WORKING GROUP 9 MARCHESINI, PISTOLESI, BOLOGNINI 
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variable in width from few meters to some hundreds of meters; less frequently 
they correspond to clean-cut single surface faults. 
The regional frame 
On fig. 2 it is possible to distinguish a Southern portion with Mesozoic 
carbonate reservoirs exposed at the surface over few basement patches, and a 
Northern portion where the shaly and clastic formations of the cap-rock extend 
uninterrupted. 
Base-metal mineralisations occur in the Southern half. There, drilling is 
reported to have penetrated granitoid rocks at shallow depth. Natural occur 
ences of steam (now captured) reach the surface only in the cap-rock area, 
and are rather concentrated in the central part of the region. 
The gravity picture (fig. 4) is consistent with the idea that the region lies on 
a large basement platform, limited to the NE., by a steep negative gradient, 
towards the great trough of the Appennines and the Po valley. The steam-area 
itself lies on a local negative anomaly trending NW-SE. 
Another very remarkable feature shown by fig. 1, is the uplift of the Plio 
cene beds, as expressed by iso-uplift contours. The agreement between the 
Bouguer map and the Pliocene uplift picture is striking. The location of the 
steam-area in relation to them could hardly be referred to chance. All the 
more so if it is observed that the newly discovered Amiata steam-field also lies 
on a similar negative gravity anomaly and similar Pliocene uplift. In fact, 
there occur two steam-producing areas showing the following coincident 
features: 1. Local negative anomaly; 2. Local Pliocene uplift; 3. Local thermic 
positive anomaly. One of them (Amiata) shows, in addition, evidence of acidic 
effusions during early Quarternary. 
Fracture analysis from airphotographs 
The airphotographs used for interpretation were taken with a Fairchild 
camera 24 X 24 cm, with focal length of 154 mm, at the average scale 1 : 12,500. 
An excellent base-map was afforded by the 1 : 25,000 scale official map of 
Italy by the I.G.M. The photo interpreters were geologists very familiar with 
the area. 
Visible fractures, and linears most probably due to fractures, were annotated, 
under the stereoscope, on the photographs, and then transferred to the base 
map. Azimuths and lengths were measured and statistically processed, at first 
by surface units of 0° 2' Lat. and Long, (approximately 9.5 sq.km, rectangles) 
to build the regional picture. Groupings by stratigraphic or other units were 
computed later according to various purposes of the investigation. A major 
division was set at the length value of 1 km, computing separately the short 
(micro) and the long (macro) fractures. The computed values were plotted as 
percentages on polar diagrams, by classes of 10° azimuth. The diagrams 
were set at the centers of the corresponding lat.long, rectangles on a geographic 
grid. The upper half of each diagram carries the values obtained from macro-