The Geometry and Evolution of Supra-Salt Normal Fault Arrays

Abstract

The structural style and evolution of normal fault arrays above mobile salt is less understood compared to those developed in rifts where strong variations in mechanical stratigraphy are absent. This research was performed to refine the understanding of the growth of supra-salt extensional fault arrays and how salt may impact the growth history and structural style of such arrays. To investigate the three-dimensional growth history of supra-salt fault arrays, detailed qualitative and quantitative analyses have been performed of fault geometry/morphology, throw distribution and syn-kinematic strata of fault arrays in two different salt basins (the techniques used in the analyses are presented in Paper I), the Egersund Basin, offshore Norway (Paper II & IV) and Santos Basin offshore Brazil (Paper III). The first paper discusses how different techniques of fault analyses can be used to determine the style of growth of seismic-scale syn-sedimentary normal faults. It is presently known that normal faults can either grow via sympathetic increase in displacement and length (‘isolated model’) or by rapid establishment of their nearfinal length prior to significant displacement accrual (‘coherent model’). Application of quantitative techniques exemplified in Paper I can aid the assessment of the relative roles of the isolated and coherent fault models during crustal extension. Paper II discusses how growth of normal faults in multilayer sequences can occur by the use of a 3D seismic case study of a supra-salt fault array in the Egersund Basin, offshore Norway. The supra-salt fault array was initiated by re-activation of a pre-existing sub-salt fault array during the Late Triassic, mainly through kinematic coupling with the sub-salt fault array, before being buried during the Late Jurassic to Turonian times. Only parts of the fault array were reactivated during the Turonian, in response to salt mobilisation driven by basin inversion. Based on the three-dimensional fault analyses, we conclude that a combination of basement faulting and salt (re-) mobilisation is the driving mechanisms behind fault activation and reactivation. Reactivated faults are located where the underlying salt is thick, while the non-reactivated faults are found where salt is depleted. Even though the sub- and supra-salt faults are mainly geometrically decoupled through the salt, our findings suggest that a kinematic coupling existed as sub-salt faults still affected nucleation and localization of the cover faults. Paper III describes the evolution of supra-salt fault arrays within the São Paulo Plateau in the Santos Basin. Kinematic analyses reveal that the fault arrays formed during two discrete phases of deformation (Albian-to-Miocene fault arrays above salt anticlines and -plateau, and Oligocene-to-Present fault arrays above salt-walls). Albian to Miocene faulting likely reflects a combination of thin-skinned overburden extension, which drove reactive diapirism in relatively distal parts of the basin, and outer-arc extension-related faulting driven by regional contraction and overburden shortening. In contrast, the Oligocene-to-Present faults likely formed due to thin-skinned overburden extension above mature salt walls. We show that: (i) supra-salt fault growth can be protracted and polyphasal; (ii) it can be difficult to determine the triggers and drivers for supra-salt normal fault nucleation and growth; and (iii) domains of extension and contraction on saltbearing passive margin may not be mutually exclusive and may overlap in time and space. Paper IV discusses the geometry and growth of a normal fault array formed during the birth, growth and death of an array of salt structures. We show that the suprasalt fault array and salt structures developed in response to; (i) movement on a thick-skinned, basin bounding fault in the Late Triassic, causing salt mobilisation by base salt tilting and gliding, which in turn caused the nucleation and initial fault growth by thin-skinned extension. Overburden gravity gliding and stretching drove reactive diapirism and salt wall growth that continued into the Late Jurassic, when the salt stocks nucleated. The faults reached their near-full length during this time. The amount of fault growth during reactive diapirism during the Late Triassic-Late Jurassic was controlled by salt flow into the widening salt walls (though extension was driving the reactive diapirism); (ii) subsequent to reactive diapirism, diapir collapse due to along-strike migration of salt within the wall drove further fault growth, principally by displacement accrual in the Early Cretaceous, with active diapirism occurring simultaneously along strike; (iii) active diapirism occurring until the Neogene, which drove further salt wall collapse and fault growth near the diapirs. Our study supports physical model predictions, showcasing a threedimensional example of how protracted, multiphase salt diapirism can influence the structure and growth of normal fault arrays. This study has demonstrated how qualitative and detailed quantitative fault analyses of supra-salt fault arrays can be used to track the flow of salt and provide a contribution to the understanding of overburden deformation linked to salt mobilisation. Further, this research offers insight into the complex interplay between salt diapirism and faulting, showcasing three-dimensional natural examples of how multiphase salt diapirism can influence the structure and growth of normal fault arrays. Detailed investigation of the dynamics of supra-salt deformation, have implications for the prediction the four-dimensional flow of salt and can thereby provide us with a better understanding of the tectono-stratigraphic evolution of salt-influenced sedimentary basins.