ON THE EXISTENCE OF A GEOTHERMAL ANOMALY IN SOUTHERN PORTUGAL

A geothermal anomaly for which the heat flow density (HFD) values are more than three times the normal average value for the Earth has been reported to exist in Hercynian structures in southern Portugal in the area of the intersection of the Messejana fault and the Ferreira-Fical ho overthrust. Such high HFD values would imply that the thermal regim e of the crust is different from that found in other Hercynian regions . To try to identify the geothermal anomaly in geoelectrical terms and to understand its origin and relation to the regional geological fram ework, a magnetotelluric survey in the frequency range 0.031-128 Hz wa s carried out in the area that includes the zone of high HFD values. T he results of the survey are discussed in conjunction with magnetic, g ravimetric and seismic information as well as synthetic calculations o f temperature distributions within the crust. It is concluded that the geothermal anomaly is probably a shallow feature and does not represe nt the regional thermal regime of the crust in southern Portugal.