Aulacogenes are long-lived trenchlike basins first described on the Russian and Siberian platforms. Subsidence in known aulacogenes was mainly Proterozoic, but many persisted as moderate negative structures in the Phanerozoic and others rebounded vertically to become linear ridges. Thus, important facies changes in Phanerozoic sediments may be located by projecting the trends of exposed aulacogenes into the subsurface.

Two lower Proterozoic aulacogenes have been recognized in the northwestern Canadian shield--one with a WSW trend that dips beneath Phanerozoic cover at Great Slave Lake and a second with a NNW trend that dips beneath the cover of Victoria Island from Bathurst Inlet.

Aulacogenes are much longer lived than other intracratonic basins. They are fault bounded, linear, and narrow, commonly 200 mi long but only 25 mi wide. They trend at high angles to the craton margin, where they merge with a contemporaneous orthogeosyncline that borders the craton. Their rock sections thicken outward from the center of the craton to as much as 40,000 ft near the margin. They contain mostly miogeosynclinal and exogeosynclinal facies, but with the addition of basic volcanics and fanglomerate at several levels. Alluvial sediment transport in aulacogenes is mainly parallel with their trends, not transverse as in the geosyncline. The sedimentary beds have mild compressive folds paralleling the boundary faults, but low-angle overthrusts, typical of the geosyncline, are abse t.

Aulacogenes, unlike geosynclines, are not accounted for by global plate tectonics. The dipping of laterally adjacent sediments toward aulacogenes, the compressive deformation within aulacogenes, and the abrupt descent of the Mohorovicic discontinuity beneath aulacogenes indicate foundering of a narrow slice of continental crust into the mantle (as opposed to tensional block faulting) to produce aulacogenes. If this interpretation is correct, aulacogenes should be bounded by high-angle reverse faults, rather than by normal faults.

Aulacogenes, characteristic of the Proterozoic, may represent a stage in the evolution of the continental portions of global plates intermediate between the pervasive mobility of the Archean and the persistent true cratons of the Phanerozoic in which only relict aulacogenes are found.

End_of_Article - Last_Page 627------------