Joint Assembly 2017
The Relationship between Sub- and Supra-salt Deformation in Salt-Influenced Rifts: Observations from the Halten Terrace, offshore Norway.
Alexander J. Coleman *1, Christopher A.-L. Jackson1, Oliver B. Duffy2
1Basins Research Group (BRG), Department of Earth Science and Engineering, Imperial College, Prince Consort Road, London, SW7 2BP, U.K. ()
2Bureau of Economic Geology, The University of Texas at Austin, University Station, Box X, TX 78713-7508, USA.
The structural style and evolution of salt-influenced rifts significantly differs from those that form in simple, predominantly ‘brittle’ crust. For example, ductile, evaporite-rich units can effectively decouple brittle deformation in sub- and supra-salt strata, resulting in the sub-salt fault becoming mechanically decoupled from, but responsible for, a forced fold in the supra-salt strata. Salt-influenced rifts therefore contain greater degrees of folding when compared to their non-salt-influenced counterparts, which is often ignored when estimating extension below and above salt. Fundamental to determining whether sub- and supra-salt fault systems balance (i.e. sub- ≈ supra-salt extension), and the relative contributions of thin-skinned, gravity-driven and thick-skinned, whole-plate stretching driven deformation, is the ability to accurately measure extension in salt-influenced rifts.
We address these issues and seek to understand how extensional strain is partitioned between faulting and folding using high quality 2D and 3D seismic and well data from the Halten Terrace, offshore Norway. Given that the salt in this location is relatively thin and immobile compared to other salt-influenced basins in the North Sea, diapirism is minimal and no allochthonous salt bodies are developed. This permits the study of salt-influenced rift structures without significant structural overprinting.
In this study, we: (1) describe the structural style and evolution of the sub- and supra-salt fault populations, (2) apply various methods to estimate extension at sub- and supra-salt levels, and (3) deconvolve the contributions of purely thin- and thick-skinned strain in deforming the overburden. We find that despite similar amounts of extension in sub- and supra-salt strata, the supra-salt strata preferentially accommodate strain by folding, whereas sub-salt strata tend to fault. This suggests that while the system is kinematically linked, strain is expressed differently above and below the salt. These results highlight that different structural styles that are stratigraphically-separated by salt should not be interpreted as isolated systems.