November 7th 2017

How continents rift apart

The continental shelf in light blue on both sides of the North Atlantic. The passive margins coincide largely with the continental shelf. Illustration: Google Earth
Complex interactions between new and old fault systems explain how continents break apart. Geological number crushers have developed numerical models to explain how the boundaries of continents - the so-called passive margins - are extended and thinned over millions of years until they become separated by a new ocean.

"We find that fault activity to a large degree controls this continental thinning", explains researcher and team leader Susanne Buiter at the Geological Survey of Norway (NGU). She is one of the researchers who have now published the new findings in Nature Communications.

Faults are broken surfaces in rocks, that are usually built through repeated sudden, sharp movements, such as earthquakes. A fault forms the boundary between two blocks of rock, or continental crust, that have moved in relation to each other.

- During continental extension faults may be active, inactive, and active again, they can be cut by new faults, change shape, and so create separate blocks in the bedrock. Extension is not a steady and smooth process, but instead it takes place in phases that are divided over several tens of millions of years and that leave their imprint on the continental margin", says Buiter.

Thinner towards the ocean

The extended, or rifted, continental crust of a passive margin becomes thinner towards the ocean, intersected by faults and overlain by sedimentary basins. In such areas, as on the Norwegian continental shelf, we can find large oil and gas resources.

"Once we know exactly how these processes have taken place over time, we hope in the long term to be able to better predict where it is most relevant to look for oil and gas", says Susanne Buiter.

She points out the numerical modelling of the research group has helped to understand the formation of passive margins precisely by calculating the evolution of the fault systems. "It was a little difficult to get the first rift phase in place in the models. Once we managed that, the further complex development and interactions of the faults followed eventually", said Buiter.

The research was undertaken at the Geological Survey of Norway in Trondheim in a project funded by the Norwegian Research Council through their basic-research funding scheme (FRINATEK 213399/F20 awarded to Buiter). First author John Naliboff now works at the University of California, Davis, USA.

The new numerical model shows great agreement to the seismic profiles of the Gossa height (top illustration) in the Møre Basin of the Mid-Norwegian margin. But in addition to being visually alike, the model explains the evolution of the fault systems over time. The red and yellow fields mark active faults (rate by which strain accumulates), while the blue indicates where faults have been formed (accumulated deformation).