ABSTRACT

How to form hyper-extended continental rifted margins  
Susanne Buiter (Geological Survey of Norway)

Continental rifted margins differ in margin width, onshore topography, and fault patterns. For example, it has been suggested that the mid-Norwegian margin may be hyper-extended, thus displaying highly thinned continental crust that extends towards the oceanic domain over a wide region. Other continental margins, such as in the northern Norway and Flemish Cap regions, are much narrower. Numerical experiments of continental rifting show that the width of rifted margins varies with crustal rheology and extension velocity. A relatively “strong” lower crust leads to a short margin accompanied by high rift flank uplift. A “weak” lower crust at moderate extension rates forms a long, hyper-extended crust. However, unless along-strike differences in crustal strength are invoked, these experiments cannot explain along-strike differences in margin width.

I will discuss 2D numerical experiments of continental margin formation and use these to illustrate how hyper-extension may be further promoted by natural variations in the rift system. I first highlight the impact of collision-phase inheritance on rifted margin architecture with experiments of a Wilson Cycle of subduction, collision and extension. Thermal inheritance in the form of elevated temperatures in the collisional crustal nappe stack promotes hyper-extension through its weakening effect on crustal rheology. I then show that sedimentation may alter margin architecture for cases with an intermediate strength lower crust by shifting from fast break-up with a sharply tapered margin to prolonged rifting with the development of hyper-extended crust. Natural along-strike variations in collision-phase inheritance combined with the interplay between crustal rheology and surface processes can therefore provide an explanation for variations in margin architecture observed in rifted margins such as the Atlantic margin systems.