Structural and dynamic features of a transtension basin: a late Ypresian example on Tunisian Platform

In Tunisia, Ypresian carbonate deposits occur on a platform preserving vest ige of an inherited fault network. The fault network delineates blocks of d ifferent sizes accounting for the platform morphology and in turn for varia tion in thickness of the Ypresian sequence. The nummulitic limestone carapa ce of the Ypresian sequence is fractured by faults and joints of various or ientations which are systematically sealed by marry beds of early Lutetian age. This indicates that the fault network was reactivated during late Ypre sian. Geometric and kinematic study indicates that this strike slip late re activation is accomodated by normal fault. These structures originated in s oft sediment undergoing diagenesis. Depending on bed competency, both ducti le and brittle deformation features were recognized in the fault. This supe rficial tectonic event represents, a recent reactivation of ancient fractur es cartographically located on the boundaries of late Ypresian megablocks. Kinematic study of the deformation within and along the boundaries of one o f these blocks, (Ousseltia block), indicates a late Ypresian, early Lutetia n strike-slip distensive faulting dynamic. Stress-field orientation indicat es a rapid re-orientation in time from a predominantly NW-SE extensional te ctonic to a NE-SW extentional event. Stratigraphic dating of that tectonica l crisis coincides with a turbulent period of relative motions between Euro pe and Africa.