How does a narrow rift become a greatly wide graben system?

In general we think that rifts are generated on magmatic systems. However, there are many rifts on magma-poor systems. 30 years ago, Lister et al. said that a wide graben system is generated by large detachment faults which have very low fracture angles. After then Renero et al. issued new theory of a wide graben system. They mentioned that sequential faulting is the key of asymmetry and wide conjugate margins in 2010. Eventually, in 2014 Brune at al. realized that the reason why sequential faulting is occurred. The essential factor of this situation is the rift migration. Especially, it forms hyper-extended continental crusts which are very thin and have up to 200km width.

Brune et al. published a paper entitled “Rift migration explains continental margin asymmetry and crustal hyper-extension” at 『nature communications』 in 2014. They performed new numerical models which consider rheologies and extension velocities appropriate for Central South Atlantic segment and West Iberia-Newfoundland. They proved that hyper-extended and continental crusts and asymmetry margins can be generated by a migration of very narrow rift which makes sequential faulting. Also, they found this migration depends on lower crustal flow.

This geological system is important in academic and economic field. Although they already have many field-based studies, few people study about these graben systems like Central South Atlantic segment and West Iberia-Newfoundland. Brune at al. want people to more easily study this system. So they provided this new numerical model about wide graben systems involved hyper-extended continental crust and asymmetry conjugate margins.

Figure 1 The normal fault becomes the narrow rift. While it migrates, graben system is extended.

In this study, Brune at al. grasp the significant cause of a wide graben system. Controlling the rift migration and the thermos profile of the system, they analyze strain rate, viscosity and horizontal velocity of this system. They can conclude how hyper-extended continental crust generates.

I mentioned they found that narrow rift migration is important in hyper-extended system. This migration depends on four factors: initiation of rift, maintenance of rift migration, rift migration velocity and initial thermal profile. Firstly, initiation of rift makes the crust thin and it causes hot asthenospheric upwelling. Sequentially, ‘exhumation channel’ is formed in a pocket of weak lower crust at the fault tip. This channel changes a lateral strength gradient which is driving force of rift migration. And then, a maintenance of rift migration is necessary to widen the graben system. This is because sequential faulting which generates the asymmetry conjugated margins originates in the maintenance. Thirdly, rift migration velocity determines the bulk of exhumation channel. High velocity leads to a Large channel that makes more hyper-extended system. Lastly, initial thermal profile influences the size of rift and the degree of symmetry.

This study is so important that we should progress. Moreover, they based in Iberia/Newfoundland and Angola/Brazil conjugates which are studied in field. Because these margins are in the Atlantic, we can investigate how the ocean started up segmentation. Also, East African rift valley looks like this system. Scientist who study in East African rift valley can be inspire by this study.