Evidence for the reactivation of a pre-existing zone of weakness and its contributions to the evolution of the Küçük Menderes Graben: a study on the Ephesus Fault, Western Anatolia, Turkey

Abstract

Detailed geological mapping at the western end of the Küçük Menderes Graben (KMG) in western Turkey reveals two main sedimentary packages separated by a regional unconformity: (1) Miocene units and (2) KMG basin-fill deposits of Quaternary age. The older basin-fill is represented by colluvial/alluvial fan facies association, which was controlled by a NW–SE-trending and SW-facing dip-slip normal fault (the Ortaburun Fault) and the conformably overlying fluvial and lacustrine deposits. The younger basin-fill, besides being characterised by lateral colluvial fan and axial river sediments deposited under the control of the Ephesus Fault, a southern basin-margin fault character, is also responsible for the evolution of the modern KMG. The analysis of structural, stratigraphic and sedimentological data indicates that three different deformation phases dominated during the Miocene to Recent time in the region: (i) during Early–Middle Miocene Phase 1 deformation (D1), the NW–SE-trending Ephesus Fault moved as a left-lateral strike-slip fault and occurred within the NE–SW-trending dextral shear zone of the İzmir-Balıkesir Transfer Zone (İBTZ) time; (ii) Middle Miocene Phase 2 deformation (D2) was associated with Ortaburun Fault, the northern basin-margin fault of the ancient basin. During this phase, the sinistral Ephesus Fault was reactivated as a dip-slip normal fault; and (iii) Phase 3 deformation (D3) was related with the formation of the KMG during the Quaternary and the Ephesus Fault forms the southern margin of the modern KMG. Structural evidence and computed palaeostress results show that the main extension direction during phase 1 and phase 3 has changed from north-west to north-east direction. This further suggests an approximately 60° rotation around the vertical axes and is attributed to the fact the region lies within the strike-slip dominated İBTZ.