NUMERICAL MODELING OF SALT DIAPIRISM - INFLUENCE OF THE TECTONIC REGIME

In the context of sedimentary basin mechanics, salt diapirism has usually been described as a phenomenon initiated and driven by gravity instabilities associated with buoyant forces or differential loading. This result holds essentially for an approximation of linear theologies of both salt and overburden. This has been contested by recent analog models and analytical studies: buoyancy alone cannot drive salt diapirism through a brittle overburden. Regional extension or compression could be a necessary mechanism likely to promote salt diapirism. Numerical models are presented here to describe the relative effects of gravity and lateral forces on the dynamics of salt diapirism. The numerical scheme is based on a viscoplastic approximation of deformations and an updated Lagrangian description of kinematics, which allows a study of large non linear deformations of salt bodies and the evolution of material interfaces. This scheme takes into account brittle and ductile deformations of sediments. Moreover, it describes the initiation and evolution of shear bands which play a crucial role in basin mechanics and, especially in the dynamics of salt diapirism. We show why extension is the most propitious driving mechanism of diapirism: necking and faulting induced by extension promote and drive salt diapirism. In the case of a brittle overburden, this result holds with or without buoyancy forces. This demonstrates that buoyancy, which alone is not sufficient to break a brittle overburden, is not a necessary mechanism either. Buoyancy, however, enhances diapirism induced by lateral forces. In contrast with extension, salt diapirism during compression is strongly constrained by the rheology of the overburden: a brittle mode of deformation favours faulting rather than buckling. Induced thrusting and thickening of the overburden inhibit massive diapirism and favour the confinement of small salt plugs. Finally, the post-extension stability of a mature diapir produced by extension is presented.