The 3D nature of passive continental margins

Rifted continental margins form when continents are stretched until they break. The North Atlantic Ocean, for example, formed after intermittent extension over a few hundred million years finally separated North America and Europe. The geologic processes involved in continental break-up are important for understanding the fundamentals of plate tectonics, assessing earthquake and volcanic hazards during continental extension, and providing a tectonic framework for interpreting the geology of hydrocarbon deposits along continental margins. Geophysical data and interpretations reveal that continental break-up processes produce complex regions of faulting and sedimentation at continental margins. Variations between a few hundred to almost thousand km occur between and along margins in their conjugated widths from the relatively undisturbed continent to the oceanic crust. The Norwegian margin, for example, shows deformation widths between approximately 250 to 500 km. Previous studies suggested that the deformation widths of rifted margins have a relationship to their obliquity of divergence. We tested this hypothesis by analysing the rift obliquity and deformation widths for 25 conjugate rifted margin segments in the Atlantic and Indian Oceans. We used the plate reconstruction software GPlates (www.gplates.org) for different plate rotation models to estimate the direction and magnitude of rifting from the initial phases of continental rifting until break-up. Our rifted margin deformation width corresponds to the distance between the onshore maximum topography and the offshore limit where the continental crust is thinner than approximately 10 km. We find a weak positive correlation between the obliquity of rifting and deformation width: Highly oblique margins are narrower than orthogonal margins. This correlation may imply that the required force for continental break-up is less for oblique margins, which has been argued for on the basis of numerical models. Numerical experiments of rifted margins have highlighted that deformation widths of margins not only depend on rift obliquity, but also on extension rate and material behaviour (rheology). Our experiments illustrate how long extended margins form in continents that are rheologically weak (for example, because they are warm) and that are extended at moderate velocities. To date, however, such numerical models typically assume that the continents are homogeneous with flat-lying lithologies. In nature, however, we observe that rifted margins tend to originate on continental collision zones. For example, extension between Norway and Greenland followed closely after Silurian continent-continent collision had built the Caledonian mountains. We used numerical experiments to investigate the effects of collisional inheritance on rifted margin architecture. Our experiments illustrate that continental rifts can utilise collisional thrust faults and/or the weak former subduction interface to exhume deep crustal rocks to the surface. In tandem, elevated temperatures in the collisional orogen can weaken its crustal rheology sufficiently to localise rifting. A fascinating result of our experiments is that inheritance also lies in the sub-lithospheric mantle, where mantle flow currents produced during oceanic subduction influence rift development from below. The level of variations in rifted margin architecture because of structural and thermal inheritances is at least of the same order as caused by variations in extension velocity and crustal rheology, if not surpassing those.

Passive margins form after rifting and break-up of a continent. They are called 'passive', but they can still experience active deformation in terms of movements, faulting and volcanism. Such active processes directly impact communities along rifted margi ns worldwide. In addition, passive margins offer great economic opportunities as many of the world's oil and gas reservoirs occur on continental shelf areas. These reasons point to the necessity of a deep understanding of the processes that shaped passive margins. Passive continental margins may form in orthogonal extension, but because plate motions are rarely exactly orthogonal to plate boundaries, it is common that plate margins experience oblique relative motions during at least part of their evolutio n. An example of oblique rifting is the early phase of extension between Norway and Greenland (in the Jurassic-Cretaceous), which was characterised by a large component of strike-slip movement. This makes margin development a 3D problem. In this project, we ask the question how the oblique character of many passive margins determined their geometry, faulting style and topographic evolution. We will attempt to answer this question by a unique combination of computer models, laboratory experiments and knowl edge from seismic and geological observations. The laboratory experiments will be built of sand and silicone (resembling brittle and ductile crustal materials) in a new modelling laboratory at the University of Bern in Switzerland. The numerical experimen ts use a new 3D modelling software developed jointly at the Geological Survey of Norway and GNS Science in New Zealand. Our experiments will provide a new view on margin development, which is of direct relevance to margin hazard, hydrocarbon exploration a nd landscape development studies.