A First Record of a Strike-slip Basin in Western Anatolia and Its Tectonic Implication: The Cumaovası Basin

Authors: BORA UZEL, HASAN SÖZBİLİR

Abstract: The Cumaovası basin, formerly known as the Çubukludağ Graben, is located at the western end of Gediz and Küçük Menderes grabens in the west Anatolian extensional province. It is 5-17-km wide and 35-km-long, NNE-SSW-trending, asymmetric basin that was formed under the control of strike-slip and oblique-slip normal faults. The basin contains two different infills that are separated by an angular unconformity: (1) an ancient basin fill consisting of Lower Miocene-Lower Pliocene sequences which accumulated in a fluvio-lacustrine depositional setting, and deformed by NE-SW-trending strike-slip faulting; and (2) a modern basin fill consisting of Plio-Quaternary units that are controlled by synchronous strike-slip and normal faulting. Movement on these faults resulted in deposition of the gently tilted Plio-Pleistocene Görece formation and the recent horizontal alluvium. The Görece formation consists of reddish alluvial fan deposits, while the unconformably overlying recent alluvium is made up of alluvial fan, alluvial plain, and fluvial deposits. Metamorphic rocks of the Menderes Massif and rocks of the Bornova mélange form the basement to the basin fill units. Structural data from both of the northern and southern margin-bounding faults of the Cumaovası basin are consistent with oblique-slip normal faults with sinistral and dextral strike-slip components. The western margin is bounded by strike-slip faults. The geomorphologic indicators along the western margin of Cumaovası basin and kinematic analysis on the striated fault planes support two senses of movements, each having opposite kinematic indicators. Quantitative indications are presented for the polyphase evolution of the Cumaovası basin, with a change from transpressional to transtensional tectonics, due to an anticlockwise rotation of the stress tensor around vertical axis. The earliest period of evolution is represented by sinistral strike-slip faulting that is documented in outcrops along the Orhanlı Fault Zone. This transpressional regime is inferred to be responsible for the NW-SE shortening associated with NE-SW extension. The younger structural data obtained from the Cumaovası basin support a mixture of normal and strike-slip movement in a transtensional tectonic regime that formed under an approximately N-S extensional direction associated with an approximately E-W compression. This transtensional phase is still ongoing in the region, as indicated by active fault planes and focal mechanisms of shallow earthquakes. The transition from transpressional to transtensional tectonic regime in the Cumaovası basin has been a consequence of local stress field inversion in the region. This local inversion should be taken into account during discussion of the regional tectonics of western Anatolia.

Keywords: Cumaovası strike-slip basin, transpression, transtension, extension, western Anatolia

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