A N-S-trending Active Extensional Structure, the Şuhut (Afyon) Graben: Commencement Age of the Extensional Neotectonic Period in the Isparta Angle, SW Turkey

Authors: ALİ KOÇYİĞİT, ŞULE DEVECİ

Abstract: The Şuhut graben is an 8-11-km-wide, 24-km-long, N-S-trending, active extensional structure located on the southern shoulder of the Akşehir-Afyon graben, near the apex of the outer Isparta Angle. The Şuhut graben developed on a pre-Upper Pliocene rock assemblage comprising pre-Jurassic metamorphic rocks, Jurassic-Lower Cretaceous platform carbonates, the Lower Miocene-Middle Pliocene Afyon stratovolcanic complex and a fluvio-lacustrine volcano-sedimentary sequence. The eastern margin of the Şuhut graben is dominated by the Afyon volcanics and their well-bedded fluvio-lacustrine sedimentary cover, which is folded into a series of NNE-trending anticlines and synclines. This volcano-sedimentary sequence was deformed during a phase of WNW-ESE contraction, and is overlain with angular unconformity by nearly horizontal Plio-Quaternary graben infill. Palaeostress analyses of slip-plane data recorded in the lowest unit of the modern graben infill and on the marginal active faults indicate that the Şuhut graben has been developing as a result of ENE-WSW extension since the latest Pliocene. The extensional neotectonic period in the Isparta Angle started in the latest Pliocene. All margins of the Şuhut graben are determined and controlled by a series of oblique-slip normal fault sets and isolated fault segments. More active faults which are capable of creating destructive earthquakes with magnitudes of Mw=6.3 and Mw=6.5 include the Ağzıkara, Güneytepe, Çobankaya and the Yarışlı faults, as in the occurrence of two devastating historical earthquakes, those of 1766 and 1862, which were seismic events with intensity of VII and X, respectively. The Yarışlı fault, however, still remains seismically inactive. In addition, the finer-grained modern graben infill is thixotropic and so these active faults and the finer-grained alluvial sediments have to be taken into account in both earthquake risk analysis and city planning design in Şuhut County.

Keywords: Isparta Angle, fold, Şuhut graben, oblique-slip normal fault, palaeostress analysis, SW Turkey

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