What is "aulacogen"

n. (label en geology) A tectonic trough on a craton that is formed as the failed arm of a triple-rift junction.

In geology, an aulacogen or aulocogen 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 system within the continental lithosphere.