Triple junctions

In plate tectonics theory during the breakup of a continent, three divergent boundaries form, radiating out from a central point (the triple junction). One of these divergent plate boundaries fails (see aulacogen) and the other two continue spreading to form an ocean. The opening of the south Atlantic Ocean started at the south of the South American and African continents, reaching a triple junction in the present Gulf of Guinea, from where it continued to the west. The NE-trending Benue Trough is the failed arm of this junction. The junction of the Red Sea, the Gulf of Aden and the East African Rift centered in the Afar Triangle (the Afar Triple Junction) is a Ridge-Ridge-Ridge (R-R-R) triple junction.

The term triple-junction has come to refer to any point where three tectonic plates meet.

An aulacogen is a failed arm of a triple junction.

The failed arm of a triple junction can be either a transform fault that has been flooded with magma, or more commonly, an inactive rift zone. Aulacogen formation starts with the termination of an active rift zone, which leaves behind a graben-like formation. Over time, this formation starts to subside and eventually minor volcanism starts to take place. The final inversion stage takes place when tectonic stress on the aulacogen changes from tensional to compressional forming horsts.

Aulacogens can become a filled graben, or sedimentary basin surrounded by a series of normal faults. These can later become the pathway for large river systems such as the Mississippi River. The rock forming an aulacogen is brittle and weak from when the rift zone was active, causing occasional volcanic or seismic activity. Because this is an area of weakness in the crust, aulacogens can become reactivated into a rift zone. An example of a reactivated aulacogen is the East African Rift or the Ottawa-Bonnechere Graben in Ontario and Quebec, Canada, an ancient aulacogen that reactivated during the breakup of Pangaea. Abandoned rift basins that have been uplifted and exposed onshore, like the Lusitanian Basin, are important analogues of deep-sea basins located on conjugated margins of ancient rift axes.