon Graben (AAG) is a 4-20-km-wide, 130-km-long NW-trending depression that separates central Anatolia in the NE and the Isparta Angle (IA) in the SW. Its southwestern margin-bounding fault determines the northeast edge of the outer IA that was previously interpreted as a compressional neotectonic structure, whereas our field evidence and recent seismic data substantiated that it is an oblique-slip normal fault characterising an extensional neotectonic regime. The AAG has an episodic and asymmetrical evolutionary history. This is indicated by two superimposed graben infills and structures. The older infill is folded, thrust-faulted and Early-early Middle Miocene in age. The younger infill is undeformed (nearly horizontal), Plio-Quaternary in age, and overlies the older infill with angular unconformity. Total throw amounts accumulated on both SW and NE margin-bounding faults, namely the Akşehir Master Fault (AMF) and the Karagöztepe Master Fault (KMF) since the Late Pliocene and Early Pleistocene, are 870 m and 200 m, respectively. Assuming a uniform motion, these values indicate motion rates of 0.3 mm/yr and 0.2 mm/yr, respectively, and the asymmetrical nature of the AAG. Kinematic analysis of surface slip data of both the AMF and KMF showed an oblique-slip motion with a minor right-lateral strike-slip component, and a NE-SW- directed extension. They also fit well with results of focal mechanism solutions of two recent seismic events, namely the 2000 December 15 Sultandağı (Mw=6.0) and the 2002 February 3 Çay (Mw=6.5) earthquakes. They have been sourced from the reactivation of the Akşehir-Pınarkaya and Sultandağı-Maltepe sections of the AMF. The Çay earthquake caused devastating damage to structures and loss of life in the region. The Çay earthquake has also led to the development of ground ruptures and surface deformation. The geometry of the ground ruptures and focal mechanism solutions of both earthquakes proved once more that the southern margin-bounding fault of the AAG or the northeastern edge of the IA is an oblique-slip normal fault. Consequently, all of these field and seismic data reveal an extensional neotectonic regime through the northeast edge of the outer IA despite the previously reported compressional neotectonic regime.

Keywords: Akşehir-Afyon Graben, Isparta Angle, extensional neotectonic regime, Sultandağı and Çay (Afyon) earthquakes

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