| Abstract |
Similar to the western Great Basin of the USA, western Turkey is a region of abundant geothermal activity currently undergoing significant extension but with relatively little volcanism. Thus, the geothermal activity in western Turkey 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 geothermal systems in this region. Much of the activity is associated with enhanced dilation on ~E-W-striking normal faults induced by a complex combination of forces, including slab roll-back in the Hellenic subduction zone and collision of the Arabian plate with Eurasia. Although faults control most of the geothermal activity in western Turkey, few detailed investigations have been conducted on the specific structural controls of individual fields. Because knowledge of such structures may facilitate development of exploration models and strategies, we have embarked upon a regional study of the controls on geothermal activity, which includes detailed analysis of fields in the Gediz-Ala?ehir graben and reconnaissance studies of several others. Our findings indicate a variety of structural controls in western Turkey 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. Several other fields, including Kur?unlu Canyon and Çamurhamami near Salihli occur at dilational fault intersections between major graben-bounding normal faults and oblique-slip, transversely oriented transfer faults. These systems occupy small dilational jogs, or pull aparts, along transverse faults near the intersection with a major detachment fault, as the transverse faults are refracted across the interface between basement rocks in the detachment footwall and Neogene sediments in the hanging wall. The Eynal geothermal system near Simav is hosted by a complex normal fault system with multiple steps and intersections. Similar to the Great Basin (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. |