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Role of salt in the geometry of extensional ramp synclines: insights from analogue models

Abstract

The widespread extensional deformation that took place in the Western Europe and north-Atlantic during the Jurassic to Cretaceous resulted in the formation of several rift systems that, in some cases, evolved to oceanic realms (i.e., Tethys and Atlantic oceans). Later, during uppermost Cretaceous to Cenozoic some of these basins were partially inverted and in some cases incorporated into different orogens as the Pyrenees.

Some of these extensional basins show a broad syncline-shape filled by thick sedimentary successions deposited overlying an extremely thinned crust (i.e., Parentis, Voring, More, Orphan, Organyà or Columbrets basins). The development of these syncline basins has been associated to the displacement of low-angle lithospheric-scale extensional faults that, with a ramp/flat geometry, controlled the formation of the so-called hangingwall ramp synclines. The shape and kinematics of such lithospheric-scale faults have been usually established using the architecture of the syn-kinematic layers that reflect changes in the fold geometry and depocenter location. Almost all of these interpretations assume a complete coupling of the hangingwall rocks and a layer parallel flexural slip deformation mechanism. But these basins could include evaporitic layers, which act as an effective décollement and control the development of salt structures. During a subsequent shortening stage, the main extensional faults were inverted and part of this contractional deformation was also observed by the evaporites acting as a contractional detachment. When this occurs, the extensional basins are transported and in the most extreme cases incorporated into the orogen (eg. Organyà Basin in Pyrenees).

The GEOMODELS Analogue Laboratory (University of Barcelona) in combination with the Numerical and 3D Modelling Laboratory provides to the GEOMODELS Research Institute a powerful tool that allow to simulate a wide variety of tectonic settings: extension, compression, strike-slip, basement faults, inversion tectonics, double-wedges, salt tectonics, gravitational movements, etc… The analogue experiments presented in this work have been carried outthanks to the modular modeling table of this laboratory.

Using an experimental approach (scaled sandbox models) the aim of this work is threefold: 1) to determine the geometrical features of the hangingwall above convex upwards ramp of a low angle extensional fault, and consequently, of the syn-kinematic layers filling the developing ramp syncline; 2) to decipher the role played by the presence of a pre-kinematic salt layer in the development of these syncline basins; and 3) to characterize the contractional deformation that took place in the syncline to solve the role of the detachment layer during the inversion. To achieve this goal and experimental program including four different sand-box models has been carried out in the GEOMODELS Analogue Modelling Laboratory, University of Barcelona. The experimental setup consisted in an horizontal box with two end walls, one of them fixed whereas the other was moved by a motor-driven worm-screw at a displacement rate of 4 mm/hour. A rigid wooden footwall formed by two panels with different dips flattening at depth was attached to the fixed wall. Above the footwall, a flexible plastic sheet stuck to the moving wall acted as a detachment. The hangingwall was modeled with two different infill configurations: one only using dry quartz-sand and other including a silicone layer at the upper part of the pre-kinematic unit. The model that includes silicone layer was repeated applying two different amounts of extension and also inversion.

Our results show that, effectively, the flat/ramp shape of the low angle extensional fault controls the geometry and the kinematic evolution of hangingwall ramp synclines. Regarding this, the experiments also demonstrate that the presence of a viscous silicone interlayer changes significantly the kinematic of the basin: the sand successions located above the silicone layer show a different structure with a different migration path of the basin depocenters. Thus, in models without silicone, antithetic faults formed in the hangingwall rollovers affect the entire model and the syn-kinematic layers show a lateral migration of the depocenters towards the fixed wall. By contrast, in models with silicone, hangingwall faults do not propagate above this layer, there are salt structures developed in both syncline margins and the location of the depocenters remain invariable until the depletion of the viscous layer in the syncline limbs. After this depletion that produces the formation of primary welds, the kinematics of this model becomes similar to the model without silicone. During the inversion, models show that low shortening produces basically the contractional reactivation of the main fault uplifting the synclinal basin. In this scenario, if salt is rather continuous, occurs an incipient reactivation of the silicone layer as a contractional detachment. By contrast, high shortening produces the total inversion of the detachment faults and the pop-up of the extensional basin.