An overview on the main stratigraphic and structural features of a geothermal area: the case of Nazilli-Buharkent section of the Büyük Menderes Graben, SW Turkey

Abstract

The study area is Nazilli-Buharkent section of the Büyük Menderes Graben. This is one of the most active intracontinental extensional structures shaping the south-western Anatolian graben–horst system, which comprises the southwestern frontal part of the Anatolian platelet. The Büyük Menderes Graben is about 3–30 km wide, 170 km long and approximately E–W-trending depression included in the back-arc section of the northerly-dipping south Aegean-Cyprian subduction zone. The Büyük Menderes Graben of episodic origin has two morphotectonic configurations: (1) a wider, uplifted, dissected and deformed initial configuration of late Early–late Middle Miocene age; and (2) the narrower, linear, undeformed and continuous recent configuration of Quaternary age. These two configurations are here termed as the Büyük Menderes palaeotectonic graben and the Büyük Menderes modern (neotectonic) graben, respectively. These two grabens are represented by two sedimentary packages separated by an intervening angular unconformity. The 1.1-km-thick older sedimentary package is deformed (steeply tilted to folded) and consists of, from bottom to top, unsorted boulder-block basal clastics and coal-bearing flood plain to lacustrine deposits of late Early–late Middle Miocene age. The younger sedimentary package is undeformed and consists of very thick (up to 0.9 km) debris flow and fluvial deposits of Quaternary age. The palaeotectonic configuration of the Büyük Menderes Graben is bounded by the Hasköy-Künüpe and İğdecik structural segments of the Büyük Menderes detachment fault. However, the modern configuration of the Büyük Menderes Graben is bounded by the Kuyucak and Menderes high-angle normal fault zones, composed of numerous fault segments, which cut and displace deeply the detachment fault. Hot water springs occur along the traces of the detachment faults. But they are not observed along the traces of modern graben-boundary faults. This reveals that the low-angle detachment fault is being still used by the geothermal fluids. Indeed, it is a fact that the geothermal fluids are shared by both the older detachment and younger high-angle normal faults at their intersection at depth, because the faults are the most suitable ways for the underground circulations of both cold meteoric waters and geothermal fluids, i.e. the geothermal system in south-western Turkey is fault-controlled. Consequently, the geothermal potential of this extensional domain is quite high based on the criteria such as the active tectonic and related faults, sources of high-heat, reservoir rocks with high porosity, reservoir cap rocks with no or low permeability and cold meteoric water supply enough. In order to obtain huge volume of geothermal fluids, first of all, whole steps of the geothermal exploration have to be completed, and then the site of borehole(s) has to be chosen so that it penetrates the intersection of low-angle detachment fault, high-angle normal fault(s) and the reservoir rocks overlain by the thick and impermeable cap rock(s).