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aulacogen A long-lived, sediment-filled graben oriented at a high angle to either a neighbouring modern ocean or a neighbouring orogenic belt. The sediments in an aulacogen are largely characterized by a lack of major deformation, although late strike-slip movement on the boundary faults is known to deform the sediments extensively. Aulacogens have been interpreted as forming within failed rifts (see RIFT VALLEY) of triple junctions and thus are taken to indicate plate tectonic activity. They have been identified as far back as the early Proterozoic and are of world-wide distribution.