Abstract: Tectonic History of Athapuscow Aulacogen (Middle Precambrian), Great Slave Lake, Northwest Territories

Paul F. Hoffman

Recent 1:50,000-scale geologic mapping of the entire Athapuscow aulacogen, complementing earlier stratigraphic work, provides the basis for a detailed reconstruction of its tectonic history.

1. Alkaline-intrusive stage: about 2,200 m.y. ago, at least two highly differentiated and strongly alkaline igneous complexes were intruded and, shortly thereafter, cut by a northeast-trending diabase dike swarm, at least 400 km long, that straddles the contact between 2,500-m.y. old granites on the northwest and mylonites of uncertain affinities on the southeast.

2. Rift-valley stage: a narrow rift, floored by reduced sediments and pillow basalts (Union Island Group), formed at the granite-mylonite contact. Block faulting enlarged the rift valley, which featured a medial horst and a north branch (Taltheilei Narrows), in which rivers carried clastic sediments (Sosan Group) toward a new ocean opening at the southwest end of the rift.

3. Downwarp stage: faulting abated but subsidence continued, producing a broad downwarp in which marine shales (Kahochella Group) transgressed the clastic deposits. Numerous basaltic and basalt-felsic centers (Seton Volcanics) developed along old fault lines during the rift-to-downwarp transition. As paleolatitude decreased, a stromatolitic carbonate bank (Pethei Group) developed along the northwest margin of the downwarp. A marly submarine slope led to a deep medial trough, into which graywacke turbidites (Blanchet Formation) were shed from a rising coast range at the southwest end of the trough. The trough ultimately filled with salt, above which shallow-water carbonate rocks and red beds were deposited. 4. Uplift stage: a profound structural inversion occurred as the medial trough began to rise and the axis of subsidence shifted to what had been the northwest margin of the downwarp. The salt dissolved and solution collapse produced a polyphase red-bed-carbonate megabreccia (Stark Formation). The uplift was denuded by mass wasting of the megabreccia and contemporaneous gravity sliding of nappes, hundreds of square kilometers in area, beneath the megabreccia. Kahochella-Pethei nappes were the first emplaced, above and behind which came Sosan nappes, and finally nappes of metamorphosed clastic sediments and bimodal volcanic rocks (Wilson Island Group) unknown in the autochthon. Red beds continued to pour into the aulacogen, transgressing the megabreccia. Sedimentation ended with effusio of subaerial basalts (Pearson Formation) and intrusion, localized at the base of the megabreccia, of numerous calcalkaline intermediate laccoliths about 1,800 m.y. old.

5. Folding stage: mild transverse compression of the aulacogen produced a broad synclinorium.

6. Transcurrent-fault stage: following a period of erosion, the synclinorium was transected longitudinally by a system of dextral strike- and oblique-slip faults, of which the most prominent has 60-km displacement. Alluvial fanglomerates (Et-then Group), locally in homoclines of great apparent thickness, were deposited during faulting.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma