Caracteres structuraux et dynamiques d'un bassin en transtension; la plate-forme tunisienne a l'Ypresien terminal

Abstract

Abstract In Tunisia, Ypresian carbonate deposits occur on a platform preserving vestige of an inherited fault network. The fault network delineates blocks of different sizes accounting for the platform morphology and in turn for variation in thickness of the Ypresian sequence. The nummulitic limestone carapace of the Ypresian sequence is fractured by faults and joints of various orientations which are systematically sealed by marly beds of early Lutetian age. This indicates that the fault network was reactivated during late Ypresian. Geometric and kinematic study indicates that this strike slip late reactivation is accomodated by normal fault. These structures originated in soft sediment undergoing diagenesis. Depending on bed competency, both ductile and brittle deformation features were recognized in the fault. This superficial tectonic event represents, a recent reactivation of ancient fractures cartographically located on the boundaries of late Ypresian megablocks. Kinematic study of the deformation within and along the boundaries of one of these blocks, (Ousseltia block), indicates a late Ypresian, early Lutetian strike-slip distensive faulting dynamic. Stress-field orientation indicates a rapid re-orientation in time from a predominantly NW-SE extensional tectonic to a NE-SW extensional event. Stratigraphic dating of that tectonical crisis coincides with a turbulent period of relative motions between Europe and Africa.