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. Geology (from Greek γη- (ge-, the earth) and λογος (logos, word, reason)) is the science and study of the Earth, its composition, structure, physical properties, history and the processes that shape it. ... A triple junction is the point where three tectonic plates diverge. ... Plate tectonics (from the Greek word for one who constructs and destroys, τεκτων, tektoon) is a theory of geology developed to explain the phenomenon of continental drift and is currently the theory accepted by the vast majority of scientists working in this area. ... It has been suggested that this article or section be merged with Rift valley. ... USGS image A graben is a depressed block of land bordered by parallel faults. ...

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. 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. This article may be too technical for most readers to understand. ... Seismic map New Madrid Seismic Zone. ... Depiction of Rodinia at time of initial breakup. ...