In geology, an aulacogen is a failed arm of a triple junction of a plate tectonics rift system. A triple junction beneath a continental plate initiates a three way breakup of the continental plate. As the continental break-up develops one of the three spreading ridges typically fails or stops spreading. The resulting failed rift is called an aulacogen and becomes a filled graben within the continent and is a zone of structural weakness.
The Mississippi embayment with the associated New Madrid Seismic Zone is an example of an ancient aulacogen that dates back to the breakup of the ancient continent Rodinia, as is the Rio Grande Rift. On the Southwestern European margin (offshore Portugal) is located another abandoned rift basin (Lusitanian Basin) that evolved at the same time of the Canadian Grand Banks region, where the Hibernia oil field is located. 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.