Continents rifts are fault bounded basins produced by extension of continental crust. it can be single (like rift in East Africa) or multiple (basin and range province in the western U.S.).
Aulacogens are the failed or less active arms of triple junction.
Rifts are of different origins and occur in different regional tectonic settings. Rifts that form in cratons, such as the East African rift system are commonly associated with domal uplifts,although the timing of doming may vary. Geophysical data indicate that both the crust and the lithosphere are thinned beneath rifts and that most or all crustal thinning occurs in the ductile lower crust over a much broader area than represented by the surface expression of the rift.
Rifts can be classified into 2 categories:
A) Active Rift
B) Passive Rift
Active rifts are characterised by early uplift and basement stripping resulting from Crustal expansion due to deep heat source whereas in the case of Passive rifts, upliftment is confined to the stretched and faulted near surface region and the shoulder of the rift zone. Active rifts contain large volumes of volcanic rocks whereas Passive rifts contain immature clastic sediments and in some instances minor volcanic rocks. In the Active rifts upliftment extends for 100’s of kms beyond the rift zone and the zone of thinning is several times wider than the rift width. In the Passive rifts the lithosphere thinning is laterally confined to the rift zone.
Although the active rifts may form in number of tectonic settings, and they commonly exhibit similar overall patterns of development. The major stages are as follows:
- Development of broad shallow depression prior to doming or volcanism.
- Diapiric Asthenosphere or a mantle plume is forcefully injected into the base of the lithosphere.
- Buoyant isostatic uplift of heated lithosphere leads to doming.
- Doming and extensional forces cause crust attenuation and thinning of the lithosphere.
- Episodic dyke injection and volcanism alternate with faulting.
- Rift valley may develop and may be associated with voluminous felsic magmatism.
- An Active rift may be aborted at a relatively early stage of its development or it may continue to open and evolve into an ocean basin, such as the Red Sea.
There are three types of both Active and Passive Rifts. Active RIfts are represented by ocean ridges, continental rifts and aulacogens and back arc basins. As continental rifts and back arc rifts continue to evolve into oceanic ridges, and all three types of passive rifts(Transform fault related, collision related and Arc related) can evolve into active rifts.
Rock Assemblages in Continental RIfts: Continental rifts are characterised by immature terrigenous clastic sediments and bimodal volcanics.