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Aulacogens are long-lived deeply subsiding troughs, at times fault-bounded, that extend at high angles from geosynclines far into adjacent foreland platforms. They are normally located where the geosyncline makes a reentrant angle into the platform. Their fill is contemporaneous with, as thick as, and lithologically similar to the foreland sedimentary wedge of the geosyncline but in addition has periodically erupted alkalic basalt and fanglomerate. Although many aulacogens have suffered mild compressional deformation, tectonic movement within them is mainly vertical; large-scale horizontal translations are rare. Aulacogens are known throughout the Proterozoic and Phanerozoic, and incipient aulacogens occur at reentrants on modern continental margins.

The 1700-to-2200-million-year-old Athapuscow Aulacogen of Great Slave Lake began as a deeply subsiding transverse graben during the early miogeoclinal stage of the Coronation Geosyncline. During the orogenic stage of the geosyncline, the aulacogen became a broader downwarp that received abnormally thick exogeosynclinal sediments from the orogenic belt. The aulacogen was compressed mildly, prior to a final stage involving transcurrent faulting, one-sided uplift, and continental fanglomerate sedimentation. The aulacogen is distinguished from the foreland sedimentary wedge of the geosyncline by having paleocurrents parallel rather than transverse to its structural trend, by having high-angle faults rather than low-angle thrusts, by its alkalic basalt volcanism, and by the lack of metamorphism.

It is hypothesized that deep-mantle convective plumes produce three-armed radial rift systems (rrr triple junctions) in continents stationary with respect to the plumes. If only two of the arms spread to produce an ocean basin, the third remains as an abandoned rift extending into the continental interior from a reentrant on the new continental margin. For example, the Benue Trough, located in the Gulf of Guinea reentrant on the west coast of Africa, may be such an abandoned rift arm formed during the Cretaceous period at the time of initial rifting of Africa and South America. Inasmuch as new continental margins are predestined to become geosynclines, such abandoned rift arms are juvenile aulacogens. In this model, aulacogens and geosynclines have a common origin but differ in the extent of rifting.

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