EXTENSION AND INVERSION OF SALT-BEARING RIFT SYSTEMS (Invited Presentation)

We used physical models to investigate structural evolution of extensional rifts containing syn-rift evaporites and their subsequent inversion. Extension was generated by a stretching rubber sheet covered with a thin silicone detachment. A series of en-échelon weak slabs (silicone polymer) were placed above the basal detachment to focus extension and form several offset graben systems, with interlocking transfer zones. Our salt analog was placed in these offset graben and allowed to settle. A uniform salt fringe was then emplaced that spanned the entire rift system, and this was covered with a thin sedimentary roof before continued extension.

During post-salt extension, deformation in the subsalt section remained focused on the graben-bounding fault systems whereas deformation in suprasalt sediments was mostly detached, forming a sigmoidal extensional minibasin system across the original segmented graben array. Little brittle deformation was observed in the post-salt section. Sedimentary loading from the minibasins drove salt up onto the footwalls of the subsalt faults, forming diapirs and salt-ridge networks on the intra-rift high blocks. Salt remobilization and expulsion from beneath the extensional minibasins was enhanced along and up the major relay/transfer zones that separated the original sub-salt grabens, forming major diapirs in these locations.

Inversion of this salt-bearing rift system produced strongly decoupled shortening belts in basement and suprasalt sequences. Suprasalt deformation geometries and orientations were strongly controlled by the salt diapir and ridge network produced during extension and subsequent downbuilding. Thrusts were typically localized at minibasin margins where the overburden was thinnest and salt had risen diapirically on the horst blocks. Subsalt shortening consisted of strongly inverted sub-salt grabens, which uplifted the suprasalt minibasins, and new popup structures. Primary welds formed as suprasalt minibasins touched down onto the inverted graben. Model geometries compare favorably to examples from the Moroccan High Atlas.