GEOTHERMAL RECONNAISSANCE OF THE MARMARA SEA REGION (NW TURKEY) - SURFACE HEAT-FLOW DENSITY IN AN AREA OF ACTIVE CONTINENTAL EXTENSION

Northwestern Anatolia is an area of significant neotectonic activity. The Northern Anatolian Fault splits into different branches and the te ctonic regime changes from translation to a combination of extension a nd translation. High seismicity as well as frequent and in places larg e hot springs are related to these tectonic features. The present stud y aims at characterising the geothermal situation of this area by deta iled heat dow density mapping and thermal spring studies, High-resolut ion temperature logs and measurements of rock thermal conductivity wer e used to determine the terrestrial heat flow density. Because of the shallowness of the available drillholes (maximum depth of 200 m), hydr aulic influences on the thermal profiles had to be taken into account. Conductive heat how density was calculated considering possible conve ctive effects due to vertical water movements. In spite of these hydra ulic influences, the temperature distribution showed stability over a time period of at least one year. Stationary models could therefore be applied. Terrestrial surface heat flow density varies regionally from 35 to 115 mW/m(2), with a mean value of 60 mW/m(2). The heat how dens ity map shows that the extensional tectonics of the region south of th e Marmara Sea is characterised by higher heat how density values. Regi ons with low tectonic activity such as parts of European Turkey or reg ions with compressional and translational tectonics have average or sl ightly lower values. The spatial pattern of thermal springs in the Mar mara Sea region does not coincide with the terrestrial heat flow densi ty distribution. Their appearance is bound to neotectonic faults (= lo cally enhanced vertical permeabilities). (C) 1998 Elsevier Science B.V . All rights reserved.