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.