The EGT'85 seismic experiment in Tunisia: a reconnaissance of the deep structures

Abstract

In July 1985, deep seismic measurements were carried out in close European-Tunisian cooperation along the extension of the main European Geotraverse (EGT) seismic refraction line through Tunisia up to the Saharian platform. The execution and results of this experiment are described and presented in this paper. Eight shots from seven different shotpoints were recorded by 120 mobile stations deployed in a network of nine reversed profile segments, with a total surveyed length exceeding 1300 km. The 6-km/s isoline is found everywhere at great depth, usually between 10 and 15 km. The main feature of the crust, sediments excluded, is its low average velocity (6 km/s), with no clear evidence for any high-velocity lower crust, except maybe in the Kairouan area. For the most part of continental Tunisia, the Moho depth varies between 30 and 35 km, with a maximum depth of 37 km in the Kasserine area. To the north and northeast, the continental crust merges into the thinned crust of the Sardinian Channel and Pelagian Sea. This crust has a typically higher mean velocity and a minimum thickness of only 13 km in the central Sardinian Channel, where the Moho depth is 21 km. In the upper mantle, we derive consistent velocity values in the 7.9–8.1-km/s range. Offsets observed in P n-wave travel time curves may indicate steps in the Moho beneath the Tellian chains. Finally, observations of two sea shots at large distance (250–500 km) reveal the presence of an upper-mantle reflector under central Tunisia, at a depth of 87 km, with an unusually high apparent velocity of 9.4 km/s below.