Experimental approach (analogue modelling) of thin- to thick-skinned inversion of extensional basins with pre-rift salt

The structural style of inverted rift basins is controlled by the inherited structures and stratigraphic elements but also by the presence of salt layers or welded equivalents. Salt acts as a main detachment during extension and, depending on its thickness, different degrees of linkage develop between the basement and overburden. The presence and distribution of salt structures, the linkage between the basement and overburden, and the continuity of salt on these salt-bearing rifted basins have a strong impact on thick- to thin-skinned deformation during inversion. As the weakest rock of the basin infill, salt acts as a contractional detachment and buried diapirs rejuvenate during early inversion. With increasing shortening thick-skinned deformation folds and uplifts the basins while the diapirs are squeezed and welded by thin-skinned deformation.

Using an approach based on systematic analogue models, this work analyses how extensional basins develop above a pre-rift salt layer and how the inherited salt structures evolve during subsequent inversion. A first set of models only affected by extensional deformation was carried out examining how the variation of different parameters such as salt and overburden thicknesses impact the structural style of salt structures developed during thick-skinned extension. Afterwards, some of these models were repeated to understand how pre-existing extensional and salt structures condition the evolution during total inversion tectonics. The experimental apparatus consists of five metal fault blocks simulating a domino basement-fault system that rotate counter-clockwise during extension and clockwise during inversion. Deformation was transferred to the blocks by a motor worm-screw at a constant velocity of 4.6 mm/h until reaching 10 cm of total extension. During the inversion phase, the same velocity was applied until reach total inversion of the basins. A layered unit of sand capped by a uniform-thickness polymer layer and additional layers of sand simulated the pre-kinematic unit. While different sand layers were added during extension, no syn-inversion sedimentation was considered.

The results of this study show that the structural style during inversion is highly conditioned by the inherited extensional configuration but also by the salt thickness that condition the degree of coupling/decoupling of the pre- and syn-kinematic successions. The study also revealed that the thickness of the overburden has a minor impact during the inversion of the basins. Such is the case that in models with either thin or thick overburden succession, the extensional geometry might be preserved if the salt is thick independently of the overburden thickness. Contrary, models with a thin salt layer are characterized by a total inversion of the ramp-syncline basin that as an inversion anticline is developed, crestal collapse extensional faults minimize the developed structural relief. Finally, the analogue modelling allowed to understand how compression caused primary weld reactivation, diapir rejuvenation, salt thickening and/or thrust emplacement. The reactivation of some of these salt-related structures is extremely impacted by the salt thickness distribution that resulted from the extensional phase. Therefore, to characterize structural style and understand the evolution of the basin it is needed an understanding of the inherited salt-related structures.