Formation and subsequent inversion of extensional salt-detached ramp-syncline basins developed above ramp-flatramp faults

Abstract

The widespread extensional deformation that took place during Jurassic to Cretaceous times in Western Europe and the North Atlantic margin resulted in the formation of several rift systems. Some of the resulting basins associated with these rifts show broad synclines detached on pre- or synkinematic Permian or Triassic salts and filled by thick sedimentary successions. They are rarely fault bounded, instead they are bounded by salt structures that are generally parallel to the major subsalt structures. As such, the formation of these extensional systems requires the presence of i) a subsalt extensional fault with significant dip changes and ii) an evaporitic unit above the extensional fault, which partially or completely decouples the suprasalt basin from the subsalt extensional fault. Moreover, the complexity of these scenarios further increases when some of these basins, during latest Cretaceous and Cenozoic times, were partially inverted or incorporated into fold-and-thrust belt.Synclinal basins have a significant exploration potential when their extensional geometry is preserved and when they have undergone positive tectonic inversion. However, in some cases, their subsalt geometry may not be fully recognizable, especially when the imaging of the subsalt seismic data is poor. The shape and kinematics of such faults have usually been established using the architecture of synkinematic units and by assuming complete coupling of the hangingwall rocks. Therefore, there are fault interpretations that do not consider the role of deep salt layers, which clearly act as an effective detachment, decoupling sub- and suprasalt deformations.This thesis provides a review of the formation and evolution extensional synclinal basins and an overview of the widespread-recognized salt-detached ramp-syncline basins. To obtain a deeper understanding of the geometry and kinematic evolution of these salt- detached ramp-syncline basins, the principal aim of this thesis is to decipher the factors that are involved in the development of these basins during both extension and inversion.To achieve this goal we carried out a deep investigation about the state of the art of extensional analog models, then we performed two experimental programs consisting in different sandbox models. The experimental results, together with the new used analytic tools, reveal that the kinematic evolution of the salt-detached ramp-syncline basins during extension and inversion depends on the interaction of different factors that may function simultaneously. Our results show that the main structure formed at the end of the extension is a salt-detached ramp-syncline. Its formation and evolution not only depends on the subsalt fault type and geometry, but also on the subsalt fault displacement, salt migration, salt thickness and degree of decoupling. In addition, the inversion of these salt-detached ramp- syncline basins is also controlled by syninversion sedimentation, erosion and the kinematics of the inherited extensional and salt structures (or its equivalent welding). The inversion of these basins resulted in a major thick-skinned fault-bend anticline with thin-skinned contractional structures.Our results are used to improve the interpretation of the Mesozoic Columbrets Basin (Western Mediterranean) and serve a guide for seismic sections of inverted Mesozoic salt- detached ramp-syncline basins on the Atlantic margins, where subsalt faults are not well- imaged, and thus the suprasalt geometries must be used to infer the subsalt structure.

L’extensió Juràssica – Cretàcica Inferior que va tenir lloc a l’Europa Occidental i al marge Nord Atlàntic va resultar en la formació de nombrosos sistemes de rift. Les conques associades a aquests rifts es mostres com a sinclinals amples, reomplerts per grans successions sedimentaries i desenganxats a un nivell salí (Pèrmic o Triàsic Superior). Aquestes conques rarament estan directament associades a falles, en canvi es troben limitades per estructures salines que generalment son paral∙leles a les estructures del basament. Per tant, la formació d’aquests sistemes extensionals requereix la presencia de i) una falla de basament extensiva amb diferents cabussaments i ii) una unitat evaporítica a sobre d’aquesta falla, la qual genera un desacoblament parcial o total respecte la conca i la falla extensiva. A més a més, la complexitat d’aquests contextos geològics augmenta quan algunes d’aquestes conques es troben parcialment invertides o fins i tot incorporades en cinturons de plecs i encavalcament durant el Cretaci Superior i Cenozoic.Les dades sísmiques i de camp permeten precisar la geometria d’aquestes conques sinclinals, però no la de les falles extensionals que las van originar. La geometria i la cinemàtica de las falles extensives associades ha aquestes conques, normalment s’ha establert utilitzant la geometria dels materials més superficials i assumint un acoblament total entre el basament i la cobertora. Aquestes interpretacions no consideren el paper que pot juga un nivell salí profund que clarament desacobla la deformació entre el basament i la cobertora.L’objectiu principal d’aquesta tesi es investigar els principals factors que controlen la formació i evolució de les conques sinclinals, utilitzant com a eina principal la modelització analògica. Per assolir aquest objectiu s’ha realitzat una revisió sobre conques sinclinals i una revisió de models analògics extensius realitzats fins ara. A partir d’aquests antecedents s’han realitzat dos programes experimentals. Els resultats d’aquests mostren com la formació i evolució de les conques sinclinals depenen de diversos factors que actuen simultàniament durant l’extensió i la posterior inversió.Aquesta tesi es pot utilitzar com a guia per reduir les incertes durant la interpretació de la geometria del basament associat a conques sinclinals desenvolupades en contextos salins, sobretot en els casos on les dades sísmiques disponibles no tenen bona resolució i cal utilitzar la geometria de la cobertora per reconèixer l’estructura del basament.