Styles of Rifting

Type One:-

Type one rifting occurs as splitting of the continent which is a precursor to the development of a new ocean. This is probably a result of tensile stress generated in response to buoyant uplift from a mantle plume. An example of this is the East African rift valley.

Type Two:-

Type two occurs by the linking together of a series of rift-rift-rift triple junctions. As this triple junction develops one of the rifts becomes inactive and is preserved in the continent as a failed rift system or an aulacogen. The aulacogen can then be reactivated at a later stage when a continent-continent collision occurs. The other two arms of the junction continue to evolve and join up to form a rift system. The formation of triple junctions has been rare in geological history but the East African rift valley is a current example.

The image above shows:

A: Doming by a mantle plume
B: A triple rift junction is initiated
C: Two of the rifts develop into an ocean and the third becomes a failed arm (aulacogen)
D: Less likely is that all three arms develop into oceans.

 

Type Three:-

Type three rift valleys form as a result of tensional regions forming as offsets from a large-scale compressional regime in a collision area. In areas distant from the colliding plates suture, tensional zones can develop as a response to strain on the system (Indentation tectonics). These rifts can be distinguished from aulacogens as they have no pre-collisional history in their geology. Examples of this type of rift are the Rhine Graben and the Baikal Rift.

Glossary:-

Indentation tectonics:- The development of failure and slip lines, which can be predicted mathematically. An analogy of this type of faulting is the lines of failure which develop when a plastic medium is indented by a rigid die. The pattern which the slip lines form is dictated by the shape of the indenter and the lateral constraints placed on the medium.

The image above shows an indentation experiment performed using plasticine as the plastic medium.

Aulacogens:-  Long-lived sedimentary troughs which are deeply subsided and generally fault-bounded. Aulacogens have a tendency to re-activate if renewed faulting and subsidence occurs. These are generally thought to form as a result of a triple rift system. An Aulacogen can also emerge from an unstable ridge-ridge-transform fault triple junction.

The image above shows:

a) A triple junction
b) A triple junction were one arm has failed (aulacogen)
c) An aulacogen emerging from and unstable ridge-ridge-transform triple junction.