THE SOUSS BASIN (TIZI-NTEST FAULT ZONE, W ESTERN HIGH ATLAS, MOROCCO)- RESULT OF A TECTONIC INVERSION CONTROLLED BY A DEEP DETACHMENT

Analysis of seismic reflection profiles and boreholes performed in the Souss Basin, which constitutes the southern foreland of the western H igh Atlas and the western continuation of the Tizi n'Test Fault Zone, allows an accurate inventory of several structures related to the rift ing and opening of the Central Atlantic, and inversion related to the collision of Africa and Europe to be made. The most conspicuous struct ure is a deep north-northwestward gently dipping detachment, from whic h the El Klea and Biougra Faults diverge, striking northeast with a no rthwesterly dip. These faults are normal, have a Triassic to Middle Ju rassic age, and delimit half-graben structures tilted to the south-sou theast. The amount of extension (beta) is at least 1.19. The obliquity of the faults with respect to the trend of the final rift gives them appearance of transfer faults. The Tertiary to present-day inversion i s outlined by the reactivation of the detachment and some associated f ormer normal faults as the El Klea Fault, and by the creation of new d ecollement levels at the base, or within, the Triassic evaporites. The present study supports the idea that most of the High Atlas structure s may be largely controlled by upper to mid-crustal gently northward d ipping detachment planes. (C) 1997 Elsevier Science Limited.