Shterev et al. GEOTHERMAL RESOURCES AND SYSTEMS IN THE STRUMA (STRYMON) RIFT VALLEY (BULGARIA AND GREECE) Kostadin Ivan Dimitar Laboratory of Mineral-Thermal Waters, NCFTR, 1618 Sofia Geological Institute, Bulgarian Academy of Sciences, 1113 Sofia University of Mining and Geology, 1156 Sofia Key words: geothermal exploration, geothermal resources, Struma rift valley, Bulgaria, Greece Introduction The Struma rift valley (graben system) is one of the most interesting and rich in thermal waters regions on the Balkan Peninsula. The geothermal activity is manifested by about 100 natural and borehole thermal sources, and many temperature and hydrogeochemical anomalies provoked by hidden deposits of thermal waters in the basement, boards and sedimentary filling of the grabens. The Struma geothermal zone is a perfect subject for fundamental studies of the structural, hydrodynamic and geochemical characteristics of non-stratified (fault- and fracture-bounded) hydrogeothermal deposits and circulation systems that have developed in tectonically active granite-metamorphic terrains outside active contemporary volcanic structures and belts. Such studies are needed also for exploration and evaluation of the geothermal resources in view of their utilisation in the tourist industry, urban needs, agriculture (geothermal greenhouses), bottling industry, and other economic activities important for Bulgaria and Greece. Industrial natural C 0 2 will be possibly available in the Greek part of the valley. The present paper is a summary of our estimations and prognoses for the character, spatial distribution, amounts and quality of the geothermal resources emplaced and reproducing themselves in the Struma rift valley. Detailed studies on some particularly important and impressive hydrogeothermal deposits in different parts of the valley will be subject of special publications. 2. Methods and information base The present study is based on deductive interpretations and summaries of existing evidence about the geological structure, the hydrogeology and geothermal conditions of the Struma rift valley and the surrounding horsts. Methods and ideas already presented in our previous publications and prognoses on the thermal waters and hydrogeothermal systems and provinces (Shterev, 197 1, 1984, 1989; Shterev and Penev, 1991) are also used as well as all available data for hydrogeological studies and shallow geothermal drilling (Sidirokastro, Lithotopos, Nigrita, Serres). Precious information has been supplied also by the two deep oil-prospecting boreholes in the Serres graben Stry-2). 3. Geological setting of the Struma (Strymon) rift valley in view of its geothermal perspectives The Struma rift valley is a segment of a complex Palaeogene - Neogene fault system in the central parts of the Balkan Peninsula. After the last compressional phase in the Early Miocene, it developed in Neogene and Quaternary time under extensional conditions as a NNW-SSE continental rift (Zagorchev, 1992) extending over more than 200 km in Bulgaria and Greece (Fig. 1). The basement of the grabens is widely exposed in the surrounding horsts. It consists of amphibolite-facies metamorphic rocks (gneisses, migmatites, schists, amphibolites, marbles) of supposed Precambrian age, intruded by Palaeozoic, Late Cretaceous and Palaeogene granitoids, and partially covered by Palaeogene volcanic and sedimentary formations. The principal fault sets strike NNW-SSE (Struma Lineament), SSW-NNE to SW-NE, and WNW-ESE to W-E. The grabens are usually elongated along the strike of the lineament although some of them follow the strike of oblique or transverse fault sets Thus, the horst-and-graben pattern of the region consists of slightly elongated but almost isometric horsts surrounded by grabens The neotectonic Serbo-Macedonian Swell formed in the beginning of the Miocene, and has been subject to continuous subsidence since Late Badenian or Sarmatian times. During a first stage (Late Badenian - Sarmatian), a small area along the Struma Lineament has been the site of lacustrine sedimentation. The second stage (Maeotian?) followed to the south of the threshold of the Middle- Mesta fault zone (lineament) with a marine ingression, and to the north, with alluvial to lacustrine sedimentation. New intense vertical block movements began in Pontian time when coarse proluvial and alluvial sediments filled the grabens during the third stage (Pontian - Romanian) A fourth stage (Pleistocene and Holocene) was marked by the most prominent horst uplift The proluvial deposits of this stage form huge fans along the active Quaternary faults. The vertical amplitude (uplift) along the main neotectonic faults reached 3.5 - 4 km. The total thickness of the Neogene and Pleistocene deposits in the grabens varied from 1 to 2 km in the northern (Blagoevgrad, Simitli, Sandanski) grabens, and up to 3 - km, in the Serres Graben. The grabens are usually asymmetric, with steep (60 - bounding normal faults However, the dip of some faults gradually decreases towards south, and the extension perpendicular to the rift increases considerably In consequence, block rotation and mutual adjustments led to an en-echelon arrangement, and the grabens were tilted against the fault zones with great vertical offset Some of the fault structures and fault intersections control earthquake foci at depths of 2 - 10 km, and in rare cases, down to 45 - 50 km The fault structures and the movements in the basement and at the sides of the Struma rift valley control a generalised and deep circulation of meteoric waters The result is the formation of stratified (fault-and-fracture bounded) hydrogeothermal deposits and systems that contain and reproduce low-mineralised thermal waters with nitrogen gas composition Some of the fault structures in the Serres graben emit intense flows of endogenous Carbonated or thermal waters with increased mineralisation are formed around them Permeable sedimentary rocks at the deep stratigraphic levels of the Serres and Sandanski grabens represent potential autonomous layered reservoirs of thermal waters as well as possible recipients of hot waters that penetrate from the basement through permeable faults or lithofacies windows. Layered reservoirs of thermal waters possibly exist also in karstified marble bodies in the basement of the Serres graben, and to a lesser extent, of the Sandanski graben 1185