FAVORABLE STRUCTURAL SETTINGS OF GEOTHERMAL SYSTEMS IN THE AEGEAN EXTENSIONAL PROVINCE OF WESTERN TURKEY

The Aegean extensional province of western Turkey is a region of abundant geothermal activity currently undergoing significant extension. The geothermal activity is generally associated with enhanced dilation on ~E-W-striking normal faults induced by a complex combination of forces, including slab roll-back in the Hellenic and Cyprean subduction zones and collision of the Arabian plate with Eurasia and resulting tectonic escape of the Anatolian block. Recent magmatism has been relatively sparse in western Turkey; thus, the geothermal activity is generally not driven by magmatic heat sources, at least not within the middle or upper crust. Instead, faults accommodating deep circulation of hydrothermal fluids of meteoric origin are the primary control on the geothermal systems. It is therefore critical to evaluate which type of faults and which parts of faults are most favorable for geothermal activity. Better characterization of the structural controls is needed to enhance exploration strategies, particularly for discovering blind geothermal resources (i.e., with no surface hot springs) and for selecting the most promising drilling sites at individual geothermal fields. We have therefore been analyzing the controls on geothermal activity in several areas across western Turkey, including the Gediz-Alasehir, Simav, and Menderes grabens.

Our findings indicate a variety of favorable structural settings but with several recurrent themes. The two hottest fields in Turkey (Germencik and Kizildere) are focused near the ends of the major normal fault zone that bounds the Menderes graben. We suggest that this fault zone breaks into multiple splays, or horsetails, as it terminates, thus generating a belt of higher fracture density and permeability that accommodates significant fluid flow. The Eynal geothermal system near Simav is hosted by a complex normal fault system with multiple steps and intersections. Several fields, including Germencik and Kursunlu Canyon near Salihli, occur at the intersections between major graben-bounding normal faults and oblique-slip, transversely oriented transfer faults. The Kursunlu Canyon field appears to occupy a small dilational jog, or pull apart, at the intersection of a transverse sinistral-normal fault and currently inactive Gediz detachment fault. However, the main geothermal upwelling for Kursunlu Canyon may lie a few kilometers away from surface springs along a major step in the active, steeply dipping graben-bounding fault zone, where multiple fault splays within the step-over generate a subvertical zone of higher fracture density and permeability. Upon intersecting the detachment fault, fluids in the upwelling may follow permeable carbonate and breccia along the gently dipping detachment rather than rising to the surface directly above the upwelling.

Similar to the Great Basin in the western USA, most systems in western Turkey occupy discrete steps in fault zones or lie in belts of intersecting, overlapping, and/or terminating faults. The similarities in favorable settings between these two active areas of continental extension suggest that conceptual exploration models can be developed for geothermal activity in particular tectonic settings. Further characterization of the structural controls on geothermal activity will help to reduce the risks of drilling while also facilitating discovery of blind geothermal systems.