Aulacogen

An aulacogen is a failed arm of a triple junction. Aulacogens are a part of plate tectonics where oceanic and continental crust is continuously being created, destroyed, and rearranged on the Earth’s surface. Specifically, aulacogens are a rift zone, where new crust is formed, that is no longer active. The term aulacogen is derived from Greek aulax 'furrow' and was suggested by the Soviet geologist Nikolay Shatsky in 1946. A triple junction is the point where three tectonic plates meet; the boundaries of these plates are characterized by divergence, causing a rift zone or spreading center; a transform fault; or convergence causing subduction or uplift of crust and forming mountains. 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. The inversion of ancient, buried aulacogens can exert a dramatic effect on crustal deformation. 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.