Australia: The Land Where Time Began

A biography of the Australian continent 

Continental Rifts & Rifted Margins

At continental rifts, regions of extensional deformation, the full thickness of the lithosphere has been deformed, under the influence of deviatoric tension. Major features of the lithosphere are called rifts, and smaller-scale extensional structures are not covered by this term, they can be found associated with virtually any type of deformation.

In the breakup of continents, rupture of the lithosphere can lead to extension and the opening of new ocean basins, rifts being the initial stage. Where rupture of the lithosphere is complete, rifts eventually become inactive, with the formation of rifted or passive continental margins. The thinning of the continental crust, and the the dissipation of heat that had been transferred from the asthenosphere to the plate during rifting, leads to subsidence of the margins below sea level by isostatic compensation. Failed rifts, aulacogens, are those that do not reach the point at which generation of oceanic crust is achieved, the rift becoming inactive at some stage of its evolution. The Connecticut Valley in the northeastern US, from the Mesozoic, is an example of a failed rift. The North Sea Basin is another.

The internal structure, history and dimensions have been found to vary greatly by research at active rifts (Ruppel, 1995). Differences in the strength and rheology of the lithosphere at the initiation of rifting, as well as processes influencing these properties as rifting progresses, explain much of the variability. Rifts tend to form narrow zones of localised strain that are less than 100 km wide at places with thick lithosphere that is cool and strong. Examples of this rift type are the Rhine Graben, the East African Rift system and the Baikal Rift. Rifts tend to form wide zones with delocalised strain, distributed across zones several hundred kilometres wide, in regions of thin lithosphere that is hot and weak. This rift type is seen in the basin and Range Province and the Aegean Sea. Volcanic activity may be associated with both rift types. Magmatism in which large volumes of magma are produced, and continental flood basalt eruptions, characterise some rift segments, such as those in Kenya, Ethiopia and Afar. In other places that are considered to be magma starved, the western branch of the East African Rift system and the Baikal Rift, the volumes of volcanic rocks are very small.

See Source 1 for more detailed information on all aspects of plate tectonics

Sources & Further reading

1. Kearey, Philip, Klepeis, Keith A. & Vine, Frederick J., 2009, Global Tectonics, 3rd Edition, Wiley-Blackwell.