Gravity tectonics or plate tectonics: example of the Numidian flysch, Tunisia

Abstract

Summary: The Numidian flysch unit is the largest tectonic unit in the western Mediterranean, extending from Gibraltar to southern Italy. Displacement of the unit is supposedly to the S, a result of either gravity tectonics or plate tectonics. Both interpretations have their adherents, and to solve this dilemma the Numidian flysch in Tunisia was studied. Field data indicate surprisingly that the Numidian flysch in northeastern Tunisia (Mogods–Hedil area) did not move at all, which renders the question of the emplacement mechanism irrelevant. In northwestern Tunisia (Kroumiria) the flysch moved presumably only a few km to the S, not 50 km as was thought previously. Analysing this small displacement it appears unlikely that gravity tectonics was of importance because move ment over the fault was probably upward; moreover, sedimentological data do not indicate a high area from which sliding could have occurred. A plate tectonic interpretation cannot explain, however, why flysch movement was spasmodic instead of continuous and contemporaneous with plate movement, or why plate deformation was restricted to the thin flysch unit.