DIFFERENT STYLES OF REMAGNETIZATION IN DEVONIAN SEDIMENTS FROM THE NORTH-WESTERN SAHARA (ALGERIA)

Sediments of Devonian age located in three areas of North Africa have been affected to different degrees by the Hercynian Orogeny. They show various remagnetizations of Carboniferous, Permian and Pleistocene age. In the Ben-Zireg anticline near the Variscan front in Algeria, the main phase of magnetic overprinting occurs during the Tournaisian. It is superimposed on a Famennian primary direction carried by magnetite. In the Beni-Abbes area to the South of the Variscan belt, griotte limestones of Famennian age show a strong recent remagnetization with few residual Permian directions. This overprinting shows a shallower inclination than the present-day field, with a small westward declination. Remagnetization is associated to magnetic transformation of sulphides. The transition of sulphides into magnetite-type minerals, as revealed in the thermomagnetic curves from rocks of the Variscan Belt, preserved the carrier against future magnetic overprinting. Compared to partially remagnetized rocks, this mechanism is referable to recrystallization during deformation.